i . i m i I m mm 1 I I - ' 1 THE MICROSCOPE: REVELATIONS. WORKS BY DR. CARPENTER. I. PRINCIPLES OF COMPARATIVE PHYSIOLOGY. A NEW AMERICAN, FROM THE FOURTH AND REVISED LONDON EDITION. With more than three hundred beautiful illustrations. In one large and hand- some octavo volume of 752 pages. Without pretending to it, it is an Ency- clopaedia of the subject, accurate and com- plete in all respects — a truthful reflection of the advanced state at which the science has now arrived. — Dublin Quarterly Jour- nal of Medical Science. A truly magnificent work — in itself a perfect physiological study. — Rankings Abstract. This work stands without its fellow. It is one, few men in Europe could have undertaken ; it is one, no man, we believe, could have brought to so successful an issue as Dr. Carpenter. We feel that this ab- stract can give the reader a very imperfect idea of the fulness of this work, and no idea of its unity, of the admirable manner in which material has been brought, from the most various sources, to conduce to its completeness, of the lucidity of the reason- ing it contains, or of the clearness of lan- guage in which the whole is clothed.—— Medical Times. II. PRINCIPLES OF HUMAN PHYSIOLOGY. WITH THEIR CHIEF APPLICATIONS TO PSYCHOLOGY, PATHOLOGY, THERAPEUTICS, HYGIENE, AND FORENSIC MEDICINE. A new American, from the last and revised London Edition, with nearly three hundred illustrations. Edited, with additions, by F. Gurney Smith, M.D. In one large and handsome volume of about nine hundred pages. The most complete work on the science in our language. — Jim. Med. Journal. The most complete work now extant in our language. — 2V. O. Med. Register. The best text-book in the language on this extensive subject. — London Med. Times. A complete cyclopaedia of this branch of science. — N. Y. Med. Times. The most complete exposition of physio- logy which any language can at present give. — Brit, and For. Med.-Chirurg. Review. III. ELEMENTS (OR MANUAL) OF PHYSIOLOGY. INCLUDING PHYSIOLOGICAL ANATOMY. Second American, from the last and revised London Edition, with one hundred and ninety illustrations. In one handsome octavo volume of 566 pages. In his former works it would seem that I Those who have occasion for an elemen- hehad exhausted the subject of Physiology, tary treatise on Physiology, cannot do bet- In the present, he gives the essence, as it ter than to possess themselves of the manual were, of the whole. — N. Y. Journal of of Dr. Carpenter. — Medical Examiner. Medicine. PREPARING. PRINCIPLES OF GENERAL PHYSIOLOGY, including ORGANIC CHEM- ISTRY, and HISTOLOGY. With a general sketch of the Vegetable and Animal Kingdoms. With several hundred illustrations. In one large and handsome octavo volume. In his last edition of the "Comparative Physiology" and "Human Physiology," the author found it desirable to omit the chapters connected with " General Physio- logy." He has therefore undertaken to prepare a volume devoted exclusively to that subject, forming an introduction to the other works, or, taken in conjunction with them, constituting a complete and extended system of Physiology, in all its branches. THE MICROSCOPE: REVELATIONS. BY WILLIAM B. CARPENTER, M.D., F.R.S., F.G.S., EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON PROFESSOR OF MEDICAL JURISPRUDENCE IN UNIVERSITY COLLEGE J PRESIDENT OF THE MICROSCOPICAL SOCIETY OF LONDON ; ETC. WITH AN APPENDIX CONTAINING THE APPLICATIONS OF THE MICROSCOPE TO CLINICAL MEDICINE, ETC. BY FRANCIS GURNEY SMITH, M.D., PROFESSOR OF THE INSTITUTES OF MEDICINE IN THE MEDICAL DEPARTMENT OF PENNSYLVANIA COLLEGE, ETC. ILLUSTRATED BY FOUR HUNDRED AND THIRTY-FOUR PHILADELPHIA: BLANCHARD AND LEA. 1856. Entered, according to Act of Congress, in the year 1856. BY BLANCHARD AND LEA, In the Clerk's Office of iiie District Court for the Eastern District of Pennsylvania. C. SHERMAN & SON, PRINTERS, 19 St. James Street. PREFACE TO THE AMERICAN EDITION. THE American Edition of Dr. Carpenter's Work has been reprinted with the Author's sanction, from advance sheets fur- nished by him to the American publishers. In assuming the supervision of the press, the Editor has been careful to leave the work as it came from the Author's hands. Such additions as seemed most required by the Students of this country have been made in the form of an Appendix. The reader will find Dr. Carpenter's reasons for omitting the Clinical Applications of the Microscope, fully detailed in his Preface ; but as the various works on this subject are not readily accessible on this side of the Atlantic, it was thought that a selection from them, in a compendious form, might enhance the usefulness of the work. Free use has been made, therefore, of the excellent manuals of Beale and Bennett ; and the various kindred treatises and jour- nals have also been drawn upon. All that this portion of the work claims is to present a general view of those subjects which seem to be most required by the student of the Microscope. The growing interest in this important field of inquiry will, it is hoped, afford sufficient apology for its introduction. vi PREFACE TO THE AMERICAN EDITION. A short account of American Microscopes, their modifications and accessories, has also been added, and the whole Appendix has been fully illustrated, through the liberality of the Pub- lishers, by the addition of upwards of one hundred wood- engravings. FRANCIS GURNEY SMITH, M.D. 428 WALNUT STREET, June, 1856. PREFACE. THE rapid increase which has recently taken place in the use of the Microscope, — both as an instrument of scientific research, and as a means of gratifying a laudahle curiosity and of obtain- ing a healthful recreation, — has naturally led to a demand for information, both as to the mode of employing the instrument and its appurtenances, and as to the objects for whose minute examination it is most appropriate. And as none of the existing Treatises, either British or Foreign, on the Microscope and its Uses, have seemed to the Author fully adapted to meet this demand (some of them being almost exclusively concerned with the Microscope itself, and others with some special branch or branches of Microscopic study), he has felt encouraged to carry out a plan which he had formed several years since ; by endea- voring to combine, within a moderate compass, that information in regard to the use of his "tools" which is most essential to the working Microscopist, with such an account of the objects best fitted for his study, as might qualify him to comprehend what he observes, and might thus prepare him to benefit science, whilst expanding and refreshing his own mind. In his account of the various forms of Microscopes and Acces- sory Apparatus, the Author has not attempted to describe every- thing which is in use in this country; still less, to go into details respecting the construction of foreign instruments. He is satis- fied that in all which relates both to the mechanical and the optical arrangements of their instruments, the chief English Microscope-makers are decidedly in advance of their Conti- nental rivals ; and on this point he speaks not only from his own conviction, but from the authority of a highly accomplished German Microscopist, who has recently visited London for the Vlll PREFACE. express purpose of making the comparison. Even among the products of English skill, it was necessary for him to make a selection ; and he trusts that he will be found to have done ade- quate justice to all those who have most claim to an honorable mention. The great objection to English Microscopes, especially on the western side of the Atlantic, seems to be their costliness ; and as it can be affirmed with truth, that the instruments of Nachet, Oberhauser, and other Continental makers, are adequate for all essential purposes, a general preference is given to the latter (as the Author understands) among the Microscopists of the United States. He feels sure, however, that no one who has ever been accustomed to work with a well- constructed English Microscope will ever give the preference to a foreign instrument ; and he is glad to be able to announce that one of the best London firms is now prepared to supply a Microscope of excellent quality at a price very little exceeding that paid for Continental instruments, of far superior capabilities. (See p. 103, note.) In treating of the Applications of the Microscope, the Author has constantly endeavored to meet the wants of those who come to the study of the minute forms of Animal and Vegetable life with little or no previous scientific preparation, but who desire to gain something more than a mere sight of the objects to which their observation may be directed. Some of these may perhaps object to the general tone of his work as too highly pitched, and may think that he might have rendered his descriptions simpler by employing fewer scientific terms. But he would reply to such, that he has had much opportunity of observing, among the votaries of the Microscope, a desire for such information as he has attempted to convey (of the extent of which desire, the success of the " Quarterly Journal of Microscopical Science" is a very gratifying evidence) ; and that the use of scientific terms cannot be dispensed with, since there are no others in which the facts can be expressed. As he has made a point of explaining these, in the places where they are first introduced, he cannot think that any of his readers need find much difficulty in appre- hending their meaning. The proportion of space allotted to the various departments, has been determined, not so much by their Physiological im- portance, as by their special interest to the Microscopist ; and the remembrance of this consideration will serve to account for PREFACE. IX much that might otherwise appear strange. The Author has thought it particularly needful to restrain himself, in treating of certain very important subjects which are fully discussed in trea- tises expressly devoted to them (such, for example, as the struc- ture of Insects, and the Primary Tissues of Vertebrata), in order that he might give more space to those on which no such sources of information are readily accessible. For the same reason he has omitted all reference to the applications of the Microscope to Pathological inquiry; a subject which would interest only one division of his readers, and on which it would have been impossible for him to compress, within a sufficiently narrow compass, a really useful summary of what such readers can readily learn elsewhere. So again, the application of the Micro- scope to the detection of Adulterations in Food, &c., is a topic of such a purely special character, and must be so entirely based on detailed descriptions of the substances in question, that he has thought it better to leave this also untouched. It has been the Author's object throughout, to guide the pos- sessor of a Microscope to the intelligent study of any department of Natural History, that his individual tastes may lead him to follow out, and his particular circumstances may give him faci- lities for pursuing. And he has particularly aimed to show, under each head, how small is the amount of reliable knowledge already acquired, compared with that which remains to be attained by the zealous and persevering student. Being satis- fied that there is a large quantity of valuable Microscope-power at present running to waste in this country, — being applied in such desultory observations as are of no service whatever to science, and of very little to the mind of the observer, — he will consider the labor he has bestowed upon the production of this Manual as well repaid, if it should tend to direct this power to more systematic labors, in those fertile fields which only await the cultivator to bear abundant fruit. In all that concerns the working of the Microscope, the Author has mainly drawn upon his own experience, which dates back almost to the time when Achromatic Object-glasses were first constructed in this country. He would be ungrateful, however, if he were not to acknowledge that he has derived many valu- able hints from the Practical Treatises of Mr. Quekett and Dr. Beale, and from the Micrographic Dictionary of Messrs. Griffith and Henfrey. And among the works by which he has been X PREFACE. specially aided in treating of the Applications of the instrument, he would especially name Mr. Quekett's valuable Lectures on Histology (Vegetable and Animal), Mr. Ralfs's beautiful Mono- graph on the British Desmidiese, Prof. W. Smith's on the Diato- macese (which will, when complete, be quite worthy to take rank with the preceding), and the Micrographic Dictionary. All the Illustrations have been drawn by Mr. W. Bagg, and have been engraved under his superintendence ; and the Author ventures to affirm, that for fidelity as well as for beauty of execution, they will bear comparison with any Microscopic de- lineations yet executed on wood. A large proportion of the subjects are original ; the sources of all that are not so, are spe- cified in the list (p. xvii). The Author feels that some apology is due for the long delay which has attended the appearance of this work. When it was first announced as forthcoming, his full intention was to apply himself immediately to its production ; but the unexpected de- mand for new editions of his two large Treatises on Physiology, required that the whole of his disposable time and attention should be given up during two years to carrying these through the press. When he at last found himself free to apply himself to the "Microscope," he fully expected that the forward state of his preparations would enable him to complete it by October, 1855. But in this expectation he has been disappointed by the occurrence of two severe attacks of indisposition, which com- pelled him for a time to suspend all mental exertion, and have rendered it necessary for him carefully to abstain from overtask- ing himself; so that he feels assured that those who have kindly waited for the appearance of this volume, will not, when ac- quainted with these circumstances, blame him for a delay, the causes of which have lain so completely beyond his control. UNIVERSITY HALL, LONDON, Feb. 9, 1856. TABLE OF CONTENTS. INTRODUCTION. PAGE History of the Microscope and Microscopic Research, . . .33 CHAPTER I. Optical Principles of the Microscope, . . ... 65 CHAPTER II. CONSTRUCTION OF THE MICROSCOPE. General Principles, ........ 86 Simple Microscopes, ....... 90 Ross's, ......... 91 Gairdner's, . . . . . . . . 92 Field's, ......... 93 Quekett's, ........ 94 Compound Microscopes, ....... 96 Field's, ........ 97 Highley's, 99 Nachet's, . . .... .. 100 Smith and Beck's Student's, . . . . . .102 Do. Dissecting, .... ^ .": 103 Warington's Universal, ..... ^ . 105 Ross's, 108 Powell and Lealand's, . . . . . . .111 Smith and Beck's, . . . . . . . 113 Nachet's Binocular, . . . . . . .114 CHAPTER III. ACCESSORY APPARATUS. Draw-Tube, ......... 116 Erector, ......... 119 Micrometer, . . . . . . . . .120 Goniometer, . . . . . . . . 124 Indicator, ......... 124 Camera Lucida, . . . . . . . . 124 Object-Glass Holder, .. . . . . . .127 Xli CONTENTS. PAGE Object-Marker, 127 Lever Stage. . . . • • • • • .128 Object-Finder, ....... Magnetic Stage, . . . . . • • .130 Diaphragm-Plate, ..... Achromatic Condenser, . . . . . . .131 Keflecting Prisms, . . . 133 White Cloud Illuminator, ..... .134 Oblique Illuminators, ...... 135 Black-Ground Illuminators, ....... 138 Polarizing Apparatus, . ... . . . . 140 Illuminators for Opaque Objects, ...... 143 Stage-Forceps, ........ 146 Glass Stage-Plate, ..... . 147 Aquatic Box, . . . . . » . . 148 Zoophyte Trough, . . . . . . . .149 Compressorium, ...... .150 Dipping-Tubes, •„ , . • . . . .152 Forceps, » ... 153 CHAPTER IV. MANAGEMENT OP THE MICROSCOPE. Support, ......... 154 Light, 154 Position of Light, .-^ . . . . . . 157 Care of the Eyes, ...... .158 Care of the Microscope, . . . . . . . 159 General Arrangements, . . . . . . . 160 Arrangement for Transparent Objects, . . . . .168 Arrangement for Opaque Objects, . . . . . 175 Errors of Interpretation, . . . . . . .180 Comparative Values of Object-Glasses ; Test-Objects, . . . 192 Determination of Magnifying Power, ..... 199 CHAPTER V. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. Microscopic Dissection, . . . . . . .201 Cutting Sections of Soft Substances, ..... 204 Cutting Sections of Harder Substances, ..... 205 Grinding and Polishing of Sections, ..... 207 Chemical Actions, . . . . . . . .211 Glass Slides, ........ 214 Thin Glass, ......... 214 Varnishes and Cements, . . . . . . . 217 Mounting Objects Dry, . . . * .. .221 Mounting Objects in Canada Balsam, . . ' v . . 224 Preservative Fluids, . . . . . . . .232 Mounting Objects in Fluid, . . . "; , . . 234 Cement-Cells, ...... . . 235 Thin-Glass Cells, . . . . . . 236 CONTENTS. Xlll Plate-Glass and Shallow Cells, . . . . . .237 Deep and Built-up Cells, . . 239 Mounting Objects in Cells, ....... 240 Importance of Cleanliness, . . . . . • . 242 Labelling and Preserving of Objects, ..... 242 Collection of Objects, ....... 243 CHAPTER VI. MICROSCOPIC FORMS OF VEGETABLE LIFE. — PROTOPHYTES. Boundary between Animal and Vegetable Kingdoms, . . . 247 Characters of Vegetable Cell, 248 Life-History of Simplest Protophytes, . . . . .251 Volvocineae, ........ 259 Desmidiaceae, . • • • • . • • • 265 Diatomacese, ........ 277 Palmellacere, . . . . • . . . .307 Ulvacete, ......... 309 Oscillatoriacese, . . . . . . . .311 Siphonaceae, . . . • • • • • 313 Confervaceae, . • • • • • • .317 Conjugateaa, . . . . • . • •' 319 Chaetophoraceae, ...... . . 321 Batrachospermeae, . . . . . . * '; 321 Characea3, * . . .322 CHAPTER VII. MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. Alg33, 327 Lichens, ......... 333 Fungi, ......... 334 Hepaticae, ........ 344 Mosses, ......... 348 Ferns, ......... 352 Equisetaceae, . . . . . . ».<;*- . 358 CHAPTER VIII. MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. Elementary Tissues, ........ 360 Structure of Stem and Root, . . . . . 376 Structure of Cuticle and Leaves, ...... 388 Structure of Flowers, ....... 395 CHAPTER IX. MICROSCOPIC FORMS OF ANIMAL LIFE: — PROTOZOA; ANIMALCULES. Protozoa, ......... 404 Rhizopoda, ........ 410 Infusoria, ......... 412 Rotifera, ......... 426 XIV CONTENTS. Foraminifera, Polycystina, Sponges, Hydra, Hydrozoa, Acalephae, Anthozoa, Structure of Shell and Spines, Echinoderm Larvae, CHAPTER X. FORAMINIFERA, POLYCYSTINA, AND SPONGES. CHAPTER XL ZOOPHYTES. CHAPTER XII. ECHINODERMATA. Polyzoa, Compound Tunicata, CHAPTER XIII. POLYZOA AND COMPOUND TUNICATA. CHAPTER XIV. MOLLUSCOUS ANIMALS GENERALLY. Shells of Mollusca, . Tongues of Gasteropods, . Development of Gasteropods, Ciliary motion on Gills, . Organs' of Sense of Mollusks,' Chromatophores of Cephalopods, Entozoa, Turbellaria, Annelida, Pycnogonidae, . Entomostraca, Suctoria, Cirrhipeda, Shell of Decapoda, Metamorphosis of Decapoda, CHAPTER XV. ANNULOSA OR WORMS. CHAPTER XVI. CRUSTACEA. PAGE 436 449 452 457 461 467 472 477 486 492 500 506 517 521 526 527 528 527 533 535 539 541 547 548 550 551 CONTENTS. XV CHAPTER XVII. INSECTS AND ARACHNIDA. PAGE Great number and variety of Objects afforded by Insects, . . . 554 Structure of Integument, ...... 555 Tegumentary Appendages, ....... 556 Parts of the Head, ....... 561 Circulation of the Blood, ....... 569 Respiratory Apparatus, . . . . . . . 570 Wings, 574 Feet, 576 Stings and Ovipositors, ....... 578 Eggs, . . . . . . . . 579 Acarida, ......... 581 Parts of Spiders, ..... .583 CHAPTER XVIII. VERTEBRATED ANIMALS. Primary Tissues, . . . . , . . . 584 Bone, ........ 584 Teeth, 588 Scales of Fish, . . . . . . . 591 Hairs, ......... 594 Feathers, ........ 597 Hoofs, Horns, &c., ...... 598 Blood, 598 White and Yellow Fibres, . . . . . .602 Skin, Mucous, and Serous Membranes, .... 604 Epidermis, ........ 605 Pigment-Cells, ....... 605 Epithelium, ........ 606 Fat, . . . . . . . . . . 607 Cartilage, ........ 608 Glands, ........ 609 Muscle, . . . . . . . . . 611 Nerve, . . . . . . . . 614 Circulation of the Blood, . . . . . . .616 Injected Preparations, . . . . . . . 619 Vessels of Respiratory Organs, ...... 624 CHAPTER XIX. APPLICATIONS OF THE MICROSCOPE TO GEOLOGY. Fossilized Wood, Coal, . . . . . . .625 Fossil Foramiriifera, Chalk, ...... 631 Nummulites, ........ 634 Orbitoides, . . . . . . . . 637 Organic materials of Rocks, ...;.. 639 Structure of Fossil Bones, Teeth, &c., ..... 641 XVI CONTENTS. CHAPTER XX. INORGANIC OR MINERAL KINGDOM. POLARIZATION. PAGE Mineral objects, ........ 644 Crystallization of Salts, ....... 645 Objects suitable for Polariscope, ...... 645 APPENDIX. Microscope as a means of Diagnosis, . . . . 650 Examination of the Nervous System, ..... 653 Muscular System, ..... 655 Respiratory System, .... 657 Morbid Lung, ..... .658 Examination of the Glandular System, ..... 662 Liver, ......... 662 Kidney, ......... 663 Morbid Kidney, ........ 664 Salivary Glands, ........ 664 Thymus and Thyroid Glands, . . . . . .664 Adipose Tissue, ........ 664 Fatty Degeneration, . . . . . . . .665 Examination of Vascular and Absorbent Systems, . . . 666 Skin, Mucous and Serous Membrane, . . . 667 The Eye, ...... 671 The Hard Tissues, . . . . . .672 Morbid Growths, ..... 674 Animal Fluids, ...... 681 Serous and Dropsical Fluids, . . . . . . 699 Injections, . . . . . . . . .700 Microscopes of American Manufacture, ..... 704 LIST OF ILLUSTRATIONS. PAGE 1. Refraction of Parallel Rays by Plano-convex Lens, . . .67 2. Ditto by Double-convex Lens, . . 67 3. Refraction of Converging Rays, . . . . .68 4. Refraction of Diverging Rays, ..... 68 5. Formation of Images by Convex Lenses, . . . .70 6. Spherical Aberration, ...... 70 7. Chromatic Aberration, . . . . . . .72 8. Section of the Achromatic Object-glass, .... 75 9. Effect of Covering-glass, ...... 76 10. Action of Simple Microscope. ..... 78 11. Simplest" form of Compound Microscope, . . . .82 12. Complete Compound Microscope, ..... 82 13. Huyghenian Eye-piece, ....... 83 14. Ross's Simple Microscope, ...... 91 15. Gairdner's Simple Microscope, ...... 92 16. Field's Simple Microscope, ...... 94 17. Quekett's Dissecting Microscope, ..... 95 18. Field's Compound Microscope, ..... 98 19. Highley's Hospital Microscope, . . . . .99 20. Nachet's Compound Microscope, ..... 101 21. Smith and Beck's Student's Microscope, . . , .102 22. Ditto Dissecting Microscope, . . . . 104 23. Warington's Universal Microscope, . . . . .105 24. Ditto Ditto, . . . . . .106 25. Ditto Ditto, . . . . . .106 26. Ditto Ditto, ...... 107 27. Ross's Large Compound Microscope, ..... 108 28. Powell and Lealand's Ditto, . . . . . . Ill 29. Smith and Beck's Ditto, . . . . . .113 30. Arrangement of Binocular Prisms, . . ., : . . 115 31. Nachet's Binocular Microscope, . . . . . .116 32. Draw-tube with Erector, . . . . . .119 33. Jackson's Eye-piece Micrometer, . . . . .122 34. Microscope arranged for Drawing, . . . . . 125 35. Ross's Achromatic Condenser, . . . . . .133 36. Smith and Beck's Ditto, ...... 133 37. Powell and Lealand's Ditto, . . . . . .134 38. White-Cloud Illuminator, . . . . . . 135 39. Amici's Prism, ........ 137 40. Parabolic Illuminator, . . . . . . 139 41. Fitting of Polarizing Prism, . . . . . .141 42. Fitting of Analyzing Prism, . . . . . 141 43. Selenite Object-Carrier, ....... 142 44. Bull's-Eye Condenser, . . . . . . 143 45. Ordinary Condensing Lens, ...... 144 46. Side Reflector, ....... 145 2 XVlii LIST OF ILLUSTRATIONS. PAGE 47. Stage-Forceps, . . t* . « • • .147 48. Aquatic Box, . , . . .« 49. Zoophyte-Trough, , . ,»•„ %« - • • .150 50. Compressorium, . • . • V . . 151 51. Dipping-Tubes, . . ... « . . .152 52. Forceps, . . . . • 53. Section of Adjusting Object-Glass, . .^ . .165 54. Arrangement of Microscope for Transparent Objects, 55. Ditto Ditto Opaque Objects, . . .177 56. Spring Scissors, *Vl ' ,;i . . . . . 203 57. Curved Scissors, ....... 204 58. Valentin's Knife, . . . 59. Section-Instrument, ....... 206 60. Lever of Contact, . . . . . . . 216 61. Slider-Forceps, . ... . . . . .226 62. Spring-Press, /' . r & • • • 227 63. Turn-Table, . V. .•«,.- . . . 236 64. Plate-glass Cells, 238 65. Tube-Cells, 239 66. Built-up Cells, . . ?r. '. . . 240 67. Palmoglcea macrococca, after Braun, . . . . .251 68. Protococcus pluvialis, after Cohu, . , . . 69. Vblvox globator, after Ehrenberg, . .'" ! . . . 259 70. Development of Volvox, after Williamson, . . . . 262 71. Various species of Staurastrum, after Ralfs, .... 266 72. Closterium lunula, after S. Gr. Osborne, .... 267 73. Development of Pediastrum granulatum, after Braun, . .271 74. Various forms of Pediastrum, after Ralfs, .... 75. Conjugation of Cosmarium botrytis, after Ralfs, . . .274 76. Ditto of Clostcrium, after Ralfs, . . . . 275 77. Didymoprium Grevillii, after Ralfs, . . . . .275 78. Portion of Isthmia nervosa, after Smith, .... 280 79. Triceratiumfavus, after Smith, ..... 281 80. Pleurosigma angulatum, after Wenham, . . . . 282 81. Biddulphia pulchella, after Smith, . . . . .283 82. Conjugation of Epiihemia, after Thwaites, .... 83. Conjugation of Aulacoseira, after Thwaites, .... 286 84. Actinocyclus undulatus, after Smith, .... 289 85. Heliopelta, . . . . . . . .290 86. Arachnoidiscus Ehrenbergii, after Smith, . . .. . 291 87. Campylodiscus costatus, after Smith, . . . •. > • .292 88. Surirella constricta, after Smith, ..... 293 89. Gompkonema geminatum, after Smith, ..... 294 90. Ditto, * more highly magnified, after Smith, . 295 91. Licmophoraflabellata, after Smith, . . . . .296 92. Meridian circulare and Bacillaria paradoxa, after Smith, . . 296 93. Achnanihes longipes, after Smith, . ,. >k . . 297 94. Diatoma vulgare, after Smith, . . . . . 298 95. Grammatophora serpentina, after Smith, .... 298 96. Isthmia nervosa, after Smith, . ; ' . . . 298 97. Meloseira subflexilis, after Smith, . . . . .299 98. Meloseira varians, after Smith, . . . . . 299 99. Mastogloia Smithii, after Smith, . .' '. ; . . 300 100. Mastogloia lanceolata, after Smith, ..... 300 101. Fossil Diatomacece from Oran, after Ehrenberg, . . .302 102.. 'Fossil Diatomacece from Mourne Mountain, after Ehrenberg, . 304 ID 3. Hcematococcus sanguineus, after Hassal, . . . .308 104. Development of Ulva, after Klitzing, . . . . 309 105. Zoospores of Ulva, after Thuret, . . , . . .310 106. Zoospores of Achlya, after linger, . . . . . 316 107. Cell-multiplication of Conferva, after Mohl, . . . .318 LIST OF ILLUSTRATIONS. XIX PAGE 108. Zygnema quininum, after Kiitzing, ..... 320 109. Chcetophora elegans, after Thuret, . . . . . 321 110. Batrachospermum moniliforme, . . . . . .322 111. Nitella flexilis, after Slack, ...... 323 112. Antheridia of Chara, after Thuret, . . . . .325 113. Mesogloia vermicularis, after Payer, . . . . 327 114. Sphacelaria cirrliosa, . . . . . . .329 115. Receptacle of Fucus, after Thuret, ..... 329 116. Antheridia and antherozoids of Fucus, after Thuret, . . . 330 117. Tetraspores of Carpocaulon, after Kiitzing, . . . 332 118. Torula Cerevisice, after Mandl, . . . . . .335 119. Sarcina ventriculi, after Robin, ..... 336 120. Botrytis bassiana, after Robin, ...... 337 121. Enterobryus spiralis, after Leidy, ..... 338 122. Structure of Enterobryus, after Leidy, ..... 339 123. Fungoid Vegetation, from Passulus, after Leidy, . . . 340 124. Stysanus caput-medusce, after Payer, ..... 341 125. Puccinia graminis, . . . . . . . 342 126. jEcidium tussilaginis, after Payer, ..... 343 127. Clavaria crispula, after Payer, ..... 344 128. Fructification of Marchantia, after Payer, . . . 345 129. Stomata of Marchantia, after Mirbel, .... 345 130. Conceptacles of Marchantia, after Mirbel, .... 346 131. Archegonia of Marchantia, after Payer, .... 347 132. Elater and spores of Marchantia, after Payer, .... 348 133. Portion of Leaf of Sphagnum, . . . . . 349 134. Structure of Mosses, after Jussieu, ..... 349 135. Antheridia and antherozoids of Polytrichum, after Thuret, . . 350 136. Mouth of Capsule of Funaria, .-'••> • . • .351 137. Peristome of Fontinalis, after Payer, . . . .351 138. Ditto of Bryum, ditto, . . . . .351 139. Ditto of Cindidium, ditto, . . . . . 351 140. Petiole of Fern, . ....... 352 141. Sori of Polypodium, after Payer, ..... 353 142. Ditto of Hcemionitis, ditto, ...... 353 143. Sorus and Indusium of Aspidium, ..... 354 144. Ditto of Deparia, after Payer, . . . . 354 145. Development of Prothallium of Pteris, after Suminski, . . 355 146. Antheridia and antherozoids of Pteris, after Suminski, . . 356 147. Archegoniura of Pteris, after Surainski, .... 357 148. Spores of Equisctum, after Payer, ..... 358 149. Section of leaf of Agave, after Hartig, . . . . 361 150. Section of Aralia (rice-paper), . ..*... 362 151. Stellate Parenchyma of Rush, ..... 362 152. Cubical parenchyma of Nuphar, ..... 362 153. Circulation in hairs of Tradescantia, after Slack, . . . 366 154. Testa of Star- Anise, . . . . . . .367 155. Section of Cherry-stone, . . . . . 368 156. Section of Coquilla-nut, . . . . . . .368 157. Spiral cells of Oncidium, . . . . . . 369 158. Spiral fibres of Collomia, . . . . . .369 159. Cells of Pceony, filled with starch, ..... 370 160. Starch-grains, under polarized light, ..... 370 161. Glandular fibres of Coniferous Wood, .... 373 162. Vascular tissue of Italian Reed, after Schleid en, . . . 374 163. Transverse Section of stem of Palm, .... 377 164. Ditto of Wanghie Cane, . . . . .377 165. Ditto of Clematis, . . . . . 379 166. Ditto of Cedar, . . . . . .379 167. Ditto of Buckthorn, ..... 380 168. Ditto Ditto more highlv magnified, . . 380 XX LIST OF ILLUSTRATIONS. PAGE 169. Transverse Section of Hazel, • . . . . -380 170. Ditto of Fossil Conifer, .... 171. Vertical section of Fossil Conifer, radial, .... 382 172. Ditto Ditto, tangential, 173. Ditto of Mahogany, ... . 382 174. Transverse Section of Fossil- wood, ..... 175. Vertical Section of Ditto, 176. Transverse Section of Fossil-wood, ..... 177. Vertical Section Ditto, . . . . .383 178. Transverse Section of Aristolochia, (t) .... 385 179. Ditto of Burdock, . . . . .385 180. Cuticle of Yucca, ....... 181. Ditto of Indian Corn, . ..'.<. . . . 388 182. Ditto of Apple, after Brongniart, . .... 389 183. Ditto ofEochea, " . . . . . .390 184. Vertical Section of leaf of Eochea, after Brongniart, . . 390 185. Cuticle of Iris, Ditto, .... 392 186. Vertical Section of leaf of Iris, Ditto, . 393 187. Longitudinal Section of ditto, Ditto, .... 393 188. Cuticle of Petal of Geranium, . . . . . 395 189. Pollen-grains of Althcea, &c., . . . . . .398 190. Seeds of Poppy, &c., . M . . 402 191. Amceba princeps, after Ehrenberg, ..... 405 192. Actinophrys sol, after Clarapede, ..... 407 193. Various forms of RMzopods, after Ehrenberg, . . . .411 194. Kerona silums, after Milne-Edwards, . . . . 414 195. Paramecium caudatum, Ditto, . . . . .414 196. Group of Vorticella, after Ehrenberg, . . . . 416 197. Fissiparous Multiplication of Chilodon, after Ehrenberg, . .418 198. Metamorphoses of Vorticella, after Stein, . . . . 419 199. Ditto of Trichoda, after Haime, * ... 420 200. Brachionus pala, after Milne-Edwards, • .... 427 201. Rotifer vulgaris, after Ehrenberg, . » . . .429 202. Stephanoceros Eichornii, Ditto, ..... 432 203. Noteus quadricornis, Ditto, . ..''•.:> . . . 434 204. Gromia oviformis, after Schulze, . . . . . 437 205. Rosalina ornata, after Schulze, . . . . . 439 206. Orbitolites complanatus, . . .< . . . 441 207. Animal of simple type of ditto, . -V, .... 442 208. Animal of complex type of ditto, . '" ' \ • '< • *^1' . . 444 209. Section of JFaujasina, after Williamson, .;: . . . . 447 210. Podocyrtis Schomburgkii, after Ehrenberg, . . . 450 211. Rhopalocanium ornatum, Ditto, . . . . .450 212. Haliomma Humboldtii, Ditto, . . . . . 450 213. Perichlamydium prcetextum, Ditto, ,"• . . . . 450 214. Polycystina from Barbadoes, Ditto, . . . . . 451 215. Stylodictya gracilis, Ditto, . . . . .452 216. Astromma Aristotelis, Ditto, . ... . . 452 217. Structure of Grantia, after Dobie, "V. ..> . . . 453 218. Portion of Halichondria, .. . .• . . . 454 219. Siliceous spicules of Pachymatisma, . ..vs« . . . 454 220. Hydra fusca, after Milne-Edwards, ;•• .; * ^-3' . . 45^ 221. Ditto in gemmation, after Trembley, .. .' „<> . . 460 222. Medusa-buds of Syncoryna, after Sars, . . • ';"•• . . 460 223. Campanularia gelatinosa, after Van Beneden, .... 464 224. Sertularia cupressina, after Johnston, .... 466 225. Thaumantias pilosella, after E. Forbes, « ,3 . . . 467 226. Development of Medusa-buds, after Dalyell. .v . . 468 227. Development of Mediisce, Ditto, . -4 » ' . . . 4^9 228. Ci/dippe and Beroe, after Milne-Edwards, I 'fo . . 470 229. Noctihica miluiriv, after Quatrefages, . ;'-U t •„ •-.-' . .571 LIST OF ILLUSTRATIONS. Xxi PAGE 230. Spicules of Alcyonium and Gorgonia, ..... 472 231. Ditto of Gorgonia guttata, . . . . . 472 232. Spicules of Muricea elongata, ...... 472 233. Filiferous capsules of Actinia, &c., after Gosse, . . . 475 234. Section of Shell of Echinus, . . . , . .478 235. Calcareous reticulation of Spine of Echinus, . . . 478 236. Ambulacral disk of Echinus, ...... 478 237. Transverse Section of Spine of Acrodadia, . . . . 479 238. Spines of Spatangus, . . . . . . .481 239. Calcareous skeleton of Astrophyton, .... 482 240. Ditto of Holothuria, . . . . .485 241. Ditto of Synapta, . . . . . 486 242. Ditto of Chirodota, . . . . .486 243. Bipinnarian larva of Star-fish, after Muller, . . . 487 244. Pluteus-larva of Echinus, after Miiller, . . . . .489 245. Cells of Lepralice, after Johnston, ..... 492 246. Laguncula repens, after Van Beneden, ..... 493 247. Bird's-head processes of Cellularia and Bugula, after Johnston and Busk, 498 248. Amaroucium prolifei^um, after Milne-Edwards, . . . 501 249. Botryllus violaceus, Ditto . . . .502 250. Perophora, after Lister, ....... 503 251. Transverse Section of Pinna, ..... 507 252. Membranous basis of Ditto . . . . . .507 253. Vertical Section of Ditto . . ... . 507 254. Oblique Section of Ditto . . . . . .508 255. Section of hinge-tooth of Mya, ..... 509 256. Nacre of Avicula, . . . . . 510 257. Tubular shell-structure of Anomia, ..... 512 258. Vertical Section of shell of Unio, ..... 512 259. Internal surface of shell of Teretoatula, .... 513 260. External Ditto Ditto . . . . .513 261. Vertical Sections of Ditto ..... 514 262. Horizontal Section of shell of Terebratula bullata, . . .514 263. Ditto Ditto of Megerlia lima, . . . 514 264. Ditto Ditto of Spiriferina rostrata, . . . 514 265. Palate of Helix hortensis, . . . . . 518 266. Ditto of Zonites cellarius, . . . . . .518 267. Ditto of Trochus zizyphinus, . . . . . 519 268. Ditto of Doris tuberculata, . . . . . .519 269. Ditto of Buccinum, under polarized light, .... 521 270. Embryoes of Nudibranchs, after Alder and Hancock, . . . 522 271. Embryonic development of Purpura, .... 523 272. Latter stages of the same, .... llM . 525 273. Structure of Polycelis, after Quatrefages, . . . , .vr> 534 274. Circulation of Terebella, after Milne-Edwards, . . . . 535 275. Ammothea pycnogonoides, after Quatrefages, . .*---; f! ,». 540 276. Cyclops quadricornis, after Baird, . ... . . 543 277. Development of Balanus, after Bate, .... 549 278. Metamorphoses of Carcinus, after Couch, . . . .551 279. Scale of Morpho Menelaus, ...... 557 280. Battledoor scale of Polyommalus argus, after Quekett. . . . 557 281. Scales of Podura plumbea, . . . .'. . 558 282. Hairs of Myriapod and Dermestes, ..... 560 283. Head and eyes of See, . . . . . . 561 284. Section of Eye of Melolontha, after Strauss-Durckheim, . . 562 285. Antenna of Cockchafer, ...... 565 286. Portions of ditto more highly magnified, . . . 565 287. Tongue of Fly, ....... 566 288. Tongue, &c., of Honey Bee, . . . . . .567 289. Proboscis of Vanessa, ...... 568 290. Tracheal system of Nepa, after Milne-Edwards, . . .571 XXII LIST OF ILLUSTRATIONS. PAGE 291. Tracheae of Dytiscus, . . . . . . 571 292. Spiracle of Fly, . . . . V . . .572 293. Spiracle of Larva of Cockchafer, . * •»- . . .572 294. Foot of Fly, after Hepworth, . . . . . .577 295. Foot of Dytiscus, . . \ *,-* . . 577 296. Eggs of Insects, after Burmeister, ' .- - . . . .580 297. Foot, with combs, of Spider, ,/>< 298. Ordinary and glutinous threads of Spider, .... 583 299. Minute Structure of Bone, after Wilson, .... 585 300. Lacunae of Ditto, highly magnified, after Mandl, . . .586 301. Section of Bony scale of Lepidosteus, .... 587 302. Vertical Section of Tooth of Lamna, after Owen, . . .588 303. Transverse Ditto of Pristis, ditto, ... 588 304. Ditto Ditto of Myliobates, . . . .589 305. Vertical Section of Human Tooth, after Mandl, ... 590 306. Portion of Skin of Sole, . . .«,< •-. . . . .592 307. Scale of Sole, . . . -.. - - *••- ... 592 308. Hair of Musk-Deer, . ruft.j . . . . .595 309. Hair of Sable, . . .<-. .^-, ... 595 310. Hairs of Squirrel and Indian Bat, . . . . .595 311. Transverse section of Hair of Pecari, .... 595 312. Structure of Human Hair, after Wilson, .... 596 313. Transverse section of Horn of Rhinoceros, . • ".••• < . . 598 314. Blood-corpuscles of Frog, after Donne, . Y: ' . . . 599 315. Ditto of Man, Ditto, . »• . . 599 316. Fibrous Membrane of Egg-shell, . fc>i , . . . 603 317. White Fibrous Tissue, ...... 603 318. Yellow Fibrous Tissue, . . *rt* . v - . . 603 319. Pigment-cells of Choroid, after Henle, 605 320. Pigment-cells of Tadpole, after Schwann, . . . .606 321. Ciliated Epithelium, after Mandl, . : . . ys . . 607 322. Areolar and Adipose Tissue, after Mandl, „ . . .608 323. Cartilage of Ear of Mouse, . *. * ... 609 324. Cartilage of Tadpole, after Schwann, -i:-, . . . .609 325. Striated Muscular Fibre, >. - V . . . 612 326. Ultimate fibrilla3 of Ditto, . . . i v •' . . 612 327. Capillary Circulation in Web of Frog's foot, after Wagner, . 618 328. Intestinal villi of Monkey, . . . . . .622 329. Various forms of Capillary network, after Berres, . . . 623 330. Portion of Gill of Eel, . . ! ,,./. . . . . 624 331. Interior of Lung of frog, » ,. • ... . . 624 332. Section of Lung of Fowl, . . •»: ;- . . 625 333. Section of Human Lung, ...... 626 334. Microscopic organisms in Levant Mud, after Williamson, . . 630 335. Ditto in Chalk from Gravesend, after Ehrenberg, 631 336. Ditto Ditto from Meudon, Ditto . . 632 337. Vertical section of Nummulite, . . v > v~> . 635 338. Portion of Ditto, more highly magnified, •-*•••- »• . . 636 339. Horizontal section of Ditto, . . . • -•. ; w:- . 637 340. Section of Orbitoides Prattii parallel to its surface, . . . 637 341. Portions of Ditto, more highly magnified, . . ,' . 638 342. Vertical section of Ditto, . . . . . .638 343. Eye of Trilobtte, after Buckland, . . . -..-$ . 640 344. Section of Tooth of Labyrinthodon, after Owen, . . . 641 345. Crystallized Silver, ....... 645 346. Oily Matter in Fibres of Brain (Todd and Bowman), . . .653 347. Corpora Amylacea of Pineal Gland (Sieveking and Jones) , . 654 348. Fatty Degeneration of Nerve-Fibre, ..... 654 349. Striated Muscular Fibre, ...... 654 350. Elongated Muscle-Cells (Non-Striated), . . . .656 351. Sarcolemma, . . . . . „' . . 656 LIST OF ILLUSTRATIONS. XX111 PAGE 352. Structure of Lung, . . . . . . .657 353. Capillaries of Lung, ...... 658 354. Tubercle Corpuscles, . . . . . . .658 355. Yellow Tubercle (Bennett), ..... 658 JJ5^ j Scrofulous Pus (Bennett), . . . . . .658 358. Section of Gray Granulations, ..... 659 359. Cretaceous and Cheesy Variety of Tubercle (Bennett), . . 659 360. Pigmentary Matter mixed with Tubercle (Bennett), . . 659 361 ) op2 ( Structure of Tubercular Mass from Cerebellum, . . . 659 363. Tubercle Corpuscle from Lung, ..... 660 364. Plastic or Pyoid Corpuscle, . . . . . .660 365. Granular Corpuscle from Cerebral Swelling, . . . 660 366. Corpuscles from Reticulum of Cancer, ..... 660 367. Lung in first Stage of Pneumonia (Da Costa), . . . 661 368. Ditto 2d Ditto Ditto . . . .661 369. Ditto Ditto Ditto ... 661 370. Ditto 3d Ditto Ditto .... 662 371. Transverse Section of Lobule of Human Liver (Leidy), . . 662 372. Hepatic Cells (Leidy), ....... 663 373. Urine from tube and Epithelial Cell, .... 663 374. Adipose Tissue, . . . . . . .665 375. Fatty Degeneration of Vessels of Brain, .... 666 OH*' [• Atheromatous Deposits in Vessels, . . . . . 667 378. Fibre-Cells passing into Fibres, . . . . . 669 379. Fibrous Tissue formed from Fibre-Cell, . . . .669 380. Altered Epithelial Cells from Ulcer of Lip, ... 669 381. Epithelial and Fibre-Cells from same, . . . . .669 382. Pus Corpuscles, ....... 677 383. Ditto after Acetic Acid, . . . . .678 384. Free Cancer Nuclei, ...... 678 OQK •) jjgg j- Polygonal Cancer-Cells (Donaldson), . . . 679 387. Caudate Cancer-Cells (Donaldson), .... 680 388. Fusiform Cells, ........ 680 389. Concentric Cancer-Cells, . . . * ' ;- 681 390. Compound, a Mother Cell, . . . . .. . 681 391. Agglomerated Nuclei (Donaldson), . . , . 681 392. Corpuscles of Blood, . . . . . .681 393. Same after exposure to Air, . . . ... 681 394. Colorless Corpuscles, . . . . . ' ••* • .682 395. Blood in Leucocythaemia, . . . , . . 682 396. Caudate Blood-Corpuscles, . . ... . . . 683 397. Milk and Colostrum-Granules, . . .'*'.' 683 398. Healthy Milk-Globules, . . . . ;'. . . 684 399. Salivary Corpuscles, Epithelial Scales and Granules, . . 684 400. Tubercle-Corpuscles (Bennett), . •- . . . 686 401. Sputum of Calcareous Matter, ..... 686 402. Sputum in Acute Pneumonia, ...... 686 403. Pus-Globules, ....... 687 404. Scrofulous Pus, . . . . . . . .687 405. Mucous Corpuscles and Epithelium in Urine, . . . 690 406. Secreting Canal and Epithelium from Kidney, .... 690 407. Mucus, Pus, Blood, and Epithelium in Leucorrhoea, . . 691 408. Fibrinous Casts from Tubuli Uriniferi, . . . . .691 409. Ditto Ditto Ditto in Bright's Disease, . 692 416. Tube containing a Homogeneous Cast, ..... 692 411. Spermatozoa, ....... 692 412. Torula from Urine of Diabetes, . . . . . .693 XXIV LIST OF ILLUSTRATIONS. PAGK 413. Oil-Globules from Urine, ...... 693 414. Lithic Acid Crystals, . . . . . . .693 415. Various forms of Lithates, ...... 694 416. Crystal of Cystine, . . . . . . .694 417. Oxalate of Lime, . ... . . . . 695 418. Same (various forms), . . •* . . . . 695 419. Dumb-Bell Crystals, ...... 695 420. Urate of Ammonia, ....... 696 421. Triple Phosphate. ....... 696 422. Stellar form of Triple Phosphate, . . . . .696 423. Starch Corpuscles after Partial Digestion, . 698 424. Microscopic Appearance of Cancerous Juice from Urine, . . 699 425. Fibres and Corpuscles from Ovarian Fluid,. . . . 699 426. Spencer's Trunnion Microscope, ..... 705 427. Queen's Student's Microscope, ..... 706 428. Grunow's Student's Microscope, ..... 707 429. Ditto Ditto smaller size, ... 708 430. Ditto first-class Microscope, ... .709 431. Diagram of Smith's Inverted Microscope, . . . . 710 432. Dr. J. L. Smith's Inverted Microscope, . . . . .711 433. ' 434 >• Dr. J. L. Smith's Goniometer and Micrometer, . . . 712-13 THE MICROSCOPE, INTRODUCTION. No one who attentively examines the progress of any depart- ment of Science, save such as are (like Mathematics or Meta- physics) of a purely abstract character, can fail to perceive how much it is dependent upon the perfection of its instruments. There are few instances, in fact, in which the invention of a new instrument, or the improvement of an old one, has not given a fresh stimulus to investigation ; even where it has done no more than afford that degree of precision to the results of inquiries already in progress, which alone could enable them to be made available as data for philosophical reasoning. But there are many cases in which such inventions or improvements have opened out entirely new paths of scientific research, leading to fertile fields of investigation whose very existence had been pre- viously unknown, to rich mines of discovery whose treasures had lain uncared for because entirely unsuspected. A few examples of this general truth may not be inappropriate, by way of pre- face to the brief notice which it is intended to give in the present Introduction, of the most important epochs in the history, as well of the Microscope itself, as of its application to the purposes of scientific research. Thus in taking a retrospective survey of the history of Astro- nomy, we find that every great advance in our knowledge of the Celestial Universe, has been preceded by improvements, either in those instruments for measuring space and time, by which the places of the Heavenly Bodies are determined, the rate of their movements estimated, and a basis for the computation of their distances ascertained ; or, again, in the. telescope, by which our power of sight is so wonderfully augmented, that we are enabled, when gazing through it into the unfathomable depths of space, to take cognizance of world beyond world and system beyond system, whose remoteness cannot be expressed by any form of words that shall convey a distinct idea to the mind, and to bring the members of our own group within such visual proximity to 3 34 INTRODUCTION. ourselves, that we can scrutinize their appearance nearly as well as if they had actually been brought a thousand times nearer to us. For it was the increased precision of celestial observations on the places and movements of the Planets, which furnished the data whereon Kepler was enabled to base his statement of the laws of their motion. It was the application of the pendulum to the measurement of short intervals of time, that enabled Galileo to ascertain the law of Falling Bodies. And it was not until the precise measurement of a degree upon the surface of the Earth had furnished the means of determining both its own diameter and its distance from the Moon, that Newton was enabled to verify and establish his grand conception, of the identity of that force which deflects the planets from a rectilineal course into elliptical orbits, with that which draws a stone to the ground ; and thus to establish that Law of Universal Gravitation, which still remains the most comprehensive, as well as the most simple, of all the generalizations, within which the intellect of man has been able to comprehend the phenomena of Nature. So, again, it was only when the elder Herschel had developed new powers in the telescope, that Sidereal Astronomy could be pursued with any view much higher than that of mapping the distribution of the stars in the celestial sphere ; and the present state of our knowledge of double, triple, and other combinations of stars, with their mutually adjusted movements, of the multiform clus- ters of luminous points which seem like repetitions of our own firmament in remote depths of space, and of those nebulous films which may be conceived to be new worlds and systems in pro- cess of formation, has only been rendered attainable by the im- provements which have been subsequently made in the construc- tion of that majestic instrument. If we glance at the mode in which the fabric of our existing Chemistry has been upreared, we at once see that it could not have attained its present elevation and stability, but for the in- strumentality of the perfected balance; by whose unerring indi- cations it was that the first decisive blow was given to the old "phlogistic" theory, that the foundation was laid for true ideas of chemical combination, that the Laws of that Combination were determined, and that the Combining Equivalents of different elementary substances were ascertained ; and by whose means alone can any of those analytical researches be prosecuted, which are not only daily adding to our knowledge of the composition of the bodies which surround us, and suggesting the most im- portant applications of that knowledge to almost every depart- ment of the Arts of Life, but which are preparing a broad and secure foundation for a loftier and more comprehensive system of Chemical Philosophy. So, again, the balance of torsion, the ingenious invention of Cavendish and Coulomb, enables the Physical philosopher not merely to render sensible, but to subject to precise measurement VALUE OF INSTRUMENTS OF RESEARCH. 35 and subdivision, degrees of force that are far too feeble to affect the nicest balance of the ordinary construction, even if it were possible to bring them to act upon it; and strange as it may seem, it has been in such a balance that the Earth itself has been weighed, and that a basis has been thus afforded for the compu- tation of the weights of the different Planets and even of the Sun ; whilst in the opposite direction it is employed to furnish those data in regard to the intensity of the electric and magnetic forces, on which alone can any valid theory of their operation be constructed. The galvanometer, again, in which the minutest Electric dis- turbances are rendered sensible by the deflection of the magnetic needle, has not only brought to light a vast class of most inte- resting electric changes which were previously unsuspected (one of the most remarkable of these being the existence of electric currents in the nerves of living animals, first ascertained by M. du Bois-Reymond), but has enabled those changes to be esti- mated with a marvellous amount of exactness ; thus furnishing to observations made by its means, a precision which is quite unattainable in any other mode, and which is absolutely essential to the establishment of any valid theory of electric action. And this same instrument is scarcely less valuable, as serving, by a particular modification of it, for the detection and estimation of changes of Temperature far too minute to be measured by the ordinary thermometer ; thus affording the requisite means of ex- actness to observation, in a department of science to which at first sight it appeared to have no relation. " What an important influence," says Sir John Herschel, "may be exercised over the progress of a single branch of science, by the invention of a ready and convenient mode of executing a definite measurement, and the construction and common intro- duction of an instrument adapted for it, cannot be better exem- plified than by the instance of the reflecting goniometer ; this simple, cheap, and portable little instrument has changed the whole face of Mineralogy, and given it all the characters of one of the exact sciences." Of all the instruments which have been ( yet applied to scien- tific research, there is perhaps not one which has undergone such important improvements within so brief a space of time, as the Microscope has received during the second quarter of the present century ; and there is certainly none whose use under its im- proved form has been more largely or more rapidly productive of most valuable results. As an optical instrument, the Microscope is now at least as perfect as the Telescope ; for the 6-feet para- bolic speculum of Lord Rosse's gigantic instrument, is not more completely adapted to the Astronomical survey of the heavenly bodies, than the achromatic combination of lenses so minute that they can scarcely be themselves discerned by the unaided eye, is to the scrutiny of the Physiologist into the mysteries of life 36 INTRODUCTION. and organization. Nor are the revelations of the one less sur- prising to those who find their greatest charm in novelty, or less interesting to those who apply themselves to the study of their scientific bearings, than are those of the other. The universe which the Microscope brings under our ken, seems as unbounded in its limit as that whose remotest depths the Telescope still vainly attempts to fathom. Wonders as great are disclosed in a speck of whose minuteness the mind can scarcely form any dis- tinct conception, as in the most mysterious of those nebulae whose incalculable distance bafiles our hopes of attaining a more inti- mate knowledge of their constitution. And the general doctrines to which the labors of Microscopists are manifestly tending, in regard to the laws of Organization and the nature of Vital Action, seem fully deserving to take rank in comprehensiveness and im- portance with the highest principles yet attained in Physical or Chemical Science. As the primary object of this treatise is to promote the use of the Microscope, by explaining its construction, by instructing the learner in the best methods of employing it, and by pointing out the principal directions in which these may be turned to good account, any detailed review of its history would be mis- placed. It will suffice to state, that whilst the simple microscope or magnifying-glass was known at a very remote period, the compound microscope, — the powers of which, like those of the telescope, depend upon the combination of two or more lenses, — was not invented until about the end of the sixteenth century ; the earlier microscopes having been little else than modified tele- scopes, and the essential distinction between the two not having been at first appreciated. Still, even in the very imperfect form which the instrument originally possessed, the attention of scien- tific men was early attracted to the Microscope ; for it opened to them a field of research altogether new, and promised to add largely to their information concerning the structure of every kind of organized body. The Transactions of the Royal Society contain the most striking evidence of the interest taken in mi- croscopic investigations two centuries ago. Their early volumes, as Mr. Quekett truly remarks, " literally teem" with improve- ments in the construction of the Microscope, and with discoveries made by its means. The Micrographia of Robert Hooke, pub- lished in 1667, was, for its time, a most wonderful production ; but this was soon surpassed by the researches of Leeuwenhoek, whose name first appears in the Philosophical Transactions, in the year 1673. That with such imperfect instruments at his command, this accurate and pains-taldng observer should have seen so much and so ivell, as to make it dangerous for any one even now to announce a discovery, without having first con- sulted his works, in order to see whether some anticipation of it may not be found there, must ever remain a marvel to the micro- scopist. This is partly to be explained by the fact, that he EARLY REVELATIONS OF THE MICROSCOPE. 37 trusted less to the compound microscope, tlian to single lenses of high power, the use of which is attended with difficulty, but which are comparatively free from the errors inseparable from the first-named instrument in its original form. The names ot Grew and Malpighi, also, appear as frequent contributors to the early volumes of the Philosophical Transactions ; the researches of the former having been chiefly directed to the minute struc- ture of Plants, and those of the latter to that of Animals. Both were attended with great success. The former laid the founda- tion of our anatomical knowledge of the Vegetable tissues, and described their disposition in the roots and stems of a great variety of plants and trees ; besides making out many important facts in regard to their physiological actions. The latter did the same for the Animal body ; and seems to have been the first to witness the marvellous spectacle of the movement of Blood in the capillary vessels of the Frog's foot, — thus verifying by ocular demonstration that doctrine of the passage of blood from the smallest arteries to the smallest veins, which had been pro- pounded as a rational probability by the sagacious Harvey. Glimpses of the invisible world of Animalcular life were oc- casionally revealed to the earlier Microscopists, by which their curiosity must have been strongly excited ; yet they do not ap- pear to have entered on this class of investigations, with any large portion of that persevering zeal which they devoted to the analysis of the higher forms of organic structure. Its won- ders, however, were gradually unfolded ; so that in the various treatises on the Microscope published during the eighteenth century, an account of the plants and animals (but especially ot the latter) too minute to be seen by the unaided eye, occupies a conspicuous place. It was towards the middle of that period, that M. Trembley of Geneva first gave to the world his researches on the "Fresh-water Polype " or Hydra; the publication of which maybe considered to have marked a most important epoch in the history of microscopic inquiry. For it presented to the natu- ralist the first known example of a class of animals (of which the more delicate and flexible Zoophytes are, so to speak, the ske- letons) whose claim to that designation had been previously doubted or even denied, the terms " sea-mosses," " sea ferns/* &e., having been applied to them, not merely as appropriately indicat- ing their form and aspect, but as expressive of what even the most eminent Zoologists, as well as Botanists, considered to be their vegetable nature. And it presented to the Physiologist an en- tirely new type of animal life; the wonderful nature of which was fitted not only to excite the liveliest interest, but also to effect a vast extension in the range of the ideas entertained up to that time regarding its nature and capacities. For what animal previously known, could propagate itself by buds like a plant, — could produce afresh any part that might be cut away, — could form any number of new heads by the completion of the halves 38 INTRODUCTION. into which the previous heads had been slit (thus realizing the ancient fable of the Hydra), — could even regenerate the whole from a minute portion, so that when the body of one individual was positively minced into fragments, each of these should grow into a new and complete polype, — could endure being turned inside out, so that what \vas previously the external surface should become the lining of the stomach, and vice versa , — and could sustain various other kinds of treatment not less strange (such as the grafting of two individuals together, head to head, or tail to tail, or the head of the one to the tail of another), not only without any apparent injury, but with every indication, in the vigor of its life, of being entirely free from suffering or damage? (See Chap. XI). — It was by our own countryman, Ellis, that the discoveries of Trembley were first applied to the elucida- tion of the real animal nature of the so-called Corallines ;J the structure of which was so carefully investigated by him, that subsequent observers added little to our knowledge of it, until a comparatively recent period. The true animalcules were first systematically studied, in the latter part of the last century, by Gleichen, a German micro- scopist, who devised the ingenious plan of feeding them with particles of coloring matter, so as to make apparent the form and position of their digestive cavities ; and this study was after- wards zealously pursued by the eminent Danish naturalist, Otho Fred. Miiller, to the results of whose labors in this field but little was added by others, until Professor Ehrenberg entered upon the investigation with the advantage of greatly improved instruments. It was at about the same period with Miiller, that Vaucher, a Genevese botanist, systematically applied the Microscope to the investigation of the lower forms of Vegetable life ; and made many curious discoveries in regard both to their structure and to the history of their lives. He was the first to notice the ex- traordinary phenomenon of the spontaneous movement of the Zoospores of the humbler aquatic plants, which is known to be the means provided by nature for the dispersion of the race (Chaps. VI, VH) ; but being possessed with the idea (common to all Natu- ralists of that period and still very generally prevalent) that spon- rhi " taneous motion evinces Animal life, he interpreted the facts which he observed, as indicating the existence of a class of beings which are Plants at one phase of their lives, and animals at another, — a doctrine which has since been completely set aside by the ad- vance of physiological knowledge. Notwithstanding this and other errors of interpretation, however, the work of Vaucher on the "Fresh-water Confervse" contains such a vast body of accu- rate observation on the growth and reproduction of the Micro- scopic Plants to the study of which he devoted himself, that it is quite worthy to take rank with that of Trembley, as having laid 1 The structures to which this term is now scientifir.ally restricted, are really vege- table. SUPPOSED FALLACIES OF THE MICROSCOPE. 39 the foundation of all our scientific knowledge of these very inte- resting forms. Although the curious phenomenon of " conju- gation" had been previously observed by Mtiller, yet its con- nection with the function of Reproduction had not been even suspected by him ; and it was by Vaucher that its real import was first discerned, and that its occurrence (which had been re- garded by Miiller as an isolated phenomenon, peculiar to a single species) was found to be common to a large number of humble aquatic forms of vegetation. But little advance was made upon the discoveries of Vaucher in regard to these, save by addition to the number of their specific forms, until a fresh stimulus had been given to such investigations by the improvement of the instru- ment itself. At present, they are among the most favorite ob- jects of study among a large number of observers, both in this country and on the Continent ; and are well deserving of the attention which they receive. Less real progress seems to have been made in Microscopic inquiry, during the first quarter of the present century, than during any similar period since the invention of the instrument. The defects inseparable from its original construction, formed a bar to all discovery beyond certain limits ; and although we are now continually meeting with new wonders, which patient and sagacious observation would have detected at any time and with any of the instruments then in use, yet it is not surprising that the impression should have become general, that almost every- thing which it could accomplish had already been done. The instrument fell under a temporary cloud from another cause ; for having been applied by Anatomists and Physiologists to the determination of the elementary structure of the animal body, their results were found to be so discordant, as to give rise to a general suspicion of a want of trustworthiness in the Microscope, and in everything announced upon its authority. Thus both the instrument and its advocates were brought into more or less discredit ; and as they continue to lie under this, in the estima- tion of many, to the present day, it will be desirable to pause here for awhile, to inquire into the sources of that discrepancy, to consider whether it is avoidable, and to inquire how far it should lead to a distrust of Microscopic observations, carefully and saga- ciously made, and accurately recorded. It is a tendency common to all observers, and not by any means peculiar to Microscopists, to describe what they believe and infer, rather than what they actually witness. The older Micro- scopic observers were especially liable to fall into this error; since the want of definiteness in the images presented to their eyes, left a great deal to be completed by the imagination. And when, as frequently happened, Physiologists began with theorizing on the elementary structure of the body, and allowed them- selves to twist their imperfect observations into accordance with their theories, it was not surprising that their accounts of what 40 INTRODUCTION. they professed to have seen should be extremely discordant. But from the moment that the visual image presented by a well-con- structed Microscope, gave almost as perfect an idea of the object, as we could have obtained from the sight of the object itself, if enlarged to the same size and viewed with the unassisted eye, Microscopic observations admitted of nearly the same certainty as observations of any other class ; it being only in a comparatively small number of cases, that a doubt can fairly remain about any question of fact, as to which the Microscope can be expected to inform us.1 Another fallacy, common like the last to all observations, but with which the Microscopic observations of former times were perhaps especially chargeable, arises from a want of due atten- tion to the conditions under which the observations are made. Thus one observer described the Human Blood-corpuscles as flattened disks resembling pieces of money, another as slightly concave on each surface, a third as slightly convex, a fourth as highly convex, and a fifth as globular ; and the former preva- lence of the last opinion, is marked by the habit which still lingers in popular phraseology, of designating these bodies as "blood-globules." Yet all microscopists are now agreed, that their real form, when examined in freshly drawn blood, is that of circular disks, with slightly concave surfaces ; and the diversity in previous statements was simply due to the alteration effected in the shape of these disks, by the action of water or other liquids added for the sake of dilution ; the effect of this being to render their surfaces first flat, then slightly convex, then more highly convex, at last changing their form to that of perfect spheres. But microscopical inquiries are not in themselves more liable to fallacies of this description, than are any other kinds of scientific investigation ; and it will always be found here, as well as elsewhere, that — good instruments and competent observers being presupposed — the accordance in results will be precisely proportional to the accordance of conditions, that is, to the simi- larity of the objects, the similarity of the treatment to which they may be subjected, and the similarity of the mode in which they may be viewed.2 The more completely, therefore, the statements of Microscopic observers are kept free from those fallacies, to which observa- tions of any kind are liable, wherein due care has not been taken 1 One of the most remarkable of the quastiones vexatce at present agitated, is the nature of the markings on the siliceous valves of Diatomaccce (Chap. VI); some ob- servers affirming those spots of the surface to be elevations, which others consider to be depressions. The difference is here one of interpretation, rather than of direct observa- tion ; the nature of the case preventing that kind of view of the object, which could leave no doubt as to the fact; and the conclusion formed being one of inference from a variety of appearances, which will differently impress the minds of different individuals. 2 In objects of the most difficult class, such as the Diatomacece, this last point is one of fundamental importance; very different appearances being presented by the same object, according to the mode in which it is illuminated, and the focal adjustment of the object- glass under which it is examined. — See Chap. VI. MODERN REVELATIONS OF THE MICROSCOPE. 41 to guard against them, the more completely will it be found that an essential agreement exists among them all, in regard to the facts which they record. And although the influence of precon- ceived theories still too greatly modifies, in the minds of some, the descriptions they profess to give of the facts actually presented to their visual sense, yet on the whole it is remarkable to what a unity of doctrine the best Microscopists of all countries are con- verging, in regard to all such subjects of this kind of inquiry, as have been studied by them with adequate care and under simi- lar conditions. Hence it is neither fair to charge upon the Microscopists of the present day the errors of their predecessors ; nor is it just to lay to the account of the instrument, what entirely proceeds from the fault of the observer, in recording, not what he sees in it, but what he fancies he can see. It was at the commencement of the second quarter of the present century, that the principle of Achromatic correction, which had long before been applied to the Telescope, was first brought into efficient operation in the construction of the Microscope ; for although its theoretical possibility was well known, insupera- ble difficulties were believed to exist in its practical application. The nature of this most important improvement will be explained in its proper place (Chap. I) ; and at present it will be sufficient to say, that within eight or ten years from the date of its first introduction, the character of the Microscope had been in effect so completely transformed, that it became an altogether new instrument; and from being considered but little better than a scientific toy, it soon acquired the deserved reputation of being one of the most perfect instruments ever devised by Art for the investigation of Nature. To this reputation it has a still greater claim at the present time ; and though it would be ha- zardous to deny the possibility of any further improvement, yet the statements of theorists as to what may be accomplished, are so nearly equalled by what has been effected, that little room for improvement can be considered to remain, unless an entirely new theory shall be devised, which shall create a new set of possibilities. Neither Botanists or Zoologists, Anatomists or Physiologists, were slow to avail themselves of the means of perfecting and ex- tending their knowledge, thus unexpectedly put into their hands ; and the records of Scientific Societies, and the pages of Scientific Journals, have ever since teemed, like the early Transactions of the Royal Society, with discoveries made by its instrumentality. All really philosophic inquirers soon came to feel, how vastly the use of the improved Microscope must add to their insight into every department of Organic Nature ; and numbers forth- with applied themselves diligently to the labor of investigation. Old lines of research, which had been abandoned as unlikely to lead to any satisfactory issue, were taken up again with the con- fident expectation of success, which the result has shown to have 42 INTRODUCTION. been well grounded ; and new paths were soon struck out, each of which, leading into some region previously unexplored, soon cleared the way to others which became alike productive ; thus laying open an almost unlimited range of inquiry, which the time that has since elapsed has served rather to extend than to contract, and which the labor that has been devoted to it has rather amplified than exhausted. A slight sketch of what has already been accomplished by the assistance of the Microscope, in the investigation of the phenomena of Life, seems an appro- priate Introduction to the more detailed account of the instru- ment and its uses, which the present Treatise is designed to embrace. The comparative simplicity of the structure of Plants, and the relatively large scale of their elementary parts, had allowed the Vegetable Anatomist, as we have seen, to elucidate some of its most important features, without any better assistance than the earlier Microscopes were capable of supplying. And many of those humbler forms of Cryptogamic vegetation, which only manifest themselves to the unaided eye when by their multiplication they aggregate into large masses, had been made the objects of care- ful study, which had yielded some most important results. Hence there seemed comparatively little to be done by the Microscopist in Botanical research; and it was not immediately perceived what was the direction in which his labors were likely to be most productive. Many valuable memoirs had been published, from time to time, on various points of vegetable structure ; the increased precision and greater completeness of which, bore testimony to the importance of the aid which had been afforded by the greater efficiency of the instrument employed in such re- searches. But it was when the attention of Vegetable Physiolo- gists first began to be prominently directed to the history ot development, as the most important of all the subjects which pre- sented themselves for investigation, that the greatest impulse was given to Scientific Botany ; and its subsequent progress has been largely influenced by that impulse, both in the accelerated rate at which it has advanced, and in the direction which it has taken. Although Robert Brown had previously observed and recorded certain phenomena of great importance, yet it is in the Memoir of Prof. Schleiden, first published in 1837, that this new movement may be considered to have had its real origin ; so that, whatever may be the errors with which his statements (whether on that occasion, or subsequently) are chargeable, there cannot be any reasonable question as to the essential service he has rendered to science, in pointing out the way to others, on whose results greater reliance may be placed. It was by Schlei- den that the fundamental truth was first broadly enunciated, that as there are as many among the lowest orders of Plants, in which a single cell constitutes the entire individual, each living for and by itself alone, so each of the cells, by the aggregation of MICROSCOPIC STUDY OF THE CRYPTOGAMIA. 43 which any individual among the higher Plants consists, has an independent life of its own, besides the " incidental " life which it possesses as a part of the organism at large : and that the doctrine was first proclaimed, that the life-history of the indi- vidual cell is therefore the very first and absolutely indispensable basis, not only for Vegetable Physiology, but (as was even then foreseen by his far-reaching mental vision) for Comparative Physiology in general. The first problem, therefore, which he set himself to investigate, was — hoiv does the cell itself originate? It is unfortunate that he should have had recourse for its solu- tion, to some of those cases in which the investigation is attended with peculiar difficulty, instead of making more use of the means and opportunities which the "single-celled" plants afford; and it is doubtless in great part to this cause, that we are to attribute certain fallacies in his results, of which subsequent researches have furnished the correction. In no department of Botany, has recent Microscopy been more fertile in curious and important results, than in that which relates to the humblest forms of Cryptogamia that abound not only in our seas, rivers, and lakes, but even more in our marshes, pools, and ditches. For, in the first place, these present us with a number of most beautiful and most varied forms, such as on that account alone are objects of great interest to the Micro- scopist; this is especially the case with the curious group (ranked among Animalcules by Prof. Ehrenberg), which, from the bipar- tite form of their cells, has received the designation of Desmidiece. In another group, that of Diatomacece (also still regarded as Ani- malcules, not only by Ehrenberg, but by many other Naturalists), not only are the forms of the plants often very remarkable, but their surfaces exhibit markings of extraordinary beauty and symmetry, which are among the best " test-objects" that can be employed for the higher powers of the instrument (Chap. IY) : moreover, the membrane of each cell being coated externally with a film of silica, which not only takes its form, but receives the impress of its minutest markings, the siliceous skeletons remain unchanged after the death of the plants which formed them, sometimes accumulating to such an amount, as to give rise to deposits of considerable thickness at the bottoms of the lakes or pools which they inhabit; and similar deposits, commonly designated as beds of "fossil animalcules," are not unfrequently found at a considerable distance from the surface of the ground, on the site of what must have probably once been a lake or estu- ary, occasionally extending over such an area, and reaching to such a depth, as to constitute no insignificant part of the crust of the globe. — It is not only in these particulars, however, that the foregoing and other humble Plants have special attrac- tions for the Microscopist ; for the study of their living actions brings to view many phenomena, which are not only well calcu- lated to excite the interest of those who find their chief pleasure 44 INTRODUCTION. in the act of observing, but are also of the highest value to the Physiologist, who seeks to determine from the study of them what are the acts wherein Vitality may be said essentially to consist, and what are the fundamental distinctions between Ani- mal and Vegetable life. Thus it is among these plants, that wre can best study the history of the multiplication of cells by "binary subdivision," which seems to be the most general mode of growth and increase throughout the Vegetable kingdom ; and it is in these, again, that the process of sexual generation is pre- sented to us under its simplest aspect, in that curious act of "conjugation" to which reference has already been made (p. 39). But further, nearly all these Plants have at some period or other of their lives, a power of spontaneous movement ; which in many instances so much resembles that of Animalcules, as to seem un- mistakably to indicate their animal nature, more especially as this movement is usually accomplished by the agency of visible cilia : and the determination of the conditions under which it occurs, and of the purposes it is intended to fulfil, is only likely to be accomplished after a far more extensive as well as more minute study of their entire history, than has yet been prose- cuted, save in a small number of instances. It is not a little remarkable, that in several of the cases in which the life-history of these plants has been most completely elucidated, they have been found to present a great variety of forms and aspects at different periods of their existence, and also to possess several different methods of reproduction ; and hence it can be very little doubted, that numerous forms which are commonly reputed to be distinct and unrelated species, will prove in the end to be nothing else than successive stages of one and the same type. One of the most curious results attained by Microscopic inquiry of late years, has been the successive transfer of one group of reputed Animalcules after another, from the Animal to the Vege- table side of the line of demarcation between the two kingdoms ; and although, as to the precise points across which this line should be drawn, there is not yet a unanimous agreement, yet there is now an increasing accordance as to its general situation, which, even a few years since, was energetically canvassed. Those who are acquainted with the well-known Volvox (com- monly termed the " globe-animalcule ") will be surprised to learn that this, with its allies, constituting the family Volvocineoe, is now to be considered as on the Vegetable side of the boundary. (On the subjects of this paragraph, see Chap. VI.) Not only this lowest type of Vegetable existence, but the Cryp- togamic series as a whole, has undergone of late years a very close scrutiny, which has yielded results of the highest impor- tance; many new and curious forms having been brought to light (some of them in situations in which their existence might have been least anticipated), and some of the most obscure por- tions of their history having received an unexpectedly clear eluci- MICROSCOPIC STUDY OF THE CRYPTOGAMIA. 45 dation. Thus the discovery was announced by M. Audouin in 1837, that the disease termed muscardine, which annually carried off large numbers of the silkworms bred in the south of France, really consists in the growth of a fungous vegetation in the interior of their bodies, the further propagation of which may be almost entirely prevented by appropriate means; in the succeeding year, the fact was brought forward by several Microscopists, that yeast also is composed of vegetable cells, which grow and multi- ply during the process of fermentation ; and subsequent re- searches have shown that the bodies of almost all animals, not even excepting Man himself, are occasionally infested by Vege- table as well as by animal Parasites, many of them remarkable for their beauty of configuration, and others for the variety of the forms they assume. The various parasites which attack our cultivated plants, again, — such as the " blights" of corn, the po- tato fungus, and the vine fungus, — have received a large measure of attention from Microscopists, and much valuable information has been collected in regard to them. It is still a question, how- ever, which has to be decided upon other than microscopic evi- dence, how far the attacks of these fungi are to be considered as the causes of the diseases to which they stand related, or whether their presence (as is undoubtedly the case in many parallel in- stances) is the effect of the previously unhealthy condition of the plants which they infest ; the general evidence appears to the author to incline to the latter view. Of all the additions which our knowledge of the structure and life-history of the higher types of Cryptogamic vegetation has received, since the achromatic microscope has been brought to bear upon them, there is none so remarkable as that which re- lates to their Reproductive function. For the existence in that group of anything at all corresponding to the sexual generation of Flowering Plants, was scarcely admitted by any Botanists ; and those few who did affirm it were unable to substantiate their views by any satisfactory proof, and were (as the event has shown) quite wrong as to the grounds on which they based them. Various isolated facts, the true meaning of which was quite un- recognized, had been discovered from time to time, — such as the existence of the moving filaments now termed "antherozoids," in the " globules" of the Chara (first demonstrated by Mr. Varley in 1834), and in the " antheridia" of Mosses and Liverworts (as shown by linger and Meyen in 1837), and the presence of " antheridia" upon what had been always previously considered the embryo- frond of the Ferns (first detected by Nageli in 1844) ; but of the connection of these with the generative function, no valid evi- dence could be produced, and the sexual reproduction of the Cryptogamia was treated by many Botanists of the greatest eminence, as a doctrine not less chimerical, than the doctrine of the sexuality of Flowering Plants had appeared to be to the opponents of Linnaeus. It was by the admirable researches 46 INTRODUCTION. of Count Suminski upon the development of the Ferns (1848), that the way was first opened to the right comprehension of the reproductive process in that group ; and the doctrine of the fertilizing powers of the "antherozoids," once established in a single case, was soon proved to apply equally well to many others. ]STot a year has since elapsed, without the pro- duction of new evidence of the like sexuality in the several groups of the Cryptogamic series ; this having been especially furnished by Hofmeister in regard to the higher types, by Thuret and Decaisne as to the marine Algse, and by Tulasne with re- spect to Lichens and Fungi ; and the doctrine may now be con- sidered as established beyond the reach of cavil from any but those, who, having early committed themselves dogmatically to the negative opinion, have not the candor to allow due weight to the evidence on the affirmative side. With the study of the Reproduction of these plants, that of the history of their Develop- ment has naturally been connected ; and some of the facts already brought to light, especially by the study of certain forms of Fun- gous vegetation, demonstrate the extreme importance of this inquiry in settling the foundations of Classification. For whereas the arrangement of Fungi, as of other Plants, has been based upon the characters furnished by their fructification, these charac- ters have been found by Tulasne to be frequently subject to varia- tions so wide, that one and the same individual shall present two or more kinds of fructification, such as had been previously con- sidered to be peculiar to distinct orders. In this department of study, which has been scarcely at all cultivated by Microscopists of our own country, there is a peculiarly wide field for careful and painstaking research, and a sure prospect of an ample har- vest of discovery. (On the subjects of the two preceding para- graphs, see Chap. VII.) Although it has been in Cryptogamic Botany, that the zealous pursuit of Microscopic inquiry has been most conducive to scien- tific progress, yet the attention of Vegetable Anatomists and Physiologists has been also largely and productively directed to the minute- structure and life-history of Flowering Plants. For although some of the general features of that structure had been made out by the earlier observers, and successive additions had been made to the knowledge of them, previously to the new era to which reference has so often been made, yet all this knowledge required to be completed and made exact, by a more refined ex- amination of the Elementary Tissues than was before possible ; and little was certainly known in regard to those processes of growth, development, and reproduction, in which their activity as living organisms consists. All the researches which have been made upon this point, tend most completely to bear out the general doctrine so clearly set forth by Schleiden, as to the independent vitality of each integral part of the fabric; and among the most curious results of the inquiries which have been MICROSCOPIC STUDY OF FLOWERING PLANTS. 47 prosecuted in this direction, may be mentioned the discovery, that the movement of " rotation" of the protoplasma (or the viscid granular fluid at the expense of which the nutritive act seems to take place) within the cells, which was first observed by the Abbe Corti in the Chara in 1776, is by no means a unique or exceptional case ; for that it may be detected in so large a num- ber of instances, among Phanerogamia no less than among Cryp- togamia, as apparently to justify the conclusion that it takes place in Vegetable cells generally, at some period or other of their evolution. In studying the phenomena of Vegetable Nu- trition, the Microscope has been most effectually applied, not merely to the determination of changes in the form and arrange- ment of the elementary parts, but also to the detection of such changes in their composition, as ordinary Chemistry would be quite at fault to discover ; each individual cell being (so to speak) a laboratory in itself, within which a transformation of organic compounds is continually taking place, not only for its own requirements, but for those of the economy at large ; and these changes being at once made apparent by the application of chemical reagents to microscopic specimens whilst actually under observation. Hence the Vegetable Physiologist finds, in this Microscopic Chemistry, one of his most valuable means of tracing the succession of the changes in which Nutrition consists, as well as of establishing the chemical nature of particles far too minute to be analyzed in the ordinary way; and he derives further assistance in the same kind of investigation, from the application of Polarized Light (§ 63), which immediately enables him to detect the presence of mineral deposits, of starch-granules, and of certain other substances which are peculiarly affected by it. One of the most interesting among the general results of such researches, has been the discovery that the true cell-wall of the Plant (the " primordial utricle" of Mohl) has the same albumi- nous composition as that of the Animal ; the external cellulose envelope, which had been previously considered as the distinc- tive attribute of the Vegetable cell, being in reality but a secre- tion from its surface. Of all the applications of the Microscope, however, to the study of the life-history of the Flowering plant, there is none which has excited so much interest, or given rise to so much discussion, as the nature of the process by which the Ovule is fecundated by the penetration of the pollen-tube. This question, in the opinion of the author, may be considered as now" determined ; and the conclusion arrived at is one so strictly in harmony with the general results obtained by the study of the (apparently) very different phenomena presented by the Genera- tive process of the Cryptogamia, that it justifies the Physiologist in advancing a general doctrine as to the nature of the function, which proves to be no less applicable to the Animal kingdom than it is to the Vegetable. (See Chap. VIII.) Among the objects of interest so abundantly offered by the 48 INTRODUCTION. Animal Kingdom to the observation of Microscopists furnished with vastly improved instruments of research, it was natural that those minuter forms of Animal life which teem in almost every stationary collection of water, should engage their early atten- tion ; and among those Naturalists who applied themselves to this study, the foremost rank must undoubtedly be assigned to the celebrated German Microscopist, Prof. Ehrenberg. For al- though it is now unquestionable that he has committed numer- ous errors, — many doctrines which at first gained considerable currency on the strength of his high reputation, having now been abandoned by almost every one save their originator, — yet when we look at the vast advances which he unquestionably made in our knowledge of Animalcular life, the untiring industry which he has displayed in the study of it, the impulse which he has given to the investigations of others, and the broad foundation which he has laid for their inquiries in the magnificent works in which his own observations are recorded, we cannot but feel that his services have been almost invaluable, since, but for him, this department of microscopic inquiry would certainly have been in a position far behind that to which it has now advanced. Yet, great as has been the labor bestowed by him and by his followers in the same line of pursuit, it has become increasingly evident of late years that our knowledge of Infusory Animalcules is still in its infancy ; that the great fabric erected by Prof. Ehrenberg rests upon a most insecure foundation ; and that the Anatomy, Physiology, and systematic arrangement of these beings need to be restudied completely, ab initio. For, in the first place, there can be no doubt whatever, that a considerable section of the so- called Animalcules belongs to the Vegetable kingdom ; consist- ing, as already pointed out, of the motile forms of the humbler Plants, of which a very large proportion pass, at some period of their existence, through a stage of activity that serves for their diffusion. Moreover, in another group, whose character has been entirely misconceived by the great German Microscopist, and was first clearly discriminated by M. Dujardin, there is neither mouth nor stomach of any kind ; the minute plants and animals which serve it as food, being incorporated, as it were, with the soft animal jelly, which constitutes the almost homogeneous body; and this jelly further extending itself into " pseudopodial" prolongations, whereby these alimentary particles are laid hold of and drawn in. It was by the same distinguished French Mi- croscopist, that the important fact was first discovered, that animals of this Rhizopod type are really the fabricators of those minute shells, which, from their Nautilus-like aspect, had been previously regarded as belonging to the highest class of the Mol- luscous Sub-Kingdom ; and the whole of this most interesting group, which had received from M. D'Orbigny (who first per- ceived the speciality of its nature, and made a particular study of it) the designation of Foraminifera, has thus had its place in MICROSCOPIC STUDY OF ANIMALCULES. 49 the Animal scale most strangely reversed ; being at once de- graded from a position but little removed from Vertebrated ani- mals, to a level in some respects even lower than that of the ordinary Animalcules. But even when Prof. Ehrenberg's class of Polygastrica has been thus reduced, by the removal of those forms which are true Plants, and by the detachment of such as belong to the Ehizopod group, we find that our knowledge of its real nature is almost wholly to be gained; since little else has yet been accomplished than a description of a multitude of forms, of whose history as living beings scarcely anything else is known, than that they take food into the interior of their bodies by means of an oral orifice, that they digest this food and appropriate it to their own growth, and that they multiply themselves by binary subdivision. Now there is a very strong analogical probability, that many even of the most dissimilar forms of these Animalcules will prove to be different states of one and the same ; for their multiplication by binary subdivision being not a true generative process, but being merely (so to speak) the growth of the individual, we may be al- most certain that sooner or later a new phase will present itself, consisting in the evolution of proper sexual bodies, which will perform a true generative act, the products of which may be very probably quite different from the forms we are accustomed to regard as peculiar to each species. The attention of several eminent Microscopists at the present time is strongly fixed upon this part of the inquiry ; which can only be efficiently prosecuted, by limiting the range of observation for a time to a small number of forms, and pursuing these through all the phases of their ex- istence. Among the most important of Prof. Ehrenberg's unquestioned discoveries, we are undoubtedly to place that of the compara- tively high organization of the Rotifera, or Wheel- Animalcules and their allies ; for which, though previously confounded with a similar Infusoria, he asserted and vindicated a claim to a far more elevated rank. For although in this instance, too, some of his descriptions have been shown to be incorrect, and many of his inferences to be erroneous, and although subsequent observ- ers are not agreed among themselves as to many important par- ticulars, yet all assent to the general accuracy of Prof. Ehrenberg's statements, and recognize the title of the Rotifera to a place not far removed from that of the Vermiform tribes. A parallel dis- covery was made about the same time by MM. Audouin and Milne Edwards, in regard to the JFlustrce and their allies, which had previously ranked among those flexible Zoophytes popularly known as " corallines," and are often scarcely to be distinguished from them in mode of growth or general aspect j1 but which 1 "You go down," says Mr. Kingsley, " to any shore after a gale of wind, and pick up a few delicate little sea-ferns. You have two in your hand (Sertularia operculata and Gemellaria loriculata), which probably look to you, even under a good pocket mag- nifier, identical or nearly so. But you are told, to your surprise, that however like the 4 50 INTRODUCTION. were separated as a distinct order by these observers, on account of their possession of a second orifice to the alimentary canal, and the general tendency of their plan of organization to that which characterizes the inferior Mollusca. The importance of this distinction was at once recognized ; and the group received the designation of Polyzoa from Mr. J. Y. Thompson, and of Bryozoa from Prof. Ehrenberg. The organization of this very interesting group was further elucidated, some yearsv subsequent- ly, by the admirable observations of Dr. Arthur Farre upon a newly-discovered form (named by him Boiverbankia), the trans- parence of whose envelopes allowed its internal structure to be distinctly made out ; and the additional features which he de- tected, were all such as to strengthen the idea already entertained of its essentially Molluscan character. This idea received its final and complete confirmation from the admirable researches of M. Milne Edwards on the Compound Ascidians, which are the lowest animals whose Molluscous nature had been previously ac- knowledged ; these having been discovered by him to agree with Zoophytes in their plant-like attribute of extension by " gemma- tion" or budding, and to present, in all the most important fea- tures of their organization, an extremely close approximation to the Bryozoa. Thus whilst Microscopic research has degraded the Foraminifera from their supposed rank with the Nautilus and Cuttle-fish, to the level of the Sponge, it has raised the "Wheel-Animalcules into proximity with the aquatic "Worms, and the humble " Sea-Mat," formerly supposed to be a Plant, to a position not much below that of the Oyster and Mussel.1 Another most curious and most important field of microscopic inquiry has been opened up in the discovery of the Transforma- tions which a large proportion of the lower animals undergo, during the early stages of their existence ; and notwithstanding that it has even yet been very imperfectly cultivated, the unexpect- ed result has been already attained, that the fact of " metamor- phosis,"— previously known only in the cases of Insects and Tad- poles, and commonly considered as an altogether exceptional phe- nomenon,— is almost universal among the inferior tribes ; it being a* rare occurrence for the offspring to come forth from the egg in a condition bearing any resemblance to that which characterizes the adult, and the latter being in general attained only after a long series of changes, in the course of which many curious phases are presented. One of the earliest and most remarkable discoveries which was made in this direction, — that of the meta- morphosis of the Cirrhipeds (Barnacles and their allies) by Mr. J. Y. Thompson, — proved of most important assistance in the de- termination of the true place of that group, which had previously been a matter of controversy ; for although in their outward dead horny polypidoms which you hold may be, the two species of animal which have formed them, are at least as far apart in the scale of creation as a quadruped is from a fish." 1 With reference to the subjects of the three preceding paragraphs, see Chaps. IX, X, XI. MICROSCOPIC STUDY OF LOWER ANIMALS. 51 characters they bear such a resemblance to Mollusks, that the Barnacles which attach themselves to floating timber, and the Acorn-shells which incrust the surfaces of rocks, are unhesitat- ingly ranked by Shell-collectors among their " multivalves," yet the close resemblance which exists between their early forms and the little "Water-Fleas which swarm in our pools, makes it quite certain that the Barnacles not only belong to the Articulated in- stead of to the Molluscous series, but that they must be ranked in close proximity to the Entomostracous division of the Crustacea, if not actually as members of it. To the same discoverer, moreover, we owe the knowledge that even the common Crab undergoes metamorphoses scarcely less strange, its earliest form being a lit- tle creature of most grotesque shape, which had been previously described as an adult and perfect Entomostracan ; so that, al- though scarcely any two creatures can apparently be more unlike than a Barnacle and a Crab, they have (so to speak) the same starting-point ; the difference in their ultimate aspect chiefly arising from the difference in the proportionate development of parts which are common to both. A still more remarkable series of Metamorphoses has recently been shown by Prof. Muller to exist among the Echinoderms (Star-fish, Sea-urchins, &c.) ; whose development he has studied with great perseverance and sagacity. Thus the larva of the Star-fish is an active free-swimming animal, having a long body with six slender arms on each side, from one end of which the young star-fish is (so to speak) budded off; and when this has attained a certain stage of development, the long twelve-armed body separates from it and dies oft', its chief func- tion having apparently been, to carry the young Star-fish to a distance from its fellows, and thus to prevent overcrowding by the accumulation of individuals in particular spots, which would be liable to occur if they never had any more active powers of locomotion than they possess in their adult state. Scarcely less remarkable are the changes which are to be witnessed in the greater number of aquatic MollusJcs, almost all of which, however inert in their adult condition, possess active powers of locomotion in their larval state ; some being propelled by the vibratile move- ment of cilia disposed upon the head somewhat after the fashion of those of Wheel-animalcules, and others by the lateral strokes of a sort of tail which afterwards disappears like that of a tadpole. Among the Annelids or marine worms, again, there is found to be an extraordinary dissimilarity, though of a somewhat dif- ferent nature, between the larval and the adult forms : for they commonly come forth from the egg in a condition but little ad- vanced beyond that of Animalcules ; and, although they do not undergo any metamorphosis comparable to that of Insects, they pass through a long series of phases of development (chiefly consisting in the successive production of new joints or segments, and of the organs appertaining to these) before they acquire their complete type. In nearly all the foregoing cases it may be re- 52 INTRODUCTION. marked, that the larval forms of different species bear to one another a far stronger resemblance than exists among adults, the distinguishing characters of the latter being only evolved in the course of their development ; and every new discovery in this direction only gives fresh confirmation to the great law of development early detected by the sagacity of Von Baer, that the more special forms of structure arise out of the more general, and this by a gradual change. The meaning of this law will become ob- vious hereafter, when some of the principal cases to which it applies shall have been brought in illustration of it (Chap. XII). A still more curious series of discoveries has been made, by means of the Microscope, in regard to the early development of the Medusan Acalephs (jelly-fish, &c.), and the relationship that exists between them and the Hydraform Zoophytes ; — two groups of animals, which had been previously ranked in different classes, and had not been supposed to possess anything in common. For it has been clearly made out by the careful observations of Sars, Siebold, Dalyell, and others, that those delicate arborescent Zoophytes, each polype of which is essentially a Hydra, not only grow by extending themselves into new branches, like plants, sometimes also budding off detached gemma?, which multiply their kind by developing themselves into Zoophytic forms like those whence they sprang ; but also produce peculiar buds hav- ing all the characters of Medusa*, which contain the proper gene- rative organs of the Zoophyte, but which, usually detaching themselves from the stock that bore them, swim freely through the ocean as minute jelly-fish, without exhibiting the slightest trace of their originally attached condition. The Medusae in due time produce fertile eggs; and each egg developes itself, not into the form of its immediate progenitor, but into that of the Zoo- phyte from which the Medusa was budded off. And thus a most extraordinary alteration of forms is presented, between the Zoo- phyte, which may be compared to the growing or vegetating stage of a Plant (its polypes representing the leaf-buds), and the Medusa, the development of which marks its flowering stage. So again, from the investigation of the early history of those larger forms of "jelly-fish" with which every visitor to the sea- coast is familiar, it has been rendered certain that they too are developed from Polype larvae, usually of very minute size, which give off Medusa buds ; so that whilst they are best known to us in their Medusan state, and the Hydraform Zoophytes in their polypoid state, each of these groups is the representative of a certain stage in the life-history of one and the same tribe of these curious beings, which, when complete, includes both states, as will hereafter be shown in more detail (Chap. XI). Changes very similar in kind, and in many respects even more remark- able, have been found by microscopic inquiry to take place among the Entozoa (intestinal worms) ; but being interesting only to professed Naturalists and scientific Physiologists, they scarcely call for particular notice in a treatise like the present. MICROSCOPIC STUDY OF LOWER ANIMALS. 53 It has not been among the least important results of the new turn which Zoological inquiry has thus taken, that a far higher spirit has been introduced into the cultivation of this science, than previously pervaded it. Formerly it was thought, both in Zoology and in Botany, that Classification might be adequately based on external characters alone ; and the scientific acquire- ments of a Naturalist were estimated rather by the extent of his acquaintance with these, than by any knowledge he might possess of their internal organization. The great system of Cuvier, it is true, professed to rest upon organization as its basis ; but the acquaintance with this which was considered requisite for the purpose, was very limited in its amount and superficial in its character ; and no Naturalist formerly thought of studying the history of Development as a necessary adjunct to the Science of Classification. How essential a knowledge of it has now become, however, if only as a basis for any truly natural arrangement of Animals, must have become apparent from the preceding sketch ; and it has thus come to be felt and admitted amongst all truly philosophic Naturalists, that the com- plete study of any particular group, even for the purposes of classification, involves the acquirement of a knowledge, not only of its intimate structure, but of its entire life-history. And thus Natural History and Physiology, — two departments of the great Science of Life, which the Creator inextricably blended, but which Man has foolishly striven to separate, — are now again being brought into their original and essential harmony ; and it is coming to be thought more credible, to give a complete eluci- dation of the history of even a single species, than to describe any number of new forms, about which nothing else is made out save what shows itself on the surface. Thus every Microscopist, however limited may be his oppor- tunities, has a wide range of observation presented to him in the study of the lower forms of Animal life; with the strongest incite- ment to persevering and well-directed inquiry, that the anticipation of novelty, and the expectation of valuable results, can afford. For, notwithstanding the large number of admirable records which have been already published (chiefly, we must admit with regret, by Continental Naturalists) upon the developmental history of the lower tribes of Animals, there is no one of the subjects that have been just passed in review, of which the knowledge hith- erto gained can be regarded as more than a sample of that which remains to be acquired. Records like those already referred tor might easily be multiplied a hundred fold, with infinite advan- tage to Science ; if those Microscopists who spend their time in desultory observation, and in looking at some favorite objects over and over again, would but concentrate their attention upon some particular species or group, and work out its entire history with patience and determination. And the observer himself would find this great advantage in so doing, — that an inquiry 54 INTRODUCTION. thus pursued gradually becomes to him an object of such attrac- tive interest, that he experiences a zest in its pursuit to which the mere dilettante is an entire stranger, — besides enjoying all the mental profit, which is the almost necessary result of the thorough performance of any task that is not in itself unworthy. And what can be a more worthy occupation, than the attempt to gain an insight, however limited, into the operations of Creative Wisdom? — these being not less wonderfully displayed among the forms of Animal life which are accounted the simplest and. least attractive, than in those which more conspicuously solicit the attention of the Student of Nature, by the beauty of their aspect or the elaborateness of their organization. It has not been, however, in the study of the minuter forms of Animal life alone, that the Microscope has been turned to valu- able account; for the Anatomists and Physiologists who had made the Human fabric the especial object of their study, and who had been led to believe that the knowledge accumulated by their repeated and persevering scrutiny into every portion accessible to their vision, was all which it lay within their power to attain, have found in this new instrument of research, the means of advancing far nearer towards the penetralia of Organ- ization, and of gaining a much deeper insight into the mysteries of Life, than had ever before been conceived possible. For every part of the entire organism has been, so to speak, decom- posed into its elementary tissues, the structure and actions of each of which have been separately and minutely investigated ; and thus a new department of study, which is known as Histology (or Science of the Tissues), has not only been marked out, but has already made great advances towards completeness. In the pursuit of this inquiry, the Microscopists of our day have not limited themselves to the fabric of Man, but have extended their researches through the entire range of the Animal kingdom ; and in so doing, have found, as in every other department of Nature, a combination of endless variety in detail, with a mar- vellous simplicity and uniformity of general plan. Thus the bones which constitute the skeleton of the Yertebrated animal, — however different from each other in their external configura- tion, in the arrangement of their compact and their cancellated portions, and other such particulars as specially adapt them for the purposes they have to perform in each organism, — all consist of a certain kind of tissue, distinguished under the microscope by features of a most peculiar and interesting kind ; and these features, whilst presenting (like those of the Human counte- nance) a certain general conformity to a common plan, exhibit (as Prof. Quekett has shown) such distinctive modifications of that plan in the different classes and orders of the Yertebrated series, that it is generally possible by the microscopic examina- tion of the merest fragment of a bone, to pronounce with great probability as to the natural family to which it has belonged. Still MICROSCOPIC STUDY OF ANIMAL TISSUES. 55 more is this the case in regard to the Teeth, whose organic struc- ture (originally detected by Leeuwenhoek) has been newly and far more completely elucidated by Profrs. Purkinje, Retzius, Owen, and Tomes ; for the inquiry into the comparative struc- ture of these organs, which has been prosecuted by Prof. Owen, in particular, through the entire range of the Vertebrated series, has shown that, with an equally close conformity to a certain general plan of structure, there are at the same time still wider diversities in detail, which are so characteristic of their respective groups, that it is often possible to discriminate, not only families, but even the genera and species, by careful attention to the minute features of their structure. Similar inquiries, with results in many respects analogous, have been carried out by the Author, in regard to the Shells of Mollusks, Crustaceans, and Echinoderms ; his researches having not only demonstrated the regularly-organized structure of these protective envelopes (which had been previously affirmed to be mere inorganic exudations, presenting in many instances a crystalline texture), but having shown that many natural groups are so distinctively character- ized by the microscopic peculiarities they present, that the in- spection of a minute fragment of Shell will often serve to deter- mine, no less surely than in the case of bones and teeth, the position of the animal of which it formed a part. The soft parts of the Animal body, moreover, such as the cartilages which cover the extremities of the bones and the ligaments which hold them together at the joints, the muscles whose contraction deve- lopes motion and the tendons which communicate that motion, the nervous ganglia which generate nervous force and the nerve- fibres which convey it, the skin which clothes the body and the mucous and serous membranes which line its cavities, the assimi- lating glands which make the blood and the secreting glands which keep it in a state of purity, — these, and many other tissues that might be enumerated, are severally found to present character- istic peculiarities of structure, which are more or less distinctly recognizable throughout the Animal series, and which bear the strongest testimony to the Unity of the Design in which they all originated. As we descend to the lower forms of Animal life, however, we find these distinctions less and less obvious ; and we at last come to fabrics of such extreme simplicity and homogene- ousness, that every part seems to resemble every other in struc- ture and action ; no provision being made for that " division of labor" which marks the higher types of organization, and which, being the consequence of the development of separate organs each having its special work to do, can only be effected where there is a "differentiation" of parts, that gives to the entire fabric a character of heterogeneousness. The Microscopic investigation whose nature has thus been sketched, has not only been most fruitful in the discovery of individual facts, but has led to certain general results, of great 56 INTRODUCTION. value in Physiological Science. Among the most important of these, is the complete metamorphosis which has been effected in the ideas previously entertained regarding living action; such, having been essentially based on the Circulation of the blood, as the only vital phenomenon of which any direct cognizance could be gained through the medium of the senses. For it gradually came to be clearly perceived, that in the Animal as in the Plant, each integral portion of the Organism possesses an independent Life of its own, in virtue of which it performs a series of actions pe- culiar to itself, provided that the conditions requisite for those actions be supplied to it ; and that the Life of the body as a whole (like a symphony performed by a full orchestra) consists in the harmonious combination of its separate instrumental acts, — the circulation of the blood, instead of making the tissues, sim- ply affording the supply of prepared nutriment, at the expense of which they evolve themselves from germs previously existing. This general doctrine was first put prominently forwards by Schwann, whose " Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants," published in 1839, marks the commencement of a new era in all that department of Animal Physiology, which comprises the simply vegetative life of the organized fabric. These researches, avow- edly based upon the ideas advanced by Schleiden, were prose- cuted in the same direction as his had been; the object which this admirable observer and philosophic reason er specially pro- posed to himself, being the study of the development of the Animal tissues. He found that although their evolution cannot be watched while in actual progress, its history may be traced out by the comparison of the successive stages brought to light by Microscopic research ; and in so far as this has been accom- plished for each separate part of the organism, the structure and actions of its several components, however diverse in their fully developed condition, are found to resemble each other more and more closely, the more nearly these parts are traced back to their earliest appearance. Thus we arrive in our retrospective survey, at a period in the early history of Man, at which the whole em- bryonic mass is but a congeries of cells, all apparently similar and equal to each other ; and going still further back, it is found that all these have had their origin in the subdivision of a single primordial cell, which is the first defined product of the generative act. On this single cell, the Physiologist bases his idea of the most elementary type of Organization ; whilst his actions present him with all that is essential to the notion of Life. And in pur- suing the history of the germ, from this, its simplest and most homogeneous form, to the assumption of that completed and per- fected type which is marked by the extreme heterogeneousness of its different parts, he has another illustration of that law of pro- gress from the general to the special, which is one of the highest principles yet attained in the science of Vitality. MICROSCOPIC STUDY OF ANIMAL DEVELOPMENT. 57 But, further, the Physiologist, not confining his inquiries to Man, pursues the like researches into the developmental history of other living beings, and is soon led to the conclusion that the same is true of them also ; each Animal, as well as each Plant, having the same starting-point in the single cell ; and the dis- tinctive features by which its perfected form is characterized, how striking and important soever these may be, arising in the course of its development towards the condition it is ultimately to present. In the progress of that evolution, those fundamental differences which mark out the great natural divisions of the Animal and the Vegetable kingdoms respectively, are the first to manifest themselves ; and the subordinate peculiarities which distinguish classes, orders, families, genera, and species, succes- sively make their appearance, usually (but not by any means constantly) in the ordei^of importance which Systematists have assigned to them. And it is in thus pursuing, by the aid which the Microscope alone can afford to his visual power, the history of the Organic Germ, from that simple and homogeneous form which seems common to every kind of living being, either to that complex and most heterogeneous organism which is the mortal tenement of Man's immortal spirit, or only to that hum- ble Protophyte or Protozoon, which lives and grows and multi- plies without showing any essential advance upon its embryonic type, — that the Physiologist is led to his grandest conception of the Unity and All-Comprehensive nature of that Creative Design, of which the development of every individual Organism, from the lowest to the highest, is a separate exemplification, at once perfect in itself, and harmonious with every other. It has been the purpose of the foregoing sketch, to convey an idea, not merely of the services which the Microscope has already rendered to the collector of facts in every department of the Sci- ence of Life, but also of the value of these facts as a foundation for philosophical reasoning. For it is when thus utilized, that observations, whether made with the Microscope or with the Telescope, or by any other instrumentality, acquire their highest value, and excite the strongest interest in the mind. But as it is not every one who is prepared by his previous acquirements to appreciate such researches, according to the scientific estimate of their importance, it may be well now to address ourselves to that large and increasing number, who are disposed to apply themselves to Microscopic research as amateurs, following the pursuit rather as a means of wholesome recreation to their own minds, than with a view to the extension of the boundaries of existing knowledge ; and to those in particular who are charged, whether as parents or as instructors, with the direction and train- ing of the youthful mind. All the advantages which have been urged at various times, 58 INTRODUCTION. with so much sense and vigor,1 in favor of the study of Natural History, apply with full force to Microscopical inquiry. What better encouragement and direction can possibly be given to the exercise of the .observing powers of a child, than to habituate him to the employment of this instrument upon the objects which immediately surround him, and then to teach him to search out novelties among those less immediately accessible ? The more we limit the natural exercise of these powers, by the use of those methods of education which are generally considered to -be spe- cially advantageous for the development of the Intellect, — the more we take him from fields and woods, from hills and moors, from river-side and sea-shore, and shut him up in close school- rooms and narrow play-grounds, limiting his attention to ab- stractions, and cutting him off even in his hours of sport from those sights and sounds of Nature wjaich seem to be the ap- pointed food of the youthful spirit, — the more does it seem im- portant that he should in some way be brought into contact with her, that he should have his thoughts sometimes turned from the pages of books to those of Creation, from the teachings of Man to those of God. Now if we attempt to give this direction to the thoughts and feelings in a merely didactic mode, it loses that spon-taneousness which is one of its most valuable features. But if we place before the young a set of objects which can scarcely fail to excite their healthful curiosity, satisfying this only so far as to leave them still inquirers, and stimulating their interest from time to time by the disclosure of such new wonders as arouse new feelings of delight, they come to look upon the pur- suit as an ever-fresh fountain of happiness and enjoyment, and to seek every opportunity of following it for themselves. There are no circumstances or conditions of life, which need be altogether cut off from these sources of interest and improve- ment. Those who are brought up amidst the wholesome influ- ences of a country life, have, it is true, the greatest direct oppor- tunities of thus drawing from the Natural Creation the appropriate nurture for their own spiritual life. But the very familiarity of the objects around them, prevents the^e from exerting their most wholesome influence, unless they be led to see how much there is beneath the surface even of what they seem to know best ; and in rightly training them to look for this, how many educational objects, — physical, intellectual, and moral, — may be answered at the same time ! " A walk without an object," says Mr. Kings- ley, " unless in the most lovely and novel scenery, is a poor exercise ; and as a recreation utterly nil. If we wish rural walks to do our children any good, we must give them a love for rural sights, an object in every walk ; we must teach them — and we can teach them — to find wonder in every insect, sublimity in every hedge-row, the records of past worlds in every pebble, and 1 By none more forcibly than by Mr. Kingsley, in his recent little volume entitled " Glaueus, or the Wonders of the Shore." EDUCATIONAL VALUE OF THE MICROSCOPE. 59 boundless fertility upon the barren shore ; and so, by teaching them to make full use of that limited sphere in which they now are, make them faithful in a few things, that they may be fit hereafter to be rulers over much." What can be a more effectual means of turning such opportunities to the best account, than the employment of an aid which not only multiplies almost infinitely the sources of interest presented by the objects with which our eyes are most familiar, but finds inexhaustible life where all seems lifeless, ceaseless activity where all seems motion- less, perpetual change where all seems inert ? Turn, on the other hand, to the young who are growing up in our great towns, in the heart of the vast Metropolis, whose range of vision is limited on every side by bricks and mortar, who rarely see a green leaf or a fresh blade of grass, and whose knowledge of animal life is practically limited to the dozen or two of creatures that every- where attach themselves to the companionship of Man, and shape their habits by his. To attempt to inspire a real love of Nature by books and pictures, in those who have never felt her influences, is almost hopeless. A child may be interested by accounts of her wonders, as by any other instructive narrative ; but they have little of life or reality in his mind, — far less than has the story of adventure which appeals to his own sympathies, or even than the fairy tale which charms and fixes his imagination. But here the Microscope may be introduced with all the more advantage, as being almost the only means accessible under such circumstances, for supplying what is needed. A single rural or even suburban walk will afford stores of pleasurable occupation for weeks, in the examination of its collected treasures. A large glass jar may be easily made to teem with life, in almost as many and as varied forms as could be found by the unaided eye in long and toilsome voyages over the wide ocean; and a never-ending source of amusement is afforded by the observation of their growth, their changes, their movements, their habits. The school-boy thus trained, looks forward to the holiday which shall enable him to search afresh in some favorite pool, or to explore the wonders of some stagnant basin, with as much zest as the keenest sportsman longs for a day's shooting on the moors, or a day's fishing in the best trout stream ; and with this great advantage over him, — that his excursion is only the beginning of a fresh stock of enjoyment, instead of being in itself, the whole. This is no imaginary picture, but one which we have constantly under our eyes ; and no argument can be needed to show the value of such a taste, to such, at least, as have set clearly before their minds the objects at which they should aim in the great work of Education. For we have not merely to train the intel- lectual powers and to develope the moral sense ; but to form those tastes — those " likes and dislikes" — which exercise a more abiding and a more cogent influence on the conduct, than either the reason or the mere knowledge of duty. It is our object to foster 60 INTRODUCTION. all the higher Aspirations, to keep in check all that is low and de- grading. But the mind must have recreation and amusement ; and the more closely it is kept by the system of education adopted, to the exercise of any one set of powers, the more potent will be that reaction which will urge it, when restraint is removed, to activity of some other kind ; and the more important is it, that this reaction should receive a direction to what is healthful and elevating, instead of to what is weakening and degrading. It is quite a mistake to imagine that those evil habits which result from a wrong exercise of the natural powers, a wrong direction of the natural tendencies, can be effectually antagonized by the simple effort at repression. The constant exercise either of ex- ternal coercion or of internal restraint, tends to keep the atten- tion directed towards the forbidden object of gratification ; the malady is only held in check, not cured ; and it will break out, perhaps with augmented force, whenever the perpetually-present impulses shall derive more than ordinary strength from some casual occurrence, or the restraining power shall have been tem- porarily weakened. The only effectual mode of keeping in check the wrong, is by making use of these same powers and tendencies in a right mode ; by finding out objects whereon they may be beneficially exercised ; and by giving them such a direc- tion and encouragement, as may lead them to expend themselves upon these, instead of fretting and chafing under restraint, ready to break loose at the first opportunity. There is no object on which the youthful energy can be employed more worthily, than in the pursuit of Knowledge ; no kind of knowledge can be made more attractive, than that which is presented by the Works of Creation ; no source is more accessible, no fountain more inexhaustible ; and there is none which affords, both in the mode of pursuing it, and in its own nature, so complete and beneficial a diversion from the ordinary scholastic pursuits. If there be one class more than another, which especially needs to have its attention thus awakened to such objects of interest, as, by drawing its better nature into exercise, shall keep it free from the grovelling sensuality in which it too frequently loses itself, it is our Laboring population ; the elevation of which is one of the great social problems of the day. On those who are actively concerned in promoting and conducting its education, the claims and advantages of the Study of Nature can scarcely be too strongly urged ; since experience has fully proved, — what might have been a priori anticipated, — that where the taste for this pursuit has been early fostered by judicious training, it be- comes so completely a part of the mind, that it rarely leaves the individual, however unfavorable his circumstances may be to its exercise, but continues to exert a refining and elevating in- fluence through his whole subsequent course of life. Now for the reasons already stated, the Microscope is not merely a most valuable adjunct in such instruction, but its assistance is essential EDUCATIONAL USES OF THE MICROSCOPE. 61 in giving to almost every Natural object its highest educational value ; and whilst the country Schoolmaster has the best oppor- tunities of turning it to useful account, it is to the city School- master that, in default of other opportunities, its importance as an educational instrument should be the greatest. It was from feeling very strongly how much advantage would accrue from the introduction of a form of Microscope, which should be at once good enough for Educational purposes, and cheap enough to find its wTay into every well-supported School in town and coun- try, that the Author suggested to the Society of Arts in the summer of 1854, that it should endeavor to carry out an object so strictly in accordance with the enlightened purposes which it is aiming to effect ; and this suggestion having been considered worthy of adoption, a Committee, chiefly consisting of experienced Microscopists, was appointed to carry it into effect. It was de- termined to aim at obtaining two instruments ; — a simple and low-priced microscope for the use of Scholars, to whom it might be appropriately given as a reward for zeal and proficiency in the pursuit of Natural History, not in books, butin the field; — and a compound Microscope for the use of Teachers, of capacity suffi- cient to afford a good view of every kind of object most likely to interest the pupil or to be within the reach of the instructor. Notwithstanding the apprehensions generally expressed, that no instruments at all likely to answer the intended purpose could possibly be produced at the prices specified, the result has proved the fallacy; for among several instruments of greater or less efficiency, sent in competition for the award, the Committee was able to select a Simple and a Compound Microscope fully an- swering their expectations, and henceforth to be supplied to the public at a cost so low as to place these instruments (it may be hoped) within the reach of almost every one to whom they are likely to be of service. An account of these two Microscopes will be given hereafter. (Chap. H* §§ 29, 31.) It is not alone, however, as furnishing an attractive object of pursuit for the young — fitted at once to excite a wholesome taste for novelty, ever growing with what it feeds on, and to call forth the healthful exercise of all those powers, both physical and mental, which can minister to its gratification, — that Natu- ral History studies in general, and Microscopic inquiry in particular, are to be specially commended as a means of intel- lectual and moral discipline ; for there is no capacity, however elevated, to which they do not furnish ample material for the exercise of all its best powers, no period of life which may not draw from them its purest pleasures. Even to observe well is not so easy a thing as some persons imagine. Some are too hasty, imagining that they can take in everything at a glance, and hence often forming very erroneous or imperfect notions, which may give an entirely wrong direction not only to their own views but to those of others, and may thus render necessary an amount 62 INTRODUCTION. of labor for the ultimate determination of the truth, many times as great as that which would have sufficed in the first instance, had the original observations been accurately made and faithfully recorded. Others, again, are too slow and hesitating ; and fix their attention too much upon details, to be able to enter into the real significance of what may be presented to the vision. Although ignorance has doubtless much to do in producing both these faults, yet they both have their source ill mental tendencies which are not corrected by the mere acquisition of knowledge, and which are very inimical, not merely to its fair reception, but also to the formation of a sound judgment upon any subject whatever. The habit of guarding against them, therefore, once acquired in regard to Microscopic observation, will be of invalu- able service in every walk of life. Not less important is it (as has been already shown), to keep our observations free alike from the bias of preconceived ideas, and from the suggestive influence of superficial resemblances ; and here, too, we find the training which Microscopical study affords, especially when it is prosecuted under the direction of an experienced guide, of the highest value in forming judicious habits of thought and action. To set the young observer to examine and investigate for him- self, to tell him merely where to look and (in general terms) what to look for, to require from him a careful account of what he sees, and then to lead him to compare this with the descriptions of similar objects by Microscopists of large experience and un- questionable accuracy, is not only the best training he can receive as a Microscopist, but one of the best means of preparing his mind for the exercise of its powers in any sphere whatever. It cannot be too strongly or too constantly kept in view, that the value of the results of Microscopic inquiry will depend far more upon the sagacity, perseverance, and accuracy of the ob- server, than upon the elaborateness of his instrument. The most perfect Microscope ever made, in the hands of one who knows not how to turn it to account, is valueless ; in the hands of a careless, a hasty, or a prejudiced observer, it is worse than value- less, as furnishing new contributions to the already large stock of errors that pass under the guise of scientific truths. On the other hand, the least costly Microscope that has ever been con- structed, how limited soever its powers, provided that it gives no false appearances, shall furnish to him who. knows what may be done with it, a means of turning to an account, profitable alike to science and t6 his own immortal spirit, those hours which might otherwise be passed in languid ennui, or in frivolous or degrading amusements,1 and even of immortalizing his name by the discovery of secrets in Nature as yet undreamed of. A ver}7 1 '"I have seen," says Mr. Kingsley, "the cultivated man, craving for travel and suc- cess in life, pent up in the drudgery of London work, and yet keeping his spirit calm, and his morals perhaps all the more righteous, by spending over his Microscope even- ings which would too probably have gradually been wasted at the theatre/' EDUCATIONAL USES OF THE MICROSCOPE. 6-3 large proportion of the great achievements of Microscopic re- search that have been noticed in the preceding outline, have been made by the instrumentality of microscopes which would be generally condemned in the present days as utterly unfit for any scientific purpose ; and it cannot for a moment be supposed, that the field which Nature presents for the prosecution of in- quiries with instruments of comparatively limited capacity, has been in any appreciable degree exhausted. On the contrary, what has been done by these and scarcely superior instruments, only shows how much there is to be done. The author may be excused for citing, as an apposite example of his meaning, the curious results he has recently obtained from the study of the development of the Purpura lapillus (rockwhelk), wrhich will be detailed in their appropriate place (Chap. XII) ; for these were obtained almost entirely by the aid of single lenses, the Compound Microscope having been only occasionally applied to, for the verification of what had been previously worked out, or for the examination of such minute details as the power employed did not suffice to reveal. But it should be urged upon such as are anxious to do service to science, by the publication of discoveries which they suppose themselves to have made with comparatively imperfect instru- ments, that they will do well to refrain from bringing these for- ward, until they shall have obtained the opportunity of verifying them with better. It is, as already remarked, when an object is least clearly seen, that there is most room for the exercise of the imagination ; and there wras sound sense in the reply once made by a veteran observer, to one who had been telling him of won- derful discoveries which another was said to have made "in spite of the badness of his Microscope," — "No, sir, it was in conse- quence of the badness of his Microscope." If those who observe, with however humble an instrument, will but rigidly observe the rule of recording only what they can clearly see, they can neither go far astray themselves, nor seriously mislead others. Among the erroneous tendencies which Microscopic inquiry seems especially fitted to correct, is that which leads to the esti- mation of things by their merely sensuous or material greatness, instead of by their value in .extending our ideas and elevating our aspirations. For we cannot long scrutinize the u world of small" to which we thus find access, without having the convic- tion forced upon us, that all size is but relative, and that mass has nothing to do with real grandeur. There is something in the extreme of minuteness, which is no less wonderful, — might it not almost be said, no less majestic ? — than the extreme of vastness. If the mind loses itself in the contemplation of the immeasurable depths of space, and of the innumerable multi- tudes of stars and systems by which they are peopled, it is equally lost in wonder and admiration, when the eye is turned to those countless multitudes of Hying beings which a single drop of 64 INTRODUCTION. water may contain, and when the attention is given to the won- drous succession of phenomena which the life-history of every individual among them exhibits, and to the order and constancy which this presents. Still more is this the case, when we direct our scrutiny to the penetration of that universe which may be said to be included in the body of Man, or of any one of the higher forms of organized being, and survey the innumerable assemblage of elementary parts, each having its own independent life, yet each working in perfect harmony with the rest, for the completion of the wondrous aggregate which the life of the whole presents. In the study of the one*class of phenomena, no less than in the survey of the other, we are led towards that Infinity, in comparison with which the greatest and the least among the objects of Man's regard are equally insignificant ; and in that Infinity alone can we seek for a Wisdom to design, or a Power to execute, results so vast and so varied, by the orderly co-opera- tion of the most simple means. CHAPTER I. OPTICAL PRINCIPLES OF THE MICROSCOPE. 1. ALL Microscopes in ordinary use, whether simple or com- pound, depend for their magnifying power on that influence exerted by lenses in altering the course of the rays of light pass- ing through them, which is termed refraction.1 This influence takes place in accordance with the two following laws, which are fully explained and illustrated in every elementary treatise on optics.2 1. A ray of light passing from a rarer into a denser medium, is refracted towards a line drawn perpendicularly to the plane which divides them ; and vice versa. II. The sines of the angles of incidence and refraction (that is, of the angles which the ray makes with the perpendicular before and after its refraction) bear to one another a constant ratio for each substance, which is known as its index of refraction. It follows from the first of these laws, that a ray of light enter- ing any denser medium perpendicularly, undergoes no refraction, but continues in its straight course ; and from the second, that the rays nearest the perpendicular are refracted less than those more distant from it. The " index of refraction" is determined for different substances, by the amount of the refractive influence which they exert upon rays passing into them, not from air, but from a vacuum; and in expressing it, the sine of the angle of refraction is considered as the unit, to which that of the angle of incidence bears a fixed relation. Thus when we say that the " index of refraction" of Water is 1-336, we mean that the sine of the angle of incidence of a ray passing into water from a vacuum, is to that of the angle of refraction, as 1*336 to 1, or almost exactly as 1J to 1, or as 4 to 3. And thus, the angle of incidence being given, that of the angle of refraction may be found by dividing it by the index of refraction. 2. On the other hand, when a ray emerges from a dense medium into a rare one, it is bent /row the perpendicular, accord- 1 It is not considered necessary in the present Treatise, to describe the reflecting Microscope of Amici; since this, although superior to the Microscopes in use previously to its introduction, has been completely superseded by the application of the Achromatic principle to the ordinary Microscope. 2 See especially Dr. Golding Bird's "Manual of Natural Philosophy,'' Chap. XXII. 66 OPTICAL PRINCIPLES OF THE MICROSCOPE. ing to the same ratio; and to find the course of the emergent ray, the sine of the angle of incidence must be multiplied by the "index of refraction," which will give the sine of the angle of refraction. ~Now when an emergent ray falls very obliquely^ pon the surface, the refraction which it would sustain in passing forth, .tending as it does to deflect it still farther from the per- pendicular, becomes so great that the ray cannot pass out at all, and is reflected back from the plane which separates the two media, into the one from which it was emerging. This internal reflection will take place, whenever the product of the sine of the angle of incidence, multiplied by the index of refraction, exceeds the sine of 90°, which is the radius of the circle ; and therefore the " limiting angle," beyond which an oblique ray suffers in- ternal reflection, varies for different substances in proportion to their respective indices of refraction. Thus, the index of refrac- tion of water being 1-336, no ray can pass out of it into a vacuum,1 if its angle of incidence exceed 48° 28', since the sine of that angle, multiplied by 1-336, equals the radius ; and in like manner, the "limiting angle" for flint-glass, its index of refraction being 1*60, is 38° 41'. This fact imposes certain limits upon the performance of microscopic Lenses ; whilst at the same time it enables the optician to make most advantageous use of glass Prisms for the purpose of reflection; the proportion of the light which they throw back being much greater than that returned from the best polished metallic surfaces, and the brilliancy of the reflected image being consequently higher. Such prisms are of great value to the Microscopist for particular purposes, as will hereafter appear (§§ 40, 41, 57, 60). 3. The lenses employed in the construction of Microscopes are chiefly convex; those of the opposite kind, or concave, being only used to make certain modifications in the course of the rays pass- ing through convex lenses, whereby their performance is rendered more exact (§§ 10, 12). It is easily shown to be in accordance with the laws of refraction already cited, that when a "pencil" of parallel rays, passing through air, impinges upon a convex surface of glass, the rays will be made to converge ; for they will be bent towards the centre of the circle, the radius being the perpendicu- lar to each point of curvature. The central or axial ray, as it coincides with the perpendicular, will undergo no refraction ; the others will be bent from their original course in an increasing degree, in proportion as they fall at a distance from the centre of the lens; and the effect upon the whole will be such, that they will be caused to meet at a point, called the focus, some distance beyond the centre of curvature. This effect will not be materi- 1 The reader may easily make evident to himself the internal reflection of water, by nearly filling a wineglass with water, and holding it at a higher level than his eye, so that he sees the surface of the fluid obliquely from beneath; no object held above the water will then be visible through it, if the eye be placed beyond the limiting angle ; whilst the surface itself will appear as if silvered, through its reflecting back to the eye the light which falls upon it from beneath. REFRACTION BY CONVEX LENSES. 67 ally changed by allowing rays to pass into air again through a plane surface of glass, perpendicular to the axial ray (Fig. 1); a lens of this description is called a plano-convex lens; and it will hereafter be shown to possess properties, which render it very useful in the construction of microscopes. But if, instead of passing through a plane surface, the rays re-enter the air through a second convex surface, turned in the opposite direction, as in a double-convex lens, they will be made to converge still more. This will be readily comprehended, when it is borne in mind that the contrary direction of the second surface, and the FIG. 1. FIG. 2. Parallel rays, falling on a plano-convex lens, Parallel rays, falling on a double-convex lens, brought to a focus at the distance of its diameter; brought to a focus in its centre ; conversely, rays and conversely, rays diverging from that point, diverging from that point, rendered parallel, rendered parallel. contrary direction of its refraction (this being from the denser me- dium instead of into it), antagonize each other ; so that the second convex surface exerts an influence on the course of the rays pass- ing through it, which is almost exactly equivalent to that of the first. Hence the focus of a double-convex lens will be just half the distance, or (as commonly expressed) will be at half the length, of the focus of a plano-convex lens having the same cur- vature on one side (Fig. 2). 4. The distance of the focus from the lens will depend, not merely upon its degree of curvature, but also upon the refracting power of the substance of which it may be formed; since the lower the index of refraction, the less will the oblique rays be deflected towards the axial ray, and the more remote will be their point of meeting ; and conversely, the greater the refractive index, the more will the oblique rays be deflected towards the axial ray, and the nearer will be their point of convergence. A lens made of any substance whose index of refraction is 1-5, will bring parallel rays to a focus at the distance of its diameter of curva- ture, after they have passed through one convex surface (Fig. 1), and at the distance of its radius of curvature, after they have passed through two convex surfaces (Fig. 2) ; and as this ratio almost exactly expresses the refractive power of ordinary Glass, we may for all practical purposes consider the " principal focus" (as the focus for parallel rays is termed), of a double-convex lens OS OPTICAL PRINCIPLES OF THE MICROSCOPE. to be at the distance of its radius, that is, in its centre of curva- ture, and that of a plano-convex lens to he at the distance of twice its radius, that is, at the other end of the diameter of its sphere of curvature. 5. It is evident from what has preceded, that as a double-con- vex lens brings parallel rays to a focus in its centre of curvature, it will on the other hand cause those rays to assume a parallel direction, which are diverging from that centre before they im- pinge upon it (Fig. 2) ; so that, if a luminous body be placed in the principal focus of a double-convex lens, its divergent rays, falling on one surface of the lens as a cone, will pass forth from its other side as a cylinder. Again, if rays already converging fall upon a double-convex lens, they will be brought together at a point nearer to it than its centre of curvature (Fig. 3) ; whilst, if the incident rays be diverging from a distant point, their focus will be more distant from the lens than its principal focus (Fig. 4). FIG. 3. FIG. 4. Rays already converging, brought to- gether at a point nearer than the principal focus ; and rays diverging from a point within the principal focus, still diverging, though in a diminished degree. Rays diverging from points more distant than the principal focus on either side, brought to a focus be- yond it; if the point of divergence be within the circle of curvature, the focus of convergence will be beyond it ; and vice versa. The further from the point from which they diverge, the more nearly will the rays approach the parallel direction; until, at length, when the object is very distant, its rays in effect become parallel, and are brought together in the principal focus. If the rays which fall upon a double-convex lens, be diverging from the farther extremity of the diameter of its sphere of curvature, they will be brought to a focus at an equal distance on the other side of the lens ; but the more the point of divergence is approximated to the centre or principal focus, the further removed on the other side will be the point of convergence, until, the point of diver- fence being at the centre, there is no convergence at all, the rays eing merely rendered parallel. If the point of divergence be within the principal focus, they will neither be brought to con- verge nor be rendered parallel, but will diverge in a diminished degree (Fig. 3). The same principles apply equally to a plano- convex lens; allowance being made for the double distance of its principal focus. They also apply to a lens whose surfaces have REFRACTION BY CONVEX AND CONCAVE LENSES. 69 different curvatures ; the principal focus of such a lens is found by multiplying the radius of one surface by the radius of the other, and dividing this product by half the sum of the same radii. The rules by which the foci of convex lenses may be found, for rays of different degrees of convergence and diver- gence, will be found in works on Optics. 6. The refracting influence of concave lenses will evidently be precisely the opposite of that of convex. Kays which fall upon them in a parallel direction, will be made to diverge as if from the principal focus, which is here called the negative focus. This will be, for a plano-concave lens, at the distance of the diameter of the sphere of curvature ; and for a double-concave, in the centre of that sphere. In the same manner, rays which are con- verging to such a degree, that, if uninterrupted, they would have met in the principal focus, will be rendered parallel ; if con- verging more, they will still meet, but at a greater distance ; and if converging less, they will diverge as from a negative focus at a greater distance than that for parallel rays. If already diverg- ing, they will diverge still more, as from a negative focus nearer than the principal focus ; but this will approach the principal focus, in proportion as the distance of the point of divergence is such, that the direction of the rays approaches the parallel. 7. If a lens be convex on one side and concave on the other, forming what is called a meniscus, its effect will depend upon the proportion between the two curvatures. If they are equal, as in a watch glass, no perceptible effect will be produced ; if the convex curvature be the greater, the effect will be that of a less powerful convex lens : and if the concave curvature be the more considerable, it will be that of a less powerful concave lens. The focus of convergence for parallel rays in the first case, and of divergence in the second, may be found by dividing the product of the two radii by half their difference. 8. Hitherto we have considered only the effects of lenses upon a " pencil" of rays issuing from a single luminous point, and that point situated in the line of its axis. If the point be situated above the line of its axis, the focus will be below it, and vice versd. The surface of every luminous body may be regarded as compre- hending an infinite number of such points, from -every one of which a pencil of rays proceeds, and is refracted according to the laws already specified ; so that a perfect but inverted image or picture of the object is formed upon any surface placed in the focus, and adapted to receive the rays. It will be evident from what has gone before, that if the object be placed at twice the distance of the principal focus, the image, being formed at an equal distance on the other side of the lens ( § 5), will be of the same dimensions with the object: whilst, on the other hand, if the object (Fig. 5, a b) be nearer the lens, the image A B will be farther from it, and of larger dimensions ; but if the object A B be farther from the lens, the image a b will be nearer to it, and 70 OPTICAL PRINCIPLES OF THE MICROSCOPE. FIG. 5. 6 *£ — smaller than itself. Further, it is to be remarked, that the larger the image in proportion to the object, the less bright it \vill be, because the same amount of light has to be spread over a greater sur- face ; whilst an image that is smaller than the object, will be more brilliant in the same proportion. 9. The knowledge of these general facts will en- able us readily to under- Formation of Images by convex lenses. ^^ ^ ordinary opem_ tion of the Microscope; but the instrument is subject to cer- tain optical imperfections, the mode of remedying which can- not be comprehended without an acquaintance with their nature. One of these imperfections results from the une- qual refraction of the rays which have passed through lenses, whose curvatures are equal over their whole surfaces. If the course of the rays passing through an ordinary convex 6), it will be found that they do not all meet exactly in the foci already stated, but that the focus F of the rays A B, A B, which have passed through the periphe- ral portion of the lens, is much closer to it than that of the rays ab, ab, which are nearer the line of its axis ; so that, if a screen be held lens be carefully laid down (] FIG. 6. Diagram illustrating Spherical Aberration. in the former, the rays which have passed through the central portion of the lens will be stopped by it before they have come to a focus ; and if the screen be carried back into the focus of the latter, the rays which were most distant from the axis will have previously met and crossed, so that they will come to it in a state of divergence, and will pass to c and d. In either case, there- fore, the image will have a certain degree of indistinctness ; and there is no one point to which all the rays can be brought by a single lens of spherical curvature. The difference between the focal points of the central and of the peripheral rays, is termed the Spherical Aberration. It is obvious that, to produce the de- sired effect, the curvature requires to be increased around the centre of the lens, so as to bring the rays which pass through it more speedily to a focus ; and to be diminished towards the cir- cumference, so as to throw the focus of the rays influenced by it to a greater distance. The requisite conditions may be theoreti- cally fulfilled by a lens, one of whose surfaces, instead of being spherical, should be a portion of an ellipsoid or hyperboloid of SPHERICAL AND CHROMATIC ABERRATION. 71 certain proportions ; but the difficulties in the way of the mecha- nical execution of lenses of this description are such, that, for practical purposes, this plan of construction is altogether un- available. 9 a. Various means have been devised for reducing the Aber- ration of lenses of spherical curvature. It may be considerably diminished, by making the most advantageous use of ordinary lenses. Thus, the aberration of a plano-convex lens, whose con- vex side is turned towards parallel rays, is only TVbths of its thickness ; whilst, if its plane side be turned towards them, the aberration is 4J times the thickness of the lens. Hence, in the employment of a plano-convex lens, its convex surface should be turned towards a distant object, when it is used to form an image by bringing to a focus parallel or slightly diverging rays ; but it should be turned towards the eye, when it is used to render parallel the rays which are diverging from a very near object. The single lens having the least spherical aberration, is a double- convex whose radii are as one to six : when its flattest face is turned towards parallel rays, the aberration is nearly 3J times its thickness; but when its most convex side receives or trans- mits them, the aberration is only TJ-wtlis of its thickness. The aberration is further diminished, by reducing the aperture or working-surface of the lens, so as to employ only the rays that pass through the central part, which, if sufficiently small in pro- portion to the whole sphere, will bring them all to nearly the same focus. The use of this may be particularly noticed in the object-glasses of common (non-achromatic) Microscopes ; in which, whatever be the size of the lens itself, the greater portion of its surface is rendered inoperative by a stop, which is a plate with a circular aperture interposed between the lens and the rest of the instrument. If this aperture be gradually enlarged, it will be seen that, although the image becomes more and more illuminated, it is at the same time becoming more and more in- distinct; and that, in order to gain defining power, the aperture must be reduced again. Now this reduction is attended with two great inconveniences ; in the first place, the loss of intensity of light, the degree of which will depend upon the quantity transmitted by the lens, and will vary, therefore, with its aper- ture; and, secondly, the diminution of the "angle of aperture," that is, of the angle (a b c, Fig. 8) made by the most diverging of the rays of the pencil issuing from any point of an object, which can enter the lens ; on the extent of which angle depend some of the most important qualities of a Microscope (§ 100). 10. The Spherical Aberration may be got rid of altogether, however, by making use of combinations of lenses, so disposed that their opposite aberrations shall correct each other, whilst magnifying power is still gained. For it is easily seen that, as the aberration of a concave lens is just the opposite of that of a convex lens, the aberration of a convex lens placed in its most OPTICAL PRINCIPLES OF THE MICROSCOPE. favorable position may be corrected by a concave lens of much less power in its most unfavorable position ; so that, although the power of the convex lens is weakened, all the rays which pass through this combination will be brought to one focus. It is by a method of this kind, that the Optician aims to correct the spherical aberration, in the construction of those combinations of lenses which are now employed as object-glasses, in all Com- pound Microscopes that are of any real value as instruments of observation. But it sometimes happens that this correction is not perfectly made ; and the want of it becomes evident, in the fog by which the distinctness of the image of the object, and especially the precision of its outlines, is obscured. 11. But the spherical aberration is not the only imperfection with which the optician has to contend in the construction of microscopes. A difficulty equally serious arises from the unequal refrangibility of the several colored rays, which together make up white or colorless light,1 so that they are not all brought to the same focus, even by a lens free from spherical aberration. It is this difference in their refrangibility, which causes their complete separation by the prism into a spectrum ; and it mani- fests itself, though in a less degree, in the image formed by a convex lens. For if parallel rays of white light fall upon a con- vex surface, the most refrangible of its component rays, namely, the violet, will be brought to a focus at a point somewhat nearer to the lens than the principal focus, which is the mean of the whole ; and the converse will be true of the red rays, which are the least refrangible, and whose focus will therefore be more dis- tant. Thus in Fig. 7, the rays of white light, A B, A B, which fall on the peripheral portion of the lens, are so far de- composed, that the violet rays are brought to a focus at c, and crossing there, diverge again and pass on towards F F. On the other hand, the red rays are not brought to a focus until D, Diagram illustrating Chromatic Aberration. and CrOSS the diver°'in°* violet rays at E E. The foci of the intermediate rays of 'the spec- trum (indigo, blue, green, yellow, and orange) are intermediate between these two extremes. If the image be received upon a screen placed at c, the focus of the violet rays, violet will pre- dominate in its own color, and it will be surrounded by a pris- matic fringe in which blue, green, yellow, orange, and red may be successively distinguished. If, on the other^hand, the screen be placed at D, the focus of the red rays, the image will have a 1 It has been deemed better to adhere to the ordinary phraseology, when speaking of this fact, as more generally intelligible than the language in which it might be more scientifically described, and at the same 'time leading to no practical error. FIG. 7. CHROMATIC ABERRATION. 73 predominantly red tint, and will be surrounded by a series of colored fringes in inverted order, formed by the other rays of the spectrum, which have met and crossed.1 The line E E, which joins the points of intersection between the red and the violet rays, marks the "mean focus," that is the situation in which the colored fringes will be narrowest, the " dispersion" of the colored rays being the least ; whilst the interval c D, which separates the foci of the extreme rays, is termed the Chromatic Aberration of the lens. As the axial ray A' E' undergoes no refraction, neither does it sustain any dispersion ; and the nearer the rays are to the axial ray, the less dispersion do they suffer. Again, the more oblique the direction of the rays, whether they pass through the central or the peripheral portion of the lens, the greater will be the refraction they undergo, and the greater also will be their dispersion ; and thus it happens that when, by using only the central part of a lens (§ 12), the chromatic aberration is reduced to its minimum, the central part of a picture may be tolerably free from false colors, whilst its marginal portion shall exhibit broad fringes.2 12. The Chromatic Aberration of a lens, like the Spherical, may be diminished by the contraction of its aperture, so that only its central portion is employed. But the error cannot be got rid of entirely by any such reduction, which, for the reasons already mentioned, is in itself extremely undesirable. Hence it is of the first importance in the construction of a really efficient Microscope, that the chromatic aberration of its "object-glasses" (in which the principal dispersion is liable to occur) should be entirely corrected, so that the largest possible aperture should be given to these lenses, without the production of any false colors. No such correction can be accomplished even theoretically in a single lens ; but it may be effected by the combination of two or more, advantage being taken of the different relations which the refractive and the dispersive powers bear to each other in different substances. For if we can unite with a convex lens, whose dis- persive power is low as compared with its refractive power, a concave of lower curvature, whose dispersive power is relatively high, it is obvious that the dispersion of the rays occasioned by the convex lens may be effectually neutralized by the opposite dispersion of the concave (§ 6) ; whilst the refracting power of the convex is only lowered by the opposite refraction of the con- cave, in virtue of the longer focus of the latter. No difficulty stands in the way of carrying this theoretical correction into practice. For the "dispersive" power of flint-glass bears so much larger a ratio to its refractive power than does that of 1 This experiment is best tried w'ith a lens of long focus, of which the central part is covered with an opaque stop, so that the light passes only through a peripheral ring; since, if its whole aperture be in use, the regular formation of the fringes is interfered with by the spherical aberration, which gives a different focus to the r,ays passing through each annular zone. 2 This is well seen in the large pictures exhibited by Oxy-hydrogen Microscopes. 74 OPTICAL PRINCIPLES OF THE MICROSCOPE. crown-gl&ss, that a convex lens of the former, the focal length of which is 7| inches, will produce the same degree of color as a convex lens of crown-glass, whose focal length is 4J inches. Hence a concave lens of the former material and curvature, will fully correct the dispersion of a convex lens of the latter ; whilst it diminishes its refractive power only to such an extent as to make its focus 10 inches. The correction for chromatic aberra- tion in such a lens would be perfect, if it were not that, although the extreme rays, violet and red, are thus brought to the same focus, the dispersion of the rest is not equally compensated ; so that what is termed a secondary spectrum is produced, the images of objects seen through such a lens being bordered on one side with a purple fringe, and on the other with a green fringe. Moreover such a lens is not corrected for spherical aberration ; and it must of course be rendered free from this, to be of any real service, however complete may be the freedom of its image from false colors. The double correction may be accomplished theoretically by the combination of three lenses, namely, a double- concave of flint placed between two double-convex of crown, ground to certain curvatures ; and this method has long been employed in the construction of the large object-glasses of Tele- scopes, which are, by means of it, rendered Achromatic, — that is, are enabled to exert their refractive power without producing either chromatic or spherical aberration. 13. It has only been of late years, however, that the construc- tion of Achromatic object-glasses for Microscopes has been con- sidered practicable ; their extremely minute size having been thought to forbid the attainment of that accuracy which is neces- sary in the adjustment of the several curvatures, in order that the errors of each separate lens which enters into the combina- tion, may be effectually balanced by the opposite errors of the rest. The first successful attempt was made in this direction, in the year 1823, by M. Selligues of Paris; the plan which he adopted being that of the combination of two or more pairs of lenses, each pair consisting of a double-convex of crown-glass, and a plano-concave of flint. In the next year, Mr. Tulley of London, without any knowledge of what M. Selligues had ac- complished, applied himself (at the suggestion of Dr. Goring) to the construction of achromatic object-glasses for the microscope ; and succeeded in producing a single combination of three lenses (on the telescope plan), the corrections of which were extremely complete. This combination, however, was not of high power, nor of large angular aperture ; and it was found that these ad- vantages could not be gained, without the addition of a second combination. Prof. Amici at Modena, also, who attempted the construction of microscopic object-glasses as early as 1812, but, despairing of success, had turned his attention to the application of the reflecting principle to the Microscope, resumed his original labors on hearing of the success of M. Selligues; and, by work- ACHROMATIC OBJECT-GLASS. 75 ing on his plan, he produced, in 1827, an achromatic combination of three pairs of lenses, which surpassed anything of the same kind that had been previously executed. From that time, the superiority of the plan of combining three pairs of lenses (Fig. 8 ; 1, 2, 3), which should be so adjusted as to correct each other's errors, to the telescopic combinations adopted by Mr. Tulley, may be considered to have been completely established ; and English opticians, working on this method, soon rivalled the best productions of Continental skill. 14. It was in this country that the next im- I* portant improvements originated; these being the result of the theoretical investigations of Mr. J. J. Lister,1 which led him to the discovery of certain properties in achromatic combinations, that had not been previously detected. Acting upon the rules which he laid down, practical opticians at once succeeded in producing com-SectionofanAchromatic binations far superior to any which had been previously executed, both in wideness of aperture, flatness of field, and perfectness of correction ; and continued progress has been since made in the same direction, by the like combination of theoretical acumen with manipulative skill.2 For the subse- quent investigations of Mr. Lister have led him to suggest new combinations, which have been speedily carried into practical execution ; and there is good reason to believe that the limit of perfection has now been nearly reached, since almost everything which seems theoretically possible has been actually accom- plished. The most perfect combinations at present in use for high powers, consist of as many as eight distinct lenses ; namely, in front, a triplet composed of two plano-convex lenses of crown- glass, with a plano-concave of dense flint between them ; next, a doublet, composed of a double-convex of crown, and a double- See his Memoir in the "Philosophical Transactions," for 1829. 2 The first British Opticians (after Mr. Tulley) who applied themselves to the con- struction of Achromatic object-glasses for microscopes, were Mr. Ross and Mr. Powell. Mr. James Smith did not enter the field until some time afterwards; but, having the advantage of Mr. Lister's special superintendence, he soon equalled, in the lower powers at least, the best productions of his predecessors. With Mr. Ross, his son has been subsequently associated : with Mr. Powell, his brother-in-law, Mr. Lealand ; and with Mr. Smith, Mr. Beck, a nephew of Mr. Lister. These three firms have constantly kept up an honorable rivalry, which has been very advantageous to the perfectionnement of the Microscope; and have maintained a position which is still far in advance of that of all other manufacturing Opticians in this country or the Continent. The lenses pro- duced by each are distinguished by excellencies of their own ; and it would be scarcely possible fairly to assign an absolute preference to either above the others. Among the amateurs who have occupied themselves in the construction of microscopic Achromatics, Mr. Wenham has been the most successful. An American rival has recently been announced, in the person of Mr. Spencer; who, taking advantage of all that had been previously accomplished, is said to have produced combinations not only equalling, but, in some important particulars, surpassing those of English makers. Only one of these, however, has found its way (the author believes) to this country; and not having had the opportunity of seeing it himself, he can only judge of it by report. (See Appendix.) 76 OPTICAL PRINCIPLES OF THE MICROSCOPE. FIG. 9. concave of flint; and at the back, another triplet, consisting of two double-convex lenses of crown, with a double-concave of flint interposed between them. By the use of this combination, an angular aperture of no less than 170° has been obtained with an objective of l-12th inch focus; and it is obvious that as an increase of divergence of no more than 10° would bring the ex- treme rays into a straight line with each other, they would not enter the lens at all ; so that no further enlargement of the aper- ture can be practically useful. 15. The enlargement of the angle of aperture, and the greater completeness of the corrections, first obtained by the adoption of Mr. Lister's principles, soon rendered sensible an imperfection in the performance of these lenses under certain circumstances which had previously passed unnoticed ; and the important discovery was made by Mr. Ross, that a very obvious dif- ference existed in the precision of the image, according as the object is viewed with, or with- out a covering of talc or thin glass; an object-glass which is perfectly adapted to either of these conditions, being sensi- bly defective under the other. The mode in which this differ ence arises, is explained by Mr. Ross as follows.1 Let o, Fig. 9, be any point of an object ; o P the axial ray of the pencil that diverges from it ; and o T, o T', two diverging rays, the one near to, the other remote from, the axial ray. Now if G G G a represent the section of a piece of thin glass, intervening between the object and the object-glass, the rays o T and o T' will be re- fracted in their passage through it, in the directions T R, T' R' ; and on emerging from it again, they will pass on towards E and E'. Now if the course of these emergent rays be traced back- wards, as by the dotted lines, the ray E R will seem to have issued from x, arid the ray F/ R' from Y ; and the distance x Y is an aberration quite sufficient to disturb the previous balance of the aberrations of the lens composing the object-glass. The requi- site correction may be effected, as Mr. Ross pointed out, by giv- ing to the front pair (Fig. 8, i) of the three of which the objective is composed, an excess of positive aberration (i. e. by under-cor- recting it), and by giving to the other two pairs (2, 3) an excess of negative aberration (i. e. by over-correcting them), and by making the distance between the former and the latter susceptible of alteration. For when the front pair is approximated most nearly to the other two, and its distance from the object is increased, 1 " Transactions of the Society of Arts," vol. li. CORRECTION FOR COVERING OF THE OBJECT. 77 its positive aberration is more strongly exerted upon the other pairs, than it is when the distance between the lenses is increased, and the distance between the front pair and the object is dimi- nished. Consequently, if the lenses be so adjusted that their cor- rection is perfect for an uncovered object, the front pair being removed to a certain distance from the others, its approximation to them will give to the whole combination an excess of positive aberration, which will neutralize the negative aberration occa- sioned by covering the object with a thin plate of glass.1 It is obvious that this correction will be more important to the perfect performance of the combination, the larger is its angle of aper- ture ; since, the wider the divergence of the oblique rays from the axial ray, the greater will be the refraction which they will sustain in passing through a plate of glass, and the greater there- fore will be the negative aberration produced, which will, if un- corrected, seriously impair the distinctness of the image. And it is consequently not required for low powers, whose angle of aperture is comparatively small ; nor even for the higher, so long as their angle of aperture does not exceed 50°. As a large pro- portion of the lenses made by foreign Opticians do not range beyond this, the adjustment in question may be dispensed with; and even where the angle is much larger, if the corrections be made perfect for a thickness of glass of l-100th of an inch (which is about an average of that with which objects of the finer kind are usually covered), they will not be much deranged by a dif- ference of a few hundredths of an inch, more or less, in that amount. 16. We are now prepared to enter upon the application of the optical principles which have been explained and illustrated in the foregoing pages, to the construction of microscopes. These are distinguished as simple, and compound; each kind having its peculiar advantages to the Student of Mature. Their essential difference consists in this ; — that in the former, the rays of light which enter the eye of the observer proceed directly from the object itself, after having been subject only to a change in their course ; whilst in the latter, an enlarged image of the object is formed by a lens, which image is viewed by the observer through a simple microscope, as if -it were the object itself. The simple microscope may consist of one lens ; but (as will be presently shown) it may be formed of two, or even three ; these, however, are so disposed as to produce an action upon the rays of light corresponding to that of a single lens. In the compound micro- scope, on the other hand, not less than two lenses must be em- ployed; one to form the enlarged image of the object, and this, being nearest to it, is called the object-glass; whilst the other again magnifies that image, being interposed between it and the eye of the observer, and is hence called the eye-glass. A perfect 1 The mode in which this adjustment is effected, will be more fitly described here- after (§8-2). 78 OPTICAL PRINCIPLES OF THE MICROSCOPE. object-glass, as we have seen, must consist of a combination of lenses ; and the eye-glass, as we shall presently see (§ 21), is best constructed by placing two lenses in a certain relative position, forming what is termed an eye-piece. These two kinds of in- strument need to be separately considered in detail. 17. Simple Microscope. — In order to gain a clear notion of the mode in which a single lens serves to "magnify" minute objects, it is necessary to revert to the phenomena of ordinary vision. An eye free from any defect has a considerable power of adjust- ing itself, in such a manner as to gain a distinct view of objects placed at extremely varying distances ; but the image formed upon the retina will of course vary in size with the distance of the object; and the amount of detail perceptible in it will fol- low the same proportion. To ordinary eyes, however, there is a limit within which no distinct image can be formed, on account of the too great divergence of the rays of the different pencils which then enter the eye ; since the eye is usually adapted to re- ceive, and to bring to a focus, rays which are parallel or but slightly divergent. This limit is variously stated at from five to ten inches ; we are inclined to think from our own observations, that the latter estimate is nearest the truth ; that is, although a person with an ordinary vision may see an object much nearer to his eye, he will see little if any more of its details, since what is gained in size will be lost in distinctness. Now the utility of a convex lens interposed between a near object and the eye, con- sists in its reducing the divergence of the rays forming the seve- ral pencils which issue from it ; so that they enter the eye in a state of moderate divergence, as if they had issued from an ob- ject beyond the nearest limit of distinct vision ; and a well-de- fined picture is consequently formed upon the retina. But not only is the course of the several rays in each pencil altered as regards the rest, by this refracting process, but the course of the pencils themselves is changed, so that they enter the eye under an angle corresponding with that at which they would have ar- rived from a larger object situated at a greater distance. The picture formed upon the retina, therefore,"by any object (Fig. 10), FIG> 10. corresponds in all respects with one which would have been made by the same ob- ject a b increased in its di- mensions to A B, and view- ed at the smallest ordinary distance of distinct vision. A "short-sighted" person, however, who can see ob- jects distinctly at a distance of two or three inches, has the same power in his eye Diagram illustrating the action of the Simple Microscope, along foy reason of its greater convexity, as that which the person of ordinary vision SIMPLE MICROSCOPE. 79 gains by the assistance of a convex lens which shall enable him to see at the same distance with equal distinctness. It is evident, therefore, that the magnifying power of a single lens, depending as it does upon the proportion between the distance at which it renders the object visible, and the nearest distance of unaided distinct vision, must be different to different eyes. It is usually estimated, however, by finding how many times the focal length of the lens is contained in ten inches ; since, in order to render the rays from the object nearly parallel, it must be placed nearly in the focus of the lens (Fig. 2) ; and the picture is referred by the mind to an object at the ordinary distance. Thus, if the focal length of a lens be one inch, its magnifying power for each dimension will be ten times, and consequently a hundred super- ficial ; if its focal distance be only one-tenth of an inch, its mag- nifying power will be a hundred linear, or ten thousand superfi- cial. The use of the convex lens has the further advantage of bringing to the eye a much greater amount of light, than would have entered the pupil from the enlarged object at the ordinary distance, provided its own diameter be greater than that of the pupil ; but this can only be the case when its magnifying power is low. 18. It is obviously desirable, especially when lenses of very high magnifying power are being employed, that their aperture should be as large as possible ; since the light issuing from a minute object has then to be diffused over a large picture, and will be proportionally diminished in intensity. But the shorter the focus, the less must be the diameter of the sphere of which the lens forms a part ; and unless the aperture be proportionally diminished, the spherical and chromatic aberrations will inter- fere so much with the distinctness of the picture, that the ad- vantages which might be anticipated from the use of such lenses will be almost negatived. Nevertheless, the Simple Microscope has been an instrument of extreme value in anatomical research, owing to its freedom from those errors to which the Compound Microscope, as originally constructed, was necessarily subject ; the greater certainty of its indications being evident from the fact, that the eye of the observer receives the rays sent forth by the object itself, instead of those which proceed from an image of that object. A detail of the means employed by different individuals, for procuring lenses of extremely short focus, though possessing much interest in itself, would be misplaced here ; since. recent improvements, as will presently be shown, have superseded the necessity of all these. It may be stated, how- ever, that Leeuwenhoeck, De la Torre, and others among the older microscopists, made great use of small globules procured by fusion of threads or particles of glass. The most important suggestion for the improvement of the Simple Microscope com- posed of a single lens, proceeded some years ago from Dr. Brewster, who proposed to substitute diamond, sapphire, garnet, 80 OPTICAL PRINCIPLES OF THE MICROSCOPE. and other precious stones of high refractive power, for glass, as the material of single lenses. A lens of much longer radius of curvature might thus be employed, to gain an equal magnifying power ; and the aperture would admit of great extension, with- out a proportional increase in the spherical and chromatic aber- rations. This suggestion has been carried into practice with complete success, as regards the performance of lenses executed on this plan ; but the difficulties of various kinds in the way of their execution, are such as to render them very expensive ; and as they are. not superior to the combination now to be described, they have latterly been quite superseded by it. This combina- tion, first proposed by Dr. Wollaston, and known as his doublet, consists of two plano-convex lenses, whose focal lengths are in the proportion of one to three, or nearly so, having their convex sides directed towards the eye, and the lens of shortest focal length nearest the object. In Dr. Wollaston 's original combina- tion, no perforated diaphragm (or " stop") was interposed; and the distances between the lenses was left to be determined by ex- periment in each case. A great improvement was subsequently made, however, by the introduction of a " stop" between the lenses, and by the division of the power of the smaller lens be- tween two (especially when a very short focus is required) so as to form a triplet, as was first suggested by Mr. Holland.* When combinations of this kind are well constructed, both the spheri- cal and the chromatic aberrations are so much reduced, that the angle of aperture may be considerably enlarged without much sacrifice of distinctness ; and hence for all powers above l-4th inch focus, doublets and triplets are far superior to single lenses. The performance of even the best of these forms of Simple mi- croscope, however, is so far inferior to that of a good Compound microscope as now constructed upon the achromatic principle, that no one who has the command of the latter form of instru- ment would ever use the higher powers of the former. It is for the prosecution of observations, and for the carrying on of dis- sections, which only require low powers, that the Simple micro- scope is to be preferred ; and, consequently, although doublets and triplets afforded the best means of obtaining a high magni- fying power, before Achromatic lenses were brought to their pre- sent perfection, they are now comparatively little used. 19. Another form of simple magnifier, possessing certain ad- vantages over the ordinary double-convex lens, is that commonly known by the name of the " Coddington" lens. The first idea of it was given by Dr. Wollaston, who proposed to cement to- gether two plano-convex, or hemispherical lenses, by their plane sides, with a stop interposed, the central aperture of which should be equal to l-5th of the focal length. The great advantage of such a lens is, that the oblique pencils pass, like the centre ones, at right angles with the surface ; and that they are consequently but little subject to aberration. The idea was further improved 1 "Transactions of the Society of Arts," vol. xlix. STANHOPE LENS — COMPOUND MICROSCOPE. 81 upon by Mr. Coddington, who pointed out that the same end would be much better answered by taking a sphere of glass, and grinding a deep groove in its equatorial part, which should be then filled with opaque matter, so as to limit the central aper- ture. Such a lens gives a large field of view, admits a considera- ble amount of light, and is equally good in all directions ; but its powers of definition are by no means equal to those of an achromatic lens, or even of a doublet. This form is chiefly use- ful, therefore, as a hand magnifier, in which neither high power nor perfect definition is required ; its peculiar qualities rendering it superior to an ordinary lens of the same power, for the class of objects for which such lenses are applied in this mode. We think it right to state that many of the magnifiers sold as " Cod- dington" lenses are not really (as we have satisfied ourselves) portions of spheres, but are manufactured out of ordinary double- convex lenses, and will be destitute, therefore, of many of the above advantages. It may be desirable to allude to the magni- fier known under the name of the "Stanhope" lens, which some- what resembles the " Coddington" in appearance, but differs from it essentially in properties. It is nothing more than a double-convex lens, having two surfaces of unequal curvatures, separated from each other by a considerable thickness of glass ; the distance of the two surfaces from each other being so ad- justed, that when the most convex is turned towards the eye, minute objects placed on the other surface shall be in the focus of the lens. This is an easy mode of applying a rather high magnifying power to scales of butterflies' wings and other similar flat and minute objects, which will readily adhere to the surface of the glass ; and it also serves to detect the presence of the larger animalcules, or of crystals in minute drops of fluid, to ex- hibit the " eels" in paste or vinegar, &c. &c. ; but it is almost en- tirely destitute of value as an instrument of scientific research, and can scarcely be regarded in any higher light than as an in- genious philosophical toy.1 20. Compound Microscope. — In its most simple form, this instru- ment consists of only two lenses, the " object-glass" and the " eye- glass :" the former, c D (Fig. 11), receiving the rays of light direct from the object, A B, which is brought into near proximity to it, forms an enlarged and inverted image A' B' at a greater distance on the other side ; whilst the latter, L M, receives the rays which are diverging from this image, as if they proceeded from an , object actually occupying its position and enlarged to its dimen- sions, and these it brings to the eye at E, so altering their course as to make that image appear far larger to the eye, precisely as in the case of the simple microscope (§ 16). It is obvious that by the use of the very same lenses, a considerable variety of magnifying power may be obtained, simply by altering their 'The principal forms of construction of Simple Microscopes, will be described in the next chapter. 6 82 OPTICAL PRINCIPLES OF THE MICROSCOPE. FIG. J 1. FIG. 12. position in regard to each other and to the object; for if the eye- glass be carried further from the object-glass, whilst the object is approximated nearer to the latter, the image A' B' will be formed at a greater distance from it, and its dimensions will consequently be augmented. If on the other hand, the eye-glass be brought nearer to the object-glass, whilst the object is removed further from it, the distance of the image will be shortened, and its dimensions proportionably diminished. We shall hereafter see that this mode of varying the magnifying power of compound microscopes may be turned to good account in more than one mode (§§ 43, 44) ; but there are limits to the use which can be advantageously made of it. — The amplification may also be varied by altering the magni- fying power of the eye-glass ; but here, too, there are limits to the increase; since defects of the object-glass, which are not perceptible when its image is but moderately enlarged, are brought into injurious promi- nence when the imperfect im- age is amplified to a much greater extent. In practice, it is generally found much better to vary the power, by employ- ing object-glasses of different f$ci ; an object-glass of long focus forming an image, which is not at many times the dis- tance of the object from the other side of the lens, and which, therefore, is not of many times its dimension ; whilst an object-glass of short focus requires that the object should be so nearly approxi- mated to it, that the distance of the image is a much higher multiple of that of the object, and its dimensions are propor- Diagram of the com- . , , , / to the flat side of the Aquarium. It is obvi- ous that the very same instrument, turned from the horizontal into the vertical position, by at- taching the clamp (as in Fig. 24) to the edge of a wooden strutt rising vertically from a hori- zontal slab, instead of to the edge of a hori- zontal table, becomes extremely well suited for examining objects which are in course of dissection in a trough too large to be con- veniently transferred to the stage of the micro- scope, for looking over minute shells spread out on a sheet of paper, and for other purposes for which a special form of dissecting microscope has been devised by Messrs. Powell and Lea- land. But again, by turning up the J. shaped support constructed for the last-named purpose, so that it shall rest (as it were) on two legs like the Greek A, and then clamping the stem that carries the body to its highest edge, the instru- ment acquires a position very suitable for ordi- nary microscope work ; and nothing is wanted to adapt it to this, save the addition of a stage and a mirror, each of ... . , which may be so con- Warington's Universal Microscope, arranged for ordinary use. , •> «, . . structed as to fit into a WARING TON'S UNIVERSAL MICROSCOPE. 107 brass socket let into the wooden support, thus completing the Microscope in the form represented in Fig. 25. This is not the last of the adaptations of which the instrument is capable; for the wooden support remaining at the same inclination, the body may be brought to the perpendicular, by shifting its^ stem in the clamp and by altering its angle at the cradle-joint ; whilst a horizontal po- sition may be given to the stage, by fitting it into another socket (Fig. 26) ; in this arrangement, FlG 26 moreover, the stage acquires an increase of firmness, from the bearing of a plate that projects at right angles from its under surface, upon the inclined face of the wooden support. Thus a dissecting microscope is form- ed, which has many of the ad- vantages of that of Messrs. Smith~andBeck; being subject, however, to the important drawback, that the mirror can- not be so placed as to reflect the light upwards through the axis of the microscope. (A means of remedying this, how- ever, might perhaps be con- trived without much difficulty or cost.) On the left side of the slanting support, at a short distance above the stage, is a hole intO Which may be fitted Warington's Universal Microscope, arranged for either the stem of a condens- ing lens for opaque objects, or the stem of the stage-forceps ; either or both of which may also be fitted into holes in the front corners of the stage. The stage is provided with a sliding ledge for the support of objects in an inclined position ; and it might also be furnished, if required, with a diaphragm plate. One of the chief merits of the instrument, however, being lightness and portability, it would not be desirable to encumber it with many accessories. For convenience of packing, the shorter portion ot the J_ piece may be connected with the longer by strong pins fitted into sockets, instead of being permanently fixed, so that the two can be readily disconnected and one part laid flat upon the other ; and the whole apparatus will then lie within a very small compass. The distinctive peculiarity of this instrument consists in the extreme simplicity of the means by which a variety of useful ends are obtained. It is scarcely one that should be re- commended to the beginner ; since it is in several respects not BO well adapted for ordinary work, as the forms already described. But it is a most valuable addition to the Microscopic apparatus of 108 CONSTRUCTION OF THE MICROSCOPE. FiG. 27. the Naturalist ; and may be constructed at so trifling an expense, to work with any objectives he already may possess, that a con- siderable demand may be anticipated for it.1 37. We now pass to an entirely different class of instruments, — those of which the aim is, not simplicity but perfection ; not the production of the best effect with limited means, but the at- tainment of everything that the Microscope can accomplish, .with- out regard to cost or complexity. This object has been certainly carried out by the Opticians of our own country, much more com- pletely than by those of the Continent ; and it seems but fair to- wards the three principal London makers, by whose labors the present admirable results have been attained, that the pattern finally adopted by each should be here delineated and described. Without any invidious preference, the first place may fairly be assigned to the Large Compound Microscope of Mr. Ross; not only as being the one which was first brought (in all essen- tial features at least) to its present form, but also because it is that which contains the greatest number of provisions for investi- gating objects in a variety of different modes. The general plan of Mr. Ross's Mi- croscope will be seen to be essentially the same with that which has been followed by Mr. Field in the simple form of this instru- ment first described (§ 31), as weH as by many other makers ; but it is carried out with the greatest at- tention to solidity of construction, in those parts especially which are most liable to tremor ; and we are informed by Mr. Eoss, that every part has been tested by the "inverted pendulum" ' This instrument has been made for Mr. Warington and for the Author by Mr. Salmon, 100 Fenchurch Street ; who supplies it, on either plan, without objectives or case, but with condenser and stage-forceps, for 3 guineas. ROSS'S LARGE COMPOUND MICROSCOPE. 109 (an instrument contrived to indicate otherwise insensible vibra- tions), and either strengthened or reduced as might be found necessary, so as to obtain an equality ^ of vibration between the stage and the optical part, which will prevent any percepti- ble tremor in the image. The " coarse" adjustment is made by the large milled head situated just behind the summit of the up- rights, which turns a pinion working into a rack cut on the back of a very strong flattened stem, that carries the transverse arm at its summit ; a second milled head (which is here concealed by the stage fittings) is attached to the other end of the axis of the pinion (as in Fig. 18), so as to be worked with the left hand. The " fine" adjustment is effected by the milled head on the transverse arm just behind the base of the "body;" this acts upon the " nose" or tube projecting below the arm, wherein the objectives are screwed. The other milled head seen at the summit of the stem, serves to secure the transverse arm to this, and may be tightened or slackened at pleasure, so as to regulate the travers- ing movement of the arm ; this movement is only allowed to take place in one direction, namely, towards the right side, being checked in the opposite by a "stop," which secures the coinci- dence of the axis of the body with the centre of the stage and with the axis of the illuminating apparatus beneath it. It is in the movements of the stage, that the greatest contrivance is shown ; these are three, namely, a traversing movement from side to side, a traversing movement from before backwards, and a rotatory movement. The traversing movements, which allow the platform carrying the object to be shifted about an inch in each direction, are effected by the two milled heads situated at the right of the stage ; and these are placed side by side, in such a position that one may be conveniently acted on by the fore- finger, and the other by the middle finger, the thumb being readily passed from one to the other. The traversing portion of the stage carries the platform whereon the object is laid, which has a ledge at the back for it to rest against ; and this platform has a sliding movement of its own, from before backwards, by which the object is first brought near to the axis of the micro- scope, its perfect adjustment being then obtained by the traversing movement. To this platform, and to the traversing slides which carry it, a rotatory movement is imparted by a milled head, placed underneath the stage on the left hand side ; for this milled head turns a pinion which works against the circular rack (seen in the figure) whereby the whole apparatus above is carried round about a third of a revolution, without in the least disturbing the place of the object, or removing it from the field of the micro- scope. This rotatory movement is useful for two purposes ; first, in the examination of very delicate objects by oblique lights, in order that, without disturbing the illuminating apparatus, the effect of the light and shadow may be seen in every direction, whereby important additional information is often gained ; and, 110 CONSTRUCTION OF THE MICROSCOPE. secondly, in the examination of objects under polarized light, a class of appearances being produced by the rotation of the object between the prisms, which is not developed by the rotation of either of the prisms themselves. Below the stage, and in front of the stem that carries the mirror, is a dovetail sliding bar, which is moved up and down by the milled head shown at its side ; this sliding bar carries what is termed by Mr. Ross the " secondary stage" (omitted in the figure for the sake of simpli- city), which consists of a cylindrical tube for the reception of the achromatic condenser, the polarizing prism, and other fittings ; to this secondary stage, also, a rotatory motion is communicated by the turning of a milled head ; and a traversing movement of limited extent is likewise given to it by means of two screws, one on the front and the other on the left hand side of the frame which carries it, in order that its axis may be brought into perfect coincidence with the axis of the " body." The special advantages of this instrument consist in its perfect steadiness, in the admirable finish of its workmanship, and in the variety of movements which may be given both to the object and to the fittings of the secondary stage. Its disadvantages consist in the want of porta- bility, necessarily arising from the * substantial mode of its con- struction ; and in the multiplicity of its movable parts, which presents to the beginner an aspect of great complexity. This complexity, however, is much more apparent than real ; for each of these parts has an independent action of its own, the nature of which is very soon learned ; and the various milled heads are so disposed, that the hand readily (and at last almost instinc- tively) finds its way from one to the other, so as to make any re- quired adjustment, whilst the eye is steadily directed to the ob- ject. To the practised observer, therefore, this multiplication of adjustments is a real saving of time and labor, enabling him to do perfectly and readily what might otherwise require much trouble, besides affording him certain capabilities which he would not otherwise possess at all. 38. Powell and Lealands Compound Microscope. — This instru- ment, represented in Fig. 28, is far lighter than the preceding in its general "build," without being at all deficient in steadi- ness ; it has not, however, some of those improvements for which Mr. Ross's plan of construction is especially adapted. The three-legged stand gives a firm support to the trunnions that carry the tube to which the stage is attached, and from which a triangular stem is raised, by the rack-and-pinion movement set in action by the double milled head, whereby the " coarse" adjustment of the focus is obtained. The triangular stem carries at its summit the transverse arm, which contains (as in Mr. Ross's Microscope) the lever action of the "fine" adjustment ; and this is acted on by the milled head at the back of the arm, whence also pass two oblique stays, which, being attached to the upper part of the body, assist in preventing its POWELL AND LEALAND'S MICROSCOPE. Ill vibration. The stage is provided with a traversing movement in each direction, to the extent of about three-quarters of an inch ; this is effected on the plan known as Turrell's, in which the two milled heads ~ oft i ^ xi *IG- ^°- are placed on the same axis, instead of side by side, one of them being also re- peated on the left hand of the stage, so that the movements may be communi- cated either by the right hand alone, or by both hands in com- bination. The plat- form which carries the object is made to elide, as in the preced- ing case, on the sum- mit of the traversing apparatus ; and it has not only a ledge whereon the object may rest, but also a " spring clip" for se- curing it whenever the stage may be placed in a vertical position. This plat- form, moreover, is so connected with the traversing apparatus, that it may be turned roundin the direction Powell and Lealand,s Large Compound Microscope. of its plane; but as this rotation takes place above, instead of beneath the traversing apparatus, there is no security that the centre of rotation shall coincide with the axis of the optical portion of the instrument ; so that, unless this adjustment have been previously made, the object will be thrown completely out of the field of view when the platform is made to revolve. Hence, although this movement is of great use in facilitating the full examination of an object, by enabling the observer to bring it into the field of view in every variety of position, it does not serve, like the rotatory movement of Mr. Ross's stage, to change the position of the object in regard to the illuminating apparatus, without disturb- ing the observer's view of it. The condenser for transparent objects, the polarizing apparatus, &c., are here fitted to the under 112 CONSTRUCTION OF THE MICROSCOPE. side of the principal stage itself, instead of to an independent or secondary stage ; an arrangement which, though convenient as regards compactness, admits of less variety of adjustment than is afforded by the latter plan. The mirror, instead of being swung loosely upon two centres, is pivoted to one end of a quadrant of brass, of which the other end is pivoted to a strong pin that projects from the sliding tube ; a spring being so at- tached to each of these pivots, as to give to the movements of the mirror that suitable degree of stiffness, which shall prevent it from being disturbed by a passing touch. No instrument can be better adapted than this to all the ordinary wants of the Mi- croscopist ; there are very few purposes which it cannot be made to answer ; and there are many who will consider that its defi- ciency as to these is counterbalanced (to say the least) by its comparative simplicity and portability, as well as by its lower cost. For the sake, however, of such as may desire the power of obtaining a more oblique illumination, than is permitted by the construction of the stage in the instrument just described, Messrs. P. and L. have recently brought out a new pattern, in which the thickness of the stage is greatly reduced, a sub-stage is provided for the reception of the condenser and other fittings, and the mirror is mounted on a doubly extending arm. 39. Smith and Beck's Large Microscope. — The general plan of this instrument (Fig. 29) nearly resembles that of the " dissecting microscope" of the same makers, already noticed (§ 35), so far, at least, as regards the mode of supporting the body, and of effecting the focal adjustments; whilst in the construction of the stage, and in the arrangement of the fittings beneath, it differs from all the microscopes hitherto described. The stage is fur- nished with the usual traversing movements ; but it is distin- guished by its thinness; and this is of importance in certain cases, as admitting of a more oblique illumination than could otherwise be obtained, and also as allowing the construction of the achromatic condenser (§ 56) to be much simplified. The platform for the object is fitted upon the traversing apparatus, in the same mode as in the microscope last described, and possesses the same kind of rotatory movement. Beneath the stage is a continuation of the gun-metal "limb" which carries the body; and this is ploughed out into a groove for the reception of a sliding-bar, which carries what may be termed the " secondary body," namely, a short tube (seen beneath the stage), capable of being moved up and down by a milled head, and fitted for the reception of the achromatic condenser, polarizing apparatus, &c. This "secondary body" consequently answers the same purpose as the " secondary stage" of Mr. Ross's microscope, and its rela- tions to the other parts of the instrument are essentially the same ; but it differs in the following particulars : — first, that by being made to work in a groove which is in perfect correspon- dence with that wherein the principal "body" works (this corre- SMITH AND BECKS LARGE MICROSCOPE. 113 spondence being secured by the action of the planing machine that ploughs both grooves), the " secondary" body always has its axis so perfectly continuous with that of the first, that no special adjustment is need- PIG 29. ed to " centre" the greater part of the illuminating apparatus ; and, secondly, that the tube will carry the achromatic condenser at its upper end, the polaris- ing prism at its lower, and the selenite plates between the two, a combination that cannot be made in any other instrument (§ 63). Moreover, as all these fittings are received into a tube of which the exact size and position are assured, the makers of this instrument can supply ad- ditional apparatus at any time, with the certainty of its accurate adjustment. This "secondary body," however, has not the rota- tory movement possessed by Mr. Ross's " secondary stage ;" and to the limited class of purposes, there- fore, which that move- ment is adapted to serve, it Cannot be adapted. Smith and Beck's Large Compound Microscope. The mirror is hung in the usual way between two centres; but the semicircle that carries these, instead of being at once pivoted to the tube which slides upon the cylindrical stem, is attached to an intermediate arm ; and by means of this it may be placed in such a position as to reflect light very ob- liquely upon the object, and thus to bring out a new set of ap- pearances, with which it is very important in certain cases to be acquainted. ^ In regard to weight and complexity, this instrument holds a position intermediate between the two last described. The mode in which the body is supported, appears to the author decidedly preferable to that adopted by the other makers ; and though it has the disadvantage of separating the focal adjust- ments from each other and from the stage motions more widely than is the case in the two preceding instruments, yet the differ- ence is scarcely perceptible in practice. The milled heads acting on the former are both of them in positions in which they are 11.4 CONSTRUCTION OF THE MICROSCOPE. easily reached by the left hand, when the elbow is resting on the table ; whilst the right hand finds the milled heads of the travers- ing stage and of the secondary body in close proximity to each other. The imperfection of the means of giving rotation to the object, constitutes in this, as in Powell and Lealand's microscope, a point of inferiority to Eoss's ; the number of cases in which such a movement is important, however, is by no means consi- derable. On the other hand, the arrangement of the illuminating apparatus in Smith and Beck's Microscope, seems to the author to present some decided advantages over that adopted by either of the other makers ; and in point of general excellence of work- manship, this instrument cannot be surpassed. Without any invidious comparisons, it may be safely said that whoever desires to possess a first-class Microscope, cannot do bet- ter than select one of the three instruments last described ; the excellence of the optical performance of the lenses supplied by their respective makers, being so nearly on a par, that the choice may be decided chiefly by the preference which the taste of the purchaser, or the nature of the researches on which he may be engaged, may lead him to entertain, for one or other of the plans of construction which has now been brought under notice. 40. Nachet's Binocular Microscope. — Since that remarkable invention of Prof. Wheatstone, the Stereoscope, has led to a general appreciation of the value of binocular vision, in conveying to the mind a notion of the solid forms of bodies, various attempts have been made to apply the same principle to the Microscope. To any one who understands the principle of the Stereoscope, a little consideration will make it obvious that this end might be theoretically attained, by placing two microscope-bodies at such an angle of inclination, that their respective object-glasses should point to the same object, whilst their eye-pieces should be at the ordinary distance of the right and left eyes from each other; but this practical difficulty will obviously and necessarily arise, in bringing the two microscopes into the requisite convergence, — that the axes of the instruments cannot be approximated suffi- ciently closely at their lower ends, unless the objectives employed should be of a focus so long, that the value of such an instrument would be extremely limited. It was early seen, therefore, that the only feasible method would be to use but a single objective for both bodies ; but to bisect the pencils of rays emerging from this lens, so as to cause all those which have issued from the ob- ject in such a direction as to pass through either half of it, to be refracted into the body situated on that side ; so that the two eyes, applied to the two eye-pieces respectively, shall receive through the two halves of the objective, two magnified images of , the object differing from each other in perspective projection, as if the object, actually enlarged to the dimensions of its image, had been viewed by both eyes at once at a moderate distance. NACHETS BINOCULAR MICROSCOPE. 115 That such a method would produce the Stereoscopic effect, might be anticipated from the result of the very simple experiment of covering the right-hand or the left-hand half of an object-glass of low power, during the examination of any object that lies in oblique perspective ; for the two views of it thus obtained, will be found to present just the kind and degree of difference which is observable in stereoscopic pictures. The first attempt to put this plan into execution, seems to have been that of Prof. Riddell, of New Orleans ; but the results of his method, as followed by opticians on the European side of the Atlantic, were far from answering the expectations excited by his own description of them. The subject was both theoretically and practically inves- tigated by Mr. Wenham, with much ability (Transactions of the Microscopical Society, new series, Vol. II, p. 1) ; and a Binocular Microscope on a pattern suggested by him, was constructed by Messrs. Smith and Beck. This, too, was far from satisfactory in its performance, having two capital defects ; namely, first, that the view which it gave was often pseudoscopic^ the projecting por- tions of the object appearing to be depressed, and vice versd ; and second, that the two bodies being united at a fixed angle of con- vergence, the distance between their axes could not be con- veniently adapted to the varying distances of the eyes of different individuals. The construction adopted by M. Cachet, however, is much more successful. His method is to divide the pencil of rays issuing from the objec- tive, by means of a prism (Fig. 30, p) whose section is an equilateral triangle ; for the rays a b on the right side, which enter the flattened sur- face presented to them, are reflected, by impinging very obliquely against one of the internal faces of the prism, towards the left, emerging again from the prism, as they had entered it, almost at right angles ; and in like manner the rays af bf on the left side are reflected towards the right. Each of these pencils is received by a similar prism, Which again Changes itS direC- Arrangement of Prisms in Nachet's Binocular tion, so as to render it parallel to its original course ; and thus the two halves a b and a' V of the original pencil are completely separated from each other to any interval that may be required, this interval being determined by the distance between the central and the lateral prisms. In FIG. 30. 116 CONSTRUCTION OF THE MICROSCOPE. Fig. 31 is shown the Binocular Microscope constructed by M. FIG. si. Nachet upon this plan. The arrangement of the base and stage is that commonly employed in French vertical Microscopes ; and a stem rises from the back of it, with which the double body is connected by a rack-and-pinion movement that gives the focal adj ustment. The apparatus of prisms shown in Fig. 30, is placed between the object-glass and the lower ends of the bodies ; and by means of a double-threaded screw acted on by a milled head between the two bodies, they may be separated from, or approximated towards, each other; so that the distance between their axes may be brought to coincide with the dis- tance between the axes of the eyes of the individual observer. The author can con- firm by his own experience the statement of M. Cachet, that this instrument is en- tirely free from that tendency to produce pseudoscopic effects, which is the great drawback in Prof. Riddell's and in Mr. Wenham's arrangements ; and it comes so near the theoretical standard of perfection, when used with low powers of moderate Binocular Microscope, aperture, that its performance may be con- sidered highly satisfactory. Its definition, however, when used with higher powers of larger angular aperture, has not yet been rendered sufficiently good, to enable it to afford a satisfactory view of the more difficult class of test-objects; and it maybe doubted whether, considering the number of deflections which the rays undergo in their course, such perfect definition is to be anticipated. For although their general course on entering and emerging from each prism may be perpendicular to its surfaces, so that they suffer no refraction, many of them will be slightly oblique, and will therefore undergo not only refraction, but also some amount of chromatic dispersion. And it is moreover to be recollected, that when high powers are being employed, and especially such as are of large angular aperture, the smallest de- parture from exactitude in the focal adjustment gives indistinct- ness to the image. Now the special object of this instrument being to convey to the mind the notion of the solid. forms of objects, of which some parts project more than others, it is obvi- ous that the rays proceeding from the projecting parts cannot be so. nearly brought to the same focus with those from the receding, as to produce an even tolerably distinct image of both at once. It seems likely to be only with objectives of comparatively low power and small angular aperture, that images suitable for the N A CIIE^ S BINOCULAR MICROSCOPE. 117 production of Stereoscopic effects will be produced ; but for cer- tain classes of objects, this mode of exhibition is most admirably adapted, the solid forms of the Potycystina (Chap.X), for example, being brought out by it (especially when they are viewed as opaque, not as transparent objects) with such a reality, as to make them rese*mble carved ivory balls which the hand feels ready to grasp. 41. The same method of dividing the pencil of rays issuing from the object glass, by a separating prism placed in its course, has been applied by M. Nachet to another purpose, — that of en- abling two or more observers to look at the same object at once, which is often a matter not only of considerable convenience, but also of great importance, especially in the demonstration of dissections. The account given by M. Cachet of the construc- tion of this instrument, as adapted for two persons, will be found in the " Quarterly Journal of Microscopical Science," Yol. II, p. 72 ; he has subsequently devised another arrangement, by which the form of the separating prism is adapted to divide the pencil into three or even four parts, each of which may be di- rected into a different body, so as to give to several observers at one time a nearly identical image of the same object. Of course, the larger the number of secondary pencils into which the pri- mary pencil is thus divided, the smaller will be the share of light which each observer will receive ; but this reduction does not in- terfere with the distinctness of the image, and may be in some degree compensated by a greater intensity of illumination. (See Appendix for a description of American instruments and mo- difications.) CHAPTER III. ACCESSORY APPARATUS. 42. IN describing the various pieces of accessory apparatus with which the Microscope may be furnished, it will be con- venient in the first place to treat of those which form (when in use) part of the instrument itself, being Appendages either to its Body or to its Stage, or serving for the Illumination of the ob- jects which are under examination ; and secondly to notice such as have for their function to facilitate that examination, by en- abling the microscopist to bring the Objects conveniently under his inspection. SECTION 1. APPENDAGES TO THE MICROSCOPE. 43. Draw-Tube. — It is advantageous for many purposes, that the Eye-piece should be fitted, not at once into the " body" of the Microscope, but into an intermediate tube ; the drawing out of which, by augmenting the distance between the object-glass and the image which it forms in the focus of the eye-glass, still further augments the size of the image in relation to that of the object (§ 20). For although the magnifying power cannot be thus increased with advantage to any considerable extent, yet, if the corrections of the object-glass have been perfectly adjusted, its performance is not seriously impaired by a moderate lengthen- ing of the body ; and this may be conveniently had recourse to on many occasions, in which some amplification is desired, inter- mediate between the powers furnished by any two objectives. Thus if one object-glass give a power of 80 diameters, and another a power of 120, by using the first and drawing the eye- piece, its power may be increased to 100. Again, it is often very useful to make the object fill up the whole, or nearly the whole, of the field of view : thus if an object that is being viewed by transmitted rays, is so far from transparent as to require a strong light to render its details visible, the distinctness of its details is very much interfered with, if, through its not occupying the peripheral part of the field, a glare of light enter the eye around its margin ; and the importance of this adjustment is even greater, if opaque objects mounted on black disks are being viewed by DRAW-TUBE — ERECTOR. 119 the Lieberkiihn (§ 65), since if any light be transmitted to the eye direct from the mirror, in consequence of the disk failing to occupy the centre field, it greatly interferes with the vividness and distinctness of the image of the object. In the use of the Micrometric eye-pieces to be presently described (§§ 45, 46), very great advantage is to be derived from the assistance of the draw- tube ; as enabling us to make a precise adjustment between the divisions of the stage micrometer, and those of the eye-piece mi- crometer ; and as admitting the establishment of a more con- venient numerical relation between the two, than could be other- wise secured without far more elaborate contrivances. More- over, if, for the sake of saving room in packing, it be desired to reduce the length of the body, the draw-tube affords a ready means of doing so ; since the body may be made to " shut up," like a telescope, to little more than half its length, without any impairment of the optical performance of the instrument when mounted for use. 44. Erector. — It is only, however, in the use of the Erector, that the full value of the draw-tube, and the advantage of giving to it a rack-and-pinion movement of its own (§ 35), come to be fully appreciated. This instrument, first applied to the Compound Microscope by Mr. Lister, consists of a tube about three inches long, having a meniscus at one end and a plano-convex lens at the other (the convex sides being upwards in each case), with a diaphragm nearly half way between them ; and this is screwed into the lower end of the draw-tube, as shown in Fig. 32. Its effect is (like the corresponding erector of the Telescope), to an- tagonize the reversion of the image formed by the object-glass, by producing a second reversion, so as to make the image presented to the eye correspond in position with the object. The passage of the rays through two additional lenses, of course occasions a certain loss of light l>y reflection from their surfaces, besides subjecting them to aberrations whereby the distinctness of the image is somewhat impaired; but this need not be an obstacle to its use for the class of purposes for which it is especially adapted in other respects (§ 35), since these seldom require a very high degree of defining power. By the position given to the Erector, it is made subservient to another pur- pose, of great utility ; namely, the procuring a very extensive range of magnifying power, without any change in the objective. For when the draw-tube, with the erector fitted to it, is com- pletely pushed in, the acting length of the body (so to speak) is so greatly reduced by the formation of the first image much nearer the objective, that, if a lens of 8-10ths of an inch focus be employed, an object of the diameter of 1J inch can be taken in, 120 ACCESSORY APPARATUS. and enlarged to no more than 4 diameters ; whilst, on the other hand, when the tube is drawn out to its whole length, the ob- ject is enlarged 100 diameters. Of course every intermediate range can be obtained, by drawing out the tube mcjre or less ; and the facility with which this can be accomplished, renders such an instrument most useful in various kinds of research, especially those in which it is important, after finding an object with a lower power, to examine it under a higher amplification ; since this may be done, without either a change of objectives, or a transfer of the object to another microscope fitted with a dif- ferent power. It is when the draw-tube is thus made subservient to the use of the Erector, that the value of its rack-and-pinion adjustment is most felt ; for by giving motion to the milled head which acts upon this (Fig. 22) with one hand, whilst the other hand is kept upon the milled head which moves the whole body (it being necessary to shorten the distance between the object and the objective, in proportion as the distance of the image from the objective is increased), the observer — after a little prac- tice in the working together of the two adjustments — may al- most instantaneously alter his power to any amount of amplifica- tion which he may find the object to require, without ever losing a tolerably distinct view of it. This can scarcely be accomplished without the rack movement ; since, if both hands be required to make the alteration of the draw-tube, the readjustment of the focus must be effected subsequently. 45. Micrometer. — Although some have applied their micro- metric apparatus' to the stage of the microscope, yet it is to the Eye-piece that it may be most advantageously adapted.1 The cobweb micrometer, invented by Ramsden for Telescopes, is pro- bably, when well constructed, the most perfect instrument that the Microscopist can employ. It is made by stretching across the field of a "positive" eye-piece (§ 23) two very delicate paral- lel wires or cobwebs, one of which can be separated from the other by the action of a fine-threaded screw, the head of which is divided at its edge into a convenient number of parts, which successively pass by an index as the milled head is turned. A por- tion of the field of view on one side is cut off at right angles to the cobweb threads, by a scale formed of a thin plate of brass having notches at its edge, whose distance corresponds to that of the threads of the screw, every fifth notch being made deeper than the rest for the sake of ready enumeration. The object being brought into such a position that one of its edges seems to touch the stationary thread, the other thread is moved by the micrometer screw, until it appear to lie in contact with the other edge of the object; the number of entire divisions on the scale 1 The Stage-micrometer constructed by Fraiinhofer is employed by many continental Microscopists ; but it is subject to this disadvantage, — that any error in its performance is augmented by the whole magnifying power employed ; whilst a like error in the Eye- piece-micrometer is increased by the magnifying power of the eye-piece alone. MICROMETER E Y E - P I E C E. 121 shows how many complete turns of the screw must have been made in thus separating the threads ; while the number to which the index points on the milled head, shows what fraction of a turn may have been made in addition. It is usual, by employ- ing a screw of 100 threads to the inch, to give to each division of the scale the value of l-100th of an inch, and to divide the milled head into 100 parts ; but the absolute value of the divi- sions is of little consequence, since their micrometric value de- pends upon the objective with which the instrument maybe em- ployed. This must be determined by means of a ruled slip of glass laid upon the stage ; and as the distance of the divisions even in the best ruled slip is by no means uniform,1 it is advisa- ble to take an average of several measurements, both upon dif- ferent slips, and upon different parts of the same slip. Here the draw-tube will be of essential use, in enabling the microscopist to bring the value of the divisions of his Micrometer to even num- bers. Thus, suppose that with a l-4th-inch object-glass, the tube being pushed in, a separation of the lines by one entire turn and 37-100ths of another were needed to take in the space be- tween two lines on the ruled slip, whose actual distance is l-1000th of an inch ; then it is obvious that 137 divisions on the milled head are equivalent with that powder to a dimension of 1- 1000th of an inch, or the value of each division is 1-137, 000th of an inch. But as this is an awkward number for calculation, the magnifying power may be readily increased by means of the draw-tube, until the space of l-1000th of an inch shall be repre- sented by a separation of the cobweb threads to the extent of 150 divisions ; thus giving to each division the much more con- venient value of l-150,000th of an inch. The Microscopist who applies himself to researches requiring micrometric measure- ment, should determine the value of his Micrometer with each of the objectives he is likely to use for the purpose ; and should keep a table of these determinations, recording in each case the extent to which the tube has been drawn out, as marked by the graduated scale of inches which it should possess. The accuracy with which measurements may be made with this instrument, is not really quite so minute as it appears to be ; for it is found practically that when the milled head is so graduated, that, by moving it through a single division, the cobweb threads are se- parated or approximated by no more than l-10,000th of an inch, it needs to be moved through four divisions, for any change in the position of the threads to be made sensible to the eye. Con- sequently, if three entire turns, or 300 divisions, were found to separate the threads so far as to coincide with a distance of l-1000th of an inch on the ruled glass under a l-8th of an inch 1 Of the degree of this inequality, some idea may be formed from the statement of Hannover, that the value of the different divisions of a glass ruled by Chevalier to 1-lGOth of a millimetre, varied between the extreme .ratios of 3T36, the mean of all being 34. 122 ACCESSORY APPARATUS. objective, although each division of the milled head, will thus represent l-300,000th of an inch, yet the smallest measurable space will be four times that amount, or 1-75, 000th of an inch. With the l-12th inch objective, the smallest measurable space may be about l-100,000th of an inch. 46. The expensiveness of the cobweb-micrometer being an im- portant obstacle to its general use, a simpler method is more com- monly adopted, which consists in the insertion of a transparent scale into the focus of the eye-piece, on which the image of the object is seen to be projected. By Mr. Eoss, who first devised this method, the "positive" eye-piece was employed, and a glass plate ruled in squares was attached beneath its field-glass, at such a distance that it and the image of the object should be in focus together ; and the value of these squares having been determined with each of the objectives, in the manner already described, the size of the object was estimated by the proportion of the square that might be occupied by its image. While the use of the posi- tive eye-piece, however, renders the definition of the ruled lines peculiarly distinct, it impairs the definition of the object; and the " negative" or common Huyghenian eye-piece is now gene- rally preferred. The arrangement devised by Mr. G. Jackson allows the divided glass to be introduced into the ordinary eye- piece (thus dispensing with the necessity for one specially adapted for micrometry), and greatly increases the facility and accuracy with which the eye-piece scale may be used. This scale is ruled like that of an ordinary measure (i. e. with every tenth line long, and every fifth line half its length), on a slip of glass, which is so fitted into a brass frame (Fig. 33, B), as to have a slight motion towards either end ; one of its extremities is pressed upon by a small fine milled-head- ed screw which works through the frame, and the other by a spring (concealed in the figure) which antagonizes the screw. The scale thus mounted is introduced through a pair of slits in the eye-piece tube, immediately above the diaphragm (Fig. 33, A), so as to occupy the centre of the field ; and it is brought accurately into focus by unscrew- ing the glass nearest to the eye, until the lines of the scale are clearly seen. The value FIG. 33. Mr. Jackson's Eye-piece Micrometer. MICROMETER EYE-PIECE. 123 of the divisions of this scale must be determined, as in the for- mer instance, by means of a ruled stage-micrometer, for each objective employed in micrometry (the drawing out of the eye- piece tube enabling the proportions to be adjusted to even and convenient numbers) ; and this having been accomplished, the scale is brought to bear upon the object to be measured, by moving the latter as nearly as possible into the centre of the field, and then rotating the eye-piece in such a manner, that the scale may lie across that diameter which it is desired to measure. The push- ing-screw at the extremity of the scale being then turned, until one edge of the object is in exact contact with one of the long lines, the number of divisions which its diameter occupies is at once read off by directing the attention to the other edge, — the operation, as Mr. Quekett justly remarks, being nothing more than laying a rule across the body to be measured. This method of measurement may be made quite exact enough for all ordi- nary purposes, provided, in the first place, that the eye-piece scale be divided with a fair degree of accuracy; and secondly, that the value of its divisions be ascertained (as in the case of the cobweb-micrometer) by several comparisons with the scale laid upon the stage. Thus if, by a mean of numerous observa- tions, we establish the value of each division of the eye-piece scale to be l-12,500th of an inch, then, if the image of an object be found to measure 3J of those divisions, its real diameter will be 3J x T2500 or l-3571st of an inch.1 Now as, with an objec- tive of l-12th inch focus, the value of the divisions of the eye- piece scale may be reduced to l-25,000th of an inch, and as the eye can estimate a fourth 'part of one of the divisions with tole- rable accuracy, it follows that a magnitude of as little as l-100,000th of an inch can be measured with a near approach to exactness, and that this instrument cannot be fairly considered as ranking much below the cobweb-micrometer in minute accuracy. At any rate, it is sufficiently precise (when due care is employed) for all ordinary purposes; and it has the great advantage of cheapness and simplicity. "Whatever method be adopted, if the measurement be made in the Eye-piece, and not on the stage, it will be necessary to make allowance for the adjustment of the object-glass to the thickness of the glass that covers the object, since its magnifying power is considerably affected by the separa- tion of the front pair of lenses from those behind it (§ 83). It will be found convenient to compensate for this alteration, by altering the draw- tube in such a manner as to neutralize the effect produced by the adjustment of the objective ; thus giving one uniform value to the divisions of the eye-piece scale, what- 1 The calculation of the dimensions is most simplified by the adoption of a decimal scale ; the value of each division being made, by the use of the draw-tube adjustment, to correspond to some aliquot part of a ten-thousandth or a hundred-thousandth of an inch, and the dimensions of the object being then found by simple multiplication : — Thus (to take the above example) the value of each division in the decimal scale is '00008, and the diameter of the object is -000?8. 124 ACCESSORY APPARATUS. ever may be the thickness of the covering-glass : the amount of the alteration required for each degree must of course be deter- mined by a series of measurements with the stage-micrometer, and should be recorded on the table of the micrometric values of the several objectives. 47. Goniometer. — When the Microscope is employed in re- searches on minute crystals, a means of measuring their angles is provided by the adaptation of a goniometer to the eye-piece. The simplest form (contrived by Schmidt and made by Ross) which answers sufficiently well for all ordinary purposes, essen- tially consists merely of a "positive" eye-piece, with a single cobweb-thread stretched across it diametrically in a circular frame capable of rotation; the edges of this frame are graduated in de- grees, and a vernier is also attached to the index, whereby frac- tional parts of degrees may be read off. By rotating the frame carrying the thread, so that it shall lie successively in the direc- tions of the two sides of the crystal, the angle which they form is at once measured by the difference of the degrees to which the index points on the two occasions. For the cobweb-thread, a glass plate, ruled with parallel lines at about the l-50th of an inch asunder, may be advantageously substituted ; since it is not then necessary to bring the crystal into such a position as to lie along the diametrical thread, but its angle may be measured by means of any one of the lines to which it happens to be nearest. If a higher degree of precision be required than this instrument is fitted to afford, the Double-refracting G-oniometer, invented by Dr. Leeson, may be substituted ; for a description of which (too long to be introduced here) the reader is referred to Dr. L.'s ac- count in the "Proceedings of the Chemical Society," Partxxxiii, and to Mr. Quekett's "Practical Treatise on the Microscope." (See Appendix for a description of a Micrometer and Goniometer, by Prof. J. L. Smith.) 48. Indicator. — When the Microscope is used for the purpose of demonstrating to others such objects as may not be at once distinguished by the uninitiated eye, it is very useful to introduce into the eye-piece, just over the diaphragm, a small steel hand pointing to nearly the centre of the field ; to whose extremity the particular portion of the image which the observer is intended to look at, is to be brought by moving the object. The hand may be so attached, as to be readily turned back when not re- quired ; leaving the field of the eye-piece quite free. This little contrivance, which was devised by Mr. J. Quekett, is appropri- ately termed by him the indicator. 49. Camera Lucida. — Various contrivances may be adapted to the eye-piece, in order to enable the observer to see the image projected upon a surface whereon he may trace its outlines. The one most generally employed is the Camera Lucida prism con- trived by Dr. Wollaston for the general purposes of delineation ; this being fitted on the front of the eye-piece, in place of the CAMERA LUCID A. 125 FIG. 34. " cap" by which it is usually surmounted. The Microscope being placed in a horizontal position, as shown in Fig. 34, the rays which pass through the eye-piece into the prism, sustain such a total reflection from its oblique surface, that they come to its upper horizontal sur- face at right an- gles to their pre- vious direction ; and the eye being so placed over the edge of this surface, that it, receives these rays from the prism through part of the pupil, whilst it looks beyond the o™ rl/-»-TTv> -t-^ o Microscope arranged with Camera Lucida for Drawing or Micrometry. piism, cio v\ 11 to «t white-paper surface on the table, with the other half, it sees the image so strongly and clearly projected upon that surface, that the only difficulty in tracing it arises from a certain incapacity which seems to exist in some individuals, for seeing the image and the tracing-point at the same time. This difficulty (which is common to all instruments devised for this purpose) is lessened by the interposition of a slightly convex lens in the position shown in the figure, between the eye and the paper, in order that the rays from the paper and tracing-point may diverge at the same angle as those which are received from the prism ; and it may be generally got over altogether, by experimentally modi- fying the relative degrees of light received from the object and from the paper. If the image be too bright, the paper, the tracing-point, and the outline it has made, are scarcely seen ; and either less light may be allowed to come from the object, or more light (as by a taper held near) may be, thrown on the paper and tracing-point. Sometimes, on the other hand, measures of the contrary kind must be taken. Instead of the prism, some microscopists prefer a speculum of polished steel, of smaller size than the ordinary pupil of the eye, fixed at an angle of 45° in front of the eye-piece ; and this answers exactly the same purpose as the preceding, since the rays from the eye-piece are reflected vertically upwards to the central part of the pupil placed above the mirror, whilst, as the eye also receives rays from the paper and tracer, in the same direction, through the peripheral portion of the pupil, the image formed by the microscope is visually pro- jected downwards, as in the preceding case. This disk, the in- 126 ACCESSORY APPARATUS. vention of the celebrated anatomist Soemmering, is preferred by some microscopic delineators to the camera lucida. The fact is, however (as the author can testify from his own experience), that there is a sort of "knack" in the use of each instrument, which is commonly acquired by practice alone ; and that a person habi- tuated to the use of either of them, does not at first work well with another. A different plan is preferred by some micro- scopists, which consists in the substitution of a plate of neutral- tint or darkened glass for the oblique mirror ; the eye receiving at the same time the rays of the microscopic image, which are obliquely reflected to it from the surface of the glass, and those of the paper, tracing-point, &c., which come to it through the glass. It is so extremely useful to the microscopist, to be able to take outlines with one or other of these instruments, that every one would do well to practise the art. Although some persons at once acquire the power of seeing the image and the tracing- point with equal distinctness, the case is more frequently other- wise ; and hence no one should allow himself to be baffled by the failure of his first attempt. It will sometimes happen, especially when the prism is employed, that the want of power to see the pencil is due to the faulty position of the eye, too large a part of it being over the prism itself. When once a good position has been obtained, the eye should be held there as steadily as possi- ble, until the tracing shall have been completed. It is essential to keep in view, that the proportion between the size of the tracing and that of the object, is affected by the height of the eye above the paper; and hence that if the microscope be placed upon a support of different thickness, or the eye-piece be elevated or depressed by a slight inclination given to the body, the scale will be altered. This it is of course peculiarly important to bear in mind, when a series of tracings is being made of any set of objects which it is intended to delineate on a uniform scale ; or when the camera lucida (or any similar arrangement) is employed for the purpose of Micrometry. All that is requisite to turn it to this account, is an accurately divided stage-micrometer, which, being placed in the position of the object, enables the observer to see its lines projected upon the surface upon which he has drawn his outline ; for if the divisions be marked upon the paper, the average of several be taken, and the paper be then divided by parallel lines at the distance thus ascertained (the spaces being subdivided by intermediate lines, if desirable), a very accurate scale is furnished, by which the dimensions of any objects drawn in outline under the same power may be minutely determined. Thus if the divisions of a stage-micrometer, the real value of each of which is l-200th of an inch, should be projected with such a magnifying power, as to be at the distance of an inch from one another on the paper, it is obvious that an ordinary inch-scale applied to the measurement of an outline, would give its dimen- sions in two-hundredths of an inch, whilst each fifth of that scale OBJECT-GLASS HOLDER — OBJECT- MARKER. 127 would be the equivalent of a thousandth of an inch. When a sufficient magnifying power is used, and the scale thus made is minutely divided, great accuracy may be obtained. It has been by the use of this method, that Mr. Gulliver has made his admi- rable series of measurements of the diameters of the Blood-cor- puscles of different animals. 50. Object- G-lass Holder. — In Microscopes of the old construc- tion, whose objectives were single lenses, these were not unfre- quently mounted near the periphery of a circular disk pivoted to the lower end of the body, in such a manner that any desired power might at once be brought into use by merely rotating the disk. Since the introduction of achromatic object-glasses, this method has been until recently abandoned ; every "power" being separately connected with the extremity of the body, so as not to admit of any substitution, save by screwing off one objective and screwing on another. The old method, however, has been par- tially reverted to by Mr. C. Brooke ; who has contrived a holder into which two objectives may be screwed, and which, being attached to the unose" of the body, enables either of them to be brought into position, by simply turning the arm on its pivot. This is an extremely convenient arrangement, and might easily be carried further if desired ; since, by having a tri-radiate or quadri-radiate arm, three or four powers might be thus brought into use successively, with as much facility as two. The principal objection to the general use of such an appendage, lies in the nicety of workmanship that is required to obtain that exact " centering," which is needed to bring the axis of the objective into precise continuity with that of the body ; and unless this be attained, the performance of the instrument is greatly impaired. In microscopes of the old construction, the other imperfections were so great, that none but an excessive deficiency in this respect would attract attention. The convenience of such an instantaneous change in the power of the objective is very great; since it is continually desirable to obtain a general view of an object with a low power, and to examine the parts of it in detail under a higher amplification, with as little expenditure of time and trouble as possible. 51. Object-Marker. — All Microscopists occasionally, and some continually, feel the need of a ready means of finding, upon a glass slide, the particular object or portion of an object, which they desire to bring into view ; and various contrivances have been suggested for the purpose. Where different magnifying powers can be readily substituted one for another, as by the use of the Erector (§ 44) or of the Object-glass holder (§ 50), no special means are required ; since, when the object has been found by a low power, and brought into the centre of the field, it is rightly placed for examination by any other objective. Even this slight trouble, however, may be saved by the adoption of more special methods ; among the simplest of which is marking the position of 128 ACCESSORY APPARATUS. the object on the surface of the thin glass which covers it. The readiest mode of doing this, when the object is large enough to be distinguished by the naked eye, is to make a small ring round it with a fine camel-hair pencil dipped in Indian ink ; but when the object is not thus visible, the slide must be laid in position on the stage, the object "found" in the microscope, the con- denser adjusted to give a bright and defined circle of light, and then, the microscope-body being withdrawn, the black ring is to be marked around the illuminated spot. The same end, how- ever, may be more precisely as well as more neatly accomplished, by attaching an object-marker to the objective itself. That of Mr. Tomes consists simply of an ivory cap, fitting over the l-4th- inch objective, having its extremity narrowed down (like that of the objective itself), but perforated in the centre, so as to form a minute ring; the object having been "found" and brought into the centre of the field, the cap is placed upon the objective, the ring is blackened with Indian ink, and then, being carefully brought by the focal adjustment into contact with the surface of the glass, it stamps on this a minute circle enclosing the object. A more elaborate contrivance of a similar kind, for marking a circle round the object by a diamond point, attached to a cap fitting on the objective, has been recently described by Mr. Bridg- man (" Quarterly MicroscopicalJournal," vol. iii, p. 237); this has the advantage of admitting a variation in the size of the circle, and also of substituting a delicate line for the broad ring which may partly obscure some neighboring object; but, on the other hand, the very delicacy of the diamond marks prevents them from being readily distinguished, and some kinds of glass are so apt to "star" when marked with a diamond point, that cracks or splinters may extend from the circle over the object it is intended to indicate. The most unobjectionable and satisfac- tory mode of "finding" an object, however, is, in the Author's opinion , that which is afforded by a graduation of the movable parts of the stage, in the manner to be presently described (§ 53). 52. Lever Stage. — The general arrangement of the Traversing Stage, now usually adapted to all high class Microscopes, ha's been already explained (§§ 37-39) ; and though the details are differently constructed by the several makers, yet the general principle is, that a lateral or horizontal movement is given to the object-platform by one milled head, and a front to back or verti- cal movement (the microscope being supposed to be placed in an inclined position) by another. The stage may be so constructed, however, that motion shall be given to the object-platform by means of a lever acting upon it in any required direction ; this being accomplished by making the object-platform slide laterally on an intermediate plate, and by making the latter slide verti- cally upon the fixed stage-plate which forms the basis of the whole; each pair of plates being connected by dovetailed slides and grooves. Thus the object-platform may be readily made to LEVER STAGE — OBJECT-FINDER. 129 traverse, not merely horizontally or vertically, but, by the simul- taneous sliding of both plates, in any intermediate direction. This is especially convenient in following the movements of Ani- malcules, &c., for which purpose this lever-stage is to be pre- ferred to the ordinary form : its use being attended with this particular facility, that, as the motion of the hand is reversed by the lever, so that the object moves in the opposite direction, and as the motion of the object is again reversed to the eye by the microscope, the image moves in the same direction as the hand; and thus, with a little practice, even the most rapid swimmer may be kept within the field by the dexterous management of the lever. For general purposes, however, the ordinary tra- versing stage will be found most convenient. 53. Object- Finder. — Either kind of movable stage admits of a simple addition, which very much facilitates the " finding" of minute objects mounted in slides, that are not distinguishable by the naked eye ; such, for example, as the particular forms that present themselves in Diatomaceous deposits. This "finder" consists of two graduated scales, one of them vertical, attached to the fixed stage-plate, and the other horizontal, attached to an arm carried by the intermediate plate ; the first of these scales enables the observer to " set" the vertically sliding plate to any determinate position in relation to the fixed plate, while the second gives him the like power of setting the horizontally sliding plate by the intermediate. In order to make use of these scales, it is of course necessary that the sliding and rotating platform on which the object immediately rests, should be always brought into one constant position upon the traversing plates beneath ; this is accomplished by means of a pair of stops, against which it should be brought to bear. So, again, this sliding plate or ob- ject-platform should itself be furnished with a "stop" for the glass slide to abut against, so as to secure this being always laid in the same position. These stops may be made removable, so as not to interfere with the ordinary working of the stage. Now supposing an observer to be examining a newly-mounted slide, containing any objects which he is likely to wish to find on some future occasion; he first lays the slide on the object-platform, with its lower edge resting on the ledge, and its end abutting against the lateral stop, and brings the object-platform itself into its fixed place against the stops ; then if, on sweeping through the slide, he meet with any particular form worthy of note, he reads off its position upon the two scales, and records it in any convenient mode. The scale may be divided to 50ths of an inch, and each of these spaces may be again halved by the eye ; the record may perhaps be best made thus, — Triceratiu m favus f |t ; — the upper number always referring to the upper scale, which is the horizontal, and the lower to the vertical. Now whenever the Microscopist may wish again to bring this object under exa- mination, he has merely to lay the slide in the same position on 9 130 ACCESSORY APPARATUS. the platform, to bring the platform itself into its fixed place on the traversing plate below, and then to adjust the traversing plates themselves by their respective scales. Even a non-movable stage may have a similar pair of scales adapted to it ; the vertical scale being so placed, as to mark the position into which the object- platform is brought by sliding it up or down ; and the horizontal scale being marked upon the object-platform itself, so as to allow the observer to note the precise position of the end of the glass slide. Thus let it be supposed that, by shifting the slide from side to side, and by moving the object-platform up or down, a certain object has been brought into the field ; if the place of the object-platform and of the slide be then noted by the vertical and the horizontal scales, the object may be found at any future time without difficulty, by readjusting the slide and the object- platform to the same numbers.1 The numbers referring to each object may either be marked upon the slides themselves, like the names of the objects, or may be recorded with these in a separate list, referring to the slides by figures alone. The gene- ral adoption of such a plan, though involving a little more labor at first, would prove in the end to be a great saving both of time and trouble. 54. Magnetic Stage. — If a stage be unprovided with a travers- ing movement of any kind, there is no means of allowing the object to be moved in all directions with smoothness and facility, and yet of holding it in any position in which the Microscopist desires to retain it, more convenient and more ready of applica- tion, than is furnished by magnetic attraction. A magnetic stage was originally proposed by Mr. King of Bristol ; but seems to have been first brought into efficient practical action by Mr. G. Busk. His plan consists in attaching two semicircular mag- nets to the under side of the stage, so as nearly to surround its aperture, and in inserting, for the conveyance of the magnetic force to the upper side, four soft iron pegs, which slightly pro- ject above its surface ; over these an object-bearer of soft iron, with its under surface ground smooth and true, will slide so readily, as to admit of very easy and precise adjustment of the 1 The first of the above plans, to the utility and accuracy of which the Author can bear strong testimony, was suggested by Mr. Okeden in the "Quart. Microsc. Journal," vol. iii, p. 166. The second had been previously suggested by Mr. E. G. Wright in the same Journal, vol. i, p. 302; the descriptions of both are made clear by figures. Other " finders" are described and figured by Mr. J. Tyrrel, Mr. T. E. Amyot, and Mr. Briclg- man, at pp. 234 and 302-304 of the last-named volume. It appears to the Author that Mr. Okeden's plan might be adopted with very little trouble or expense in every Mi- croscope possessed of a stage movement, and Mr. Wright's in every Microscope with a fixed stage but movable object-carrier ; and that it would be very desirable for every microscope thafmay be made hereafter, to be furnished with such scales. If the dif- ferent makers could agree upon some common system of graduation, in the same way as Microscopists have adopted 3 inches by 1 as the standard dimension of object- slides, much trouble would be saved to observers at a distance from one another, who might wish to examine each others' objects ; for the numerical reference attached to each object would then enable it to be found by every observer, whose stage should be graduated upon the same method. MAGNETIC STAGE — DIAPHRAGM-PLATE. 131 place of the object by a steady and practised hand. Another ar- rangement, which has some advantages over the preceding, has been proposed by Mr. J. B. Spencer ; this consists in surround- ing the aperture of the stage by a ring of soft iron, the surface of which projects very slightly above the brass plate, the mag- nets (cut out of sheet-steel) being attached to the under side of the object-bearer. This method is perhaps more readily applica- ble than Mr. Busk's to the stage of any microscope, and will pro- bably interfere less with its other fittings.1 55. Diaphragm-Plate. — No microscope stage should ever be without a diaphragm-plate fitted to its under surface, for the sake of restricting the amount of light reflected from the mirror, and of limiting the angle at which its rays impinge on the object (see Figs. 18 and 21). This plate should always be at least half an inch below the object, since it is otherwise comparatively in- operative ; and thus, whilst it may be fixed immediately beneath a movable stage whose thickness serves to remove it sufficiently far, it should be fixed on the end of a short tube forming a sort of well on the under side of the stage, when this consists of but a single fixed plate. The diaphragm-plate should be perforated with holes of several different sizes, in the largest of which it is convenient to fit a ground-glass (this, by means of a screw-socket, may be made removable at pleasure), the use of which is to dif- fuse a soft and equable light over the field, when large trans- parent objects (such as sections of wood) are under examination ; between the smallest and the largest aperture, there should be an unperforated space, to serve as a dark background for opaque objects. The diaphragm-plate itself, the "well" of the stage, in fact every part through which light passes to the object from be- neath, should be blackened, in order to avoid the interference that would be occasioned by irregularly reflected rays. The edge of the diaphragm-plate should be notched at certain intervals, and a spring catch fitted so as to drop into the notches, in order that each aperture may be brought into its proper central posi- tion. This simple arrangement, in combination with the mirror (which should be concave on one side and plane on the other) and side-condenser (§ 64), affords to the Microscopist all the means of illuminating his objects, whether transparent or opaque, which are ordinarily requisite : to bring out the highest powers of the instrument, however, more refined methods of illumina- tion are required ; and a far greater variety of treatment is needed in the case of many objects, the determination of whose true characters is a matter of difficulty, even under every advantage which can be derived from assistance of this kind. 56. Achromatic Condenser. — In almost every case in which an objective of 1-4 th inch or any shorter focus is employed, its per- 1 For a more detailed description, with illustrative figures, of Mr. Busk's Magnetic Stage, see " Quarterly Microsc. Journal," vol. ii, p. 280 ; and for Mr. Spencer's, vol iii, p. 173. 132 ACCESSORY APPARATUS. formance is greatly improved by the interposition of an achro- matic combination between the mirror and the object, in such a manner that the rays reflected from the former shall be brought to a focus in the spot to which the objective is directed. Tliis may be accomplished sufficiently well for ordinary purposes, by adapting a French triple combination of about l-4th inch focus, to the end of a tube 1J inch long, which shall slide within an- other tube fitted to the opening in the stage, by the bayonet catch or any similar connection that gives attachment to the diaphragm-plate. If this be correctly centred in the first in- stance, and the workmanship of the microscope be good, no more expensive arrangement will be required, by such at least as may be satisfied with that degree of perfection, which suffices for the clear discernment of all but the most difficult objects. The slid- ing movement of the tube, especially if it be accomplished by a lever-action (as suggested by Mr. Quekett), is quite sufficient for the adjustment of the focus ; and the removal of the outer lens adapts it for use with objectives below 1-4 th inch, to whose per- formance it often affords important assistance. In the most per- fect arrangement of the Achromatic Condenser, however, such as is now adapted to all first-class instruments made in this country, the achromatic combination is one specially adapted to the purpose; and is so mounted as to insure the greatest ac- curacy of its adjustments. By Mr. Ross it is supported by what he terms the ''secondary stage" (§ 37); and by Messrs. Smith and Beck it is carried upon the summit of the " cylindrical fit- ting" which answers the same purpose (§ 39) ; whilst by Messrs. Powell and Lealand, it is attached by a bayonet-catch to the under side of the fixed stage-plate (§ 38). In either case it is provided with a pair of milled-headed screws (Fig. 36), which give it a slight degree of horizontal motion in transverse directions, for the purpose of procuring an accurate centring ; and where, as in Messrs. Powell and Lealand's instrument, the focal adjust- ment is not given by the movement of the carriage which bears it, a rack and pinion is attached for this purpose to the tube of the condenser itself. In order that the Achromatic Condenser should be made to afford the greatest possible variety of modifi- cations of the illuminating pencil, it requires to be furnished with a diaphragm-plate (as first suggested by Mr. Gillett) imme- diately behind its lenses ; and this should be pierced with holes of such a form and size, as to be adapted to cut off in various degrees, not merely the peripheral, but also the central parts of the illuminating pencil. The former of these purposes is of course accomplished, by merely narrowing the aperture which limits the passage of the rays through the central part of the lens ; the latter, on the other hand, requires an aperture as large as that of the lens, having its central part more or less completely occupied by a solid disk, which may so nearly fill the circle, as to leave but a mere ring through which the light may pass. Such ACHROMATIC CONDENSER. 133 FIG. 35. Ross's Achromatic Condenser. FlG. 36. apertures are shown in the diaphragm- plates in Figs. 35 and 36. The Condenser thus completed is constructed on three different plans by the three principal makers, in accordance with the differ- ent arrangements of their respective stages. By Mr. Ross, who ori- ginally carried Mr. Gillett's plans into operation, the diaphragm- plate has the shape of a short frustrum of a cone (Fig. 35), so at- tached to the condenser, that the portion of the plate which passes through it shall cut it transversely ; each aperture is indicated by a number on the dial ; and a spring-catch is so arranged, as to mark when any one of the apertures is in its right place, and to show its number. The thinness of the stage in Messrs. Smith and Beck's microscope, allows the dia- phragm-plate to be made upon the ordinary plan (Fig. 36), since it can be brought sufficiently near to the lenses of the con- denser, without coming into too close con- tiguity with the stage ; and this is obvi- ously the simplest and most convenient arrangement. By Messrs. Powell and Lea- land, again, — their stage being too thick to allow of the diaphragm-plate being placed beneath it, without removing that plate from its proper position be- hind the lenses of the condenser, — the diaphragm-plate is made so small that it can be received into the interior of the stage (Fig. 37), but is rotated by a milled head be- neath ; and the edge of this is marked by numbers, each signify- ing a particular aperture, and thus marking by its position which aper- ture is in use. As, however, the smallness of the diaphragm-plate so limits the number of apertures, that the desirable variety could not be afforded by it alone, a second plate is made to ro- tate immediately beneath it upon the same axis (like the hour and minute hands of a watch), by means of a second milled head, numbered at its edge like the first ; and the apertures in the diaphragm-plate being simple circles, the centres of these are covered by stops of different sizes, supplied by the second or " stop "-pi ate ; by which very ingenious arrangement, a great variety of combinations may be obtained, all of them indicated by the numbering on the two milled heads. 57. Reflecting Prisms. — Every mirror composed of glass silvered at the back, gives, as is well known, a double reflection ; namely, a principal image from the metallic surface, and a secondary im- Smith and Beck's Achromatic Condenser. 184 ACCESSORY APPARATUS. age from the surface of the glass in front of it. This secondary image, it has been thought, interferes with the perfect perform- ance of the achromatic condenser ; and hence, for obtaining the FIG. 37. Powell and Lealand's Achromatic Condenser. most satisfactory definition, some Microscopists prefer to direct the axis of the microscope to the source of light (the mirror being turned aside) ; whilst others, feeling the inconvenience of the position thus required, have recourse to a prism which shall give the required reflection with only a single image. The prism usually employed (having been originally applied to this purpose by M. Dujardin) has plane surfaces, and acts, therefore, as the equivalent of a plane mirror. A reflecting prism has been de- vised, however, by Mr. Abraham (optician of Liverpool), which is intended by him to take the place both of mirror and achro- matic condenser, though its action (as it seems to the author) must rather be that of the ordinary concave mirror ; this has one of its surfaces hollowed out to receive one side of a double-con- vex lens, the other side of which acts as the emergent surface of the prism, causing the rays as they pass through it to converge ; and the prism itself being composed of flint-glass, whilst the lens is of crown, no chromatic dispersion of the rays is produced, though the spherical aberration is not corrected. 58. White-Cloud Illuminators. — It being universally admitted that the light of a bright white cloud is the best of all kinds of illumination for nearly every kind of Microscopic inquiry, various attempts have been made to obtain such light, from the direct rays either of the sun or of a lamp, by what may be called an artificial cloud. Some have replaced the plane mirror by a sur- face of pounded glass or of carbonate of soda, or (more com- monly) by a disk of plaster of Paris, the latter being decidedly the preferable method ; but a sufficiently bright light is not thus obtained, unless a condenser be employed to intensify the illumi- nation of the mirror. Such a condenser may be most conveni- WHITE-CLOUD AND OBLIQUE ILLUMINATORS. 135 ently attached by a jointed arm to the frame which carries the disk, according to the method of Messrs. Powell and Lealand, shown in Fig. 38 ; the frame itself being made to fit upon the mirror, and to turn with it in every direction. Another very simple, and for many purposes very effi- FJG. 38. cient mode of obtaining a white-cloud illumination (invented by Mr. Handford) consists in coating the back of a concave plate of glass, like that employed in the ordinary concave mirror, with white zinc paint, instead of silvering it; and then mounting this in a frame, which may be fitted (like the plaster of Paris disk just described) over the ordinary mirror. A concave surface of plas- ter of Paris, moreover, might easily be obtained by casting it when fluid upon the convex surface of such a plate. When a concavity is thus given to the white surface, its performance with low powers is much improved ; but with high powers, a special condensation of the light must be adopted, and the arrangement above described seems the simplest that could be devised. It is open, however, to certain objections, which become apparent when very high powers are used and difficult objects are under examination ;" and to obtain the most perfect white-cloud illumi- nation possible, is the object of the apparatus devised by Mr. Gillett. This consists of a small camphine lamp, placed nearly in the focus of a parabolic speculum, which reflects the rays either at once upon a disk of roughened enamel or upon a second (hyperbolic) speculum which reflects them upon such a disk. A very pure and concentrated light is thus obtained ; and as the forms of the incident pencils are broken up by the roughened surface, that surface takes the place of the lamp, as the source from which the rays primarily issue. The advantage of this illu- mination is specially felt, in the examination of objects of the most difficult class under the highest powers. 59. Oblique Illuminators. — It is frequently desirable to obtain a means of illuminating transparent objects with rays of more obliquity than can be reflected to them from the mirror, even when this is thrown as much as its mounting will permit out of the axis of the Microscope (§ 39), or than can be transmitted by the ordinary achromatic condenser, even when all but its mar- ginal aperture is stopped out. Such oblique light may be used in two entirely different modes. The rays, although very far out of the axis of the microscope, may still not make too great an angle with it to fall beyond the aperture of the objective; and thus, entering its peripheral portion after their passage through the object, they will form the image in the ordinary way. The advantage of such oblique illumination, arises from its power of bringing out markings which cannot be seen when only direct rays are employed ; and when the rays come only from one side, 136 ACCESSORY APPARATUS. so as to throw a strong shadow, and either the stage or the illu- minator is made to rotate, so that the light shall fall upon the object successively in every azimuth, information may often be gained respecting the nature of these markings, which can be acquired in no other mode. But the direction given to the rays may be so oblique, that they shall not enter the object-glass at all; in this case, they serve to illuminate the object itself, which shines by the light whose passage it has interrupted ; and as the observer then receives no. other light than that which radiates from it, the object (provided it be of a nature to stop enough light) is seen bright upon a dark field. Each of these methods has its advantages for particular classes of objects ; and it is ad- visable, in all doubtful cases, to have recourse to every variety of oblique illumination that shall present the object under a different aspect. Almost every Microscopist who has especially devoted his attention to the more difficult lined or dotted objects, has de- vised his own particular arrangement for oblique illumination, and feels confident of its superiority to others. To give a full description of all, would be quite unsuitable to our present ob- ject ; those, therefore, will be specially noticed, which have al- ready acquired general approval ; whilst such as have only been recommended by individuals, will be simply referred to. As they have little in common, save their purpose, it seems scarcely possible to classify them according to any other character, than that afforded by the direction which they give to the oblique rays ; some of them bringing these to bear on the object from one side alone, and others from all sides. 60. One of the earliest methods devised for obtaining oblique light, was the eccentric prism of M. Nachet ; which, occupying the place of the achromatic condenser, and like it receiving its light from the mirror, has its surfaces so arranged, as to throw a converging pencil of rays on the under side of the object, whose axis is at an angle of about 40° with the axis of the microscope. One great convenience of this instrument lies in the power of giving revolution to the prism, by simply turning it in its socket, so as to direct the oblique rays upon the object from every side successively, without moving the stage. Its principal disadvan- tages consist in the limitation of its aperture (producing a de- ficiency of light), in the want of correction for its chromatic aber- ration," and in the absence of any power of varying the obliquity of the illuminating pencil.1 All these disadvantages seem to be remedied by the plan of oblique illumination recently proposed by Mr. Sollitt, of Hull, which consists in the employment of an Achromatic condenser of very long focus and large aperture, mounted in such a manner as to enable its axis to be inclined to that of the microscope through a wide angular range ; a con- 1 A full description of M. Nachet's prism, and a mathematical investigation of its properties, by Mr. G. Shadbolt, will be found in the "Transactions of the Microscopical Society" (1st series), vol. iii, p. 74, et seq. PRISMS FOR OBLIQUE ILLUMINATION. 187 denser of this description he states to be suitable also for all ordi- nary purposes. (" Quart. Microsc. Journ.," vol. iii, p. 87.) Such an instrument, when its axis does not form a very large angle with that of the microscope, may receive its light from the plane mirror, especially if this be so mounted as to be capable of being turned considerably out of the visual axis ; but when its position is too oblique for the light to be thus supplied to it, recourse must be had to rays either proceeding direct from their source (such as a lamp or a bright cloud), or directed at the requisite angle by a reflector placed in a suitable position. For this latter purpose, a rectangular prism (§ 57), mounted on a separate stand, will be found very convenient. By many observers, a combina- tion of the reflecting and refracting powers of a prism is preferred, which causes the rays to be at once reflected by a plane surface, and concentrated by lenticular surfaces ; so that the prism an- swers the purpose of mirror and condenser at the same time. Such a prism was first constructed by Amici ; and it may be either mounted on a separate base, or attached to some part of the microscope-stand. The mounting adopted by Messrs. Smith and Beck, and shown in Fig. 89, is a very simple and convenient one ; this consists in attaching the frame _pIG> 39, of the prism to a sliding bar, which works in dovetail grooves on the top of a cap that may be set on the cylindrical fitting beneath the stage ; the slide serves for the regulation of the distance of the prism from the axis of the micro- scope, and consequently of the obliquity of the illumination ; whilst its distance beneath the stage is adjusted by the rack-movement of the cylindrical fit- Amici's Prism for oblique i] ting. In this manner, an illuminating pencil of almost any de- gree of obliquity may be readily obtained ; but there is no pro- vision for the correction of its aberrations. Such a provision is afforded by the achromatic prism of Mr. Abraham (§ 57), which may be mounted in the manner just described. And the same object is attained by an arrangement devised by Mr. Grubb, a Dublin optician, of which Dr. Robinson, of Armagh, speaks very highly; the prism having its aberrations corrected for a lamp placed 'at a given distance in the plane of the stage ; and being mounted in such a manner as to be capable of travelling (like Mr. Sollitt's condenser) through an angular range of as much as 120° (" Quart. Microsc. Journ.," vol. iii, p. 166). In all of these me- thods, the obliquity of the illumination is practically limited by the construction of the stage, and especially by the relation which its thickness bears to the diameter of its lower aperture. The thinner the stage, and the larger its lower aperture, the more oblique will be the rays which may be transmitted through it ; and in admitting an extreme obliquity of illumination, the 138 ACCESSORY APPARATUS. thin stage recently introduced into some of the best Microscopes (§§ 38, 39) possesses a great advantage over all whose thickness is greater. On the other hand, it is when the rays are most oblique, that the greatest advantage is gained by making them fall upon the object from every side in succession ; and where this cannot be accomplished (as in the case of Cachet's prism) by the rotation of the illuminating apparatus, the rotatory movement must be given to the object. It is obvious that, for this purpose, a revolving stage which keeps the object constantly in the field (§ 37), is decidedly preferable to one which does not possess such a movement; but the means have not yet been found of obtain- ing this advantage without some sacrifice of the other. 61. "Whenever the rays are directed with such obliquity, as not to be received into the object-glass at all, but are sufficiently retained by the object, to render it (so to speak) self-luminous, we have what is known as the black ground illumination ; to which the attention of Microscopists generally was first drawn by the Rev. J. B. Reade, in the year 1838, although it had been prac- tised some time before, not only by the Author but by several other observers. For low powers whose angular aperture is small, and for such objects as do not require anymore special provision, a sufficiently good " black ground" illumination maybe obtained by means of the concave mirror alone, especially when it is so mounted as to be capable of a more than ordinary degree of ob- liquity. In this manner it is often possible, not merely to bring into view features of structure that might not otherwise be dis- tinguishable, but to see bodies of extreme transparency (such, for instance, as very minute Animalcules) that are not visible when the field is flooded (so to speak) by direct light ; these pre- senting the beautiful spectacle of phosphorescent points rapidly sailing through a dark ocean. Where the mirror cannot be placed in a position oblique enough to give this effect, a black ground illumination sufficiently good for many purposes may be ob- tained by Mr. Reade's original method ; which consisted in dis- pensing with the mirror altogether, and in placing the lamp and ordinary condensing-lens (§ 64) in such a position beneath and to one side of the stage, as to throw upon the under side of the object a pencil of rays too oblique to enter the object-glass after passing through it. Another very simple mode, which answers sufficiently well for low powers and for the larger objects which these are fitted to view, consists in the substitution, for the achromatic condenser, of a plano-convex lens of great convexity, forming a large segment of its sphere, with a central stop to cut oft" the direct rays ; for the rays passing through the marginal portion of this Spotted Lens, being strongly refracted by its high curvature, are made to converge at an angle too wide for their entrance into an objective of moderate aperture, and thus the field is left dark ; whilst all the light stopped by the object serves (as it were) to give it a luminosity of its own. Neither of the PARABOLIC AND ANNULAR CONDENSERS. 139 foregoing plans, however, will answer well for objectives of high power, having such large angles of aperture that the light must fall very obliquely to pass beyond them altogether. Thus if the pencil formed by the u spotted lens" have an angle of 60°, its rays will enter a l-4th-inch objective of 70°, and the field will not be darkened. For obtaining a greater degree of obliquity, Mr. Wenham has contrived a Parabolic Speculum,1 having its apex cut off, so that the object might be placed in the focus, to which all rays parallel to its axis are reflected ; and the direct rays being checked by a stop placed behind it, the object is illuminated only by those which are reflected to it from all sides of the interior of the parabola at a very oblique angle. As the thickness of the glass slide on which the object is mounted, was found by Mr. W. to produce a very sensible aberration in the rays converging towards it, he interposed a meniscus lens, having such a curva- ture as to produce a counteracting aberration of an opposite kind. The circular opening at the bottom of the wide tube (Fig. 40) that carries the speculum, may be fitted with a diaphragm, adapted to cover any portion of it that may be desired ; and by giving rotation to this diaphragm, rays of great obliquity may be made to fall upon the object from every azimuth in succes- sion (§ 60). A like purpose was aimed at in the Annular Con- denser of Mr. Shadbolt,2 which consists of a ring of glass, whose surface was so shaped as to present a prismatic section ; the in- clination of the outer side being such as to produce a total re- flection of the rays impinging on it, and to direct these through the inner side of the ring, so as to fall at a very oblique angle upon the object, from every azimuth of the circle. A combina- tion of both methods is adopted in the Parabolic Illuminator (Fig. 40), now supplied by Messrs. Smith and Beck ; for this consists of a paraboloid of glass, resembling FlG 40 a cast of the interior of Mr. Wenham's para- bolic speculum, but reflecting the rays which fall upon the outer surface of the glass, like Mr. Shadbolt's annular prism. It has the ad- vantage of being more easily constructed than the parabolic speculum, and is little, if at all, inferior to it in performance ; but it requires that an appropriate "stop" should be adapted to it, for each objective with which it is to be used : whilst in Mr. Wenham's speculum, the requisite adaptation for the angular aperture of the objective is made by altering the posi- tion of the stop by means of the central stem ; the effect of which alteration is to cut off a larger and larger proportion of the least ob- Parabolic illuminator, lique rays, the more nearly the stop is approximated to the ob- 1 "Transactions of the Microscopical Society" (1st Series), vol. iii, p. 85. 2 Op. cit. p. 132. 140 ACCESSORY APPARATUS. ject; and thus to illuminate it more and more exclusively by those which meet at the widest angle. In using either of these illuminators, the rays which are made to fall upon them should be parallel, consequently the plane mirror should always be em- ployed ; and when, instead of the parallel rays of daylight, we are obliged to use the diverging rays of a lamp, these should be rendered as parallel as possible, previously to their reflection from the mirror, by the interposition of the "bull's eye" con- denser (§ 64) so adjusted as to produce this effect. 62. For the exhibition of those classes of objects which are suitable for "black ground" illumination, and which are better seen by light sent into them from every azimuth, than they are by a pencil, however bright, incident in one direction only, no more simple, convenient, and efficient means could probably be found, than that which is afforded by the " spotted lens" for low powers, and by the "parabolic illuminator" for powers as high as l-4th or l-5th of an inch focus ; — the use of the latter with the highest powers, being rendered disadvantageous by the great reduction in the amount of light, occasioned by the necessity for cutting off of all the rays reflected from the paraboloid, which fall upon the object within the limits of their angle of aperture. One of the great advantages of this kind of illumination consists in this : that, as the light radiates from each part of the object as its proper source, instead of merely passing through it from a more remote source, its different parts are seen much more in their normal relations to one another, and it acquires far more of the aspect of solidity. The rationale of this is easily made apparent by holding up a glass vessel with a figured surface between one eye and a lamp or a window, so that it is seen by transmitted light alone ; for the figures of its two surfaces are then so blended together to the eye, that unless their form and distribution be previously known, it can scarcely be said with certainty which markings belong to either. If, on the other hand, an opaque body be so placed behind the vessel, that no rays are transmitted directly through it, wrhilst it receives adequate illumination from the circumambient light, its form is clearly discerned, and the two surfaces are differentiated without the least difficulty. 63. Polarizing Apparatus. — In order to examine transparent objects by polarized light, it is necessary to employ some means of polarizing the rays before they pass through the object, and'to apply to them, in some part of their course between the object and the eye, an analyzing medium. These two requirements may be provided for in different modes. The polarizer may be either a bundle of plates of thin glass, used in place of the mirror, and polarizing the rays by reflection ; or it may be a "single image" or "Mcol" prism of Iceland Spar, which is so constructed as to transmit only one of the two rays into which a beam of ordinary light is made to divaricate on passing through this substance ; or it may be a plate of Tourmaline, or one of POLARIZING APPARATUS. 141 the artificial tourmalines composed of the disulphate of iodine and quinine, now known by the designation of "Herapathite," after the name of their discoverer. Of these methods, the "Nicol" prism is the one generally preferred ; the objection to the reflecting polarizer being, that it cannot be made to rotate ; the tourmaline being undesirable, on account of the color which it imparts when sufficiently thick to produce an effective polarization ; and the crystals of Herapathite being seldom obtained perfect, of sufficient size to afford a good illumination. The polarizing prism is usually fitted into a tube (Fig. 41, A, a) with a large milled head (c) at the bottom, by which it is made to rotate in a collar (b) that is attached to the microscope ; this collar may be fitted to the under side of the stage-plate, or, wThere a secondary stage is provided, it may be attached to this ; in the microscope of Messrs. Smith and Beck, it screws into the lower part (b) of a tube (Fig. 41, B) that slides into the " cylin- drical fitting" beneath the stage (Fig. 29). The analyzer, which FIG. 41. FIG. 42. Fitting of Polarizing Prism in Smith and Beck's Fitting of Analyzing Prism Microscope. upon the Eye-piece. may be either a " Nicol" prism, a Tourmaline, or a crystal of Herapathite, is usually placed either in the interior of the micro- scope, or between the eye-piece and the eye. If it be a prism, it is mounted in a tube, which may either be screwed into the lower end of the body in the situation of the erector (Fig. 32), or may be fitted over the eye-piece in place of its ordinary cap (Fig. 42) ; in the former situation it has the advantage of not limiting the field, but it stops a considerable proportion of the light ; in the latter, it detracts much less from the brightness of the image, but cuts off a good deal of the margin *of the field. A plate of Tourmaline or Herapathite, if obtainable of sufficient size and freedom from color, has a decided advantage above the Nicol prism, as an analyzer, in being free from both these inconveniences ; and it may be set in a cap which fits over the ordinary cap of the eye-piece. For bringing out certain effects of color by the use of Polarized light (Chap. XX), it is desir- able to interpose a plate of Selenite beneath the polarizer and 142 ACCESSORY APPARATUS. the object; and it is advantageous that this should be made to revolve. A very convenient mode of effecting this, is to mount the selenite plate in a revolving collar, which fits into the upper end (a) of the tube (Fig. 41, B) that receives the polarizing prism. In order to obtain the greatest variety of coloration with different objects, films of selenite of different thickness should be employed; and this may be accomplished by sub- stituting one for another in the revolving collar. A still greater variety may be obtained by mounting three films, which sepa- rately give three different colors, in a frame resembling that in which hand-magnifiers are usually mounted, so that they may be used singly or in double or triple combinations ; as many as thirteen different tints may thus be obtained ; but the advantage of revolution is sacrificed. When the construction of the microscope does not readily admit of the connection of the selenite plate with the polarizing prism, it is convenient to make use of a plate of brass (Fig. 43) somewhat larger than the glass slides in which objects are FlG- 43- ordinarily mounted, with a ledge near one edge for the slide to rest against, and a large circular aperture into which a glass is fitted, having a film of selenite cemented to it ; this " selenite stage" object-carrier. or obj ect-carrier being laid upon the stage of the micro- scope, and the slide containing the object being placed upon it, the effect of the selenite is obtained, as in the previous arrange- ment ; and by an ingenious modification contrived by Dr. Leeson, the ring into which the selenite plate is fitted being made movable, one plate may be substituted for another, whilst rotation may be given to the ring by means of a tangent-screw fitted into the brass plate. Such a " selenite stage" answers every purpose that can be required ; but as there is no provision for using two or three plates in combination, it is necessary to have a distinct selenite plate for every modification of colors that may be desired. A very beautiful effect may be obtained with certain kinds of semi-opaque objects, by illuminating them by means of a " spotted lens" (§ 61), with a polarizer of Herapathite placed at such a dis- tance above it as to receive the converging hollow pencil near its termination in the object, and an analyzer of the usual descrip- tion,— a combination devised by Mr. Furze ;* for the solidity which this mode of oblique illumination imparts to certain ob- jects, is remarkably heightened by the play of colors afforded by the polarization of the light. When the polarizing apparatus is being employed with any but the lowest powers, it is very advan- 1 " Transactions of the Microscopical Society" (2d series), vol. iii, p. 63. BULL S-EYE CONDENSER. 143 tageous to use the achromatic condenser in combination with it; this combination, which cannot be made in ordinary microscopes, is provided for in that of Messrs. Smith and Beck, by the " cylin- drical fitting" so often referred to, which can receive the polar- izing prism at its lower end, and the achromatic condenser at its upper, whilst the selenite plate or plates may be interposed be- tween them.1 64. Illuminators for Opaque Objects. — All objects through which sufficient light cannot be transmitted to enable them to be viewed in the modes already described, require to be illuminated by rays, which, being thrown upon the surface under examination, shall be reflected from it into the microscope ; and this mode of viewing them may often be advantageously adopted in FlG 44 regard to semi-transparent or even transparent objects, for the sake of the diverse aspects it affords. Among the various methods devised for this pur- pose, the one most generally adopted consists in the use of a condensing lens, either at- tached to the microscope, or mounted upon a separate stand, by which the rays pro- ceeding from a lamp or from a bright sky are made to con- verge upon the object. For the efficient illumination of large opaque objects, such as injected preparations, it is desirable to employ a " bull's- eye" condenser (which is a plano-convex lens of short focus, two or three inches in diameter), mounted upon a separate stand, in such a man- ner as to allow of being placed in a great variety of positions. The mounting shown in Fig. 44 is perhaps one of the best Buirs-Eye condenser. that can be adopted: the frame which carries the lens is born eat the bottom upon a swivel- joint, which allows it to be turned in any azimuth ; whilst it may be inclined at any angle to the horizon, by the revolution of 1 For an account of the nature and properties of Polarized Light, which would be out of place in the present treatise, see the chapters on that subject in Dr. Golding Bird's " Manual of Natural Philosophy," Dr. Pereira's " Lectures on Polarized Light," New Ed., edited by Prof. Baden Powell, or any modern treatise on Optics. 144 ACCESSORY APPARATUS. the horizontal tube to which it is attached, around the other horizontal tube which projects from the stem ; by the sliding of one of these tubes within the other, again, the horizontal arm may be lengthened or shortened ; the lens may be secured in any posi- tion (as its weight is apt to drag it down when it is inclined, unless the tubes be made to work, the one into the other, more stiffly than is convenient) by means of a tightening collar milled at its edges ; and finally the horizontal arm is attached to a spring socket, which slides up and down upon a vertical stem. The optical effect of such a lens differs according to the side of it turned towards the light, and the condition of the rays which fall upon it. The position of least spherical aberration, is when its convex side is turned towards parallel or towards the least diverging rays ; consequently, when used by daylight, its plane side should be turned towards the object ; and the same position should be given to it, when it is used for procuring converging rays from a lamp, the lamp being placed four or five times farther off on one side, than the object is on the other. But it may also be employed for the pur- pose of reducing the diverging rays of the lamp to parallelism, for use either with the parabolic illuminator (§ 61), or with the side- reflector to be presently described ; and the plane side is then to be turned towards the lamp, which must be placed at such a dis- tance from the condenser, that the rays which have passed through the latter shall form a lumin- ous circle equal to it in size, at whatever distance from the lens the screen may be held. Even where the large " bull's- eye" condenser is provided, it is well to have a smaller con- densing lens in addition ; and this, which is usually a double- convex lens, may either be mounted on a separate base (Fig. 45), or may be attached to some part of the micro- scope. (In Messrs. Smith and Beck's large microscope, Fig. 29, two sockets with binding- screws, one for the condensing lens, the other for the side-re- flector, are seen in the "limb.") This condensing lens is suffi- Condensing Lens. cient by itself for most ordi- nary purposes; and it may also be used to obtain a greater concentration of the rays already brought into convergence by the bull's-eye (§ 93). 65. The illumination of opaque objects may be effected by re- flection, as well as by refraction ; and a very advantageous means FIG. 45. SIDE-REFLECTOR FOR OPAQUE OBJECTS. 145 FIG. 46. of using the light of a lamp for this purpose, is afforded by the Side-Reflector contrived by Mr. Ross. This is a highly polished concave speculum (Fig. 46), which can be placed above and to one side of the object; and which is so mounted as to be capable of being placed in every kind of position, according to the place of the lamp, and the degree of obliquity of the illumination required. The squared stem, with which the speculum is con- nected by several intermediate joints, may be fitted to a socket, either in the stage or in some part of the microscope-stand, like that of the smaller condensing lens. The light reflected by the speculum upon the object, may be either that which falls on it direct from the lamp, or may come to it through the intervention of the bull's-eye, arranged so as to throw parallel rays upon the speculum (§ 64). The prisms already described as in use for the illumination of transparent objects by the reflection of light from beneath, may also be employed, by an inversion of their position, for the illumination of opaque objects from above. In Continental Microscopes, the prism is frequently attached to the lower end of the body ; but this is an undesirable mode of supporting it, since the illumination is dis- turbed by every alteration in the distance between the body and the object. This seems to be provided against by the mount- ing of the prism in Mr. Grubb's micro- scope (§ 60), which allows it to be used at any angle either above or below the stage. A mode of illuminating opaque objects by a small concave speculum reflecting the light directly down upon it, was formerly side-Reflector. much in use, but is now comparatively seldom employed. This concave speculum, termed a "Lieber- kiihn" from the celebrated Microscopist who invented it, is made to fit upon the end of the objective, having a perforation in the centre for the passage of the rays from the object to the lens; and it receives its light from the mirror beneath, the object being so mounted as only to stop out the central portion of the rays that are reflected upwards. The curvature of the speculum is so adapted to the focus of the object-glass, that, when the latter is duly adjusted, the rays reflected up to it from the mirror shall be made to converge strongly upon the part of the object that is in focus ; consequently, unless (as is sometimes done) the spe- culum should be mounted on a tube sliding over the " nose" of 10 140 ACCESSORY APPARATUS. the microscope, and capable of being adjusted to the different distances required by the several objectives, a separate speculum is required for every object-glass. The disadvantages of this mode of illumination are chiefly these : — first, that by sending the light down upon the object almost perpendicularly, there is scarcely any shadow, so that the inequalities of its surface, and any minute markings which it may present, are but faintly or not at all seen; second, that the size of the object must be so limited by that of the speculum, as to allow the rays to pass to its marginal portion ; and third, that a special mode of mounting is required, to allow the light to be reflected from the mirror around the margin of the object. The first objection may be in some degree removed, by turning the mirror considerably out of the axis, so as to reflect its light obliquely upon the Lieberkiihn, which will then send it down obliquely upon the object ; the illumination, however, will not even then be so good as that which is afforded by the side-reflector. The mounting of opaque objects in wooden slides (Chapter Y), which affords in many cases the most convenient means of preserving them, completely prevents the employment of the Lieberkiihn in the examination of them; and they must either be set, for this purpose, upon disks which afford them no protection, or in glass cells with a blackened background. The cases wherein the Lieberkiihn is most useful, are those in which it is desired to examine small opaque objects, such as can be held in the stage-forceps (§ 66), or laid upon a slip of glass, with lenses of half inch focus or less; since a stronger light can be thus concentrated upon them, than can be easily obtained by side-illumination. In every such case, a black background must be provided, of such a size as to fill the field, so that no light shall come to the eye direct from the mirror, and yet not large enough to create any unnecessary ob- struction to the passage of the rays from the mirror to the specu- lum. With each Lieberkiihn is commonly provided a blackened stop of appropriate size, having a well-like cavity, and mounted upon a pin which fits into a support connected with the under side of the stage ; but though the " dark well" serves to throw out a few objects with peculiar force, yet, for all ordinary pur- poses, a spot made with black paper or black sealing-wax- varnish upon a slip of glass will answer the required purpose very effectually, the slip being simply laid upon the stage beneath the object. SECTION 2. APPARATUS FOR THE PRESENTATION OF OBJECTS* 66. Stage-Forceps. — Every Microscope should be furnished with a pair of Stage-forceps (Fig. 47) for holding minute objects beneath the object-glass. They are mounted by means of a joint upon a pin, which fits into a hole either in the corner of the stage itself, or in the object-platform ; the object is inserted by pressing the pin that projects from one of the blades, whereby it is sepa- STAGE-FORCEPS — GLASS STAGE-PLATE. 147 rated from the other; and the blades close again, so as to retain the object when the pressure is withdrawn. By sliding the wire stem which bears the forceps through its socket, and by moving that socket vertically upon its joint, and the joint horizontally FIG. 47. Stage-Forceps. upon the pin, the object may be brought into the field precisely in the position required; and it may be turned round and round, so that all sides of it may be examined by simply giving a twisting movement to the wire stem. The other extremity of the stem often bears a small brass box filled with cork, and per- forated with holes in its side ; this affords a secure hold to com- mon pins, to which disks of card, &c., may be attached, whereon objects are mounted for being viewed with the Lieberkuhn. This method of mounting was formerly much in vogue, but has been less employed of late, since the Lieberkuhn has fallen into comparative disuse. 67. Glass Stage-Plate. — Every Microscope should be furnished with a piece of plate-glass, about 4 in. by 1J in., to one margin of which a narrow strip of glass is cemented, so as to form a ledge. This is extremely useful, both for laying objects iipon (the ledge preventing them from sliding down when the micro- scope is inclined), and for preserving the stage from injury by spilling of sea-water or other saline or corrosive liquids, when such are in use. Such a plate not only serves for the examina- tion of transparent, but also of opaque objects ; the dark back- ground being furnished by the diaphragm-plate (§ 55), and the condensing-lens being so placed as to throw a side-light upon them. A small addition may be conveniently made to the glass stage-plate, which adapts it for use as a Growing-Slide. A circu- lar aperture, of about the diameter of a test-tube, is made near one end of the plate (the length of which, for this purpose, had better be not less than 5 inches), and in this is to be fitted a little cup, formed of the end of a test-tube, about three-quarters of an inch deep, in such a manner that its rim shall project a little above the surface of the plate. The cup may be closed by an ordinary cork, or (to avoid the danger of splitting it) by a disk of glass cemented to a ring of cork which shall embrace the ex- terior of the tube ; but a small aperture must be left, by grind- ing a notch in the rim of the cup, sufficient to admit the passage of two or three threads of lamp-cotton. The manner in which the "growing-slide" is used, is this: — Supposing we wish to follow the changes undergone by some minute Alga or Infusorium, which 148 ACCESSORY APPARATUS. we have just detected in a drop of liquid under examination upon an ordinary slip of glass (and covered with thin glass), — we transfer this slip to the "growing-slide," fill the cup with distilled water mixed with a small proportion of the water in which the organism was found, and then so arrange the threads (previously moistened with distilled water), that they shall pass from the cup to the edge of the liquid in which the object is con- tained. Thus, as the water evaporates from beneath the thin glass, the threads will afford a continuous supply; and the threads will not become dry, until the whole of the liquid has been absorbed by them and has been dissipated by evaporation. Fresh supplies may, of course, be introduced into the cup from time to time, as may be needed, so as to prevent any loss of liquid from beneath the thin glass ; and in this manner, the most important requisite for the continued growth of aquatic or- ganisms,—a constant supply of liquid, without an exclusion of air, — may be secured.1 68. Aquatic Box or Animalcule Cage. — This, also, is an ap- pendage with which every Microscope should be provided, so varied and so constant is its utility. It consists of a short piece of wide brass tube, fixed perpendicularly at one end into a flat plate of brass (Fig. 48) which is perforated by an aperture equal in FIG. 48. Aquatic Box or Animalcule Cage, as seen in perspective at A, and in section at B. diameter to that of the tube, and having its opposite extremity closed by a disk of glass (B b) ; over this fits a cover, formed of a piece of tube just large enough to slide rather stiffly upon that which forms the box, closed at the top by another disk of glass (B a). The cover being taken off, a drop of the liquid to be examined, or any thin object which can be most advantageously looked at in fluid, is placed upon the lower plate ; the cover is then slipped over it, and is pressed down until the drop of liquid be spread out, or the object be flattened, to the degree most con- venient for observation. If the glass disk which forms the lid be cemented or burnished into the brass ring which carries it, a small hole should be left for the escape of air or superfluous fluid ; and this hole may be closed up with a morsel of wax, if 1 See. the " Micrographic Dictionary," by Dr. Griffith and Mr. Henfrey, Introduction, p. xx. AQUATIC BOX OR ANIMALCULE CAGE. 149 it be desired to prevent the included fluid from evaporating. But as it is desirable that this glass should be thin enough to allow a l-4th-inch objective to be employed for the examination of Animalcules, &c., and as such thin glass is extremely apt to be broken, it is a much better plan to furnish the brass cover with a screw-cap, which holds the brass disk with sufficient firmness, but permits it to be readily replaced when broken ; and as the looseness of this fitting gives ample space for the escape of air or fluid around the margin of the disk, no special aperture is needed. It is always desirable, if possible, to prevent the liquid from spreading to the edge of the disk ; since any objects it may contain are very apt, in such a case, to be lost under the opaque ring of the cover ; this is to be avoided by limiting the quantity of the liquid introduced, by laying it upon the centre of the lower plate, and by pressing down the cover with great caution, so as to flatten the drop equally on all sides, stopping short when it is spreading too close to the margin. With a little practice, this object may in general be successfully attained ; but if so much superfluous liquid should have been in- troduced, that it has flooded the circumference of the inclosed space, and exuded around the edge of the disk, it is better to wipe the whole perfectly dry, and then to introduce a fresh drop, taking more care to limit its quantity and to restrain it within convenient bounds. If the box be well constructed, and the glass disks be flat, they will come into such close contact, that objects of extreme thinness may be compressed between them; hence not only may such small animals as Water-fleas (Entomo- straca) be restrained from the active movements which preclude any careful observation of their structure, — and this without any permanent injury being inflicted upon them, — but much smaller creatures, such as Wheel-Animalcules (Rotifera), or Bryozoa, may be flattened out, so as to display their internal organization more clearly, and even the larger Infusoria may be treated in like manner. The working Microscopist will find it of great advantage to possess several of these aquatic boxes, of different sizes ; and one or two of them may have the glass cover of stronger glass than the rest, and firmly fixed in its rim, so that, if the cover be made to slide equably on the box, the instrument (in hands accustomed to careful manipulation) may be made to answer the purpose of a compressorium (§ 70). 69. Zoophyte Trough. — -For the examination of living aquatic objects, too large to be conveniently received into the Aquatic Box, the Zoophyte trough contrived by Mr. Lister may be em- ployed with great advantage. This consists of a trough of the shape represented in Fig. 48, formed of plates and slips of plate- glass cemented together by marine glue ; of a loose vertical plate of glass, just so much smaller than the front or back of the in- side of the trough, as to be able to move freely between its sides; and of a horizontal slip of glass, whose length equals that of the 150 ACCESSORY APPARATUS. inside bottom of the trough, but whose breadth is inferior by the thickness of the plate just men- tioned. The trough being filled with water (fresh or salt, as the case may be), the horizontal slip is laid at the bottom, and the vertical plate is placed in contact with the front of the trough, its lower mar- gin being received into the space left at the front edge of the hori- zontal slip, which serves to hold it there, acting as a kind of hinge ; a zoophyte Trough. small ivory wedge is then inserted between the front glass of the trough and the upper part of the vertical plate, which it serves to press backwards ; but this pressure is kept in check by a little spring of bent whalebone, which is placed between the vertical plate and the back glass of the trough. By moving the ivory wedge up or down, the amount of space left between the upper part of the vertical plate and the front glass of the trough can be precisely regulated ; and as their lower margins are always in close appo- sition, it is evident that the one will incline to the other, with a constant diminution of the distance between them, from above downwards. Hence a Zoophyte, or any similar body, dropped into this space, will descend until it rests against the two sur- faces of glass, and will remain there hi a situation extremely convenient for observation ; and the regulating-wedge, by in- creasing or diminishing the space, serves to determine the level to which the object shall fall. Of these troughs, again, it is con- venient for the working Microscopist to be furnished with several, of different sizes ; and in one of them OJiara or Nitella may be kept growing in a state very convenient for observation. A similar trough may be provided for this last purpose, however, by dispensing with the vertical plate and horizontal slip alto- gether, and approximating the front and back plates so that only a very narrow space is contained between them ; in this case it is convenient to let the upper lip of the back plate project con- siderably beyond that of the front plate, as objects may then be much more readily inserted between them ; and the back plate may also be conveniently made to project beyond the sides of the trough, as would be useful, too, in the case of larger troughs. If it be wished to grow Chara, &c., in a thin trough of this kind, the trough, whenever it is not under observation, should be im- mersed in a tumbler or jar of water, since the plant will not nourish in a very limited supply. 70. Compressor ium. — The purpose of this instrument is to apply a graduated pressure to objects, whose structure can only be made out when they are thinned by extension. For such as will bear tolerably rough treatment, a well-constructed Aquatic Box may be made to answer the purpose of a compressor; but COMPRESSORIUM. 151 there is a very large class, whose organization is so delicate as to be confused or altogether destroyed by the slightest excess of pressure ; and for the examination of such, an instrument in which the degree of compression can be regulated with pre- cision, is almost indispensable. Various plans of construction have been proposed; but none among them appears to the Author to present so many advantages as the one represented in Fig. 50, the general 'plan of which was originally devised by Schiek of Berlin, but the details of which have been modified by M. de Quatrefages, who has constantly employed this instru- ment in his elaborate and most successful researches on the organization of the Marine Worms. It consists of a plate of brass between 3 and 4 inches long, and from Fr«- f)0- 1J to 1J inch broad, having a central aper- ture of from J to j of an inch. This central aperture is covered on its Upper Side by a Compressorium. disk of thin glass, which may be cemented to the brass plate by Canada balsam ; and the under side of it is bevelled away, so that the thickness of the edge shall not interfere with the approach of the objective to its margin, when that side is made the uppermost. Near one extremity of the plate is a strong vertical pin, that gives support to a horizontal bar which turns on it as on a swivel ; through the end of this bar that projects beyond the plate, there passes a screw with a milled head ; and at the other end is jointed a second bar, against one end of which the screw bears, whilst the other carries a frame holding a second disk of thin glass. This frame is a small circular plate of glass, having an aperture equal in size to that of the large plate ; to its under side, which is flat, a disk of thin glass is cemented by Canada balsam, while its upper side is bevelled off as it approaches the opening, for the purpose just now specified ; and by being swung between pivots in a semicircle of brass, which is itself pivoted" to the movable arm, it is made capable of a limited movement in any direction. The upper disk with the apparatus which supports it, having been completely turned aside round the swivel-joint, the object to be compressed is laid upon the lower disk ; the upper disk is then turned back so as to lie precisely over it, and by the action of the milled-head screw, is gradually approximated to the lower, to which the pivot movements of its frame allow it to take up a parallel position, whatever may be the inclination of the bar. As it is frequently of great importance to be able to look at either side of the object under compression, the principal plate is provided with two pins at the extremity farthest from the milled head, which, being exactly equal in length to the swivel-pin, afford, with it, a support to the instrument, when it 152 ACCESSORY APPARATUS. FIG. 51. B is so turned that the side represented as undermost in the figure, shall be uppermost ; and it is in order that high powers may be used in this case as in the other, that the disk which then covers the object is made of thin glass, instead of being (as in the original form of the instrument) a piece of thick glass plate. That a thin glass disk is more liable to fracture under pressure, than a thick one, is no serious objection to its use for this pur- pose ; since the lower one is not more likely to break than the upper one ; and either may be replaced with extreme facility, by simply warming the part of the instrument to which it is attached, so as to loosen the cement that holds it. And the advantage of being able to view an object under a high power, from either side, will be most fully appreciated by every one who has been much engaged in the class of observations, which this instrument is specially adapted to facilitate. If this Compres- sorium be made of sufficient size to admit an or- dinary glass slide between the vertical pins, an object may be subjected to compression, and afterwards removed for examination out of the compressor, without transferring it from one glass to another, which is frequently an advantage. In this case, it will be convenient that the thin glass disk should be "countersunk" into the upper surface of the principal plate, so as to form one level with it. 71. Dipping Tubes. — In every operation in which small quantities of liquid, or small ob- jects contained in liquid, have to be dealt with by the Microscopist, he will find it a very great convenience to be provided with a set of tubes of the forms represented in Fig. 51, but of somewhat larger dimensions. These were for- merly designated as " fishing tubes;" the purpose for which they were originally devised having been the fishing out of Water-Fleas, aquatic Insect-Larvae, the larger Animalcules, or other living objects distinguishable either by the un- aided eye or by the assistance of a magnifying glass, from the vessels that may contain them. But they are equally applicable, of course, to the selection of minute Plants ; and they may be turned to many other no less useful purposes, some of which will be specified hereafter. When it is desired to secure an object which can be seen either with the eye alone or with a magnifying glass, one of these tubes is passed down into the liquid, its upper orifice having been previously closed by the forefinger, until its lower orifice is close above the object; the finger being then removed, the liquid suddenly rises into the tube, probably carrying Fishing Tubes. FISHING TUBES — FORCEPS. 153 tke object up with it; and, if this is seen to be the case, by put- ting the finger again on the top of the tube, its contents remain in it when the tube is lifted out, and may be deposited on a slip of glass or on the lower disk of the aquatic box, or, if too copious for either receptacle, maybe discharged into a watch-glass. In thus fishing for any but the minutest objects, it will be generally found convenient to employ the open-mouthed tube c ; and when its contents have been discharged, if they include but a single object of the desiderated kind, this may be taken up by one of the finer tubes, A, B, or, if more convenient, the whole superfluous fluid may be sucked up by the mouth, and the object left with no more than is suitable ; or, if there be many of the desired objects in the fluid first selected, these may be taken up from it, one by one, by either of the finer tubes. 72. Forceps. — Another instrument so indispensable to the Mi- croscopist as to be commonly considered an appendage to the Microscope, is the Forceps for taking up minute objects ; many forms of this have been devised, of which one of the most con- venient is represented in Fig. 52, of something less than the FIG. 52. Forceps. actual size. As the forceps, in marine researches, have continu- ally to be plunged into sea-water, it is better that they should be made of brass or of German silver, than of steel, since the latter rusts far more readily ; and as they are not intended (like dis- secting forceps) to take a firm grasp of the object, but merely to hold it, they may be made very light, and their spring part slender. As it is essential, however, to their utility, that their points should meet accurately, it is well that one of the blades should be furnished with a guide-pin, passing through a hole in the other. The foregoing constitute, it is believed, all the most important pieces of Apparatus, which can be considered in the light of Ac- cessories to the Microscope. Those which have been contrived to afford facilities for the preparation and mounting of Objects, will be described in a future chapter (Chap. V). It may be thought that some notice ought to be taken of the Frog Plate and Fish Pan, with the former of which many Microscopes are sup- plied, whilst the latter has scarcely yet gone altogether out of use. But the Author having been accustomed to gain all the advantages of these, by methods far more simple, whilst at least equally efficacious, does not consider them as presenting any ad- vantages which render it desirable to expend time or space in giving a detailed account of them ; and he will explain the methods alluded to, under the appropriate head. CHAPTER IY. MANAGEMENT OF THE MICROSCOPE. 73. Support. — The Table on which the Microscope is placed, when in use, should be one whose size enables it also to receive the various appurtenances which the observer finds it convenient to have within his reach, and whose steadiness is such as to allow of his arms being rested upon it without any yielding; it should, moreover, be so framed, as to be as free as possible from any tendency to transmit the vibrations of the building or floor whereon it stands.1 The manner in which the Microscope itself is constructed, however, will have a great influence on the effect of any such disturbing cause ; since, if the whole instrument move together, scarcely any tremulousness will be produced in the image, by vibrations which cause it to " dance" most un- pleasantly, if the body and stage of the Microscope oscillate in- dependently of each other. Hence, in choosing a Microscope, it should always be subjected to this test, and should be unhesi- tatingly rejected if the result be unfavorable. It is of course to be borne in mind, that any vibration, either of the object or of the optical apparatus, in which the other does not partake, will be much more apparent when high magnifying powers are used, than when the object is amplified in a much less degree, the motion of the object being magnified in precisely the same ratio with the object itself; hence if, when. the microscope is thus tested with high powers, it is found to be free from fault, its steadiness with low powers may be assumed ; but, on the other hand, a Microscope which may give an image free from perceptible tremor when the lowest powers only are employed, may be quite unfit for use with the highest. 74> Light. — Whatever may be the purposes to which the Mi- croscope is applied, it is a matter of the first importance to secure a pure and adequate illumination. There is scarcely any class 1 The working Microscopist will find it a matter of great convenience to have a Table specially set apart for this purpose; furnished with drawers in which are contained the various accessories he may require for the preparation and mounting of objects. If the Microscope be one which is not very readily laken out from and put back into its case, it is very convenient to cover it with a large bell-glass; which may be so suspended from the ceiling, by a cord carrying a counterpoise at its other end, as to be raised or lowered with the least possible trouble, and to be entirely out of the way when the Microscope is in use. Similar but smaller bell-glasses are also useful for the protection of objects, which are in course of being examined or prepared, and which it is desirable to seclude from dust. ARTIFICIAL LIGHT — LAMPS. 155 of objects, for the examination of which good daylight is not to be preferred to any other kind of light ; but good lamplight is preferable to bad daylight. When daylight is employed, the Microscope should be placed near a window, whose aspect should be (as nearly as may be convenient) opposite to the side on which the sun is shining; for the light of the sun reflected from a bright cloud, is that which the experienced Microscopist will almost always prefer, the rays proceeding from a cloudless blue sky being by no means so well fitted for his purpose, and the dull lurid reflection of a dark cloud being the worst of all. The direct rays of the sun are far too powerful to be used with ad- vantage, unless its intensity be moderated, either by reflection from a plaster-of-paris or some other " white-cloud" mirror (§ 58), or by passage through some imperfectly transparent medium. The moderator contrived by Mr. Eainey for lamp or gas-light (§ 75), has been found to answer equally well for direct sun- light ; the glare and heating power of which it so effectually sub- dues, as to destroy all tendency to injure the most delicate ob- ject, or to confuse the observer's view of it ; whilst an illumina- tion is obtained by its means, whose intensity renders it superior for certain purposes to anything else. The young Microscopist is earnestly recommended to make as much use of daylight as possible ; not only because, in a large number of cases, the view of the object which it affords is more satisfactory than that which can be obtained by any kind of lamp-light, but also because it is much less " trying" to the eyes. So great, indeed, is the dif- ference between the two in this respect, that there are many who find themselves unable to carry on their observations for any length of time by lamp-light, although they experience neither fatigue nor strain from many hours' continuous work by daylight. 75. When recourse is had to Artificial light, it is of great im- portance, not only that it should be of good quality, but that the arrangement for furnishing it should be suitable to the special wants of the Microscopist. Thus, although a wax or composi- tion candle affords a very pure light, yet its use is attended with two inconveniences, which render its use very undesirable when any better light can be obtained ; — namely, the constant flicker- ing of the flame, which is not sufficiently prevented by surround- ing it with a chimney; and the continual alteration in its level, which is occasioned by the consumption of the candle. The most useful light for ordinary use, is that furnished by the steady and constant flame of the lamp, fed either with oil, camphine, or gas; the wick or burner should be cylindrical or " argand;" it should be capable of adjustment to any height above the table ; and a movable shade should be provided, by which the light may be prevented from coming direct to the observer's eyes, or from diffusing itself too widely through the room. These requisites are supplied by the lamp commonly known as the " University" 156 MANAGEMENT OF THE MICROSCOPE. or " reading" lamp, which has a circular foot with a vertical stem, on which the oil-reservoir (carrying with it the burner) and the shade, can be fixed at any convenient height. French and German lamps on the same general construction, but having the reservoir contrived on the " bird-fountain" principle, are also to be obtained, being largely imported for the use of watch-makers ; these have the advantage of burning out all their oil, which is not the case with the ordinary " reading"-lamp, as it does not burn well except when full or nearly so ; but they are usually destitute of a shade, which, however, can be easily added. Lamps of either kind are sometimes constructed on the " solar" principle, which increases the purity and intensity of the light, but at the same time not only diminishes the diameter of the flame, but also produces an inconvenient transverse "break" near its lower part. The best kind of -light which an oil-lamp can furnish, is that yielded by the " Moderator" lamps which have of late come into .such general use ; but they have this im- portant drawback, that they contain in themselves no adjustment for varying the elevation of the burner, and that their construc- tion is such as to give no facilities for any arrangement of this kind. The same objection applies to the Camphine-lamps in ordinary use ; but a small camphine-lamp has been constructed for the special use of Microscopists, which is capable of being placed on an adjustable stand, so that its flame may be raised or lowered to any desired level. The light of this lamp is whiter and more intense than that of any other, and it may be used with advantage for certain very delicate observations (§ 58) ; but for the ordinary purposes of the Microscopist it is not so convenient, the surface of flame from which the light can be received by the mirror or condenser, being limited by the peculiar construction which the combustion of camphine requires. To every one who has a supply of gas at command, the use of it for his microscope- lamp (by means of a flexible tube) strongly recommends itself, on account of its extreme convenience, and its freedom from any kind of trouble. The lamp should be constructed on the general plan already described, the burner being made to slide up and down on a stem rising perpendicularly from a foot, which also carries a shade ; and the burner should be one which affords a bright and steady cylindrical flame, either "Leslie's" or the " cone"-burner being probably the best. Even the best light supplied by a gas-lamp, however, is inferior in quality to that of a good oil-lamp ; and is more injurious and unpleasant to the eye. Hence the interposition of some kind of artificial medium, adapted to keep back the yellow rays, whose predominance in the lamp-flame is the chief source of its injurious action, is es- pecially required when gas-light is used. This may be partly effected, by the simple expedient of using a chimney of bluish glass, known as " Leblond's ;" but, in addition, it is advantageous to cause the light to pass through a screen of bluish-black or POSITION OF THE LIGHT. 157 neutral-tint glass ; and it will then be nearly purified as to quality, though much reduced in intensity.1 Mr. Rainey, who has paid great attention to the hest means of obtaining a good illumination by artificial light, recommends, as the best moderator, one piece of dark blue glass, free from any tint of red, another of very pale blue with a slight shade of green, and two of thick white plate-glass, all cemented together with Canada balsam ; this, as already stated, may be used with sun-light, as well as with lamp-light. The Microscopist who wishes to render the artificial light which he may be in the habit of using, as pure as possible, will do well to compare with daylight (as suggested by Dr. Griffith, who seems to have been the first to employ tinted glass with this object) ; furnishing himself with several pieces of glass of different shades, substituting one for another, and alter- ing their distances from the lamp,2 until he has succeeded in so tempering its rays, that the field of his Microscope, or the object under view, is not more colored when illuminated by the artificial light reflected from the mirror, than it is when the mirror is so turned as to reflect good light from a white cloud. 76. Position of the Light. — When the Microscope is used by daylight it will usually be found most convenient to place it in such a manner, that the light shall be at the left hand of the ob- server. It is most important that no light should enter his eye, save that which comes to it through the Microscope ; and the access of direct light can scarcely be avoided, when he sits with his face to the light. Of the two sides, it is more convenient to have the light on the left; first, because it is not interfered with by the right hand, when this is employed in giving the requisite direction to the mirror, or in adjusting the illuminating appara- tus ; and secondly, because, as most persons employ the right eye rather than the left, the projection of the nose serves to cut off those lateral rays, which, when the light comes from the right side, glance between the eye and the eye-piece. In order to pre- vent, still more completely, the access of false light, it is desirable, if it be otherwise convenient, that when daylight is employed, its source should be a little behind the observer : but as it will then, by falling upon the stage, interfere with the view of any object which is imperfectly transparent (§ 87), it maybe necessary to keep it from doing so, by the interposition of a screen. "When Artificial light is employed, the same general precautions should be taken. The lamp should always be placed on the left side, 1 A gas-lamp provided with these and other appurtenances for regulating the illumi- nation, and also with a water-bath and mounting-plate, is supplied by Mr. S. Highley, Fleet Street. 2 The nearer the colored glass is approximated to the flame, the less modification will it produce in its rays ; since their intensity varies in different parts of their course, in- versely with the square of their distance from the illuminating centre, whilst its in- fluence is a constant quality. Hence a pale-blue glass placed near the mirror, or be- tween the mirror and the stage, has more effect than a chimney of much deeper blue immediately surrounding the flame. 158 MANAGEMENT OF THE MICROSCOPE. unless the use of the mirror be dispensed with, or some special reason exist for placing it otherwise. If the object under exami- nation be transparent, the lamp should be placed at a distance from the eye about midway between that of the stage and that of the mirror; if on the other hand, the object be opaque, it should be at a distance about midway between the eye and the stage ; so that its light may fall, in the one case upon the mirror, in the other case upon the stage, at an angle of about 45° with the axis of the microscope. The passage of direct rays from the flame to the eye, should be guarded against by the interposition of the lamp-shade ; and no more light should be diffused through the apartment, than is absolutely necessary for other purposes. If observations of a very delicate nature are being made, it is desi- rable, alike by daylight and by lamp-light, to exclude all lateral rays from the eye, as completely as possible ; and this may be readily accomplished, by means of a shade attached to the eye- piece of the microscope. Such a shade may be made most simply of an oblong piece of card-board, having a circular hole cut in it, by which it may fit upon the eye-piece or the upper part of the body; its two ends should be turned up, so as to cut off all lateral light ; its upper side should also be turned up, so as to cut off the light from the front ; and a notch should be cut in its lower edge, in the proper position to receive the nose. It may be either painted black, or may be covered with black cloth or velvet. 77. Care of the Eyes. — Although most Microscopists acquire a habit of employing only one eye (generally the right), yet it will be decidedly advantageous to the beginner, that he should learn to use either eye indifferently ; since by employing and resting each alternately, he may work much longer, without incurring un- pleasant or injurious fatigue, than when he always employs the same. Whether or not he do this, he will find it of great im- portance to acquire the habit of keeping open the unemployed eye. This, to such as are unaccustomed to it, seems at first very em- barrassing, on account of the interference with the microscopic image, which is occasioned by the picture of surrounding objects, formed upon the retina of the second eye ; but the habit of re- stricting the attention to that impression only which is received through the microscopic eye, may generally be soon acquired ; and when it has once been formed, all difficulty ceases. Those who find it unusually difficult to acquire this habit, may do well to learn it in the first instance with the assistance of the shade just described; the employment of which will permit the second eye to be kept open without any confusion. The advantage of the practice, in diminishing the fatigue of long- continued obser- vation, is such, that no pains are ill bestowed by the Microscopist, which are devoted to early habituation to it. There can be no doubt that the habitual use of the Microscope for many hours together, especially -by lamp-light, and with high magnifying CLEANSING OF THE LENSES, ETC. 1£9 powers, has a great tendency to injure the sight. Every Micro- scopist who thus occupies himself, therefore, will do well, as he values his eyes, not merely to adopt the various precautionary measures already specified, but rigorously to observe the simple rule of not continuing to observe, any longer than he can do so without fatigue. 78. Care of the Microscope. — Before the Microscope is brought into use, the cleanliness and dryness of its glasses ought to be ascertained. If dust or moisture should have settled on the Mirror, this can be readily wiped off. If any spots should show themselves on the field of view, when it is illuminated by the mirror, these are probably due to particles adherent to one of the lenses of the Eye-piece ; and this may be determined by turning the eye-piece round, which will cause the spots also to rotate, if their source lies in it. It may very probably be sufficient to wipe the upper surface of the eye-glass (by removing its cap), and the lower surface of the field-glass ; but if, after this has been done, the spots should still present themselves, it will be neces- sary to unscrew the lenses from their sockets, and to wipe their inner surfaces ; taking care to screw them firmly into their places again, and not to confuse the lenses of different eye-pieces. Sometimes the eye-glass is obscured by dust of extreme fineness, which may be carried off by a smart puff of breath ; the vapor which then remains upon the surface being readily dissipated, by rapidly moving the glass backwards and forwards a few times through the air. And it is always desirable to try this plan in the first instance ; since, however soft the substance with which the glasses are wiped, their polish is impaired in the end by the too frequent performance of the process. The best material for wiping glass, is a piece of soft wash-leather, from which the dust it generally contains has been well beaten out. If the Object- glasses be carefully handled, and be kept in their boxes when not in use, they will not be likely to require cleansing. One of their chief dangers, however, to which they are liable in the hands of ,an inexperienced Microscopist, arises from the neglect of precaution in using them with fluids ; which, when allowed to come in contact with the surface of the outer glass, should be wiped off as soon as possible. In screwing and unscrewing them, great care should be taken to keep the glasses at a distance from the surface of the hands ; since they are liable not only to be soiled by actual contact, but to be dimmed by the vaporous ex- halation from skin which they do not touch. This dimness will be best dissipated, by moving the glass quickly through the air. It will sometimes be found, on holding an object-glass to the light, that particles either of ordinary dust, or more often of the black coating of the interior of the microscope, have settled upon the surface of its back lens ; these are best removed by a clean and dry camel-hair pencil. If any cloudiness or dust should still present itself in an object-glass, after its front and back surfaces 160 MANAGEMENT OF THE MICROSCOPE. have been carefully cleansed, it should be sent to the maker (if it be of English manufacture) to be taken to pieces, as the ama- teur will seldom succeed in doing this without injury to the work; the foreign combinations, however, being usually put together in a simpler manner, may be readily unscrewed, cleansed, and screwed together again. Not unfrequently an objective is ren- dered dim by the cracking of the cement by which the lenses are united, or by the insinuation of moisture between them ; this last defect occasionally arises from a fault in the quality of the glass, which is technically said to " sweat." In neither of these cases has the Microscopist any resource, save in an Optician ex- perienced in this kind of work ; since his own attempts to remedy the defect are pretty sure to be attended with more injury than benefit. 79. General Arrangement of the Microscope for Use. — The in- clined position of the instrument, already so frequently referred to, is that in which Observation by it can be so much more ad- vantageously carried on than it can be in any other, that this should always be had recourse to, unless particular circumstances render it unsuitable. The precise inclination that may prove to be most convenient, will depend upon the " build" of the Micro- scope, upon the height of the observer's seat as compared with that of the table on which the instrument rests, and lastly, upon the tallness of the individual ; and it must be determined in each case by his own experience of what suits him best, — that which he finds most comfortable, being that in which he will be able not only to work the longest, but to see most distinctly. The selec- tion of the object-glasses and eyerpieces to be employed, must be entirely determined by the character of the object. Large ob- jects presenting no minute structural features, should always be examined in the first instance by the lowest powers, whereby a general view of their nature is obtained ; and since, with lenses of comparatively long focus and small angle of aperture, the pre- cision of the focal adjustment is not of so much consequence as it is with the higher powers, not only those parts can be seen which are exactly in focus, but those also can be tolerably well distinguished, which are not precisely in that plane, but are a little nearer or more remote. When the general aspect of an object has been sufficiently examined through low powers, its details may be scrutinized under a higher amplification ; and this will be required in the first instance, if the object be so minute, that little or nothing can be made out respecting it, save when a very enlarged image is formed. The power needed in each par- ticular case, can only be learned by experience ; that which is most suitable for the several classes of objects hereafter to be described, will be specified under each head. In the general ex- amination of the larger class of objects, the range of power that is afforded by the "erector" in combination with the " draw-tube" (§ 44), will be found very useful ; whilst for the ready exchange USE OF DIFFERENT EYE-PIECES. 161 of a low power for a high one, great convenience is afforded by Mr. Brooke's object-glass holder (§ 50). 80. When the Microscopist wishes to augment his magnifying power, he has a choice between the employment of an Objective of shorter focus, and the use of a deeper Eye-piece. If he pos- sess a complete series of objectives, he will generally find it best to substitute one of these for another, without changing the eye- piece for a deeper one ; but if his " powers" be separated by wide intervals, he will be able to break the abruptness of the increase in amplification which they produce, by using each objective first with the shallower, and then with the deeper eye piece. Thus if a Microscope be only provided with two objectives, of 1 inch and 1-4 th inch focus respectively, and with two eye- pieces, one nearly double the power of the other (as is the case with Messrs. Smith and Beck's new Educational Microscope, p. 103, note), such a range as the following may be obtained, — 55, 100, 200, 350 diameters ; or, with two objectives of somewhat shorter focus, and with deeper eye-pieces (as is the case with an instrument in the Author's possession, constructed by Kellner of Wetzlar, whose Microscopes have acquired for themselves a deservedly high reputation),— 88, 176, 350, 700 diameters. The use of the " draw-tube" (§ 43) enables the Microscopist still fur- ther to vary the magnifying power of his instrument, and thus to obtain almost any exact number of diameters he may desire, within the limits to which he is restricted by the focal length of his objectives. The advantage, to be derived, however, either from " deep eye-piecing," or from the use of the draw-tube, will mainly depend upon the quality of the object-glass. For if it be imperfectly corrected, its errors are so much exaggerated, that more is lost in definition than is gained in amplification ; whilst, if its aperture be small, the loss of light is an equally serious drawback. On the other hand, a combination of perfect construction and wide angle of aperture, will sustain this treat- ment with so little impairment in the perfection of its image, that a magnifying power may be obtained by its use, such as, with an inferior instrument, can only be derived from an object- ive of much shorter focus combined with a shallow eye-piece.1 In making any such comparisons, it must be constantly borne in mind that the real question is, what can be seen ? It is always desirable for the purposes of research, to employ the lowest power with which the details of structure can be clearly made out; since, the lower the power, the less is the liability to error from false appearances, and the better can the mutual relations of the different parts of the object be appreciated. Hence in testing the optical quality of a Microscope, the question should always 1 The 4-10ths object-glass of Messrs. Smith and Beck was specially distinguished by the Jurors of the Great Exhibition, as affording, by the use of deep eye-pieces and the draw tube, a power fully equivalent in the resolution of difficult tests, to that which, a few years previously, could only have been given by an objective of J-8th inch. 11 162 MANAGEMENT OF THE MICROSCOPE. be, — not, what is its greatest magnifying power, — but, what is the least magnifying power under which it will show objects of a given degree of difficulty. 81. In making the Focal Adjustment, when low powers are used, it will scarcely be necessary to employ any but the coarse movement ; provided that the rack be well cut, the pinion work in it smoothly and easily, without either "spring," "loss of time," or "twist," and the milled head be large enough to give the requisite leverage. All these are requisites which should be found in every well-constructed instrument ; and its possession of them should be tested, like its freedom from vibration, by the use of high powers, since a really good coarse adjustment should enable the observer to "focus" an objective of l-8th inch with precision. What is meant by "spring" is the alteration which may often be observed to take place on the withdrawal of the hand ; the object which has been brought precisely into focus, and which so remains as long as the milled head, is between the fingers, becoming indistinct when the milled head is let go. The source of this fault may lie either in the rack-movement itself, or in the general framing of the instrument, which is so weak as to allow of displacement by the mere weight or pressure of the hand ; should the latter be the case, the " spring" may be in great degree prevented, by carefully abstaining from bearing on the milled head, which should be simply rotated between the fin- gers. By " loss of time" is meant the want of sufficient readiness in the action of the pinion upon the rack, so that the milled head may be moved a little in either direction, without affecting the body ; thus occasioning a great diminution in the sensitiveness of the adjustment. This fault may sometimes be detected in Microscopes of the best original construction, which have gradu- ally worked loose, from the constancy with which they have been in employment ; and it may often be corrected by tighten- ing the screws that bring the pinion to bear against the rack. And by "twist" it is intended to express that apparent move- ment of the object across the field, which results from a real displacement of the axis of the body to one side or the other, owing to a want of correct fitting in the working parts. As this last fault depends entirely upon bad original workmanship, there is no remedy for it ; but it is one which most seriously interferes with the convenient use of the instrument, however excellent may be its optical performance. In the use of the coarse adjust- ment with an objective of short focus, extreme care is necessary to avoid bringing it down upon the object, to the injury of one or both ; for althgugh the spring with which the tube for the reception of the object-glass is furnished, whenever the fine adjustment is immediately applied to this (§ 31), takes off the violence of the crushing action, yet such an action, even when thus moderated, can scarcely fail to damage or disturb the object, and may do great mischief to the lenses. Where no such spring- ADJUSTMENT OF FOCUS. 163 tube is furnished, the fine adjustment being otherwise provided for, or being not supplied at all, still greater care is of course required. It is here, perhaps, well to notice, for the guidance of the young Microscopist, that the actual distance between the object-glass and the object, when a distinct image is formed, is always considerably less than the nominal focal length of the object-glass ; thus, the distance of the 1 inch or 2-3 inch object- glass may be little more than half an inch; that of the 4-10 inch may be but little more than one-tenth of an inch ; that of a 1-4 or a 1-5 inch may scarcely exceed one- twentieth ; that of a 1-8 inch may not be one-fortieth ; and that of a 1-12 or a 1-16 inch may be so close as not to admit the intervention of a piece of glass no more than one-hundredth of an inch in thickness. The reason of this is, that the focal length of an Achromatic objective is estimated by that of the Single lens with which it agrees in the size of the image it forms, and therefore in magnifying power (e. g., it is said to be of 1 inch focus, when its power is equiva- lent to that of a single lens, which brings parallel rays to a point at an inch distance) ; whilst from its being composed of a com- bination of lenses, the point from which that focal distance has really to be measured, is not at the surface of its front lens, but at some distance behind it, in the interior of the combination. One more precaution it may be well to specify ; — namely, that either in changing one object for another, or in substituting one objective for another, save when powers of such focal length are employed as to remove all likelihood of injury, the "body" should be turned to one side, where the construction of the Mi- croscope admits of this displacement, or (where it does not) should have its distance from the stage increased by the " coarse movement." This precaution is absolutely necessary, when ob- jectives of short focus are in use, to avoid injury either to the lenses or to the object; and when it is habitually practised with, regard to these, it becomes so much like an "acquired instinct," as to be almost invariably practised in other cases. 82. In obtaining an exact Focal Adjustment with object- glasses of less than half an inch focus, it will be generally found convenient to employ the fine movement ; and as recourse will frequently be had to its assistance for other purposes also, it is very important that it should be well constructed and in good working order. The points to be particularly looked to in test- ing it, are for the most part the same with those already noticed in relation to the coarse movement. It should work smoothly and equably, producing that graduated alteration of the distance of the object-glass from the object, which it is its special duty to effect, without any jerking or irregularity. It should be so sensi- tive, that any movement of the milled head should at once make its action apparent, by an alteration in the distinctness of the image, when high powers are employed, without any " loss of 164 MANAGEMENT OF THE MICROSCOPE. time."1 And its action should not give rise to any twisting or displacing movement of the image, which ought not to be in the least decree disturbed by any number of rotations of the milled head, still less, by a rotation through only a few degrees. One great use of the "fine adjustment" consists in bringing into view different strata of the object, and this in such a gradual manner that their connection with one another shall be made apparent. "Whether an opaque or a transparent object be under examina- tion, only that part can be perfectly discerned under any power, which is exactly in focus ; and when high powers of large aper- ture are employed, this is the only part that can be seen at all. A minute alteration of the focus often causes so entirely different a set of appearances to be presented, that, if this alteration be made abruptly, their relation to the preceding can scarcely be even guessed at; and the gradual transition from the one to the other, which the fine adjustment alone affords, is therefore ne- cessary to the correct interpretation of either. To take a very simple case : — The transparent body of a certain animal being traversed by vessels lying in different planes, one set of these vessels is brought into view by one adjustment, another set by " focussing" to a different plane ; and the connection of the two sets of vessels, which may be the point of most importance in the whole anatomy of the animal, may be entirely overlooked, for want of a fine adjustment, the graduated action of which shall enable one to be traced continuously into the other. What is true even of low and medium powers, is of course true to a still greater degree of high powers ; for although the " coarse movement" may enable the observer to bring any stratum of the object into accurate focus, it is impossible for him by its means to secure that transitional "focussing," which is often so much more instructive than an exact adjustment at any one point. A clearer idea of the nature of a doubtful structure is, in fact, often derived from what is caught sight of in the act of changing the focus, than by the most attentive study and comparison of the different views obtained by any number of separate " focussings." The experienced Microscopist, therefore, when examining an object of almost any description, constantly keeps his finger upon the milled head of the "fine movement," and watches the effect produced by its revolution upon every feature which he distinguishes ; never leaving off, until he be satisfied that he has scrutinized not only the entire surface, but the entire thickness of the object. It will often happen, that, where different structural features present themselves on different planes, it will be difficult or even impossible to determine which of them is the nearer and *It will sometimes happen that the "fine movement" will seem not to act, merely because it has been so habitually worked in one direction rather than the other, that its screw has been turned too far. In that case, nothing more is required for its restoration to good working order, than turning the screw in the other direction, until it shall have reached about the middle of its range of action. ADJUSTMENT OF THE OBJECT-GLASS. 165 which the more remote (it being the special result of the ordinary mode of viewing objects by transmitted light, that such differ- ences are obliterated), unless, by the use of the " fine movement," it be ascertained, when they are successively brought into focus, whether the object-glass has been moved towards or away from the object. Even this, however, will not always succeed in cer- tain of the most difficult cases, in which the difference of level is so slight as to be almost inappreciable ; — as, for instance, in the case of the markings on the siliceous loricw of the Diatomacese (Fig. 80). 83. When objectives of short focus and of wide angular aper- ture are being employed, something more is necessary than exact focal adjustment; this being the Adjustment of the Object-glass itself, which is required to neutralize the disturbing effect of the glass cover upon the course of the rays proceeding from the ob- ject (§ 15). For this adjustment, it will be recollected, a power of altering the distafcce between the front pair and the remainder of the combination is required; and this power is obtained in the following manner. The front pair of lenses is fixed into a tube (Fig. 53, A), which slides over an interior tube (B) by which the other two pairs are held ; and it is drawn up or down by means of a collar (c), which works in a furrow cut in the inner tube, and upon a screw- thread cut in the outer, so that its revolution in the plane to which it is fixed by the one tube, gives a vertical movement to the other. In one part of the outer tube, an ob- long slit is made, as seen at D, into which projects a small tongue, screwed on the inner tube ; at the side of the former two Section of an Adjusting Object-Glass. horizontal lines are engraved, one pointing the word " covered ;" whilst the latter is „ , . , „ to the word "un- covered," the other to crossed by a horizontal mark, which is brought to coincide with either of the two lines by the rotation of the screw-collar, which moves the outer tube up or down. When the mark has been made to point to the line "uncovered," it indicates that the dis- tance of the lenses of the object-glass is such, as to make it suita- ble for viewing an object without any interference from thin glass ; when, on the other hand, the mark has been brought, by the revolution of the screw-collar, into coincidence with the line " covered," it indicates that the front lens has been brought into 166 MANAGEMENT OP THE MICROSCOPE. such proximity with the other two, as to produce a " positive aberration" in the objective, fitted to neutralize the "negative aberration" produced by the interposition of a glass cover of a certain thickness. It is evident, however, that unless the par- ticular thickness of glass for which this degree of alteration is suited, be always employed for this purpose, the correction can- not be exact ; and means must be taken for adapting it to every grade of thickness, which may be likely to present itself in the glass covers. Unless this correction be made with the greatest precision, the enlargement of the angle of aperture, to which our Opticians have of late applied themselves with such remarkable success, becomes worse than useless ; being a source of dimi- nished instead of increased distinctness in the details of the ob- ject, which are far better seen with an objective of greatly inferior aperture, possessing no special adjustment for the thickness of the glass. . The following general rule is given by Mr. Wenham, for securing the most efficient performance of an object-glass with any ordinary object: — " Select any dark speck or opaque portion of the object, and bring the outline into perfect focus ; then lay the finger on the milled head of the fine motion, and move it briskly backwards and forwards in both directions from the first position. Observe the expansion of the dark outline of the object, both when within, and when without, the focus. If the greater expansion, or coma, is when the object is without the focus, or furthest from the objective, the lenses must be placed further asunder, or towards the mark ' uncovered.' If the greater coma is when the object is within the focus, or nearest to the objective, the lenses must be brought closer together, or towards the mark < covered.' "When the object-glass is in proper adjustment, the expansion of the outline is exactly the same both within and without the focus." A different indication, however, is afforded by such "test-objects" as present (like the Podura- scale and the Diatomacese) a set of distinct dots or other mark- ings. For " if the dots have a tendency to run into lines when the object is placed without the focus, the glasses must be brought closer together ; on the contrary, if the lines appear when the object is within the focal point, the object must be further sepa- rated.1 When the angle of aperture is very wide, the difference in the aspect of any severe test under different adjustments be- comes at once evident ; markings which are very distinct when the correction has been exactly made, disappearing almost instantaneously when the screw-collar is turned a little way round."2 1 See " Quart. Journ. of Microsc. Science," vol. ii, p. 138. 2 Mr. Wenham remarks (loc. cit.), not without justice, upon the difficulty of making this adjustment, even in the Objectives of our best Opticians; and he states that he has himself succeeded much better, by making the outer tube the fixture, and by making the tube that carries the other pairs slide within this ; the motion being given by the action of an inclined slit in the revolving collar, upon a pin that passes through a longi- tudinal slit in the outer tube, to be attached to the inner. The whole range of adjust- ADJUSTMENT OF THE OBJECT-GLASS. 167 84. Although the most perfect correction required for each par- ticular object (which depends, not merely upon the thickness of its glass cover, but upon that of the fluid or balsam in which it may be mounted) can only be found by experimental trial, yet for all ordinary purposes, the following simple method, first de- vised by Mr. Powell, will suffice. The object-glass, adjusted to "uncovered," is to be "focussed" to the object; its screw-collar is next to be turned, until the surface of the glass cover comes into focus, as may be perceived by the spots or striae by which it may be marked ; the object is then to be again brought into focus by the ufine movement." The edge of the screw-collar being now usually graduated, the particular adjustment which any object may have been found to require, and of which a record has been kept, may be made again without any difficulty. By Messrs. Smith and Beck, however, who first introduced this graduation, a further use is made of it. By experiments such as those described in the last paragraph, the correct adjustment is first found for any particular object, and the number of divisions observed, through which the screw-collar must be moved in order to bring it back to 0°, the position suitable for an uncovered ob- ject. The thickness of the glass cover must then be measured by means of the "fine movement:" this is done by bringing into exact focus, first the object itself, and then the surface of the glass cover, and by observing the number of divisions through which the milled head (which is itself graduated) has passed in making this change. A definite ratio between that thickness of glass, and the correction required in that particular objective, is thus established ; and this serves as the guide to the requisite correction for any other thickness, which has been determined in like manner by the "fine movement." Thus, supposing a particular thickness of glass to be measured by 12 divisions of the milled head of the fine movement, and the most perfect per- formance of the object-glass to be obtained by moving the screw- collar through 8 divisions, then a thickness of glass measured by 9 divisions of the milled head, would require the screw-collar to be adjusted to 6 divisions in order to obtain the best effect. The ratio between the two sets of divisions is by no means the same for different combinations; and it ought to be determined for each objective by its maker, who will generally be the best judge of the best " points" of his lenses ; but when this ratio has been once ascertained, the adjustment for any thickness of glass with which the object may happen to be covered, is readily made by the Microscopist himself. Although this method appears some- what more complex than that of Mr. Powell, yet it is more per- fect ; and when the ratio between the two sets of divisions has been once determined, the adjustment does not really involve ment is thus performed within a third part of a revolution, with scarcely any friction, and with such an immediate transition from good to bad definition, that the best point is made readily apparent. 168 MANAGEMENT OF THE MICROSCOPE. more trouble. Another use is made of this adjustment by Messrs. Smith and Beck ; namely, to correct the performance of the ob- jectives, which is disturbed by the increase of distance between the objective and the eye-piece, that is occasioned by the use of the draw-tube (§ 43). Accordingly, they mark a scale of inches on the draw-tube (which is useful for many other purposes), and direct that for every inch the body is lengthened, the screw-collar of the objective shall be moved through a certain number of divisions. 85. Arrangement for Transparent Objects. — If the object be already " mounted" in a slide, nothing more is necessary, in order to bring it into the right position for viewing it, than to lay the slide upon the object-platform of the stage, and to sup- port it in such a position (by means of the sliding ledge or other contrivance) that the part to be viewed is, as nearly as can be guessed, in the centre of the aperture of the stage, and therefore in a line with the axis of the body. If the object be not " mounted," and be of such a kind that it is best seen dry, it may be simply laid upon the glass stage-plate (§ 67), the ledge of which will prevent it from slipping off when the microscope is inclined, and a plate of thin glass may be laid over it for its pro- tection, if its delicacy should seem to render this desirable. If, again, it be disposed to curl up, so that a slight pressure is needed to flatten or extend it, recourse may be had to the use of the aquatic box (§ 68), or of the compressorium (§ 70), no liquid, however, being introduced between the surfaces of glass. In a very large proportion of cases, however, either the objects to be examined are already floating in fluid, or it is preferable to examine them in fluid, on account of the greater distinctness with which they may be seen ; if such objects be minute, and the quantity of liquid be small, the drop is simply to be laid on a slip of glass, and covered with a plate of thin glass ; if the object or the quantity of liquid be larger, it will be better to place it in the aquatic box ; whilst, if the objects have dimen- sions which render even this inconvenient, the zoophyte trough (§ 69) will afford the best medium for its examination. If it be wished to have recourse to compression, for the expansion or flattening of the object, this may be made upon the ordinary slide, by .pressing down the thin glass cover with a pointed stick ; and this method, which allows the pressure to be applied where it may chance to be most required, will generally be found preferable for delicate portions of tissue which are easily spread out, and which, in fact, require little other compression than is afforded by the weight of the glass cover, and by the capillary attraction which draws it into proximity with the slide beneath. A firmer and more enduring pressure may be exerted by the dexterous management of a well-constructed aquatic box ; and this method is peculiarly valuable for confining the movements of minute animals, so as to keep them at rest under ARRANGEMENT FOR TRANSPARENT OBJECTS. 169 the field of the microscope, without killing them. It is where a firm but graduated pressure is required, for the flattening out of the bodies of thin semi-transparent animals, without the necessity of re- moving them from the field of the microscope, that the compresso- rium is most useful. Wherever the first and simplest of the above methods can be had recourse to, it is the preferable one ; since the object, when on a glass slide, can be subjected to the Achro- matic Condenser, Polariscope, Oblique Illumination, &c., with far more convenience than when removed to a plane above the stage, as it must be when the aquatic box is used. Whether the object be submitted to examination on a slip of glass, or in the aquatic box or compressorium, it must be first brought approximately into position, and supported there, just as if it were in a mounted slide. The precise mode of effecting this will differ, according to the particular plan of the instrument employed; thus in some, it is only the ledge itself that slides along the stage ; in others, it is a carriage of some kind, whereon the object-slide rests; in others, again, it is the entire platform itself that moves upon a fixed plate beneath. 86. Having guided his object, as nearly as he can do by the unassisted eye, into its proper place, the Microscopist then brings his light (whether natural or artificial) to bear upon it, by turning the mirror in such a direction as to reflect upon its under surface the rays which are received by itself from the sky or the lamp. The concave mirror is that which should always be first employed, the plane being reserved for special purposes ; and it should bring the rays to convergence in or near the plane in which the object lies (Fig. 54). The distance at which it should be ordinarily set beneath the stage, is that at which it brings parallel rays to a focus ; but this distance should be capable of elongation, by the lengthening of the stem to which the mirror is attached ; since the rays diverg- ing from a lamp at a short distance, are not so soon brought to a focus. The correct focal adjustment of the mirror may be FIG. 54. Arrangement of Microscope for Transparent Objects. ed of, by its formation of images of window-bars, chimneys, &c., upon any semi-transparent medium placed in the plane of the object. It is only, however, when 170 MANAGEMENT OF THE MICROSCOPE. small objects are being viewed under high magnifying powers, that such a concentration of the light reflected by the mirror is either necessary or desirable ; for with large objects, seen under low powers, the field would not in this mode be equably illuminated. The diffusion of the light over a larger area may be secured, either by shifting the mirror so much above or so much below its previous position, that the pencil will fall upon the object whilst still converging or after it has met and diverged ; or, on the other hand, by the interposition of a plate of ground glass in the course of the converging pencil, — this last method, which is peculiarly appropriate to lamp-light, being very easily had recourse to, if the diaphragm plate, as formerly recommended (§ 55), have had its largest aperture filled with such a diffused medium. The eye being now applied to the Eye-piece, and the body being "focussed," the object is to be brought into the exact position required, by the use of the traversing movement, if the stage be provided with it ; if not, by the use of the two hands, one moving the object-slide from side to side, the other pushing the ledge, fork, or holder that carries it, either forwards or backwards, as may be required. It is always to be remembered, in making such adjustments by the direct use of the hands, that, owing to the inverting action of the microscope, the motion to be given to the object, whether lateral or vertical, must be precisely opposed to that which its image seems to require, save when the Erector (§ 44) is employed. When the object has been thus brought fully into view, the Mirror may require a more accurate adjustment. What should be aimed at, is the diffusion of a clear and equable light over the entire field; and the observer should not be satisfied, until he has attained this object. If the field should be darker on the one side than on the other, the mirror should be slightly turned in such a direction as to throw more light upon that side ; perhaps in so doing, the light may be withdrawn from some part previously illuminated; and it may thus be found that the pencil is not large enough to light up the entire field. This may be owing to one of three causes : either the cone of rays may be received by the object too near to its focal apex, the remedy for which lies in an alteration in the distance of the mirror from the stage ; or, from the very oblique position of the mirror, the cone is too much narrowed across one of its diame- ters, and the remedy must be sought in a change in the position either of the microscope or of the lamp, so that the face of the mirror may not be turned so much away from the axis of vision; or, again, from the centre of the mirror being out of the optical axis of the instrument, the illuminating cone is projected obliquely, an error which can be rectified without the least difficulty. If the cone of rays should come to a focus in the object, the field is not unlikely to be crossed by the images of window-bars or chimneys, or the form of the lamp-flame may be REGULATION OF TRANSMITTED LIGHT. 171 distinguished upon it; the former must be got rid of by a slight change in the inclination of the mirror; and if the latter cannot be dissipated in the same way, the lamp should be brought a little nearer. 87. The equable illumination of the entire field having been thus obtained, the quantity of light to be admitted should be re- gulated by the Diaphragm-plate (§ 55). This must depend very much upon the nature of the object, and upon the intensity of the light. Generally speaking, the more transparent the object, the less light does it need for its most perfect display ; and its most delicate markings are frequently only made visible, when the major part of the cone of rays has been cut off. Thus the movement of the cilia, — those minute vibratile filaments, with which almost every Animal is provided in some part of its or- ganism, and which many of the humbler Plants also possess, — can only be discerned in many instances, when the light is ad- mitted through the smallest aperture. On the other hand, the less transparent objects usually require the stronger illumination which is afforded by a wider cone of rays ; and there are some (such as semi-transparent sections of fossil teeth) which, even when viewed with low powers, are better seen with the intenser light afforded by the Achromatic Condenser. In every case in which the object presents any considerable obstruction to the passage of the rays through it, great care should be taken to pro- tect it entirely from incident light; since this extremity weakens the effect of that which is received into the microscope by trans- mission. It is by daylight that this interference is most likely to occur : since, i*f the precautions already given (§ 76) respecting the use of lamp-light be observed, no great amount of light can fall upon the upper surface of the object. The observer will be warned that such an effect is being produced, by perceiving that there is a want, not only of brightness, but of clearness, in the image ; the field being veiled, as it were, by a kind of thin vapor ; and he may at once satisfy himself of the cause, by interposing his hand between the stage and the source of light, when the immediate increase of brilliancy and of distinctness will reveal to him the occasion of the previous deficiency in both. Nothing more is necessary for its permanent avoidance, than the inter- position of an opaque screen (blackened on the side towards the stage) between the window and the object ; care being of course taken, that the screen does not interfere with the passage of light to the mirror. Such a screen may be easily shaped and adapted either to be carried by the stage itself, or by the stand for the condenser; but it is seldom employed by Microscopists, as it interferes with access to the left side of the stage ; and the interposition of the hand, so often as it may be needed, is more frequently had recourse to in preference, as the more convenient expedient. The young Microscopist who may be examining transparent objects by daylight, is recommended never to omit 172 MANAGEMENT OF THE MICROSCOPE. ascertaining, whether the view which he may obtain of them, is in any degree thus marred by incident light. 88. Although the illumination afforded by the mirror alone is quite adequate for a very large proportion of the purposes for which the Microscope may be profitably employed (nothing else having been used by many of those who have made most valua- ble contributions to Science by means of this instrument), yet, when high magnifying powers are employed, and sometimes even when but a very moderate amplification is needed, great advan- tage is gained from the use of the Achromatic Condenser. The various modes in which this may be constructed, and may be fitted to the Microscope, have been already described (§ 56) ; we have now%to speak of the manner of using it. The lenses with which the Condenser is provided should be made to separate from each other, in such a manner that two or three distinct powers should be afforded ; the complete combination should be used with objectives of l-5th inch focus or less ; the front lens should be removed with objectives of from half to a quarter of an inch focus ; and the second lens may be removed, so that the back lens will be alone employed, when it is desired to use the condenser with objectives of less than half an inch focus. It is of the greatest importance that the Condenser should be ac- curately adjusted, both as to the coincidence of its optical axis with that of the Microscope itself, and as to its focal distance from the object. The centring may be most readily accom- plished, by so adjusting the distance of the condenser from the stage (by the rack-and-pinion action, or the sliding movement, with which it is always provided), that a sharp circle of light shall be thrown on any semi-transparent medium laid upon it ; then, on this being viewed through the Microscope with an ob- jective of sufficiently low power to take in the whole of it, if this circle be not found to be concentric with the field of view, the axis of the condenser must be altered by means of the milled- head tangent screws with which it is provided. The focal adjust- ment, on the other hand, must be made under the objective which is to be employed in the examination of the object, by turning the mirror in such a manner as to throw upon the visual image of the object (previously brought into the focus of the Micro- scope) an image of a chimney or window-bar, if daylight be em- ployed, or of the top, bottom, or edge of the lamp-flame, if lamp- light be in use ; such a vertical movement should be given to the condenser, as may render the view of this as distinct as possible ; and the direction of the mirror should then be sufficiently changed to displace these images, and to substitute for them the clearest light that can be obtained. It will generally be found, however, that although such an exact focussing gives the most perfect results by daylight, yet that by lamp-light the best illu- mination is obtained, when the condenser is removed to a some- what greater distance from the object, than that at which it gives USE OF ACHROMATIC CONDENSER. 173 a distinct image of the lamp. In every case, indeed, in which it is desired to ascertain the effect of variety in the method of illumination, the effects of alterations in the distance of the con- denser from the object should be tried ; as it will often happen that delicate markings become visible when the condenser is a little out of focus, which cannot be distinguished when it is pre- cisely in focus. The diaphragm-plate with which all the best forms of Achromatic Condenser are now furnished, enables the observer not only to. vary the angle of his illuminating pencils through a range of from 20° to 80°, but also to stop off the central portion of the pencil, so as to allow only its most oblique rays to pass ; and the contrast presented by the aspect of many objects, according as the size and form of the aperture in the diaphragm-plate limits the rays transmitted by the condenser to those of the central or those of the peripheral portion of the pencil, is often so marked, as to show beyonfl question the great importance of this mode of varying the illumination. When the condenser is employed, the plane Mirror may often be sub- stituted with advantage for the concave ; the chief effect of this exchange being to diminish the quantity of light, without alter- ing the angle of the illuminating pencil. It must be borne in mind, in making such an alteration, that the plane mirror reflects parallel or (if from a lamp) diverging rays, instead of the con- verging rays reflected by the concave mirror ; so that the focus of the condenser is likely to require readjustment. For objects of great delicacy and transparency, the " white-cloud" illumina- tion (§ 58) may be had recourse to with advantage ; or, if it be desired that the illuminating pencil should be free from the error imparted by the double reflection of the mirror, the mirror may be turned aside, and in its stead the lamp (if the observation be made by artificial light) may be placed in the axis of the micro- scope; or the mirror may be replaced by "Dujardin's prism" (§ 57), which will be equally available either by daylight or by lamp-light. 89. Should it be desired, however, to try the effect of very oblique light upon an object, the Achromatic Condenser must be removed (unless, as in Mr. Sollitt's arrangement, § 130, it be so constructed as to be capable of inclination to the axis of the Microscope), and other means must be employed. The simplest method, where the mirror is mounted on an "arm" (Fig. 29), is to turn it to one side so as to reflect the rays at a considerable angle ; and where this cannot be done, nearly the same effect is produced by placing the lamp in the direction from which it is desired that the oblique rays should proceed, and interposing an ordinary condensing lens between it and the object. Or, if the Microscopist be provided with the means of mounting a " Du- jardin's prism" on a separate stand, he may place it in such a position as to reflect light from any point required : and he may concentrate that light by an ordinary condenser. The possession 174 MANAGEMENT OF THE MICROSCOPE. of Amici's prism, however (which serves both as mirror and con- denser, § 60), will save the necessity of any other provision of this kind. It is when objects are thus illuminated by oblique light, and when their markings are of such a kind as to be best or to be only shown by light falling upon them in one particular direction, that we derive the greatest advantage from the power of giving a rotatory movement either to the object or to the illu- minating apparatus. Thus suppose that an object be marked by longitudinal striae, too faint to be seen by ordinary direct light ; the oblique light most fitted to bring them into view, will be that proceeding in either of the directions c or D ; that which falls upon B it in the directions A and B, tending to obscure the striae rather than to disclose them. But, moreover, if the striae should be due to furrows or prominences which have one side inclined and the other side abrupt, they will not be brought into view indifferently by light from c or from D, but will be shown best by that which makes the strongest shadow ; hence if there be a projecting ridge, with an abrupt side looking towards c, it will be best seen by light from D ; whilst if there be a furrow with a steep bank on the side of c, it will be by light from that side that it will be best displayed. But it is not at all unfrequent for the longitudinal striae to be crossed by others ; and these transverse striae will usually be best seen by the light that is least favorable for the longitudinal ; so that, in order to bring them into distinct view, either the illuminating pencil or the object must be moved a quarter round. The re- volving action with which the stage of Mr. Ross's Microscope is provided (§ 37), enables this movement to be given to the object without any displacement of its image, which, of course, exe- cutes, to the eye of the observer, a rotation in the opposite di- rection. In other microscopes, however, it is difficult to give a rotation to the object, by causing the object-platform to turn upon its axis, without throwing the object out of the field (§ 38); though this may be accomplished, by such an adjustment of the traversing movement, as shall bring the centre of the tube on which that platform turns round, into the visual axis of the microscope — or, if this adjustment cannot be conveniently made in the first instance, by keeping the right hand constantly in action upon the milled heads of the stage movement, whilst the left hand rotates the object-platform, so as, by means of the former, to correct the displacement of the object occasioned by the latter. It may be sufficient, however, to examine the object in several different positions, so that the appearances it presents in each may be compared, without thus watching the transition from one to the other. 90. There are many kinds of transparent objects, especially such as either consist of thin plates, disks, or spicules of siliceous or calcareous matter, or contain such bodies, which are peculiarly ARRANGEMENT FOR OPAQUE OBJECTS. 175 well seen under the black-ground illumination (§§ 61, 62) ; for not only does the brilliant luminosity which they then present, con- trasting remarkably well with the dark ground behind them, show their forms to extraordinary advantage ; but this mode of illumination imparts to them an appearance of solidity, which they do not exhibit by ordinary transmitted light (§ 62) ; and it also frequently brings out surface-markings, which are not other- wise distinguishable. Hence, when any object is under exami- nation, that can be supposed to be a good subject for this method, the trial of it should never be omitted. For the low powers, the use of the "spotted lens" will be found sufficiently satisfactory ; for the higher, the paraboloid should be employed (§ 61). Similar general remarks may be made, respecting the examination of ob- jects by polarized light. Some of the most striking effects of this kind of illumination, are produced upon bodies whose par- ticles have a crystalline aggregation ; and hence it may often be employed with great advantage to bring such bodies into view, when they would not otherwise be distinguished ; thus, for ex- ample, the raphides of Plants are much more clearly made out by its means, in the midst of the vegetable tissues, than they can be by any other. But the peculiar effects of polarized light are also exerted upon a great number of other organized substances, both Animal and Vegetable ; and it often reveals differences in the arrangement or in the relative density of their component par- ticles, the existence of which would not otherwise have been suspected ; hence, the Microscopist will do well to have recourse to it, whenever he may have the least suspicion that its use can give him an additional power of discrimination. 91. Arrangement for Opaque Objects. — Although a large pro- portion of the objects best suited for Microscopic examination are either in themselves sufficiently transparent to admit of being viewed by light transmitted through them, or may be made so by appropriate means, and although that method (where it can be adopted) is generally the one best fitted for the elucidation of the details of their structure, yet there are many objects of the most interesting character, the opacity of which entirely forbids the use of this method, and of which, therefore, the surfaces only can be viewed, by means of the incident rays which they reflect. These are, for the most part, objects of comparatively large di- mensions, for which a low magnifying power suffices ; and it is specially important, in the examination of such objects, not to use a lens of shorter focus than is absolutely necessary for dis- cerning the details of the structure ; since, the longer the focus of the objective employed, the less is the indistinctness produced by inequalities of the surface, and the larger, too, may be its aper- ture, so as to admit a greater quantity of light, to the great im- provement of the brightness of the image. It is surprising how little attention has been given by Opticians to the construction of objectives suitable for this purpose. In their zeal for the im- 176 MANAGEMENT OF THE MICROSCOPE. provement of the higher powers of the Microscope, they have thought comparatively little of the lower ; and in Continental Microscopes, it is rare to meet with an objective which will give even a tolerable view of a large opaque object. The Author, in- deed, well remembers the time, when it was not thought worth while, even by English Opticians, to construct Achromatic ob- ject-glasses of less than an inch focus ; and the production of objectives of 1J inch and 2 inch focus has been chiefly called for, in consequence of their value in displaying anatomical pre- parations in which the bloodvessels have been injected with coloring matter. The view which is afforded of large opaque objects, however, by a Compound Microscope, furnished with even an imperfectly corrected Achromatic object-glass, giving a magnifying power of 20 or 25 diameters, is so greatly preferable to that which is given by any Simple Microscope, that no instru- ment that is intended for general research should be unfurnished with such a power. It is especially required in Microscopes that are to be used for Educational purposes ; since it is most im- portant that the young should be trained in a knowledge of the wonders and beauties of the familiar objects around them ; and an objective of low power and wide aperture, adapted to the examination of a large surface at once, affords a means of dis- playing these, such as can be afforded in no other way, save by the use of the Erector and draw-tube (§ 44). A microscope fur- nished with these appendages, need not be supplied with an ob- jective of longer focus than 1 inch or 8-10ths inch ; but the Author would strongly recommend to such as do not possess them, that they should give to a "dividing" 1J inch or 2 inch (in which the front lens is removable, and is replaced by a per- forated cap that limits the aperture of the back lens, which is then employed by itself, having a focus of about 3 inches) a pre- ference over such as do not thus supply the extremely low power which he recommends.1 92. The mode of bringing opaque objects under view, will differ according to their "mounting," and to the manner in which it is desired to illuminate them. If the object be mounted in a " slide" of glass or wood, upon a large opaque surface, the slide must be laid on the stage in the usual manner, and the object brought as nearly as possible into position by the eye alone (§ 84). If it be not so mounted, it may be simply laid upon the glass stage-plate, resting against its ledge ; and the diaphragm- plate must then be so turned, as to afford it a black background. 1 A single pair (flint and crown) of about 2 inch focus, was constructed at the Author's request, some years since, by Messrs. Smith and Beck, for the special purpose of exhibiting injected preparations, and other opaque objects; and its performance has been so satisfactory to him, that he was induced to urge upon the Microscopic Committee appointed by the Society of Arts, that the Educational Microscope for which they in- vited competition (§ 31), should be furnished with such a power. This recommenda- tion having been adopted, the instrument selected has been specially fitted for the class of objects above alluded to. ILLUMINATION FOR OPAQUE OBJECTS. 177 FIG. 55. Arrangement of Microscope for Opaque Objects. For all ordinary purposes, a piano or double-convex lens, of about 1J inch diameter, and 2 inches focus, either mounted upon a separate stand (as in Fig. 45), or so attached by a jointed support to the Microscope itself as to admit of being placed in any required position, will answer extremely well as a Condenser. If Daylight be em- ployed, the micro- scope should be so placed that the strong- est light may fall obliquely upon the stage, and preferably from the left hand side ; there will then be no difficulty in so disposing this con- denser, as to afford an illumination suffi- cient for almost any kind of object, pro- vided the quality of the light itself be good. Direct sunlight cannot be here em- ployed, without the production of an injurious glare, and the risk of burning the .object; but the sunlight reflected from a bright cloud is the best light possible. The condenser should always be placed at right angles to the direction of the illumi- nating rays, and at a distance from the object which will be de- termined by the size of the surface to be illuminated and by the kind of light required. If the magnifying power employed be high, and the field of view be consequently limited, it will be desirable so to adjust the lens, as to bring the cone of rays to a point upon the part of the object under examination ; and this adjustment can only be rightly made whilst the object is kept in view under the microscope, the condenser being moved in va- rious modes, until that position has been found for it in which it gives the best light. If, on the other hand, the power be low, and it be desired to spread the light equably over a large field, the condenser should be placed either within or beyond its focal distance ; and here, too, the best position will be ascertained by trial. It will often be desirable, also, to vary both the obliquity of the light, and the direction in which it falls upon the object ; the aspect of which is greatly affected by the manner in which the shadows are projected upon its surface, and in which the lights are reflected from the various points of it. There are many objects, indeed, distinguished by their striking appearance when the light falls upon them on one side, which are entirely destitute both of brilliancy of color and of sharpness of outline, 12 178 MANAGEMENT OF THE MICROSCOPE. when illuminated from the opposite side. Hence it is always desirable to try the effect of changing the position of the object; which, if it be " mounted," may be first shifted by merely re- versing the place of the two ends of the slide, and then, if this be not satisfactory, may be more completely as well as more gradually altered, by making the object-platform itself revolve, where the stage is fitted with such a movement : if, however, the object be not mounted, but be simply resting on the stage-plate, it may be readily shifted by hand. With regard to the obliquity of the illuminating rays, it is well to remark, that if the object be "mounted" under a glass cover, and the incident rays fall at too great an angle with the perpendicular, a large proportion of them will be reflected, and the brilliancy of the object will be greatly impaired. 93. The same general arrangement must be made, when Artificial light is used for the illumination of opaque objects ; the lamp being placed in such a position in regard to the stage, that its rays may fall in the direction indicated in Fig. 55 ; and these rays being collected and concentrated by the condenser, as already directed. As the rays proceeding from a lamp within a short distance are already diverging, they will not be brought by the condenser to such speedy convergence, as are the parallel rays of daylight; and it must, therefore, be further removed from the object, to produce the same effect. By modifying the distance of the condenser from the lamp and from the object respectively, the cone of rays may be brought nearly to a focus, or it may be spread almost equally over a large surface, as may be desired. In the illumination of opaque objects, the inferiority of artificial to solar light is not so perceptible as in the case of transparent objects; and the former has the advantage of being more easily concentrated to the precise degree, and of being more readily made to fall in the precise direction, that may be found most advantageous. Moreover, the contrast of light and shadow will be more strongly marked, when no light falls upon the object except that proceeding from the lamp used for its illumination, than it can be when the shadows are partially lightened by the rays which fall upon the object from every quarter, as must be the case if it be viewed by daylight. If the ordinary condensing lens do not afford a sufficient illumination, the large " bull's-eye" condenser (§ 64) may be employed ; its convex side being turned towards the lamp, when it is desired to bring its rays into the most complete convergence. And, if a still more concentrated light be required for the illumination of a small object under a high power, the small condenser may be so placed as to receive the cone where it is reduced to its own size ; since, by its means, the rays may be brought to a more exact convergence than they can be by the bull's-eye alone. In this manner, very minute bodies may lie viewed as opaque objects under a tolerably high magnifying power ; provided that the EXAMINATION OF OPAQUE OBJECTS. 179 brasswork of the extremities of the objectives be so bevelled off, as to allow the illuminating cone to have access to the object.1 No method of illuminating large opaque objects by lamp-light is more effective, than the reflection of light from a concave speculum placed near the side of the object (§ 65); this not only affords a brilliant light, which may be equably spread over as large a surface as may be required, but may, by the mode in which it is jointed to its supports, be made to throw its rays upon the object at a great variety of angles, without the neces- sity of moving the lamp, whereby the direction in which the best illumination can be gained, is readily ascertained. If a more intense light and a greater concentration be required, than the speculum will afford by reflecting the diverging rays of the lamp, these may be rendered parallel or slightly convergent by the interposition of the bull's-eye condenser, which, for such a purpose, must have its plane side turned towards the lamp. This speculum cannot be so advantageously used by daylight, the ordinary condensing lens being then decidedly preferable. 94. If the object which it is desired to examine be of small size, and of a shape and character that render it unsuitable to be laid upon the glass stage-plate, or to be turned over so as to bring each side in turn into the most advantageous position, — as is the case, for example, with the capsules of Mosses, the mouths ol which cannot be conveniently brought into view in this mode, — it may be grasped in the stage-forceps (§ 66), which afford great facility for this kind of manipulation ; or, if it be too minute or delicate to be thus held, it may be taken up upon the head of a small pin, by moistening this with saliva or with a little thin gum-water ; and the pin may then be either held in the stage- forceps, or may be run into the cork at its opposite extremity. By careful manipulation, every part of such an object may be brought under view successively, and may be exposed to every variety of illumination. It is in viewing objects supported in this mode, that the utility of the Lieberkiihn (§ 65) is chiefly felt; for, as the stage-forceps needs to be shifted into different po- sitions, so that the object is sometimes raised above and some- times depressed below the level of the stage, in order to present it under a different aspect, the side illumination, whatever be its source, needs to be newly adjusted with each change in the po- sition of the object; whilst the Lieberkiihn adjusts itself, so to speak, when the object is brought into focus. If the mirror be so mounted that it can be turned considerably out of the axis of the microscope, and the aperture of the stage be sufficiently large, a light of considerable obliquity may be reflected from the Lie- 1 Since the introduction of the Parabolic illuminator, the occasions on which advantageous recourse can be had to the examination of minute objects with high powers by incident light, have become much less numerous; since these objects are for the most part sufficiently transparent to admit of being illuminated by that instrument ; and when they are so, the view of them which it affords is generally much superior to any that can be gained by the method of illumination described above. 180 MANAGEMENT OF THE MICROSCOPE. berkiihn ; thus enabling it to afford a kind of illumination, which, as already remarked, is usually much more valuable than that produced by the nearly perpendicular rays sent down by it on the object, when the mirror is placed in the axis. Whenever the Lieberkiihn is employed, care must be taken that the direct light from the mirror be entirely stopped out by the interposition of a " dark well" or of a black disk, of such a size as to fill the field given by the particular objective employed, but not to pass much beyond it. An ingenious combination of a hemispherical Lie- berkiihn with the Paraboloid (§ 61) has been devised by Mr. Wenham, for the illumination of minute opaque objects by very oblique rays,1 and Mr. C. Brooke has attached a small plane spe- culum to objectives of l-8th and l-12th inch focus (which cannot be otherwise advantageously employed with that illuminator), in such a manner that its surface is level with, or very little below, that of the outer lens, so as to reflect downwards upon the object those extreme pencils of rays which pass by the aperture of the object-glass. In either case, an oblique illumination from one side only may be obtained, by shutting off either half of the lower aperture of the paraboloid. These contrivances for the examina- tion of minute objects with high powers by incident light,- have scarcely yet received the attention they deserve. 95. ^Errors of Interpretation. — The correctness of the conclu- sions which the Microscopist will draw, regarding the nature of any object, from the visual appearances which it presents to him, when examined in the various modes now specified, will neces- sarily depend in great degree upon his previous experience in microscopic observation, and upon his knowledge of the class of bodies to which the particular specimen may belong. Not only are observations of any kind liable, as already remarked (Intro- duction, pp. 39-41), to certain fallacies arising out of the previous notions which the observer may entertain, in regard to the con- stitution of the objects or the nature of the actions to which his attention is directed ; but even the most practised observer is apt to take no note of such phenomena as his mind is not pre- pared to appreciate. Thus, for example, it cannot be doubted that many Physiologists must have seen those appearances in thin slices of Cartilage, which are now interpreted as denoting its cellular organization, without in the least degree suspecting their real import, which Schwann was the first to deduce from the study of the development of that tissue ; it was not known before his time, " what cells mean" in Animal organization ; and the retinal pictures which now suggest the idea of them to the mind of even the tyro in the study of Histology (p. 54), passed almost entirely unnoticed by keen-sighted and intelligent Micro- scopists previously to 1839. Errors and imperfections of this kind can only be corrected, it is obvious, by general advance in scientific knowledge ; but the history of them affords a useful 1 "Quart. Journ. of Microsc. Science," vol. ii, p. 155. ERRORS OE INTERPRETATION. 181 warning against hasty conclusions drawn from a too cursory ex- amination. If the history of almost any scientific investigation were fully made known, it would generally appear, that the sta- bility and completeness of the conclusions finally arrived at, had only been attained after many modifications, or even entire altera- tions, of doctrine. And it is, therefore, of such great importance to the correctness of our conclusions, as to be almost essential, that they should not be finally formed and announced, until they have been tested in every conceivable mode. It is due to Science, that it should be burdened with as few false facts and false doc- trines as possible. It is. due to other truth-seekers, that they should not be misled, to the great waste of their time and pains, by our errors. And it is due to ourselves, that we should not commit our reputation to the chance of impairment, by the pre- mature formation and publication of conclusions, which may be at once reversed by other observers better informed than our- selves, or may be proved to be fallacious at some future time, perhaps even by our own more extended and careful researches. The suspension of the adjudgment, whenever there seems room for doubt, is a lesson inculcated by all those Philosophers who have gained the highest repute for practical wisdom; and it is one which the Microscopist cannot too soon learn, or too constantly practise. 96. Besides these general warnings, however, certain special cautions should be given to the young Microscopist, with regard to errors into which he is liable to be led, by the misinterpreta- tion of appearances peculiar to objects thus viewed, even when the very best instruments are employed. Thus the sharpness of the outline of any transparent object is impaired by a change in the course of the rays that merely pass by it, which is termed Inflection or Diffraction. If any opaque object be held in the course of a cone of rays diverging from a /ocus, the shadow which it will form upon a screen held to receive it, will not possess a well-defined edge, but will have as its boundary a shaded band, gradually increasing in brightness from the part of the screen on which the shadow is most intense, to that on which the illumination is most complete. If the light be homogeneous in its quality, the shaded band will possess no colors of its own ; but if the light be decomposable, like the ordinary solar beam, the band will exhibit prismatic fringes.1 It is obvious that such a diffraction must exist in the rays transmitted through the sub- stance, as well as along the edges, of transparent objects ; and that it must interfere with the perfect distinctness, not merely of their outlines, but of their images, the various markings of which are shadows of portions that afford obstacles, more or 1 This phenomena is explained on the Undulatory Theory of light, by the disturbance which takes place in the onward propagation of waves, when subsidiary centres of undulation are developed by the impact of the principal undulations on obstacles in their course ; the chromatic dispersion being due to the inequality in the lengths of the undulations proper to the severally colored rays. 182 MANAGEMENT OF THE MICROSCOPE. less complete, to the perfectly free transmission of the rays. There are many objects of great delicacy, in which the " diffrac- tion-band" is liable to be mistaken for the indication of an actual substance ; on the other hand, the presence of an actual sub- stance of extreme transparency, may sometimes be doubted or denied, through its being erroneously attributed to the " diffrac- tion-band."1 No rules can be given for the avoidance of such errors, since they can only be escaped by the discriminative power which education and habit confer. The practised Micro- scopist, indeed, almost instinctively makes the requisite allow- ance for diffraction ; and seldom finds himself embarrassed by it, in the interpretation of the visual appearances which he obtains through a good instrument. Besides this unavoidable result of the inflection of the rays of light, there is a peculiar phenomenon attendant upon oblique illumination at certain angles in one direction ; which consists in the production of a double image, or a kind of overlying shadow, sometimes pre- senting markings equally distinct with those of the object itself. This image, which is not unlike the secondary spectrum formed by reflection from the outer surface of a silvered-glass mirror, has been called the " diffracting spectrum ;" but its origin does not really lie in the diffraction of the luminous rays, since on the one hand it cannot be explained according to the laws of diffrac- tion, and on the other it may be traced to an entirely different cause. An object thus illuminated is seen by two different sets of rays ; those, namely, of transmitted light, which pass through it obliquely from the source of the illumination to the opposite side of the object-glass ; and those of radiated light, which, being intercepted by the object, are given off' from it again in all di- rections. (The latter alone are the rays whereby the images are formed in any kind of "black-ground" illumination, §§ 61, 62.) Two different images will be formed, when the illuminating pencil is very oblique, and the angular aperture of the object- glass is wide ; one of them by the light transmitted to one ex- treme of its aperture, the other by the light radiated to its gene- ral surface ; and one or the other of these images may be stopped out, by covering that portion of the lens which receives, or that which does not receive, the transmitted pencil. This " diftract- ing-spectrum" may be produced at pleasure, in an object illu- minated by direct light and seen with a large aperture, by hold- ing a needle or a horse-hair before the front lens, so as to split the aperture into two parts. 97. Errors of interpretation arising from the imperfection of the Focal adjustment, are not at all uncommon amongst young 1 Thus the account given by Prof. Sharpey and the Author, of the structure of Muscu- lar Fibre (Chap. XVIII), has been called in question by observers who had not seen their preparations, on the ground that the " diffraction-band" had not been allowed for. To whatever the appearance in question (Fig. 326) may be due, there cannot be the ^lightest question that it does not arise from diffraction. ERRORS OF INTERPRETATION. 183 Microscopists. With lenses of high power, and especially with those of large angular aperture, it very seldom happens that all the parts of an object, however small and flat it may be, can be in focus together ; and hence the focal adjustment being exactly made for one part, everything that is not in exact focus is not only more or less indistinct, but is often wrongly represented. The indistinctness of outline will sometimes present the appear- ance of a pellucid border, which, like the diffraction-band, may be mistaken for actual substance. But the most common error is that which is produced by the reversal of the lights and shadows, resulting from the refractive powers of the object itself; thus, the bi-concavity of the blood-disks of Human (and other Mammalian) blood, occasions their centres to appear dark, when in the focus of the Microscope, through the dispersion of the light which it occasions ; but when they are brought a little within the focus, by a slight approximation of the object-glass, the centres appear brighter than the peripheral parts of the disks (Fig. 315). The same reversal presents itself in the case of the markings of the Diatomacese ; for these, when the surface is ex- actly in focus, are seen as light hexagonal spaces, separated by dark partitions ; and yet, when the surface is slightly beyond the focus, the hexagonal arese are dark, and the intervening parti- tions light (Fig. 80). The best means of avoiding errors of in- terpretation arising from this source, lies in the employment of the lowest powers with which the particular structures can be distinguished ; since, if the different parts of the surface and margin of the object can be simultaneously brought so nearly into focus that a distinct view may be gained of all of them at once, no false appearances will be produced, and everything will be seen in its real aspect. 98. A very important and very frequent source of error, which sometimes operates even on experienced Microscopists, lies in the refractive influence exerted by certain peculiarities in the form or constitution of objects, upon the rays of light trans- mitted through them ; this influence being of a nature to give rise to appearances in the image, which suggest to the observer an idea of their cause that may be altogether different from the reality. A very characteristic illustration of the fallacy resulting from external configuration, is furnished by the notion which long prevailed among Microscopic observers, and which still lingers in the public mind, of the tubular structure of the Human hair. This notion has no other foundation, than the existence of a bright band down the axis of the hair, which is due to the convergence of the rays of light occasioned by the convexity of its surface, and which is equally shown by any other transparent cylinder; and it is unmistakably disproved by the appearances presented by thin transverse sections of Hair, which show that it is not only filled up to its centre with a medullary substance, but that its centre is sometimes even darker than the surrounding 184 MANAGEMENT OF THE MICROSCOPE. part (Fig. 311). Of the fallacy which may sometimes arise from diversities in the refractive power of the internal parts of an ob- ject, we have an equally "pregnant instance" in the misinter- pretation of the nature of the lacunce and canaliculi of Bone (Fig. 300), which were long supposed to be solid corpuscles with radiating filaments of peculiar opacity, instead of being, as is now universally admitted, minute chambers with diverging pas- sages, excavated in the solid osseous substance. For just as the convexity of its surfaces will cause a transparent cylinder to show a bright axial band, so will the concavity of the internal surfaces of the cavities or tubes hollowed out in the midst of highly refracting substances, occasion a divergence of the rays passing through them, and consequently render them so dark that they are easily mistaken for opaque solids. That such is the case with the so-called "bone-corpuscles," is shown by the effects of the infiltration of Canada balsam through the osseous substance ; for when this fills up the excavations, — being nearly of the same refractive power with the bone itself, and being also quite transparent, and (in thin laminse) quite colorless, — it ob- literates them altogether. So, again, if a person who is unac- customed to the use of the microscope should chance to have his attention directed to a preparation mounted in liquid or in bal- sam, that might chance to contain air-bubbles, he will be almost certain to be so much more strongly impressed by the appear- ance of these, than by that of the object, that his first remark will be upon the number of strange-looking black rings which he sees, and his first inquiry will be in regard to their meaning. 99. No experienced Microscopist could now be led astray by such obvious fallacies as those alluded to ; but it is necessary to dwell upon them, as warnings to those who have still to go through the same education. The best method of learning to appreciate the class of appearances in question, is the comparison of the aspect of globules of Oil in water, with that of globules of Water in oil, or of bubbles of Air in water or Canada-balsam. This comparison may be very readily made by shaking up some oil with water to which a little gum has been added, so as to form an emulsion ; or by simply placing a drop of oil of turpentine and a drop of water together on a slip of glass, laying a thin glass cover upon them, and then moving the cover several times back- wards and forwards upon the slide.1 Now when such a mixture is examined with a sufficiently high magnifying power, all the globules present nearly the same appearance, namely, dark mar- gins with bright centres ; but when the test of alteration of the focus is applied to them, the difference is at once revealed; for whilst the globules of Oil surrounded by water become darker as the object-glass is depressed, and lighter as it is raised, those of 1 If this latter mode be adopted, it is preferable, as suggested by the authors pf the " JMicrographic Dictionary" (Introduction, p. xxxii), to color the oil of turpentine with alkanet, or some similar substance, for its more ready distinction. APPEARANCES OF GLOBULES OF AIR, ETC. 185 Water surrounded by oil become more luminous as the object- glass is depressed, and darker as it is raised. The reason of this lies in the fact, that the high refracting power of the oil causes each of its globules to act like a double-convex lens of very short focus ; and as this will bring the rays which pass through it into convergence above the globule (i. e. between the globule and the objective), its brightest image is given, when the object-glass is removed somewhat further from it than the exact focal distance of the object. On the other hand, the globule of wat^r in oil, or the minute bubble of air in water or balsam, acts, in virtue of its inferior refractive power, like a double-concave lens ; and as the rays of this diverge from a virtual focus below the globule (i. e. between the globule and the mirror), the spot of greatest lumi- nosity will be found, by causing the object-glass to approach within the proper focus. Now in the "protoplasm" of the cells of the lower Plants, and in the " sarcode" of the lower animals, oil-particles and vacuoles (or void spaces) are often interspersed ; and present, at first sight, so very striking a resemblance, that the inexperienced observer may well be pardoned for mistaking the " vacuoles" for larger globules of a material more refractive than the gelatinous substance around them. But the difference in the effects of alterations of focus on the two sets of appear- ances, at once serves to make evident the difference of their causes; and this, moreover, is made obvious by the effect of oblique light, which will cause the strongest shadow to exhibit itself on opposite sides, in the two cases respectively. It will be obvious that minute elevations and depressions of the surface of the object will exert an influence upon the course of the rays which it transmits, very similar to that which proceeds from the presence of globular spaces, filled with transparent substances of greater or less refracting power, in its interior ; and that the dis- crimination between the two may be made by the same' means. For if the dots appear more luminous as the object-glass is raised, and darker as it is depressed, they may be interpreted as being due to convexities upon the surface ; but if the contrary is the case, they may be referred to concavities. 100. Among the sources of fallacy by which the young Micro- scopist is liable to be misled, one of the most curious is the Mole- cular Movement which is exhibited by the particles of nearly all bodies that are sufficiently finely divided, when suspended in water or other fluids. This movement was first observed in the fine granular particles, which exist in great abundance in the contents of the pollen-grains of plants (sometimes termed the fovilla\ and which are set free by crushing these grains ; and it was imagined that they indicated the possession of some special vital endowment of these particles, analogous to that of the sper- matozoa of animals. In the year 1827, however, it was an- nounced by Dr. Bobert Brown, that numerous other substances, organic and inorganic, when reduced to a state of equally minute 186 MANAGEMENT OF THE MICROSCOPE. division, exhibit a like movement, so that it cannot be regarded as indicative of any endowment peculiar to the fovilla-granules ; and subsequent researches have shown, that there is no known ex- ception to the rule, that such motion takes place in the particles of all substances, though some require to be more finely divided than others, before they will exhibit it. Nothing is better adapted to show it, than a minute portion of gamboge, indigo, or carmine, rubbed up with water ; for the particles of these substances, which are not dissolved, but only suspended, are of sufficiently large size to be easily distinguished with a magnifying power of 250 diameters, and are seen to be in perpetual locomotion. Their movement is chiefly of an oscillatory kind ; but they also rotate backwards and forwards upon their axes, and they gradually change their places in the field of viewr. It may be observed that the movement of the smallest particles is the most energetic, and that the largest are quite motionless, whilst those of intermediate size move, but with comparative inertness. The movement is not due (as some have imagined) to evaporation of the liquid ; for it continues, without the least abatement of energy, in a drop of aqueous fluid that is completely surrounded by oil, and is therefore cut off from all possibility of evaporization ; and it has been known to continue for many years, in a small quantity of fluid enclosed between two glasses in an air-tight case. It is, however, greatly accelerated, and rendered more energetic, by Heat ; and this seems to show that it is due, either directly to some calorical changes continually taking place in the fluid, or to some obscure chemical action between the solid particles and the fluid, which is indirectly promoted by heat. It is curious that the closer the conformity between the specific gravity of the solid particles and that of the liquid, the less minute need be that reduction in their size which is a necessary condition of their movement; and it is from this that the substances just named are so favorable for the exhibition of it. On the other hand, the particles of metals, which are from seven to twelve times as heavy as water, require to be reduced to a minuteness many times greater than that of the particles of carmine or gamboge, before they become subject to this curious action. In any case in which the motions of very minute particles, of whatever kind, are in iquestion, it is necessary to make allowance for this "molecular movement;" and the young Microscopist will therefore do well to familiarize himself with its ordinary characters, by the careful observation of it in such cases as those just named, and in any others in which he may meet with it. 101. Comparative Values of Object- Glasses; Test Objects. — In estimating the comparative values of different object-glasses, re- gard must always be had to the purpose for which each is de- signed ; since it is impossible to construct a combination, which shall be equally serviceable for every requirement. It is com- monly assumed, that an Objective which will show certain test- DEFINING AND PENETRATING POWERS. 187 objects, must be very superior, for everything else, to a glass which will not show these ; but this is known to every practical Micro- scopist to be a great mistake, — the very qualities which enable it to resolve the more difficult "tests" being incompatible with those which make it most useful in all the ordinary purposes of scientific investigation. Four distinct attributes have to be specially considered, in judging of the character of an object- glass, viz. : (1) its defining power, or power of giving a clear and distinct image of all well-marked features of an object, especially of its boundaries ; (2) its penetrating power, or power of enabling the observer to look into the structure of objects;1 (3) its resolving power, by which it enables closely-approximated marking to be distinguished ; and (4) the flatness of the field which it gives. I. The "Defining power"of an objective mainly depends upon the perfection of its correctness, both for Spherical and for Chro- matic aberration (§§ 9, 15) ; and it is an attribute essential to the satisfactory performance of any objective, whatever be its other qualities. Good definition may be more easily obtained with lenses of small or moderate, than with lenses of large angular aperture ; and in the aim to extend the aperture, the perfection of the definition is not unfrequently impaired. An experienced Microscopist will judge of the defining power of a lens by the quality of the image which it gives of almost any object with which he may be familiar; but there are certain "tests," to be presently described, which are particularly appropriate for the determination of it. Any imperfection in defining power is ex- aggerated, as already pointed out (§§ 22, 80), by the use of " deep" eye-pieces ; so that, in determining the value of an objective, it is by no means sufficient to estimate its performance under a low eye-piece, an image which appears tolerably clear when mode- rately magnified, being often found exceedingly deficient in sharpness when more highly amplified. The use of the draw- tube (§ 43) affords an additional means of testing the defining power ; but this cannot be fairly had recourse to, unless an altera- tion be made in the adjustment for the thickness of the glass that covers the object (§ 84), in proportion to the lengthening of the body, and the nearer approximation of the object to the ob- jective which this involves. II. The "Penetrating power" of an object-glass (good defini- tion being of course presupposed) mainly depends upon the 1 The Author is aware that he is here employing the term " Penetration" in a sense very different from that which it was intended to convey by Dr. Goring, who first ap- plied it to designate a certain quality of Microscopic objectives. But he considers that what was termed "penetration" by Dr. Goring may be far more appropriately desig-' nated as resolving power • this term having been long in use to express the parallel attri- bute of Telescopes, as regards the separation of the diffused luminosity of Nebulae into distinct points of light. The term Penetration, having been thus set free, may well be applied (as above) in what seems its natural meaning; and the Author (who has long been in the habit of employing it in this sense) may refer to the Report of the Jury of the " Great Exhibition" of 1851, as giving an authoritative sanction to the above use of it 188 MANAGEMENT OF THE MICROSCOPE. degree of distinctness with which parts of the object that are a little out of focus can be discerned ; and' this will be found to vary greatly in different objectives, being, within certain limits, in an inverse proportion to the extent of the angle of aperture. This is very easily understood on optical principles. The central rays of any pencil undergo the least refraction or change in their course ; the peripheral rays, the most . The greater the change, the greater is the difference between the amounts of refraction respectively undergone by rays coming off from points at slightly different distances ; and the greater, when the focal adjustment is correct for one of these points, will be the indistinctness of the image of the other. Hence an objective of comparatively limited aperture may enable the observer to gain a view of the whole of an object, the several parts of whose structure lie at different dis- tances from it, sufficiently good to afford an adequate idea of the relation of those parts to each other; whilst if the same object be looked at with an objective of very wide angle of aperture, which only enables what is precisely in focus to be seen at all, each part can only be separately discerned, and the mutual re- lations of the whole cannot be brought into view. The want of this "penetrating power" is a serious drawback in the perfor- mance of many objectives, which are distinguished by the pos- session of other admirable qualities. The possession of a high measure of it is so essential, in the Author's opinion, to the satisfactory performance of those objectives which are to be em- ployed for the general purposes of scientific investigation, that he cannot consider its deficiency to be compensated by the posses- sion of any degree of the resolving power, whose use is compara- tively limited. III. The " Resolving power," by which very minute markings, — whether lines, striae, or dots, — are discerned and clearly sepa- rated from each other, may be said to stand in direct relation (a perfect definition being presupposed) to the extent of its angle of aperture, and consequently to the obliquity of the rays which it can receive from the several points of the surface of the ob- ject. This is not so much the case, where the markings depend upon the interposition of opaque or semi-opaque particles in the midst of a transparent substance, so that the lights and shadows of the image represent the absolute degrees of greater or less transparency in its several parts ; as it is where, the whole sub- stance being equally transparent, the markings are due to the refracting influence which inequalities of the surface exert upon the course of the rays that pass through it. It may be readily perceived, on a little reflection, that the information given about such inequalities by rays of light transmitted axially through the object, must be very inferior to that which can be gained from rays of light transmitted obliquely ; and thus it happens that, as already explained, many such markings are seen by oblique illumination (as, for instance, by the use of the central stop in RESOLVING POWER — FLATNESS OF FIELD. 189 the condenser, § 56), which could not be seen, under the same object-glass, by light transmitted more nearly in the axis of the microscope. When an object, however, is seen by transmitted light, no degree of obliquity in the illuminating rays can be use- ful, which exceeds that at which the object-glass can receive them: but the illumination of objects which are seen by radiated light (§ 62), depends upon these very rays ; and thus it is that the " black-ground" illumination by the paraboloid or by any other effective contrivance (§ 61), will often bring surface-mark- ings into view, which cannot be seen by transmitted light. An object-glass of very wide aperture, however, will receive, even with ordinary illumination, so many rays of great obliquity, that the same kind of effect will be produced, as by oblique illumina- tion with an objective of smaller aperture ; but when, with such an objective, oblique illumination is used, a greater resolving power is obtained, than any combination of smaller angular aper- ture can possess. In comparing the resolving power of different object-glasses, it is obviously essential to a correct judgment, that the illumination should be the same ; for it will often happen that an observer who knows the "points" of his own instru- ment, will "bring out" tests, which another, with object-glasses of much greater capability, does not resolve, simply for want of proper management. Moreover, it must be borne in mind that great resolving power may exist, even though the definition may be far from exact ; since the former depends more upon angle of aperture, than upon the perfection of the corrections : and yet there cannot be the slightest question, that, of two objectives ot the same focal length, one perfectly corrected up to a moderate angle of aperture, the other with the wider aperture but less per- fectly corrected, the former will be the one most suitable to the general purposes of the Microscopist. IV. The " Flatness of the field" afforded by the object-glass, is a condition of great importance to the advantageous use of the Microscope ; since the real extent of the field of view prac- tically depends upon it. Many objectives are so constructed, that, even with a perfectly flat object, the foci of the central and of the peripheral parts of the field are so different, that when the adjustment is made for one, the other is entirely indistinct. Hence, when the central portion is being looked at, no more in- formation is gained respecting the peripheral, than if it had been altogether "stopped out." With a really good object-glass, not only should the image be distinct even to the margin of the field, but the marginal portion should be as free from chromatic fringes or from indistinctness of outline, as the central portion. In many microscopes of inferior construction, the imperfection of the objectives in this respect, is masked by the contraction of the aperture of the diaphragm in the eye-piece (§ 21), which limits the dimensions of the field ; and the performance of one objective within this limit may scarcely be distinguishable from 190 MANAGEMENT OF THE MICROSCOPE. that of another, although, if the two were compared under an eye-piece of larger aperture, their difference of excellence would be at once made apparent, by the perfect correctness of one to the margin of the field, and by the entire failure of the other in every part save its centre. In estimating the relative merits of two lenses, therefore, as regards this condition, the comparison should of course be made under the same Eye-piece. Y. It may be safely affirmed, that the most perfect Object-glass is that which combines all the preceding attributes, in the highest degree in which they are compatible one with another. But, as has just been shown, two of the most important, namely — pene- trating power and resolving power, — stand in such opposite re- lations to the angular aperture, that the highest degree of which each is in itself capable, can only be attained by some sacrifice of the other; and, therefore, of two objectives which are respec- tively characterized by the predominance of these opposite qualities, one or the other will be preferred by the Microscopist, according to the particular class of researches which he may be carrying on ; just as a man who is about to purchase a horse, will be guided in his choice by the kind of work for which he destines the animal. Hence it shows, in the Author's estimation, just as limited an appreciation of the practical applications of the in- strument, to estimate the merits of an object-glass by its capa- bility of showing certain lined or dotted " tests," without any reference to its penetrating or defining power ; as it would be if a man should estimate the merits of a horse merely by the num- ber of seconds within which he could run a mile, or by the num- ber of pounds he could draw ; without any reference, in the first case, either to the weight he could carry, or to the length of time during which he could maintain his speed ; and in the second case, either to the rate of his draught, or to his power of con- tinuing the exertion. The greatest capacity for speed alone, the power of sustaining it not being required, and burden being reduced almost to nothing, is that which is sought in the Racer ; the greatest power of steady draught, the rate of movement being of comparative little importance, is that which is most valued in the cart-horse; but for the ordinary carriage-horse or roadster, the highest merit lies in such a combination of speed and power with endurance, as cannot coexist with the greatest perfection of either the two first. The Author feels it the more important that he should express himself clearly and strongly on this sub- ject, as there is a great tendency at present, both among amateur Microscopists and among Opticians, to look at the attainment of that " resolving power" which is given by angular aperture, as the one thing needful ; those other attributes which are of far more importance in almost every kind of scientific investigation, being comparatively little thought of; and he therefore ventures here to repeat the remarks he made upon this subject in his re- cent Presidential Address to the Microscopical Society, of the REAL VALUE OF ANGULAR APERTURE. 191 correctness of which he has been since assured, by the approval of many of those who have most successfully employed the Mi- croscope in Physiological investigations. " The superiority in resolving power possessed by object-glasses of large angular aper- ture, is obtained at the expense of other advantages. For even granting that there is no sacrifice of that most important ele- ment defining power (which can only be secured, with a very wide angle, by the utmost perfection in all the corrections), yet the adequate performance of such a lens can only be secured by the greatest exactness in the adjustments. Only that portion of the object which is precisely in focus, can be seen with an ap- proach to distinctness, everything that is in the least degree out of it being imbedded (so to speak) in a thick fog ; it is re- quisite, too, that the adjustment for the thickness of the glass that covers the object, should exactly neutralize the effect of its refraction ; and the arrangement of the mirror and condenser must be such as to give to the object the best possible illumina- tion. If there be any failure in these conditions, the perfor- mance of a lens of very wide angular aperture is very much in- ferior to that of a lens of moderate aperture ; and except in very experienced hands, this is likely to be generally the case. Now to the working Microscopist, unless he be studying the particu- lar classes of objects which expressly require this condition, it is a source of great inconvenience and loss of time to be obliged to be continually making these adjustments ; and a lens, which, when adjusted for a thickness of glass of 1-1 00", will perform without much sensible deterioration with a thickness either of 1-80" or of 1-120", is practically the best for all ordinary pur- poses. Moreover, a lens of moderate aperture has this very great advantage, that the parts of the object which are less perfectly in focus, can be much better seen ; and therefore that the relation of that which is most distinctly discerned, to all the rest of the object, is rendered far more apparent. Let me remind you, fur- ther, that almost all the great achievements of Microscopic re- search have been made by the instrumentality of such objectives as I am recommending. There can be no question about the large proportion of the results which Continental microscopists may claim, in nearly all departments of minute anatomical, physiolo- gical, botanical, or zoological investigation, since the introduc- tion of this invaluable auxiliary ; and it is well known that the great majority of their instruments are of extremely simple con- struction, and that their objectives are generally of very mode- rate angular aperture. Moreover, if we look at the date of some of the principal contributions which this country has furnished to the common stock, such as the < Odontography' of Professor Owen, the ' Eesearches into the Structure of Shell' carried out by Mr. Bowerbank and myself, the < Physiological Anatomy' of Messrs. Todd and Bowman, the first volume of the « Histologi- cal Catalogue,' by Professor Quekett, and the ' British Desmi- I 192 MANAGEMENT OF THE MICROSCOPE. dese' of Mr. Ralfs, we find sure reason to conclude that these re- searches must have been made with the instrumentality of lenses, which would in the present day be regarded as of very limited capacity. I hope that, in these remarks, I shall not be under- stood as in any way desirous to damp the zeal of those, who are applying themselves to the perfectionizing of achromatic objec- tives. I regard it as a fortunate thing for the progress of science, that there are individuals whose tastes lead them to the adoption of this pursuit ; who stimulate our instrument-makers to go on from one range to another, until they have conquered the diffi- culties which previously baffled them ; and then apply themselves to find out some new tests, which shall offer a fresh difficulty to be overcome. But it is not the only, nor can I regard it as the chief work of the Microscope, to resolve the markings upon the Diato- maceee, or tests of the like difficulty ; and although I should con- sider this as the highest object of ambition to our makers, if the performances of such lenses with test-objects were any fair mea- sure of their general utility, yet as I think that I have demon- strated that the very conditions of their construction render them inferior in this respect for the purposes of ordinary microscopic research, I would much rather hold out the reward of high ap- preciation (we have no other to give) to him who should produce the best ivorking microscope, adapted to all ordinary requirements, at the lowest cost. It does not seem to me an unapt simile, to compare the devotees of large angular apertures to the gentle- man of the 'turf.' It is, I believe, generally admitted, that the breeding of a class of horses distinguished by speed and < blood,' which is kept up by the devotion of a certain class of our country- men to the noble sport of racing, is an advantage to almost every breed of horses throughout the country ; tending, as it does, to develope and maintain a high standard in these particu- lars. But no one would ever think of using a race-horse for a roadster or a carriage-horse ; knowing well that the very qualities which most distinguish him as a racer, are incompatible with his suitableness for ordinary work. And so I think that the ' breeders' of first-class Microscopes (if I may so designate them) are doing great service, by showing to what a pitch of per- fection certain "kinds of excellence may be carried, and by thus improving the standard of ordinary instruments ; notwithstand- ing that, for nearly all working purposes, the latter may be prac- tically superior." 102. Test Objects. — It is usual to judge of the optical perfection of a Microscope, by its capacity for exhibiting certain objects, which are regarded as tests of the merits of its object-glasses ; these tests being of various degrees of difficulty ; and that being accounted the best instrument, which shows the most difficult of such tests. Now it must be borne in mind, that only two out of the four qualities which have been just enumerated, — namely, defining power, and resolving power, — can be estimated by any TEST-OBJECTS. 193 of these tests ; and the greater number of them, being objects whose surface is marked by lines, striae, or dots, are tests of re- solving power, and thus of angular aperture only. Hence, as already shown, an objective may show very difficult test-objects, and yet may be very unfit for ordinary use. Moreover, these test-objects are only suitable to object-glasses of very short focus and high magnifying power ; whereas the greater part of the real work of the Microscope is done with objectives of compara- tively low power ; and the enlargement of the angular aperture, which enables even these to resolve (under deep eye-pieces) many objects which were formerly considered adequate tests for higher powers, is by no means an unmixed good. In estimating the value of an object-glass, it should always be considered for what purpose it is intended ; and its merits should be judged of according to the degree in which it fulfils that purpose. We shall therefore consider, what are the attributes proper to the several "powers" of object-glasses, — low, medium, and high; — and what are the objects by its mode of exhibiting which, it may be fairly judged. I. By object-glasses of low power, we may understand any whose focal length is greater than half an inch. The " powers" usually made in this country are of 2 in. or 1J in. focus (these being sometimes made to divide, so as to leave a power of about 3 in. focus), 1 in., and 8-10ths or 2-3ds in. ; and they give a range of amplification of from 12 to 60 diameters with the shallower eye-piece, and of from 18 to 90 diameters with the deeper. These are the objectives which are most used in the examination of opaque objects, and of transparent objects of large size and of comparatively coarse texture ; and the qualities most desirable in them, are a sufficiently large aperture to give a bright image, combined with such accurate definition as to give a clear image, with penetrating power sufficient to prevent any moderate in- equalities of surface from seriously interfering with the distinct- ness of the entire picture, and with perfect flatness of the image when the object itself is flat. For the 2 in. or 1J in. objective, no ground of judgment is better, than the manner in which it shows such an "injected" preparation as the interior of a Frog's lung (Fig. 331) or a portion of the villous coat of the Monkey's intestine (Fig. 328) ; for the aperture ought to be sufficient to give a bright image of such objects, by ordinary daylight, with- out the use of a condensing-lens ; the border of every vessel should be clearly defined, without any thickness or blackness of edge; every part of such an object that comes within the field, should be capable of being made out when the focal adjustment is adapted for any other part ; whilst, by making that adjustment a medium one, the whole should be seen without any marked indistinctness. If the aperture be too small, the image will be dark ; if it be too large, details are brought into view (such as the separateness of the particles of the vermilion injection) which 13 194 MANAGEMENT OF THE MICROSCOPE. it is of no advantage to see, whilst, through the sacrifice of pene- tration, those parts of the object which are brought exactly into focus being seen with over-minuteness, the remainder are en- veloped in a thick fog, through which even their general contour can scarcely be seen to loom ; and if the corrections be imper- fectly made, no line or edge will be seen with perfect sharpness. For defining power, the Author has found the pollen-grains of the Hollyhock, or any other flower of the Mallow kind (Fig. 189), viewed as an opaque object, a very good test; the minute spines with which it is beset, being but dimly seen with any save a good object-glass of these long foci, and being really well exhibited only by adding such power to the eye-piece, as will exaggerate any want of definition on the part of an inferior lens. For flat- ness of field, no test is better than a section of Wood (Fig. 165) or a large Echinus-spine (Fig, 237), under an eye-piece that will give a field of the diameter of from 9 to 12 inches. Such objects ought to be very well shown by the divided lens of 2 in. or 3 in. focus ; but, as its corrections are rendered imperfect by the re- moval of the front pair, its defining power is necessarily impaired, and cannot be made even tolerable, save by such a curtailment of the aperture as detracts from the brightness of the image. The general performance of object-glasses of 1 in. and 8-10ths in. focus, may be partly judged of by the manner in which they show such injections as those of the Gill of the Eel (Fig. 330) or of the Bird's Lung (Fig. 332), which require a higher magnifying power for their resolution than those previously named; still better, perhaps, by the mode in which they exhibit a portion of the wing of some Lepidopterous insect, having well-marked scales ; the same qualities should here be looked for, as in the case of the lowest powers ; and a want of either of them is to be distinguished in a similar manner. The increase of angular aperture which these lenses may advantageously receive, should render them capable of resolving all the easier "test" scales of Lepidoptera, such as those of the Morpho menelaus (Fig. 279), in which, with the deeper eye-piece, they should show the trans- verse as well as the longitudinal markings. The tongue of the common Fly (Fig. 287) is one of the best transparent objects for enabling a practised eye to estimate the general performance of object-glasses of these powers; since it is only under a really good lens, that all the details of its structure can be clearly made out ; and an objective which shows this well, may be trusted to for any other object of its kind. For flatness of field, sections of small Echinus- spines are very good tests. The exactness of the corrections in lenses of these foci, may be judged of by the examination of objects which are almost sure to exhibit color, if the correction be otherwise than perfect ; this is the case, for example, with the glandulae of Coniferous wood (Fig. 161), the centres of which ought to be clearly defined under such objec- tives, and ought to be quite free from color ; and also with the TEST-OBJECTS FOB, LOW AND MEDIUM POWER. 195 tracheae of Insects (Fig. 291), the spires of which ought to be distinctly separated from each other, without any appearance of intervening chromatic fringes. II. We may consider as object-glasses of medium power, those which range from half to one-fifth of an inch focus; whose magnifying power is from about 100 to 250 diameters under the shallower eye-piece, and from about 150 to 375 diameters with the deeper. These cannot be advantageously employed in the examination of opaque objects, save of such as are of unusual minuteness ; but their great value lies in the information they enable us to obtain, regarding the details of organized structures and of living actions, by the examination of properly prepared transparent objects by transmitted light. It is to these lenses, that the remarks already made respecting angular aperture (§ 101) especially apply ; since it is in them that the greatest difference exists, between the ordinary requirements of the scientific in- vestigator, and the special needs of those who devote themselves to the particular classes of objects for which the greatest resolv- ing power is required. A moderate amount of such power is essential to the value of every objective within the above-named range of foci ; thus, even a good half-inch should enable the markings of the larger scales of the Polyommatus argus (azure- blue butterfly) to be distinguished, these being of the same kind with those of the Menelaus, but more delicate, and should clearly separate the dots of the small or "battledoor" scales (Fig. 280) of the same insect, which, if unresolved, 'are seen as coarse longitu- dinal lines; a good 4-10ths in. should resolve the larger scales of the Podura (Fig. 281) without difficulty; and a good l-4th or l-5th in. should bring out the markings on the smaller scales of the Podura, and should resolve the markings on the Pleurosigma hippocampus into longitudinal and transverse lines. Even the 4-10ths (a power for which Messrs. Smith and Beck have attained a deserved celebrity) may be made with an angle of aperture sufficiently wide to resolve the objects named as fair tests for the powers above it ; and so the l-4th inch may, by the enlargement of its angular aperture to 120° (which has been accomplished by Mr. Ross) be made to exhibit the more difficult Diatomacese. But it will be found that, in such object-glasses, the difficulty of making the most advantageous use of them, and the loss of penetrating power which necessarily attends the excessive exten- sion of their angular aperture, are most serious drawbacks to their practical utility in the hands of the Anatomical or Physio- logical investigator ; for whose purposes, such a resolving power as will show the easier tests first enumerated, combined with perfect definition, with a fair amount of penetrating power, and with flatness of field, constitute the best combination. For de- fining power, very good tests are found in the complex hairs of many animals, such as the Indian Bat (Fig. 310, c), and the Dermestes (Fig. 282, B). And for that combination of the seve- 196 MANAGEMENT OF THE MICROSCOPE. ral attributes which the Author thinks most important, he lias found no test more valuable and positive, as regards objectives of from 4-10ths to l-5th inch focus, than Mr. Lealand's prepara- tions of Muscular fibre (Fig. 326). In every case, the objective should be tested with the deeper, as well as with the shallower eye-piece ; and the effect of this will be a fair test of its merits. Where markings are indistinguishable under a certain objective, merely because of their minuteness or their too close approxima- tion, they may be enlarged or separated by a deeper eye-piece, provided that the objective be well corrected. But if, in such a case, the image be darkened or blurred, so as to be rather deteri- orated than improved, it may be concluded that the objective is of inferior quality, having either an insufficient angular aperture, or being imperfectly corrected, or both. ' III. All object-glasses of less than l-5th of an inch focus, may be classed as high powers ; the focal lengths to which they are ordinarily constructed are l-6th, l-8th, l-12th, and l-16th of an inch respectively ; and the magnifying powers they are fitted to afford, range from about 320 to 850 diameters with the shallower eye-piece, and from 480 to 1300 diameters with the deeper. By the use of still deeper eye -pieces, a power of 2000 or more may be easily obtained ; but nothing seems to be really gained by such high amplification. Moreover, as the 1-1 2th inch object- glass may have its angular aperture extended to the utmost limits compatible with the reception of rays from any object, it does not seem that anything can be gained by a reduction of the focal distance to the l-16th inch ; and the latter being a more difficult combination, as well to construct as to use, both Opticians and Microscopists have of late years found it advantageous to limit themselves to the l-12th, which gives an amplification of about 650 diameters with the shallower eye-piece, and of about 1000 with the deeper. The use of this class of objectives is much more- restricted than that of the preceding. They are not em- ployed for the ordinary purposes of scientific investigation ; arid their value chiefly lies in the power which they afford, of tracing out certain points of minute structure, which the objectives ot medium power may only doubtfully indicate, and of exhibiting certain classes of very difficult striated or dotted objects, which they cannot resolve. Hence it is obvious that, with regard to object-glasses of this class, "resolving power" (coupled with " de- fining power") is the highest requisite, " penetration" and "flat- ness of field" being of secondary account; and that the value of an objective may here be fairly estimated by its angular aperture, provided that its aberrations be exactly corrected. Of angular aperture and definition, very good tests are afforded by the lines artificially ruled by M. Robert, and by the more "difficult" species of Diatomacese. What is known as "Robert's Test" is a plate of glass, on a small space of which, not exceeding a fiftieth of an inch in breadth, are ruled ten or more series of lines, form- TEST-OBJECTS FOR HIGH POWERS. 197 ing as many separate bands of equal breadth ; in each of these bands, the lines are ruled at a certain known distance ; and the distances are so adjusted in the successive bands, as to form a regularly diminishing series, and thus to present a succession of tests of progressively increasing difficulty. The distances of the lines differ on different plates ; all the bands in some series being resolvable under a good objective of l-4th inch focus, whilst the closest bands in others defy the resolving power of a l-12th inch objective of large aperture. Thus a " test-plate" whose widest lines are at a distance from each other of l-1000th of a Paris line, or of 1-11, 200th of an English inch, and whose closest lines are at l-5000th of a line, or l-56,000th of an inch, from each other, will serve as a very fair test for the angular aperture and defining power of object-glasses below l-4th inch focus ; the superiority of each in these particulars, being judged of by the number of bands which it will resolve into well-defined lines, and by the sharpness and clearness of these lines; while the performance of a l-4th inch objective may be accounted very satisfactory, if it will enable them all to be clearly distinguished. But if the first of the bands should have an interval of only l-4000th of a Paris line, or l-45,000th of an English inch, between its lines, and the last should have its lines approximated to l-10,000th of a Paris line, or l-112,000th of an English inch, then only a few of the easier bands will be resolved by the l-4th inch, a few more by the l-8th inch, and even the l-12th inch will probably not enable any band to be distinctly resolved, whose lines are closer than l-7000th of a Paris line, or l-79,000th of an English inch. At present, therefore, the existence of separate lines of a narrower interval than this, is a matter of faith rather than of sight; but there can be no reasonable doubt that the lines do exist ; and the resolution of them wrould evince the extraordinary superiority of any objective, or of any system of illumination, which should enable them to be distinguished. The mathematical certainty with which the degree of approximation of these lines may be ascertained, and the gradation of the series which they present, gives to M. Robert's test-plate a very high value for the determi- nation of the relative merits of different objectives, of that class, at least, in which angular aperture and definition are of the first importance ; whilst it also serves to test the degree in which these capabilities are possessed by object-glasses of medium power, in which other attributes also have to be considered. The value of the minuter Diatomacece, as furnishing, in their sur- face-markings, admirable test-objects for the highest powers of the Microscope, was first made known by Messrs. Harrison and Sollitt, of Hull, in 1841 ; and it cannot be questioned that this discovery has largely contributed to the success of the endeavors which have since been so effectually made, to perfect this class of objectives, and to find out new methods of using them to the best advantage. The nature of these markings will be described 198 MANAGEMENT OF THE MICROSCOPE. hereafter ; and it will be sufficient in this place to give a table of the average distances of the lineation of different species,1 which will serve to indicate their respective degrees of difficulty as " tests." The greater part of those which are now in use for this purpose, are comprehended in the genus Pleurosigma of Prof. W. Smith, which includes those Naviculoe whose "frustules" are dis- tinguished by their sigmoid (S-like) curvature (§ 184). 1. Pleurosigma littorale, 2. Pleurosigma Hippocampus, 3. Pleurosigma strigile, 4. Pleurosigma strigosuru, 5. Pleurosigma elongatum, 6. Pleurosigma angulatum, Y. Pleurosigma Spenceri, 8. Pleurosigma fasciola, 9. Pleurosigma obscurum, 10. Pleurosigma macrum, 11. Nitzschia sigmoidea. 12. Navicula rhomboid es, Lines in l-1000th of an inch. 24 30 long., 40 trans. 36 44 48 52 55 long., 50 trans. 64 75 85 85 85 The first seven of the foregoing may be resolved, with judi- cious management, by good l-4th or l-5th in. objectives; the remainder require the l-8th or l-12th in., for the satisfactory ex- hibition of their markings. Several very difficult tests of this description have been furnished by Prof. Bailey of West Point (U. S.), among them the very beautiful Grrammatophora subtilis- sima and the Hyalodiscus subtilis; the latter, being of discoid form, and having markings which radiate in all directions, very much like those of an engine-turned watch, is a useful test for observers who have not facilities for obtaining oblique light in any direction ; since, whatever may be the azimuth from which the oblique pencil may proceed, some portion of the disk will always 1 This table is taken from Prof. W. Smith's admirable Monograph on the Diatomaceas; and it includes most of the species usually employed as tests. These should always be mounted between two pieces of thin glass, according to the method hereafter to be described (§ 122), in order to avoid, as much as possible, the production of aberrations in the illuminating pencil. The number of lineations must be considered as an average, the extremes sometimes varying to a considerable amount on either side. A much higher estimate is given by Messrs. Harrison and Sollitt in the " Quart. Journ. of Mi- crosc. Science," vol. ii, p. 62 ; the Pleurosigma fasciola being reckoned by them to contain 90 lines in 1-1 000th of an inch, the Nitzschia sigmoidea 100 lines, and a species cited as Navicula arcus (which can scarcely be the one so named by Ehrenberg, and termed by Prof. W. Smith Eunotia arcus) no less than 130. The last they speak of as "so ex- tremely difficult, that, in order even to catch a glimpse of its delicate markings, the observer must be in possession of glasses of a very large angle of aperture and the finest definition, have the most careful management of oblique light, and in addition be pos- sessed of a large share of patience." The Author cannot but believe that there is some error in these measurements; since, as the well-defined lines upon Nobert's test-plate have not yet been resolved, when they have approximated more closely than the highest numbers mentioned in Prof. W. Smith's table, it can scarcely be imagined possible that the delicate markings of a Navicula should even be "glimpsed," if they be as much closer than those of the species previously accounted most difficult, as those of the latter are than those of the easiest. DETERMINATION OF MAGNIFYING POWER. 199 be in the best possible position in regard to the light, whereas, in the case of other finely-lined tests, it is only when the most favorable position has been attained, perhaps after tedious and troublesome trials, that the markings are displayed.1 103. Determination of Magnifying Power. — The last subject to be here adverted to, is the mode of estimating the magnifying power of Microscopes, or, in other words, the number of times that any object is magnified. This will of course depend upon a comparison of the real size of the object, with the apparent size of the image ; but our estimate of the latter will depend upon the distance at which we assume it to be seen, since, if it be projected at different distances from the eye, it will present very different dimensions. Opticians generally, however, have agreed to consider ten inches as the standard of comparison ; and when, therefore, an object is said to be magnified 100 diameters, it is meant that its visual image, projected at 10 inches from the eye (as when thrown down by the Camera Lucida, § 49) upon a surface at that distance beneath, has 100 times the actual dimen- sions of the object. The measurement of the magnifying power of Simple or Compound Microscopes by this standard is attended with no difficulty. All that is required is a stage-micrometer accurately divided to a small fraction of an inch (the l-100th will answer very well for low powers, the l-1000th for high), and a common foot-rule divided to tenths of an inch. The micro- meter being adjusted to the focus of the objective, the rule is held parallel with it, at the distance of ten inches from the eye. If the second eye be then opened, whilst the other is looking at the object, the circle of light included within the field of view, and the object itself, will be seen faintly projected upon the rule ; and it will be very easy to mark upon the latter the appa- rent distances of the divisions on the micrometer, and thence to ascertain the magnifying power. Thus, supposing each of the divisions of l-100th of an inch to correspond with 1J inch upon the rule, the linear magnifying power is 150 diameters; if it cor- respond with half an inch, the magnifying power would be 50 diameters. If, again, each of the divisions of the l-1000th inch micrometer correspond to 6-10ths of an inch upon the rule, the magnifying power is 600 diameters ; and if it correspond to 1T2^ inch, the magnifying power is 1200 diameters. In this mode of measurement, the estimate of parts of tenths on the rule can only be made by guess ; but greater accuracy may be obtained by projecting the micrometer- scale with the Camera Lucida at the distance of ten inches from the eye, marking the intervals on paper, taking an average of these, and repeating this with the compasses ten times along the inch-scale. Thus, if the space given by one of the divisions of the l-1000th-inch micrometer, 1 See Prof. Bailey's interesting memoirs in Vols. II and VII of the " Smithsonian Con- tributions to Knowledge." 200 MANAGEMENT OF THE MICROSCOPE. repeated ten times along the rule, gave 6 inches and 2J tenths, the value of each division would be '625 of an inch, and the magnifying power 625. The superficial magnifying power is ot course estimated by squaring the linear ; but this is a mode of statement never adopted by scientific observers, although often employed to excite popular admiration, or to attract customers, by those whose interest is concerned in doing so.1 1 An ingenious method has been devised by Prof. Harting, of Utrecht, for determining " the utmost limits of penetrating and separating power possessed by a Microscope,'' by using as test-objects the very reduced images of various bodies formed by air-bubbles in gum-mucilage. The mode of obtaining and employing these images for the above purpose, will be found in the " Quarterly Journal of Microscopical Science," vol i, p. 292. CHAPTER V. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. UNDER this head it is intended to give such general directions respecting the preparation, mounting, and collection of Objects, as will supersede the necessity of frequent repetition when each particular class is described ; and also to enumerate the materials and appliances, which will be required or found advantageous. SECTION 1. PREPARATION OF OBJECTS. 104. Microscopic Dissection. — The separation of the different parts of an Animal or Vegetable structure by dissection, so as to prepare any portion for being minutely examined under the Microscope should be accomplished, so far as may be found practicable, with the naked eye ; but the best mode of doing this, will depend in great degree upon the size and character of the object. Generally speaking, it will be found advantageous to carry on the dissection under water, with which alcohol should be mingled where the substance has been long immersed in spirit. The size and depth of the vessel should be proportioned to the dimensions of the object to be dissected ; since, for the ready access of the hands and dissecting instruments, it is con- venient that the object should neither be far from its walls, nor lie under any great depth of water. Where there is no occasion that the bottom of the vessel should be transparent, no kind of dissecting-trough is more convenient, than that which every one may readily make for himself, of any dimensions he may desire, by taking a piece of sheet gutta percha of adequate size and stoutness, warming it sufficiently to render it flexible, and then turning up its four sides, drawing out each corner into a sort of spout, which serves to pour away its contents when it needs emptying. The dark color of this substance enables it to furnish a background, which assists the observer in distinguishing deli- cate membranes, fibres, &c., especially when magnifying lenses are employed ; and it is hard enough, without being too hard, to allow of pins being fixed into it, both for securing the object, and for keeping apart such portions as it is useful to put on the stretch. When glass or earthenware troughs are employed, a piece of sheet-cork, loaded with lead, must be provided, to an- 202 PREPARATION OF OBJECTS. swer the same puposes. In carrying on dissections in such a trough, it is frequently desirable to concentrate additional light upon the part which is being operated on, by means of the smaller condensing lens (Fig. 45) ; and when magnifying power is wanted, it may be supplied either by a single lens, mounted after the manner of Boss's Simple Microscope (Fig. 14, B), or by a Compound body mounted as in one of Mr. Warington's ar- rangements (Fig. 24). Portions of the body under dissection, being floated off when detached, may be conveniently taken up from the trough by placing a slip of glass beneath them (which is often the only mode in which delicate membranes can be satis- factorily spread out) ; and may be then placed under the micro- scope for minute examination, being first covered with thin glass, beneath the edges of which is to be introduced a little of the liquid wherein the dissection is being carried on. Where the body under dissection is so transparent, that more advantage is gained by transmitting light through it, than by looking at it as an opaque object, the trough should have a glass bottom; and for this purpose, unless the body be of unusual size, some of the glass "cells" to be hereafter described (§§ 136, 137) will usually answer very well. The finest dissections may often be best made upon ordinary slips of glass ; care being taken to keep the object sufficiently surrounded by fluid. For work of this kind, no simple instrument is more generally serviceable than Mr. Quekett's Dissecting Microscope (Fig. 17) ; but if higher magni- fying powers be needed than this will conveniently afford, re- course may be had to Smith and Beck's Dissecting Microscope (Fig. 29), which for this purpose should always be furnished with the Erector (Fig. 32). A particular arrangement of the light, devised many years since by the Author, will enable an expert dissector to prosecute his work with the naked eye, to an extent for which a lens would otherwise be required. This consists in giving to the object the same kind of black-ground illumination, as is now in common use for a very different purpose ; and nothing more is necessary to afford it, than to attach to the under side of the stage a sort of " well," composed of a tube blackened in its interior, about 1J inch long, of the same diameter as the opening of the stage-plate, into the lower extremity of which a diaphragm or a ground-glass may be fitted, for the purpose of diminishing or of softening the light. The slide being laid upon the stage, and the mirror being so turned as to illuminate the object, the eye is to be so placed (the arm carrying the magnifiers being turned to one side) that the object is seen against the dark background afforded by the side of the well. In this manner, fibres of extreme minuteness, or other particles of extraordinary delicacy, can be clearly distinguished, such as could otherwise be scarcely discerned at all without the assistance of a magnifier. And the further the dissection can be carried in this mode, the less difficulty will be found in completing it, when the simple or INSTRUMENTS FOR MICROSCOPIC DISSECTION. 203 compound Microscope is brought to bear upon it. Whenever a dissection is being made upon the stage of a microscope, it is desirable that support should be provided for the hands on either side. This may be given by books or blocks of wood piled up to the requisite height ; but in place of flat "rests," it is much more convenient to provide a pair of inclined planes, sloping away from the stage at an angle of about 30° below the horizon. These may be either solid blocks of wood, or (which is much less cumbrous) they may be made of two boards hinged together, one giving the inclined plane, which rests at one end upon the table, while the other, standing vertically, affords the requisite elevation to the extremity which abuts against the stage. 105. The instruments used in Microscopic 'dissection, are for the most part of the same kind as those which are needed in ordinary minute Anatomical research, such as scalpels, scissors, forceps, &c. ; the fine instruments used in operations upon the eye, however, will commonly be found most suitable. A pair of delicate scissors curved to one side, is extremely convenient for cutting open tubular parts ; these should have their points blunted ; but other scissors should have fine points. A pair of fine-pointed scissors (Fig. 56), one leg of which is fixed in alight handle, and the other kept FJG 56 apart from it by a spring, so as to close by the pressure of the finger and to open of it- self, will be found (if the blades be well sharpened on spring-scissors. a hone) much superior to any kind of knives, for cutting through delicate tissues with as little disturbance of them as possible : Swammerdam is said to have made great use of this instrument in his elaborate insect-dissec- tions. Another cutting instrument much used by some dissectors, may be designated as a miniature of the shears used in shearing sheep, or as a cutting-forceps ; the blades of such an instrument may be prevented from springing too far asunder, by means of a regulating-screw (as in the " microtome" of M. Strauss-Durck- heim) or by some other kind of check ; and the cutting action, being executed by the opposed pressure of the finger and thumb, may be performed with great precision. A pair of small straight forceps, with fine points, and another pair of curved forceps, will be found useful in addition to the ordinary dissecting-forceps. Of all the instruments contrived for delicate dissections, how- ever, none are more serviceable than those which the Microsco- pist may make for himself out of ordinary needles. These should be fixed in light wooden handles1 (the cedar sticks used for 1 Special needle-holders (like miniature port-crayons) have been made for this pur- pose; and although they afford the facility of lengthening or shortening the acting point of the needle at will, and also of carrying a reserve store of needles at the other end, yet the Author would decidedly recommend the use of the wooden handles, of which a large stock may be obtained for a trifle. 204 PREPARATION OP OBJECTS. camel-hair pencils, OB the handles of steel-pen-holders, will answer extremely well), in such a manner that their points should not project far,1 since they will otherwise have too much "spring:" much may be done by their mere tearing action; but if it be desired to use them as cutting instruments, all that is ne- cessary is to give them an edge upon a hone. It will sometimes be desirable to give a finer point to such needles, than they ori- ginally possess ; this also may be done upon a hone. A needle with its point bent to a right angle, or nearly so, is often useful ; and this may be shaped by simply heating the point in a lamp or candle, giving to it the required turn with a pair of pliers, and then hardening the point again by reheating it and plunging it into cold water or -tallow. 106. Cutting Sections of Soft Substances. — Most important in- formation respecting the structure of many substances, both Animal and Vegetable, may be obtained by cutting sections of them, thin enough to be viewed as transparent FIG. 57. objects. Where the substances are soft, no other instrument is necessary for this purpose, than a sharp knife, which may be best made with a thin two-edged blade like that of a lancet ; conside- rable practice is needed, however, to make effec- tual use of it ; and some individuals acquire a degree of dexterity, which others never succeed in attaining. In cutting sections of Animal tis- sues, which, owing to the quantity of water they contain, do not present a sufficiently firm resis- tance, it is often desirable to half-dry these, by exposing small pieces freely to the air, with the aid of a gentle warmth if required ; when this desiccating process has been carried sufficiently Sr. far, thinner sections can be cut, than could pos- sibly have been made in the original state of the tissue ; and the texture, after a short maceration in water, almost entirely recovers its pristine characters. There are certain tis- sues, however, which will not bear to be thus treated, and of which it is sufficient to examine an extremely minute portion ; and for making sections of these, such a pair of scissors as is re- presented in Fig. 57 will often be found very useful; since, owing to the curvature of the blades,2 the two extremities of a 1 The following is the mode in which the Author has found it convenient to mount his needles for this and other purposes: — The needle being held firmly in a pair of pliers grasped by the right hand, its point may be forced into the end of a cedar or other stick held in the left, until it has entered to the depth of half an inch or more; the needle is then cut off to the desired length (the eye end being thus got rid of) ; and being then drawn out, the truncated end is forced into the hole previously made by the point, until it cannot be made to penetrate further, when it will be found to be very se- curely fixed. The end of the handle which embraces it, may then be bevelled away round its point of insertion. 2 It is difficult to convey by a drawing the idea of the real curvature of this instru- ment, the blades of which, when it is held in front view, curve — not to either side — CUTTING THIN SECTIONS. 205 section taken from a flat surface will generally be found to .thin away, although the middle of it may he too thick to exhibit any structure. Where only a moderate degree of thinness is re- quired, either in consequence of the transparence of the tissue, or because it is not desired to exhibit its minutest details, the two-bladed knife contrived by Prof. Valentin (Fig. 58) may be employed with advantage. The blades are attached to each other at their lower end by a screw, in such a manner that their " spring." tends to keep them apart; and their distance is regu- lated by pushing the little rivet backwards or forwards in the FIG. 58. Valentin's Knife. slit through which it works. The knife should be dipped in water before using, or, still better, the section should be made under water, as the instrument works much better when wet ; after use, it should be carefully washed and dried, a piece of soft leather being passed between the blades. If any water have found its way into the part through which the rivet works, the movable blade should be detached by taking out its screw, and each blade should be cleaned separately.1 107. Cutting Sections of Harder Substances. — There is a large class of substances, both Animal and Vegetable, which are too hard to admit of sections being made in the manner just de- scribed, but of which extremely thin slices can be made by a sharp cutting instrument, if only they be properly held and sup- ported, more especially when the thickness of the section can be regulated by a mechanical contrivance ; such are, in particular, the Stems and Roots of Plants, and the Horns, Hoofs, Cartilages, and similarly firm structures of Animals. Various costly ma- chines have been devised for this purpose, some of them charac- terized by great ingenuity of contrivance and beauty of work- manship ; but every purpose to which these are adapted, will be found to be answered by a very simple and unexpensive little instrument, which may either be held in the hand, or (which is preferable) may be firmly attached by means of a T-shaped piece of wood (as in Fig. 59), to the end of a table or work-bench. but towards the observer ; these scissors being, as the French instrument-makers say, courbes sur le plat. As an example of the utility of such an instrument to the Micro- scopist, the Author may cite the curious demonstration given a few years since, by Dr. Aug. Waller, of the structure of the gustative papillae, by snipping oft" the papillae from the living human tongue, which may be done with no more pain than the prick of a pin would occasion. 1 An improved form of this instrument is constructed by Mr. Matthews of Portugal Street ; the blades being made with a convex instead of a straight edge, their distance from each other being regulated by a milled -head screw, and their separation for clean- ing being more easily accomplished. 206 PREPARATION OF OBJECTS. This instrument essentially consists of an upright hollow cylinder of brass, with a kind of piston FlG- 59« which is pushed from below upwards by a fine-threaded screw, turned by a large milled head; at the upper end, the cylinder terminates in a brass table, which is made to present a perfectly flat surface. At one side is seen a small milled head, which acts upon a "binding- screw," whose extremity pro- jects into the cavity of the cylin- der, and serves to compress and steady anything that it holds. A cylindrical stem of wood, a section instrument. piece of horn, whalebone, carti- lage, &c., is to be fitted to the interior of the cylinder, so as to project a little above its top, and is to be steadied by the " binding-screw ;" it is then to be cut to a level by means of a sharp knife or razor, laid flat upon the table. The milled head is next to be moved through such a por- tion of a turn, as may very slightly elevate the substance to be cut, so as to make it project in an almost insensible degree above the table ; and this projecting part is to be sliced off with a knife, previously dipped in water. The best knife for this purpose is a razor, ground flat (instead of concave) on one side, but having still a concave surface on the other ; the flat side is to be laid downwards upon the table ; and the motion given to the edge should be a combination of drawing and pressing. (It will be generally found that better sections are made, by working the knife from the operator, than towards him.) When one slice has been thus taken off, it should be removed from the blade by dipping it into water, or by the use of a camel-hair brush ; the milled head should be again advanced, and another section taken ; and so on. Different substances will be found both to bear and to require different degrees of thickness ; and the amount that suits each can only be found by trial. It is advantageous to have the large milled head graduated, and furnished with a fixed index ; so that this amount having been once determined, the screw shall be so turned as to always produce the exact elevation required. Where the substance of which it is desired to obtain sections by this instrument, is of too small a size or of too soft a texture to be held firmly in the manner just described, it may be placed between the two vertical halves of a cork of suitable size to be pressed into the cylinder; and the cork, with the object it grasps, is then to be sliced in the manner already described, the small section of the latter being carefully taken oft the knife, or floated away from it, on each occasion, to prevent it from being CUTTING AND GRINDING THIN SECTIONS. 207 lost among the lamellae of cork which are removed at the same time. The special methods of preparation which are re- quired in the case of the various substances, of which sections may be conveniently cut by this instrument, will be described under their several heads. 108. Grinding and Polishing of Sections. — Substances which are too hard to be sliced with a cutting instrument in the manner last described, — such as bones, teeth, shells, corals, fossils of all kinds, and even some recent vegetable tissues, — can only be re- duced to the requisite thinness for Microscopical examination, by grinding down thick sections, until they become so thin as to be transparent. The general method of making such pre- parations will be here described;1 but those special details of management which particular substances may require, will be given when these substances are respectively described. The first thing to be done, will usually be to procure a section of the substance, as thin as it can be safely cut. Most substances not siliceous may be divided by the fine saws used by artisans for cutting brass ; but there are some bodies (such as the enamel of teeth, and porcellanous shells), which, though merely calcareous, have their mineral particles arranged in such a peculiar state of aggregation, as to make it very difficult and tedious to divide them in this mode ; and it is much the quicker operation to slit them with a disk of soft iron (resembling that used by the lapi- dary) charged at its edge with diamond-dust, which may be driven in an ordinary lathe. Where waste of material is of no account, a very expeditious method of obtaining pieces fit to grind down, is to detach them from the mass with a strong pair of " cutting-pincers," or, if it be of small dimensions, with " cut- ting-pliers;" and a flat surface must then be given to it, either by holding it to the side of an ordinary grindstone, or by rubbing it on a plate of lead (cast or planed to a perfect level) charged with emery, or by a strong toothed file, the former being the most suitable for the hardest substances, the latter for the toughest. There are certain substances, especially calcareous fossils of wood, bone, and teeth, in which the greatest care is required in the performance of these preliminary operations, on account of their extreme friability ; the vibration produced by the working of the saw or the file, or by grinding on a rough surface, being sufficient to disintegrate even a thick mass, so that it falls to pieces under the hand ; such specimens, therefore, it is requisite to treat with great caution, dividing them by the smooth action of the wheel, and then rubbing them down upon nothing rougher than a very fine ^ grit." Where (as often happens) such speci- mens are sufficiently porous to admit of the penetration of Canada balsam, it will be desirable, after soaking them in tur- 1 The following directions do not apply to Siliceous substances; as sections of these can only be prepared by those who possess a regular Lapidary's apparatus, and who have been specially instructed in the use of it. 208 PREPARATION OF OBJECTS. pentine for a while, to lay some liquid balsam upon the parts through which the section is to pass, and then to place the speci- men before the fire or in an oven for some little time, so as first to cause the balsam to run in, and then to harden it ; by this means the specimen will be rendered much more fit for the processes it has afterwards to undergo. It not unfrequehtly happens, that the small size, awkward shape, or extreme hardness of the body, occasions a difficulty in holding it either for cutting or grinding; in such a case, it is much better to attach it to the glass in the first instance, by any side that happens to be flattest ; and then to rub it down by means of the "hold" of the glass upon it, until the projecting portion has been brought to a plane, and has been prepared for permanent attachment to the glass. This is the method which is generally most convenient to pursue with re- gard to small bodies ; and there are many which can scarcely be treated in any other way, than by attaching a number of them to the glass at once, in such a manner as to make them mutually support one another.1 109. The mode in which the operation is then to be proceeded with, depends upon whether the section is to be ultimately set up in Canada balsam (§ 125), or is to be mounted dry (§ 122), or in fluid (§ 132). In the former case, the following is the plan to be pursued. The flattened surface is to be polished, by rub- bing it with water on a " Water-of-Ayr" stone, on a hone or " Turkey" stone, or on a new stone recently introduced under the name of the "Arkansas" stone; the first of the three is the best for all ordinary purposes ; but the two latter, being much harder, may be employed for substances which resist it.2 When this has been sufficiently accomplished, the section is to be at- tached with Canada balsam to a slip of thick, well-annealed glass ; and, as the success of the final result will often depend upon the completeness of its adhesion to this, the means of most effec- tually securing that adhesion will now be described in detail. Some Canada balsam, previously rendered somewhat stiff by the evaporation of part of its turpentine, is to be melted on the glass slip, so as to form a thick drop, covering a space somewhat larger than the area of the section ; and it should then be set aside to cool, during which process, the bubbles that may have formed in it will usually burst. When cold, its hardness should be tested, which is best done by the edge of the thumb-nail ; for it should be with difficulty indented by its pressure, and yet should not be 1 Thus, in making horizontal and vertical sections of Foraminifera, as it would be im- possible to cut them through, they must be laid close together in a bed of hardened Canada balsam on a slip of glass, in such positions, that, when rubbed down, the plane of section shall traverse them in the desired directions; and one flat surface having been thus obtained for each, this must be turned downwards, and the other side ground away. 2 As the flatness of the polished surface is a matter of the first importance, that of the stones themselves should be tested from time to time ; and whenever they are found to have been rubbed down on any one part more than on another, they should be flat- tened on a paving-stone with fine sand, or on the lead-plate with emery. GRINDING AND POLISHING THIN SECTIONS. 209 so resinous as to be brittle. If it be too soft, as indicated by its too ready yielding to the thumb nail, it should be boiled a little more; if too hard, which will be shown by its chipping, it should be re-melted and diluted with more fluid balsam, and then set aside to cool as before. When it is found to be of the right con- sistence, the section should be laid upon its surface, with the polished side downwards ; the slip of glass is next to be gradu- ally warmed until the balsam is softened, special care being taken to avoid the formation of bubbles ; and the section is then to be gently pressed down upon the liquefied balsam, the pressure being at first applied rather on one side than over its whole area, so as to drive the superfluous balsam in a sort of wave towards the other side, and an equable pressure being finally made over the whole. If this be carefully done, even a very large section may be attached to glass, without the intervention of any air- bubbles ; if, however, they should present themselves, and they cannot be expelled by increasing the pressure over the part be- neath which they are, or by slightly shifting the section from side to side, it is better to take the section entirely off, to melt a little fresh balsam upon the glass, and then to lay the section upon it as before. 110. When the section has been thus secured to the glass, and the attached part thoroughly saturated (if it be porous) with hard Canada balsam, it may be readily reduced in thickness, either by grinding or filing as before, or, if the thickness be excessive, by taking oft' the chief part of it at once by the slitting-wheel. So soon, however, as it approaches the thinness of a piece of ordi- nary card, it should be rubbed down with water on one of the smooth stones previously named, the glass slip being held be- neath the fingers with its face downwards, and the pressure being applied with such equality, that the thickness of the section shall be (as nearly as can be discerned) equal over its entire surface. As soon as it begins to be translucent, it should be placed under the Microscope (particular regard being had to the precaution specified in § 86), and note taken of any inequality ; and then, when it is again laid upon the stone, such inequality may be brought down, by making special pressure with the fore-finger upon the part of the slide above it. When the thinness of the section is such as to cause the water to spread around it between the glass and the stone, an excess of thickness on either side may often be detected, by noticing the smaller distance to which the liquid extends. In proportion as the substance attached to the glass is ground away, the superfluous balsam which may have exuded around it will be brought into contact with the stone ; and this should be removed with a knife, care being taken, how- ever, that a margin be still left round the edge of the section. As the section approaches the degree of thinness which is most suitable for the display of its organization, great care must be taken that the grinding process be not carried too far ; and fre- 14 210 PREPARATION OF OBJECTS. quent recourse should be had to the Microscope, which it is con- venient to have always at hand, when work of this kind is being carried on. There are many substances whose intimate structure can only be displayed in its highest perfection, when a very little more reduction would destroy the section altogether ; and every Microscopist who has occupied himself in making such prepara- tions, can tell of the number which he has sacrificed in order to attain this perfection. Hence if the amount of material be limited, it is a good rule to stop short as soon as a good section has been made, and to lay it aside — "letting well alone" — whilst the at- tempt is being made to procure a better one ; if this should fail, another attempt may be made, and so on, until either success has been attained, or the whole of the material has been con- sumed,— the first section, however, still remaining : whereas, if the first, like every successive section, be sacrificed in the attempt to obtain perfection, no trace will be left to " show what has been." In judging of the appearance of sections in this stage under the Microscope, it is to be remembered that its transpa- rence will subsequently be considerably increased by mounting in Canada balsam (§ 125) ; this is particularly the case with fossils, to which a deep hue has been given by the infiltration of some coloring matter ; and with any substances whose particles have a molecular aggregation, that is rather amorphous than crystalline. When a sufficient thinness has been attained, the section may generally be "mounted" in Canada balsam; and the mode in which this must be managed, will be detailed hereafter (§ 129). 111. As there are certain substances, however, the view of whose structure is impaired by mounting in Canada balsam, and which should therefore be mounted either dry or in fluid, a dif- ferent method of procedure must be adopted with them. If tolerably thin sections of them can be cut in the first instance, or if they are of a size and shape to be held in the hand whilst they are being roughly ground down, there will be no occasion to attach them to glass at all ; is is frequently convenient to do this at first, however, for the purpose of obtaining a "hold" upon the specimen ; but the surface which has been thus at- tached, must afterwards be completely rubbed away, in order to bring into view a stratum which the Canada balsam shall not have penetrated. As none but substances possessing considera- ble toughness, such as bones and teeth, can be treated in this manner, and as these are the substances which are most quickly reduced by a coarse file, and are least liable to be injured by its action, it will be generally found possible to bring the sections to a considerable thinness, by laying them upon a piece of cork or soft wood held in a vice, and operating upon them first with a coarser and then with a finer file. When this cannot safely be carried further, the section must be rubbed down upon that one of the fine stones already mentioned (§ 109), which is found best to suit it ; as long as the section is tolerably thick, the finger may be used to press and move it : but as soon as the finger USE OF CHEMICAL EEAGENTS. 211 itself begins to come into contact with the stone, it must be guarded by a flat slice of cork, or by a piece of gutta percha, a little larger than the object, Tinder either of these, the section may be rubbed down until it has been reduced to the requisite degree of tenuity ; but even the most careful working, on the finest-grained stone, will leave its surface covered with scratches, which not only detract from its appearance, but prevent the de- tails of its internal structure from being as readily made out, as they can be in a polished section. This polish may be imparted, by rubbing the section with putty powder (peroxide of tin) and water, upon a leather strap, made by covering the surface of a board with buff-leather, having three or four thicknesses of cloth, flannel, or soft leather beneath it ; this operation must be per- formed on both sides of tbe section, until all the marks of the scratches left by the stone shall have been rubbed out; when the specimen will be fit for mounting, after having been carefully cleansed from any adhering particles of putty powder. 112. Chemical Actions. — One important part of the preparation of Microscopic objects, is often effected by the use of Chemical Reagents. These may be employed, either for the sake of re- moving substances of which it is desired to get rid, in order to bring something else into view ; or for the sake of detecting the presence of particular substances in the object under examina- tion. Thus, in order to obtain the animal basis of Shell, Bone, Tooth, &c., it is necessary to dissolve away the calcareous portion of these tissues by the use of acids ; a mixture of nitric and muriatic acids is preferable ; and this should be added, little by little, to a considerable bulk of water, until a disengagement of gas be perceived*to commence from the surface of the specimen. Care should always be taken not to hurry the process by adding too much acid, since, when the animal membrane is of very deli- cate consistence, it is liable to be dissolved ; and in some cases it is better to allow the action to go on for many weeks, adding only a drop or two of acid at a time. When siliceous particles are to be removed (such as those which form the loricce of the Diatomacese), for the sake of leaving the organic membrane in a state adapted to separate examination, hydrofluoric acid must be employed as the menstruum. It is sometimes necessary, on the other hand, to get rid of the organic matter, for the sake of ob- taining the mineral particles in a separate state, as in the case of the spicules of Sponges, Gorgonise, &c., this may be done either by incineration, or (which is generally preferable) by boiling or macerating for a long time in a solution of caustic potash. In separating from Guano, again, the siliceous skeletons of Diato- naaceee, &c., which it may contain, muriatic and nitric acids are largely used, to dissolve away every part of the mass on which they will act ; the microscopic organisms for which search is made, being contained in a few grains of sediment which are left when a pound of pure guano is thus treated. 212 PREPARATION OF OBJECTS. 113. In applying Chemical Reagents to Microscopic objects for the purpose of testing, it is necessary to use great care not to add too much at once ; and it is better that the test-bottle itself should afford the means of regulating the quantity, than that an additional rod or tube should be required. Two modes have been devised for this purpose. One consists in drawing the neck of the test-bottle to a capillary orifice, and covering it with a cap which fits around it ; and the fluid is caused to flow from this, drop by drop, by the warmth of the hand applied to the bottle, which causes an expansion of the air it may contain.1 When these bottles are emptied, they must be refilled by expelling the air by heat, and placing the capillary orifice under the surface of the fluid to be introduced, which will then be forced in as the bottle cools ; this process may need to be repeated two or three times (care being taken that the heat applied be not so great as to crack the bottle) ; but it is better not to fill the bottle more than half-full, in order that air enough may be left for the* warmth of the hand to act upon. The other arrangement for applying minute quantities of test-liquids, consists in the elongation of the stopper, which is drawn to a fusiform point, so as to serve as the test-rod for its own bottle.2 This enables either a mere trace, or several ordinary drops, of the reagent to be applied at once ; for the elongated stopper will take up a considerable quantity, a larger or smaller proportion of which (as desired), may be left behind, by bringing the lower part of the stopper into contact with the inside of the neck of the bottle, as it is being withdrawn. Whichever plan is made use of, great care should be taken to avoid carrying away from the slide to which the test-liquid is applied, any loose particles which may be updn it, and which may be thus transferred to some other object, to the great per- plexity of the Microscopist. It is better, indeed, not to deposit the drop of test-liquid on the slide in immediate contact with the substance to which it is to be applied ; but to bring the two into contact after the test-bottle has been withdrawn. 114. The following are the Test-Liquids most frequently needed. 1. Solution of Iodine in water (1 gr. of iodine, 3 grs. of iodide of potassium, 1 oz. of distilled water) turns starch blue, and cel- lulose brown ; it also gives an intense brown to albuminous sub- stances. 2. Dilute Sulphuric Acid (one of acid to two or three parts of water) gives to cellulose that has been previously dyed with iodine, a blue or purple hue ; also, when mixed with a solution of sugar, it gives a rose-red hue, more or less deep, with nitro- genous substances and with bile (Pettenkofer's test). 1 A set of 12 test-bottles on this plan, packed in a box, is supplied by Mr. Highley, of Fleet Street. 2 Bottles of this pattern, which was devised by Dr. Griffith, are sold by Mr. Ferguson, of Giltay salt and 2 grs. of corrosive sublimate, to a quart of water, — or, in cases where the coagulating action of corrosive sublimate on albuminous matters would be an objection, the substitution of 20 grains of arsenious acid) may be used in its stead ; or Thwaites's fluid, or Dr. Beale's modification of it, or Deane's Gelatine may be tried. It is often quite impossible to predicate beforehand what preservative fluid will answer best for a particu- lar kind of preparation ; and it is consequently desirable, where there is no lack of material, always to mount the same object in two or three different ways, -marking on each slide the method I 234 MOUNTING OF OBJECTS. employed, and comparing the specimens from time to time, so as to judge how each is affected. 132. Of Mounting Objects in Fluid. — As a general rule, it is desirable that objects which are to be mounted in fluid, should be soaked in the particular fluid to be employed, for some little time before mounting; since, if this precaution be not taken, air- bubbles are very apt to present themselves. It is sometimes necessary, in order to secure the displacement of air contained in the specimen, to employ the air-pump in the mode already directed (§ 128) ; but it will sometimes be found sufficient to im- merse the specimen for a few minutes in alcohol (provided that this does not do any detriment to its tissues), which will often penetrate where water will not make its way; and when the spirit has driven out the air, the specimen may be removed back to water, which will gradually displace the spirit. "When Deane's Gelatine is used, however, all that can be done, will be to drain the object of its superfluous water before applying the liquefied medium ; but as air-bubbles are extremely apt to arise, they must be removed by means of the air-pump, the gelatine being kept in a liquid state by the use of a vessel of hot water, as in the case of Canada balsam. In dealing with the small quantities of fluid required in mounting microscopic objects, it is essential for the operator to be provided with the means of transferring very small quantities from the vessel containing it, to the slide, as well as of taking up from the slide what may be lying superfluous upon it. The straight and curved-pointed "dipping-tubes" (Fig. 51, A, B) may be made to answer this purpose ; but it is much better that tubes for this purpose be furnished with a bulb, like that of the Chemist's "pipette," and that their orifices be drawn to a fine point. The fluid is drawn into the tubes by suction, and expelled by the pressure of the breath ; the curved-pointed tube will generally be the best for introducing fluid beneath the flass cover, and the straight-pointed for simply filling cells or >r taking up superfluous fluid. The Author has of late found very great convenience in the use of a small glass syringe, the orifice of which is slightly curved and drawn to a fine capillary point ; for as the syringe works independently of the mouth, its orifice may be applied in any way that may be found convenient; and when the mouth is freed from the efforts of suction and ejection, the eyes can be better employed in watching the opera- tion. Besides the pipettes and the syringe, some blotting-paper, of the most bibulous kind that can be procured, will be found very useful. 133. There are certain objects of extreme thinness, which re- quire no other provision for mounting them in fluid, than an ordinary glass slide, a thin glass cover, and some gold-size or asphalte (§ 120). The object having been laid in its place, and a drop of the fluid laid upon it (care being taken that no air- space remains beneath the under side of the object and the sur- MOUNTING OBJECTS IN FLUID. 235 face of the slide), the glass cover is then to be laid upon it, one side being first brought into contact with the slide, and the other being gradually lowered, in such a manner that the air shall be all displaced before the fluid. If any air-bubbles remain in the central part of the space between the cover and the slide, the former must be raised again, and more fluid should be introduced ; but if the bubbles be near the edge, a slight pressure on that part of the cover will often suffice to expel them, or the cover may be a little shifted so as to bring them to its edge. There are some objects, however, whose parts are liable to be displaced by the slightest shifting of this kind ; and it is more easy to avoid making air-bubbles, by watching the extension of the fluid as the cover is lowered, and by introducing an additional supply when and where it may be needed, than it is to get rid of them after- wards without injury to the object. When this end has been satisfactorily accomplished, all that is needed is first to remove all superfluous fluid from the surface of the slide, and from around the edge of the cover, with a piece of blotting-paper, taking care not to draw away any of the fluid from beneath the cover, or (if any have been removed accidentally) to replace what may be deficient; and then to make a circle of asphalte or gold- size around the cover, taking care that it "wets" its edges, and ad- vances a little way upon its upper surface. When this first coat is dry, another should be applied, particular care being taken that the cement shall fill the angular furrow at the margin of the cover. In laying on the second coat, it will be convenient, if the cover be round, to make use of the whirling-table (Fig. 63) ; and if the slide be so carefully laid upon ' it, that the glass cover is exactly concentric with its axis, the whirling-table may be used even for the first application of the varnish ; a slight error in this respect, however, may occasion the displacement of the cover. By far the greater number of preparations which are to be pre- served in liquid, however, should be mounted in a " Cell" of some kind, which forms a well of suitable depth, wherein the preservative liquid may be retained. This is absolutely necessary in the case of all objects, whose thickness is such as to prevent the glass cover from coming into close approximation with the slide ; and it is desirable, wherever that approximation is not such as to cause the cover to be drawn to the glass slide by ca- pillary attraction, or wherever the cover is sensibly kept apart from the slide by the thickness of any portion of the object. Hence it is only in the case of objects of the most extreme tenuity, that the "cell" can be advantageously dispensed with; — the dan- ger of not employing it, in many cases in which there is no diffi- culty in mounting the object without it, being that after a time the cement is apt to run in beneath the cover, which process is pretty sure to continue, when it may have once commenced. 134. Cement- Cells. — When the Cells are required for mounting very thin objects, they may be advantageously made of varnish 236 MOUNTING OF OBJECTS. only, by the use of the ingenious little instrument (Fig. 63) con- trived by Mr. Shadbolt. This consists of a small slab of ma- hogany, into one end of FIG 63. which is fixed a pivot, whereon a circular turn- table of brass, about three inches in diameter, is made to rotate easily, a rapid motion being given Shadbolt's Turn-table for making Cement-Cells. to it by the application of the fore-finger to the milled head seen beneath. The glass slide being laid upon the turn-table, in such a manner that its two edges shall be equidis- tant from the centre (a guide to which is afforded by a circle of an inch in diameter, traced upon the brass), and being held by the springs with which it is furnished, a camel-hair pencil dipped in the varnish to be used (Brunswick black or Asphalte is the best) is held in the right hand, so that its point comes into con- tact with the glass, a little within the guiding circle just named. The turn-table being then put into rotation with the left hand, a ring of varnish of suitable breadth is made upon the glass ; and if the slide be set aside in a horizontal position, this ring will be found, when dry, to have lost the little inequalities it may have at first presented, and to present a very level surface. If a greater thickness be desired than a single application will con- veniently make, a second layer may be laid on after the first is dry. It is convenient to prepare a number of these cells at once, since, when "the hand is in," they will be made more dexter- ously than when the operation is performed only once; and it will be advantageous to subject them to the warmth of a slightly heated oven, whereby the flattening of their surface will be more completely assured. The Microscopist will find it a matter ot great convenience to have a stock of these cells always by him, ready prepared for use. 135. Thin Glass Cells. — For the reception of objects too thick for varnish cells, but not thicker than ordinary thin glass, it is advantageous to construct cells of glass ; and these may be made in one of two ways, either by grinding down the cross sections of glass tubes (§ 137) until they have been reduced to the desired thinness, or by perforating a plate of thin glass with an aperture of the desired size ; and then cementing the ring or the plate to the glass slide with marine glue. The former plan is liable to the objection, that in reducing the glass rings to the desired thinness, they are extremely liable to crack or break, and that their attainable forms are limited. The latter will generally answer very well, if care be taken in the selection of &flat piece of thin glass ; and the perforation, if due precaution be em- ployed, may be made of any size or form that may be desired. For making round cells, the perforated pieces that sometimes re- PLATE GLA«SS CELLS. 237 main entire after the cutting of disks (§ 117) may be employed, the disks often falling out of themselves when the glass is laid aside for a few days ; and thus the same piece of thin glass may afford a plate, which, when cemented to a glass slide forms a cell, and a disk suitable as the cover to a cell of somewhat smaller size. There is great danger, however, of the cracking of the surrounding glass, especially when the disk is of large size ; and it will generally be found a saving of trouble, to employ the method recommended by Dr. L. Beale. This consists in attach- ing a piece of thin glass to one of the glass rings of which the deeper cells are made (§ 137), of any form that may be desired, by means of marine glue, first laid upon the latter, and melted upon the hot plate ; when the glue is quite cold, the point of a round or semicircular file is sharply thrust through the centre of the thin glass, which is carefully filed to the size of the interior of the ring ; and the ring being then heated a second time on the hot plate, the thin glass plate may be readily detached from it, and at once cemented upon the glass slide. The success of this simple process depends upon the very firm and intimate ad- hesion of the thin glass to the ring, which prevents any crack from running into the part of the thin glass that is attached to it, however roughly the file may be used. By having many of the rings on the hot plate at once, and operating with them in turn, a great number of cells can be made in a short time ; and such large thin cells may be made in this mode, as could scarcely be fabricated (on account of the extreme brittleness of this glass) by any other. A press, consisting of two plates of brass screwed together, holding the thin glass between them, has been devised by Mr. C. Brooke for the same purpose ; but the foregoing method has the advantage, not only of requiring no special ap- paratus, but also of enabling the form and size of the perforation to be readily varied. After the thin glass has been cemented to the slide, it is desirable to roughen its upper surface, by rubbing it upon a leaden or pewter plate (§ 108) with fine emery ; since the gold-size or other varnish adheres much more firmly to a " ground" than to a polished surface. Although the thin glass cell requires much more trouble in its preparation than the cement cell, yet it is decidedly to be preferred for any very choice objects; since, if any air should find admission, it is more readily detected ; and the remounting of the object may be accomplished in the same cell, with very little disturbance of its position. 136. Plate Glass and Shallow Cells.— For mounting objects of somewhat greater thickness than can be included within thin glass cells, shallow cells may be made by drilling apertures of the de- sired size in pieces of plate-glass of the requisite thickness, and by attaching these with marine glue to glass slides (Fig. 64). Such holes may be made not merely circular (A) but oval (c) ; and a very elongated perforation may be made, by drilling two 238 MOUNTING *OF OBJECTS. Fro. 64. holes at the required distance and then connecting them by cut- ting out the intermediate space (B). These operations, however, can scarcely be performed by any but regular glass-cutters, and, being troublesome, are expensive ; hence the plate-glass cells have been generally superseded, either by tube-cells (§ 137) or by built-up cells. Although the former may be reduced to any degree of shallowness A that may be desired, and are made of most of the sizes and forms that can be ordinarily needed, yet for extra sizes or B peculiar forms, shallow cells may be easily built up after the following very simple and effective method. A piece of plate glass, of a thickness that - shall give the desired depth to the cell, is to be cut to the dimensions of its outside wall ; and a strip is then to \ be cut off with the diamond from each of its edges, of such breadth as shall leave the in- terior piece equal in its dimensions to the cavity of the cell, that is desired. This piece being rejected, the four strips are then to be cemented upon the glass slide in their original position, so that the diamond cuts shall fit together with the most exact pre- cision ; and the upper surface is then to be ground flat with emery upon the pewter plate, and left rough as before. This plan answers admirably for constructing such large shallow cells as are required for the mounting of Zoophytes and similar ob- jects. Having had occasion, during the last few months, to mount a large number of objects in shallow cells, the Author has adopted the recommendation of a friend, to make use of cells which are sunk by grinding out a concave in the thickness of a glass plate. These, until recently, were costly ; but they are now made in large quantities, and their price has been so much reduced, that they can be obtained more cheaply than any other kind. For objects whose shape adapts them to the form and depth of the concavity, these cells will be found peculiarly advantageous ; since they do not hold air-bubbles so tenaciously as do those with per- pendicular walls ; and there is no cemented plate or ring to be loosened from its attachment, either by a sudden jar or by the lapse of time. For transparent objects, however, they are^less suitable (unless manufactured with more care than is usually given to them) than they are for opaque ; since the concave bottom is seldom so highly polished, as to be free from scratches and rough- TUBE-CELLS. 239 FIG. 65. nesses, which greatly interfere with the appearance of the picture. Cells of this kind may be obtained, from Messrs. Jackson, Ox- ford Street, either of round or oval form, and not only ground out of slides of the usual size (3 in. by 1) and thickness, but also hollowed in pieces of plate-glass of larger dimensions. 137. Deep and Built-up Cells. — The deep cells which are re- quired for mounting Injections and other microscopic prepara- tions of considerable size and thickness, may^Q made by drilling through a piece of thick plate glass (Fig. 64, D) ; but for the reason already given, the drilled cells are now seldom used, their place having been taken, either by tube-cells, or by the deep built-up cells to be presently described. The tube-colls are made by cut- ting transverse sections of thick-walled glass tubes of the required size, grinding the surfaces of these rings to the desired thinness, and then cementing them to the glass slides with marine glue. Not only may round cells (Fig. 65, A, B), of any diameter and any depth that the Microscopist can possibly require, be made by this simple method, but oval, square-shaped, or oblong cells (c, D) are now made, of the forms and sizes that he is most likely to want, by flattening the round glass- tube whilst hot, or by blow- ing it within a mould. The facility with which such cells may be made, and the se- curity they afford, have caused the deep cells built up of separate strips of glass c (Fig. 66) to be comparatively little employed, except in cases where some very un- usual size or shape (A) may be required, or where it is D necessary that not merely the top and bottom, but also the sides of the object, should be clearly seen (B). The perfect construction of these requires a nicety of workmanship which few amateurs possess, and the expenditure of more time than Microscopists generally have to spare ; and as it is consequently preferable to obtain them ready made, directions for making them need not here be given. A new plan of making deep cells, however, has been lately intro- duced by Dr. L. Beale ; which, though it does not give them side walls possessing the same flatness with those of the built-up cells, adapts them to serve most of the purposes for which these are required, and makes them more secure against leakage; whilst it has the advantage of being so easy and simple, that Tube-Cells, Round and Quadrangular. 240 MOUNTING OF OBJECTS. Built-up Cells. any one may put it into practice. A long strip of plate glass is to be taken, whose breadth is equal to the desired depth of the cell, and whose length must be FIG. 6fi. equal to the sum of that of all its sides. This strip is to be carefully bent to a right angle in the blow-pipe flame, at three points previously indicated by marks so placed as to show where the angles should fall ; and the two ends, which will thus be brought into contact at right angles, are to be fused together. Thus a large square well, slightly rounded at the angles, will be formed ; and this, being very brittle, should be allowed to cool very gradually, or, still better, should be annealed in an oven. It must then be ground quite true on its upper and lower edges, either on the lead plate with emery, or on a flat stone with fine sand ; and it may then be cemented to a glass slide in the usual way. 138. Mounting Objects in Cells. — In mounting an object in a cell, the first attention will of course be given to the cleanness of the interior of the cell, and of the glass cover which is to be placed on -it ; this having been secured, the cell is to be filled with fluid by the pipette or syringe ; and any minute air-bubbles which may be seen adhering to its bottom or sides, must be removed by the needle : the object, previously soaked in fluid resembling that with which the cell is filled, is then to be placed in the cell, and should be carefully examined for air-bubbles on all sides, and also by looking up from below. When every pre- caution has been taken to free it from these troublesome in- truders, the cover may be placed on the cell, one side being first brought down upon its edge and then the other ; and if the cell have been previously brimming over with fluid (as it ought to be) it is not likely that any air-space will remain. If, however, any bubbles should present themselves beneath the cover, the slide should be inclined, so as to cause them to rise towards the highest part of its circumference, and the cover slipped away from that part, so as to admit of the introduction of a little ad- ditional fluid by the pipette or syringe ; and when this has taken the place of the air-bubbles, the cover may be slipped back into its place.1 All superfluous fluid is then to be taken 1 Mr. Quekett and some other practised manipulators recommend that the edges of the cell and that of the disk of glass be smeared with the gold-size or other varnish em- ployed, before the cell is filled with fluid ; but the Author has found this practice ob- jectionable, for two reasons, — first, because it prevents the cover from being slipped to one side (which is often desirable), without its being soiled by the varnish, — and se- cond, because when the edge of the cell has been thus made to " take" the varnish, that which is afterwards applied for the closure of the cell is more likely to run in, than if the whole of the surface covered by the glass is moistened with water. MOUNTING OBJECTS IN CELLS. 241 up with blotting-paper ; and particular care should be taken thoroughly to dry the surface of the cell and the edge of the cover, since the varnish will not hold to them if they be in the least damp with water. Care must also be taken, however, that the fluid be not drawn away from between the cover and the edge of the cell on which it rests ; since any deficiency here is sure to be filled by varnish, the running in of which is particu- larly objectionable. These minutiae having been attended to, the closure of the cell may be at once effected, by carrying a thin layer of gold-size or asphalte around and upon the edge of the glass cover, taking care that it touches every point of it, and fills the angular channel which is left around its margin. If the wall of the cell be very thin, it will be advantageous to include it in the ring of varnish ; so that this shall hold down the cover, not only on the cell, but on the slide beneath. The Author has found it advantageous, however, to delay closing the cell for some little time after the superfluous fluid has been drawn off; for as soon as evaporation beneath the edges of the cover begins to diminish the quantity of fluid in the cell, air-bubbles often begin to make their appearance, which were previously hidden in the recesses of the object; and in the course of half an hour, a considerable number are often collected. The cover should then be slipped aside, fresh fluid be introduced, the air-bubbles removed, and the cover put on again ; and this operation should be repeated, until it fails to draw forth any more air-bubbles. It will, of course, be observed, that, if the evaporation of fluid should proceed far, air-bubbles will enter beneath the cover ; but these will show themselves on the surface of the fluid ; whereas those which arise from the object itself, are found in the deeper parts of the cell. Much time may be saved, however, and the freedom of the preparation from air-bubbles may be most effec- tually secured, by placing the cell, after it has been filled in the first instance, in the vacuum of an air-pump ; and if several ob- jects are being mounted at once, they may all be subjected to the exhausting process at the same time. The application of the varnish should be repeated after the lapse of a few hours, and may be again renewed with advantage several times in the course of a week or two ; care being taken that each layer covers the edges, as well as the whole surface, of that which pre- ceded it. 139. The presence of air-bubbles, in any preparation mounted in fluid, is to be particularly avoided, not merely on account of its interference with the view of the object, but also because, when air-spaces, however small, once exist, they are almost cer- tain to increase, until at last they take the place of the entire fluid, and the object remains dry. Even with the most ex- perienced manipulators, however, this misfortune not unfre- quently occurs ; being sometimes due to the obstinate entangle- ment of air-bubbles in the object, when it was originally mounted ; 16 242 MOUNTING OF OBJECTS. and sometimes to the perviousness of some part of the cement, which has allowed a portion of the contained flu id to escape, and air to find admission. In either case, so soon as an air-bubble is seen in such a preparation, the attempt should be made to prevent its increase, by laying on an additional coat of varnish ; but if this should not be successful, the cover should be taken off, and the specimen remounted, so soon as the fluid has escaped to such a degree as to leave any considerable portion of it uncovered. 140. Importance of Cleanliness. — The success of the result of any of the foregoing operations is greatly detracted from, if, in consequence of the adhesion of foreign substances to the glasses wrhereon the objects are mounted, or to the implements used in the manipulations, any extraneous particles are brought into view with the object itself. Some such will occasionally present themselves, even under careful management ; especially fibres of silk, wool, cotton, or linen, from the handkerchiefs, &c., with which the glass slides may have been wiped, and grains of starch, which often remain obstinately adherent to the thin glass covers kept in it. But a careless and uncleanly manipulator will allow his objects to contract many other impurities than these ; and espe- cially to be contaminated by particles of dust floating through the air, the access of which may be readily prevented by proper precautions. It is desirable to have at hand a well-closed cup- board furnished with shelves, or a cabinet of well-fitted drawers, or a number of bell-glasses upon a flat table, for the purpose of securing our glasses, objects, &c., from this contamination, in the intervals of the work of preparation ; and the more readily accessible these receptacles are, the more use will the Microsco- pist be likely to make of them. Great care ought, of course, to be taken, that the liquids employed for mounting should be freed, by effectual filtration, from all floating particles ; and both these and the Canada balsam should be kept in well-closed bottles. 141. Labelling and Preserving of Objects. — Whenever the mounting of an object has been completed, its name ought to be at once marked on it, and the slide should be put away in its appropriate place. Some inscribe the name on the glass itself, with a writing diamond ; whilst others prefer to gum a label ! on the slide ; and others, again, cover one or both surfaces of the slide with colored paper, and attach the label to it. In the case of objects mounted dry or in balsam, the latter method has the advantage of rendering the glass cover more secure from dis- placement by a slight blow or "jar," when the varnish or balsam may have become brittle by the lapse of years. Instead, how- ever, of attaching the white label on which the name of the ob- ject is written, outside the colored paper with which the slide is 1 Very neat gummed labels, of various sizes and patterns suitable to the wants of the Microscopist, are sold by the " Drapers' Stationers" in the City. COLLECTION OF OBJECTS. 243 covered, it is better to attach the label to the glass, and to punch a hole out of the colored paper, sufficiently large to show the name, in the part corresponding to it ; in this manner the label is prevented from falling off, which it frequently does when attached to the glass without protection, or to the outside of the paper cover. When objects are mounted in fluid, either with or without cells, paper coverings to the slides had better be dis- pensed with ; and besides the name of the object, it is desirable to inscribe on the glass that of the fluid in which it is mounted. For the preservation of objects, the pasteboard boxes now made at a very reasonable cost, with wooden racks, to contain 6, 12, or 24 slides, will be found extremely useful. In these, however, the slides must always stand upon their edges; a position which, besides interfering with that ready view of them which is re- quired for the immediate selection of any particular specimen, is unfavorable to the continued soundness of preparations mounted in fluid. Although such boxes are most useful, indeed almost indispensable, to the Microscopist, for holding slides which he desires (for whatever purpose) to keep for a while constantly at hand, yet his regularly classified series is much more conve- niently stored in a Cabinet containing numerous very shallow drawers, in which they lie flat and exposed to view. Such cabinets are now prepared for sale under the direction of our principal Opticians, with all the improvements that experience has suggested. In order to prevent the warping of the thin wood of which the bottoms of the drawers are usually made, whereby their sliding action is obstructed, it has been found ad- vantageous to substitute strained canvas or papier mache. Again, in order to antagonize the disposition of the slides to slip one over another in the opening or shutting of the drawers, it has been found preferable to arrange them in such a manner, that they lie with their ends (instead of their long sides) towards the front of the drawer, and to interpose a cross-strip of wood, lying parallel to the front of the drawer, between each row. It is very convenient, moreover, for the front of the drawer to be furnished with a little tablet of porcelain, on which the name of the group of objects it may contain can be written in pencil, so as to be readily rubbed out; or a small frame may be attached to it, into which a slip of card may be inserted for the same purpose. SECTION 3. COLLECTION OF OBJECTS. 142. A large proportion of the objects with which the Micro- scopist is concerned, are derived from the minute parts of those larger organisms, whether Vegetable or Animal, the collection of which does not require any other methods than those pursued by the ordinary Naturalist. With regard to such, therefore, no special directions are required. But there are several most in- teresting and important groups, both of Plants and Animals, 244 COLLECTION OF OBJECTS. which are themselves, on account of their minuteness, essentially microscopic; and the collection of these requires peculiar methods and implements, which are, however, very simple, — the chief element of success lying in the knowledge where to look, and what to look for. In the present place, general directions only will be given ; the particular details relating to the several groups, being reserved for the account to be hereafter given of each. 143. All the Microscopic organisms in question, being aquatic, must be sought for in pools, ditches, streams, or other collections of water ; through which some of them freely move, whilst others attach themselves to the stems and leaves of aquatic plants, or even to pieces of stick or decaying leaves, &c., that may be float- ing on the surface or submerged beneath it, while others, again, are to be sought for in the muddy sediments at the bottom. Of those which have the power of free motion, some keep near the surface, whilst others swim in the deeper waters ; but the situa- tion of many depends entirely upon the light, since they rise to the surface in sunshine, and subside again afterwards. The Col- lector will therefore require a means of obtaining samples of water at different depths, and of drawing to himself portions of the larger bodies to which the microscopic organisms may be attached. For these purposes, nothing is so convenient as a rod about five feet long, which may be divided into two pieces jointed together ; and the farther extremity of this rod should be pierced with a hole, passing for some distance into its length.1 Into this hole, as a socket, may be fitted either of the three implements which the Collector may happen to require. If he desires to take up samples of the water, he will need a wide-mouthed bottle, containing about 2 oz. This may be attached to the extremity of the rod, by simply passing round its neck a strap of thin whale- bone or sheet gutta percha, the two ends of which are to be brought together and inserted into the socket, in which they may be secured by a plug of soft wood or cork. The bottle being held sideways with its mouth partly below the water, the surface may be skimmed ; or, if it be desired to bring up a sample of the liquid from below, or to draw into the bottle any bodies that may be loosely attached to the submerged plants, the bottle is to be plunged into the water with its mouth downwards, carried into the situation in which it is desired that it should be filled, and then suddenly turned with its mouth upwards. If, again, the organisms which it may be desired to collect, are of sufficient size to be strained out of the water by a piece of fine muslin, a ring-net should be fitted into the socket of the rod. This may be made by sewing the muslin bag to a ring of stout wire, fur- nished with a projecting stem which may be inserted by means of%a cork into the socket of the rod. But it is more convenient 1 Cheap fishing-rods are now sold at the toy shops, which answer this purpose ex- tremely well, the last or slenderest joint being laid on one side; its socket in the last joint but one. being well adapted to receive the fittings above described. COLLECTION OF OBJECTS. 245 that the muslin should he made removable ; and this may he pro- vided for (as suggested in the " Micrographic Dictionary," Intro- duction, p. xxiv) by the substitution of a wooden ring, grooved on its outside, for the wire ring ; the muslin being strained upon it by a ring of vulcanized India rubber, which lies in the groove, and which may be readily slipped off and on, so as to allow a fresh piece of muslin to be put in the place of that which has been last used. For bringing up portions of larger Plants, either for the sake of examining their own structure, or for obtaining the growths which may be parasitic upon them, a cutting-hook, shaped somewhat like a sickle, may be fitted into the socket of the rod. 144. The Collector should also be furnished with a number of bottles, into which he may transfer the samples thus obtained. These it will be convenient to have of two kinds ; one set wide- mouthed, and capable of being closely corked, for minute Plants ; the other set with narrower mouths, having short pieces of tube passed through the corks, for the purpose of containing Animal- cules without depriving them of air. The former kind, however, may be safely employed for Animalcules, if they be not above two-thirds filled (so as to leave an adequate air-space), and be not kept long closed. Such bottles should be fitted into cases, in which several may be carried at once without risk of breakage.1 Whilst engaged in the search for Microscopic objects, it is desi- rable for the collector to possess a means of at once recognizing the forms which he may gather, where this is possible, in order that he may decide whether the " gathering" is, or is not, worth preserving; for this purpose either a powerful " Coddington" or u Stanhope" lens (§ 19), or a Gairdner's Simple Microscope (§ 28), will be found most useful, according to the class of objects of which the collector is in search. The first will answer very well for Zoophytes and the larger Diatomacese ; but the second or third will be needed for Desmidiaceae, the smaller Diatomaceae, and Animalcules. 1 The bottles in which smelling-salts are now commonly sold, having the corks fitted into disks of turned wood, are very convenient, both in size and shape, for the purposes of the Microscopist ; cases containing 3, 4, 6, or 8 such bottles, are made by Mr. Ferguson, of Giltspur Street. The wide-mouthed bottles with screw caps, made by the York Glass Company, are also extremely convenient. CHAPTER VI. MICROSCOPIC FORMS OF VEGETABLE LIFE.— PROTOPHYTES. 145. IN commencing our survey of these wonders and beau- ties of Life and Organization, which are revealed to us by the assistance of the Microscope, it seems on every account the most appropriate to turn our attention in the first instance to the Vegetable Kingdom ; and to begin with those humblest mem- bers of that kingdom, whose form and structure, and whose very existence, in many cases, are only known to us through its use. For those who desire to make themselves familiar with microscopic appearances, and to acquire dexterity in microscopic manipulation, cannot do better than educate themselves by the study of those comparatively simple forms of organization, which the Vegetable fabric presents ; since a facility in minute dissec- tion and in microscopic analysis may be thus acquired, which will save much expenditure of time and labor, that might be unprofitably applied, without such apprenticeship, to the attempt to unravel the complexities of Animal organization. But fur- ther, the scientific Histologist (p. 49) looks to the careful study of the structure of the simplest forms of Vegetation, as furnish- ing the key (so to speak) that opens the right entrance to the study of the elementary Organization, not merely of the higher Plants, but of the highest Animals. And in like manner, the scientific Physiologist looks to the complete knowledge of their life history, as furnishing the surest basis for those general no- tions of the nature of Vital Action, which the advance of science has shown to be really well founded, only when they prove equally applicable to both kingdoms. But further, a peculiar interest attaches itself at the present time, to everything which throws light upon the debated question of the boundary between the two kingdoms ; a question which is not less keenly debated among Naturalists, than that of many a disputed frontier has been between adjacent Nations. For many parts of this border country have been taken and retaken several times ; their inha- bitants (so to speak) having first been considered, on account of their general appearance, to belong to the Vegetable Kingdom, — then, in consequence of some movements being observed in them, being claimed by the Zoologists, — then, on the ground of their DISTINCTION BETWEEN PLANTS AND ANIMALS. 247 evidently plant-like mode of growth, being transferred back to the Botanical side, — then, owing to the supposed detection of some new feature in their structure or physiology, being again claimed as members of the Animal Kingdom, — and lastly, on the discovery of a fallacy in these arguments, being once more laid hold of by the Botanist, with whom, for the most part, they now remain. For the attention which has been given of late years to the study of the humblest forms of Vegetation, has led to the knowledge of so many phenomena, among what must be undoubtedly regarded as Plants, which would formerly have been considered unquestionable marks of animality, that the discovery of the like phenomena among the doubtful beings in question, so far from being any evidence of their animality, really affords a probability of the opposite kind. 146. In the present state of Science, it would be very difficult, and is perhaps impossible, to lay down any definite line of de- marcation between the two kingdoms ; since there is no single character by which the Animal or Vegetable nature of any or- ganism can be tested. Probably the one which is most generally applicable, among those lowest organisms which most closely approximate to one another, is — not, as formerly supposed, the presence or absence of spontaneous motion, but the dependence of the being for nutriment upon organic compounds already formed, which it takes (in some way or other) into the interior of its body, or its possession of the power of obtaining its own alimentary matter by absorption, from the inorganic elements on its exterior. The former is the characteristic of the Animal Kingdom as a whole ; the latter is the attribute of the Vegetable ; and although certain apparently exceptional cases may exist, yet these do not seem to occur among the group in wrhich such a means of distinction is most useful to us. For we shall find that those Protozoa, or simplest Animals, which seem to be com- posed of nothing else than a mass of living jelly (Chaps. IX, X), are supported as exclusively, either upon other Protozoa, or upon Protophyta, which are humble Plants of equal simplicity, as the highest Animals are upon the flesh of other animals, or upon the products of the Vegetable Kingdom ; whilst these Pro- tophy tes, in common with the highest Plants, draw their nourish- ment from water, carbonic acid, and ammonia, and are distin- guished by their power of liberating oxygen, through the decomposition of carbonic acid, under the influence of sunlight. And we shall, moreover, find, that even such Protozoa as have neither stomach nor mouth, receive their alimentary matter direct into the very substance of their bodies, in which it under- goes a kind of digestion ; whilst the Protophyta absorb through their external surface only, and take in no solid particles of any description. With regard to motion, which was formerly con- sidered the distinctive attribute of animality, we now know, not merely that many Protophytes (perhaps all, at some period or 248 MICROSCOPIC FORMS OF VEGETABLE LIFE. other of their lives) possess a power of spontaneous movement, but also that the instruments of motion, when these can be dis- covered, are of the very same character in the Plant as in the Animal ; being little hair-like filaments termed cilia (from the Latin cilium, an eyelash), by whose rhythmical vibration the body of which they form part is propelled in definite directions. The peculiar contractility of these cilia cannot be accounted for in either case, any better than in the other ; all we can say is, that it seems to depend upon the continued vital activity of the living substance of which these filaments are prolongations ; and that this contractile substance has a composition essentially the same in the Plant as in the Animal. 147. The plan of organization throughout the Vegetable kingdom presents this remarkable feature of uniformity, — that the fabric of the highest and most complicated Plants, consists of nothing else than an aggregation of the bodies termed cells, every one of which, amongst the lowest and simplest forms of Vegetation, may maintain an independent existence, and may multiply itself most indefinitely, so as to form vast assemblages of similar bodies. And the essential difference between the plans of structure in the two cases lies in this, — that the cells produced by the self-multiplication of the primordial cell of the Protophyte, are all mere repetitions of it and of one another, each living by and for itself, — whilst those produced by the like self-multiplication of the primordial cell in the Oak or Palm, not only remain in mutual connection, but undergo a progressive " differentiation," a fabric being thereby developed, which is composed of a number of distinct organs (stem, leaves, roots, flowers, &c.), each of them characterized by specialities not merely of external form but of intimate structure (the ordinary type of the cell undergoing various modifications, to be described in their proper place, Chap. VIII), and performing actions pe- culiar to itself, which contribute to the life of the Plant as a whole. Hence, as was first definitely stated by Schleiden (see Introduc- tion, p. 43), it is in the life-history of the individual cell, that we find the true basis of the study of Vegetable Life in general. And we shall now inquire, therefore, what information on this point we derive from Microscopic research. In its most com- pletely developed form, the Vegetable Cell may be considered as a closed membranous bag or vesicle, containing a fluid cell-sap ; and thus we have to consider separately the cell-wall and the cell- contents. The " cell- wall" is composed of two layers, of very different composition and properties. The inner of these, which has received the name of primordial utricle, appears to be the one first formed, and most essential to the existence of the cell ; it is extremely thin and delicate, so that it escapes attention so long as it remains in contact with the external layer ; and it is only brought into view when separated from this, either by develop- mental changes (Fig. 107), or by the influence of reagents which CHARACTERS OF THE VEGETABLE CELL. 249 cause it to contract by drawing forth part of its contents (Fig. 175). Its composition is indicated, by the effects of reagents, to be albuminous; that is, it agrees with the formative substance of the Animal tissues, not only in the proportions of oxygen, hydro- gen, carbon, and nitrogen which it contains, but also in the nature of the compound formed by the union of these elements. The external layer, on the other hand, though commonly re- garded as the proper " cell- wall," is generated on the surface of the primordial utricle, after the latter has completely inclosed the cavity and its contents, so that it takes no essential part in the formation of the cell. It is usually thick and strong in com- parison with the other, and may often be shown to consist of several layers. In its chemical nature it is altogether dissimilar to the primordial utricle ; for it is essentially composed of cellu- lose, a substance containing no nitrogen, and nearly identical with starch. The relative offices of these two membranes are very different ; for whilst there are many indications that the primordial utricle continues to participate actively in the vital operations of the cell, it seems certain that the cellulose-wall takes no concern in them, but is only their product ; its function being simply protective. The contents of the vegetable-cell, being usually more or less deeply colored, have received the col- lective designation of endochrome, (or internal coloring substance) ; and they essentially consist of a layer of colorless " protoplasm" (or organizable fluid, containing albuminous matter in combina- tion with dextrine or starch-gum) in immediate contact with the primordial utricle, within which is the more watery cell-sap, particles of chlorophyll or coloring substance being diffused through both, or through the former only. 148. But although these component parts may be made out without any difficulty in a large proportion of Vegetable cells, yet they cannot be distinguished in some of those humble organ- isms, which are nearest to the border ground between the two kingdoms. For in them we find the u cell-wall" very imperfectly differentiated from the " cell-contents;" the former not having by any means the firmness of a perfect membrane, and the latter not possessing the liquidity which elsewhere characterizes them. And in some instances, the cell appears to be represented only by a mass of endochrome, so viscid as to retain its external form without any limitary membrane, though the superficial layer seems to have a firmer consistence than the interior substance ; and this may or may not be surrounded by a gelatinous-looking envelope, which is equally far from possessing a membranous firmness, and^yet is the only representative of the cellulose-wall. This viscid endochrome consists, as elsewhere, of a colorless protoplasm, through which coloring particles are diffused, some- times uniformly, sometimes in local aggregations, leaving parts of the protoplasm uncolored. The superficial layer, in particular, is frequently destitute of color; and the " primordial utricle" ap- 250 MICROSCOPIC FORMS OF VEGETABLE LIFE. pears to be formed by its solidification. In the interior of the viscid mass, are commonly found vacuoles, which are distinguished from the surrounding substance by their difference in refracting power ; these, however, are not usually void spaces, but are cavi- ties in the protoplasm occupied by fluid of a more watery con- sistence; and this " vacuolation" of the interior, which increases until the cell-contents have almost entirely lost their original viscidity and are of a more watery character, seems to take place part passu with the consolidation of the exterior into distinct membranous walls ; so that the development of a perfect cell out of a rudimentary mass of endochrome, may be stated to consist essentially in the gradual differentiation of its substance, which was at first a nearly homogeneous viscid mass, into the solid cell-wall and the liquid cell-contents. It is interesting to ob- serve, at the very outset of our inquiry into the nature of Organ- ization and Vital action, so characteristic an illustration of the great law of Von Bar, already referred to (pp. 52, 55). 149. Now among the Protophytes or simplest Plants, on the examination of which we are about to enter, there are many, of which every single cell is not only capable of living in a state of isolation from the rest, but even normally does so ; and thus, in the ordinary phraseology, every cell is to be accounted a ''dis- tinct individual." There are others, again, of which shapeless masses are made up by the aggregation of contiguous cells, which, though quite capable of living independently, remain attached to each other by the mutual fusion (so to speak) of their gela- tinous investments. And there are others, moreover, in which a definite adhesion exists between the cells, and in which regular plant-like structures are thus formed, notwithstanding that every cell is still but a repetition of every other, and is capable of living independently if detached, so as to answer to the designation of a " unicellular" or single-celled plant. These different conditions we shall find to arise out of the mode in which each particular species multiplies by binary subdivision (§ 150) : for where the pair of cells that is produced by the segmentation of the previous cell, undergo a complete separation from one another, they will henceforth live quite separately; but if, instead of undergoing this complete fusion, they should be held together by the inter- vening gelatinous envelope, a shapeless mass results from re- peated subdivisions not taking place on any determinate plan ; and if, moreover, the binary subdivision should always take place in a determinate direction, a long narrow filament (Fig. 104, D), or a broad flat leaf-like expansion (Fig. 104, G), may be generated. To such extended fabrics, the term " unicellular plants" can scarcely be applied with propriety, since they may be built up of many thousands or millions of distinct cells, which have no disposition to separate from each other spontaneously. Still they correspond with those which are strictly unicellular, in the absence of any differentiation, either in structure or in GENERAL CHARACTERS OF PROTOPHYTES. 251 actions, between their component cells; each one of these being a repetition of the rest, and no relation of mutual dependence existing among them. All such organisms may well be included under the general term of Protophytes, by which it is convenient to designate the primitive or elementary forms of Vegetation ; and we shall now enter, in such detail as the nature of the pre- sent Treatise allows, into the history of those forms of the group, which present most of interest to the Microscopist, or which best serve to illustrate the general doctrines of Physiology. 150. The life-history of one of these Unicellular Plants, in its most simple form, can scarcely be better exemplified than in the Palmoglcea macrococca (Kutzing); one of those humble kinds of vegetation which spreads itself as a green slime over damp stones, walls, &c. When this slime is FIG. 67. examined with the microscope, it is found to consist of a multitude of green cells (Fig. 67, A), each surrounded by a ge- latinous envelope ; the cell, which does not seem to have any distinct membranous wall, is filled with granular particles of a green color ; and a "nucleus" may some- times be distinguish- ed through the midst of these. When treat- ed with tincture of iodine, however, the green contents of the cell are turned to a brownish hue, and a dark-brown nucleus is distinctly shown (G). Other cells are seen (B), which are considerably elongated, some of them beginning to present a sort of hour-glass contraction across the middle ; in these is commencing that curious multiplication by duplicative subdivision, which is the mode in which increase nearly always takes place throughout the Vegetable kingdom ; and when cells in this condition are treated with tincture of iodine, the nucleus is seen to be undergoing the like elongation and constriction (H). A more advanced state of the process of subdivision is seen at c, in which the constriction has proceeded to the extent of com- pletely cutting off the two halves of the cell, as well as of the nucleus (i) from each other, though they still remain in mutual contact ; but in a yet later stage, they are found detached from each other (D), though still included within the same gelatinous Various phases of development of Palmoglcea macrococca: A, full-grown cell ; B, c, D, E, successive stages of binary subdivision; F, row of cells, produced by succession of subdivisions ; G, H, i, cells treated by iodine; K, L, M, cells in conjunction. 252 MICROSCOPIC FORMS OF VEGETABLE LIFE. envelope. Each new cell then begins to secrete its own gela- tinous envelope ; so that, by its intervention, the two are usually soon separated from one another (E). Sometimes, however, this is not the case; the process of subdivision being quickly re- peated, before there is time for the production of the gelatinous envelope, so that a series of cells (F), hanging on one to another, is produced. There appears to be no definite limit to this kind of multiplication ; and extensive areas may be quickly covered, in circumstances favorable to the growth of the plant, by the products of the duplicative subdivision of one primordial cell. This, however, is simply an act of G-rowth, precisely analogous to that by which any one of the higher forms of Vegetation ex- tends itself, and differing only in this, that the cells produced by each act of cell-subdivision in the present case exactly resemble that from which they sprang ; whilst in the case of more highly organized Plants, they gradually become differentiated to a greater or less degree, so that special " organs" are evolved, which take upon themselves dissimilar yet mutually dependent parts, in the economy of the entire organism, 151. The process which represents the G-eneration of the higher Plants is here performed in a manner so simple, that it would not be recognized as such, if we were not able to trace it up through a succession of modes of gradually increasing complexity, until we arrive at the elaborate operations which are concerned in the production and fertilization of the seeds of Flowering Plants. For it consists in nothing else than the reunion or fusion together of any pair of cells (KJ, — a process which is termed Conjugation; and it is characteristic of this humble plant, and shows how imperfect must be the consistence of its cell-mem- brane, that this seems to enter into the fusion, no less completely than the cell-contents. The communication is at first usually made by a narrow neck or bridge (K) ; but before long it extends through a large part of the contiguous boundaries (L) ; and at last the two cells are seen to be completely fused into one mass (M); which is termed the spore. Each "spore" thus formed is the "primordial cell" of a new generation, into which it evolves itself by successive repetitions of the process of binary subdivi- sion. It is curious to observe, that during the conjugating pro- cess, a production of oil-particles takes place in the cells ; these at first are small and distant, but gradually become larger and approximate more closely to each other, and at last coalesce so as to form oil-drops of various sizes, the green granular matter disappearing; and the color of the conjugated body changes, with the advance of this process, from green to a light yellowish- brown. When the spore begins to vegetate, on the other hand, producing a pair of new cells by binary subdivision, a converse change takes place ; the oil-globules disappear, and green granu- lar matter takes their place. Now this is precisely what occurs in the formation of seed among the higher Plants ; for starchy STILL AND MOTILE CONDITIONS OF PROTOPHYTES. 258 substances are transformed into oil, which is stored up in the seed for the nutrition of the embryo, and is applied, during ger- mination, to the purposes which are at other times answered by starch or chlorophyll. The growth of this little plant appears to be favored by cold and damp ; its generation, on the other hand, is promoted by heat and dryness; and it is obvious that the spore-cell must be endowed with a greater power of resisting this, than the vegetating plant has, since the species would other- wise be destroyed by every drought. 152. If the preceding sketch really comprehends the whole life-history of the humble plant to which it relates, this history is much more simple than that of other forms of vegetation, which, without appearing to possess an essentially higher struc- ture, present themselves under a much greater variety of forms and conditions. One of the most remarkable of these varieties is the motile condition, which seems to be common, in some stage or other of their existence, to a very large proportion of the lower forms of aquatic vegetation ; and which usually de- pends upon the extension of the primordial utricle into one or two thread-like filaments (Fig. 68, H-L), endowed with the power of executing rhythmical contractions, whereby the cell is im- pelled through the water. As an illustration of this peculiar mode of activity, which was formerly supposed to betoken Ani- mal life, a sketch will be given of the history of a plant, the Protococcus pluvialis, which is not uncommon in collections of rain-water,1 and which, in its motile condition, has been very 1 The Author had under his own observation, about eight years ago, an extraordinary abundance of what he now feels satisfied must have been this plant, in a rain-water cistern, which had been newly cleaned out. His notice was attracted to it, by seeing the surface of the water covered with a green froth, whenever the sun shone upon it. On examining a portion of this froth under the Microscope, he found that the water was crowded with green cells in active motion; and although the only bodies at all re- sembling them, of which he could find any description, were the so-called Animalcules, constituting the genus Chlamydomonas of Prof. Ehrenberg, and very little was known at that time of the " motile" conditions of Plants of this description, yet of the vegetable nature of these bodies he could not entertain the smallest doubt. They appeared in freshly collected rain-water, and could not, therefore, be deriving their support from organic matter; under the influence of light, they were obviously decomposing carbonic acid and liberating oxygen, and this influence he found to be essential to the continu- ance of their growth and development, which took place entirely upon the Vegetative plan. Not many days after the Protophyte first appeared in the water, a few Wheel- Animalcules presented themselves; these fed greedily upon it, and increased so rapidly (the weather being very warm) that they soon became almost as crowded as the cells of the Protococcus had been ; and it was probably due in part to their voracity, that the plant soon became less abundant, and before long disappeared altogether. Had the Author been then aware of its assumption of the "still" condition, he might have found it at the bottom of the cistern, after it had ceased to present itself at the surface. The account of this Plant given above, is derived from that of Dr. Cohn, in the " Nova Acta Acad. Nat. Curios." (Bonn, 1850), torn, xxii; of which an abstract by Mr. George Busk is contained in the "Botanical and Physiological Memoirs," published by the Ray Society for 1853. This excellent observer states that he kept his plants for observation in little glass vessels, having the form of a truncated cone, about two inches deep, and one inch and a quarter in diameter, with a flat bottom polished on both sides, and filled with water to the depth of from two to three lines. "It was only in vessels of this kind," he says, "that he was able to follow the development of a number of various 254 MICROSCOPIC FORMS OF VEGETABLE LIFE. commonly regarded as an Animalcule, its different states having been described under several different names. 153. In the first place, the color of these cells varies consider- ably; since, although they are usually green at the period of their most active life, they are sometimes red; and their red form has received the distinguishing appellation of ITcematococcus. Very commonly the red coloring matter forms only a central mass of greater or less size, having the appearance of a nucleus (as shown in Fig. 68, E) ; and sometimes it is reduced to a single granular point, which has been erroneously represented by Prof. Ehrenberg as the eye of these so-called Animalcules. It is quite certain that the red coloring substance is very nearly related in FIG. 68. Various phases of development of Protococcus pluvialis : — A, an encysted cell, which has passed into the " still" condition; B, division of a "still" cell into two; c, another mode of division into two, each primordial vesicle having developed a cellulose envelope around itself, whilst yet within the original cell; D, division of an encysted cell into four; E, division of an encysted cell into eight; F. division of an encysted cell into thirty-two segments; G, motile gonidia (zoospores) after their escape from the original cell; H, a primordial utricle, without cellulose envelope, furnished with two cilia; i. a similar primordial utricle, with distinct cellulose envelope, and threads of protoplasm extending towards it; K, an encysted primordial utricle, pointed at both ends, and furnished wilhtwo cilia; L, an encysted primordial utricle, of which nearly half is composed of a colorless granular substance, enclosing a red body resembling a nucleus. its chemical character to the green, and that the one may be converted into the other; though the conditions under which this conversion may take place are not precisely known. In the still form of the cell, with which we may commence the history cells throughout its whole course." Probably he would have found the glass tube cells represented in Fig. 65, if he had been acquainted with them, to answer his purpose just as well as these specially constructed vessels. STILL AND MOTILE CONDITIONS OF PROTOCOCCUS. 255 of its life, we find a mass of endochrome, consisting of a color- less protoplasm, through which red or green colored granules are more or less uniformly diffused; on the surface of this endochrome, the colorless protoplasm is condensed into a more consistent layer, forming an imperfect "primordial utricle;" and this is surrounded by a tolerably firm layer, which seems to con- sist of cellulose or of some modification of it. Outside this (as shown in Fig. 68, A), when the " still" cell is formed by a change in the condition of a cell that has been previously " motile," we find another envelope, which seems to be of the same nature, but which is separated by the interposition of aqueous fluid; this, however, may be altogether wanting. The multiplication of the "still" cells by self-division takes place as in the previous instance; the endochrome, enclosed in its primordial utricle, first undergoing separation into two halves (as seen at B), and each of these halves subsequently developing a cellulose enve- lope around itself, and undergoing the same division in its turn. Thus 2, 4, 8, 16 new cells are successively produced; and these are sometimes set free by the complete dissolution of the enve- lope of the original cell ; but they are more commonly held together by its transformation into a gelatinous investment, in which they remain imbedded. Sometimes the contents of the primordial utricle subdivide at once into four segments (as at D), of which every one forthwith acquires the characters of an inde- pendent cell ; but this, although an ordinary method of multi- plication among the "motile" cells, is comparatively rare in the "still" condition. Sometimes, again, the cell-contents of the "still" form subdivide at once into eight portions, which, being of small size, and endowed with motile power, may be con- sidered as "zoospores;" it is not quite clear what becomes of these ; but there is reason to believe that some of them retain their motile powers, and, after increasing in size, develope an investing cyst, like the free primordial utricles to be presently described ; that others produce a firm cellulose envelope, and become "still" cells; and that others (perhaps the majority) perish without any further change. 154. When the ordinary self-division of the " still" cells into two segments has been repeated four times, so as to produce 16 cells — and sometimes at an earlier period — the new cells thus produced assume the "motile" condition ; being liberated before the development of the cellulose envelope, and becoming fur- nished with two long vibratile filaments, which appear to be extensions of the primordial utricle (H). In this condition, it seems obvious that the colorless protoplasm is more developed relatively to the coloring matter, than it is in the "still" cells; it generally accumulates in the part from which the vibratile filaments or cilia proceed, so as to form a sort of transparent beak (H, K, L); and it usually contains "vacuoles," occupied only by clear aqueous fluid, which are sometimes so numerous as to 256 MICROSCOPIC FORMS OF VEGETABLE LIFE. take in a large part of the cavity of the cell, so that the colored contents seem only like a deposit on its walls. Before long, this "motile" primordial utricle acquires a peculiar saccular invest- ment, which seems to correspond with the cellulose envelope of the "still" cells, but which is not so firm in its consistence (i, K, L). Thread-like extensions of the protoplasm, sometimes con- taining colored globules, are not unfrequently seen to radiate from the primordial utricle towards the exterior of this envelop- ing bag (i) ; these are rendered more distinct by iodine, and can be made to retract by means of reagents ; and their existence seems to show, on the one hand, that the transparent space through which they extend themselves is only occupied by a watery liquid, and on the other, that the layer of protoplasm which constitutes the primordial utricle, is far from possessing the tenacity of a completely formed membrane. The vibratile filaments pass through the cellulose envelope, which invests them with a sort of sheath ; and in the portion that is within this sheath, no movement is seen. During the active life of the " motile" cells, the vibration of these cilia is so rapid, that it can be recognized only by the currents it produces in the water, through which the cells are rapidly propelled ; but when the mo- tion becomes slacker, the filaments themselves are readily dis- tinguishable; and they maybe made more obvious by the addition of iodine. The multiplication of these motile cells may take place in various modes, giving rise to a great variety of appear- ances. Sometimes they undergo a regular binary subdivision, whereby a pair of motile cells is produced (c), each resembling its single predecessor in possessing the cellulose investment, the transparent beak, and the vibratile filaments, before the solution of the original investment. Sometimes, again, the contents of the primordial cell undergo a segmentation in the first instance into four divisions (D) ; which may either become isolated by the dissolution of their envelope, and may separate from each other in the condition of free primordial utricles (u), developing their cellulose investments at a future time ; or may acquire their cel- lulose investments (as in the preceding case) before the solution of that of the original cell ; and sometimes, even after the disap- pearance of this, and the formation of their own independent in- vestments, they remain attached to each other at their beaked extremities, the primordial utricles being connected with each other by peduncular prolongations, and the whole compound body having the form of a +. This quaternary segmentation appears to be a more frequent mode of multiplication among the "motile" cells, than the subdivision into two; although as we have seen, it is less common in the "still" condition. So, also, a primary segmentation of the entire endochrome of the "mo- tile" cells, into 8, 16, or even 32 parts, may take place (E, F), thus giving rise to as many minute primordial cells. These, when they are set free, and possess active powers of movement, rank VARIOUS CONDITIONS OF PROTOCOCCUS. 257 as "zoospores" (G): which may either develope a loose cellulose investment or cyst, so as to attain the full dimensions of the or- dinary motile cells (i, K), or may become clothed with a dense envelope, and lose their vibratile cilia, thus passing into the "still" condition (A); and this last transformation may even take place, before they are set free from the envelope within which they were produced, so that they constitute a mulberry-like mass, which fills the whole cavity of the original cell, and is kept in motion by its cilia. 155. All these varieties, whose relation to each other has been clearly proved by watching the successional changes that make up the history of this one Plant, have been regarded as consti- tuting, not merely distinct species, but distinct genera of Animal- cules; such us, Chlamydomonas, Euglena, Trachelomonas, G-yges, Q-onium, Pandorina, Botryocystis, Uvella, Syncrypta, Monas, As- tasia, Bodo, and probably many others.1 Certain forms, such as the "motile" cells (i, K, L), appear in a given infusion, at first exclusively and then principally ; they gradually diminish, be- come more and more rare, and finally disappear altogether, being replaced by the " still" form. After some time, the number of the "motile" cells again increases, and reaches, as before, an ex- traordinary amount ; and this alteration may be repeated several times in the course of a few weeks. The process of segmentation is often accomplished with great rapidity. If a number of motile cells be transferred from a larger glass into a small capsule, it will be found, after the lapse of a few hours, that most of them have subsided to the bottom ; in the course of the day, they will all be observed to be upon the point of subdivision ; on the fol- lowing morning, the divisional brood will have become quite free ; and on the next, the bottom of the vessel will be found covered with a new brood of self-dividing cells, which again pro- ceed to the formation of a new brood, and so on. The activity of motion and the activity of multiplication, seem to stand, in some degree, in a relation of reciprocity to each other ; for the self-dividing process takes place with greater rapidity in the "still" cells, than in the "motile." 1 In the above sketch, the Author has presented the facts described by Dr. Cohn, under the relation which they seemed to him naturally to bear; for the membrane which immediately surrounds the primordial utricle of the "still*1 cells, appears to him to be essentially the same with the sacculated investment of the " motile" form, since it differs only in its greater density, and in the absence of the interposed fluid. It is dis- tinctly stated by Cohn, towards the conclusion of his Memoir, that the one like the other consists of cellulose, since both alike give the characteristic blue color with a very dilute solution of iodine, and with moderately diluted sulphuric acid ; yet he speaks of what he terms the " encysted xoospore" (i, K, L), as being formed by one cell uithin another, the outer cell having a true cell-membrane and aqueous contents, but being destitute of primordial utricle; whilst the inner cell has denser, colored contents, but is without the true cell-membrane. Having enjoyed, since the above was written, the opportunity of personally communicating with Dr. Cohn with regard to the question to which it refers, the Author is glad to be able to state, that Dr. Cohn's later observations have led him to adopt a view of the relationship of the "still"' and "motile" forms, which is in essential accordance with his own. . 17 258 MICROSCOPIC FORMS OF VEGETABLE LIFE. 156. What are the precise conditions which determine the transition from one state to the other, cannot yet be precisely stated ; but the influence of certain agencies can be predicted with tolerable certainty. Thus it is only necessary to pour the water containing these organisms, from a smaller and deeper into a larger and shallower vessel, at once to determine segmen- tation in numerous cells, — a phenomenon which is observable also in many other Protophytes. The "motile" cells seem to be favorably affected by light, for they collect themselves at the sur- face of the water and at the edges of the vessel ; but when they are about to undergo segmentation, or to pass into the " still" condition, they sink to the bottom of the vessel, or retreat to that part of it in which they are least subjected to light. "When kept in the dark, the "motile" cells undergo a great diminution of their chlorophyll, which becomes very pale, and is diffused, in- stead of forming definite granules ; they continue their move- ment, however, uninterruptedly, without either sinking to the bottom, or passing into the still form, or undergoing segmenta- tion. A moderate warmth, particularly that of the vernal sun, is favorable to the development of the u motile" cells ; but a tem- perature of excessive elevation prevents it. Eapid evaporation of the water in which the " motile" forms of Protococcm may be contained, kills them at once ; but a more gradual loss, such as takes place in deep glasses, causes them merely to pass into the " still" form ; and in this condition, — especially when they have assumed a red hue, — they may be completely dried up, and may remain in a state of dormant vitality for many years. It is in this state that they are wafted about in atmospheric currents, and that, being brought down by the rain into pools, cisterns, &c., they may present themselves where none had been previously known to exist; and there, under favorable circumstances, they may undergo a very rapid multiplication, and may maintain them- selves until the water is dried up, or some other change occurs which is incompatible with the continuance of their vital activity. They then very commonly become red throughout, the red color- ing substance extending itself from the centre towards the cir- cumference, and assuming an appearance like that of oil-drops ; and these red cells, acquiring thick cell-walls and a mucous en- velope, float in flocculent aggregations on the surface of the water. This state seems to correspond with the "winter spores" of other Protophytes ; and it may continue until warmth, air, and moisture, cause the development of the red cells into the ordinary " still" cells, green matter being gradually produced, until the red substance forms only the central part of the en do- chrome. After this occurs the cycle of changes which has been already described ; and the Plant may pass through a long series of these, before it returns to the state of the red thick- walled cell, in which it may again remain dormant for an unlimited period. Even this cycle, however, cannot be regarded as completing the VOLVOCINE^l. 259 history of the species before us ; since it does not include the performance of any true Generative act. There can be little doubt that, in some stage of its existence, a " conjugation" of two cells occurs, as in the preceding case ; and the attention of ob- servers should be directed to its discovery, as well as to the de- tection of other varieties in the condition of this interesting little Plant, which will be probably found to present themselves before and after the performance of that act. 157. From the composite motile forms of the preceding type, the transition is easy to the group of Vol- vocinece, — an assemblage of minute Plants Fm 69 of the greatest interest to the Microscopist, on account both of the Animalcule-like activity of their movements, and of the freat beauty and regularity of their forms, he most remarkable example of this group, is the well-known Volvox globator (Fig. 69), or "globe-animalcule;" which is not un- common in fresh-water pools, and which, attaining a diameter of l-30th of an inch, may be seen with the naked eye, when the drop containing it is held up to the light, voivox ciobator. swimming through the water which it in- habits. Its onward motion is usually of a rolling kind ; but it sometimes slides smoothly along, without turning on its axis ; whilst sometimes, again, it rotates like a top, without changing its position. When examined with a sufficient magnifying power, the Volvox is seen to consist of a hollow sphere, com- posed of a very pellucid material, which is studded at regular intervals with minute green spots, and which is often (but not constantly) traversed by green threads connecting these spots to- gether. From each of the spots proceed two long cilia ; so that the entire surface is beset with these vibratile filaments, to whose combined action its movements are due. Within the external sphere, there may generally be seen from two to twenty other globes, of a darker color, and of varying sizes ; the smaller of these are attached to the inner surface of the investing sphere, and project into its cavity ; but the larger lie freely within the cavity, and may often be observed to revolve by the agency of their own ciliary filaments. After a time, the original sphere bursts, the contained sphericles swim forth and speedily develope themselves into the likeness of that within which they have been evolved; their component particles, which are at first closely aggregated together, being separated from each other by the in- terposition of the transparent pellicle. It was long supposed that the Volvox was a single Animal ; and it was first shown to be a composite fabric, made up of a repetition of organisms in all respects similar to each other, by Prof. Ehrenberg ; who, however, considered these organisms as Monads, and described 260 MICROSCOPIC FORMS OF VEGETABLE LIFE. them as each possessing a mouth, several stomachs, and an eye ! Our present knowledge of their nature, however, leaves no doubt of their Vegetable character ; and the peculiarity of their history renders it desirable to describe it in some detail. 158. Each of the so-called "Monads" is in reality a some- what flask-shaped mass of endochrome, about l-3000th of an inch in diameter; consisting, as in the previous instances, of chlorophyll-granules, diffused through a colorless protoplasm (Fig. 70, i, L) ; and bounded by a layer of condensed protoplasm, which represents a primordial utricle, but is obviously far from having attained a membranous consistence. It is prolonged out- wardly (or towards the circumference of the sphere) into a sort of colorless peak or proboscis, from which proceed two long vibratile cilia (L); and it is invested by a pellucid or " hyaline" envelope (i, d) of considerable thickness, the borders of which are flattened against those of other similar envelopes (E, c c), but which does not appear to have the tenacity of a true mem- brane. It is impossible not to recognize the precise similarity between the structure of this body, and that of the motile " en- cysted" cell of Protococcus pluvialis (Fig. 68, K) ; there is not, in fact, any perceptible difference between them, save that which arises from the regular aggregation, in Volvox, of the cells which normally detach themselves from one another in Protococcus. The presence of cellulose in the hyaline substance is not indi- cated, in the ordinary condition of Volvox, by the iodine and sulphuric acid test, though the use of " Schulz's solution" gives to it a faint blue tinge ; there can be no doubt of its existence, however, in the hyaline envelope of what has been termed Vol- vox aureus, which is in reality nothing but the " winter spore" of Volvox glolator. The cilia and endochrome, as in the motile forms of Protococcus, are tinged of a deep brown by iodine, with the exception of one or two particles in each cell, which, being turned blue, may be inferred to be starch ; and when the contents of the cell are liberated, bluish flocculi, apparently in- dicative of the presence of cellulose, are brought into view by the action of sulphuric acid and iodine. All these reactions are strictly vegetable in their nature. When the cell is approaching maturity, its endochrome always exhibits one or more " vacuoles" (Fig. 70, i, a a), of a spherical form, and usually about one-third of its own diameter ; and these " vacuoles" (which are the so-called " stomachs" of Prof. Ehrenberg) have been observed by Mr. G. Busk to undergo a very curious rhythmical contraction and dila- tation at intervals of about 40 seconds ; the contraction (which seems to amount to complete obliteration of the cavity of the vacuole) taking place rapidly or suddenly, whilst the dilatation is slow and gradual. This curious action ceases, however, as the cell arrives at its full maturity ; a condition which seems to be marked by the greater consolidation of the primordial utricle, by the removal or transformation of some of the chlorophyll (the STRUCTURE OF VOLVOX GLOBATOR. 261 greater part of the coloring matter contracting into a small irre- gular mass, which adheres to the bottom or side of the cell, leaving the rest of the cavity clear and transparent), and the formation of the red spot (6), which obviously consists, as in Protococcus, of a peculiar modification of chlorophyll. 159. Each mass of endochrome normally communicates with those in nearest proximity with it, by extensions of its own sub- stance, which are sometimes single and sometimes double (E, b b)y and these connecting processes necessarily cross the lines of division between their respective hyaline investments. The thickness of these processes varies very considerably ; for some- times they are broad bands, and in other cases mere threads ; whilst they are occasionally wanting altogether. This difference seems partly to depend upon the age of the specimen, and partly upon the abundance of nutriment which it obtains ; for, as we shall presently see, the connection is most intimate at an early period, before the hyaline investments of the cells have increased so much as to separate the masses of endochrome to a distance from one another (B, c, D); whilst in a mature individual, in which this separation has taken place to its full extent, and the nutritive processes have become less active, the masses of endo- chrome very commonly assume an angular form, and the con- necting processes are drawn out into threads (as seen at E), or they retain their globular form, and the connecting processes altogether disappear. The influence of reagents, or the infiltra- tion of water into the interior of the hyaline investment, will sometimes cause the connecting processes (as in Protococcus^ § 154), to be drawn back into the central mass of endochrome ; and they will also retreat on the mere rupture of the hyaline in- vestment; from these circumstances it may.be inferred, that they are not enclosed in any definite membrane. On the other hand, the connecting threads are sometimes seen as double lines, which seem like tubular prolongations of a consistent membrane, with- out any protoplasmic granules in their interior. It is obvious, then, that an examination of a considerable number of specimens, exhibiting various phases of conformation, is necessary to de- monstrate the nature of these communications ; but this may be best made out by attending to the history of their development, which we shall now describe. 160. The spherical body of the young Volvox (Fig. 78, A) is composed of an aggregation of somewhat angular masses of en- dochrome (5), separated by the interposition of hyaline substance ; and the whole seems to be enclosed in a distinctly membranous envelope, which is probably the distended hyaline investment of the primordial cell, within which, as will presently appear, the entire aggregation originated. In the midst of the polygonal masses of endochrome, one mass (a), rather larger than the rest, is seen to present a circular form ; and this, as will presently appear, is the originating cell of what is hereafter to become a 262 MICROSCOPIC FORMS OF VEGETABLE LIFE. new sphere. The growing Yolvox at first increases in size, not only by the interposition of new hyaline substance between its component masses of endochrome, but also by an increase in these masses themselves (B, a), which come into continuous con- nection with each other by the coalescence of processes (b) which FIG. 70. Various Stages in the Development of Volvox Globator. they severally put forth ; at the same time, an increase is ob- served in the size of the globular cell (c), which is preliminary to its binary subdivision. A more advanced stage of the same developmental process is seen at c ; in which the connecting pro- cesses (a a) are so much increased in size, as to establish a most intimate union between the masses of endochrome, although the increase of the intervening hyaline substance carries these masses apart from one another; whilst the endochrome of the central DEVELOPMENT OF VOLVOX GLOBATOR. 263 globular cell lias undergone segmentation into two halves. In the stage represented at D, the masses of endochrome have been still more widely separated by the interposition of hyaline sub- stance ; each has become furnished with its pair of ciliary fila- ments ; and the globular cell has undergone a second segmenta- tion. Finally at^ E, which represents a portion of the spherical wall of a mature Yolvox, the endochrome masses are observed to present a more scattered aspect, partly on account of their own reduction in size, and partly through the interposition of a greatly increased amount of hyaline substance, which is secreted from the surface of each mass ; and that portion which belongs to each cell, standing to the endochrome mass in the relation of the cellulose coat of ordinary cells to their primordial utricle, is frequently seen to be marked out from the rest by delicate lines of hexagonal areolation ( front view' c' , 1 frustule 111 the act of self-division. are among the commonest forms of Diatomaceae. In Licmophora (Fig. 91) we meet with a dif- ferent mode of growth ; for the newly formed part of the stipes, instead of itself becoming double with each act of self-division of the frustule, increases in breadth, while the frustules them- selves remain coherent ; so that a beautiful fan-like arrangement is produced. A splitting away of a few frustules seems occasion- ally to take place, from one side or the other, before the elonga- tion of the stipes ; so that the entire plant presents us with a more or less complete flabella or fan upon the summit of the branches, with imperfect nabellse or single frustules irregularly scattered throughout the entire length of the footstalk. This beautiful plant is marine, and is parasitic upon sea-weeds and zoophytes. 296 MICROSCOPIC FORMS OF VEGETABLE LIFE. 186. In the group at which we now arrive, there is more or less of permanent connection between the frustules themselves ; and this may depend merely upon the cohesion of their surfaces, or may be occasioned by the persis- FIG. 91. tence of the connecting membrane of the valves after the completion of the self- division of the frustules. To the former division belong several genera, the form of whose frustules is more or less elongated, so that the filament formed by their co- hesion is a flattened band. If the two extremities of the frustule be of equal breadth, as in Bacillaria,) the band will be straight; but if one be broader than the other, so that the frustule in front view has a cuneate or wedge- like form, the filament will be curved, as in the beauti- ful Meridian circulars (Fig. 92, A). Although these, when gathered and placed under the microscope, pre- sent the appearance of circles overlying one another, they really grow in a helical (screw-like) form, making several continuous FIG. 92. A, Meridian circulare :— B, Bacillaria paradoxa. turns. This Diatom abounds in many localities in this country; but there is none in which it presents itself in such rich luxu- riance, as in the mountain brooks about "West Point in the United States, the bottoms of which, according to Prof. Bailey, FRAGILLARIA — ACHNANTHES. 297 FIG. 93. " are literally covered in the first warm days of spring with a ferruginous colored mucous matter, about a quarter of an inch thick, which on examination by the microscope, proves to be filled with millions and millions of these exquisitely beautiful siliceous bodies. Every submerged stone, twig and spear of grass is enveloped by them ; and the waving plume-like appear- ance of a filamentous body covered in this way is often very ele- gant." The genus Bacillaria, so named from the staff-like form of its frustules, is now limited to the species B. paradoxa (Fig. 92, B), whose remarkable movements have been already de- scribed (§ 179). Owing to this displacement of the frustules, its filaments seldom present themselves with straight parallel sides, but nearly always in forms more or less oblique, such as those represented above. This curious object is an inhabitant of salt or of brackish water. Many of the species formerly ranked under this genus are now referred to the genus Diatoma (§ 187) ; to which also the genus Fragillaria is nearly allied, the difference between them lying chiefly in the mode of adhe- sion of the frustules. These in Fragillaria form long straight filaments with parallel sides ; the filaments, however, as the name of the genus im- plies, very readily break up into their component frustules, often separating at the slightest touch. Its various species are very common in pools and ditches. Among the numerous genera be- longing to this group, we may stop to notice Achnanthes; some of the species of which, par- ticularly A. longipes (Fig. 93), are furnished with a single nearly straight stipes, attached to one end of the lower margin of the first frustule of the filament. There is a curious difference in the markings of the valves of the upper and lower frustules ; for while both are traversed by strife, which are resolvable under a sufficient power into rows of dots, as well as by a longitu- dinal line, which sometimes has a nodule at each end (as in Navicula), the lower valve («), has also a transverse line, forming a stauros or cross, which is wanting in the upper valve (e). A persistence of the connecting membrane may sometimes be observed in this genus, so as to form an additional connection between the cells; thus, in Fig. 93, it not only holds to- gether the two new frustules resulting from the subdivision of the lowest cell, a, which are not yet completely separated the one from the other, but it may be observed to invest the two frus- tules b and gonidia, whilst the latter are known as macrogomdia. The offices of these different classes of reproductive bodies are only now beginning to be understood; and the inquiry is one so fraught with Physiological interest, and, from the facility of growing these plants in artificial Aquaria, may be so easily pur- sued, that it may be hoped that Microscopists will apply them- selves to it so zealously, as not long to leave any part of it in obscurity. 199. The family Conjugates agrees with that of the Confervacece in its mode of growth, but differs from it in the plan in which its generative process is performed ; this being accomplished by an act of "conjugation," resembling that which has been de- scribed in the simplest Protophytes. These plants are not found so much in running streams, as in waters that are perfectly still, such as those in ponds, reservoirs, ditches, or marshy grounds ; and they are for the most part unattached, floating freely at or near the surface, especially when buoyed up by the bubbles of gas which are liberated from the midst of them under the influ- ence of solar light and heat. In an early stage of their growth, whilst as yet the cells are undergoing multiplication by subdi- vision, the endochrome is commonly diffused pretty uniformly through their cavities (Fig. 109, A) ; but as they advance towards the stage of conjugation, the endochrome ordinarily arranges itself in regular spirals (B), but occasionally in some other forms. The act of "conjugation" usually occurs between the cells of two distinct filaments, that happen to lie in proximity to each other ; and all the cells of each filament generally take part in it at once. The adjacent cells put forth little protuberances, which come into contact with each other, and then coalesce by the breaking down of the intervening partitions, so as to establish a free passage between the cavities of the conjugating cells. In some genera of this family (such as Mesocarpus), the conjugating 320 MICROSCOPIC FORMS OF VEGETABLE LIFE. cells pour their endochromes into a dilatation of the passage that has been established between them; and it is there that they com- mingle, so as to form the spore or the embryo-cell. But in the Zygnema (Fig. 108), which is among the commonest and best known forms of Conjugatese, the endochrome of one cell passes over entirely into the cavity of the other; and it is within the latter that the spore is formed (c), the two endochromes coa- FIG. 108. A Various stages of the history of Zygnema quininum:— A, three cells, a, b, c, of u young filament, of which b is undergoing subdivision ; B, two filaments in the first stage of conjugation, showing the spiral disposition of their endochromes, and the protuberances from the conjugating cells ; c, com- pletion of the act of conjugation, the endochromes of the cells of the filament a having entirely passed over to those of filament b, in which the sporangia are formed. lescing into a single mass, around which a firm envelope gradu- ally makes its appearance. Further, it may be generally observed, that all the cells of one filament thus empty themselves, whilst all the cells of the other filaments become the recipients ; here, therefore, we seem to have a foreshadowing of the sexual dis- tinction of the generative cells into " sperm-cells" and "germ- cells," which we have just seen to exist in the Confervaceae (§ 198). And this transition will be still more complete, if (as Itzigsohn has affirmed) the endochrome of certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly color- less spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to the antherozoids of the truly sexual Protophytes.1 1 This group of plants seems to serve as the connecting link between those simple Protophytes in which the sexes are not yet differentiated, and those higher forms in which the distinction between the " sperm-cells" and " germ-cells1' is very apparent. For let it be supposed that in Sphceroplea (§ 198) a conjugation of two adjacent cells were to take place, at that stage in their development in which the endochrome is uniformly arranged in rings, no differentiation of sexes yet showing itself, — the process would in all respects correspond with that of the ordinary Conjugatefe. Again, whilst in Mesocarpus, the two conjugating cells appear to take (as in the Desmidece, § 169) a precisely similar share in the formation of their product, the first stage of differentiation REPKODUCTION OF CH.ETO PHOR ACEJE. 321 200. The ChcetopJioracece constitute another beautiful and in- id plants, of which some species found in fresh and pure water, teresting little group of Confervoid plants, of which some species inhabit the sea, whilst others are fou: FIG. 109. rather in that of gently mov- ing streams, however, than in strongly flowing currents. Ge- nerally speaking, their filaments put forth lateral branches, and extend themselves into arbo- rescent fronds ; and one of the distinctive characters of the group is afforded by the fact, that the extremities of these branches are usually prolonged into bristle-shaped processes (Fig. 109). As in many pre- ceding cases, these plants mul- tiply themselves by the con- version of the endochrome of certain of their cells into "zoo- spores;" and these, when set free, are seen to be furnished with four large cilia. " Rest- ing-spores" have also been seen in many species ; and it is probable that these, as in Con- fervaceee, are true generative products of the fertilization of the contents of "germ-cells" by " antherozoids" developed within " sperm-cells" (§ 198). Nearly allied to the preceding are the Batrachospermece, whose name is indicative of the strong resemblance which their beaded filaments bear to frog-spawn ; these exhibit a somewhat greater complexity of structure, and afford objects of extreme beauty to the Microscopist. The plants of this family are all inhabitants of fresh water, and they are chiefly found in that which is pure and gently flowing. "They are so extremely flexible," says Dr. Hassall, "that they obey the slightest motion of the fluid which surrounds them ; and nothing can surpass the ease and grace of their movements. into sperm-cells and germ-cells is manifested in Zygnema, by the passage of the whole endochrome of those of one filament into the cavities of the other, and by the formation of the spores within the latter. In Spirogyra, moreover, the endochrome of one set of cells becomes converted into antherozoids before conjugation, whilst that of the other aggregates into a sporangial mass; thus exhibiting the second stage of differentiation. Further, there are certain species which agree with the ordinary Conjugates in their general habit, and which form " resting-spores" like theirs, but in which no conjugation has been observed; and it seems not improbable that in these, as in Sphseroplea, the antherozoids make their way out of the sperm-cell by minute apertures in its wall, and swim freely about before finding their way into the germ-cell through the apertures in its wall; — still, however, performing by this means the very same act, as that which is accomplished by the more direct process of conjugation, — viz., the introduction of the contents of the sperm cell into the interior of the germ-cell. 21 Branches of ChcetopJtora elegans. in the act of discharging ciliated zoospores, which are seen, as in motion, on the right. 322 MICROSCOPIC FORMS OF VEGETABLE LIFE. FIG. 110. When removed from the water, they lose all form, and appear like pieces of jelly, without trace of organization ; on immersion, however, the branches quickly resume their former disposition." Their color is for the most part of a brownish-green ; but some- times they are of a reddish or bluish purple. The central axis of each plant is originally composed of a single file of large cylindrical cells laid end to end ; but this is subsequently in- vested by other cells, in the manner to be presently described. It bears, at pretty regular intervals, whorls of short radiating branches, each of them composed of rounded cells arranged in a bead-like row (Fig. 110), and sometimes subdividing again into two, or themselves giving off lateral branches. Each of the pri- mary branches originates in a little protuberance from the primi- tive cell of the central axis, precisely after the manner of the lateral cells of Conferva glomerata (§ 198) ; as this protuberance increases in size, its cavity is cut off by a septum, so as to render it an independent cell ; and by the continual repetition of the process of duplicative subdivision, this single cell be- comes converted into a beaded filament. Certain of these branches, however, instead of radiating from the main axis, grow downwards upon it, so as to form a closely fitting in- vestment, that seems properly to belong to it. Some of the radiating branches grow out into long transparent points, like those of Chaetophoracese ; and it does not seem by any means improbable, that these, like the " horns" of Vaucheria (§ 197), are really antheridia. For within certain cells of other branches, " resting-spores" are formed ; by the agglomeration of which are produced the large dark bodies, that are seen in the midst of the whorls of branches (Fig. 110). 201. This seems the most appropriate place to consider a group of humble plants, having a peculiar interest for Microscopists,— that, namely, of Characece; in which we have a vegetative appa- ratus as simple as that of the Protophytes already described, whilst their reproductive apparatus is even more highly developed than that of the proper Algae. They are for the most part in- habitants of fresh waters, and are found rather in such as are still, than in those which are in motion ; one species, however, may be met with in ditches whose waters are rendered salt by communication with the sea. They may be easily grown for Batractospermum monUifarme. ROTATION IN CELLS OF CHAR A. 323 the purposes of observation, in large glass jars exposed to the light; all that is necessary being to pour off the water occasion- ally from the upper part of the vessel (thus carrying away a film that is apt to form on its surface), and to replace this by fresh water. Each plant is composed of an assemblage of long tubi- form cells, placed end to end ; with a distinct central axis, around which the branches are disposed at intervals with great regular- ity (Fig. Ill, A). In one of the genera, Nitella, the stem and FIG. ill. Nitttta flexilis :— A, stern and branches of the natural size; a, b, c, d, four verticils of branches issuing from the stem; e, /, subdivision of the branches;— B, portion of the stem and branches en- larged; o, b, joints of stem; c, d, verticils; e, f, new cells sprouting from the sides of the branches ; ft, h, new cells sprouting at the extremities of the branches. branches are simple cells, which sometimes attain the length of several inches ; whilst in the true Chara, each central tube is surrounded by an envelope of smaller ones, which is formed as in Batrachospermese, save that the investing cells grow upwards as well as downwards from each joint, and meet each other on the stem half-way between the joints. Some species have the power of secreting carbonate of lime from the water in which they grow, if this be at all impregnated with calcareous matter; and by the deposition of it beneath their teguments, they have gained their popular name of " stone-worts." These humble plants have attracted much attention, in consequence of the facility with which the "rotation," or movement of fluid in the interior of the individual cells, may be seen in them. Each cell, 324 MICROSCOPIC FORMS OF VEGETABLE LIFE. in the healthy state, is lined by a layer of green oval granules, which cover every part, except two longitudinal lines that remain nearly colorless (Fig. Ill, B); and a constant stream of semi-fluid matter, containing numerous jelly-like globules, is seen to flow over this green layer, the current passing up one side, changing its direction at the extremity, and flowing down the other side, the ascending and descending spaces being bounded by the transparent lines just mentioned. That the currents are in some way directed 'by the layer of granules, appears from the fact noticed by Mr. Yarley,1 that if accident damages or removes them near the boundary between the ascending and descending currents, a portion of the fluid of the two currents will inter- mingle bypassing the boundary; whilst, if the injury be repaired by the development of new granules on the part from which they had been detached, the circulation resumes its regularity, no part of either current passing the boundaiy. In the young cells, however, the rotation may be seen, before their granular lining is formed. The rate of the movement is affected by any- thing which influences the vital activity of the plant ; thus, it is accelerated by moderate warmth, whilst it is retarded by cold ; and it may be at once checked by a slight electric discharge through the plant. The moving globules, which consist of starchy matter, are of various sizes ; being sometimes very small, and of definite figure, whilst in other instances they are seen as large irregular masses, which appear to be formed by the aggre- gation of the smaller particles.3 The production of new cells, for the extension of the stem or branches, or the origination of new whorls, is not here accomplished by the subdivision of the parent cell, but takes place by the method of out-growth (Fig. Ill, B, e,f, /) that seem to spring from the floor, — these cells being what are seen from above, when the observer looks down through the central aperture just mentioned. If the vertical section should happen to traverse one of the peculiar bodies which occupies the centres of the divisions, it will bring into view a structure of remarkable complexity. Each of these stomata (as they are termed, from the Greek «0^, a needle) is very appropriate to one of the most common states in which these bodies present themselves, that, namely, of bundles of needle-like crystals, lying side by side in the cavity of the cell ; such bundles are well seen in the cells lying immediately beneath the cuticle of the bulb of the medicinal Squill. It does not ap- ply, however, to other forms which are scarcely less abundant ; thus, instead of bundles of minute needles, single large crystals, octohedral or prismatic, are frequently met with ; and the pris- matic crystals are often aggregated in beautiful stellate groups. One of the most common materials of raphides, is oxalate of lime, which is generally found in the stellate form ; and no plant yields these stellate raphides so abundantly as the common Rhu- barb, the best specimens of the dry medicinal root containing as much as 35 per cent, of them. In the cuticle of the bulb of the Onion, the same material occurs under the octohedral or the prismatic form. In other instances, the calcareous base is com- bined with tartaric, citric, or malic acid ; and the acicular raphides are said to consist usually of phosphate of lime. Some raphides are as long as l-40th of an inch, while others measure no more than l-1000th. They occur in all parts of plants, — the wood, pith, bark, root, leaves, stipules, sepals, petals, fruit, and even in the pollen. They are always situated in cells, and riot, as some have stated, in the intercellular passages ; the cell-membrane, however, is often so much thinned away, as to be scarcely dis- tinguishable. Certain plants of the Cactus tribe, when aged, have their tissues so loaded with raphides as to become quite brittle ; so that when some large specimens of 0. senilis, said to be a thousand years old, were sent to Kew Gardens, from South America, some years since, it was found necessary for their pre- servation during transport, to pack them in cotton, like jewelry. 372 STRUCTURE OF PHANEROGAMIC PLANTS. It is not yet known what office the raphides fulfil in the economy of the Plant ; or whether they are to be considered in any other light, than as non-essential results of the vegetative processes. For as all these processes require the introduction of mineral bases from the soil, and themselves produce organic acids in the substance of the plant, it may be surmised that the accidental union of the components will occasion the formation of raphides wherever such union may occur ; and this view is supported by the fact, that the late Mr. E. Quekett succeeded in artificially producing raphides within the cells of "rice paper" (§223), by first filling these with lime water by means of the air-pump, and then placing the paper in weak solutions of phosphoric and oxalic acids. The artificial raphides of phosphate of lime were rhombo- hedral ; while those of oxalate of lime were stellate, exactly re- sembling the natural raphides of the Rhubarb.1 231. A large proportion of the denser parts of the fabric of the higher Plants, is made up of the substance which is known as Ligneous Tissue, or Woody Fibre. This, however, can only be regarded as a very simple variety of the " cellular ;" for it is com- posed of peculiarly elongated cells (Fig. 171, a «), usually pointed at their two extremities so as to become spindle-shaped, whose walls have a special tendency to undergo consolidation by the internal deposit of sclerogen. It is obvious that a tissue consist- ing of elongated cells, adherent together by their entire length, and strengthened by internal deposit, must possess much greater tenacity than any tissue in which the cells depart but little from the primitive spherical form ; and we accordingly find Woody Fibre introduced, wherever it is requisite that the fabric should ?ossess not merely density, but the power of resistance to tension. n the higher classes of the Vegetable kingdom, it constitutes the chief part of the stem and branches, where these have a firm and durable character; and even in more temporary structures, such as the herbaceous stems of annual plants, and the leaves and flowers of almost every tribe, this tissue forms a more or less important constituent, being especially found in the neighbor- hood of the spiral vessels and ducts, to which it affords protection and support. Hence the bundles or fasciculi composed of these elements, which form the skeletons of leaves, and which give " stringiness" to various esculent vegetable substances, are com- monly known under the name of fibro-vascular tissue. In their young and unconsolidated state, the ligneous cells seem to con- duct fluids with great facility in the direction of their length : and in the Coniferous tribe whose stems and branches are desti- 1 The materials of the above paragraph are derived from the excellent section on this subject in Prof. Quekett's "Lectures on Histology." Besides the vegetables therein named as affording good illustrations of different kinds of Raphides, may be mentioned, the parenchyma of the leaf of Jlgave, Jlloe, Cycas, Encephalartos, &c., the cuticle of the bulb of the Hyacinth, Tulip, and Garlic (and probably of other bulbs), the bark of the Apple, Cascarilla, Cinchona, Lime, Lontst, and many other trees, the pith of Eleagmts, and the testa of the seeds of jLnagallis and the Elm. SPIRAL VESSELS. 373 FIG 161. tnte of vessels, they afford the sole channel for the ascent of the sap. I^ut after their walls have become thickened by internal deposit, they are no longer subservient to this function ; nor, in- deed, do they 'then appear to fulfil any other purpose in the vegetable economy, than that of affording mechanical support. It is this which constitutes the difference between the alburnum or " sap-wood," and the duramen or "heart-wood," of Exogenous stems (§ 238). A peculiar set of markings, seen on the woody fibres of the Coniferce, and of some other tribes, is represented in Fig. 161 ; in each of these spots, the inner circle appears to mark a deficiency of the lining deposit, as in the porous cells of other plants ; whilst the outer circle indi- cates the boundary of a lenticular cavity, which intervenes between the adjacent cells at this point, and which contains a small globular body that may be sometimes detached. Of the purpose of these minute bodies interposed between the wood-cells, nothing is known ; there can be no doubt, however, from the definiteness and con- stancy of their arrangement, that they fulfil some important object in the eco- section of coniferous wood nomy of the plants in which they occur; the direction of the fibres, showing and there are varieties in this arrangement so characteristic of different tribes, that it is sometimes possible to determine, by the microscopic inspec- tion of a minute fragment, even of a fossil wood, the tribe to which it belonged. The woody fibre thus marked, is often designated as " glandular." 232. All the more perfect forms of Phanerogamia contain, in some part of their fabric, the peculiar structures which are known as Spiral Vessels.1 These have the elongated shape of woody fibres; but the internal deposit, as in the "spiral cells" (§ 228), takes the form of a spiral fibre winding from end to end, remain- ing distinct from the cell-wall, and retaining its elasticity ; this fibre may be single, double, or even quadruple, — this last charac- ter presenting itself in the very large elongated fibre-cells of the Nepenthes (Chinese pitcher-plant). These cells are especially found in the delicate membrane ("medullary sheath") surround- ing the pith of Exogens, and in the midst of the woody bundles occurring in the stem of Endogens; thence they proceed in each case to the leaf-stalks, through which they are distributed to the leaves. By careful dissection under the microscope, they may be separated entire ; but their structure may be more easily dis- 1 So long, however, as they retain their original cellular character, and do not coalesce with each other, these fusiform spiral cells cannot be regarded as having anymore claim to the designation of vessels, than have the elongated cells of the ligneous tissue. 374 STRUCTURE OF PHANEROGAMIC PLANTS. FIG. played, by cutting round, but not through, the leaf-stalk of the Strawberry, Geranium, &c., and then drawing the parts asunder. The membrane composing the tubes of the vessels will thus be broken across ; but the fibres within, being elastic, will be drawn out and unrolled. Spiral vessels are sometimes found to convey liquid, whilst in other cases they contain air only ; the conditions of this difference are not yet certainly known. 233. Although fluid generally finds its way with tolerable facility through the various forms of Cellular tissue, especially in the direction of the greatest length of its cells, a more direct means of connection between distant parts is required for an active circulation. This is afforded by what has been termed Vasiform tissue, which consists merely of cells laid end to end, the partitions between them being more or less obliterated, so that a continuous Duct is formed. The origin of these ducts in cells is occasionally very evident, both in the contraction of their calibre at regular intervals, and in the persistence of the remains of their partitions (Fig. 175, b b) ; but in most cases it can only be ascertained by studying the history of their development, neither of these indications being traceable. The component cells appear to have been sometimes simply membranous, but more commonly to have possessed the fibrous type (§ 228). Some of the ducts formed from the latter (Fig. 162, 2) are so like continuous spiral vessels, as to be scarcely distinguishable from them, save in the want of elasticity in the spiral fibre, which causes it to break when the attempt is made to draw it out. This would seem to have taken place, in some instances, from the natural elongation of the cells by frowth; the fibre being bro- en up into rings, which sometimes lie close together, but more commonly at con- siderable intervals ; such a duct is said to be annular (Fig. 162, i). Intermediate forms between the spiral and annular ducts, which show the derivation of the latter from the former, are very fre- quently to be met with. The spires are sometimes broken up still more completely, and the fragments of the fibre extend in various directions, so as to meet^and form an irregular net- Longitudinal section of stem of Italian Seed : — a, cells of the pith; b, fibro-vascular bundle, containing, 1, annular duct; 2, spiral duct; 3, dotted duct, with woody fibre ; c, cells of the integument. PREPARATION OF VEGETABLE TISSUE. 375 work lining the duct, which is then said to be reticulated. The continuation of the deposit, however, gradually contracts the meshes, and leaves the walls of the duct marked only by pores like those of porous cells (§ 227) ; and canals upon this plan, commonly designated as dotted ducts, are among the most com- mon forms of vasiform tissue, especially in parts of most solid structure and least rapid growth (Fig. 162, 3). The " scalari- form" ducts of Ferns (§ 218) are for the most part of the spiral type; but spiral ducts are frequently to be met with also in the rapidly growing leaf-stalks of Flowering plants, such as the Rhubarb. Not unfrequently, however, we find all forms of ducts in the same bundle, as seen in Fig. 162. The size of these ducts is occasionally so great, as to enable their openings to be distinguished by the unaided eye. They are usually largest in stems, whose size is small in proportion to the surface of leaves which they support, such as the common Cane, or the Vine ; and generally speaking they are larger in woods of dense texture, such as Oak or Mahogany, than in those of which the fibres, being softer, can themselves be subservient to the con- veyance of fluid. They are entirely absent in the Coniferse. 234. The Vegetable Tissues, whose principal forms have been now described, but among which an immense variety of detail is found, may be either studied as they present themselves in thin sections of the various parts of the plant under examination, or in the isolated condition in which they are obtained by dissec- tion. The former process is the most easy, and yields a large amount of information ; but still it cannot be considered that the characters of any tissue have been properly determined, until it has been dissected out. Sections of some of the hardest vegetable substances, such as "vegetable ivory," the "stones" of fruit, the "shell" of the cocoa-nut, &c. (§ 227), can scarcely be obtained except by slicing and grinding (§ 108) ; and these may be mounted either in Canada balsam or in weak spirit. In cases, however, in which the tissues are of only moderate firm- ness, the section may be readily and most effectually made with the " Section-Instrument" (§ 107) ; and there are few parts of the Vegetable fabric which may not be advantageously examined by this means, any very soft or thin portions being placed in it between two pieces of cork. In certain cases, however, in which even this compression would be injurious, the sections must be made with a sharp knife, the substance being laid upon a slip of glass. In dissecting the Vegetable tissues, scarcely any other instrument will be found really necessary, than a pair of needles (in handles), one of them ground to a cutting edge. The adhe- sion between the component cells, fibres, &c., is often sufficiently weakened by a few hours' maceration, to allow of their readily coming apart, when they are torn asunder by the needle points beneath the simple lens of a dissecting microscope. But if this should not prove to be the case, it is desirable to employ some 376 STRUCTURE OF PHANEROGAMIC PLANTS. other method, for the sake of facilitating their isolation. None is so effectual as the boiling of a thin slice of the substance under examination, either in dilute nitric acid, or in a mixture of nitric acid and chlorate of potass. This last method (which was devised by Schultz) is the most rapid and effectual, requiring only a few minutes for its performance ; but as oxygen is libe- rated with such freedom as to give an almost explosive character to the mixture, it should be put in practice with extreme caution. After being thus treated, the tissue should be boiled in alcohol, and then in water ; and it will then be found very easy to tear apart the individual cells, ducts, &c., of which it may be com- posed. These may be preserved by mounting in weak spirit. 235. Structure of the Stem and Root. — It is in the stems and roots of Plants, that we find the greatest variety of tissues in combination, and the most regular plans of structure ; and sec- tions of these, viewed under a low magnifying power, are objects of peculiar beauty, independently of the scientific information which they afford. The Axis (under which term is included the Stem with its branches, and the Root with its ramifications) always has for the basis of its structure a dense cellular paren- chyma; though this, in the advanced stage of development, may constitute but a small proportion of it. In the midst of this parenchyma we generally find fibro-vascular bundles ; that is, fasciculi of woody fibre, with ducts of various kinds, and (very commonly) spiral vessels. It is in the mode of arrangement of these bundles, that the fundamental difference exists between the stems that are commonly designated as Endogenous, and those which are (more appropriately) termed Exogenous; for in the former, the bundles are dispersed throughout the whole diameter of the axis, without any peculiar plan, the intervals between them being filled up by cellular parenchyma ; whilst in the latter they are arranged side by side, in such a manner as to form a hollow cylinder of wood, which includes within it that portion of the cellular substance which is known as pith, whilst it is itself enclosed in an envelope of the same substance, that forms the bark. These two plans of axis formation, — respec- tively characteristic of those two great groups into which the Phanerogamia are subdivided, namely the Monocotyledonous, and the Dicotyledonous, — will now be more particularly described. 236. When a transverse section (Fig. 163) of a Monocotyledo- nous stem is examined microscopically, it is found to exhibit a number of fibro-vascular bundles, disposed without any regu- larity in the midst of the mass of cellular tissue, which forms (as it were) the matrix or basis of the fabric. Each bundle contains two, three, or more large ducts, which are at once distinguished by the size of their openings ; and these are surrounded by woody fibre, and spiral vessels, the transverse diameter of which is so extremely small, that the portion of the bundles which they form is at once distinguished in transverse section, by the close- STRUCTURE OF M ONO C 0 T Y L ED 0 N 0 U S STEM. 377 ness of its texture (Fig. 164). The bundles are least numerous in the centre of the stem, and become gradually more approxi- mated towards the circumference : but it frequently happens that the portion of the area in which they are most compactly FIG. 163. FIG. 164. Transverse section of stem of young Palm. Portion of transverse section of stem of Wunghie Cane. arranged, is not absolutely at its exterior, this portion being itself surrounded by an investment composed of cellular tissue only; and sometimes we find the central portion, also, com- pletely destitute of fibro-vascular bundles ; so that a sort of indication of the distinction between pith, wood, and bark is here presented. This distinction, however, is very imperfect ; for we do not find either the central or the peripheral portions ever separable, like pith and bark, from the intermediate woody layer. In its young state, the centre of the stem is always filled up with cells ; but these not unfrequently disappear after a time, except at the nodes, leaving the stem hollow, as we see in the whole tribe of Grasses. When a vertical section is made of a woody stem (as that of a Palm) of sufficient length to trace the whole extent of the fibro-vascular bundles, it is found that whilst they pass at their upper extremity into the leaves, they pass at the lower end, also, towards the surface of the stem, and assist, by their interlacement with the outer bundles, in forming that extremely tough investment, which the lower ends of these stems present. The fibro-vascular bundles once formed, receive no further additions ; and the augmentation of the stem in diameter depends upon the development of new woody bundles, in continuity with the leaves which are successively evolved at its summit. It was formerly supposed that these successively 378 STRUCTURE OF PHANEROGAMIC PLANTS. formed bundles descend in the interior of the stem through its entire length, until they reach the roots ; and as the successive development of leaves involves a successive development of new bundles, the stem was imagined to be continually receiving addi- tions to its interior, whence the term Endogenous was given to this type of stem-structure. From the fact just stated, however, regarding the course of the fibro- vascular bundles, it is obvious that such a doctrine cannot be any longer admitted ; for those which are most recently formed only pass into the centre of the stem during the higher part of their course, and usually make their way again to its exterior at no great distance below ; and thus the lower and older portions of a Palm-stem really do receive very little augmentation in diameter, while a rapid elon- gation is taking place at its summit. In fact, the dense unyield- ing nature of the fabric, which is formed by the interlacement of the fibro-vascular bundles at or near the surface of the trunk, would prevent any such augmentation by expanding pressure from within. 237. In the stems of Dicotyledonous Phanerogamia, we find a method of arrangement of the several parts, which must be re- garded as the highest form of the de- velopment of the axis, being that in which the greatest differentiation exists. A distinct division is always seen in a transverse section, between the three concentric arese of the pith, the wood, and the bark ; the first (a) being central and the last (b) peripheral, and these having the wood interposed between them, its circle being made up of wedge- shaped bundles (d, d), kept apart by the bands (c, c) that pass between the pith and the bark. The Pith (Fig. 165, a) is c c, plates of ceiiuiar tissue (medui- almost in variably composed of cellular betweeu the W°°dy tissue onlv> wllich usually presents (in transverse section) a hexagonal areola- tion. When newly formed, it has a greenish hue, and its cells are filled with fluid ; but it gradually dries up and loses its color ; and not unfrequently its component cells are torn apart by the rapid growth of their envelope, so that irregular cavities are found in it ; or, if the stem should increase with extreme rapidity, it becomes hollow, the pith being reduced to fragments which are found adhering to its interior wall. The pith is immediately surrounded by a delicate membrane, consisting almost entirely of spiral vessels, and termed the " medullary sheath." 238. The Woody portion of the stem (Fig. 165, b, b) is made up of woody fibres, usually with the addition of ducts of various kinds ; these, however, are absent in one large group, the Coni- Diagram of the first formation of an Exogenous stem; a, pith; b 6, hark ; EXOGENOUS STEM. 379 ferce or Fir tribe with its allies (Figs. 166, 170-172), in which the woody fibres are of unusually large diameter, and have the pe- culiar "glandular" markings already described (§ 231). In any stem or branch of more than one year's growth, the woody gtruc- Transverse section of young stem of Clematis:— a, pith; b,b,b, woody bundles; c, c, c, medullary rays. ture presents a more or less distinct appearance of division into concentric rings, the number of which varies with the age of the tree (Fig. 167). The composition of the several rings, which are the sections of so many cylindrical layers, is uniformly the same, however different their thickness ; the arrangement of the two principal elements, however, — namely the woody fibre and the ducts, — varies in different species ; the ducts being sometimes FIG. 166. Portion of transverse section of stem of Cedar: — a, pith ; b, b, wood ; c, bark. almost uniformly diffused through the whole layer, but in other instances being confined to its inner part; while in other cases, again, they are dispersed with a certain regular irregularity (if such an expression may be allowed), so as to give a curiously figured appearance to the transverse section (Figs. 167, 168). The general fact, however, is, that the ducts predominate towards the inner side of the ring (which is the part of it first formed), and 380 STRUCTURE OF PHANEROGAMIC PLANTS. FIG. 167. that the outer portion of each layer is almost exclusively composed of woody tissue ; such an arrange- ment is seen in Fig. 165. This alter- nation of ducts and woody fibre fre- quently serves to mark the succes- sion of layers, when, as is not un- common, there is no very distinct line of separation between them. The number of layers is usually considered to correspond with that of the years during which the stem or branch has been growing; and this is, no doubt, generally true in regard to the trees of temperate climates. There appears strong reason to believe, however, that such is not the universal rule ; and that we should be more correct in stating that each layer indicates an " epoch of vegetation;" which, in tempe- rate climates, is usually (but not invariably) a year, but which is commonly much less in the case of trees flourishing in tropical regions. For ex- ample, we not unfrequently meet with stems, in which the place of a layer of the ordinary breadth is occupied by two nar- row layers ; the line of demarcation between them having appa- rently been formed by a temporary interruption to the process of growth, in the middle of the period through which the formation of wood extends. Such an interruption might occur from heat Transverse section of stem of Buckthorn (R/iamnus). FIG. 108. Portion of the preceding figure, more highly magnified. FlG. 169. Portion of transverse section of stem of Hazel, showing, in the portion a, &, c, six narrow layers of wood. and drought, in a tree that flourishes best in a cold damp atmo- sphere, or from a fall of temperature in a tree that requires heat ; and in a variable season, it might recur several times. Some- thing of this kind would appear to have been the cause of the MEDULLARY RAYS. 881 peculiar appearance presented by a section of Hazel- stem (in the Author's possession), of which a portion is represented in Fig. 169; for between two layers of the ordinary thickness, there in- tervenes a band whose breadth is altogether less than that of either of them, and which is yet composed of no fewer than six layers, four of them (c) being very narrow, and each of the other two being about as wide as these four together. The inner layers of wood are the oldest, and the most solidified by matters de- posited within their component cells and vessels ; hence they are spoken of collectively under the designation duramen or " heart- wood." On the other hand, it is through the cells and ducts of the outer and newer layers, that the sap rises from the roots towards the leaves ; and these are consequently designated as alburnum or " sap-wood." The line of demarcation between the two is sometimes very distinct, as in Lignuin-vitae and Cocos wood ; and. as a new layer is added every year to the exterior of the alburnum, an additional layer of the innermost part of the alburnum is every year consolidated by internal deposit, and is thus added to the exterior of the duramen. More generally, however, this consolidation is gradually effected, and the al- burnum and duramen are not separated by any abrupt line of division. 239. The Medullary Rays which cross the successive rings of wood connecting the cellular substance of the pith with that of the bark, and dividing each ring FIG. 170. of wood into wedge-shaped seg- ments, are thin plates of cellular tissue (Fig. 165, c, c), not usually extending to any great depth in the vertical direction. It is not often, however, that their cha- racter can be so clearly seen in a transverse section, as it is in that just referred to; for they are usually compressed so closely, as to appear darker than Portion of transverse section of large stem of the WedgCS of WOOdy tisSU6 be- coniferous Wood (fossil), showing part of two tweeti which thev intervene annual layers, divided at a, a, and traversed by /T7,. i /™ -« />n\ i J/L • i very thin but numerous Medullary Rays. (*,*&*• 166-169); and their real nature is best understood by a comparison of longitudinal sections made in two different direc- tions,— namely, radial and tangential, — with the transverse. Three such sections of a fossil Coniferous wood in the Author's pos- session are shown in Figs. 170-172. The stem was of such large size, that, in so small a part of the area of its transverse section as is represented in Fig. 170, the medullary rays seem to run parallel to each other, instead of radiating from a common centre. They are very narrow ; but are so closely set together, that only two or three rows of woody fibres (no ducts being here 382 STRUCTURE OF PHANEROGAMIC PLANTS. present) intervene between any pair of them. In the longitu- dinal section taken in a radial direction (Fig. 171), and conse- Fro. 171. FIG. 172. Fig. 171. Portion of vertical section of the same wood, taken in a radial direction, showing the glandular woody fibres, without ducts, crossed by the Medullary Rays, a, a. Fig. 172. Portion of vertical section of the same wood, taken in a tangential direction, so as to cut across the Medullary Rays. quently passing in the same course with the medullary rays, these are seen as thin plates (a, a, a) made up of superposed cells very much elongated, and crossing, in a hori- FIG. 173. zorital direction, the glandular woody fibres which lie parallel to one another vertically. And in the tangential section (Fig. 172), which passes in a direction at right angles to that of the medullary rays, and therefore cuts them across, we see that each of the plates thus formed has a very limited depth from above downwards, and is composed of no more than one thickness of horizontal cells. A section of the stem of Mahogany, taken in the same direction as the last (Fig. 173), gives a very good view of the cut ends of the medullary rays, as they pass between the woody fibres ; and they are seen to be here of somewhat greater thickness, being composed of two or three rows of cells, ar- ranged side by side. In another fossil wood, whose transverse section is shown in Fig. 174, and its tangential section in Fig. 175, vertical section of Mahogany. the me(juiiaiy rays are seen to occupy a much larger part of the substance of the stem ; being seen in the transverse section as broad bands (a a, a a), intervening between the closely set woody fibres, among which the large ducts are scattered ; whilst in the tangential, they are observed to be not EXOGENOUS STEM: — MEDULLARY RAYS. 383 only deeper than the preceding from ahove downwards, but also to have a much greater thickness. This section also gives an excellent view of the ducts b b, b 6, which are here plainly seen FIG. 174. FIG. 175. a a a b b Fig. 174. Portion of transverse section of Fossil Wood, showing the Medullary Rays, a a, a a, a a, running nearly parallel to each other, and the openings of the large Ducts in the midst of the woody fibres. Fig. 175. Portion of vertical (tangential) section of the same Wood, showing the woody fibres separated by the Medullary Rays, and by the large Ducts, b b, b b. to he formed by the coalescence of large cylindrical cells, lying end to end. In another fossil wood in the Author's possession, the medullary rays constitute a still larger proportion of the Fro. 176. FIG. 177. Fig. 176. Portion of transverse section of a Fossil Wood, remarkable for the very large size of the Medullary Rays, 6, b, which separate the woody plates, a, a. Fig. 177. Vertical (tangential) section of the same wood, showing, a, a, the woody bundles, and 6, 6, the Medullary Rays. stem ; for in the transverse section (Fig. 176), they are seen as very broad bands (5, b), alternating with plates of woody struc- 384 STRUCTURE OF PHANEROGAMIC PLANTS. ture («, a), whose thickness is often less than their own ; whilst in the tangential section (Fig. 177), the cut extremities of the medullary rays occupy a very large part of the area, having ap- parently determined the sinuous course of the woody fibres ; instead of looking, as in Fig. 172, as if they had forced their way between the woody fibres, which there hold a nearly straight and parallel course on either side of them. 240. The Bark may be usually found to consist of three princi- pal layers ; the external, or epiphloeum^ also termed the suberous (or corky) layer ; the middle, or mesophloeum, also termed the "cellular envelope;" and the internal, or endophlceum, which is more commonly known as the liber. The two outer layers are entirely cellular ; and are chiefly distinguished by the form, size, and direction of their cells. The epiphlaeum is generally com- posed of one or more layers of colorless or brownish cells, which usually present a cubical or tabular form, and are arranged with their long diameters in the horizontal direction : it is this which, when developed to an unusual thickness, forms Cork, a substance which is by no means the product of one kind of tree exclusively, but which exists in greater or less abundance in the bark of every exogenous stem. The mesophloeum consists of cells, usually of green color, prismatic in their form, and disposed with their long diameters parallel to the axis ; it is more loosely arranged than the preceding, and contains " intercellular passages," winch often form a network of canals, that have been termed "latici- ferous vessels ;" and although usually less developed than the suberous layer, it sometimes constitutes the chief thickness of the bark. The liber or inner bark, on the other hand, usually contains woody fibre in addition to the cellular tissue and latici- ferous vessels of the preceding ; and thus approaches more nearly in its character to the woody layers, with which it is in close proximity on its inner surface. The liber may generally be found to be made up of a succession of thin layers, equalling in number those of the wood, the innermost being the last formed ; but no such succession can be distinctly traced in the cellular envelope, or in the suberous layer; although it is certain that they, too, augment in thickness by additions to their interior, whilst their external portions are frequently thrown off in the form of thickish plates, or detach themselves in smaller and thinner flakes. The bark is always separated from the wood by the cambium layer, which is the part wherein all new growth takes place: this seems to consist of mucilaginous semifluid matter ; but it is really made up of cells of a very delicate tex- ture, which gradually undergo transformations, whereby they are for the most part converted into woody tissue, ducts, spiral vessels, &c. These materials are so arranged, as to augment the fibro-vascular bundles of the wood on their external surface ; thus forming a new layer of alburnum, wrhich encloses all those that preceded it ; whilst they also form a new layer of liber, on BARK. 885 the interior of all those which preceded it ; they also extend the medullary rays, which still maintain a continuous connection be- tween the pith and the bark; but a portion remains unconverted, so as always to keep apart the liber and alburnum. This type of stem structure is termed Exogenous ; a designation which ap- plies very correctly to the mode of increase of the woody layers, although (as we have just seen) the liber is formed upon a truly endogenous plan. 241. Numerous departures from the normal type are found in particular tribes of Exogens. Thus in some, the wood is not marked by concentric circles, their growth not being inter- rupted by any seasonal change. In other cases, again, each woody zone is separated from the next, by the interposition of a thick layer of cellular substance. Sometimes wood is formed in the bark (as in Calycanthus), so that several woody columns are produced, which are quite independent of the principal woody axis, but cluster around it. Occasionally the woody stem is divided into distinct segments, by the peculiar thickness of cer- tain of the medullary rays ; and in the stem of which Fig. 178 represents a transverse section, these cellular plates form four large segments, disposed in the manner of a Maltese cross, and alternating with the four woody segments, which they equal in size. 242. In its first- developed state, the Exogenous stem con- sists, like the so-called endogenous, of cellular tissue only ; but FIG. 179. FIG. 178. Fig. 178. Transverse section of the stem of a Climbing-plant (Aristolochia?) from New Zealand. Fig.179. Portion of transverse section of Burdock (Arctium), showing one of the fibro-vascular bundles, that lies beneath the cellular integument. after the leaves have been actively performing their functions for a short time, we find a circle of fibro-vascular bundles, as repre- sented in the diagram, p. 378, interposed between the central (or medullary) and the peripheral (or cortical) portions of the cellular matrix ; these fibro-vascular bundles being themselves 25 386 STRUCTURE OF PHANEROGAMIC PLANTS. ^ separated from each other by plates of cellular tissue, which still remain to connect the central and the peripheral portions .of the matrix. This first stage in the formation of the Exogen- ous axis, in which its principal parts — the pith, wood, bark, and medullary rays — are marked out, is seen even in the stems of herbaceous plants, which are destined to die down at the end of the season (Fig. 179) ; and sections of these, which are very easily prepared, are most interesting Microscopic objects. In such stems, the difference between the " Endogenous" and the " Exogenous" types is manifested in little else than the disposi- tion of the fibro-vascular layers ; which are scattered through nearly the whole of the cellular matrix (although most abundant towards its exterior), in the former case ; but are limited to a circle within the peripheral portion of the cellular tissue, in the latter. It is in the further development which takes place during succeeding years in the woody stems of perennial Exogens, that those characters are displayed, which separate them most com- pletely from the Ferns and their allies, whose stems contain a cylindrical layer of fibro-vascular bundles, as well as from (so- called) Endogens. For whilst the fibro-vascular layers of the latter, when once formed, undergo no further increase, those of Exogenous stems are progressively augmented by the metamor- phosis of the cambium layer ; so that each of the bundles which once lay as a mere series of parallel cords beneath the cellular investment of a first year's stem, may become in time the small end of a wedge-shaped mass of wood, extending continuously from the centre to the exterior of a trunk of several feet in diameter, and becoming progressively thicker as it passes out- wards. The fibro-vascular bundles of Exogens are therefore spoken of as "indefinite ;" whilst those of Exogens and Aero- gens (Ferns, &c.) are said to be " definite" or " closed." 243. The structure of the Roots of Endogens and Exogens is essentially the same in plan with that of their respective stems. Generally speaking, however, the roots of Exogens have no pith, although they have medullary rays ; and the succession of dis- tinct rings is less apparent in them, than it is in the stems from which they diverge. In the delicate radical filaments which pro- ceed from the larger root-fibres, a central bundle of vessels will be seen, enveloped in a sheath of cellular substance ; and this investment also covers in the end of the fibril, which is usually somewhat dilated, and composed of peculiarly succulent tissue, forming what is termed the spongiole. The structure of the radical filaments may be well studied in the common Duckiveed, every floating leaf of which has a single fibril hanging down from its lower surface. 244. The structure of Stems and Roots cannot be thoroughly examined in any other way than by making sections in different directions with the Section -instrument. The general directions already given (§ 107) leave little to be added respecting this SECTIONS OF STEMS. 387 special class of objects ; the chief points to be attended to being the preparation of the stems, &c., for slicing, the sharpness of the knife and the dexterity with which it is handled, and the method of mounting the sections when made. The wood, if green, should first be soaked in strong alcohol for a few days, to get rid of the resinous matter; and it should then be macerated in water for some days longer, for the removal of its gum, before being submitted to the cutting process. If the wood be dry, it should first be softened by soaking for a sufficient length of time in water, and then treated with spirit and afterwrards with water, like green wood. Some woods are so little affected even by prolonged maceration, that boiling in water is necessaiy to bring them to the degree of softness requisite for making sec- tions. No wood that has once been dry, however, yields such, good sections, as that which is cut fresh. When a piece, of the appropriate length, has been placed in the grasp of the Section- instrument (wedges of deal or other soft wood being forced in with it, if necessary for its firm fixation), a few thick slices should first be taken, to reduce its surface to an exact level; the surface should then be wetted with spirit, the micrometer-screw moved through a small part of a revolution, and the slice taken oif with the razor, the motion given to which should partake both of drawing and pushing. A little practice will soon enable the operator to discover, in each case, how thin he may venture to cut his sections, without a breach of continuity; and the micrometer-screw should be turned so as to give the required elevation. If the surface of the wood has been sufficiently wetted, the section will not curl up in cutting, but will adhere to the surface of the razor, from which it is best detached by dipping the razor in water, so as to float away the slice of wood, a camel-hair pencil being used to push it off, if necessary. All the sections that may be found sufficiently thin and perfect, should be put aside in a bottle of weak spirit, until they be mounted. For the minute examination of their structure, it is generally much better to preserve them in fluid, than to mount them either dry or in Canada balsam; and no fluid answers better than weak spirit. Where a mere general view only is needed, the dry mounting answers the purpose sufficiently well. It is only in the case of the section being unusually opaque, that mounting it in Canada balsam can be of any service whatever; and in general it is rather injurious than useful, making the section so transparent that its features can scarcely be discerned. Trans- verse sections, however, when charred by heating between two plates of glass until they turn brown, may be mounted with advantage in Canada balsam, and are then very showy specimens for the solar or gas-microscope. The number of beautiful and interesting objects which may be thus obtained, at the cost of a very small amount of trouble, can scarcely be conceived save by those who have made a special study of these wonderful struc- 388 STRUCTURE OF PHANEROGAMIC PLANTS. tares. Even the commonest trees, shrubs, and herbaceous plants, yield specimens that exhibit a varied elaboration of design, which cannot but strike with astonishment even the most cursory observer ; and there is none in which a careful study of sections, made in different parts of the stem, and especially in the neigh- borhood of the "growing point," will not reveal to the eye of the scientific Physiologist, some of the most important phenomena of Vegetation. Fossil Woods, when well preserved, are almost invariably silicified, and require, therefore, to be cut and polished by a Lapidary. Should the Microscopist be fortunate enough to meet with a portion of a calcified stem, in which the organic structure is preserved, he should proceed with it after the man- ner of other hard substances which need to be reduced by grind- ing (§§ 108-110). 245. Structure of the Cuticle and Leaves. — On all the softer parts of the higher Plants, save such as grow under water, we find a superficial layer, different in its texture from the parenchyma beneath, and constituting a distinct membrane, known as the FIG. 180. FIG. 181. Cuticle of Leaf of Yucca. Cuticle of Leaf of Indian Corn (Zea Mais). Cuticle.1 This membrane is composed of cells, the walls of which are flattened above and below, whilst they adhere closely to each other laterally, so as to form a continuous stratum (Fig. 184, a, a). Their shape is different in almost every tribe of Plants ; thus in the cuticle of the Yucca (Fig. 180), Indian Corn (Fig. 181), Iris (Fig. 183), and most other Monocotyledons, the cells are elongated, and present an approach to a rectangular contour ; their margins being straight in the Yucca and Iris, but minutely 1 The term epidermis is applied to this membrane by many Vegetable Physiologists, on account of the analogy it seems to present to the epidermis of Animals; but as epidermis means a membrane that lies upon the derm or " true skin," and as no such subjacent layer exists in the Plant, the transference of the designation is altogether inap- propriate. It would be much more correct to designate by the name cutis or derm what is ordinarily denominated the Cuticle ; and to reserve the term epidermis for the thin pellicle which may be sometimes detached from it (§ 247). STRUCTURE OF THE CUTICLE. 389 sinuous or crenated in the Indian Corn. In most Dicotyledons, on the other hand, the cells of the cuticle depart less from the form of circular disks ; but their margins usually exhibit large irregular sinuosities, so that they seem to fit together like the pieces of a dissected map, as is seen in the cuticle of the Apple (Fig. 182, b, b). Even here, however, the cells of the portion of FIG. 182. Portion of the Cuticle of the inferior surface of the Leaf of the Apple, with the layer of parenchyma in immediate contact with it: — a, a, elongated cells of the cuticle overlying the veins or nerves of the leaf; b, 6, ordinary cuticle cells overlying the parenchyma; c, c, stornata; rf, d, green cells of the parenchyma, forming a very open network near the lower surface of the leaf. the cuticle (a, a) that overlies the veins of the leaf, have an elon- gated form, approaching that of the wood-cells of which these veins are chiefly composed ; and it seems likely, therefore, that the elongation of the ordinary cuticle-cells of Monocotyledons has reference to that parallel arrangement of the veins, which their leaves almost constantly exhibit. The cells of the cuticle are colorless, or nearly so, no chlorophyll being formed in their interior; and their walls are generally thickened by secondary deposit, especially on the side nearest the atmosphere. This deposit is of a waxy nature, and consequently renders the mem- brane very impermeable to fluids ; the retention of which within the soft tissue of the leaf is obviously the purpose to be answered by the peculiar organization of the cuticle. In most European Plants, the cuticle contains but a single row of cells, which are usually, moreover, thin-sided ; whilst in the generality of tro- pical species, there exist two, three, or even four layers of thick- sided cells ; this last number being seen in the Oleander, the cuticle of which, when separated, has an almost leathery firm- ness. The difference in conformation is obviously adapted to the conditions of growth under which these plants respectively exist; since the cuticle of a plant indigenous to temperate climates, would not afford a sufficient protection to the interior structure against the rays of a tropical sun ; whilst the diminished heat of this country would scarcely overcome the resistance presented by the dense and non-conducting tegument of a 390 STRUCTURE OF PHANEROGAMIC PLANTS. species formed to exist in tropical climates. A very curious modification of the cuticle is presented by the RocTiea falcata, commonly known as the "ice-plant;" a designation it owes to FIG. 183. FIG. 184. Portion of the Cuticle of the upper surface of the leaf of Moehea falcata, as seen at A from its inner side, and at B from its outer side: — a, a, small cells forming the inner layer of the cuticle; b, 6, large prominent cells of the outer layer; c, c, stomata, disposed between the latter. the peculiar appearance of its surface, which looks as if it were covered with frozen dewdrops. This appearance is occasioned by the presence of a layer of very large oval cells (Figs. 183, 184, b, b), which lie detached one from another upon the surface of the ordinary cuticle, a, a. In other instances, the cuticle is partially invested by a layer of scales, which are nothing else than flattened cells, often having a very peculiar form ; whilst in numerous cases, again, we find the surface be- set with hairs, which occasion- ally consist of single elongated cells, but are more commonly made up of a linear series, attached end to end, as in Fig. 153. Sometimes these hairs bear little glandular bodies at their extremities, by the secretion of which a peculiar viscidity is given to the surface of the leaf, as in the Sundew (Drosera) ; in other instances, the hair has a glandular body at its base, with whose secretion it is moistened ; so that when this secretion is of an irritating quality, as in the Nettle, it constitutes a " sting." A great variety of such organs may be found, by a microscopic examination of the surface of the leaves of Plants having any kind of superficial investment to the cuticle. Many connecting links are found between hairs and scales, such as the "stellate hairs" of the Deutzia scabra, which a good deal resemble those within the air-chambers of the Yellow Water-lily (Fig. 152). Portion of a vertical section of the same Leaf, showing the small cells, a, a, of the inner layer of cuticle; the large cells, 6, 6, of the outer layer; c, one of the stomata; <2, d, cells of the parenchyma; L, lacuna between the parenchymatous cells, into which the stoma opens. STRUCTURE OF CUTICLE AND STOMATA. 391 246. The Cuticle in many Plants, especially those belonging to the Grass tribe, has its cell-walls impregnated with silex, like that of the Equisetum (§ 220) ; so that, when all the organic matter has been got rid of by heat or by acids, the forms of the cuticle-cells, hairs, stomata, &c., are still marked out in silex, and are most beautifully displayed by Polarized light. Such silicified cuticles are found on the husks of the " grains" yielded by these plants : and there is none in which a larger proportion of mineral matter exists, than that of Rice, which contains some curious elongated cells with toothed margins. The hairs with which the palece (chaff-scales) of most Grasses are furnished, are strengthened by the like siliceous deposit ; and in the Festuca pratensis, one of the common meadow grasses, the palese are also beset with longitudinal rows of little cup-like bodies formed of silica. The cuticle and scaly hairs of Deutzia also contain a large quantity of silex; and are remarkably beautiful objects for the Polariscope. 247. Externally to the cuticle, there usually exists a very deli- cate transparent pellicle, without any decided traces of organiza- tion, though occasionally somewrhat granular in appearance, and marked by lines that seem to be impressions of the junctions of the cells with which it was in contact. When detached by maceration, it not only comes off from the surface of the cuticle, but also from that of the hairs, &c., which this may bear. This membrane, the proper Epidermis (p. 388, note), is obviously formed by the agency of the cells of the cuticle ; and it seems to consist of the external layers of their thickened cellulose walls, which have coalesced with each other, and have separated them- selves from the subjacent layers, by a change somewhat analogous to that which occurs in the Palmellese (§ 194), the outer walls of whose original cells seem to melt away into the gelatinous in- vestment, that surrounds the "broods" which have originated in their subdivision. 248. In nearly all Plants which possess a distinct Cuticle, this is perforated by the minute openings termed Stomata (Figs. 182, 183, c, c) ; which are bordered by cells of a peculiar form, dis- tinct from those of the cuticle, and more resembling in character those of the tissue beneath. These boundary-cells are usually somewhat kidney-shaped, and lie in pairs (Fig. 185, I, b), with an oval opening between them ; but by an alteration in their form, the opening may be contracted or nearly closed. In the cuticle of Yucca, however, the opening is bounded by two pairs of cells, and is somewhat quadrangular (Fig. 180) ; and a like doubling of the boundary-cells, with a narrower slit between them, is seen in the cuticle of the Indian Corn (Fig. 181). In the stomata of no Phanerogamic Plant, however, do we meet with any conformation at all to be compared in complexity with that which has been described as existing in the humble Mar- 392 STRUCTURE OF PHANEROGAMIC PLANTS. chantia (214). FIG. 185 Stomata are usually found most abundantly (and sometimes exclusively) in the cuticle of the lower surfaces of leaves, where they open into the air-chambers that are left in the parenchyma which lies next the inferior cuticle ; in leaves which float on the surface of water, however, they are found in the cuticle of the upper surface only ; whilst, in leaves that ha- bitually live entirely submerged, as there is no distinct cuticle, so there are no stomata. In the erect leaves of Grasses, the Iris Portion of the Cuticle of the leaf of the Iris • *> 0 .-, p -, nl germanica, torn from its surface, and carrying tribe, &C., they are tOUttd equally away with it a portion of the parenchymatous (or nearly SO) Oil both SUrfaCCS. layer in immediate contact with it:— a, a, elon- A o-Anm-fll fopf fhmr avA looaf gated cellsofthecuticle;M,cellsofthestomata; ^S ^ g61161?1 Iact> ™eJ "J c, c, cells of the parenchyma; d, d. impressions UUmerOUS 111 SUCCUlent .Plants, formed by their contact, on the epidermic cells; w]lose moisture, obtained in a e. e, lacunae in the parenchyma, corresponding to , . ' -, . -, , the stomata. scanty supply, is destined to be retained in the system ; whilst they abound most in those which exhale fluid most readily, and therefore absorb it most quickly. It has been estimated that no fewer than 160,000 are contained in every square inch of the under surface of the leaves of Hydrangea and of several other plants ; the greatest number seeming always to present itself in species, the upper surface of whose leaves is entirely destitute of these organs. In Iris germanica, each surface has nearly 12,000 stomata in every square inch ; and in Yucca, each surface has 40,000. In Oleander, Banksia, and some other plants, the stomata do not open directly upon the lower surface of the cuticle, but lie in the deepest part of little pits or depressions which are excavated in it, and which are lined with hairs ; the mouths of these pits, with the hairs that line them, are well brought into view by taking a thin slice from the surface of the cuticle with a sharp knife ; but the form of the cavities, and the position of the stomata, can only be well made out in vertical sections of the leaves. 249. The internal structure of Leaves is best brought into view by making vertical sections, that shall traverse the two layers of cuticle and the intermediate cellular parenchyma ; por- tions of such sections are shown in Figs. 184, 186, and 187. In close apposition with the cells of the upper cuticle (Fig. 186, «, a], which may or may not be perforated with stomata (c, c, d, d), we find a layer of soft thin-walled cells, containing a large quantity of chlorophyll; these cells usually press so closely against one another, that their sides become mutually flattened, and no spaces are left, save where there is a definite air-chamber into which the stoma opens (Fig. 184, L) ; and the compactness of INTERNAL STRUCTURE OF LEAVES. 393 FIG. 186. this superficial layer is well seen, when, as often happens, it ad- heres so closely to the cuticle, as to be carried away with this when it is torn away (Fig. 185, «» containing the spores, is dis- . P charged; — F. transformation of encysted Vorticella (B) into Other instances, however, form of Acmeta; b, nucleus;— G, stalked Acineta form of Onlv a sinS"! e offsDrinS* is Vorticella, enclosing a young one, the result of the transfor- ,®P .,* mation of the nucleus;— H, young free Vorticella; a, b, c, as developed from the " nu- in Flg. i; 0t posier[or circlet of ciiia. cleus" of the original cell- body, which offspring may have an entirely dissimilar form ; and this latter change occurs in Vorticella, in conjunction with other HOW individuals OI the 420 MICROSCOPIC FORMS OF ANIMAL LIFE. peculiarities of a very remarkable kind. For the encysted Vor- ticella becomes changed into the form of an Acineta (closely re- sembling that of Actinophrys), as shown in Fig. 198, F; and this may acquire a new stalk, so as to correspond with a Podophrya (G). At the same time, its band-like nucleus becomes entirely metamorphosed into a free body of ovate form, which carries at its narrower end a circlet of long vibrating cilia, while its more obtuse end is perforated by a mouth which communicates with a distinct oral cavity. In the interior of this offspring (H), we already observe a long oval nucleus (b) and a round contractile vesicle (), supposed to be rudimentary eyes, and having the mouth (a) at its extre- mity ; this prominence may be considered, therefore, as a true head, notwithstanding that it is not clearly distinguishable from the body. This head also bears upon its under surface a projecting tubular organ (d), which was thought by Professor Ehrenberg to be a siphon for the admission of water to the cavity of the body for the purpose of respiration ; this, however, is certainly not the case, the tube being imperforate at its extremity; and there seems much more probability in the idea of Dujardin, that it re- presents the antennce or palpi of higher Artie ulata, the single organ being replaced in many Rotifera by a pair, of which each is furnished at its extremity with a brush-like tuft of hairs that can be retraced into the tube. The oesophagus, which is narrow in the Rotifer, but which is dilated into a crop in Stephanoceros (Fig. 202) and in some other genera, leads to the masticating apparatus, which in these animals is placed far behind the mouth, and in close proximity to the stomach. It consists of a pair of stirrup-shaped jaws (Fig. 201, e) each having from one to five teeth (in Rotifer, two), which appear to contain mineral matter and to be of harder texture than the rest of the fabric ; these jaws are put in action by powerful muscles, and are so moved that all the food which passes into the stomach is subject to be divided and torn by their teeth. In many Rotifera, the conformation of this masticating apparatus is extremely complicated. The form of the alimentary canal varies; this being sometimes a simple tube, passing without enlargement or constriction from the masticat- ing apparatus to the anal orifice at the posterior part of the body ; whilst in other instances there is a marked distinction between the stomach and intestinal tube, the former being a large globu- lar dilatation immediately below the jaws, whilst the latter is cylindrical and comparatively small. The alimentary canal of Rotifer most resembles the first of these types, but presents a dilatation (Z) close to the anal orifice, which may be considered as a cloaca; that of Brachionus (Fig. 200) is rather formed upon ROTIFER A, THEIR GENERAL STRUCTURE. 429 FIG. 201. the second. Connected with the alimentary canal are various glandular appendages, more or less developed ; sometimes clus- tering round its walls as a mass of se- parate follicles, which seems to be the condition of the glandular investment (g) of the alimentary canal in Rotifer; in other cases having the form of coecal tubuli. Some of these open into the stomach close to the termination of the oesophagus, and have been supposed to be salivary or pancreatic in their character; whilst others, which dis- charge their secretion into the intesti- nal tube, have been regarded, and pro- bably with correctness, as the rudi- ment of a liver. In a curious animal- cule of this class, minutely described by Mr. Dalrymple,1 although the mouth, masticating apparatus, and stomach are constructed upon the re- gular type of the genus Notommata, to which it seems nearly allied, yet there is neither intestine nor anal ori- fice, and the indigestible matters are rejected through the mouth. This, so far as is yet known, is a solitary example of the existence of this cha- racter of degradation in the class Ro- tifera. There does not appear to be any special circulating apparatus in these animals ; but the fluid which is contained in the general cavity of the body, between the exterior of the alimentary canal and the in- ner tegumentary membrane, is probably to be regarded as nu- tritive in its character; and its aeration is provided for by a peculiar apparatus, which seems to be a rudimentary form of the u water vascular system," that attains a high development in the class of Worms. On either side of the body there is usually to be observed a long flexuous tube (Fig. 200), which extends from a contractile vesicle common to both and opening into the cloaca (Fig. 201, «, i\ towards the anterior region of the body, where it frequently subdivides into branches, one of which may arch over towards its opposite side, and inosculate with a corresponding branch from its tube. Attached to each of these tubes are a number of peculiar organs (usually from two to eight on each side), in which a trembling movement is seen, very like that of a flickering flame ; these appear to be pear-shaped sacs, attached 1 " Philos. Transact." 1849, p. 339. Rotifer vulgaris, as seen at A, wilh the wheels drawn in, and at B with ihe wheels expanded; a. mouth; 6, eye- spots ; c, wheels ; d, calcar (antenna ?) : e,jaws and teeth; /, alimentary canal ; >j. glandular (?) mass enclosing it; A, longitudinal muscles; i, t, tubes of water vascular system; k, young ani- mal; /, cloaca. 430 MICROSCOPIC FORMS OF ANIMAL LIFE. by hollow stalks to the main tube, having a long cilium in the interior of each, that is- attached by one extremity to the interior of the sac, and vibrates with a quick uudulatoiy motion in its cavity; and there can be little doubt that their purpose is to keep up a constant movement in the contents of the aquiferous tubes, whereby fresh water may be continually introduced from without, for the aeration of the fluids of the body.1 There is much uncertainty with regard to the structures which Prof. Ehrenberg has described as ganglia and nerves ; and it seems doubtful if there is more than a single nervous centre in the neighborhood of the single, double, or multiple red spots, which are seen upon the head of the liotifera, and which, cor- responding precisely in situation with those that in the higher Articulata are unquestionably eyes, are probably to be regarded as rudiments of visual organs. 279. The Reproduction of the Rotifera has not yet been com- pletely elucidated. There is no instance, in this group, in which multiplication by gemmation or spontaneous fission is certainly known to take place ; but the occurrence of clusters formed by the aggregation of a number of individuals of Conochilus, adhe- rent by their tails, and enclosed within a common lorica, would seem to indicate that these clusters, like the aggregations of Polygastrica, Bryozoa, and Tunicata, must have been formed by continuous growth from a single individual. The ordinary method of multiplication, however, is commonly supposed to be by a proper generative act ; as distinct sexes have been disco- vered in several individuals, and the act of sexual union has been witnessed. The condition of the male of the remarkable genus described by Mr. Dalrymple (loc. cit.) is a most extraordinary one ; for it possesses no mandibles, pharynx, oesophagus, stomach, nor hepatic glands ; having, in fact, no other organs fully deve- loped, than those of generation. It would appear, therefore, quite unfit to obtain aliment for itself ; and its existence is pro- bably a very brief one, being continued only so long as the store of nutriment supplied by the egg remains unexhausted. In Rotifer, however, as in by far the larger proportion of the class, no males have been discovered ; and it remains doubtful whether the two sexes are united in the same individual, or whether the males are produced only at certain times. The female organ consists but of a single ovarian sac, which frequently occupies a large part of the cavity of the body, and which opens at its lower end by a narrow orifice into the cloaca. Although the number of eggs in these animals is so small, yet the rapidity with which the whole process of their development and maturation is accom- plished, renders the multiplication of the race very rapid. The egg of the Hydatina is extruded from the cloaca within a few 1 See Mr. Huxley's account of these organs, in his description of Lacinularia socialis, " Transact of Microsc. Soc." Ser. 2, vol. i. Other observers have supposed that the pyriform sacs communicate with the general cavity of the body; bnt the Author has much confidence in the correctness of Mr. Huxley's statements on this point. ROTIFER A, THEIR STRUCTURE AND REPRODUCTION. 431 hours after the first rudiment of it is visible ; and within twelve hours more the shell hursts, and the young animal comes forth. In the Rotifer and several other genera, the development of the emhryo takes place whilst the egg is yet retained within the body of the parent (Fig. 201, &), and the young are extruded alive ; whilst in some other instances, the eggs, after their extru- sion, remain attached to the posterior extremity of the body (Fig. 200), until the young are set free. In general it would seem that, whether the rupture of the egg-membrane takes place before or after the egg has left the body, the germinal mass within it is developed at once into the form of the young animal, which resembles that of its parent; no preliminary metamorphosis being gone through, nor any parts developed which are not to be permanent. The transparency of the egg-membrane, and also of the tissues, of the parent Rotifer, allows the process of deve- lopment to be watched, even when the egg is retained within the body ; and it is curious to observe, at a very early period, not merely the red eye spot of the embryo, but also a distinct ciliary movement. The multiplication ofHydatina (in which genus three or four eggs are deposited at once, and their development com- pleted out of the body) takes place so rapidly, that, according to the estimate of Prof. Ehrenberg, nearly seventeen millions may be produced within twenty-four days from a single individual. Even in those species which usually hatch their eggs within their bodies, a different set of ova is occasionally developed, which are furnished with a thick glutinous investment: these, which are extruded entire, and are laid one upon another, so as at last to form masses of considerable size in proportion to the bulk of the animals, seem not to be destined to come so early to maturity, but very probably remain dormant during the whole winter sea- son, so as to produce a new brood in the spring. These " winter eggs" are inferred by Mr. Huxley, from the history of their de- velopment, to be really gemmce produced by a non-sexual opera- tion ; while the bodies commonly called ova, he considers to be true generative products. Dr. Cohn has recently informed the Author, however, that he has ascertained by direct experiment upon those species in, which the sexes are distinct, that the bodies commonly termed ova (Figs. 200, 201), are really internal gemmce, since they are reproduced, through many successions, without any sexual process, just like the external gemmae of Hydra (§ 301), or the internal gemmse of Entomostraca and Aphides (Chap. XVI). And this view appears to himself to be more accordant with general physiological analogy than that of Mr. Huxley ; since, in the other instances referred to, as in the Roti- fera, the multiplication by gemmation goes on rapidly whilst food and warmth are abundantly supplied ; but gives place to the true generative process, when the nutritive activity is lowered by their withdrawal. 280. Certain Rotifera, among them the common Wheel- Ani- 432 MICROSCOPIC FORMS OF ANIMAL LIFE. FTG. 20*2. malcule, are remarkable for their tenacity of life, even when re- duced to the state of most complete dryness ; for they can be kept in this condition for any length of time, and will yet revive very speedily upon being moistened. Experiments have been carried still farther with the allied tribe of Tardigrades ; indi- viduals of which have been kept in a vacuum for thirty days, with sulphuric acid and chloride of calcium (thus suffering the most complete desiccation that the Chemist can effect), and yet have not lost their capability of revivification. This fact, taken in connection with the extraordinary rate of increase mentioned in the preceding paragraph, removes all difficulty in accounting for the extent of the diffusion of these animals, and for their oc- currence in incalculable numbers in situations where, a few days previously, none were known to exist. For their entire bodies may be wafted in a dry state by the atmosphere, from place to place ; and their return to a state of active life, after a desiccation of unlimited duration, may take place whenever they meet with the requisite conditions, — moisture, warmth, and food. It is probable that the ova are capable of sustaining treat- ment even more severe than the fully developed animals can bear ; and that the race is frequently continued by them, when the latter have perished. 281. The principles on which the various forms that belong to this class should be systematically arranged, have not yet been satisfactorily de- termined. By Prof. Ehren- berg, the disposition of the ciliated lobes or wheel- organs, and the enclosure or non-en- closure of the body in a lorica or case, were taken as the basis of his classification ; but as his ideas on both these points are inconsistent with the actual facts of organization, the ar- rangement founded upon them cannot be received. Another division of the class has been propounded by M. ste han os Eichvrnii Dujardm, which is based on the several modes of life of the most characteristic forms. And in a third, more recently put forth by Prof. Leydig, the general configuration of the body, ROTIFERA — FLOSCULARIANS AND ME LIC E RTI ANS. 433 with the presence, ahsence, and conformation of the foot (or tail), are made to furnish the characters of the subordinate groups. Either of the two latter is certainly more natural than the first, as bringing together for the most part the forms which most agree in general organization, and separating those which differ; and we shall adopt that of M. Dujardin as most suitable to our present purpose. I. The first group includes those that habitually live attached by the foot, which is prolonged into a pedicle ; and it includes two families, the Floscularians and the Melicertians, both of which bear a certain general resemblance to the Vorticellince (§ 268) on the one hand, and to Zoophytes (Chap. XI) on the other. For they are commonly found attached to the stems and leaves of aquatic plants, by a long pedicle or footstalk, which bears a somewhat bell-shaped body; and in one of the most beautiful species, the Stephanoceros Eichornii (Fig. 202), this body has five long tentacles, beset with tufts of short bristly cilia, reminding us of the ciliated tentacles of the Bryozoa (Chap. XIII), whilst the body seems to be enclosed in a cylindrical cell, resembling that of Hydrozoa and Bryozoa. ' A comparison of this with other forms, however, shows that these tentacles are only extensions of the ciliated lobes which are common to all the members of these families; and the so-called "cell" is not formed by a thickening and separation of the outer tegument, but by a gelatinous secre- tion from it; so that, as the rest of the organization is essentially conformable to the Rotiferous type, no such passage is really established by this animal towards other groups, as it is com- monly supposed to form. In one respect, Floscularia is still more aberrant; for the long bristly filaments with which its lobes are beset, are not capable of rhythmical vibration, and cannot, there- fore, be properly termed " cilia." The body of M elicerta is pro- tected by a most curious cylindrical tube, composed of little rounded pellets agglutinated together ; this is obviously an arti- ficial construction ; and Mr. Gosse has been fortunate enough to have an opportunity of watching the animal whilst engaged in building it up. Beneath a projection on its head which he terms the chin, there is observed a small disk-like organ, in which, when the wheels are at work, a movement is seen very much resembling that of a revolving ventilator. Towards this disk, the greater proportion of the solid particles that may be drawn from the surrounding liquid into the vortex of the wheel-organs, are driven by their ciliary movement, a small part only being taken into the alimentary canal; and there they accumulate, until the aggregation (probably cemented by a glutinous secretion furnished by the organ itself) acquires the size and form of one of the globular pellets of the case ; the time ordinarily required being about three minutes. The head of the animal then bends itself down, the pellet-disk is applied to the edge of the tube, the newly formed pellet is left attached there, and, the head being 28 434 MICROSCOPIC FORMS OF ANIMAL LIFE. lifted into its former position, the formation of a new pellet at once commences. II. The next of M. Dujardin's primary groups (ranged by him, however, as the third), consists of the ordinary Rotifer and its allies, which pass their lives in a state of alternation between the conditions of those attached by a pedicle, of those which habitu- ally swim freely through the water, and of those which creep or crawl over hard surfaces. As these have already been fully de- scribed, it is not requisite to dwell longer upon them. III. The next group consists of those Eotifers which seldom or never attach themselves by the foot, but habitually swim freely through the water; and putting aside the peculiar aberrant form Albertia, which has only been found as a parasite in the intestines of worms, it may be divided into two families, the Brachionians and the Furcularians. The former are for the most part distin- guished by the short, broad, and flattened form of the body (Figs. 200, 203) ; which is, moreover, enclosed in a sort of cuirass, Fia. 203. Noteus quadricornis ; A, dorsal view; B, side view. formed by the consolidation of the external integument. This cuirass is often very beautifully marked on its surface, and may be prolonged into extensions of various forms, which are some- times of very considerable length. The latter (corresponding almost exactly with the Hydatinece of Prof. Ehrenberg) derive their name from the bifurcation of the foot into a sort of two- bladed forceps ; their bodies are ovoidal or cylindrical, and are enclosed in a flexible integument, which is often seen to wrinkle itself into longitudinal and transverse folds, at equidistant lines. To this family belongs the Hydatina senta, which is one of the largest of the Rotifera, and which was employed by Prof. Ehren- berg as the chief subject of his examination of the internal struc- ture of this group ; as does also the Notommata, the curious con- dition of whose male has been already referred to (§ 279). ROTIFERA — TARDIGRADA. 435 IV. The fourth of M. Dujardin's primary orders consists of the very curious tribe, first carefully investigated by M. Doyere, to which the name of Tardigrada has been given, on account of the slowness of their creeping movement. Their relation to the true Rotifera, however, is not at all clear; and many naturalists regard them as altogether distinct. They are found in the same loca- lities with the Rotifers, and, like them, can be revivified after desiccation (§ 280) ; but they have a vermiform body, divided trans- versely into five segments, of which one constitutes the head, whilst each of the others bears a pair of little fleshy protube- rances, furnished with four curved hooks, and much resembling the pro-legs of a caterpillar. The head is entirely unpossessed of ciliated lobes ; and it is only in the presence of a pair of jaws somewhat resembling those of Rotifera, and in the correspon- dence of their general grade of organization, that they bear any structural relation to the class we have now been considering. They may be pretty certainly regarded as a connecting link between the Rotifera and the Worms ; but they should probably be ranked on the worm side of the boundary. 282. Notwithstanding that all the best informed Zoologists are now agreed in ranking the class of Rotifera in the Articulated series, yet there is still a considerable discordance of opinion as to the precise part of that series in which they should stand. For whilst Prof. Leydig, who has recently devoted much atten- tion to the study of the class, regards them as most allied to the Crustacea, and terms them " Ciliocrustaceans," Mr. Huxley, with (as it seems to the Author) a clearer insight into their real nature, has argued that they are more connected with the Annelida, through the resemblance which they bear to the early larval forms of that class (Fig. 274). Considered in this light, the Tar- digrada might seem to represent a more advanced phase of the same developmental history.1 1 In addition to the classical works of Ehrenberg and Dujardin, the following Me- moirs should be consulted (besides those already referred to) by such as wish to acquaint themselves with the best researches upon the Rotifera: — Leydig in "Siebold and Kolliker's Zeitschrift," Band VI, heft 1 ; Goss on Melicerta ringens, in " Transact, of Microsc. Soc." Ser. I, vol. iii, p. 58; and "Quart. Journ. of Microsc. Sci." vol. i, p. 71 ; Williamson on Melicerta ringens, Op. cit. p. 1; and Huxley on Lacinularia socialis, in " Transact, of Microsc. Soc." Ser. II, vol. i, p. 1. CHAPTER X. FORAMINIFERA, POLYCYSTINA, AND SPONGES. 283. RETURNING now to the lowest or Rhizopod type of Animal life (§ 264), we have to direct our attention to three very remark- able series of forms, almost exclusively marine, under which that type manifests itself; all of them being distinguished by a skeleton of greater or less density; and this skeleton being generally so consolidated by mineral deposit, as to retain its form and inti- mate structure, long after the animal to which it belonged has ceased to live, even for those undefined periods in which they have been imbedded as fossils in strata of various geological ages. In the first of these groups, the Foraminifera, this skeleton con- sists of a calcareous shell (save in a few exceptional cases, in which the lorica remains flexible), that invests the sarcode body, but is usually perforated with numerous minute apertures. In the second group, also, the Polycystina, there is an investing shell, perforated with numerous apertures ; but this shell is sili- ceous, instead of calcareous. In the third group, on the other hand, — that of Porifera, or Sponges, — the skeleton is usually composed of a network of horny fibres, strengthened either by calcareous or siliceous spicules, and having the soft animal body, which is composed of an aggregate of Amoeba-like cells, in its interstices : in this group, moreover, we have a departure from the Rhizopod type, in the fact that certain parts of the free sur- faces are furnished with cilia, whereby currents of water are sustained, that serve both for nutrition and for respiration. 284. Foraminifera. — The beings now known under this desig- nation, possess, for the most part, polythalamous or " rnany-cham- bered" cells (Fig. 336), so strongly resembling those of Nautilus, Spirula, and other Cephalopod Mollusks, that it is not surprising that the older Naturalists, to whom the structure of these animals was entirely unknown, ranked them under that class. As such they were described by M. D'Orbigny (to whom we owe much of our knowledge of this group), in all his earlier publications; and they were distinguished from the ordinary Cephalopods that possess a single siphon passing from chamber to chamber, by the designation Foraminifera, which originally imported that the communications between the chambers are commonly made by GENERAL ORGANIZATION OF F 0 R AM IN IF ER A. 437 FIG. 204. several such apertures, though it is now more commonly under- stood as applying to the sieve-like structure often presented by the external shell. It was by M. Dujardin, in 1835, that the structure of these animals was first shown to be conformable to the Ilhizopod type ; and notwithstanding the opposition to his views which has been set up by Prof. Ehrenberg (who, with in- explicable pertinacity, has con- tinued to rank them among his Bryozoa, Chap. XIII), they have been confirmed by all subse- quent observers, and more espe- cially by the recent researches of Prof. Schulze,1 who has given admirable descriptions of the animals of several different kinds of Foraminifera, derived from observation of them during their living state. The confor- mity of the Foraminifera to the ordinary Rhizopod type, is best seen in those forms, such as Grromia (Fig. 204), in which there is no multiplication of chambers; and it is made ob- vious by an examination of the accompanying figure, that there is no other essential difference between Grromia and Arcella or Diffugia (Fig. 193), than that which lies in the greater length and slenderness of the pseudo- podial prolongations of the sar- code body in the former, as com- pared with those of the latter. The food is obtained by the ex- tension of these pseudopodia in Various directions from the mouth of the shell ; and the ab- sence of any membrane investing them is clearly indicated by their fusion or coalescence when two or more happen to come into contact, as well as by the vagueness of the expansions into which they are occasionally seen to spread out. These instru- ments entangle and lay hold of the minute bodies which serve as food to the animals, consisting of Diatomacese, Desmidiese, the smaller forms of Confervse, &c. ; and they draw these, by their contraction, into the substance of the body, within which they may be seen through the transparent shell. It is not by any means constantly, that their indigestible residua are cast forth 1 "tiber den Organismus der Polythalamien (Foraminiferen).'' Leipzig, 1854. o^/ormi*, with its pseudopodia extended. 438 FORAMINIFERA, POLYCYSTINA, AND SPONGES. again, as we have seen them to be from the surface of the body of the Actinophrys (§ 262) ; for they sometimes accumulate in considerable numbers, so as even to choke up a large part of the cavity. The sarcode body is occasionally seen to extend itself, as shown in Fig. 204, around the exterior of the shell; and pseudopodia are put forth from this extension, as well as from the ordinary outlet. Although nothing is certainly known of the mode of propagation of these animals, yet it is probable, from the analogy of the composite forms, that they multiply them- selves by the detachment of gemmae, composed of portions of sarcode, which in time form an envelope or shell. Nothing has been yet seen, that in the least corresponds to a true Generative process. 285. By far the greater number of Foraminifera are composite fabrics, evolved by a process of continuous gemmation, each gemma remaining in connection with the body by which it was put forth ; and according to the plan on which this gemmation takes place, will be the configuration of the composite body thereby produced. Thus if the bud should be put forth from the aperture of Gromia, in the direction of the axis of its body, and a second shell should be formed around this bud, in conti- nuity with the first, and this process should be successionally re- peated, a straight rod-like shell will be produced, having many chambers communicating with each other by the openings that originally constituted their mouths, the mouth of the last formed chamber being the only aperture through which the sarcode body, thus composed of a number of segments connected by a peduncle or " stolon" of the same material, can now project itself or draw in its food. The successive segments may be all of the same size, or nearly so, in which case the entire rod will approach the cylindrical form, or will resemble a line of beads ; but it often happens that each segment is somewhat larger than the pre- ceding, so that the composite shell has a conical form, the apex of the cone being the original segment, and its base the one last formed. The method of growth now described, is common to a large number of Foraminifera, chiefly belonging to the genera Nodosaria, Dentaliwa, Morginulina, which M. D'Orbigny has ranked together in an order, under the designation of Stichos- tegues ; it is, however, frequently seen also in the advanced stages of growth of such as usually begin upon the spiral plan ; and there are various considerations (into which this is not the place to enter) which satisfy the Author, that the mere direction of in- crease is a character of very subordinate importance in the group, instead of being one of such fundamental value as to serve for the basis of classification. 286. If each of the successively formed segments, instead of being developed exactly in the axis of its predecessor, should be directed a little to one side, it is obvious that a curved instead of a straight rod will be produced ; and this curvature may increase, COMPOSITE FORMS OF FO R AMINIFE R A. 439 until it becomes a spiral (Fig. 205). The character of this spiral will depend in great degree upon the enlargement or non-enlarge- ment of the successively formed chambers ; for sometimes it opens out very rapidly, every whorl being considerably broader than that which it surrounds, in consequence of the great excess of the size of each segment over that of its predecessor, as is ge- nerally the case in Cristellaria ; whilst in other instances there is so little difference between the successive segments, after the spiral has made two or three turns, that the breadth of each whorl scarcely exceeds that of its predecessor, as is well seen in Faujasina (Fig. 209), as also in Nummulite (Chap. XIX). An intermediate condition is presented by Rosalina (Fig. 205), which FIG. 205. Rosalina ornata, with its pseudopodia extended. may be taken as an example of a very large group of Foramini- fera, composed of those whose plan of growth is helical or spiral, and ranged by M. D'Orbigny under the designation Helicostegues. In this genus, as in a large proportion of its congeners, we find the shell perforated with numerous apertures, through which pseudopodia can be extended from any of the segments that are not enclosed by others, as well as from the mouth or aperture of the outer segment ; and when this is the case, it does not appear that the sarcode body is so often extended over the exterior of the shell, as it is when the shell has no perforations for the put- ting forth of these extensions. There are generally indications, however, in the structure of the shell itself, — new layers being often formed over the innermost whorls of the spiral, or partial 440 FORAMINIFERA, POLYCYSTINA, AND SPONGES. deposits being added, sometimes in the shape of bosses, spines, or other outgrowths, — that a soft substance, capable of originating such new formation, must occasionally spread itself over the whole external surface of the previous segments. And it is not difficult to understand how this may come to pass, when it is borne in mind that the gelatinous sarcode, however fine may be the threads into which it divides itself, may readily form a con- tinuous layer by the coalescence of those threads. The group of Helicostegues is subdivided by M. D'Orbigny into two families, the Nautiloidce, and the Turbinoidce ; the first and most important consisting of those in which the successive whorls all lie in the same plane, so that the shell is " equilateral" (like that of a Nau- tilus), as is the case with Nummulites and their allies (such as Nonionina, Assilina, Operculina, &c., between which and Num- mulites the differences are but very slight); whilst the second contains those in which the spiral passes obliquely round an axis, so that the shell becomes "inequilateral" (like that of a snail or periwinkle), as is the case with Rotalia (Fig. 335), Faujasina (Fig. 209) and Rosalina (Fig. 205). 287. Putting aside less important variations in the plan of gemmation, we have now to notice one that seems essentially dis- tinct from the preceding ; that, namely, in which the new seg- ments are added in concentric rings, each surrounding its pre- decessors, so as to form flattened disks. As an example of this curi- ous type of Foraminiferous structure, the Orlitolite may be cited ; which, long known as a very abundant fossil in the early ter- tiaries of the Paris basin, has lately proved to be scarcely less abundant in certain parts of the existing ocean. The largest specimens of it, sometimes attaining the size of a sixpence, have hitherto been obtained only from the coast of New Holland and various parts of the Polynesian Archipelago ; but disks of com- paratively minute size (from the diameter of an ordinary pin's head, to that of a small pea) and of simpler organization, are to be found in almost all Foraminiferous sands and dredgings from the shores of the warmer regions of the globe, being especially abundant in those of some of the Philippine Islands, of the Red Sea, of the Mediterranean, and especially of the ^Egean. When such disks are subjected to Microscopic examination, they are found (if uninjured by abrasion) to present the structure repre- sented in Fig. 206 ; where we see on the surface (by incident light) a number of rounded elevations, arranged in concentric circles around a sort of nucleus (which has been laid open in the figure to show its internal structure) ; whilst at the margin we observe a row of rounded projections, with a single aperture or pore in each of the intervening depressions. In very thin disks, the structure is often brought into view, by mounting them in Canada balsam, and transmitting light through them, sufficiently well to render any other mode of preparation unnecessary ; but in those which are too opaque to be thus seen through, it is suffi- FORAMINIFERA — ORBITOLITE. 441 Simple disk ot Orbitolites cmnplanatus, laid open to show its interior structure :— a, central cell; b, circum- ambient cell surrounded by concentric zones of cells, connected with each other by annular and radiating passages. cient to rub down one of the surfaces upon a stone, and then to mount the specimen in balsam. Each of the superficial eleva- tions will then be found to be the roof or cover FIG. 206. of an ovate cavity or cell, which communicates by means of a lateral passage with the cavity on either side of it in the same ring; so that each circu- lar zone of cells might be described as a continuous annular passage, dilated into cavities at intervals. On the other hand, each zone communicates with the zones that are inter- nal and external to it, by means of passages in a radiating direction; and it is curious that these passages run, not from the cells of the inner zone to those of the outer ; but from the connect- ing passages of the former to the cells of the latter ; so that the cells of each zone alternate in position with those of the zones that are internal and external to it. The radial passages from the outermost annulus make their way at once to the mar- gin, where they terminate, forming the "pores" which (as al- ready mentioned) are to be seen on its exterior. The central nucleus, when rendered sufficiently transparent by the means just adverted to, is found to consist of a central cell (a), usually somewhat pear-shaped, that communicates by a narrow passage with a much larger circumambient cell (#), which nearly sur- rounds it, and which sends off* a variable number of radiating passages towards the cells of the first zone, which forms a com- plete ring around the nucleus.1 288. The idea of the nature of the living occupant of these cavities, which might be suggested by the foregoing account of their arrangement, is fully borne out by the results of the exami- nation of the sarcode body, that may be obtained by the macera- tion in dilute acid (so as to remove the shelly investment) of specimens of Orbitolite, that have been gathered fresh from the sea-weeds to which in the living state they are found adherent, and have been kept in spirit. For this body is found to be coin- posed (Fig. 207) of a multitude of segments of "sarcode," pre- 1 Although the above may be considered the typical form of the Orbitolite, yet, in a very large proportion of specimens, the first few zones are not complete circles, the early growth having taken place rather in a spiral than in a radial direction ; between these two plans, there is every variety of gradation ; and even where the spiral is mos< distinctly marked in the first instance, the additions soon come to be made in concent/io zones. 442 FORAMINIFEKA, POLYCYSTINA, AND SPONGES. FIG. 207. senting not the least trace of higher organization in any part, and connected together by " stolons" of the like substance. The " central" pear-shaped segment, a, is seen to have budded off its "circumambient" segment, 5, by a narrow footstalk or stolon; and this circumambient segment, after passing almost entirely round the central one, has budded off three stolons, which swell into new segments from which the first annulus is formed. Scarcely are any two specimens precisely alike, as to the mode in which* the first annulus originates from the " circum- ambient" segment; for some- times a score or more of radial passages extend them- selves from every part of the margin of the latter (and this, as corresponding with the plan of growth afterwards followed, is probably the typical arrangement), whilst in other cases (as in the ex- ample before us) the number of these primary offsets is extremely small. Each zone is seen to consist of an as- semblage of ovate segments, whose height (which could not be shown in the figure) corresponds with the thick- ness of the disk ; these seg- ments, which are all exactly similar and equal to one another, are connected by annular "stolons;" and each zone is connected with that on its exterior, by radial extensions of those stolons, passing off between the segments. Although no opportunity has yet been obtained, for a microscopic examination of these animals in their living state, yet there can be no reasonable doubt that the radial extensions of the outermost zone issue forth as pseudopodia from the mar- ginal pores; and that they search for and draw4n alimentary materials, in the same manner as do those of other Foramini- fera ; the whole of the soft body, which has no communication whatever with the exterior, save through these marginal pores, "being nourished by the transmission of the products of diges- tion from segment to segment and from zone to zone, through similar bands of gelatinous substance. In all cases in which the growth of the disk takes place with normal regularity, it is pro- bable that a complete circular zone is added at once. When the earcode body has increased beyond the capacity of its enveloping disk, it may be presumed that its pseudopodial extensions, pro- ceeding from the marginal pores, coalesce, so as to form a corn- composite Animal of simple type of Orbitolites complanatus : — a, central mass of sarcode ; 6, cir- cumambient mass, giving off peduncles, in which originate the concentric zones of segments con- nected by annular bands. FORAMINIFERA — ORBITOLITE. 443 plete annulus of sarcode round the margin of the outermost zone ; and it is probable that it is by a deposit of calcareous matter in the surface-portion of this annulus, that the new zone of shelly substance is formed, which constitutes the walls of the cells and passages occupied by the soft sarcode body. Thus we find this simple type of organization giving origin to fabrics of by no means microscopic dimensions, in which, however, there is no other differentiation of parts than that concerned in the formation of the shell; every segment and every stolon (with the exception of the two forming the "nucleus") being, so far as can be ascertained, a precise repetition of every other, and the segments of the nucleus differing from the rest in nothing else than their form. The equality of the endowments of the segments is shown by the fact, of which accident has repeatedly furnished proof, — that a small portion of a disk, entirely sepa- rated from the remainder, will not only continue to live, but will so increase as to form a new disk ; the loss of the nucleus not appearing to be of the slightest consequence, from the time that active life is established in the outer zones. In what manner the multiplication and reproduction of the species are accom- plished, we can as yet do little more than guess; but from appearances sometimes presented by the sarcode body, it seems reasonable to infer that "gemmules," corresponding with the "zoospores" of Protophytes (§ 197), are occasionally formed by the breaking up of the sarcode into globular masses ; and that these, escaping through the marginal pores, are sent forth to develope themselves into new fabrics. Of the mode wherein that sexual operation is performed, however, in which alone true Generation consists, nothing whatever is known. 289. One of the most curious features in the history of this animal, is its capacity for developing itself into a form, which, whilst fundamentally the same as that previously described, is very much more complex. In all the larger specimens of Orbi- tolite, we observe that the marginal pores, instead of constituting but a single row, form many rows, one above another ; and be- sides this, the cells of the two surfaces, instead of being rounded or ovate in form, are usually oblong and straight-sided, their long diameters lying in a radial direction. "When a vertical section is made of such a disk, it is found that these oblong chambers constitute two superficial layers, between which are interposed columnar chambers of a rounded form ; and these last are con- nected together by a complex series of passages, the arrangement of which will be best understood from the examination of a part of the sarcode body that occupies them (Fig. 208). For the ob- long superficial chambers are occupied by segments of sarcode, c c, d d, lying side by side so as to form part of an annulus, but each of them being disconnected from its neighbors, and com- municating only by a double footstalk with the two circular sto- lons a af, b bf, which obviously correspond with the single stolon 444 FORAMINIFERA, POLYCYSTINA, AND SPONGES. FIG. 208. of the simple type (Fig. 207). These indirectly connect together, not merely all the superficial cells of each zone, but also the columnar segments of the intermediate layer; for these columns (e e, e e) terminate above and below in the annular stolons, sometimes passing directly from one to the other, but some- times going out of the direct course to coalesce with another column. The columns of the successive zones (two sets of which are shown in the figure) communicate with each other by threads of sarcode, in such a manner, that (as in the simple type) each column is thus brought into connection with two columns of the zone next interior, to which Portion of composite Animal of complex it alternates in position. Similar type of Orbitolites complanatus :-a a', b b', the ^ i i m o ^ n «• off from fh P Oil t Pr- upper and lower annular bands of two con- TnreaQfe> paSSing O centric zones; cc, the upper layer of super- mOSt ZO116, tlirOUgh the multiple ficial segments, and dd the lower layer, con- ranpres of marmnal porCS, WOUld nected with the annular bands of both zones; n V ji -IT -v-r ee,e'e\ vertical segments of the two zones. doubtleSS act aS pSCUdopodia. JSTOW this plan of growth is so different from that previously described, that there would at first seem ample ground for distinguishing the simple and the complex types as two species. But the test furnished by the examination of a large number of specimens, which ought never to be passed by when it can possibly be appealed to, furnishes these very singular results : — 1st, That the two forms must be considered as specifically iden- tical ; since there is not only a gradational passage from one to the other, but they are often combined in the same individual, the inner and first formed portion of a large disk frequently pre- senting the simple type, whilst the outer and later formed part has developed itself upon the complex : — 2d, That although the last mentioned circumstance would naturally suggest that the change from the one plan to another may be simply a feature of advancing age, yet this cannot be the case ; since the complex plan sometimes evolves itself even from the very first (the nucleus, though resembling that of the simple form, sending out two or more tiers of radiating threads), whilst, more frequently, the sim- ple prevails for an indefinite number of zones, and then changes itself in the course of a few zones into the complex. The mode in which this change occurs is not a little curious. In the first place, the short segments, threaded (so to speak) upon their an- nular stolon, undergo elongation, and the annular stolon itself becomes double, being first, as it were, split in two, and its upper and lower halves being separated by the interpolation of a length- FORAMINIFERA — NUMMULITES, ETC. 445 ening piece to each columnar segment. These additional pieces are at first very short ; but with the growth of every new zone, they commonly increase in length ; and this interpolated portion comes to constitute the principal part of the thickness of the disk. While this change is going on, another is taking place in the position of the superficial portions of the segments which are above and below the annular stolons ; for these, whilst at first seeming to be mere continuations of the columns beneath, and being connected (like them) with the stolons of their own zones alone, are so displaced, in the course of two or three zones, as to arch over the space between the zones (as shown in Fig. 208), and to connect themselves with the stolon, not only of their own zone, but of the next. It has been thought desirable to enter into the foregoing detail, since a more striking instance could scarcely be drawn from any department of Natural History, of the wide range of variation that may occur within the limits of one and the same species; and the Microscopist needs to be specially put on his guard as to this point in respect to the lower types of Animal, as to those of Vegetable life, since the determi- nation of form seems to be far less precise among such, than it is in the higher types.1 290. The type of Foraminiferous structure which has been just described, is in many respects peculiar, and may be con- sidered as verging towards the Sponges. It is obvious, from what has been said of the extreme freedom with which the several segments of the sarcode body communicate with each other, that they form one whole, in a far greater degree than they do in the ordinary composite Foraminifera, whose segments are more completely separated, and are very commonly connected only by a few very slender threads of sarcode. Indeed if we were to imagine a discoidal mass of sarcode to be traversed by a reticulated calcareous skeleton, somewhat resembling that open areolar texture which forms the shell of the Echinida (§ 312), and this network were to possess somewhat of that regularity in the disposition of its successively formed parts, which is presented to us in the spines of that group (Fig. 237), we should have no unapt representation of the calcareous skeleton of the Orbitolite, and of its relation to the animal which it envelopes and protects. Now there are certain Sponges which have a reticular skeleton composed of mineral matter (§ 296), differing from that of the Orbitolite in little else than the want of the zonular arrangement which marks successive epochs of growth ; and we shall see that the constitution of the soft body is essentially the same in one case as in the other. A remarkable connecting link between the Orbitolite and certain Sponges, seems, in fact, to be presented 1 For a full account of the Organization of the Orbitolite, and of the various conditions under which it presents itself, see the Author's memoir upon that genus in " Philos. Transact." 1856. 446 FORAMINIFERA, POLYCYSTINA, AND SPONGES. to us in the curious Thalassicolla* discovered by Mr. Huxley, and since observed by Prof. Muller, which seems also to have rela- tions to the Potycystina (§ 292). 291. The essential simplicity of the animal, and the absence of anything like structure in the shell, of the Orbitolite, ob- viously place it much lower in the scale than those Foraminifera, which have the segments not only more completely divided, but enclosed in a shell which is itself distinctly organized. Such is the case with Nummulites and their nearest allies among the recent forms. For in these, as will be more fully shown here- after (Chap. XIX), each segment has its own separate envelope of shell, so that the partition between any two adjacent chambers is double ; the chambers are so far cut off from one another, as to communicate only by very narrow passages ; but means are afforded, by which even the innermost chambers are brought into tolerably direct relation with the exterior. For in this type of structure, we observe that those parts of their walls which look towards the outer surfaces, are perforated with immense numbers of pores resembling those of Fig. 205, but more numerous, minute, and closely set ; and where the walls are thick, these pores are continued as tubes through their entire substance (Figs. 209, 338). However fine they may be, the extraordinary tenuity of the threads into which the sarcode is occasionally seen to divide itself (Fig. 204), shows that this need not be an ob- stacle to the passage of pseudopodial prolongations through them. But further, in the spaces between the walls of contiguous cham- bers, we find a system of large tubes, which make their way directly from the central to the peripheral portion of the disk, and which, communicating on the one hand with the innermost chambers, and on the other with the margin (being extended and carried on to it whenever the previous edge is covered by a new growth), serve to bring the former into a very direct rela- tion with the external sources of supply of nutriment and oxygen. A strikingly developed example of this system of "interseptal" canals is presented in the genus Faujasina (Fig. 209) ;2 where the large size of the canals, and the extreme simplicity of their arrangement, enable them to be very readily traced out. In some instances the arrangement becomes extremely complex: 1 See " Annals of Natural History," 2d Ser. vol. viii, p. 433 ; and " Quart. Journ. of Microsc. Science," vol. iv, p. 72. 2 See Prof. Williamson's Memoir on the Faujasina, in the "Transact, of Microsc. Soc." 2d Ser. vol. i. As the correctness of the account of the interseptal system of canals, which has been given by the Author (in his Memoir on Nummulite, &c., in " Quart. Geol. Journ." Feb. 1850), and confirmed by the researches of Prof. W. and himself upon numerous recent types, has been called in question by no less an authority than Prof. Schulze, who has, in consequence, altogether ignored this important cha- racter in his classification, the Author thinks it right to state, that although the above figure is copied from one of those which illustrate Prof. Williamson's Memoir, yet it is the almost precise counterpart of a section of the same species prepared by himself. The incredulity of Prof. Schulze and others, upon this point, simply depends upon their want of aptitude in the preparation of sufficiently thin sections. COLLECTION AND SELECTION OF FORAMINIFERA. 447 FIG. 209. Section of Faujasina near its base and parallel to 5t:-showin? ««, the radiating interseptal canals; 6, their inlernal bifurcations; c, a transverse branch; d tubular wall of the chambers. this is especially the case, where, as frequently happens, there is an interstitial calcareous skeleton to be nourished (often extend- ing itself into outgrowths of various forms and sizes), in addition to the immediate investments of the seg- ments; and it becomes ob- vious from the far greater development of this canal system wherever such is the case, that this interstitial skeleton is chiefly if not entirely maintained through the instrumentality of the "stolons" of sarcode which occupy these canals, and whose remains may often be distinctly traced in the dried shell.1 Now this, the highest type of Foramini- fei'OUS Structure, is not Only r>rP^PTltPrl hv Slioh RTliral Dy Spiral forms aS Nummulite, but is found also in a genus that is conformable, in its concentric plan of growth, to Orbitolite. Hence it is obvious that no arrangement founded, as in that of M. D'Orbigny, upon a character of such secondary importance as the direction of gemmation, is likely to be in accordance with physiological features which a perfect knowledge of the animal might be expected to afford; and as these can be partly judged of from the structure of the shell, it seems obvious that this ought to be made the first consideration. To carry out a classi- fication on such a basis, however, will involve a large amount of patient investigation.2 292. Many of the Foraminifera attach themselves in the living state to Sea-weeds, Zoophytes, &c. ; and they should, therefore, be carefully looked for on such bodies, especially when it is de- sired to observe their internal organization and their habits of life. They are often to be collected in much larger numbers, however, from the sand or mud dredged up from the sea-bottom, or even from that taken from between the tide marks. In a paper containing some valuable hints on this subject,3 Mr. Legg 1 The Author has been enabled to make out this curious point completely, in the Calcarina, a little body resembling a spur-rowel. For he has obtained ample evidence that the spire with its regularly added segments, and interstitial skeleton extending itself into radiating spines, may grow quite independently of one another. The proof will be submitted in future Memoirs to the Royal Society. a To this labor, the Author has been for some years devoting a portion of his very limited leisure, in conjunction with his friend Prof. Williamson, of Manchester ; and the results of their united labors will appear at the earliest practicable period, in the Ray Society's publications. 3 " Transactions of the Microscopical Society," 2d Series, vol. ii, p. 19. 448 FORAMINIFERA, POLYCYSTINA, AND SPONGES. mentions that, in walking over the Small Mouth Sand, which is situated on the north side of Portland Bay, he observed the sand to he distinctly marked with white ridges, many yards in length, running parallel with the edge of the water ; and upon examining portions of these, he found Foraminifera in considera- ble abundance. One of the most fertile sources of supply that our own coasts afford, is the " ouze" of the Oyster-beds, in which large numbers of living specimens will be found; the variety of specific forms, however, is usually not very great. In separating these bodies from the particles of sand, mud, &c., with which they are mixed, various methods may be adopted, in order to shorten the tedious labor of picking them out, one by one, under the Simple Microscope ; and the choice to be made among these will mainly depend upon the condition of the Foraminifera, the importance (or otherwise) of obtaining them alive, and the nature of the substances with which they are mingled. Thus, if it be desired to obtain living specimens from the oyster-ouze, for the examination of their soft parts, or for preservation in a vivarium, much time will be saved by stirring the mud (which should be taken from the surface only of the deposit) in a jar with water, and then allowing it to stand for a few moments ; the finer par- ticles will remain diffused through the liquid, while the heavier will subside ; and as the Foraminifera (in the present case) be- long to the latter category, they will be found almost entirely free from extraneous matter, at the bottom of the vessel, after the operation has been repeated two or three times. It would always be well to examine the first deposit let fall by the water that has been poured away ; as this may contain the smaller and lighter forms of Foraminifera. But supposing that it be only desired to obtain the dead shells from a mass of sand brought up by the dredge, a very different method should be adopted. The whole mass should be exposed for some hours to the heat of an oven, and be turned over several times, until it is found to have been thoroughly dried throughout; and then, after being al- lowed to cool, it should be stirred in a large vessel of water. The chambers of their shells being now occupied by air alone (for the bodies of such as were alive will have shrunk up almost to nothing), the Foraminifera will be the lightest portion of the mass ; and they will be found floating on the water, while the particles of sand, &c., subside. Another method, devised by Mr. Legg, consists in taking advantage of the relative sizes of different kinds of Foraminifera and of the substances that ac- company them. This, which is especially applicable to the sand and rubbish obtainable from sponges (which may be got in large quantity from the sponge merchants), consists in sifting the whole aggregate through successive sieves of wire-gauze, com- mencing with one of ten wires to the inch, which will separate large extraneous particles, and proceeding to those of 20, 40, 70, and 100 wires to the inch, each (especially that of 70) retaining GENERAL STRUCTURE OF POLYCYSTINA. 449 a much larger proportion of Foraminiferous shells than of the accompanying particles ; so that, a large portion of the extra- neous matters being thus got rid of, the final selection he- comes comparatively easy. Certain forms of Foraminifera are found attached to shells, especially bivalves (such as the Cha- macece), with foliated surfaces ; and an extensive examination of those of the Indian Seas, when brought home "in the rough," has yielded to Mr. W. K. Parker some most valuable and novel results, which will be made public in due time. 293. The final selection of specimens for mounting, should always be made under some appropriate form of Single Micro- scope (§§ 27-30) ; a fine camel-hair pencil, with the point wetted between the lips, being the instrument which may be most con- veniently and safely employed, even for the most delicate speci- mens. In mounting Foraminifera as microscopic objects, the method to be adopted must entirely depend upon whether they are to be viewed by transmitted or by reflected light. In the former case, they should be mounted in Canada balsam ; the various precautions to prevent the retention of air bubbles which have been already described (§ 128), being carefully observed. In the latter, no plan is so simple, easy, and effectual, as the attaching them with a little gum to a blackened disk of card, and guarding them by a perforated wooden slide (§ 123). They should be fixed in various positions, so as to present all the dif- ferent aspects of the shell, particular care being taken that its mouth is clearly displayed ; and where, as will often happen, the several individuals differ considerably from one another, special care should be taken to form series illustrative of their range of variation, and of the mutual connections of even the most di- verse forms. For the display of the internal structure of Fora- minifera, it will often be necessary to make extremely thin sec- tions, in the manner already described (§§ 108-110) ; and much time will be saved, by attaching a number of specimens to the glass at once, and by grinding them down together. For the preparation of sections, however, of the extreme thinness that is often required, those which have been thus reduced should be transferred to separate glasses, and finished oif each one by itself. 294. Polycystina. — It is probable that we are to refer to the same general type, an extensive group of very interesting microscopic bodies, possessing great beauty and variety of form and structure (Figs. 210-216) of whose essential character extremely little is known. These are minute siliceous shells, which appear, from the recent observations of Prof. Muller, to contain in the living state an olive-brown " sarcode," extending itself into pseudo- podial prolongations (resembling those of Actinophrys, § 262), that pass through the large apertures by which the shells are perforated. The sarcode body does not seem always to fill the shell ; being stated by Prof. Muller to occupy, in the Encyrtidium 29 450 FORAMINIFERA, POLYCYSTINA, AND SPONGES, FIG. 210. FIG. 211. of Messina, only the upper part or vault of the shell, and to be very regularly divided into four lobes. It is a peculiar feature in these Polycystinse, that their shells are often pro- longed into spines or other projections, which are some- times arranged in such a manner as to give them a very singular aspect (Figs. 210, 211). It seems pro- bable that these creatures are almost as widely diffused at the present time as are the Foraminifera ; although, from their greater minute- ness, they have not been so often recognized. For having been first discovered by Prof. Ehrenberg at Cux- haven on the North Sea, they were afterwards found by him in collections made in the Antarctic Seas, and have been recently described by Prof. Bailey as presenting themselves (with Foraminifera and Diato- maceae) in the deposits brought up by the sounding-lead from the bottom of the Atlantic Ocean, at depths of from 1000 to 2000 fathoms. They appear to have been much more abundant, how- ever, during the later geological periods ; for not only have certain forms (among them Haliomma, Fig. 212) been detected by Fig. 210. Podocyrtis Schomburgkii. Fig. 211. Rhopalocanium ornatum. FIG. 212. FIG. 213. Fig. 212. Haliomma Humboldtii. Fig. 213. Pbrichlamydium prcetextum. Prof. Ehrenberg in the chalks and marls of Sicily and Greece, and of Oran in Africa, and also in the diatomaceous deposits of Bermuda and Richmond (Virginia) ; but a large proportion of the rock that prevails through an extensive district in the island of Barbadoes, has been found by him to be composed of Poly- POLYCYSTINA OF BARBADOES. 451 cystina, mingled with Diatomacere, with a few calcareous Fora- minifera, and writh calcareous earth which was probably derived FIG. 214. Fossil Polycystina, &c., from Barbadoes:— a, Podocyrtis mitra; 6, Rhabdolithus sceptrum; c, Lychnocanium falciferum; d. Enoyrtidium tubulus; e, Flustrella concentrica; /, Lychnocanium lucerna; #, Encyrtidium elegans; h, Dictyospyris clathrus; i, Encyrtidium mongolfieri; fc, Stepha- nolithis spiuescens; Z, S. nodosa; m, Lithocyclia ocellus; n, Cephalolithis sylvina; o, Podocyrtis cothurnata ; p, Rhabdolithes pipa. from the decomposition of corals, &c., so as to form, according to the relative proportions of these constituents (which differed in different parts of the deposit), a tripoli-like sandstone, whitish and very friable, a compact calcareous sandstone, and strata of a marly character sometimes containing semi-opal. Previously to this last discovery, which was made in the year 1846 (the mate- rials having been furnished by the geological researches of Sir R. H. Schomburgk), 39 species of Polycystina had been esta- blished by Prof. E. ; but in the Barbadoes deposit, he has detected no fewer than 282 forms which he considers to be specifically distinct, besides 25 species of Diatomacese and Foraminifera, and 54 forms which he cannot distinctly determine, but which he classes under the provisional designations of CreolitJiaria and Phytolitharia, making 361 in all, of which more than 300 were previously unknown. The 282 species of Polycystina are arranged by Prof. E. in seven families, which include forty-four genera ; but it is obvious that in our present state of almost entire igno- rance of the structure and physiology of the animals to which these shells belong, no classification can be otherwise than pro- visional.— Few Microscopic objects are more beautiful than an assemblage of the most remarkable forms of Barbadian Polycys- 452 FORAMINIFERA, POLYCYSTINA, AND SPONGES. tina, especially when seen brightly illuminated upon a black ground; since (for the reason formerly explained, § 62) their FIG. 215. FIG. 216. Fig. 215. Stylodictya gracilis. Fig. 216. Astromma Aristotelis. "solid forms" become much more apparent than they are when these objects are examined by light transmitted through them. And the "black ground illumination," either by the "spotted lens" or by the "paraboloid" (§ 61), is much to be preferred for this purpose, to the ordinary mode of illuminating opaque objects by incident light from a condenser, although this may be advan- tageously had recourse to, by the Microscopist who is unprovided with these appurtenances. No class of objects is more suitable than these to the "Binocular Microscope" (§ 40); the stereosco- pic projection of which causes them to be presented to the mind's eye in complete relief, so as to bring out with the most marvel- lous and beautiful effect all their delicate sculpture, reminding the observer (to compare small things with great) of the finest specimens of the hollow ivory balls carved by the Chinese.1 295. Sponges. — Although this tribe has been bandied from the Animal to the Vegetable kingdom, and back again, several times in succession, yet its claim to a place among the Protozoa may now be considered as pretty certainly determined, by the infor- mation derived from Microscopic examination of its minute structure. For in the living Sponge, the skeleton, usually com- posed of a fibrous network strengthened by spicules of mineral matter, is clothed with a soft flesh; and this flesh has been found by Dujardin and all subsequent observers to consist of an aggregation of Amoeba-like bodies (Fig. 217, B), some of which (as Mr. Dobie has shown)2 are furnished with one or more long 1 For a fuller description of this group, see Prof. Ehrenberg's Memoirs in the " Transac- tions of the Berlin Academy'' for 1846, 1847, and his recently published" Microgeologie ;" also "Ann. of Nat. Hist" 1847. 2 Goodsir's Annals of Anatomy and Physiology," No. 2, May, 1852. GENERAL STRUCTURE OF SPONGES. 453 cilia, closely resembling those of Volvox (Fig. 70, i), by the agency of which, a current of water is kept up through the passages and canals excavated in the substance of the mass. FIG. 217. Structure of Grautia compressa: — A, portion moderately magnified, showing general arrangement of triradiate spicules and intervening tissue ; — B, small portion highly magnified, showing ciliated cells. And from the observations of Mr. Carter1 upon the early development of Sponges, it appears that they begin life as soli- tary Amaebce, and that it is only in the midst of aggregations formed by the multiplication of these, that the characteristic Sponge-structure makes its appearance, the formation of spicules being the first indication of such organization. The ciliated cells seem to form the walls of the canals by which the whole fabric of the Sponge is traversed ; these canals, which are very irregular in their distribution, may be said to commence in the small pores of the surface, and to terminate in the large vents ; and a current is continually entering at the former, and passing forth from the latter, during the whole life of the Sponge, bring- ing in alimentary particles and oxygen, and carrying out excre- mentitious matter. 296. The skeleton which gives shape and substance to the mass of sarcode particles that constitutes the living animal, is composed, in the Sponges with which we are most familiar, of an irregular reticulation of fibres. The arrangement of these may be best made out, by cutting thin slices of a piece of Sponge submitted to firm compression, and viewing these slices, mounted upon a dark ground, with a low magnifying power, under incident light. Such sections, thus illuminated, are not merely striking objects, but serve to show, very characteristically, the general disposition of the larger canals and of the smaller areolse with which they communicate. In the ordinary Sponge, the fibrous skeleton is almost entirely destitute of spicules, the absence of which, in fact, is one important condition of that 1 " Annals of Natural History," Ser. 2, vol. iv. 454 FORAMINIFERA, POLYCYSTINA, AND SPONGES. flexibility and compressibility on which its uses depend. When spicules exist in connection with such a skeleton, they are either altogether imbedded in the fibres, or they are implanted into them at their bases, as shown in Fig. 218. In the curious and beautiful Dictyochalix pumiceus of Barbadoes, however, the en- tire network of fibres is composed of silex, and is so transpa- rent that it looks as if composed of spun glass. There are many Sponges in which no fibrous network can be discerned, the spicules lying imbedded in the midst of the sarcode mass ; such is the case in G-rantia (Fig. 217, A), whose triradiate spicules are composed of carbonate of lime. Sponge-spicules are much more frequently siliceous than calcareous ; and the variety of forms presented by the siliceous spicules is much greater than that which we find in the comparatively small number of species in which they are composed of carbonate of lime. The long needle-like spicules (Fig. 219) which are extremely abundant in several Sponges, lying close together in bundles, are sometimes straight, sometimes slightly curved ; they are sometimes pointed at both ends, sometimes at one only ; one or both ends may be furnished with a head like that of a pin, or may carry three or more diverging points, which sometimes curve back so as to form hooks (Fig. 334, H). When the spicules project from the horny framework, they are usually somewhat conical in form, and their surface is often beset with little spines, arranged at regular intervals, giving them a jointed appearance (Fig. 218). Sponge- spicules frequently occur, however, under forms very different from the preceding ; some being short and many-branched ; and the branches being themselves very commonly stunted into mere tubercles (some examples of FIG. sis. which type are presented in Fig. 334, A, c) ; whilst others are stel- late, having a central body with conical spines projecting from it in all directions (as at D of the same figure). Great varieties FIG. 219. Fig. 218. Portion of Halichondria (?) from Madagascar, with spicules projecting from the fibrous network. Fig. 219. Siliceous Spicules of Pachymatisma. present themselves in the stellate form, according to the relative predominance of the body and of the rays ; in those represented SPONGES, MODE OF EXAMINING THEM. 455 in Fig. 219, the rays, though very numerous, are extremely short ; in other instances the rays are much longer, and scarcely any central nucleus can be said to exist. The varieties in the form of Sponge-spicules are, in fact, almost endless; and a single sponge often presents two or more (as shown in Fig. 219), the stellate spicules usually occurring either in the interspaces between the elongated kinds, or in the external crust. In one curious Sponge described by Mr. Bowerbank (the Dusideia fra- gilis\ the spicules are for the most part replaced by particles of sand, of very uniform size, which are found imbedded in the horny fibre. The spicules of Sponges cannot be considered, like the "raphides" of Plants (§ 230), simply as deposits of mineral matter in a crystalline state. For the forms of many of them are such as no mere crystallization can produce ; many of them possess internal cavities, which contain organic matter; and the calcareous spicules, whose mineral matter can be readily dissolved away by an acid, are found to have a distinct animal basis. Hence it seems probable, that each spicule was origi- nally a cell or segment of sarcode, which has undergone calcifica- tion, and by the self-shaping power of which, the form of the spicule is mainly determined. 297. Of the Reproductive process in Sponges, much has yet to be learned. The following is perhaps the most probable account of it : — Multiplication by gemmation is effected by the detachment of minute globular particles of sarcode from the interior of the canals, where they sprout forth as little protu- berances, whose footstalks gradually become narrower and nar- rower until they give way altogether ; these gemmules, like the zoospores of Algae, possess cilia, and issuing forth from the vents, transport themselves to distant localities, where they may lay the foundation of new fabrics. But according to the recent observations of Mr. Huxley,1 a true sexual generation also takes place, as might be anticipated; both ova and sperm-cells being found imbedded in the substance of the Sponge. The bodies distinguished as capsules, which are larger than the gemmules, and which usually have their investment strengthened with siliceous spicules very regularly disposed, are probably the pro- ducts of this operation. They contain numerous globular par- ticles of sarcode, every one of which, when set free by the rup- ture of its envelope, becomes an independent Amoeba-like body, and may develope itself into a complete Sponge. 298. With the exception of those that belong to the genus Spongilla, all known Sponges are marine ; but they differ very much in habit of growth. For whilst some can only be obtained by dredging at considerable depths, others live near the surface, whilst others attach themselves to the surfaces of rocks, shells, &c., between the tide-marks. The various species of Grrantia, in which alone of all the marine Sponges has ciliary movement • '^ •' l "Ann. of Nat. Hist." Ser. 2, vol. vii. 456 FORAMINIFERA, POLYCYSTINA, AND SPONGES. yet been seen, belong to this last category. They have a pecu- liarly simple structure, each being a sort of bag whose wall is so thin that no system of canals is required, the water absorbed by the outer surface passing directly towards the inner, and being expelled by the mouth of the bag. The cilia may be plainly distinguished with a l-8th inch objective, on some of the cells of the gelatinous substance scraped from the interior of the bag ; or they may be seen in situ, by making very thin transverse sections of the substance of the Sponge.1 It is by such sections alone, that the internal structure of sponges, and the relation of their spicular and horny skeletons to their fleshy substance, can be demonstrated. In order to obtain the spicules in an isolated condition, however, the animal matter must be got rid of, either by incineration, or by chemical reagents. The latter method is preferable, as it is difficult to free the mineral residue from car- bonaceous particles by heat alone. If (as is commonly the case) the spicules are siliceous, the Sponge may be treated with strong nitric or nitro-muriatic acid, until its animal substance is dis- solved away ; if, on the other hand, they be calcareous, a strong solution of potass must be employed instead of the acid. The operation is more rapidly accomplished by the aid of heat ; but if the saving of time be not of importance, it is preferable on seve- ral accounts to dispense with it. The spicules, when obtained in a separate state, should be mounted in Canada balsam. Sponge-tissue may often be distinctly recognized in sections of agate, chalcedony, and other siliceous accretions, as will here- after be stated in more detail (Chap. XIX). 1 See Dobie, loc. cit. ; and Bowerbank in " Transactions of Microscopical Society/' 1st Ser. vol. iii, p. 137. CHAPTER XI. ZOOPHYTES. 299. THE term Zoophyte, although sometimes used in a wider signification, is properly restricted to the class of Polypifera, or polype-bearing animals, whose composite skeletons or "poly- paries" have more or less of a plant-like form ; even the Potyzoa (or Bryozoa) being now excluded, on account of their truly Molluscan structure (Chap. XIII), notwithstanding the zoophytic character of their forms and of their habits of life. The true Zoophytes may be divided into two primary groups, the Hydrozoa and the Anthozoa ; the Hydra (or fresh-water polype) standing as the type of the one, and the Sea-Anemone as the representa- tive of the other. As the Hydrozoa are essentially microscopic animals, they need to be described with some minuteness ; whilst in regard to the Anthozoa, only those points can be dwelt on, which are of special interest to the Microscopist. 300. The Hydra is to be searched for in pools and ditches, where it is most commonly to be found attached to the leaves or stems of aquatic plants, floating pieces of stick, &c. Two species are common in this country, the H. viridis or green polype, and the H. vulgaris, which is usually orange-brown, but sometimes yellowish or red (its color being liable to some variation accord- ing to the nature of the food on which it has been subsisting) ; a third less common species, the H. fusca, is distinguished from both the preceding by the length of its tentacula, which in the former are scarcely as long as the body, whilst in the latter they are, when fully extended, many times longer (Fig. 220). The body of the Hydra consists of a simple bag or sac, which may be regarded as a stomach, and which is capable of varying its shape and dimensions in a very remarkable degree ; sometimes extending itself in a straight line, so as to form a long narrow cylinder, at other times being seen (when empty) as a minute contracted globe, whilst, if distended with food, it may present the form of an inverted flask or bottle, or even of a button. At the upper end of this sac is a central opening, the "mouth," and this is surrounded by a circle of tentacula or " arms," usually from six to ten in number, which are arranged with great regu- larity around the orifice. The body is prolonged at its lower 458 ZOOPHYTES. FIG. 220. end into a narrow base, which is furnished with a suctorial disk ; and the Hydra usually attaches itself by this, whilst it allows its tendril-like tentacula to float freely in the water, like so many fishing-lines. The wall of the body is composed of cells, im- bedded in a kind of sarcode ; and it consists of two principal layers, an outer and more com- pact, of which the cells form a tolerably even surface, and an inner that lines the stomach, into the cavity of which some of the cells project. Between these layers, there is a space chiefly occupied by " sarcode," having many vacuoles or lacunae (which often seem to communicate with one another) excavated in its substance. The arms are made up of the same materials as the body ; but their surface -is beset with little wart-like prominences, which, when carefully examined, are found to be composed ot clusters of small "thread-cells," having a single large cell with a long spiculum in the centre of each. The structure of these thread-cells or "urticating or- gans" will be described hereafter (§ 310) ; at present it will be enough to point out, that this ap- paratus, repeated many times on each tentacle, is doubtless in- tended to give to the organ a great prehensile power ; the minute filaments forming a rough surface, adapted to prevent the object from readily slipping out of the grasp of the arm, whilst the central spiculum is projected into its substance, and probably conveys into it a poisonous fluid secreted by a vesicle at the base of the dart. The latter inference is founded upon the oft-re- peated observation, that if the living prey seized by the tentacles have a body destitute of hard integument, as is the case with the minute aquatic Worms which constitute a large part of its aliment, this speedily dies, even if, instead of being swallowed, it escapes from their grasp ; on the other hand, minute Euto- mostracous Crustacea, Insects, and other animals with hard en- velopes, may escape without injury, even after having been de- tained for some time in the polype's embrace. The contractility of the tentacula (the interior of which is traversed by a canal Hydra fusca, with a young bud at 6, and a more advanced bud at c. HYDRA — ITS MULTIPLICATION BY BUDS. 459 which communicates with the cavity of the stomach) is very remarkable, especially in the Hydra fusca ; whose arms, when extended in search of prey, are not less than seven or eight inches in length ; whilst they are sometimes so contracted, when the stomach is filled with food, as to appear only like little tubercles around its entrance. By means of these instruments, the Hydra is enabled to derive its subsistence from animals, whose activity, as compared with its own slight powers of loco- motion, might have been supposed to remove them altogether from its reach ; for when, in its movements through the water, a minute worm or a water-flea happens to touch one of the ten- tacula of the polype, spread out as these are in readiness for prey, it is immediately seized by this, other arms are soon coiled around it, and the unfortunate victim is speedily conveyed to the stomach, within which it may frequently be seen to continue moving for some little time. Soon, however, its struggles cease, and its outline is obscured by a turbid film, which gradually thickens, so that at last its form is wholly lost. The soft parts are soon completely dissolved, and the harder indigestible por- tions are rejected through the mouth. A second orifice has been observed at the lower extremity of the stomach ; but this would not seem to be properly regarded as anal, since it is not used for the discharge of such exuviae ; it is probably rather to be considered as representing, in the Hydra, the entrance to that ramifying cavity, which, in the compound Hydroida, brings into connection the lower extremities of the stomach of all the in- dividual polypes (Fig. 223). A striking proof of the simplicity of the structure of the Hydra, is the fact that it may be turned inside out like a glove ; that which was before its external tegu- ment becoming the lining of its stomach, and vice versd. 301. The ordinary mode of multiplication in this animal, is by a gemmation resembling that of Plants. Little bud-like pro- cesses (Fig. 220, ft, m branches, of the natural size; B, a small portion enlarged, tliy J ^U1J& !JuljPe showing the structure of the animal; a, terminal branch trildeS itself. The OHgill bearing polypes; 6, polype bud partially developed; c, Qf fae bodies Commonly horny cell, containing the expanded polype, a; e, ovarian , . capsule, containing medusiform gemmae in various stages DUG errOUCOUSly CleSlg- of development ;/, fleshy substance extending through the rioted i(OVarian Capsules" stem and branches, and connecting the different polype- / \ • xi •] ,. "U 4. cells and ovarian capsules; g, annular constrictions at the \e)l 1S CXaCtly S] base of the branches. their destination is very different. Within them are evolved, by a budding process, the generative organs of the COLLECTION AND OBSERVATION OF HYDROZOA. 465 Zoophyte ; and these sometimes develope themselves into the form of independent Medusae, which completely detach them- selves from the stock that bore them, make their way out of the capsule, and swim forth freely, to mature their sexual products (some developing spermatozoa, and others ova) and give origin to a new generation of polypes ; whilst in other cases, these flower-buds, whose Medusan structure is less distinctly pro- nounced, do not completely detach themselves, but expand one after another at the mouth of the capsule, withering and dropping off after they have matured their generative pro- ducts ; and in other cases, again, the Medusan conformation is altogether obscured by want of development, the sexual act being performed by those bodies whilst they are still enclosed within their capsules. There is reason to believe that the male and female Medusoids are always developed within separate cap- sules, possibly on distinct polypidoms ; the males give forth spermatozoa ; whilst the females prepare ova, which, when ferti- lized by the entrance of spermatozoa, develope themselves into ciliated "gemmules," and these, escaping from the capsules, soon evolve themselves into true polypes. This last is the only mode of generation that has been yet witnessed among the Ser- tularidce ; for no free Medusoids have been observed to make their way out of the capsules of any members of this family (Fig. 224), within which may be seen several bodies that are commonly reputed to be eggs, but are really imperfectly de- veloped gemmae of the Medusan type. It is worthy of notice, that the horny capsule has been shown by Prof. E. Forbes, to be essentially a metamorphosed branch, whose numerous small cells have coalesced (as it were) into a single large one ; this is made obvious by a careful comparison of the forms under which it presents itself, in different members of these two families. 305. There are few parts of our coasts, which will not supply some or other of the beautiful and interesting forms of Zoophytic life which have been thus briefly noticed, without any more trouble in searching for them, than that of examining the sur- faces of rocks, stones, sea-weeds, and dead shells between the tide-marks. Many of them habitually live in that situation; and others are frequently cast up by the waves from the deeper waters, especially after a storm. Many kinds, however, can only be obtained by means of the dredge. For observing them during their living state, no means is so convenient as the zoophyte- trough (§ 69), invented for that express purpose by Mr. Lister, to whom we owe not only many improvements in the Microscope and its appurtenances, but also some of the earliest and best ob- servations upon this class of Zoophytes which the application of the Achromatic principle permitted. Before mounting them for preservation as microscopic objects, the Author has found it best to keep them for some time in strong spirit; after a prolonged maceration in which, they may be mounted in spirit sufficiently 30 466 ZOOPHYTES. dilute to be destitute of any injurious action on the cement. Goadby's fluid also may be used ; but the preservation of the soft parts is not quite so FIG. 224. complete with it as with spirit. The size of the cell must of course be propor- tioned to that of the object; and if it be desired to mount such a specimen as may serve for a characteristic illustra- tion of the mode of growth of the species it represents, the large shallow cells, whose walls are made by cement- ing four strips of glass to the plate that forms the bottom (§ 136), will generally be found preferable. The horny polyparies of the Sertularidse, when mounted in Canada balsam, are beautiful objects for the Polariscope; but in order to prepare them suc- cessfully, some nicety of ma- nagement is required. The following are the outlines of the method recommended by Dr. Golding Bird, who very successfully practised it. The specimens selected, which should not exceed two inches in length, are first to be submitted, while immersed in water of 120°, to the vacuum of an air-pump. The ebullition which will take place within the cavities, will have the effect of freeing the polyparies from dead polypes and other animal matter ; and this cleansing process should be re- peated several times. The specimens are then to be dried, by first draining them for a few seconds on bibulous paper, and then by submitting them to the vacuum of an air-pump, within a thick earthenware ointment pot fitted with a cover, which has been previously heated to about 200° ; by this means, the specimens are very quickly and completely dried, the water being evapo- rated so quickly that the cells and tubes hardly collapse or wrinkle. The specimens are then to be placed in camphine, and again subjected to the exhausting process, for the displacement of the air by that liquid; and when they have been thoroughly saturated, they should be mounted in Canada balsam in the usual mode. When thus prepared, they become very beautiful trans- parent objects for low magnifying powers; and they present a gorgeous display of colors when examined by polarized light, with the interposition of a plate of selenite. These objects are Sertularia cupressina:—A, natural size; magnified. B, portion RELATIONSHIP OF MEDUSA TO HYDROZOA. 467 peculiarly fitted for Mr. Furze's combination of the polarizing plate with the spotted lens (§ 63); as they then exhibit all the richness of coloration which the former developes, with the pe- culiar solidity or appearance of projection which they derive from the use of the latter. 306. No result of Microscopic research has been more unex- pected, than the discovery of the close relationship subsisting between the Hydroid Zoophytes and the Medusoid Acalephse (or jelly-fish). We have seen that many of the small free-swimming Medusans, belonging to that simple tribe of which TJiaumantias (Fig. 225) may be taken as a representative, are really to be considered as the detached sexual apparatus of the Zoophytes from which they have been budded off, endowed with indepen- dent organs of nutrition and locomotion, whereby they become capable of maintaining their own ex- istence and of developing their gene- FIG. 225. rative products. The general confor- mation of these organs will be under- stood from the accompanying figure. Many of this group are very beautiful objects for Microscopic examination, being small enough to be viewed entire in the zoophyte-trough. There are few parts of the coast on^ which they may , not be found, especially On a Calm « naked-eyed" Medusce :-a a, oral warm day, by skimming the surface of temacuia; 6, stomach; c, gastro- the sea with a fine muslin net attached vascujar cana!s; havi"g the1ova- _ . . . i f> i nes) <* "> on either side, and ter- tO a ring, Which may either be fixed tO mina,ing in the marginal canal.ee. the end of a stick held in the hand, or may be fastened by a string to the stern of the boat as a tow-net. In either case, the net should be taken up from time to time, held so as to allow the water it contains to drain through it, and then turned inside out (so that what was previously its internal surface shall now be the external), and moved about in a bucket of water, so that any minute animals adhering to it may be washed off. When we turn from these small and simple forms, to the large and highly-developed Medusans which are com- monly known as "jelly-fish," we find that their history is essen- tially similar; for their progeny have been ascertained to develope themselves in the first instance under the polype form, and to lead a life which in all essential respects is zoophytic ; their development into Medusae taking place only in the closing phase of their existence, and then rather by gemmation from the original polype, than by a metamorphosis of its own fabric. The embryo emerges from the cavity of its parent, within which the first stages of its development have taken place, in the con- dition of a ciliated gemmule, of rather oblong form, very closely resembling an Infusory animalcule, but destitute of a mouth. One end soon contracts and attaches itself, however, so 468 ZOOPHYTES. as to form a foot ; the other enlarges and opens to form a mouth, four tubercles sprouting around it, which grow into tentacula ; whilst the central cells melt down to form the cavity of the stomach. Thus a Hydra-like polype is formed, which soon acquires many additional tentacula ; and this, according to the observations of Sir J. G. Dalyell, leads in every important par- ticular the life of a Hydra, propagates like it by repeated gem- mation, so that whole colonies are formed as offsets from a single stock, and can be multiplied like it by artificial division, each segment developing itself into a perfect Hydra. There seems to be no definite limit to its continuance in this state, or to its power of giving origin to new polype buds ; but under conditions not yet ascertained, the Strobila (as it is termed) ceases to propagate by ordinary gemmation, and enters upon an entirely new series of changes. In the first place, the body becomes more cylindrical in form than it previously was ; then a constriction or indentation is seen around it, just below the ring which encircles the mouth and gives origin to the tentacula ; and similar constrictions are soon repeated around the lower parts of the cylinder, so as to give to the whole body somewhat the appearance of a rouleau of coins ; a sort of fleshy bulb, some- what of the form of the original polype, being still left at the attached extremity (Fig. 226, A). The number of circles is FIG. 226. Successive Stages of Development of Medusa buds from Strdbila larva : — a, polype body; b, its original circle of tentacula; c, its secondary circle of tentacula; d, proboscis of most advanced Medusa disk; e, polype bud from side of polype body. indefinite, and all are not formed at once, new constrictions appearing below, after the upper portions have been detached ; as many as 30 or even 40 have thus been produced in one speci- men. The constrictions then gradually deepen, so as to divide the cylinder into a pile of saucer-like bodies ; the division being most complete above, and the upper disks usually presenting some increase in their diameter : and whilst this is taking place, the edges of the disks become divided into lobes (B), each lobe soon presenting the cleft with the supposed rudimentary eye (more probably an auditory organ) at the bottom of it, which is POLYPOID DEVELOPMENT OF MEDUSA. 469 to be plainly seen in the detached Medusae (Fig. 227, c). Up to this period, the tentacula of the original polype surmount the highest of the disks ; but before the detachment of the topmost disk, this circle disappears, and a new one is -developed at the summit of the bulb which remains at the base of the pile (c, c). At last, the topmost and largest disk begins to exhibit a sort of convulsive struggle; it becomes detached and swims freely away ; and the same series of changes takes place from above downwards, until the whole pile of disks is detached and con- verted into free swimming Medusae. But the original polypoid body still remains ; and may return to its polype-like and original mode of gemmation (D, rms, whose polypes, when expanded, present the likeness of "animal flowers:" and it consists of two principal subdivisions, — the Asteroida, or Alcyonian zoophytes, whose polypes, having only six or eight broad short tentacula, present a star-like aspect when expanded, — and the Helianthoida, whose polypes, having numerous tentacula disposed in several rows, bear a resemblance to sun-flowers or other composite blossoms. Of the first of these orders, which contains no solitary species, a characteristic ex- ample is found in the Alcyonium digitatum of our coasts, which is commonly known under the name of " dead-man's toes," or FIG. 230. FIG. 231. FIG. 232. 230. Spicules of Alcyonium and Gwgonia. FIG. 231. Spicules of Gorgonia guttata. FIG. 232. Spicules of Muricea elongata. " sea-paps." When a specimen of this is first torn from the rock to which it has attached itself, it contracts into an unshapely mass, whose surface presents nothing but a series of slight de- pressions arranged with a certain regularity. But after being ASTEROID AND HELIANTHOID POLYPES. 473 immersed for a little time in a jar of sea-water, the mass swells out again, and from every one of these depressions an eight- armed polype is protruded, "which resembles a flower of ex- quisite beauty and perfect symmetry. In specimens recently taken, each of the petal-like tentacula is seen with a hand-glass to be furnished with a row of delicately slender pinnae or fila- ments, fringing each margin, and arching outwards ; and with a higher power, these pinnae are seen to be roughened, throughout their whole length, with numerous prickly rings. After a day's captivity, however, the petals shrink up into short, thick, un- shapely masses, rudely notched at their edges" (Gosse). When a mass of this sort is cut into, it is found to be channelled out, somewhat like a sponge, by ramifying canals; the vents of which open into the stomachal cavities of the polypes, which are thus brought into free communication with each other, — a character that especially distinguishes this order. A movement of fluid is kept up within these canals, as may be distinctly seen through their transparent bodies, by means of cilia lining the internal surfaces of the polypes ; but no cilia can be discerned on their external surfaces. The tissue of this spongy polypidom is strengthened throughout, like that of Sponges (§ 296), with mine- ral spicules (always, however, calcareous), which are remarkable for the elegance of their forms ; these are disposed with great regularity around the bases of the polypes, and even extend part of their length upwards on their bodies. The presence of such spicules is, in fact, a very constant character throughout this group. Thus in the Grorgonia or Sea-fan, whilst the central part of the polypidom is consolidated into a horny axis, the soft flesh which clothes this axis is so full of tuberculated spicules, espe- cially in its outer layer, that, when this dries up, the spicules form a thick yellowish or reddish incrustation upon the horny stem ; this is, however, so friable, that it may be easily rubbed down between the fingers, and, when examined with the Micro- scope, it is found to consist of spicules of different shapes and sizes, more or less resembling those shown in Figs. 230-232, sometimes colorless, but sometimes of a beautiful crimson, yel- low, or purple. These spicules are best seen by the methods of illumination that give a black ground, on which they stand out with great brilliancy. They are, of course, to be separated from the animal substance in the same manner as the calcareous spi- cules of Sponges (§ 298); and they should be mounted, like them, in Canada balsam. It is interesting to remark that the hard cal- careous stem of the Red Coral, which takes the place of the horny axis of the Sea-fan, is found, by the examination of very thin sections, to be made up of a solid aggregation of separate spicules, closely resembling those of Alcyonian zoophytes in general. The spicules always possess an organic basis ; as is proven by the fact, that when their lime is dissolved by dilute acid, a gelatinous- 474 ZOOPHYTES. looking residuum is left, which preserves the form of the spicule, and is probably to be considered as a cell in an early stage of formation, its wall not yet being differentiated as a distinct mem- brane. 310. Of the order Helianthoida, the common Actinia or " Sea- Anemone" may be taken as the type ; the individual polypes of all the composite structures included in the group being con- structed upon the same model. In by far the larger proportion of these Zoophytes, the bases of the polypes, as well as the soft flesh that connects together the members of aggregate masses, are consolidated by calcareous deposit into stony corals ; and the surfaces of these are beset with cells, usually of a nearly circular form, each having numerous lamellae radiating from its centre towards its circumference, which are formed by the consolidation of the lower portions of the radiating partitions, that divide the space intervening between the stomach and the general integu- ment of the animal into separate chambers. This arrangement is seen on a large scale in the Fungia or "mushroom coral" of tropical seas, which is the stony base of a solitary anemone-like polype ; on a far smaller scale, it is seen in the little Caryophyllia, a like solitary polype of our own coasts, which is scarcely dis- tinguishable from an Actinia by any other character than the presence of this disk, and also on the surface of many of those stony corals known as "Madrepores;" whilst in some of these the individual polype-cells are so small, that the lamellated ar- rangement can only be made out when they are considerably magnified. Portions of the surface of such corals, or sections taken at a small depth, are very beautiful objects for the lower powers of the Compound Microscope, the former being viewed by reflected and the latter by transmitted light. And thin sec- tions of various fossil Corals of this group are very striking objects for the lower powers of the Oxyhydrogen Microscope. The chief point of interest to the Microscopist, however, in the structure of these animals, lies in the extraordinary abundance and high development of those "filiferous capsules," or "thread- cells," the presence of which on the tentacles of the Hydraform polypes has been already noticed (§ 300), and which are also to be found, sometimes sparingly, sometimes very abundantly, in the tentacles surrounding the mouth of the Medusae, as well as on other parts of their bodies. If a tentacle of any of the Sea- Anemonies, so abundant on our coasts (the smaller and more transparent kinds being selected in preference), be cut off, and be subjected to gentle pressure between the two glasses of the aquatic box or of the compressorium, multitudes of little dart- like organs will be seen to project themselves from its surface near its tip ; and if the pressure be gradually augmented, many additional darts will every moment come into view. Not only do these organs present different forms in different species ; but FILIFEROUS CAPSULES OF POLYPES. 475 FTG. 233. even in one and the same individual very strongly marked diver- sities are shown, of which a few examples are given in Fig. 233. At A, B, c, and D, is shown the ap- pearance of the "filiferous capsules," whilst as yet the thread lies coiled up in their interior ; whilst at E, F, G, H, are seen a few of the most striking forms which they exhibit, when the thread or dart has started forth. The most probable account of their organization seems to be, that each is a cell, of which one end is extended into the thread-like or dart-like prolongation, but which is doubled in upon itself, in such a manner that the armature appears to be contained in its interior; and that the springing out of the dart is due to the eversion of the portion of the cell which had previously been press- ed inwards. These thread-cells are found, however, not merely in the tentacles and other parts of the ex- ternal integument of Helianthoid Zoophytes, but also in the long fila- ments which lie in coils within the chambers that surround the stomach, in contact with the sexual organs which are attached to the lamellae dividing the chambers. It was for- merly supposed that the last-named organs were always ovaria, and that the long and slender filaments con- tain sperm-cells and are consequently the male organs. But since it has been proved that the peculiar " fili- ferous CapSUles" Which lie Side by . -. . \ «, -n*7 side in these filaments are really identical in structure with those which are found in the skin, the idea of their sexual nature has been abandoned ; and a more careful examination of the organs attached to the walls of the chambers has shown that these are not always ovaries, but that they sometimes contain sperm-cells, the two sexes being here divided, not united, in the same indi- vidual. "What can be the office of the filiferous filaments thus contained in the interior of the body, it is difficult to guess at. They are often found to protrude from rents in the external tegu- ment, when any violence has been used in detaching the animal from its base ; and when there is no external rupture, they are Filiferous Capsules of Helianthoid Po- •A, B, Corynactis Allmanni ; c, E, F, Caryophyllia Smithii; D, G, Actinia crassi- cornis ; H, Actinia Candida. 476 ZOOPHYTES. often forced through the wall of the stomach into its cavity, and may be seen hanging out of the mouth. The largest of these capsules, in their unprotected state, are about l-300th of an inch in length ; and the thread or dart, in Corynactis Allmanni, when fully extended, is not less than l-8th of an inch, or thirty-seven times the length of the capsule.1 1 For the fullest description of these curious bodies, as well as for much other valuable information upon Zoophytes, see Mr. Gosse's " Naturalist's Rambles on the Devonshire Coast." Those who may desire to acquire a more systematic and detailed acquaintance with this group, may be especially referred to the following Treatises and Memoirs: — Dr. Johnston's "History of British Zoophytes,'' Prof. Owen's" Lectures on the Compara- tive Anatomy and Physiology of the Invertebrate Animals," Prof. Rymer Jones's '•General Outline of the Organization of the Animal Kingdom," Prof. Milne Edwards's "Recherches sur les Polypes," Prof. Van Beneden " Sur les Tubulaires,'' and "Sur les Campanulaires," in "Mem. de 1'Acad. Roy. de Bruxelles," torn. xvii. Sir J. G. Dalyell's u Rare and Remarkable Animals of Scotland," vol. i. Trembley's " Mem. pour servir £ 1'histoire d'un genre de Polype d'Eau douce," M. Hollard's "Monographic du Genre JLc.tinia." in " Ann. des Sci. Nat." Se"r. 3, torn, xv, Mr. Mummery, " On the development of Tubularia indivisa" in " Transact, of Microsc. Soc." 2d Ser. vol. i, p. 28, and Prof. Max. Schultze, "On the Male Reproductive Organs of C amp anul aria geniculata" in •' Quart. Journ. of Microsc. Sci." vol. iii, p 59. CHAPTER XII. OF ECHINODERMATA. 311. As we ascend the scale of Animal life, we meet with such a rapid advance in complexity of structure, that it is no longer possible to acquaint one's self with any organism by microscopic examination of it as a whole ; and the dissection or analysis which becomes necessary, in order that each separate part may be studied in detail, belongs rather to the Comparative Anatomist than to the ordinary Microscopist. This is especially the case with the Echinus (sea-urchin), Asterias (star-fish), and other mem- bers of the class Echinodermata ; since even a general account of their complex organization would be quite foreign to the purpose of this work ; whilst there are certain parts of their structure, which furnish microscopic objects of such beauty and interest that they cannot by any means be passed by; besides which, recent observations on their embryonic forms have revealed a most unexpected order of facts, the extension and verification of which will be of the greatest service to science, — a service that can only be effectually rendered by well-directed Microscopic research in fitting localities. 312. It is in the structure of that calcareous skeleton, which probably exists, under some form or other, in every member of this class, that the Microscopist finds most to interest him. This attains its highest development in the Echinida ; in which it. forms a box-like shell, or "test," composed of numerous polygo- nal plates jointed to each other with great exactness, and beset on its external surface with " spines," which may have the form of prickles of no great length, or may be stout club-shaped bodies, or, again, may be very long and slender rods. The inti- mate structure of the shell is everywhere the same ; for it is com- posed of a network, which consists of carbonate of lime with a very small quantity of animal matter as a basis, and which ex- tends in every direction (i. e. in thickness, as well as in length and breadth), its areolce or interspaces freely communicating with each other (Fig. 234). These "areolre," and the solid structure which surrounds them, may bear an extremely variable propor- 478 OF ECHINODERMATA. FIG. 234. posed: — a a, portions of a deeper layer. FIG. 235. tion, one to the other; so that in two masses of equal size, the one or the other may greatly predominate ; and the texture may have either a remarkable lightness and porosity, if the network be a very open one, like that of Fig. 235, or may possess a considerable degree of compactness if the solid portion be strengthened. Generally speaking, the different layers of this network, which are connected together by pillars that pass from one to the other in a di- * rection perpendicular to their plane, are so arranged that the perforations in one shall correspond to the inter- mediate solid structure in the next; and their transparency is such, that when we are examining a section thin enough to con- tain only two or three such layers, it is easy, by properly " focussing" the Microscope, to bring either one of them into distinct view. From this very simple but very beautiful ar- rangement, it comes to pass that the plates of which the entire "test" is made up, possess a very considerable degree of strength, notwithstanding that their porousness is such, that if a portion of a fractured edge, or any other part from which the investing membrane has been removed, be laid upon fluid of almost any description, this will be rapidly sucked up into its substance. A very beau- tiful example of the same kind of calcareous skeleton, having a more regular conformation, is furnished by the disk or ro- sette which is contained in the tip of every one of the tubular suckers put forth by the living Echinus from the ambulacral pores of its shell. If the entire disk be cut off, and be mounted when dry in Canada balsam, the calcareous rosette maybe seen sufficiently well; but its beautiful struc- ture is better made out, when the animal membrane that en- One of the segments of the calcareous skeleton of on " miuw ***^ Ambulacral disk of Echinus. clOSCS it has DQQU got rid OI Transverse Section of the medullary portion of Spine of Acrocladia, showing its more open network. FTG. 236. CALCAREOUS SKELETON OF ECHINIDA. 479 by boiling in caustic potass ; and the appearance of one of the five segments of which it is composed, when thus prepared, is shown in Fig. 236. 313. The most beautiful display of this reticulated structure, however, is shown in the structure of the "spines" of JSchinus, Cidaris, &c. ; in which it is combined with solid ribs or pillars, disposed in such a manner as to increase the strength of these organs ; a regular and elaborate pattern being formed by their intermixture, which shows considerable variety in different species. When we make a thin transverse section of almost any spine belonging to the genus Echinus (the small spines of our British species, however, being exceptional in this respect), we are at once made awrare of the existence of a number of concen- tric layers, arranged in a manner that strongly reminds us of the concentric rings of an Exogenous tree (Fig. 167). The number of these layers is extremely variable; depending not merely upon the age of the spine, but (as will presently appear) upon the part of its length from which the section happens to be taken. The centre is usually occupied by a very open network (Fig. 235) ; and this is bounded by a row of transparent spaces (like those at a af, b bf, c c', Fig. 237), which, on a cursory inspec- tion, might be supposed to be void spaces, but which on a closer examination are found to be the sections of solid ribs or pillars, which run in the direction of the length of the spine, and form the exterior of every layer. Their solidity becomes very obvi- ous, when we either examine a section of a spine whose substance is pervaded (as often happens) with a coloring matter of some FlG. 237. / g a y h' Portion of transverse section of Spine of Acrocladia mammillaia. depth, or when we look at a very thin section by the " black- ground" illumination. Around the innermost circle of these solid pillars, there is another layer of the calcareous network, which again is surrounded by another circle of solid pillars; and this arrangement may be repeated many times, as shown in Fig. 237, the outermost row of pillars forming the projecting ribs that are very commonly to be distinguished on the surface of the spine. Around the cup-shaped base of the spine is a membrane 480 OF ECHINODERMATA. which is continuous with that covering the surface of the shell, arid which serves not merely to hold down the cup upon the tubercle over which it works, but also, by its contractility, to move the spine in any required direction. This membrane is probably continued onwards over the whole surface of the spine, although it cannot be clearly traced to any distance from the base ; and the new formations may be presumed to take place in its substance. Each new formation completely ensheaths the old; not merely surrounding the part previously formed, but also projecting considerably beyond it ; and thus it happens that the number of layers shown in a transverse section, will depend in part upon the place of the section. For if it cross near the base, it will traverse every one of the successive layers from the very commencement ; whilst, if it cross near the apex, it will traverse only the single layer of the last growth, notwithstanding that, in the club-shaped spines, this terminal portion may be of considerably larger diameter than the basal ; and in any inter- mediate part of the spine, so many layers will be traversed as have been formed since the spine first attained that length. The basal portion of the spine is enveloped in a reticulation of a very close texture, without concentric layers ; forming the cup or socket which works over the tubercle of the shell. The combi- nation of elegance of pattern with richness of coloring, renders well-prepared specimens of these spines among the most beauti- ful objects that the Microscopist can anywhere meet with. The large spines of the various species of the genus Acrocladia fur- nish sections most remarkable for size and elaborateness as well as for depth of color (in which last point, however, the deep purple spines of Echinus lividus are pre-eminent) ; but for exqui- site neatness of pattern, there are no spines that can approach those of Eehinometra heteropora and E. lucunter. The spines of Heliocidaris variolaris are also remarkable for their beauty. No succession of concentric layers is seen in the spines of the British Echini, probably because (according to the opinion of the late Sir J. Gr. Dalyell) these spines are cast off and renewed every year ; each new formation thus going to make an entire spine, instead of making an addition to that previously existing. Most curious indications are sometimes afforded by sections of Echinus-spines, of an extraordinary power of reparation inherent in these bodies. For irregularities are often seen in the trans- verse sections, which can be accounted for in no other way, than by supposing the spines to have received an injury when the irregular part was at the exterior, and to have had its loss of substance supplied by the growth of new tissue, over which the subsequent layers have been formed as usual. And sometimes a peculiar ring may be seen upon the surface of a spine, which indicates the place of a complete fracture, all beyond it being a new growth, whose unconformableness to the older or basal portion is clearly shown by a longitudinal section. SPINES OF SPATANGUS. 481 314. The spines of Cidaris present a marked departure from the plan of structure exhibited in Echinus ; for not only are they destitute of concentric layers, but the calcareous network which forms their principal substance, is ensheathed in a solid cal- careous cylinder perforated with tubules, which seems to take the place of the separate pillars of the Echini. This is usually found to close in the spine at its tip also ; and thus it would appear that the entire spine must be formed at once, since no addition could be made either to its length or to its diameter, save on the outside of this sheath, where it is never to be found. The sheath itself often rises up in prominent points or ridges on the surface of these spines ; thus giving them a character by which they may be distinguished from those of Echini. The slender, almost filamentary spines of Spatangus (Fig. 238), and the innumerable minute hair-like processes attached to the shell of Clypeaster, are composed of the like regularly reticulated sub- FIG. 238. Spines of Spatongus. stance; and many of these are very beautiful objects for the lower powers of the Microscope, when examined by reflected light, and laid upon a black ground, without any further pre- paration. It is interesting also to find that the same structure presents itself in the curious Pedicellarice (forceps-like bodies mounted on long stalks), which are found on the surface of many Echinida, and the nature of which has been a source of much perplexity to Naturalists, some maintaining that they are parasites, whilst others consider them as proper appendages of the Echinus itself. The complete conformity which exists be- tween the structure of their skeleton and that of the animal to which they are attached, would seem to remove all reasonable doubt of their being truly appendages to it, as observation of their actions in the living state would indicate. Another ex- ample of the same structure is found in the peculiar system of plates which surrounds the interior of the oral orifice of the shell, and which gives support to the five teeth that may often be seen projecting externally through that orifice; the whole forming what is known as the "lantern of Aristotle." The 31 482 OF ECHINODERMATA. FIG. 239. texture of the plates or jaws resembles that of the shell in every respect, save that the network is more open ; but that of the teeth is much more compact. The latter have been described by Mr. Quekett as consisting of a substance not altogether un- like the " dentine" of the teeth of higher animals, save that the tubuli, though sometimes parallel, usually have more of a reti- culated arrangement, and sometimes dilate into irregular " la- cunae" or spaces excavated in the hard substance.1 The Author is not prepared to speak with confidence on this point ; but he is disposed to think that the structure of the teeth is essentially the same as that of the shell, save in the interspaces of the network being much narrower ; and that the appearance of tubuli (in which Mr. Quekett has not been able to make out distinct walls) is due merely to the elongation of these interspaces. 315. The calcareous plates which form the less compact skeletons of the Asteriada (star-fish and their allies) and of the Opkiurida (sand- stars and brittle-stars), have the same texture as those of the shell of Echinus. And this presents itself, too, in the spines or prickles of their surface, when these (as in the large Groniaster equestris) are large enough to be furnished with a calcareous framework, and are not mere projections of the horny integument. An example of this kind, furnished by the Astro- pliyton (better known as the Euryale), is represented in Fig. 239. The spines with which the arms of the species of Ophiocoma (brittle- star) are beset, are often remarkable for their beauty of conformation ; that of 0. rosula, one of the most common kinds, might serve (as Prof. E. Forbes justly remarked) in point of lightness and beauty, as a model for the spire of a cathedral. 316. The calcareous skeleton is very highly developed in the Crinoidea ; their stems and branches being made up of a cal- careous network, closely resembling that of the shell of the Echinus. This is extremely well seen, not only in the recent Pentacrinus Oaput Medusce, a somewhat rare animal of the West Indian seas, but also in a large proportion of the fossil Crinoidea, whose remains are so abundant in many of the older geological formations ; for, notwithstanding that these bodies have been penetrated in the act of fossilization by a mineral infiltration, which seems to have substituted itself for the original fabric (a regularly crystalline cleavage being commonly found to exist in the fossil stems of Encrinites, &c., as in the fossil spines of Echinidans), yet their organic structure is often most perfectly preserved. In the circular stems of finer mites, the texture of 1 %{ Lectures on Histology," vol. ii, p. 234. Calcareous plate and claw of Astrophyton (EuryaJe). SECTIONS OF ECHINUS SPINES. 488 the calcareous network is uniform, or nearly so, throughout ; but in the pentangular Pentacrini, a certain figure or pattern is formed by variations of texture in different parts of the trans- verse section ; and the patterns, though formed upon one gene- ral plan, are sufficiently diverse in different species, to enable these to be recognized by the examination of a transverse section of a single joint of the stem. 317. The structure of the shells, spines, and other solid parts of the skeleton of Echinodermata can only be displayed by thin sections, made upon the general plan already described (§§ 109, 110). But their peculiar texture requires that certain precautions should be taken ; in the first place, in order to prevent the sec- tion from breaking, whilst being reduced to the desirable thin- ness ; and in the second, to prevent the interspaces of the net- work from being clogged by the particles abraded in the re- ducing process. A section of the shell, spine, or other portion of the skeleton, should first be cut with a fine saw, and rubbed on a flat file until it is about as thin as an ordinary card, after which it should be smoothed on one side by friction with water on a Water-of-Ayr stone. It should then be carefully dried, first on white blotting-paper, afterwards by exposure for some time to a gentle heat, so that no water may be retained in the inter- stices of the network, which would oppose the complete pene- tration of the balsam. Next, it is to be attached to a glass slip by balsam hardened in the usual manner ; but particular care should be taken, first, that the balsam be brought to exactly the right degree of hardness, and second, that there be enough, not merely to attach the specimen to the glass, but also to saturate its substance throughout. The right degree of hardness is that at which the cement can be with difficulty indented by the thumb-nail ; if it be made harder than this, it is apt to chip off the glass in grinding, so that the specimen also breaks away ; and "if it be softer, it holds the abraded particles, so that the openings of the network becomes clogged with them. If, when rubbed down nearly to the required thinness, the section appears to be uniform and satisfactory throughout, the reduction may be completed without displacing it ; but if (as often happens) some inequality in thickness should be observable, or some minute air-bubbles should present themselves between the glass and the under surface, it is desirable to loosen the specimen by the ap- plication of just enough heat to melt the balsam (special care being taken to avoid the production of fresh air-bubbles), and to turn it over so as to attach the side last polished to the glass, taking care to remove or to break with the needle-point any air- bubbles that there maybe in the balsam covering the part of the glass on which it is laid. The surface now brought uppermost is then to be very carefully ground down ; special care being taken to keep its thickness uniform through every part (which may be even better judged of by the touch than by the eye), and to 484 OF ECHINODERMATA. carry the reducing process far enough, without carrying it too far. Until practice shall have enabled the operator to judge of this by passing his finger over the specimen, he must have con- tinual recourse to the microscope during the later stages of his work ; and he should bear constantly in mind, that, as the speci- men will become much more transparent when mounted in balsam and covered with glass, than it is when the ground sur- face is exposed, he need not carry his reducing process so far as to produce at once the entire transparency he aims at, the attempt to accomplish which would involve the risk of the de- struction of the specimen. In "mounting" the specimen, liquid balsam should be employed, and only a very gentle heat (not sufficient to produce air-bubbles, or to loosen the specimen from the glass) should be applied ; and if, after it has been mounted, the section should be found too thick, it will be easy to remove the glass cover, and to reduce it further, care being taken to harden the balsam which has been newly laid on, to the proper degree. 318. If a number of sections are to be prepared at once (and it is often useful to do this for the sake of economy of time, or in order to compare sections taken from different parts of the same spine), this may be most readily accomplished by laying them down, when cut off by the saw, without any preliminary preparation save the blowing the calcareous dust from their sur- faces, upon a thick slip of glass well covered with hardened balsam ; a large proportion of its surface may thus be occupied by the sections attached to it, the chief precaution required being that all the sections come into equally close contact with it. Their surfaces may then be brought to an exact level, by rubbing them down, first upon a flat piece of grit (which is very suitable for the rough grinding of such sections), and then upon a large Water-of-Ayr stone whose surface is "true." When this level has been attained, the ground surface is to be well washed and dried, and some balsam previously hardened is to be spread over it, so as to be sucked in by the sections, a moderate heat being at the same time applied to the glass slide ; and this being in- creased to a sufficient degree to loosen the sections without over- heating the balsam, the sections are to be turned over one by one, so that the ground surfaces are now to be attached to the glass slip, special care being taken to press them all into close contact with it. They are then to be very carefully rubbed down, until they are nearly reduced to the required thinness ; and if, on examining them from time to time, their thinness should be found to be uniform throughout, the reduction of the entire set may be completed at once ; and when it has been carried suffi- ciently far, the sections, loosened by warmth, are to be taken up upon a camel-hair brush dipped in turpentine, and transferred to separate slips of glass whereon some liquid balsam has been pre- viously laid, in which they are to be mounted in the usual man- CALCAREOUS STRUCTURE OF HOLOTHURIDA. 485 ner. It more frequently happens, however, that, notwithstanding every care, the sections, when ground in a number together, are not of uniform thickness, owing to some of them being under- laid by a thicker stratum of balsam than others are ; and it is then necessary to transfer them to separate slips, before the reducing process is completed, attaching them with hardened balsam, and finishing each section separately. 319. It now remains for us to notice the curious and often very beautiful structures, which represent, in the order Holothurida, the solid calcareous skeleton of the orders already noticed. All the animals belonging to this order are distinguished by the flexibility and absence of firmness of their envelopes ; and ex- cepting in the case of certain species which have a set of cal- careous plates, supporting teeth, disposed around the mouth, very much as in the Echinida, we do not find among them any representation that is apparent to the unassisted eye, of that skeleton which constitutes so distinctive a feature of the class generally. But a microscopic examination of their integument at once brings to view the existence of great numbers of minute isolated plates, every one of them presenting the characteristic reticulated structure, which are set with greater or less closeness in the substance of the skin. Various forms of the plates which thus present themselves in Holothuria are shown in Fig. 240 ; and FIG. 240. Calcareous plates in skin of Holothuria. at A is seen an oblique view of the kind marked a, more highly magnified, showing the very peculiar manner wherein one part is superposed on the other, which is not at all brought into view when it is merely seen through in the ordinary manner. In the Synapta^ one of the long-bodied forms of this order, which does not occur upon our own coasts, but is abundant in the Adriatic Sea, the calcareous plates of the integument have the regular form shown at A, Fig. 241 ; and each of these carries the curious anchor-like appendage, c, which is articulated to it by the notched piece at the foot, in the manner shown (in side view) at B. The anchor-like appendages project from the surface of the skin, and may be considered as representing the spines of Echinida. Nearly allied to the Synapta is the Chirodota, of which one species (the C. digitata), although previously accounted a very rare inhabi- 486 OF ECIIINODERMATA. tant of oar seas, has lately been found in considerable numbers at Torquay (by Mr. Kingsley), and might probably be met with FIG. 241. c FIG. 242. Wheel-like plates from skin of Chirodota violacea. Calcareous skeleton of Synapta : — A, plate imbedded in skin ; B, ihe same, with its anchor-like spine attached; c, anchor like spine separated. more frequently if carefully searched for. Not having had the opportunity of examining a specimen of this animal, the Author is unable to say whether or not its integument possesses the very re- markable wheel-like plates, repre- sented in Fig. 242, which are found in the skin of Chirodota violacea, a species inhabiting the Mediterra- nean. These plates are objects of singular beauty and delicacy, being especially remarkable for the very minute notching (scarcely to be discerned in the figures without the aid of a magnifying glass) which is traceable round the inner margin of their "tires." There can be scarcely any reasonable doubt, that every member of this order has some kind of calca- reous skeleton, disposed in a manner conformable to the examples now cited ; and it would be very valuable to determine how far the very marked peculiarities by which they are respectively distinguished, are characteristic of genera and species. The plates may be obtained separately, by the usual method of treat- ing the skin with a solution of potass; and they should be mounted in Canada balsam. But their position in the skin can only be ascertained by making sections of the integument, both vertical and parallel to its surface ; and these sections, when dry, are most advantageously mounted in the same medium, by which their transparency is greatly increased. All the objects of this class are most beautifully displayed by the black-ground illumi- nation (§§ 61, 62) ; and the same method, when applied to very thin sections of Echinus- spines, brings out some effects of mar- vellous beauty. 320. Echinoderm Larvce. — We have now to notice that most remarkable set of objects, furnished to the Microscopic inquirer by the larval forms of this class, for our present knowledge of which, imperfect as it still is, we are almost entirely indebted to LARVAL FORMS OF ECHINODERMATA. 487 the painstaking and widely extended investigations of Prof. M tiller. All that our limits permit, is a notice of two of the most curious forms of these larvae, by way of sample of the wonderful phenomena which his researches have brought to light ; so as (it may be hoped) to excite such an interest among those Microscopists in particular who may have the opportunity of pursuing these inquiries, as may induce them to apply them- selves perseveringly to them, and thus to supply the numerous links which are at present wanting in the chain of developmental history. The peculiar feature by which the early history of the Echinoderms generally seems to be distinguished, is this, — that the embryonic mass of cells is converted, not into a larva which subsequently attains the adult form by a process of metamor- phosis, but into a peculiar zooid, which seems to exist for no other purpose than to give origin to the Echinoderm by a kind of internal gemmation, and to carry it to a distance by its active locomotive powers, so as to prevent the spots inhabited by the respective species from being overcrowded by the accumulation of their progeny. The larval zooids are formed upon a type quite different from that which characterizes the adults ; for in- stead of a radial symmetry, they exhibit a bilateral, the two sides being precisely alike, and each having a ciliated fringe along the greater part of the whole of its length. The two fringes are united by a superior and an inferior transverse ciliated band; and between the two, FIG 243 the mouth of the zooid is al- ways situated. Further, al- though the adult Star-fish and Sand-stars have neither intesti- nal tube nor anal orifice, their larval zooids, like those of other Echinoderms, always possess both. The external forms of these larvse, however, vary in a most remarkable degree, owing to the unequal evolution of their different parts ; and there is also a considerable di- versity in the several orders, as to the proportion of the fabric of the larva which enters into the composition of the adult form. In the fully deve- loped Star-fish and Sea-urchin, the only part retained is a por- tion of the stomach and intes- tine, which is pinched off, so to speak, from that of the Larval zooid. 321. One of the most remarkable forms of Echinoderm Bipinnaria asterigera, or Larva of Star- Fish :— a, mouth; a', oesophagus: 6, intesti- nal tube and anal orifice ; c, furrow in which the mouth is situated; d d', bilobed peduncle ; 1, 2, 3, 4, 5, 6. 7, ciliated arms. 488 OF ECHINODEKMATA. larvae is that which has received the name of Bipinnaria (Fig. 243), from the symmetrical arrangement of its natatory organs. The mouth (a), which opens in the middle of a transverse fur- row, leads through an oesophagus V to a large stomach, around which the body of a Star-fish is developing itself ; and on one side of this mouth is observed the intestinal tube and anus (5). On either side of the anterior portion of the body, are six or more narrow fin-like appendages, which are fringed with cilia ; and the posterior part of the body is prolonged into a sort of pedicle, bilobed towards its extremity, which also is covered with cilia. The organization of this larva seems completed, and its movements through the water are very active, before the mass at its anterior extremity presents anything of the aspect of the Star-fish ; in this respect corresponding with the movements of the "pluteus" of the Echinida (§ 322). The temporary mouth of the larva does not remain as the permanent mouth of the Star-fish ; for the oesophagus of the latter enters on what is to become the dorsal side of its body, and the true mouth is sub- sequently formed by the thinning away of the integument on its ventral surface. The young Star-fish is separated from the bipinnarian larva, by the forcible contractions of the connecting pedicle, as soon as the calcareous consolidation of its integument has taken place, and its true mouth has been formed, but long before it has attained the adult condition ; and as its ulterior development has not hitherto been observed in any instance, it is not yet known what are the species in which this mode of evolution prevails. The larva continues active for several days after its detachment ; and it is possible, though perhaps scarcely probable, that it may develope another Asteroid by a repetition of this process of gemmation.1 322. In the Bipinnaria, as in other larva zooids of the Aste- riada, there is no internal calcareous framework ; such a frame- work, however, is found in the larvae of the Echinida and Ophi- urida, of which the form delineated in Fig. 244 is an example.2 The embryo issues from the ovum as soon as it has attained, by the repeated segmentation of the yolk, the condition of the " mul- berry mass ;" and the superficial cells of this are covered with cilia, by whose agency it swims freely through the water. So rapid are the early processes of development, that no more than from twelve to twenty-four hours intervene between fecundation 1 See the observations of Koren and Daniellsen (of Bergen) in the " Zoologiske Bid- rag," Bergen, 1847 (translated in the " Ann. des Sci. Nat." 3e Se"r. Zool. torn, iii, p. 347) ; and the Memoir of Prof. Miiller, " Ueber die Larven und die Metamorphose der Echinodermen," in " Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin," 1848. 2 See Prof. Muller, " Ueber die Larven und die Metamorphose der Ophiuren und Seeigel," in " Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin," 1846. See also, for the earlier stages, a Memoir by M. Derbes, in " Ann. des Sci. Nat." 3e Se*r. Zool. torn, viii, p. 80; and for the later, Krohn's " Beitrag zur Eritwickelungs- geschichte der Seeigillarven," Heidelberg, 1849, and his Memoir in " Muller's Archiv." 1851. LARVAL FOEMS OF ECHINODERMATA. 489 and the emersion of the embryo ; the division into two, four, or even eight segments taking place within three hours after impregnation. Within a few hours after its emer- sion, the embryo changes from the spherical into a sub-pyramidal form with a flattened base; and in the centre of this base is a de- pression, which gradually deepens, so as to form a mouth that communicates with a cavity in the interior of the body, which is sur- rounded by a portion of the yolk-mass that has returned to the liquid granular state. Subsequently a short intes- tinal tube is found, with an anal orifice, opening on one side of the body. The pyra- mid is at first triangular, but it afterwards becomes quad- rangular; and the angles are greatly prolonged round the mouth (or base), whilst the apex of the pyramid is some- c times much extended in the opposite direction, but is sometimes rounded off' into a kind of dome (Fig. 244, A). All parts of this curious body, and especially its most pro- jecting portions, are strength- ened by a framework of thread-like calcareous rods (e). In this condition, the embryo swims freely through the water, being propelled by the action of cilia, which clothe the four angles of the pyramid and its projecting arms, and which are sometimes thickly set upon two or four projecting lobes (/) ; and it has received the designation of Pluteus. The mouth is usually surrounded by a sort of proboscis, the angles of which are prolonged into four slender processes (g, leads to the stomach, ?, which is surrounded by biliary tubuli ; and from this passes off the intes- tine, m, which termi- nates at n in the cloaca. The long post-abdomen of integtine in cloaca; o heart. ^ pericardium. Pt ova. Compound mass of Amaroucium proliferum, with the anatomy of a single zooid: — A, thorax; B, abdomen; c, post-abdomen : — c, oral orifice ; e, branchial sac ; /, tho- racic sinus; i, anal orifice ; i', projection overhanging it; j, nervous ganglion ; .k, resophagus ; I, stomach sur- rounded by biliary tubuli; m, intestine; n. termination is principally occupied by the large ovarium, P, which contains ova in various stages of development. These, when matured and set free, find their way into the cloaca ; where two large ova are seen (one marked p' and the other immediately below rium ; p', egg ready to escape ; q, testis ; r, spermatic canal : r', termination of this canal in the cloaca. 502 POLYZOA AND COMPOUND TUNICATA. it), waiting for expulsion. In this position they receive the fertilizing influence from the testis, g, which discharges its pro- ducts by the long spermatic canal, r, that opens into the cloaca at rf. At the very bottom of the post-abdomen, we find the heart, 0, enclosed in its pericardium, o'. In the tribe we are now considering, a number of such animals are imbedded toge- ther in a sort of gelatinous mass, and covered with an integu- ment common to them all ; the composition of this gelatinous substance is remarkable as including " cellulose," which gene- rally ranks as a purely vegetable product. The mode in which new individuals are developed in this mass, is by the extension of " stolons" or creeping stems from the bases of those previ- ously existing ; and from each of these stolons several buds may be put forth, every one of which may evolve itself into the like- ness of the stock from which it proceeded, and may in its turn increase and multiply after the same fashion. A communication between the circulating systems of the different individuals is kept up, through their connecting stems, during the whole of life ; and thus their relationship to each other is somewhat like that of the several polypes on the polypidom of a Cam'panularia (§ 304). 332. In the family of Didemnians, the post-abdomen is absent, the heart and generative o. tM9. apparatus being placed by the side of the intes- tine in the abdominal portion of the body. The zooids are fre- quently arranged in star-shaped clusters, their anal orifices being all directed towards a common vent which oc- cupies the centre. This shortening is still more remarkable, however, in the family of Botryllians, whose beautiful stellate gelatinous incrustations are extremely common upon sea-weeds and submerged rocks (Fig. 249). The anatomy of these animals is very similar to that of the Amaroucium already described ; with this exception, that the body exhibits no distinction of cavities, all the organs being brought together in one, which, must be considered as thoracic. In this respect, there is an evident approximation towards the solitary species. 333. This approximation is still closerj however in the " social" Ascidians, or Clavellinidce ; in which the general plan of structure is nearly the same, but the zooids are simply connected by their Botryllus violaceus: — A, cluster on the surface of a Fucus B, portion of the same enlarged. SOCIAL ASCIDIANS. 503 stolons, instead of being included in a common investment (Fig. 250) ; so that their relation to each other is very nearly the same as that of the zooids of Laguncula (§ 322), the chief difference FIG. 250. A, Group of Perophora (enlarged), growing from a common stalk: — B, single Perophora; a, test; b, inner sac ; c, branchial sac, attached to the inner sac along the line c' c'; e, e, finger-like processes projecting inwards; /, cavity between test and internal coat ;/", anal orifice or funnel ;#, oral orifice; gf, onil tentacula ; h, downward stream of food ; h', oesophagus ; i. stomach ; fc, vent ; I, ovary (?) ; w, vessels connecting the circulation in the body with that in the stalk. being that a regular circulation takes place through the stolon in the one case, such as has no existence in the other. A better opportunity of studying the living actions of the Ascidians can scarcely be found, than that which is afforded by the genus Pero- phora, first discovered by Mr. Lister, which occurs not unfre- quently on the south coast of England and in the Irish Sea, living attached to sea-weeds, .and looking like an assemblage of minute globules of jelly, dotted with orange and brown, and linked by a silvery winding thread. The isolation of the body of each zooid from that of its fellows, and the extreme transparence of its tunics, not only enable the movements of fluid within the body to be dis- tinctly discerned, but also allow the action of the cilia that border the slits of the respiratory sac to be clearly made out. This sac is perforated with four rows of narrow oval openings, through which a portion of the water that enters its branchial ori- fice (g) escapes into the space between the sac and the mantle, and is thus discharged immediately by the funnel (/'). What- ever little particles, animate or inanimate, the current of water brings, flow into the sac, unleae stopped by the tentacula (g'} at its entrance, which do not appear fastidious. The particles which are admitted usually lodge somewhere on the sides of the sac, and then travel horizontally until they arrive at that part of it down which the current proceeds to the entrance of the sto- 504 POLYZOA AND COMPOUND TUNICA T A. mach (i), which is situated at the bottom of the sac. Minute animals are often swallowed alive, and have been observed dart- ing about in the cavity for some days, without any apparent injury either to themselves or to the creature which encloses them. In general, however, particles which are unsuited for re- ception into the stomach, are ejected by the sudden contraction of the mantle (or muscular tunic), the vent being at the same time closed, so that they are forced out by a powerful current through the branchial orifice. 334. The circulation of blood, throughout the entire class, is remarkable for the alternation which it presents, from time to time, in the direction of its flow; and this curious phenomenon may be particularly well studied in the Perophora. The creep- ing-stalk (Fig. 250) that connects the individuals of any group, contains two distinct canals, which send off branches into each peduncle. One of these branches terminates in the heart, which is nothing more than a contractile dilatation of the principal trunk ;, this trunk subdivides into vessels (or rather sinuses, which are mere channels not having proper walls of their own), of which some ramify over the respiratory sac, branching off at each of the passages between the oval slits, whilst others are first distributed to the stomach and intestines, and to the soft surface of the mantle. All these reunite, and form a trunk, which passes to the peduncle, and constitutes the returning branch. Now whilst at some periods, the heart may be seen vigorously contracting from behind forwards, so as to propel the blood along the course just described, the observer, if he con- tinue to watch, will see its pulsations becoming fainter for a few beats, and the flow in the vessels becoming slower ; and then, after a slight pause, the whole current in all its windings is re- versed. The heart gives the opposite impulse, receiving the blood from the body, and sending it back into the peduncle through the tube that previously conveyed it thence ; while the tube that had previously served to carry the returning stream, now brings the blood from the stem, and distributes it to the •branchial sac, mantle, and visceral apparatus, whence it finds its way back to the heart. After the circulation has continued for a certain time in this new direction, the intermission is re- peated; and then a reversal takes place to the first course. The average time during which the circulation persists in each di- rection, seems to be about the same for the one as for the other; but the period between the cha'nges varies as much as from thirty seconds to two minutes. Although the circulation in the different bodies is brought into connection by the common stem, yet that of each is independent of*the rest, continuing when the current through its own footstalk is interrupted by a ligature ; and the stream which returns from the branchial sac and the viscera is then poured into the posterior part of the heart, instead of entering the peduncle. This is the course which it takes in DEVELOPMENT OF THE ASCIDIANS. 505 the " solitary" Ascidians ; and also in those composite forms, whose connecting stolons do not contain bloodvessels. 335. The development of the Ascidians presents some pheno- mena of much interest to the Microscopist ; the early stages of which are observable, whilst the ova are still within the cloaca of the parent. After the ordinary repeated segmentation of the yolk, whereby a " mulberry mass," is produced, a sort of ring is seen, encircling its central portion ; but this soon shows itself as a tapering tail-like prolongation from one side of the yolk, which gradually becomes more and more detached from it, save at the part from which it springs. Either whilst the egg is still within the cloaca, or soon after it has escaped from the vent, its en- velope bursts, and the larva escapes ; and in this condition it presents very much the appearance of a tadpole, the tail being straightened out, and propelling the body freely through the water by its lateral strokes. The centre of the body is occupied by a mass of liquid yolk ; and this is continued into the interior of three prolongations which extend themselves from the oppo- site extremity, each terminating in a sort of sucker. After swim- ming about for some hours with an active wriggling movement, the larva attaches itself to some solid body by means of one of these suckers; if disturbed from its position, it at first swims about as before ; but it soon completely loses its activity, and be- comes permanently attached ; and important changes manifest themselves in its interior. The prolongations of the central yolk- substance into the anterior processes and tail are gradually drawn back, so that the whole of it is concentrated into one mass ; and the tail, now consisting only of the gelatinous envelope, is either detached entire from the body by the contraction of the connect- ing portion, or withers and is thrown off gradually in shreds. The shaping of the internal organs out of the yolk-mass, takes place very rapidly ; so that by the end of the second day of the sedentary state, the outlines of the branchial sac and of the sto- mach and intestine may be traced ; no external orifices, however, being as yet visible. The pulsation of the heart is first seen on the third day, and the formation of the branchial and anal orifices takes place on the fourth ; after which the ciliary currents are immediately established through the branchial sac and alimentary canal. The embryonic development of other Ascidians, solitary as well as composite, takes place on a plan essentially the same as the foregoing, a free tadpole-like larva being always produced in the first instance ; and in the curious Appendicularia, which occasionally presents itself on our own coasts, this larval form is retained through life.1 ' For more special information respecting the Compound Ascidians. see especially the admirable Monograph of Prof. Milne Edwards on that group, Mr. Lister's Memoir " On the Structure and Functions of Tubular and Cellular Polypi, and of Ascidiae," in the " Philos. Transact." 1834, and Mr. Huxley's Memoir " On Doliolum and Appendicu- laria," in "Philos. Transact/' 1851 ; also the Art. Tunicata in the "Cyclopaedia of Ana- tomy and Physiology. '; CHAPTER XIY. MOLLUSCOUS ANIMALS GENERALLY. THE various forms of " Shell-fish," with their " naked" or shell- less allies, furnish a great abundance of objects of interest to the Microscopist ; of which, however, the greater part may be grouped under three heads ; — namely (1), the structure of the Shell, which is most interesting in the Conchifera or "Bivalves;" (2) the structure of the Tongue of the Gasteropoda, most of which have "Univalve" shells, others, however, being "naked;" and (3) the Developmental History of the embryo, for the study of which certain of the Gasteropods present the greatest facilities. These three subjects, therefore, will be first treated of systematically ; and a few miscellaneous facts of interest will be subjoined. 336. Shells of Mollusca. — These investments were formerly re- garded as mere inorganic exudations, composed of calcareous particles cemented together by animal glue ; Microscopic exa- mination, however, has shown that they possess a distinctly or- ganic structure ; and this structure presents certain very remark- able variations, in some of the natural groups of which the Mol- luscous series is composed. We shall first describe that which may be regarded as the characteristic structure of the ordinary Bivalves ; taking as a type the group of Margaritacece, which in- cludes the "Pearl-oyster" and its allies, the common Pinna ranking amongst the latter. In all these shells, we readily dis- tinguish the existence of two distinct layers ; an external, of a brownish-yellow color; and an internal, which has a pearly or " nacreous" aspect, and is commonly of a lighter hue. The structure of the outer layer may be conveniently studied in the shell of Pinna, in which it commonly projects beyond the inner, and there often forms laminae sufficiently thin and transparent to exhibit the general nature of its organization without any artificial reduction. If a small portion of such a lamina be exa- mined with a low magnifying power, even without any prepara- tion by transmitted light, each of its surfaces will present very much the appearance of a honeycomb ; whilst its broken edge exhibits an aspect which is evidently fibrous to the eye, but which, when examined under the microscope with reflected light, resembles that of an assemblage of basaltic columns (Fig. 334, p). PRISMATIC SHELL-STRUCTURE OF BIVALVES. 507 The shell is thus seen to be composed of a vast number of prisms, having a tolerably uniform size, an , -i j0 (Peari oyster). the dip of these laminae, the closer will their edges be ; whilst the less the angle which they make with the surface, the wider will be the interval between the lines. When the sec- tion passes for any distance in the plane of a lamina, no lines will present themselves on that space. And thus the appearance of a section of nacre is such, as to have been aptly compared by Sir J. HerscheP to the surface of a smoothed deal board, in which the woody layers are cut perpendicularly to their surface in one part, and nearly in their plane in another. Sir D. Brewster (loc. cit.) appears to suppose that nacre consists of a multitude of layers of carbonate of lime alternating with animal membrane ; and that the presence of the grooved lines on the most highly polished surface, is due to the wearing away of the edges of the animal laminae, whilst those of the hard calcareous laminae stand out. If each line upon the nacreous surface, however, indicates a distinct layer of shell-substance, a very thin section of mother- of-pearl ought to contain many thousand laminse, in accordance with the number of lines upon its surface ; these being frequently no more than l-7500th of an inch apart. But when the nacre is treated with dilute acid, so as to dissolve its calcareous portion, no such repetition of membranous layers is to be found ; on the contrary, if the piece of nacre be the product of one act of shell- formation, there is but a single layer of membrane. The mem- brane is usually found to present a more or less folded or plaited arrangement; but this has generally been obviously disturbed by the disengagement of carbonic acid in the act of decalcification, which tends to unfold the plaits. There is one shell, however, — 1 " Philos. Transact." 1814. 2 " Edinb. Philos. Journal," vol. ii. STRUCTURE OF NACRE. 511 the well-known Jfaliotis splendens, — which affords us the oppor- tunity of examining the plaits without any disturbance of their arrangement, and thus presents a clear demonstration of the real structure of nacre. This shell is for the most part made up of a series of plates of animal matter, resembling tortoise-shell in its aspect, alternating with thin layers of nacre ; and if a piece of it be submitted to the action of dilute acid, the calcareous portion of the nacreous layers being dissolved away, the plates of animal matter fall apart, each one carrying with it the membranous resi- duum of the layer of nacre that was applied to its inner surface. It will usually be found that the nacre-membrane covering some of these horny plates will remain in an undisturbed condition ; and their surfaces then exhibit their iridescent lustre, although all the calcareous matter has been removed from their structure. On look- ing at the surface with reflected light under a magnifying power of 75 diameters, it is seen to present a series of folds or plaits more or less regular ; and the iridescent hues which these exhibit, are often of the most gorgeous description. If the membrane be extended, however, with a pair of needles, these plaits are un- folded, and it covers a much larger surface than before ; but its iridescence is then completely destroyed. This experiment, then, demonstrates that the peculiar lineation of the surface of nacre (on which its iridescence undoubtedly depends, as originally shown by Sir D. Brewster) is due, not to the outcropping of alter- nate layers of membranous and calcareous matter, but to the dis- position of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface. There are several bivalve shells which present what may be termed a sub- nacreous structure, their polished surfaces being covered with lines indicative of folds in the basement-membrane ; but these folds are destitute of that regularity of arrangement, which is necessary to produce the iridescent lustre. This is the case, for example, with most of the Pectinidce (or Scallop tribe), also with some of the Mytilacece (or Mussel tribe), and with the common Oyster. Where there is no indication of a regular corrugation of the shell-membrane, there is not the least approach to the nacre- ous aspect ; and this is the case with the internal layer of by far the greater number of shells, the presence of true nacre being exceptional, save in a small number of families. It is of the inner layer that those rounded concretions are usually formed, which are often found in the interior of shells, and which, when composed of nacreous substance resembling that of the lining of the shell of Avicula, are known as pearls. Such concretions are found in many other shells ; but they are usually less remarkable for their pearly lustre; and when formed at the edge of the valves, they may be partly or even entirely made up of the pris- matic substance of the external layer, and may be consequently altogether destitute of the pearly aspect. The " membranous" shell-substance, some form of which constitutes the internal layer 512 MOLLUSCOUS ANIMALS GENERALLY. FIG. 257. of most bivalve shells, is occasionally traversed by tubes, which seem to commence from the inner surface of the shell, and to pass towards the exterior. These tubes vary in size from about the l-20,000th of an inch, or even less, to about the l-2000th ; but their general diameter, in the shells in which they most abound, is about l-4000th of an inch. The direction and distri- bution of these tubes are ex- tremely various in different ge- nera : in Anomia ephippium they are scantily distributed in the in- ternal nacreous lamina, whilst in the yellow outer layer they are very abundant, forming an irre- gular network (Fig. 257), which spreads out in a plane parallel to the surface. 340. The ordinary account of the mode of growth of the shells of Bivalve Mollusca, — that they are progressively enlarged by the deposition of new laminae, each of wrhich is in contact with the internal surface of the preceding, and extends beyond it. — does not express the whole truth ; for it takes no account of the fact that most shells are composed of two layers of very different texture, and does not specify whether both these layers are thus formed by the entire surface of the mantle whenever the shell has to be extended, or whether only one is produced. An exami- nation of Fig. 258 will clearly show the mode in which the FIG. 258. Tubular Shell-structure of Anomia. Vertical section of the lip of one of the valves of the shell of Unio:—a. b, c, successive formation? of the outer layer, a' I' c', the same of the inner layer. operation is effected. This figure represents a section of one of the valves of Unio occidens, taken perpendicularly to its surface, and passing from the margin or lip (at the left hand of the figure) towards the hinge (which would be at some distance beyond the right). This section brings into view the two substances of which the shell is composed ; traversing the outer or prismatic layer in the direction of the length of its cells, and ^passing through the nacreous lining, in such a manner as to bring into view its numerous laminae, separated by the lines a a', b b', c c' , SHELLS OF TEREBRATUL^!. 513 &c. These lines evidently indicate the successive formations of this layer; and it may be easily shown, by tracing them towards the hinge on the one side, and towards the margin on the other, that at every enlargement of the shell, its whole interior is lined by a new nacreous lamina, in immediate contact with that which preceded it. The number of such laminae, therefore, in the oldest part of the shell, indicates the number of enlargements which it has undergone. The outer or prismatic layer of the growing shell, on the other hand, is only formed where the new structure projects beyond the margin of the old ; and thus we do not find one layer of it overlapping another, except at the lines of junction of two distinct formations. When the shell has attained its full dimensions, however, new laminae of both layers still continue to be added ; and thus the lip becomes thickened by successive formations of prismatic structure, each being applied to the inner surface of the preceding, instead of to its free margin. A like arrangement may be well seen in the Oyster ; with this difference, that the successive layers have but a comparatively slight adhe- sion to each other. 341. The shells of Terebratulce, and of several other genera of Brachiopoda, are distinguished by peculiarities of structure, which serve to distinguish them from all others. When thin sections of them are microscopically examined, they exhibit the appear- ance of long flattened prisms (Fig. 259, 6), which are arranged with such obliquity, that their rounded extremities crop out upon the inner surface of the shell in an imbricated (tile-like) manner (a). All true Terebmtulidce, both recent and fossil, exhibit another very remarkable peculiarity; namely, the presence of a large number of perforations in the shell, generally passing nearly per- pendicularly from one surface to the other (as is shown in vertical sections, Fig. 261), and terminating internally by open orifices FIG. 259. FIG. 260. Fig. 259. Internal surface (a), and oblique section (6), of Shell of Terebratula (Waldheimia) australis. Fig. 260. External surface of same. (Fig. 259), whilst externally they are covered by the periostracum (Fig. 260). Their diameter is greatest towards their external surface, where they sometimes expand suddenly, so as to become 33 514 MOLLUSCOUS ANIMALS GENERALLY. FIG. 261. trumpet-shaped; and it is usually narrowed rather suddenly, when, as sometimes happens, a new internal layer is formed as a lining to the preceding (Fig. 261, A, d d). Hence the diame- ter of these canals, as shown in different transverse sections of one and the same shell, will vary according to the part of its thickness which the section hap- pens to traverse. The different species of Terebratulidce, how- ever, present very striking diver- sities in the size and closeness of the canals, as shown by sec- tions taken in corresponding Vertical sections of Shell of Terebratula (W M- ^^q of til PI r ^Vlplh' thrPP PX heimia) australis .--showing at A the canals Pai1 opening by large trumpet-shaped orifices on the ampleS of thlS kllld are given outer surface, and contracting at d, d, into nar- for ^he g^g Qf comparison in row tubes; and presenting at B a bifurcation of -r-v or»ct nr» A mi i lhe canals. Figs. 262-264. These canals are occupied, in the living state, by tubular prolongations of the mantle, the interior of which is filled with a fluid containing minute cells and granules, which, from its corresponding in appearance with the fluid contained in the great sinuses of the mantle, may be considered to be the animal's blood. Hence these ceecal tubes may be inferred to FIG. 762. FIG. 263. FIG. 264. Fig. 262. Horizontal section of Shell of Terebratula bullata (fossil, oolite). Fig. 263. " " " of Megerlia lima (fossil, chalk). Fig. 264. " " of Spiriferina rostrata (triassic). possess a respiratory function ; and seem to be analogous to tubes of a very similar nature, which extend into the "test" of many Tunicata from their sinus system (§ 334). In the family RhyncJio- nellidce, which is represented by only two recent species (the Rh. psittacea and Eh. nigricans, both of which formerly ranked as Terebratulse), but which contains a very large proportion of fossil Brachiopods, these canals are entirely absent ; so that the unifor- mity of their presence in the Terebratulidee, and of their absence in the Rhynchonellidse, supplies a character of great value in the STRUCTURE OF SHELL OF GASTEROPODA. 515 discrimination of the fossil shells belonging to these two groups respectively. Great caution is necessary, however, in applying this* test; mere surface-markings cannot be relied on; and no statement on this point is worthy of reliance, which, is not based on a microscopic examination of thin sections of the shell. In .the families Spiriferidce and Strophonemidoe, on the other hand, some species possess the perforations, whilst others are destitute of them ; so that their presence or absence there only serves to mark out subordinate groups. This, however, is what holds good in regard to characters of almost every description, in other departments of Natural History, as well as in this ; a character which is of fundamental importance from its close relation to the general plan of organization in one group, being, from its want of constancy, of far less account in another.1 342. There is not by any means the same amount of diversity in the structure of the shell in the class of Gasteropoda, as that which exists among the several tribes of Conchifera ; a certain typical plan of construction being common to by far the greater number of them. The small proportion of animal matter con- tained in most of these shells^ is a very marked feature in their character ; and it serves to render other features indistinct, since the residuum left after the removal of the calcareous matter is usually so imperfect, as to give no clue whatever to the explana- tion of the appearances shown by sections. Nevertheless, the structure of these shells is by no means homogeneous, but always exhibits indications, more or less clear, of an original organic arrangement. The "porcellanous" shells are composed of three layers, all presenting the same kind of structure, but each differing from the others in the mode in which this is dis- posed. For each layer is made up of an assemblage of thin laminae placed side by side, which separate one from another, apparently in the planes of rhomboidal cleavage, when the shell is fractured ; and, as was first pointed out by Mr. Bowerbank, each of these laminae consists of a series of elongated spicules (considered by him as prismatic cells filled with carbonate of lime) lying side by side in close apposition ; and these series are disposed alternately in contrary directions, so as to intersect each other nearly at right angles, though still lying in parallel planes. The direction of the planes is different, however, in the three layers of the shell, bearing the same relation to each other as have those three sides of a cube which meet each other at the same angle ; and by this arrangement, which is better seen in the fractured edge of Qyprcea or any similar shell, than in thin sections, the strength of the shell is greatly augmented. A similar arrangement, obviously designed with the same purpose, has been shown by Mr. Tomes to exist in the enamel of the 1 For a particular account of the Author's researches on this group, see his memoir on the subject, forming part of the Introduction of Mr. Davidson's " Monograph of the British Fossil Brachiopoda," published by the Palseontographical Society. 516 MOLLUSCOUS ANIMALS GENERALLY. teeth of Rodentia. The principal departures from this plan of structure are seen in Patella, Chiton, Haliotis, Turbo, and its allies, and in the " naked" Gasteropods, many of which last, both terrestrial and marine, have some rudiment of a shell. Thus in the common Slug, Limax rufus, a thin oval plate, of calcareous texture, is found imbedded in the shield-like fold of the mantle covering the fore-part of its back ; and if this be examined in an early stage of its growth, it is found to consist of an aggregation of cell-like bodies, generally somewhat hexa- gonal in form, and consolidated by a deposit of calcareous matter, which is sometimes so arranged as to be quite transpa- rent, whilst in other instances it presents an appearance closely resembling that delineated in Fig. 255. In the epidermis of the mantle of some species of Doris, on the other hand, we find long calcareous spicules, generally lying in parallel directions, but not in contact with each other, giving firmness to the whole of its dorsal portion ; and these are sometimes covered with small tubercles, like the spicules of Gorgonia (§ 309). They may be separated from the soft tissue in which they are imbed- ded, by means of caustic potash ; and when treated with dilute acid, whereby the calcareous matter is dissolved away, an organic basis is left, retaining in some degree the form of the original spicule. This basis cannot be said to be a true cell; but it seems to be rather a cell in the earliest stage of its formation, being an isolated particle of " sarcode" without wall or cavity ; and the close correspondence between appearances presented by thin sections of various " univalve" shells, and the forms of the spicules of Doris, seems to justify the conclusion, that even the most compact shells of this group are constructed out of the like elements, in a state of closer aggregation and more definite arrangement, with the occasional occurrence of a layer of more spheroidal bodies of the same kind, like those forming the rudi- mentary shell of Limax. 343. The animals composing the class of Cephalopoda (Cuttle- fish and Nautilus tribe), are for the most part unpossessed of shells ; and the structure of the few that we meet with in the genera Nautilus, Argonauta (Paper-nautilus) and Spirula, does not present any peculiarities that need here detain us. The rudimentary shell or sepiostaire of the common Cuttle-fish, how- ever, which is frequently spoken of as the "cuttle-fish bone," exhibits a very beautiful and remarkable structure, such as causes sections of it to be* very interesting microscopic objects. The outer shelly portion of this body consists of horny layers, alternating with calcified layers, in which last may be seen an hexagonal arrangement somewhat corresponding with that in Fig. 255. The soft friable substance that occupies the hollow of this boat-shaped shell, is formed of a number of delicate plates, running across it from one side to the other in parallel directions, but separated by intervals several times wider than TONGUES OF GASTEROPODS. 517 the thickness of the plates ; and these intervals are in great part filled up by what appear to be fibres or slender pillars, passing from one plate or floor to another. A more careful examination shows, however, that instead of a large number of detached pillars, there exists a comparatively small number of very thin sinuous laminae, which pass from one surface to the other, wind- ing and doubling upon themselves, so that each lamina occupies a considerable space. Their precise arrangement is best seen by examining the parallel plates, after the sinuous laminse have been detached from them ; the lines of junction being distinctly indicated upon these. By this arrangement, each layer is most effectually supported by those with which it is connected above and below ; and the sinuosity of the thin intervening laminse, answering exactly the same purpose as the " corrugation" given to iron plates for the sake of diminishing their flexibility, adds greatly to the strength of this curious texture ; which is at the same time lightened by the large amount of space between the paral- lel plates that intervenes between the sinuosities of the laminae. The best method of examining this structure, is to make sections of it with a sharp knife in various directions, taking care that the sections are no thicker than is requisite for holding together; and these may be mounted on a black ground as opaque objects, or in Canada balsam as transparent objects. 344. The structure of Shells generally is best examined by making sections in different planes, as nearly parallel as may be possible to the surfaces of the shell ; and other sections at right angles to these ; the former may be designated as horizontal, the latter as vertical. Nothing need here be added to the full direc- tions for making such sections, which have already been given (§§ 108-110). Much valuable information may also be derived, however, from the examination of the surfaces presented by fracture. The membranous residua left after the decalcification of the shell by dilute acid, may be mounted in weak spirit or in Goadby's solution.1 345. Tongue of Crasteropod Mollusks. — The organ which is commonly known under this designation, is one of a very singu- lar nature ; and we should be altogether wrong in conceiving of it as having any likeness to that on which our ordinary ideas of 'such an organ are founded. For instead of being a projecting body, lying in the cavity of the mouth, it is a tube that passes backwards and downwards beneath the mouth ; its hinder end being closed, whilst in front it opens obliquely upon the floor ot the mouth, being (as it were) slit up and spread out, so. as to form a nearly flat surface. On the interior of the tube, as well as on the flat expansion of it, we find numerous transverse 1 For fuller details on the minute structure of the shells of the Mollusca, see the Authors memoirs on that subject in the "Reports of the British Association" for 1844 and 1847 ; also Mr. Bowerbank's memoir on the same subject in " Transact, of Micro- scopical Society," Ser. 1, vol. i; and Mr. Quekett's " Lectures on Histology," vol. ii, Chaps, xvi-xxii. 518 MOLLUSCOUS ANIMALS GENERALLY. rows of minute teeth, which are set upon flattened plates ; each principal tooth sometimes having a basal plate of its own, whilst in other instances one plate carries several teeth. Of the for- mer arrangement we have an example in the tongue of many FIG. 265. FIG. 266. Fig. 205. Portion of the left half of the Palate of the Helix hortensis; the rows of teeth near the edge separated from each other to show their form. Fig. 266. Palate of Zonites cellarius. terrestrial Gasteropods, such as the Snail (Helix) and Slug (Li- max), in which the number of plates in each row is very con- siderable (Figs. 265, 266), amounting to 180 in the large garden Slug (Limax maximus) ; whilst the latter prevails in many marine Gasteropods, such as the common Whelk (Buccinum undatum\ the tongue of which has only three plates in each row, one bear- ing the small central teeth, and the two others the large lateral teeth (Fig. 269). The length of the tongue, and the number of rows of teeth, vary greatly in different species. Generally speak- ing, the tongue of the terrestrial Gasteropods is short, and is contained entirely within the nearly globular head ; but the rows of teeth being closely set together, they are usually very numerous, there being frequently more than 100, and in some species as many as 160 or 170 ; so that the total number of teeth may mount up, as in Helix pomatia, to 21,000, and in Limax maximus, to 26,800. The transverse rows are usually more or less curved, as shown in Fig. 266, whilst the longitudinal rows are quite straight; and the curvature takes its departure on each side from a central longitudinal row, the teeth of which are symmetrical, whilst those of the lateral portions of each trans- verse row present a modification of that symmetry, the pro- minences on the inner side of each tooth being suppressed, whilst those on the outer side are increased ; this modification being observed to augment in degree, as we pass from the central line towards the edges. The tongue of the marine Gasteropods is generally longer, and its teeth larger ; and in many instances it extends far beyond the head, which may, indeed, contain but a small part of it. Thus in the common Limpet (Patella), we find the principal part of the tongue to lie folded up, but perfectly free, in the abdominal cavity, between the intestines and the TONGUES OF GASTEHOPODS. 519 muscular foot ; and in some species its length is twice or even three times as great as that of the entire animal. In a large pro- portion of cases, these tongues exhibit a very marked separation between the central and the lateral portions (Figs. 267, 269) ; the teeth of the central band being frequently small and smooth at their edges, whilst those of the lateral are large and serrated. The tongue of Trochus zizyphinus, represented in Fig. 267, is one of the most beautiful examples of this form ; not only the large teeth of the lateral bands, but the delicate leaf-like teeth of the central, portion, having their edges minutely serrated. A yet more complex type, however, is found in the tongue of Haliotis ; in which there is a central band of teeth having nearly straight edges instead of points ; then, on each side, a lateral band con- sisting of large teeth shaped like those of the shark ; and beyond this, again, another lateral band on either side, composed of several rows of smaller teeth. Very curious differences also present themselves among the different species of the same genus. Thus in Doris pilosa, the central band is almost entirely wanting, and each lateral band is formed of a single row of very large hooked teeth, set obliquely, like those of the lateral bands in Fig. 267 ; whilst in Doris tuberculata, the central band is the FIG. 268. Palate of Trochus zizyphinus. Palate of Doris tuberculata. part most developed, and contains a number of rows of conical teeth, standing almost perpendicularly, like those of a harrow (Fig. 268). 346. Many other varieties might be described, did space per- mit ; but we must be content with adding, that the form and arrangement of the teeth afford characters of great value in classification, as was first pointed out by Prof. Loveri (of Stock- holm) in 1847, and has been since very strongly urged by Dr. J. E. Gray, who considers that the structure of the tongue is one 520 MOLLUSCOUS ANIMALS GENERALLY. of the best guides to the natural affinities of the species, genera, and families of this group, since any important alteration in the form or position of the teeth must be accompanied by some corresponding peculiarity in the habits and manners of the ani- mal.1 Hence a systematic examination and delineation of the structure and arrangement of these organs, by the aid of the Microscope and Camera Lucida, would be of the greatest service to this department of Natural History. The short thick tube of the Limax and other terrestrial Gasteropods, appears adapted for the trituration of the food previously to its passing into the oesophagus ; for in these animals we find the roof of the mouth furnished with a large strong horny plate, against which the flat end of the tongue can work. On the other hand, the flattened portion of the tongue of Buccinum and its allies is used by these animals asll file, with which they bore holes through the shells of the mollusks that serve as their prey; this they are enabled to effect, by everting that part of the proboscis-shaped mouth whose floor is formed by the flattened part of the tongue, which is thus brought to the exterior, and by giving a kind of sawing motion to the organ, by means of the alternate action of two pairs of muscles, — a protractor, and a retractor, — which put forth and draw back a pair of cartilages wrhereon the tongue is supported, and also elevate and depress its teeth.2 Of the use of the long blind tubular part of the tongue in these Gasteropods, however, scarcely any probable guess can be made ; unless it be a sort of "cavity of reserve," from which a new toothed surface maybe continually supplied, as the old one is worn away, some- what as the front teeth of the Kodents are constantly being regene- rated from the surface of the pulps which occupy their hollow conical bases, as fast as they are rubbed down at their edges. 347. The preparation of these tongues for the Microscope, can, of course, be only accomplished by carefully dissecting them from their attachments within the head ; and it will be also necessary to remove the membrane that forms the sheath of the tube, when this is thick enough to interfere with its transpa- rency. The tube itself should be slit up with a pair of fine scissors, through its entire length ; and should be so opened out, that its expanded surface may be a continuation of that which forms the floor of the mouth. The mode of mounting it will depend upon the manner in which it is to be viewed. For the ordinary purposes of Microscopic examination, no method is so good as mounting in fluid; either weak spirit or Goadby's solution answering very well. But many of these tongues, especially those of the marine Gasteropods, become most beauti- 1 "Annals of Natural History,'' Ser. 2, vol. x, p. 413. 2 For additional details on the organization of the tongue and teeth of the Gasteropod Mollusks, see Mr. W. Thomson, in "Cyclop, of Anat. and Physiol." vol. iv, pp. 1142, 1 143 ; and in " Ann. of Nat. Hist." Ser. 2, vol. vii, p. 86. DEVELOPMENT OF GASTEROPODS. 521 FIG. 269. Palate of Buccinum undatum, as seen under polarized light. ful objects for the Polariscope, when they are mounted in Canada balsam ; the form and arrangement of the teeth being very strongly brought out by it (Fig. 269),. and a gorgeous play of colors being exhibited when a selenite plate is placed behind the ob- ject, and the analyzing prism is made to rotate. 348. The stomachs, also, of many Gasteropod Mollusks are furnished with teeth, which are implanted on their walls for the further reduction of the food; such teeth, very numerous but of small size, and bearing a strong resemblance to those of its tongue, are found in the stomach of the common Slug. In several marine Gasteropods, however, especially Bulla, Scyllcea, and Aplysia, the gastric teeth are individu- ally much larger, though less numerous, and constitute a very efficient reducing apparatus, especially when combined, as they frequently are, with a horny or calcareous deposit in the walls of the stomach, which converts it into a " gizzard" for the trituration of the substances that have been divided by the teeth. 349. Development of G-asteropod Mollusks. — The history of em- bryonic development may be studied with peculiar facility in certain members of this class, and presents numerous pheno- mena of great interest. The eggs (save among the terrestrial species) are usually deposited in aggregate masses, each enclosed in a common protective envelope. The nature of this envelope, however, varies greatly : thus in the common Lymnceus stagnalis, or " water-snail," of our ponds and ditches, it is nothing else than a mass of soft jelly, about the size of a sixpence, in which from 50 to 60 eggs are imbedded, and which is attached to the leaves or stems of aquatic plants ; in the Buccinum undatum, or common Whelk, it is a membranous case, connected with a con- siderable number of similar cases by short stalks, so as to form large globular masses, which may often be picked up on our shores, especially between April and June; in the Purpura lapillus, or Rock- whelp, it is a little flask-shaped capsule, having a firm horny wall, which is attached by a sort of foot to the surface of rocks between the tide-marks, great numbers being often thus found standing erect side by side ; whilst in the Nudibranchiate order generally (consisting of the Doris, Eolis, and other " sea-slugs") it forms a long tube with a membranous wall, in which immense numbers of eggs (even half a million or more) are packed closely together in the midst of a jelly-like substance, this tube being disposed in coils of various forms, 522 MOLLUSCOUS ANIMALS GENERALLY. which are usually attached to sea-weeds or zoophytes. The course of development, in the first and last of these instances, may be readily observed from the very earliest period, down to that of the emersion of the embryo ; owing to the extreme transparency of the "nidamentum," and of the egg-membranes themselves. The first change which will be noticed by the ordi- nary observer, is the " segmentation" of the yolk-mass, which divides itself (after the manner of a cell undergoing duplicative subdivision) into two parts, each of these two into two others, and so on, until a mulberry-like mass of minute yolk segments is evolved. Generally speaking, however, there may be noticed at a very early stage of this process, as performed by Gasteropod Mollusks, an inequality in the size of the segments (Fig. 271, c) ; one set, derived from the larger of the two divisions (D) into which the yolk-sphere first separates itself, being destined to form the internal organs, whilst the other set of segments, of much inferior dimensions, and formed by the subdivision of the smaller half of the original sphere, furnishes the material for the superficial parts. Soon after the " mulberry mass" has been formed, it commonly begins to exhibit a very curious alternating movement within the egg, two or three turns being made in one direction, and the same number in a reverse direction: this movement, which is due to ciliary action, is often extremely transitory in its duration ; but in the Lymnceus it continues almost up to the escape of the embryo, and, when several ova are brought into view at once under a low magnifying power, the spectacle is a very curious one. 350. A separation is usually seen at an early period, between the anterior or cephalic portion, and the posterior or visceral portion, of the embryonic mass ; and the development of the former advances with the greater activity. One of the first changes which is seen in it, consists in its extension into a sort of fin-like membrane on either side, the edges of which are fringed with long cilia (Fig. 270), whose movements may be clearly distinguished whilst the embryo is still shut up within the egg ; at a very early FlQ- 27°- period may also be discern- ed the "auditory vesicles" or rudimentary organs of hearing (§ 353), which scarce- ly attain any higher deve- lopment in these creatures during the whole of life ; and from the immediate __ neighborhood of these is EmbTyo^s of Nudibranchiate 'Gasteropods. put forth a prOJ CCtioil , which is afterwards to be evolved into the "foot" or muscular disk of the animal. While these organs are making their appearance, the shell is being formed PURPURA. 523 on the surface of tlie posterior portion, appearing first as a thin covering over its hinder part, and gradually extending itself until it becomes large enough to enclose the embryo com- pletely, when this contracts itself. The ciliated lobes are best seen in the embryoes of Nudibranchs, in which they are much larger than in Lymnseus ; and the fact of the universal presence of a shell in the embryoes of the former group, is of peculiar interest, as it is destined to be cast off very soon after they enter upon active life. These embryoes may be seen to move about as freely as the narrowness of their prison permits, for some time previous to their emersion ; and when set free by the rupture of the egg-cases, they swim forth with great activity by the action of their ciliated lobes, — these, like the wheels of Rotifera, serving also to bring food to the mouth, which is at that time unprovided with the reducing apparatus subsequently found in it. The same is true of the embryo of Lymnseus, save that its swimming movements are less active, in consequence of the in- ferior development of the ciliated lobes ; and the currents pro- duced by these seem to have reference chiefly to the provision of supplies of food, and of aerated water for respiration. The disappearance of the cilia has been observed by Mr. Hogg to be coincident with the development of the teeth to a degree sufficient to enable the young water-snail to crop its vegetable food ; and he has further ascertained, that if the growing animal be kept in fresh water alone for some time, with vegetable matter of any kind, the gastric teeth are very imperfectly developed, and the cilia are still retained.1 351. A very curious modification of the ordinary plan of deve- lopment, is presented in the Pur- pura lapillus ; and it is probable FIG. 271. that something of the same kind o B exists also in Buccinum, as also in other Gasteropods of the same ex- tensive order (Pectinibranclnata). Each of the capsules already de- scribed (§ 349) contains from 500 to 600 egg-like bodies (Fig. 271, A), imbedded in a viscid gelatinous substance ; but only from 12 to 30 embryoes usually attain complete development* and it is obvious from the large Comparative Size Early stages of embryonic development of Which these attain (Fisr. 272, B), Purpura lapiUus:— A, egg-like spherule; B,C, that each of them must include #3Stt&£S an amount Of Substance equal tO of development of early embryoes. that of a great number of the bodies originally found within the capsule. The explanation of this fact (long since noticed by Dr. J. E. Gray in regard to Bucci- 1 "Transactions of Microscopical Society," 2d Ser. vol. ii, p. 93. 524 MOLLUSCOUS ANIMALS GENERALLY. num) seems to be as follows : — Of those 500 or 600 egg-like bodies, only a small part are true ova, the remainder being merely yolk- spherules, which are destined to serve for the nutrition of the embryoes. The distinction between them manifests itself at a very early period, even in the first segmentation ; for while the yolk-spherules divide into two equal hemispheres (Fig. 271, B), the real ova divide into a larger and a smaller segment (D) ; in the cleft between these are seen the minute " directive vesicles," which appear to be always double or even triple, although, from being seen " end on," only one may be visible ; and near these is generally to be seen a clear space in each segment. The dif- ference is still more strongly marked in the subsequent divisions; for whilst the cleavage of the yolk-spherules goes on irregularly, so as to divide each into from 14 to 20 segments having no defi- niteness of arrangement (c, E, F, G), that of the ova takes place in such a manner as to mark out the distinction already alluded to between the cephalic and the visceral portions of the mass (H) ; and the evolution of the former into distinct organs very speedily commences. In the first instance, a narrow transparent border is seen around the whole embryonic mass, which is broader at the cephalic portion (i) ; next, this border is fringed with a short cilia, and the cephalic extension into two lobes begins to show itself; and then between the lobes a large mouth is formed, opening through a short, wide oesophagus, the interior of which is ciliated, into the visceral cavity, occupied as yet only by the yolk particles originally belonging to the ovum (K). Whilst these developmental changes are taking place in the embryo, the whole aggregate of segments formed by the subdivision of the yolk-spherules coalesces into one mass, as shown at A, Fig. 272 ; and the embryoes are often, in the first instance, so completely buried within this, as only to be discoverable by tearing its por- tions asunder ; but some of them may commonly be found upon its exterior ; and those contained in one capsule very commonly exhibit the different stages of development represented in Fig. 271, H-K. After a short time, however, it becomes apparent that the most advanced embryoes are beginning to swallow the yolk-segments of the conglomerate mass ; and capsules will not unfrequently be met with, in which embryoes of various sizes, as a, 5, , .-i i £> T* v .i.T~ Portion of a large Trachea of Dytiscus, with after the USUal fashion, by the some of Us principal branches. Tracheal system of Nepa (Water-scorpion): — a, head; 6, first pair of legs; c, first segment of the thorax ; d, second pair of wings ; e, second pair of legs;/, trachea! trunk; g, one of the stigmata; h, air-sac. their FIG. 291. 572 INSECTS AND ARACHNID A. winding of a continuous spiral fibre through their interior, the fibre is broken into rings, and these rings do not surround the whole tube, but are terminated by a set of arches that pass from one to another (Fig. 287, A). When a portion of one of the great trunks with some of the principal branches of the tracheal system has been dissected out, and so pressed in mounting that the sides of the tubes are flattened against each other (as has happened in the specimen represented in Fig. 291), the spire forms two layers which are brought into close apposition ; and a very beautiful appearance, resembling that of "watered silk," is produced by the crossing of the two sets of fibres, of which one overlies the other. That this appearance, however, is altogether an optical illusion, may be easily demonstrated by carefully following the course of any one of the fibres, which will be found to be perfectly regular. 392. The " stigmata" or "spiracles" through which the air enters the tracheal system, are generally visible on the exterior of the body of the Insect (especially on the abdominal segments) as a series of pores along each margin of the under surface. In most larvse, nearly every segment is provided with a pair ; but in the perfect insect, several of them remain closed, especially in the thoracic region, so that their number is often considerably reduced. The structure of the spiracles varies greatly in regard to complexity in different Insects ; and even where the general plan is the same, the details of conformation are peculiar, so that perhaps in scarcely any two species are they alike. Gene- rally speaking, they are furnished with some kind of sieve at their entrance, by which particles of dust, soot, &c., which would otherwise enter the air-passages, are filtered out ; and this sieve may be formed by the interlacement of the branches of minute arborescent growths from the borders of the spiracle, as in the common Fly (Fig. 292), or in the Dytiscus; or it may be a mem- brane perforated with minute holes, and supported upon a frame- FIG. 293. Spiracle of common Fly. Spiracle of Larva of Cockchafer. work of bars that is prolonged in like manner from the thickened margin of the aperture (Fig. 293), as in the larva of the Melo- SPIRACLES. 573 lontha (cockchafer). $"ot unfrequently, the centre of the aperture is occupied by an impervious disk, from which radii proceed to its margin, as is well seen in the spiracle of Tipula (crane-fly). In those aquatic larvae which breathe air, we often find one of the spiracles of the last segment of the abdomen prolonged into a tube, the mouth of which remains at the surface, while the body is immersed ; the larvae of the Gnat tribe may frequently be observed in this position. 393. There are many aquatic Larvae, however, which have an entirely different provision for respiration ; being furnished with external leaf-like or brush-like appendages, into which the tracheae are prolonged, so that, by absorbing air from the water that bathes them, they may convey this into the interior of the body. We cannot have a better example of this than is afforded by the larva of the common Ephemera (day-fly), the body of which is furnished with a set of branchial appendages resembling the "fin- feet" of Branchiopods (§ 368), whilst the three-pronged tail also is fringed with clusters of delicate hairs which appear to minister to the same function. In the larva of the Libellula (dragon-fly), the extension of the surface for aquatic respiration takes place within the termination of the intestine; the lining membrane of which is folded into an immense number of plaits, each contain- ing a minutely ramified system of tracheae ; the water, slowly drawn in through the anus for bathing this surface, is ejected with such violence that the body is impelled in the opposite direction ; and the air taken up by its tracheae is carried, through the system of air-tubes of which they form a part, into the remotest organs. This apparatus is a peculiarly interesting object for the Microscope, on account of the extraordinary co- piousness of the distribution of the tracheae in the intestinal folds. 394. The main trunks of the Tracheal system, with their prin- cipal ramifications, may generally be got out with little difficulty, by laying open the body of an insect or larva, under water, in a dissecting-trough (§ 104), and removing the whole visceral mass, taking care to leave as many as possible of the branches which will be seen proceeding to this from the two great longitudinal tracheae, to whose position these branches will serve as a guide. Mr. Quekett recommends the following as the most simple method of obtaining a perfect system of tracheal tubes from a larva : — a small opening having been made in its body, this is to be placed in strong acetic acid, which will soften or decompose all the viscera ; and the tracheae may then be well washed with the syringe, and removed from the body with the greatest facility, by cutting away the connections of the main tubes with the spiracles by means of fine-pointed scissors. In order to mount them, they should be floated upon the slide, on which they should then be laid out in the position best adapted for displaying them. If they are to be mounted in Canada balsam, they should be 574 INSECTS AND ARACHNIDA. allowed to dry upon the slide, and should then be treated in the usual way; but their natural appearance is best preserved by mounting them in fluid (weak spirit or Goadby's solution), using a shallow cell to prevent pressure. The finer ramifications of the tracheal system may generally be seen particularly well in the membranous wall of the stomach or intestine; and this, having been laid out and dried upon the glass, may be mounted in balsam so as to keep the tracheae full of air (whereby they are much better displayed), if care be taken to use balsam that has been previously thickened, to drop this on the object without liquefying it more than is absolutely necessary, and to heat the slide and the cover (the heat may be advantageously applied directly to the cover, after it has been put on, by turning over the slide so that its upper face shall look downwards) only to such a degree as to allow the balsam to spread and the cover to be pressed down. The spiracles are easily dissected out by means of a pointed knife or a pair of fine scissors ; they should be mounted in fluid, when their texture is soft; and in balsam, when the integument is hard and horny. 395. Wings. — These organs are essentially composed of an extension of the external membranous layer of the integument, over a framework formed by prolongations of the inner horny layer; within which prolongations, tracheae are nearly always to be found, whilst they also contain channels through which blood circulates during the growth of the wing and for a short time after its completion. This is the simple structure presented to us in the wings of Neuroptera (dragon-flies, &c.), Hymenoptera (bees and wasps), Diptera (two-winged flies), and also of many Homoptera (cicadse and aphides) ; and the principal interest of these wings as microscopic objects, lies in the distribution of their "veins" or "nerves" (for by both names are the ramifica- tions of their skeleton known), and in certain points of acces- sory structure. The venation of the wings is most beautiful in the smaller Neuroptera; since it is the distinguishing feature of this order, that the veins, after subdividing, reunite again, so as to form a close network ; whilst in the Hymenoptera and Dip- tera such reunions are rare, especially towards the margin of the wings, and the areolse are much larger. Although the mem- brane of which these wings are composed, appears perfectly homogeneous when viewed by transmitted light, even with a high magnifying power, yet, when viewed by light reflected obliquely from their surfaces, an appearance of cellular areola- tion is often discernible; this is well seen in the common Fly, in which each of these areolse has a hair in its centre. In order to make this observation, as well as to bring out the very beautiful iridescent hues which the wings of many minute insects (as the Aphides) exhibit when thus viewed, it is convenient to hold the wing in the stage-forceps, for the sake of giving it every variety of inclination; and when that position has been found, which WINGS AND ELYTRA OF INSECTS. 575 best displays its most interesting features, it should be set up as nearly as possible in the same. For this purpose it should be mounted on an opaque slide; but instead of being laid down upon its surface, the wing should be raised a little above it, its " stalk" being held in the proper position by a little cone of soft wrax, in the apex of which it may be imbedded. The wings of most Hymenoptera are remarkable for the peculiar apparatus by which those of the same side are connected together, so as to constitute in flight but one large wing ; this consists of a row ot curved hooks on the anterior margin of the posterior wing, which lay hold of the thickened and doubled-down posterior edge of the anterior wing. These hooks are sufficiently appa- rent in the wings of the common Bee, when examined with even a low magnifying power ; but they are seen better in the Wasp, and better still in the Hornet. The peculiar scaly covering of the wings of the Lepidoptera has already been noticed (§ 381); but it may here be added that the entire wings of many of the smaller and commoner insects of this order, such as the Tineidce or " clothes' moths," form very beautiful opaque objects for low powers; the most beautiful of all being the divided \vings of the Fissipennes or " plumed moths," especially those of the genus Pterophorus. 396. There are many Insects, however, in which the wings are more or less consolidated by the interposition of a layer of horny substance between the two layers of membrane. This plan of structure is most fully carried out in the Coleoptera (beetles), in which the anterior wings are so much thickened and are so little extended, that they are useless in flight, and serve merely as cases or covers for the posterior, which lie folded up beneath them when not in use ; hence these are distinguished as elytra. These elytra, when the insect is at rest, meet along the median line of the back, and cover nearly the whole upper surface of the body ; and it is upon them that the brilliant hues, by which the integument of many of these insects is distinguished, are most strikingly displayed. In the anterior wings of the Forficulidce or earwig tribe (which form the connecting link between this order and the Orthoptera), the cellular structure may often be readily distinguished when they are viewed by transmitted light, espe- cially after having been mounted in Canada balsam. The ante- rior wings of the Orthoptera (grasshoppers, crickets, &c.) although not by any means so solidified as those of Coleoptera, contain a great deal of horny matter ; they are usually rendered sufficiently transparent, however, by Canada balsam, to be viewed with transmitted light; and many of them are so colored as to be very showy objects (as are also the posterior fan-like wings) for the solar or gas-microscope, although their large size, and the ab- sence of any minute structure, prevent them from affording much interest to the ordinary Microscopist. We must not omit to mention, however, the curious sound-producing apparatus which 576 INSECTS AND ARACHNIDA. is possessed by most insects of this order, and especially by the common House- Cricket ; this consists of the "tympanum" or drum, which is a space on each of the upper wings, scarcely crossed by veins, but bounded externally by a large dark vein provided with three or four longitudinal ridges, and of the "file" or "bow," which is a transverse horny ridge in front of the tym- panum, furnished with numerous teeth ; and it is believed that the sound is produced by the rubbing of the two bows across each other, while its intensity is increased by the sounding-board action of the tympanum. The wings of the Fulgoridce (lantern- flies) have much the same texture with those of the Orthoptera, and possess about the same value as microscopic objects ; differ- ing considerably from the purely membranous wings of the Ci- cadae and Aphides, which are associated with them in the order Homoptera. In the order Hemiptera, to which belong various kinds of land and water insects that have a suctorial mouth re- sembling that of the common Bug, the wings of the anterior pair are usually of parchmenty consistence, though membranous near their tips, and are often so richly colored as to become very beau- tiful objects, when mounted in balsam and viewed by transmitted light ; this is the case especially with the terrestrial vegetable- feeding kinds, such as the Pentatoma and its allies, some of the tropical forms of which rival the most brilliant of the Beetles. The British species are by no means so interesting; and the aquatic kinds, which, next to the bed-bugs, are the most com- mon, always have a dull brown or almost black hue; even among these last, however, — of which the Notonecta (water-boatman) and the Nepa (water-scorpion) are well-known forms, — the wings are beautifully variegated by differences in the depth of that hue. 397. Feet. — Although the feet of Insects are formed pretty much on one general plan, yet that plan is subject to considerable modifications, in accordance with the habits of life of different species. The entire limb usually consists of five divisions, namely, the coxa or hip, the trochanter, the femur or thigh, the tibia or shank, and the tarsus or foot ; and this last portion is made up of several successive joints. The typical number of these joints seems to be five ; but that number is subject to reduction ; and the vast order Coleoptera is subdivided into primary groups, ac- cording as the tarsus consists of five, four, or three segments. The last joint of the tarsus is usually furnished with a pair of strong hooks or claws (Figs. 294, 295) ; and these are often ser- rated (that is, furnished with saw-like teeth), especially near the base. The under surface of the other joints is frequently beset with tufts of hairs, which are arranged in various modes, some- times forming a complete "sole ;" this is especially the case in the family Ourculionidce ; so that a pair of the feet of the " dia- mond-beetle," mounted so that one shows the upper surface made resplendent by its jewel-like scales, and the other the hairy cushion beneath, is a very interesting object. In many Insects, FEET OF INSECTS. 577 especially of the Fly kind, the foot is furnished with a pair of membranous expansions, termed pulvilli (Fig. 294); and these are beset with numerous hairs, each of which has a minute FIG. 294. disk at its extremity. This structure is evidently connected with the power which these in- sects possess, of walking over smooth surfaces in opposition to the force of gravity; yet there is still considerable un- certainty as to the precise mode in which it ministers to this faculty. Some believe that the "pulvilli" act as suckers, the insect being held up by the pressure of the air against their upper surface, when a vacuum is formed beneath; whilst Foot of Fiy, others maintain that the adhe- sion is the result of the secretion of a viscid liquid from the under side of the foot. The careful observations of Mr. Hep- worth have led him to a conclusion which seems in harmony .with all the facts of the case ; namely, that the minute disks at FIG. 295. A. Foot ofDytiscus, showing its apparatus of suckers ; a, 6, large suckers; o, ordinary suckers : — B, one of the ordinary suckers more highly magnified. the extremity of the individual hairs act as suckers, and that each of them secretes a liquid, which, though not viscid, serves to make its adhesion perfect.1 And this view of the case derives 1 See Mr. Hepworth's communications to the " Quart. Journ. of Microsc. Science," ,vol. ii, p. 158, and vol. iii, p. 312. 37 578 INSECTS AND ARACHNID A. confirmation, from the presence of a similar apparatus, on a far larger scale, on the foot of the Dytiscus (Fig. 295, A). The first joints of the tarsus of this insect are widely expanded, so as to form a nearly circular plate ; and this is provided with a very remarkable apparatus of suckers, of which one disk (a) is ex- tremely large, and is furnished with strong radiating fibres, a second (b) is a smaller one formed on the same plan (a third, of the like kind, being often present), whilst the greater number are comparatively small tubular club-shaped bodies, each having a very delicate membranous sucker at its extremity, as seen on a larger scale at B. These last seem to resemble the hairs of the Fly's foot in every particular but dimension ; and an intermediate size is presented by the hairs of many beetles, especially Curcu- lionidpe. The feet of Caterpillars differ considerably from those of perfect Insects. Those of the first three segments, which are afterwards to be replaced by true legs, are furnished with strong horny claws ; but each of those of the other segments, which are termed "pro-legs," is composed of a circular series of compara- tively slender curved booklets, by which the caterpillar is enabled to cling to the minute roughnesses of the surface of the leaves, &c., on which it feeds. This structure is well seen in the pro- legs of the common Silk-worm. 398. Stings and Ovipositors. — The Insects of the order Hymen- optera are all distinguished by the prolongation of the last seg- ment of the abdomen into a peculiar organ, which, in one division of the order, is a " sting," and in the other is an " ovipositor," — an instrument for the deposition of the eggs, which is usually also provided with the means of boring a hole for their reception. The former group consists of the Bees, "Wasps, Ants, &c. ; the latter of the Saw-flies, Gall-flies, Ichneumon-flies, &c. These two sets of instruments are not so unlike in structure, as they are in function. The " sting" is usually formed of a pair of darts, beset with barbed teeth at their points, and furnished at their roots with powerful muscles whereby they can be caused to project from their sheath, which is a horny case formed by the prolongation of the integument of the last segment, slit into two halves, which separate to allow the protrusion of the sting; whilst the peculiar " venom" of the sting is due to the ejection, by the same muscular action, of a poisonous liquid, from a bag situated near the root of the sting, which passes down a canal excavated between the darts, so as to be inserted into the punc- ture which they make. The stings of the common Bee, Wasp, and Hornet, may all be made to display this structure without much difficulty in the dissection. The " ovipositor" of such in- sects as deposit their eggs in holes ready made, or in soft animal or vegetable substances (as is the case with the Ichneu- monidce), is simply a long tube, which is enclosed, like the sting, in a cleft sheath. In the Gall-flies (Cynipidce), the extremity of the ovipositor has a toothed edge, so as to act as a kind of saw, STINGS AND OVIPOSITORS OF HYMENOPTERA. 579 whereby harder substances may be penetrated ; and thus an aperture is made in the leaf, stalk, or bud of the plant or tree infested by the particular species, in which the egg is deposited, together with a drop of fluid that has a peculiarly irritating effect upon the vegetable tissues, occasioning the production of the "galls," which are new growths that serve not only to protect the larvae, but also to afford them nutriment. The Oak is in- fested by several species of these insects, which deposit their eggs in different parts of its fabric ; and some of the small "galls" which are often found upon the surface of oak leaves, are extremely beautiful objects for the lower powers of the Microscope. It is in the Tenthredinidce, or Saw-flies, and in their allies the Siricidce, that the ovipositor is furnished with the most powerful apparatus for penetration ; and some of these insects can bore by its means into hard timber. Their " saws" are not un- like the " stings" of Bees, &c., but are broader, are toothed for a greater length, and are made to slide along a firm piece that supports each blade, like the " back" of a carpenter's " tenon- saw ;" they are worked alternately (one being protruded while the other is drawn back) with great rapidity ; and when the per- foration has been made, the two blades are separated enough to allow the passage of the eggs between them. Many other In- sects, especially of the order Diptera, have very prolonged ovi- positors, by means of which they can insert their eggs into the integuments of animals, or into other situations in which the larvee will obtain appropriate nutriment; a remarkable example of this is furnished by the Gad-fly ( Tabanus\ whose ovipositor is composed of several joints, capable of being drawn together or extended like those of a telescope, and is terminated by boring instruments; and the egg being conveyed by its means, not only into but through the integument of the Ox, so as to be im- bedded in the tissue beneath, a peculiar kind of inflammation is set up there, which (as in the analogous case of the gall-fly) forms a nidus appropriate both to the protection and to the nutri- tion of the larva. Other Insects which deposit their eggs in the ground, such as the Locusts, have their ovipositors so shaped as to answer for digging holes for their reception. The preparations which serve to display the foregoing parts, are best seen when mounted in balsam ; save in the case of the muscles and poison- apparatus of the sting, which are better preserved in weak spirit or Goadby's solution. 399. The sexual organs of Insects furnish numerous objects of extreme interest to the Anatomist and Physiologist ; but as an account of them would be unsuitable to the present work, a re- ference to a copious source of information respecting one of their most curious features, and to a list of the species that afford good illustrations, must here suffice.1 The Eggs of many Insects are 1 See the Memoirs of M. Lacaze-Duthiers " Sur I'armure ge"nitale des Insectes," in ''Ann. des Sci. Nat." Sieme Se"r. torn, xii, xiv, xvii, xviii, xix; and M. Ch. Robin's 580 INSECTS AND ARACHNIDA. objects of great beauty, on account of the regularity of their form, and the symmetry of the markings on their surface (Fig. 296). The most interesting belong for the most part to the Lepidopterous order ; and there are few among these that are not worth examination, some of the commonest (such as those of the FIG. 296. Eggs of Insects magnified : — A, Pontia napi ; B, Vanessa urticce ; c, Hippardiia tithous ; D, Argynnis Latiionia. Cabbage-butterfly, which are found covering large patches of the leaves of that plant) being as remarkable as any. Those of the puss-moth (Cerura vinula), the privet hawk-moth (Sphinx ligustri), the small tortoise-shell butterfly (Vanessa urticce}, the meadow- brown butterfly (Hipparchia janira), the brimstone-moth (Rumia cratcegata), and the silk- worm (Bombyxmori\ may be particularly specified ; and from other orders, those of the cockroach (Blatta orientalis), field cricket (Acheta campestris\ water scorpion (Nepa ranatra), bug (Cimex lectularius), cow- dung fly (Scatophaga ster- coraria), and blow-fly (Musca vomitoria). In order to preserve these eggs, they must be mounted in fluid in a cell ; since they will otherwise dry up and become misshapen. The remarkable mode of reproduction that exists among the Aphides must not pass unnoticed here, from its curious connection with the non- sexual reproduction of Entomostraca (§ 369) and Rotifera (§ 279), as also of Hydra (§ 301) and Zoophytes generally, all of which fall specially, most of them exclusively, under the observation of the Microscopist. The wingless Aphides which may be seen in the spring and early summer, may be considered as larvae or pupse (the earlier states of this insect not being distinguishable from its perfect form, except by their want of wings) ; and these larvse, which, though commonly designated as females, are really of no sex, give origin to a brood of. similar wingless Aphides, which come into the world alive, and which, before long, go through a like process of multiplication. As many as from seven to ten successive broods may thus be produced in the course of a single season ; so that from a single Aphis, it has been calcu- lated that no fewer than ten thousand million millions may be evolved within that period. In the latter part of the year, how- ever, some of these larval Aphides attain their full development into winged males and females, by which the true generative process is performed, whose products are eggs, which, when " Me"moire sur les Objets qui peuvent etre conserves en Preparations Microscopiques" (Paris, 1856), which is peculiarly full in the enumeration of the objects of interest afforded by the class of Insects. PARASITIC ACARIDA. 581 hatched in the succeeding spring, give origin to a new brood of larvae that repeat the curious life-history of their predecessors.1 The non-sexual multiplication of the larvae is obviously a pro- cess of gemmation, analogous to the multiplication of cells by subdivision ; whilst the true Generative process, analogous to the conjugation of cells, is only performed when perfect Aphides of distinct sexes have been evolved. 400. Arachnida.^- The general remarks which have been made in regard to Insects, are equally applicable to this class ; which includes, along with the Spiders and Scorpions, the tribe of Acarida, which consists of the Mites and Ticks. Many of these last are parasitic, and are popularly associated with the wingless parasitic Insects, to which they bear a strong general resem- blance, save in having eight legs instead of six. The true "mites" (Acarince) 'generally have the legs adapted for walking, and some of them are of active habits. The common cheese- mite, as seen by the naked eye, is familiar to every one ; yet few who have not seen it under a microscope, have any idea of its real conformation and movements; and a cluster of them, cut out of the cheese they infest, and placed under a magnifying power sufficiently low to enable a large number to be seen at once, is one of the most amusing objects that can be shown to the young. There are many other species, which closely resem- ble the cheese-mite in structure and habits, but which feed upon different substances ; and some of these are extremely destruc- tive. To this group belongs a small species, the Sarcoptes scabiei, whose presence appears to be the occasion of one of the most disgusting diseases of the skin — the itch, — and which is hence commonly termed the "itch insect." It is not found in the pustule itself, but in a burrow which passes off from one side of it, and which is marked by a red line on the surface ; and if this burrow be carefully examined, the creature will very commonly, but not always; be met with. It is scarcely visible to the naked eye ; but when examined under the' microscope, it is found to have an oval body, a mouth of conical form, and eight feet, of which the four anterior are terminated by small suckers, whilst the four posterior end in very prolonged bristles. The male is only about half the size of the female. The Ricinice or "ticks" are usually destitute of eyes, but have the mouth provided with lancets, that enable them to penetrate more readily the skins of animals whose blood they suck. They are usually of a flattened, round, or oval form ; but they often acquire a very large size by suction, and become distended like a blown bladder. Different species are parasitic upon different animals ; and they bury their suckers (which are often furnished with minute recurved hooks) so firmly in the skins of these, that they can hardly be detached without pulling away the skin with them. It is probably the 1 For a careful examination and philos6phical appreciation of the real nature of this process, see Dr. Waldo J. Burnett's Memoir in the " Transactions of the American Academy of Arts and Sciences." 1853. 582 INSECTS AND ARACHNIDA. young of a species of this group, which is commonly known as the "harvest bug," and which is usually designated as the Acarus autumnalis ; this is very common in the autumn upon grass or other herbage, and insinuates itself into the skin at the roots of the hair, producing a painful irritation; like other Acarida, for some time after their emersion from the egg, it possesses only six legs (the other pair being only acquired after the first moult), so that its resemblance to parasitic insects becomes still stronger. It is probable that to this group also belongs the Demodex fotticulorum, a creature which is very com- monly found parasitic in the sebaceous follicles of the human skin, especially in those of the nose. In order to obtain it, pressure should be made upon any one of these that appears enlarged and whitish, with a terminal black spot; the matter forced out will consist principally of the accumulated sebaceous secretion, having the parasites with their eggs and young min- gled with it. These are to be separated by the addition of oil, which will probably soften the sebaceous matter sufficiently to set free the animals, which may be then removed with a pointed brush; but if this mode should not be effectual, the fatty matter may be dissolved away by digestion in a mixture of alcohol and ether. The pustules in the skin of a dog affected with the " mange" have been found by Mr. Topping to contain a Demo- dex, which seems only to differ from that of the human sebaceous follicles in its somewhat smaller size ; and M. Gruby is said to have given to a dog a disease resembling the mange, if not identical with it, by inoculating it with the human parasite. The Acarida are best preserved as microscopic objects, by mounting in glycerine. 401. The number of objects of general interest, furnished to the Microscopist by the Spider tribe, is by no means considera- ble. Their eyes exhibit a condition intermediate between that of Insects and Crustaceans, and that of Vertebrata ; for they are single like the " stemmata" of the former, usually number from six to eight, are sometimes clustered together in one mass, but are sometimes disposed separately, while they present a decided approach in internal structure to the type characteristic of the visual organs of the latter. The structure of the Mouth is always mandibulate, and is less complicated than that of the mandibulate insects. The Respiratory apparatus, which, where developed at all among the Acarida, is tracheary like that of Insects, is here constructed upon a very different plan ; for the "stigmata," which are usually four in number on each side, open into a like number of respiratory sacculi, each of which contains a series of leaf-like folds of its lining membrane, upon which the blood is distributed so as to afford a large surface to the air. In the structure of the limbs, the principal point worthy of notice is the peculiar appendage with which they usually terminate ; for the strong claws, with a pair of which SPINNING APPARATUS OF SPIDERS. 583 Foot, with comb-like claws, of the commoi (Epeira). Spider FIG. 298. the last joint of the foot is furnished, have their edges cut into comb- like teeth (Fig. 297), which seem to be used by FlG- 297- the animal as cleansing instruments. One of the most curious parts of the organization of the Spiders is the " spinning appara- tus," by means of which they fabricate their elabo- rately constructed webs. This consists of the " spin- nerets," and of the glandu- lar organs in which the fluid that hardens into the thread is elaborated. The usual number of the spinnerets, which are situated at the poste- rior extremity of the body, is six ; they are little teat-like promi- nences, beset with hairy appendages ; and it is through a certain set of these appendages, which are tubular and terminate in fine-drawn points, that the glutinous secretion is forced out in a multitude of streams of extreme minuteness. These streams harden into fibrils, immediately on coming into contact with the air; and the fibrils proceeding from all the apertures of each spinneret, coalesce into a single thread. It is doubtful, however, whether all the spinnerets are in action at once, or whether those of different pairs may not have dissimilar functions; for whilst the radiating threads of a spider's web are simple (Fig. 298, A), those which lie across these, forming its concentric circles or rather poly- gons, are studded at intervals with viscid globules (B), which appear to give to these threads their peculiarly adhesive character ; and it does not seem by any means unlikely, that each kind of thread should be produced by its own pair of spinnerets. The total number of spinning tubes varies greatly, according to the species of the- spider, and the sex and age of the individual; being more than 1000 in some cases, and less than 100 in others. The size and complexity of the secreting glandulse vary in like manner: thus in the Spiders which are most remarkable for the large dimensions and regular construction of their webs, they occupy a large portion of the abdominal cavity, and are composed of slender branching tubes, whose length is in- creased by numerous convolutions; whilst in those which have only occasional use for their threads, the secreting organs are either short and simple follicles, or are undivided tubes of mode- rate length. Ordinary thread (A), and gluti- nous thread (B), of the common CHAPTER XVIII. VERTEBRATED ANIMALS. 402. WE are now arrived at that highest division of the Ani- mal Kingdom, in which the bodily fabric attains its greatest development, not only as to completeness, but also as to size ; and it is in most striking contrast with the class we have been last considering. Since not only the entire bodies of Verte- brated animals, but, generally speaking, the smallest of their integral parts, are far too large to be viewed as Microscopic objects, we can study their structure only by a separate exami- nation of their component elements ; and it seems, therefore, to be a most appropriate course, to give, under this head, a sketch of the microscopic characters of those primary tissues, of which their fabric is made up, and which, although they may be traced with more or less distinctness in the lower tribes of Animals, attain their most complete development in this group. Since the time when Schwann first made public the remarkable results of his researches (p. 56), it has been very generally believed that all the Animal tissues are formed, like those of Plants, by a metamorphosis of Cells; an exception being taken, however, by some Physiologists, in regard to the simple fibrous tissues (§ 417). The tendency of many recent investigations, however, has been to throw further doubt on the generality of this doctrine ; since they appear to indicate that many other tissues than the fibrous may be formed (like these) by the consolidation of the plasma or formative fluid, without passing through the intermediate condi- tion of cells. Hence no attempt will here be made to do more than describe the most important of those distinctive characters, which the principal tissues present, when subjected to micro- scopic examination ; and as it is of no essential consequence what order is adopted, we may conveniently begin with the structure of the skeleton,1 which gives support and protection to the softer parts of the fabric. 403. Bone. — The Microscopic characters of osseous tissue may 1 This term is used in its most general sense, as including not only the proper vertebral or internal skeleton, but also the hard parts protecting the exterior of the body, which forms the dermal skeleton. STRUCTURE OF BONE. 585 sometimes be seen in very thin natural plates of bone, sucb as in that forming the scapula (shoulder-blade) of a Mouse ; but they are displayed more perfectly by artificial sections, the details of the arrangement being dependent upon the nature of the speci- men selected, and the direction in which the section is made. Thus when the shaft of a "long" bone of a Bird or Mammal is cut across in the middle of its length, we find it to consist of a hollow cylinder of dense bone, surrounding a cavity which is occupied by an oily marrow ; but if the section be made nearer its extremity, we find the outside wall gradually becoming thinner, whilst the interior, instead of forming one large cavity, is divided into a vast number of small chambers or cancelli, which com- municate with each other and with the cavity of the FIG. 299. shaft, and are filled, like it, with marrow. In the bones of Eeptiles and Fishes, on the other hand, this " cancel- lated" structure usually ex- tends throughout the shaft, which is not so completely differentiated into solid bone and medullary cavity, as it is in the higher Vertebrata. In the most developed kinds of "flat" bones, again, such as those of the head, we find the two surfaces to be com- posed of dense plates of bone, with a " cancellated" struc- ture between them ; whilst in the less perfect type pre- sented to us in the lower Vertebrata, the whole thick- ness is usually more or less " cancellated," that is, burrowed out by medullary cavities. When we examine, under a low magnifying power> a longitu- dinal section of a ulong" bone, or a section of a "flat" bone parallel to its surface, we find it traversed by numerous canals, termed Haversian after their discoverer Havers, which are in connection with the central cavity, and are filled, like it, with marrow: in the shafts of long bones, these canals usually run in the direction of their length, but are connected here and there by cross branches ; whilst in the flat bones, they form an irregu- lar network. On applying a higher magnifying power to a thin transverse section of a long bone, we observe that each of the canals whose orifices present themselves in the field of view (Fig. 299), is the centre of a rod of bony tissue (1), usually more or less circular in its form, which is arranged around it in concentric Minute structure of Bone, as seen in transverse sec- tion : — 1, an ossicle surrounding an Haversian canal, 3, showing the concentric arrangement of the lamellae; 2, the same, with the lacunae and canaliculi ; 4, por- tions of the lamella? parallel with the external surface. 586 VERTEBRATED ANIMALS. rings, resembling those of an Exogenous Stem. These rings are marked out and divided by circles of little dark spots ; which, when closely examined (2), are seen to be minute flattened cavities excavated in the solid substance of the bone, from the two flattened sides of which pass forth a number of extremely minute tubules, one set extending inwards, or in the direction of the centre of the system of rings, and the other outwards, or in the direction of its circumference; and by the inosculation of the tubules (which are termed canaliculi) of the different rings with each other, a continuous communication is established between the central Haversian canal and the outermost part of the bony rod that surrounds it, which doubtless ministers to the nutrition ot the texture. Bloodvessels are traceable into the Haversian canals; but the "canaliculi," being far too minute to carry blood-corpuscles, can only convey a nutrient fluid that is sepa- rated from the blood for the special service of the bone. 404. The minute cavities, or lacunce (sometimes, but erro- neously termed " bone-corpus- cles," as if they were solid bodies), from which the canali- culi proceed, are highly charac- teristic of the true osseous struc- ture ; being never deficient in minutest parts of the bones the hi°*her Lacunce of Osseous suhstanoe: — a, central cavity. Q£ 6, its ramifications. , •, i j/i r> TTV i though those of Fishes are occa- sionally destitute of them. The dark appearance which they present is not due to opacity, but is simply an optical effect, dependent (like the blackness of air-bubbles in" liquids) upon the dispersion of the rays by the highly-refracting substance that surrounds them (§ 98). The size and form of the lacunae differ considerably in the several Classes of Vertebrata, and even in some instances in the Orders ; so as to allow of the determination of the tribe to which a bone belonged, by the microscopic exami- nation of even a minute fragment of it (§ 453). The following are the average dimensions of the lacunae, in characteristic ex- amples drawn from the four principal classes, expressed in fractions of an inch : — Long: Diameter. Short Diameter. Man, ..... 1-1440 to 1-2400 1-4000 to 1-8000 Ostrich, ..... 1-1333 to 1-2250 1-5425 to 1-9650 Turtle, ..... 1-375 to 1-1150 1-4500 to 1-5840 Conger-eel, .... 1-550 to 1-1135 1-4500 to 1-8000 The lacunae of Birds are thus distinguished from those of Mam- mals by their somewhat greater length and smaller breadth ; but they differ still more in the remarkable tortuosity of the canali- culi, which wind ^backwards and forwards in a very irregular manner. There is an extraordinary increase in length in the STRUCTURE OF BONE. 587 FIG. 301. lacunae of Reptiles, without a corresponding increase in breadth ; and this is also seen in some Fishes, though in general the lacunae of the latter are remarkable for their angularity of form, and the fewness of their radiations, — as shown in Fig. 301, which repre- sents the lacunae and ca- naliculi in the bony scale of the Lepidosteus ("bony pike" of the North Ame- rican lakes and rivers), with which the bones of its internal skeleton per- fectly agree in structure. The dimensions of the lacunae in any bone do not bear any relation to ,i • f 4-lrt ' 1 f Section of the bony scale of Lepidosteus: — a, showing the tne animal tO reguiar distribution of the lacuna? and of the connecting Which it belonged ; thuS canaliculi; b, small portion more highly magnified. there is little or no per- ceptible difference between their size in the enormous extinct Iguanodon, and in the smallest Lizard now inhabiting the earth. But they bear a close relation to the size of the blood-corpuscles in the several classes; and this relation is particularly obvious in the "perennibranchiate" Batrachia, the extraordinary size of whose blood-corpuscles will be presently noticed (§ 414) : — Proteus, Siren, Menopoma, Lepidosiren, Pterodactyle, 405. In preparing sections of bone, it is important to avoid the penetration of the Canada balsam into the interior of the lacunae and canaliculi ; since, when these are filled by it, they become almost invisible. Hence it is preferable not to employ this ce- ment at all, except, it may be, in the first instance ; but to rub down the section beneath the finger, guarding its surface with a slice of cork or a slip of guttapercha (§ 111) ; and to give it such a polish, that it may be seen to advantage even when mounted dry. As the polishing, however, occupies much time, the benefit which is derived from covering the surfaces of the specimen with Canada balsam may be obtained, without the injury resulting from the penetration of the balsam into its interior, by adopting the following method. A quantity of balsam proportioned to the size of the specimen is to be spread upon a glass slip, and to be 1 See Prof. J. Quekett's Memoir on this subject, in the " Transact, of the Microsc. Soc.'' Ser. 1, vol. ii; and his more ample illustration of it in the "Illustrated Catalogue of the Histological Collection in the Museum of the Roy. Coll. of Surgeons," vol. ii. Long Diameter. Short Diameter. 1-570 to -980 1-885 to 1-1200 1-290 to -480 1-540 to 1-975 1-450 to -700 1-1300 to 1-2100 1-375 to -494 1-980 to 1-2200 1-445 to -1185 1-4000 to 1-52251 588 VERTEBRATED ANIMALS. rendered stiffer by boiling, until it becomes nearly solid when cold ; the same is to be done to the thin glass cover ; next, the specimen being placed on the balsamed surface of the slide, and being overlaid by the balsamed cover, such a degree of warmth is to be applied, as will suffice to liquefy the balsam, without causing it to flow freely; and the glass cover is then to be quickly pressed down, and the slide to be rapidly cooled, so as to give as little time as possible for the penetration of the liquefied balsam into the lacunar system. The same method may be employed in making sections of Teeth. 406. Teeth. — The intimate structure of the Teeth in the several classes and orders of Yertebrata presents differences which are no less remarkable than those of their external form, arrangement and succession. It will obviously be impossible here to do more than sketch some of the most important of these varieties. The principal part of the substance of all teeth is made up of a solid tissue that has been appropriately termed Dentine. In the Shark tribe, as in many other Fishes, the general structure of this "dentine" is extremely analogous to that of bone; the tooth being traversed by numerous canals, which are continuous with the Haversian canals of the subjacent bone, and receive bloodvessels from them (Fig. 302) ; and each of these canals being surrounded by a system of tubuli (Fig. FIG. 302. FIG. 303. Fig. 302. Perpendicular section of tooth of Lamna, moderately enlarged, showing network of medullary canals. Fig. 303. Transverse section of portion of tooth of Pristis, more highly magnified, showing orifices of medullary canals, with systems of radiating and inosculating tubuli. 303), which radiate into the surrounding solid substance. These tubuli, however, do not enter lacunae, nor is there any concentric annular arrangement arouijd the medullary canals; but each system of tubuli is continued onwards through its own division. STRUCTURE OF TEETH. 589 of the tooth, the individual tubes sometimes giving off lateral branches, whilst in other in- stances their trunks bifurcate. FIG. 304. This arrangement is pecu- liarly well displayed, when sections of teeth constructed upon this type are viewed as opaque objects (Fig. 304). In the teeth of the higher Ver- tebrata, however, we usually find the centre excavated into a single cavity (Fig. 305), and the remainder destitute of vascular canals ; bat there are intermediate cases (as in the teeth of the ^reat fossil Sloths) 1 • i ,i • ,• n Iransverse Section of Tooth of Myliobates (Eagle in which the inner portion ot Ray) viewed as an opaque object. the dentine is traversed by prolongations of this cavity, conveying bloodvessels, which do not pass into the exterior layers. The tubuli of the "non-vascular" dentine, which exists by itself in the teeth of nearly all Mammalia, and which in the Elephant is known as "ivory," all radiate from the central cavity, and pass towards the surface of the tooth in a nearly parallel course. Their diameter at their largest part averages l-10,000th of an inch; their smallest branches are im- measurably fine. It is impossible that even the largest of them can receive blood, as their diameter is, far less than that of the blood-discs; but it is probable that, like the canaliculi of bone, they may absorb nutrient matter from, the vascular surface upon which their inner extremities open. The tubuli in their course present greater and lesser undulations ; the former are few in number ; but the latter are numerous, and as they occur at the same part of the course of several contiguous tubes, they give rise to the appearance of lines concentric with the centre ot radiation. These secondary curvatures probably indicate, in dentine, as in the crab's shell (§ 374) successive stages of calcifi- cation. 407. In the teeth of Man and most other Mammals, and in those of many Reptiles and some Fishes, we find two other substances, one of them harder, and the other softer, than den- tine ; the former is termed Enamel ; and the latter Cementum or Crusta Petrosa. The Enamel is composed of long prismatic cells, closely resembling those of the prismatic shell-substance for- merly described (f 336), but on a far more minute scale ; the diameter of the cells not being more, in Man, than l-5600th of an inch. The length of the prisms corresponds with the thickness of the layer of enamel ; and the two surfaces of this layer present the ends of the prisms, the form of which usually approaches the hexagonal. The course of the enamel-prisms is 590 VERTEBRATED ANIMALS. FIG. 305. Vertical section of Human Molar Tooth :— 1, enamel ; 2, cernentum or crusta peirosa ; 3, dentine or ivory; 4, osse- ous excrescence, arising from hypertrophy of cemen- tum; 6, pulp-cavity ; 6, osse- ous lacunae at outer part of dentine. more or less wavy ; and they are marked by numerous trans- verse striae, resembling those of the pris- matic shell-substance, and probably origi- nating in the same cause, — the coalescence of a series of shorter cells, to form the lengthened prism. In Man, and in Car- nivorous animals, the enamel covers the crown of the tooth only, with a simple cap or superficial layer of tolerably uniform thickness (Fig. 305, a), which follows the surface of the dentine in all its inequalities ; and its component prisms are directed at right angles to that surface, their inner extremities resting in slight but regular depressions on the exterior of the dentine. In the teeth of many of the Herbivorous animals, however, the Enamel forms (with the Cementum) a series of vertical plates, which dip down into the substance of the dentine, and present their edges alternately with it, at the grinding surface of the tooth ; and there is in such teeth no continuous layer of enamel over the crown. The pur- pose of this arrangement is evidently to provide, by the unequal wear of these three substances, — of which the enamel is the hardest, and the cementum the softest, — for the constant maintenance of a rough surface, adapted to triturate the tough vegetable substances on which these animals feed. The enamel is the least constant of the dental tissues. It is more frequently absent than present in the teeth of the class of Fishes ; it is wanting in the entire order of Ophidia (serpents) among existing Reptiles ; and it forms no part of the teeth of the Edentata (sloths, &c.) and Cetacea (whales) amongst Mammals. The Cementum, or Crusta Petrosa, has the characters of true bone ; possessing its distinctive stellate lacunae and radiating canaliculi. Where it exists in small amount, we do not find it traversed by medullary canals ; but, like dentine, it is occasionally furnished with them, and thus resembles bone in every particular. These medullary canals enter its substance from the exterior of the tooth, and conse- quently pass towards those which radiate from the central cavity in the direction of the surface of the dentine, where this pos- sesses a similar vascularity, — as was remarkably the case in the teeth of the extinct Megatherium. In the Human tooth, how- ever, the cementum has no such vascularity ; but forms a thin layer, which envelopes the root of the tooth, commencing near the termination of the capping of enamel (Fig. 305, b). In the teeth of many Herbivorous Mammals, it dips down with the enamel to form the vertical plates of the interior of the tooth ; SCALES OF FISHES. 591 and in the teeth of the Edentata, as well as of many Reptiles and Fishes, it forms a thick continuous envelope over the whole surface, until worn away at the crown. 408. Dermal Skeleton. — The skin of Fishes, of most Reptiles, and of a few Mammals, is strengthened hy plates of a horny, cartilaginous, bony, or even enamel-like texture, which are some- times fitted together at their edges, so as to form a continuous box- like envelope, whilst more commonly they are so arranged as partially to overlie one another, like the tiles on a roof; and it is in this latter case that they are usually known as scales. Al- though we are accustomed to associate in our minds the u scales" of Fishes with those of Reptiles, yet they are essentially different structures ; the former being developed in the substance of the true skin, with a layer of which, in addition to the epidermis, they are always covered; and bearing a resemblance to cartilage and bone in their texture and composition ; whilst the latter are formed upon the surface of the true skin, and are to be con- sidered as analogous to nails, hoofs, &c., and other " epidermic appendages." In nearly all the existing Fishes, the scales are flexible, being but little consolidated by calcareous deposit; and in some species they are so thin and transparent, that, as they do not project obliquely from the surface of the skin, they can only be detected by raising the superficial layer of the skin, and searching beneath it, or by tearing off the entire thickness of the skin, and looking for them near its under surface. This is the case, for example, with the common Eel, and with the Viviparous Blenny ; of either of which fish, the skin is a very interesting object when dried and mounted in Canada balsam, the scales being seen imbedded in its substance, whilst its outer surface is studded with pigment-cells. Generally speaking, how- ever, the posterior extremity of each scale projects obliquely from the general surface, carrying before it the thin membrane that encloses it, which is studded with pigment-cells ; and a por- tion of the skin of almost any Fish, but especially of such as have scales of the ctenoid kind (that is, furnished at their posterior extremities with comb-like teeth, Fig. 307), when dried with its scales in situ, is a very beautiful opaque object for the low powers of the Microscope (Fig. 306). Care must be taken, how- ever, that the light is made to glance upon it in the most ad- vantageous manner ; since the brilliancy with which it is reflected from the comb-like projections, entirely depends upon the angle at which it falls upon them. The only appearance of structure exhibited by the thin flat scale of the Eel, when examined micro- scopically, is the presence of a layer of isolated spheroidal trans- parent bodies, imbedded in a plate of like transparency ; these, from the researches of Prof. Williamson upon other scales, ap- pear not to be cells (as they might readily be supposed to be), but to be concretions of carbonate of lime. When the scale of the Eel is examined by polarized light, its surface exhibits a 592 VERTEBEATED ANIMALS. beautiful St. Andrew's cross ; and if a plate of selenite be placed behind it, and the analyzing prism be made to revolve, a re- markable play of colors is presented. 409. In studying the structure of the more highly-developed scales, we may take as an illustration that of the Carp ; in .which two very distinct layers can be made out by a vertical section, with a third but incomplete layer interposed between them. The outer layer is composed of several concentric laminae of a structureless trans- FIG. 307. parent substance, like that of cartilage ; the outermost of these laminae is the small- est, and the size of the plates increases progressively from without inwards, so FIG. 306. Portion of Skin of Sole, viewed as an opaque object. of Sole, viewed as a transparent object. that their margins appear on the surface as a series of concentric lines ; and their surfaces are thrown into ridges and furrows, which commonly have a radiating direction. The inner layer is composed of numerous laminae of a fibrous structure, the fibres of each lamina being inclined at various angles to those of the lamina above and below it. Between these two layers is inter- posed a stratum of calcareous concretions, resembling those of the scale of the Eel; these are sometimes globular or spheroidal, but more commonly " lenticular," that is, having the form of a double convex lens. The scales which resemble those of the Carp in having a form more or less circular, and in being desti- tute of comb-like prolongations, are called cycloid; and such are the characters of those of the salmon, herring, roach, &c. The structure of the " ctenoid" scales (Fig. 307), which we find in the sole, perch, pike, &c., does not differ essentially from that of the " cycloid," save as to the projection of the comb-like teeth from the posterior margin; and it does not appear that the strongly-marked division which Prof. Agassiz has attempted to establish between the "cycloid" arid the "ctenoid" orders of fishes, on the basis of this difference, is in harmony with their general organization. Scales of either kind may become con- solidated to a considerable extent, by the calcification of their soft substance; but still they never present any approach to the SCALES OF FISHES. 593 true bony structure, such as is shown in the two orders to be next adverted to. In the ganoid scales, on the other hand, the whole substance of the scale is composed of a substance which is essentially bony in its nature; its intimate structure being almost always comparable to that of one or other of the varieties which present themselves in the bones of the Vertebrate skele- ton ; and being very frequently identical with that of the bones of the same fish, as is the case with the Lepidosteus (Fig. 301), one of the few existing representatives of this order, which, in former ages of the Earth's history, comprehended a large num- ber of important families. Their name (from r covered by irabricated scales * i M , ^ -i P-, or flattened cells. IOr Whilst the lOWer part OI it re- Fig. 309. Hair of Musk-deer, consisting almost sembles the fine hair of the mouse entirely of polygonal ceiis. 1 xi • j.1 • J Fig. 310. A, Small Hair of Squirrel; B, Large or squirrel, this thins away and Hairof^imrd; c, H™ of Indian £at. then dilates again into a very thick fibre, having a central portion composed of polygonal cells, enclosed in a flattened sheath of a brown fibrous substance. The structure of the Human hair is in certain respects peculiar. When its outer surface is examined, it is seen to be traversed by irregular lines (Fig. 312, A), which are most strongly marked in foetal hairs ; and these are the indications of the imbricated ar- rangement of the flattened cells or scales which form the cortical layer. This layer, as is shown by transverse sections (c, D), is a very thin and transparent cy- linder; and it incloses the peculiar fibrous substance, that constitutes the principal part of the shaft of the hair. The constituent fibres of this substance, which Fig. 308. Hair of Suble, showing large rounded FIG. 311. Transverse Section of Hair of Pecari. 596 VERTEBRATED ANIMALS. are marked out by the delicate striae that maybe traced in longi- tudinal sections of the hair (B), maybe separated from each other by crushing the hair, especially after it has been macerated for some time in sulphuric acid; and each of them, when completely separated from its fellows, is found to be a long spindle-shaped Structure of Human Hair: — A. external surface of the shaft, showing the transverse striae and jagged boundary caused by the imbrications of the cortical substance ; B, longitudinal section of the shaft, showing the fibrous character of the medullary substance, and the arrangement of the pig- mentary matter; c, transverse section, showing the distinction between the transparent envelope, the cylinder of medullary substance, and the cellular centre; D, another transverse section showing deficiency of central cellular substance. cell. In the axis of this fibrous cylinder, there is very com- monly a band which is formed of spheroidal cells ; but this is usually deficient in the fine hairs scattered over the general sur- face of the body, and is not always present in those of the head.1 The hue of the hair is due, partly to the presence of pigmentary granules, either collected into patches, or diffused through its substance ; but partly also to the existence of a multitude of minute air-spaces, which cause it to appear as dark by transmitted and white by reflected light. The cells of the axis-band, in par- ticular, are very commonly found to contain air, giving it the black appearance shown at c. The difference between the black- ness of pigment and that of air-spaces, maybe readily determined by attending to the characters of the latter as already laid down (§§ 98, 99) ; and by watching the effects of the penetration of oil of turpentine or other liquids, which do not alter the appear- ance of pigment spots, but obliterate all the markings produced by air-spaces, these returning again as the hair dries. In mount- ing hairs as microscopic preparations, they should in the first instance be cleansed of all their fatty matter by maceration in ether ; and they may then be put up, either in weak spirit or in 1 Several writers regard this band of polygonal cells a's the "medullary" substance, and the fibrous structure which forms the principal body of the hair, as the "cortical" substance; the transparent sheath receiving some separate designation. To the Author, however, it appears perfectly clear that the transparent horny sheath, with its lines of imbrication, is the representative of the cortical substance of other hairs ; and that its entire contents, whether polygonal cells or cells elongated into fusiform fibres, must be considered as equivalent to their medullary substance. STRUCTURE OF FEATHERS. 597 Canada balsam, as may be thought preferable, the former men- struum being well adapted to display the characters of the finer and more transparent hairs, while the latter allows the light to penetrate more readily through the coarser and more opaque. Transverse sections of hairs are best made by gluing or gumming several together, and then putting them into the section-instru- ment ; those of human hair may be easily obtained, however, by shaving a second time, very closely, a part of the surface over which the razor has already passed more lightly, and by picking out from the lather, and carefully washing, the sections thus taken off. 412. The stems of Feathers exhibit the same kind of structure as hairs ; their cortical portion being the horny sheath that en- velopes the shaft, and their medullary portion being their pith- like substance which that sheath includes. In small feathers, this may usually be made very plain by mounting them in Canada balsam ; in large feathers, however, the texture is some- times so altered by the drying up of the pith (the cells of which are always found to be occupied by air alone), that the cellular structure cannot be demonstrated, save by boiling thin slices in a dilute solution of potass, and not always even then. In small feathers, especially such as have a downy character, the cellular structure is very distinctly seen in the lamince or "barbs," which are sometimes found to be composed of single files or. pear- shaped cells, laid end to end; but in larger feathers, it is usually necessary to increase the transparency of the barbs, especially when these are thick and little pervious to light, either by soak- ing them in turpentine, mounting them in Canada balsam, or boiling them in a weak solution of potass. In the feathers which are destined to strike the air with great force in the act of flight, we find the barbs fringed on each side with hair-like fila- ments or pinnoe ; on one side of each barb these filaments are toothed on one edge, whilst on the other side they are furnished with curved hooks ; and as the two sets of pinnae which spring from two adjacent barbs, cross one another at an angle, and each hooked pinna on one locks into the teeth of several of the toothed pinnae arising from the other, the barbs are connected together very firmly by this apparatus of " hooks and eyes," which reminds us of that already mentioned as to be observed on the wings of Hymenopterous Insects (§ 395). Feathers or portions of feathers of birds distinguished by the splendor of their plumage, are very good objects for low magnifying powers, when illuminated on an opaque ground; but care must betaken that the light falls upon them at the angle necessary to produce their most brilliant reflection into the axis of the microscope ; since feathers which exhibit the most brilliant metallic lustre to an observer at one point, may seem very dull to the eye of another in a different position. The small feathers of Humming birds, portions of the feathers of the Peacock, and others of a like kind, are well worthy of an examination; and the scientific 598 VERTEBKATED ANIMALS. Microscopist who is but little attracted by mere gorgeousness, may well apply himself to the discovery of the peculiar struc- ture, which imparts to these objects their most remarkable cha- racter. 413. Sections of Horns, Hoofs, Claivs, and other like modifica- tions of Epidermic structure, — which may be made by the Section instrument (§ 107), the substance to be cut having been softened, if necessary, by soaking in warm water, — do not in general afford any very interesting features, when viewed in the ordinary mode; but there are no objects on which Polarized light produces more remarkable effects, or which display a more beautiful variety of colors, when a plate of selenite is placed be- hind them, and the analyzing prism is made to rotate. A curious modification of the ordinary structure of horn, is pre- sented in the appendage borne by the Rhinoceros upon its snout, which in many points resembles a bundle of hairs, its substance being arranged in minute cylinders around a number of separate centres, which have probably been formed by independent papillae (Fig. 313). When transverse sections of these cylinders are viewed by polarized light, each of them is seen to be marked by a cross, somewhat resembling that of starch-grains ; and the lights and shadows of this cross are replaced by contrasted colors, when the selenite-plate is interposed. The substance commonly but erroneously termed Whalebone, which is formed from the surface of the membrane that lines the mouth of the whale, and has no relation* to its true bony skeleton, is almost identical in structure with Khinoceros-horn, and is similarly affected by polarized light. The central portion of each of its component fibres, like the medullary substance of hairs, contains cells that have been so little altered as to be easily recognized ; and the outer or cortical portion also may be shown to have a like structure, by macerating it in a solution of potass, and then in water. Sections of any of the horny tissues are best mounted in Canada balsam. 414. Blood. — Carrying our Microscopic survey, now, to the elementary parts of which those softer tissues are made up, that are subservient to the active life of the body, rather than to its merely me- chanical requirements, we Transverse Section of Horn of RUnoceros, viewed shall in the first place notice by Polarized Light. the isolated floating cells contained in the Blood, and known as the " blood-corpuscles." These are of two kinds; the Fm. 313. m RED CORPUSCLES or BLOOD. 599 "red," and the "white" or "colorless." The former present, in every instance, the form of a flattened disk, which is circular in Man and in most Mammalia (Fig. 315), but which is oval in Birds, Reptiles (Fig. 314), and Fishes, and in a few Mammals (all belonging to the Camel tribe) ; in the one form as in the other, this disk is a flattened cell, whose walls are pellucid and colorless, but whose contents are colored. They may be caused to swell up and burst, however, by the imbibition of water ; and the perfect transparency and the homogeneous character of their walls then become evident. The "red corpuscles" in the blood of Oviparous Yertebrata are distinguished by the presence of a distinct central spot or nucleus, which appears to be composed of an aggregation of minute granules ; this is most distinctly brought into view by treating the blood-disks with acetic acid, which renders the remaining portion extremely transparent, while it increases the opacity of the nucleus (Fig. 314, d). It is remark- able, however, that the "red corpuscles" of the blood of Mam- mals should possess no obvious nucleus ; the dark spot which is seen in their centre (Fig. 315, 6), being merely an effect of refrac- FIG. 3J4. Fig. 314. Red Corpuscles of Frog's Blood:—!, 1, their flattened face; 2, particle turned nearly edgeways; 3, colorless corpuscle ; 4, red corpuscles altered by dilute acetic acid. Fig. 315. Red Corpuscles of Human Blood: represented at «, as they are seen when rather within the focus of the microscope, and at 6, as they appear when precisely in the focus. tion, consequent upon the double concave form of the disk. When the corpuscles are treated with water, so that their form becomes first flat, and then double convex, ihe dark spot disappears; whilst, -on the other hand, it is made more evident when the concavity is increased by the partial emptying of the cell, which may be accomplished by treating the blood-corpuscles with fluids of greater density than their own contents. The size of the " red corpuscles" is not altogether uniform in the same blood; thus it varies in that of Man, from about l-4000th to the l-2800th of an inch. But we generally find that there is an average size, which is pretty constantly maintained among the different individuals of the same species ; that of Man may be stated at about l-3200th of an inch. The following Table1 exhibits the average dimen- 1 These measurements are chiefly selected from those given by Mr. Gulliver in his edition of Hewson's Works, p. 236, et seq. 600 VERTEBRATED ANIMALS. sions of some of the most interesting examples of the Red blood- corpuscles, in the four classes of Yertebrated Animals, expressed in fractions of an inch. Where two measurements are given, they are the long and the short diameters of the same corpuscle. MAMMALS. Man, 1-3200 Dog, 1-3542 Whale, 1-3099 Elephant, 1-2745 Mouse, 1-3814 Camel, 1-3254, 1-5921 Llama, 1-3361, 1-6294 Java Musk-Deer, . . . .1-12325 Caucasian Goat, 1-7045 Two-toed Sloth, . . 1-2865 BIRDS. Golden Eagle, Owl, Crow, 1-1961, 1-4000 Blue-Tit, . . . . 1-2313, 1-4128 Parrot, .... 1-1812, 1-3832 | Ostrich, 1-1649, 1-1830, 1-3400 j Cassowary, . . . . 1-1455, Heron, ." . . . . 1-1913, Fowl, 1-2102, M898J 1-4000 j Gull, 1-2097,1-4000 1-3000 1-2800 1-3491 1-3466 REPTILES. Turtle, 1-1231, 1-1882 Crocodile, .... 1-1231, 1-2286 Green Lizard, . . . 1-1555, 1-2743 Slow-worm, . . . 1-1178, 1-2666 Viper, 1-1274, 1-1800 Frog, . . Water-Newt, Siren, . . Proteus, Lepidosiren, 1-1108, 1-1821 1-814, 1-1246 1-420, 1-760 1-337 1-570, 1-941 FISHES. Perch, 1-2099, Carp, 1-2142, Gold-Fish, 1-2824 1-3429 1-1777, 1-2824 Pike, 1-2000, 1-3555 Eel, 1-1745, 1-2842 Gymnotus, 1-1745, 1-2599 Thus it appears that the smallest red corpuscles known are those of the Musk-Deer ; whilst the largest are those of that curious group of Batrachian (frog-like) Reptiles which retain their gills through the whole of life ; and the oval blood-disks of the Proteus, being above 36 times as long as those of the Musk-Deer, and pro- bably at least 20 times as broad, would cover no fewer than 720 of them. 415. The "colorless" corpuscles are more readily distinguished in the blood of Reptiles, than in that of Man ; being, in the former case, of much smaller size, as well as having a circular outline (Fig. 314, c) ; whilst in the latter, their size and contour are nearly the same, so that, as the red corpuscles themselves, when seen in a single layer, have but a very pale hue, -the de- ficiency of color does not sensibly mark their difference of nature. It is remarkable that, notwithstanding the great variations in the sizes of the red corpuscles in different species of Vertebrate d ani- mals, the size of the "colorless" is extremely constant through- out, their diameter being seldom much greater or less than l-3000th of an inch in the warm-blooded classes, and l-2500th in Reptiles. Their ordinary form is globular; but their aspect is subject to considerable variations, which seem to depend in great part upon their phase of development. Thus in their early state, in which they seem to be identical with the corpuscles found floating in Chyle and Lymph , the cell-wall can scarcely be dis- COLORLESS CORPUSCLES OF BLOOD. 601 tinguished from the large nuclear mass which it incloses; by treating the cell with water or acetic acid, however, the mem- brane is distended, and the nucleus very commonly breaks up into fragments in its interior. This last appearance seems natural to the corpuscles in a more advanced condition ; and the isolated particles are often to be seen executing an active molecular movement within the cell, which continues when they are dis- charged by the bursting of the cell, consequent upon the addition of a solution of potass. These corpuscles are occasionally seen to exhibit very curious changes of form, which remind us of those of the Amoeba (§ 261) ; a protrusion taking place from the same portion of the cell-wall, the form of which seems quite indetermi- nate ; and this being soon succeeded by another, from some dif- ferent part of the cell, the first being either drawn in again, or remaining as it was. These changes have been observed, not only in the " colorless corpuscles" of the blood of various Verte- brated animals, but also in the corpuscles floating in the circu- lating fluid of the higher Invertebrata, such as the Crab, which resemble the "colorless" corpuscles of Vertebrated blood rather than its " red" corpuscles, — these last, in fact, being altogether peculiar to the circulating fluid of Vertebrated animals. 416. In examining the Blood microscopically, it is, of course, of importance to obtain as thin a stratum of it as possible, so that the corpuscles may not overlie one another. This is best accomplished by selecting a piece of thin glass of perfect flat- ness, and then, having received a small drop of blood upon a glass slide, to lay the thin glass, not upon this, but with its edge just touching the edge of the drop ; for the blood will then be drawn in by capillary attraction, so as to spread in a uniformly thin layer between the two glasses. The inexperienced observer will be surprised at the very pale hue which the red corpuscles exhibit beneath the microscope, when seen in a single stratum; but this surprise need no longer be felt, when it is borne in mind that the thickness of the film of coloring fluid which they contain, is probably not more than l-20,000th of an inch ; and if a drop of ink, or of almost any colored liquid, however dark, be pressed out between two glasses into an equally thin film, its hue will be lightened in the same degree. The red hue of the corpuscles, however, becomes . obvious enough, when two or more layers of them are seen through at once. The " colorless corpuscles" in Human blood are usually not more than 1-350 of the " red," so that no more than one or two are likely to be in the field at once ; and these may generally be recognized most readily, by their standing apart from the rest ; for whilst the "red" corpuscles have a tendency to adhere to each other by their discoidal surfaces, the " colorless" show no such disposition. Thin films of blood may be preserved in the liquid state, with little change, by applying gold size or asphalte round the edge of the thin glass cover before evaporation has had time to take 602 VERTEBRA TED ANIMALS. place ; but it is in some respects preferable to dilute the liquid with a small quantity of Goadby's solution, its strength being so adjusted as not to produce any endosmotic change of form in the corpuscles. But it is far simpler to allow such films to dry, with- out any cover, and then merely to cover them for protection ; and in this condition the general characters of the corpuscles can be very well made out, notwithstanding that they have in some degree shrivelled by the desiccation they have undergone. And this method is particularly serviceable, as affording a fair means of comparison, when the assistance of the Microscopist is sought in determining, for Medico-legal purposes, the source of sus- picious blood stains; the average dimensions of the dried blood- corpuscles of the several domestic animals, being sufficiently different from each other and from those of Man, to allow the nature of any specimen to be pronounced upon with a high de- gree of probability. 417. Simple Fibrous Tissues. — A large proportion of every animal fabric is made up of simple fibres, whose function is to hold other parts together, or to serve as cords for the communi- cation of movement. A very beautiful example of a fibrous tissue of this kind, is furnished by the membrane of the com- mon Fowl's egg, which (as may be seen by examining an egg whose shell remains soft for want of consolidation by calcareous particles) consists of two principal layers, one serving as the basis of the shell itself, and the other forming that lining to it, which is known as the membrana putaminis. The latter may be separated, by careful tearing with needles and forceps, after prolonged maceration in water, into several matted lamellae re- sembling that represented in Fig. 316 ; and similar lamellae may be readily obtained from the shell itself, by dissolving away its lime by dilute acid. The simply fibrous structures of the body generally, however, belong to one of two very definite kinds of tissue; the "white" and the "yellow," whose appearance, com- position, and properties are very different. The white fibrous tissue, though sometimes apparently composed of distinct fibres, more commonly presents the aspect of bands, usually of a flattened form, and attaining the breadth of l-500th of an inch, which are marked by numerous longitudinal streaks, but can seldom be torn up into minute fibres of determinate size. The fibres and bands are occasionally somewhat wavy in their direc- tion ; and they have a peculiar tendency to fall into undulations, when it is attempted to tear them apart from each other (Fig. 317). This tissue is easily distinguished from the other, by the effect of acetic acid, which swells it up and renders it transparent, at the same time bringing into view certain oval nuclear cor- puscles. It is perfectly inelastic ; and we find it in such parts as tendons, ordinary ligaments, fibrous capsules, &c., whose function it is to resist tension without yielding to it. The yellow fibrous tissue exists in the form of long, single, elastic, branching fila- WHITE AND YELLOW FIBROUS TISSUE. 603 ments, with a dark decided border; which are disposed to curl when not put on the stretch (Fig. 318). They are for the most part between l-5000th and l-10,000th of an inch in diameter ; but they are often met with both larger and smaller. They frequently anastomose, so as to form a network. This tissue does not undergo any change, when treated with acetic FIG. 316. FIG. 317. Fibrous membrane from Egg-shell. White Fibrous Tissue from Ligament. FIG. 318. acid. It exists alone (that is, without any mixture of the white) in parts which require a peculiar elasticity, such as the middle coat of the Arteries, the Vocal Cords, the "ligamentum nuehse" of Quadrupeds, the elastic liga- ment which holds together the valves of a Bivalve shell, and that by which the claws of the Feline tribe are retracted when not in use ; and it enters largely into the composition of Areolar tissue. This con- sists of a network of minute fibres and bands, which are interwoven in every direction, so as to leave innumerable areolce or little spaces, which n *• -, ..-, Yellow Fibrous Tissue from Ligamentum NuchsB communicate freely with one of Caif. another. Of these fibres, some are of the yellow or elastic kind ; but the majority are composed of the white fibrous tissue ; and, as in that form of elementary structure, they frequently present the form of broad flattened bands, or membranous shreds, in which no distinct fibrous ar- rangement is visible. The proportion of the two forms varies, according to the amount of elasticity, or of simple resisting power, which the endowments of the part may require. We find this tissue in a very large proportion of the bodies of higher Animals; thus it binds together the ultimate fibres of the muscles and nerves into minute fasciculi, unites these fasciculi into larger ones, these again' into still larger ones which are 604 VERTEBRATED ANIMALS. obvious to the eye, and these into the entire muscle ; whilst it a*lso forms the membranous divisions between distinct muscles. In like manner it unites the elements of nerves, glands, &c., binds together the fat- cells into minute masses, these into larger ones, and so on ; and in this way it penetrates and forms part of all the softer organs of the body. For the display of the characters of these tissues, small and thin shreds may be cut with the curved scissors from any part that affords them ; and these must be torn asunder with needles under the simple micro- scope, until the fibres are separated to a degree sufficient to enable them to be examined to advantage under a higher magnify- ing power. 418. Skin, Mucous and Serous Membranes. — The Skin which forms the external envelope of the body, is divisible into two principal layers ; the "true skin," which usually makes up by far the larger part of its thickness, and the " cuticle," " scarf skin," or Epidermis, which covers it. At the mouth, nostrils, and other orifices of the open cavities and canals of the body, the skin passes into the membrane that lines these, which is distinguished as the Mucous membrane, from the peculiar glairy secretion of mucus by which its surface is protected. But the great closed cavities of the body, which surround the heart, lungs, intestines, &c., are lined by membranes of a different kind; which, as they secrete only a thin serous fluid from their surfaces, are known as Serous membranes. Both Mucous and Serous membranes consist, like the skin, of a proper membra- nous basis, and of a thin cuticular layer, which, as it differs in many points from the epidermis, is distinguished as the Epithe- lium (§421). The substance of the "true skin" and of the "mucous" and "serous" membranes, is principally composed of the fibrous tissues last described ; but the skin and the mucous membranes are very copiously supplied with bloodvessels and with glandulse of various kinds ; and in the skin we also find abundance of nerves and lymphatic vessels, as well as, in some parts, of hair-follicles. The distribution of the vessels in the skin and mucous membranes, which is one of the most interest- ing features in their structure, and which will come under our notice hereafter (Figs. 328, 329), is intimately connected with their several functions. In serous membranes, on the other hand, the supply of bloodvessels is more scanty, their function being simply protective. 419. Epidermic and Epithelial Cell-layers.- — The Epidermis or "cuticle" covers the exterior surfaces of the body, as a thin semi- transparent pellicle, which is shown by microscopic examination to consist of a series of layers of cells, which are continually wearing off at the external surface, and are being renewed at the surface of the true skin ; so that the newest and deepest layers gradually become the oldest and most superficial, and are at last thrown off by slow desquamation. In their progress from EPIDERMIS. 605 the internal to the external surface of the Epidermis, the cells undergo a series of well-marked changes. When we examine the innermost layer, we find it soft and granular; consisting of nuclei, in various stages of development into cells, held together by a tenacious semi-fluid substance. This was formerly con- sidered as a distinct tissue, and was supposed to be the peculiar seat of the color of the skin ; it received the designation of rete mucosum. Passing outwards, we find the cells more completely formed; at first nearly spherical in shape, but becoming poly- gonal where they are flattened one against another. As we proceed further towards the surface, we perceive that the cells are gradually more and more flattened, until they become mere horny scales, their cavity being obliterated ; their origin is indi- cated, however, by the nucleus in the centre of each. This change in form is accompanied by a change in the chemical composition of the tissue, which seems to be due to the meta- morphosis of the contents of the cells, into a horny substance identical with that of which hair, horn, nails, hoofs, &c., are composed. Mingled with the epidermic cells, we find others whicl,i secrete coloring-matter instead of horn; these are termed "pigment-cells." The most remarkable development of "pig- ment-cells" in the higher animals, is on the inner surface of the Choroid coat of the eye, where they have a very regular arrange- ment, and form several layers, known as the Pigmentum nigrum. When examined separately, these cells are found to have a polygonal form (Fig. 319, a\ FIG. 319. and to have a distinct nucleus (b) in their interior. The black col or is given by the accumulation, within the cell, of a number of flat, rounded, or oval granules, of extreme minuteness, which exhibit an active move- ment when set free from the cell, and even whilst enclosed within it. The pigment-cells are not always, however, of this simply- rounded or polygonal form ; they sometimes ceiis from present remarkable stellate prolongations, Nigrum:— a, pigment- under which form they are well seen in the ^granules concealing i • /» ii TTI /TTV r»r»»r \ mi ™e nucleus; b, the nu- skin of the Frog (Fig. 327, c, c). The gra- cieus distinct dual formation of these prolongations may be traced in the pigment-cells of the Tadpole during its meta- morphosis (Fig. 320). Similar varieties of form are to be met with in the pigmentary cells of Fishes and small Crustacea, which also present a great variety of hues; and these seem to have the power of likening their color to that of the bottom over which the animal may live, so as to serve for its conceal- ment. 420. The structure of the Epidermis may be examined in a variety of ways. If it be removed by maceration from the true skin, the cellular nature of its under-surface is at once recognized, 606 VERTEBRATED ANIMALS. FIG. 320. Pigment-cells from tail of Tadpole : — a. a, simple forms of recent origin ; 6, b, more com- plex forms subsequent- ly assumed. when it is subjected to a magnifying power of 200 or 300 diame- ters, by light transmitted through it, with this surface uppermost ; and if the epidermis be that of a negro or any other dark-skinned race, the pigment-cells will be very distinctly seen. This under-surface of the epidermis is not flat, but is excavated into pits and channels for the recep- tion of the papillary elevations of the true skin; an arrangement which is shown on a large scale in the thick cuticular covering of the Dog's foot, the subjacent papillae being large enough to be distinctly seen (when injected) with the naked eye. The cellular nature of the newly- formed layers is best seen, by examining a little of the soft film that is found upon the surface of the true skin, after the more consistent layers of the cuticle have been raised by a blister. The alteration which the cells of the external layers have undergone, tends to obscure their character; but if any fragment of epidermis be macerated for a little time in a weak solution of soda or potass, its dry scales become softened, and are filled out by imbibition into rounded or polygo- nal cells. The same mode of treatment enables us to make out the cellular structure in warts and corns, which are epidermic growths from the surface of papillae enlarged by hypertrophy. 421. The Epithelium may be designated as a delicate cuticle, covering all the free internal surfaces of the body, and thus lining all its cavities, canals, &c. Save in the mouth and other parts in which it approximates to the ordinary cuticle both in locality arid in nature, its cells usually form but a single layer ; and are so deficient in tenacity of mutual adhesion, that they cannot be detached in the form of a continuous membrane. Their shape varies greatly; for sometimes they are broad, flat, and scale-like, and their edges approximate closely to each other, so as to form what is termed a " pavement" or "tessellated" epithelium ; such cells are observable on the web of a frog's foot, or on the tail of the tadpole ; for, though covering an external surface, the soft moist cuticle of these parts has all the characters of an epithelium. In other cases, the cells have more of the form of cylinders, stand- ing erect side by side, one extremity of each cylinder forming part of the free surface, whilst the other rests upon the membrane to which it serves as a covering. If the cylinders be closely pressed together, their form is changed into prisms ; and such epithelium is often known as "prismatic." On the other hand, if the surface on which it rests be convex, the bases or lower ends of the cylinders become smaller than their free extremities ; and thus each has the form of a truncated cone rather than of a cylin- EPITHELIUM — FAT-CELLS. 607 der, and such epithelium (of which that covering the villi of the intestine, Fig. 328, is a peculiarly-good example) is termed " conical." But between these primary forms of epithelial cells, there are several intermediate gradations ; and one often passes almost insensibly into the other. Any of these forms of Epi- thelium may be furnished with cilia; but these appendages are more commonly found attached to the elongated, than to the flattened forms of epithelium- cells (Fig. 321). "Ciliated epi- thelium" is found upon the lining membrane of the air-passages in all FlG- 321- air-breathing Vertebrata ; and it also presents itself in many other situa- tions, in which a propulsive power is needed to prevent an accumula- tion of mucous or other secretions. Owing to the very slight attachment Cil{ated Epithelium; a, nucleated .-• -,-. . ~ i *- ,1 . resting on their smaller extremities; that usually exists between the epi- 6j cilia< thelium and the membranous sur- face whereon it lies, there is usually no difficulty whatever in examining it ; nothing more being necessary than to scrape the surface of the membrane with a knife, and to add a little water to what has been thus removed. The ciliary action will generally be found to persist for some hours or even days after death, if the animal has been previously in full vigor j1 and the cells that bear the cilia, when detached from each other, will swim freely about in water. If the thin fluid that is copiously discharged from the nose in the first stage of an ordinary " cold in the head," be subjected to microscopic examination, it will commonly be found to contain a great number of ciliated epithelium-cells that have been thrown off from the lining membrane of the nasal passages. 422. Fat. — One of the best examples which the bodies of higher animals afford, of a tissue composed of an aggregation of cells, is presented by the Adipose tissue ; the cells of which are distinguished by their power of drawing into themselves oleagi- nous matter from the blood. Fateells are sometimes dispersed in the interspaces of Areolar tissue ; whilst in other cases they are aggregated in distinct masses, — constituting the proper Adi- pose tissue. The individual fat-cells always present a nearly spherical or spheroidal form ; sometimes, however, when they are closely pressed together, they become somewhat polyhedral, from the flattening of their walls against each other (Fig. 322). Their intervals are traversed by a minute network of blood- vessels, from which they derive their secretion ; and it is proba- bly by the constant moistening of their walls with a watery fluid, that their contents are retained without the least transudation, 1 Thus it has been observed in the lining of the windpipe of a decapitated criminal, as much as seven days after death; and in that of the river-tortoise, it has been seen fifteen days after death, even though putrefaction had already far advanced. 608 VERTEBRATED ANIMALS. although these are quite fluid at the temperature of the living body. Fat-cells, when filled with their characteristic contents, have the peculiar appearance which has been FIG. 3-22. already described as appertaining to oil- globules (§ 99), being very bright in their centre, and very dark towards their margin, in consequence of their high refractive power ; but if, as often happens in prepara- tions that have been long mounted, the oily contents should have escaped, they then look like any other cells of the same form. Although the fatty matter which fills these cells (consisting of a mixture of stearine or of margarine with oleine) is liquid at the ordinary temperature of the body of a warm- blooded animal, yet its harder portion some- times crystallizes on cooling; the crystals shooting from a centre, so as to form a star- ?ireoiar tissue. shaped cluster. In examining the structure of Adipose tissue, it is desirable, where practicable, to have recourse to some specimen in which the fat- cells lie in single layers, and in which they can be observed with- out disturbing or laying them open ; such a condition is found, for example, in the mesentery of the mouse, and it is also occa- sionally met with in the fat deposits which present themselves at intervals in the connective tissues of the muscles, joints, &c. Small collections of fat-cells are found in the deeper layers of the true skin, and may be brought into view by vertical sections ot it. And the structure of large masses of fat may be examined by thin sections, these being placed under water in thin cells, so as to take off the pressure of the thin glass from their surface, which would cause the escape of the oil particles. No method of mounting (so far as the Author is aware) is successful in causing these cells permanently to retain their contents. 423. Cartilage. — In the ordinary forms of Cartilage, also, we have an example of a tissue •ssentially composed of cells ; but these are commonly separated from each other by an intercel- lular substance, the thickness of which differs greatly in different kinds of cartilage, and even in different stages of the growth ot any one. Thus in the cartilage of the external ear of a Bat or Mouse (Fig. 323), the cells are packed as closely together as are those of an ordinary vegetable parenchyma (Fig. 150, A); and this seems to be the early condition of most cartilages that are afterwards to present a different aspect. In the ordinary carti-. lages, however, that cover the extremities of the bones, so as to form smooth surfaces for the working of the joints, the amount of intercellular substance is usually considerable ; and the carti- lage-cells are commonly found imbedded in this, in clusters of two, three, or four (Fig. 324), which are evidently formed by STRUCTURE OF GLANDS. 609 a process of "duplicative subdivision" analogous to that by which the multiplication of cells takes place in the Vegetable Kingdom (Fig. 67). The substance of these cellular Cartilages is entirely destitute of blood- vessels ; being nourished FlG- 324- solely by imbibition from .,«*?. --(L ^v &* FIG. 32:1. Fig. 323. Fig. 324. each other cells. Cellular Cartilage of Mouse's ear. Section of the branchial Cartilage of Tadpole: — a, group of four cells, separating from ; 6, pair of cells in apposition; c, c, nuclei of cartilage-cells; d, cavity containing three the blood brought to the membrane covering their surface. Hence they may be compared, in regard to their grade of organi- zation, with the larger Algae; which consist, like them, of aggre- gations of cells held together by intercellular substance, without vessels of any kind, and are nourished by imbibition through their whole surface. There afe many cases, however, in which the structureless intercellular substance is replaced by bundles of fibres, sometimes elastic, but more commonly non-elastic; such combinations, which are termed j£6rc»-cartilages, are interposed in certain joints, wherein tension as well as pressure has to be re- sisted, as, for example, between the vertebrae of the spinal column, and the bones of the pelvis. In examining the structure of Car- tilage, nothing more is necessary than to make very thin sections with a sharp razor or scalpel, or with a Valentin's knife (§ 106), or, if the specimen be large and dense (as the cartilage of the ribs), with the section-instrument (§ 107). These sections may be mounted in weak spirit, in Goadby's solution, or in glycerine; but in whatever way they are mounted, they undergo a gradual change by the lapse of time, which renders them less fit to dis- play the characteristic features of their structure. 424. Structure of Glands. — The various secretions of the body (as the saliva, bile, urine, &c.) are formed by the instrumentality of organs termed Glands ; which are, for the most part, formed on one fundamental type, whatever be the nature of their pro- duct. The simplest idea of a gland is that which we gain from an examination of the "follicles" or little bags imbedded in the wall of the stomach ; some of which secrete mucus for the pro- tection of its surface, and others gastric juice. These little bags are filled with cells of a spheroidal form, which may be consi- 39 610 VERTEBRATED ANIMALS. dered as constituting their epithelial lining ; these cells, in the progress of their development, draw into themselves from the blood the constituents of the particular product they are to secrete; and they then seem to deliver it up, either by the- burst- ing or by the melting away of their walls, so that this product , may be poured forth from the mouth of the bag, into the cavity in which it is wanted. The liver itself, in the lowest animals wherein it is found, presents this condition. Some of the cells that form the lining of the stomach in the Hydra and Actinia, seem to be distinguished from the rest by their power of secreting bile, which gives them a brownish-yellow tinge ; in many Polyzoa, Compound Tunicata, and Annelida, these biliary cells can be seen to occupy follicles in the walls of the stomach ; in Insects, these follicles are few in number, but are immensely elongated so as to form biliary tubes, which lie loosely within the abdomi- nal cavity, frequently making many convolutions within it, and discharge their contents into the commencement of the intestinal canal ; whilst in the higher Mollusca, and in Crustacea, the folli- cles are vastly multiplied in number, and are connected with the ramifications of gland ducts, like grapes upon the stalks of their bunch, so as to form a distinct mass, which now becomes known as the liver. The examination of the biliary tubes of the Insect, or of the biliary follicles of the Crab, which may be accomplished with the utmost facility, is well adapted to give an idea of the essential nature of glandular structure. Among Vertebrated animals, the salivary glands, the pancreas (sweetbread), and the mammary glands, are well adapted to display the follicular struc- ture; nothing more being necessary than to make sections of these organs, thin enough to be viewed as transparent objects. The liver of Vertebrata, however, presents certain peculiarities of structure, which are not yet fully understood ; for although it is essentially composed, like other glands, of secreting cells, yet it has not yet been determined beyond doubt, whether these cells are contained within any kind of membranous investment. The kidneys of Vertebrated animals are made up of elongated tubes, which are straight, and lined with a pavement-epithelium, in the inner or "medullary" portion of the kidney, whilst they are con- voluted, and filled with a spheroidal epithelium, in the outer or " cortical." Certain flask-shaped dilatations of these tubes in- clude curious little knots of bloodvessels, which are known as the "Malpighian bodies" of the kidney; these are well displayed in injected preparations. For such a full and complete investiga- tion of the structure of these organs as the Anatomist and Phy- siologist require, various methods must be put in practice, which this is not the place to detail. It is perfectly easy to demonstrate the cellular nature of the substance of the liver, by simply scraping a portion of its cut surface ; since a number of its cells will be then detached. The general arrangement of the cells in the lobules, may be shown by means of sections thin enough to be MUSCULAR FIBRE. 611 transparent; whilst the arrangement of the bloodvessels can only be shown by means of injections (§ 433). Fragments of the tubules of the kidney, sometimes having the Malpighian cap- sules in connection with them, may also be detached by scraping its cut surface ; but the true relations of these parts can only be shown by thin transparent sections, and by injections of the bloodvessels and tubuli. The simple follicles contained in the walls of the stomach are brought into view by vertical sections ; but they may be still better examined by leaving small portions of the lining membrane for a few days in dilute nitric acid (one part to four of water), whereby the fibrous tissue will be so soft- ened, that the clusters of glandular epithelium lining the cells (which are but very little altered) will be readily separated. 425. Muscular Tissue. — Although we are accustomed to speak of this tissue as consisting of "fibres," yet the ultimate struc- ture of the "muscular fibre" is very different from that of the simple fibrous tissues already described. When we examine an ordinary Muscle (or piece of " flesh") with the naked eye, we observe that it is made up of a number of fasciculi or bundles of fibres ; which are arranged side by side with great regularity, in the direction in which the muscle is to act; and which are united by areolar tissue. These fasciculi may be separated into smaller parts, which appear like simple fibres; but when these are examined by the microscope, they are found to be them- selves fasciculi, composed of minuter fibres bound together by delicate filaments of areolar tissue. By carefully separating these, we may obtain the ultimate " muscular fibre." This fibre exists under two forms, the striated and the non-striated. The former is chiefly distinguished by the transversely-striated ap- pearance which it presents, and which is due to an alteration of light and dark spaces along its whole extent ; the breadth and distance of those striae vary, however, in different fibres, and even in different parts of the same fibre, according to its state of contraction or relaxation. Longitudinal striae are also fre- quently visible, which are due to a partial separation between the component fibrillse into which the fibre may be broken up. When a fibre of this kind is more closely examined, it is seen to consist of a delicate tubular sheath, quite distinct on the one hand from the areolar tissue which binds the fibres into fasciculi, and equally distinct from the internal substance of the fibre. This membranous tube, which has been termed the myolemma, is not perforated either by nerves or capillary vessels ; and forms, in fact, a complete barrier between the real elements of Muscular structure, and the surrounding parts. These elements appear to be very minute cylindrical particles with flattened faces of nearly uniform size, and adherent to each other both by their flat sur- faces and by their edges. The former adhesion is usually the most powerful; and causes the substance of the fibre, when it is broken up, to present itself in the form of delicate fibriUce, each 612 VERTEBRATED ANIMALS. FIG. 325. I 11 ID! — * 1 1! of which is composed of a single row of the primitive particle (Fig. 325). The diameter of the fibres varies greatly in different kinds of Vertebrated ani- mals. Its average is greater in Reptiles and Fishes than in Birds and Mammals, and its extremes also are wider ; thus its dimensions vary in the Frog from l-100th to l-1000th of an inch, and in ___^ _ the Skate from l-65th to ^^^•••••KiSSili^ l-300th; whilst in the Hu- Striated Muscular fibre, separating into fibrilfe. ™an SUDJCCt, the average IS about 1 -400th of an inch, and the extremes about l-200th and l-600th. 426. When the fibrillse are separately examined, under a magnifying power of from 250 to 400 diameters, they are seen to present a cylindrical or slightly beaded form ; FIG. 326. and their linearly aggregated particles then appear a to be minute cells. We observe the same alterna- tion of light and dark spaces, as when the fibrillse are united into fibres or into small bundles ; but it may be distinctly seen, that each light space is divided by a transverse line; and that there is a pellucid border at the sides of the dark spaces, as well as between their contiguous extremities (Fig. 326). This pellucid border seems to be the cell- wall; the dark space enclosed by it (which is usually bright in the centre) being the cavity of the cell, which is filled with a highly refracting substance. When the fibril is in a state of relaxa- tion, as seen at #, the diameter of the cells is greatest in the lorfgitudinal direction: but when it is contracted, the fibril increases in diameter as it diminishes in length; so that the transverse diameter of each cell becomes equal to the longitu- dinal diameter, as seen at b ; or even exceeds it. Thus the act of Muscular contraction seems to consist in a change of form in the cells of the ultimate fibrillse, consequent upon a contraction between the walls of their two extremities ; and it is interesting to observe, how very closely it thus corresponds with the contraction of certain Vege- table tissues, of which the component cells are capable of producing movements, when they are irritated, by means of a similar change of form. The diameter of the ulti- mate fibrillge will of course be subject to variations, in accord- ance with their contracted or relaxed condition ; but it seems to be otherwise tolerably uniform in different animals, being for Structure of the ultimate Fibrillm of Striated Muscu- lar fibre :— o, a fibril in a state of ordinary relaxa- tion ; b, a fibril in a stale of partial contraction. MUSCULAR FIBRE. 613 the most part about 1-1 0,000th of an inch. It has been observed, however, as high as l-5000th of an inch, and as low as l-20,000th, even when the fibre was not put upon the stretch. 427. The "smooth" or non-striated form of Muscular fibre, which is especially found in the walls of the stomach, intestines, bladder, and other similar parts, is composed of flattened bands whose diameter is usually between 2-2000th and l-3000th of an inch; and these bands are collected into fasciculi, which do not lie parallel with each other, but which cross and interlace. By macerating a portion of such muscular substance, however, in dilute nitric acid (about one part of ordinary acid to three parts of water) for two or three days, it is found that the bands just mentioned may be easily separated into elongated fusiform cells, not unlike woody fibre in shape; each distinguished for the most part by the presence of a long staff-shaped nucleus, brought into view by the action of acetic acid. These cells, in which the dis- tinction between cell-wall and cell-contents can by no means be clearly seen, are composed of a soft yellow substance, often con- taining small pale granules, and sometimes yellow globules of fatty matter. In the coats of the bloodvessels are found cells having the same general characters, but shorter and wider in form ; and although some of these approach very closely in their general appearance to epithelium-cells, yet they seem to have quite a different nature, being distinguished by their contractile endowments. 428. In the examination of Muscular Tissue, a small portion may be cut out with the curved scissors; and this should be torn up into its component fibres, and these, if possible, should be separated into their fibrillre, by dissection with a pair of needles, under the simple microscope. The general character of the striated fibre are admirably shown in the large fibres of the Frog; and by selecting a portion in which these fibres spread themselves out to unite with an aponeurotic expansion, they may often be found so well displayed in a single layer, as not only to exhibit all their characters without any dissection, but also to show their mode of connection with the simple fibrous tissue of which the aponeurosis is formed. As the ordinary cha- racters of the fibre are but little altered by boiling, this process may be had recourse to for their more ready separation, espe- cially in the case of the tongue. The separation of the fibres into their fibrillse is only likely to be accomplished, in the higher Vertebrata, by repeated attempts, of which the greater number are likely to be unsuccessful ; but it may be accomplished with much greater facility in the Eel and other fish, the tenacity of whose muscular tissue is much less. The characters of the fibrillse are not nearly so well pronounced, however, in the Fish, as in the warm-blooded Vertebrata; and among the latter, the Pig has been found by Mr. Lealand (who has been peculiarly successful in this class of preparations) to yield the best examples. 614 VEKTEBRATED ANIMALS. He lays great stress on the freshness of the specimen, which should be taken from the body as soon as possible after death ; and when a successful preparation has been made, it should be preserved in Goadby's solution. The shape of the fibres can only be properly seen in cross sections ; and these are best made by drying a piece of muscle, so that very thin slices can be cut with a sharp instrument, which on being moistened again, will resume in great part their original characters. Striated muscu- lar fibres are readily obtainable from the limbs of Crustacea and of Insects ; and their presence is also readily distinguishable in the bodies of Worms, even of very low organization ; so that it may be regarded as characteristic of the Articulated series gene- rally. On the other hand, the Molluscous classes are for the most part distinguished by the non-striation of their fibre ; there are, however, two remarkable exceptions, strongly striated fibre having been found in the Terebratula and other Brachiopods, and also in many Potyzoa. Its presence seems always related to energy and rapidity of movement; whilst the non-striated pre- sents itself, where the movements are slower and feebler in their character. 429. Nerve-substance. — Whenever a distinct Nervous system can be made out, it is found to consist of two very different forms of tissue ; namely, the vesicular, which are the essential components of the ganglionic centres, and the tubular, of which the connect- ing trunks consist. The " nerve-vesicles" or " ganglion- glo- bules" are cells, whose typical form may be regarded as globular ; but they often present an extension into one or more long pro- cesses, which give them a "caudate" or a "stellate" aspect. These processes have been traced into continuity, in some in- stances, with the axis-cylinders of nerve-tubes ; whilst in other cases they seem to inosculate with those of other vesicles. The vesicles are filled with a finely-granular substance, which extends into the prolongations ; and they also usually contain pigment- granules, which give them a reddish or yellowish-brown color ; but these are commonly absent among the lower animals. It is the presence of this pigment, however, which gives to collections of ganglion-globules in the warm-blooded Yertebrata that pecu- liar hue, which causes it to be known as the cineriiious or gray matter. Each of the nerve-tubes, on the other hand, of which the trunks are composed, consists, in its most completely deve- loped form, of a delicate membranous sheath, within which is a hollow cylinder of a material known as the " white substance of Schwann," whose outer and inner boundaries are marked out by two distinct lines, giving to each margin of the nerve-tube what is described as a "double contour." The centre or axis of the tube is occupied by a transparent substance, which is known as the "axis-cylinder;" and there is reason to believe that this last is the essential component of the nervous fibre, and that the hollow cylinder that surrounds it, serves, like the MICROSCOPIC CHARACTERS OF NERVE-SUBSTANCE. 615 tubular sheath, for its complete isolation. The contents of the membranous envelope are very soft, yielding to slight pressure ; and they are so quickly altered by the contact of water or of any liquids that are foreign to their nature, that their characters can only be properly judged of when they are quite fresh. Besides the proper tubular fibres, however, there are others, known as "gelatinous," which are considerably smaller than the preced- ing, and do not exhibit any differentiation of parts. They are flattened, soft, and homogeneous in their appearance, and con- tain numerous nuclear particles, which are brought into view by acetic acid. They can sometimes be seen to be continuous with the axis-cylinders of the ordinary fibres, and also with the radiat- ing prolongations of the vesicles ; so that their nervous character, which has been doubted by some anatomists, seems established beyond doubt. The ultimate distribution of the nerve-fibres may be readily traced in thin vertical sections of the skin, treated with solution of soda. It was formerly supposed that all its papillee are furnished with nerve-fibres, and minister to sensa- tion ; but it is now known that a large proportion (at any rate) of those furnished with loops of bloodvessels (Fig. 329, D), being destitute of nerve-fibres, must have for their special office the production of the epidermis ; whilst those which, possessing nerve-fibres, have sensory functions, are usually destitute of bloodvessels. The greater part of the interior of each sensory papilla of the skin, is occupied by a peculiar "axile body," which seems to be merely a bundle of ordinary fibrous tissue, whereon the nerve-fibre appears to terminate. The nerve-fibres are more readily seen, however, in the " fungiform" papillae of the tongue, to each of which several of them proceed ; these bodies, which are very transparent, may be well seen by snipping off minute portions of the tongue of the Frog ; or by snipping off the papillae themselves from the surface of the living Human tongue, which can be readily done by a dexterous use of the curved scissors, with no more pain than the prick of a pin would give. The transparency of any of these papillae is in- creased, by treating them with a solution of soda. 430. For the sake of obtaining a general acquaintance with the microscopic characters of these principal forms of Nerve- substance, it is best to have recourse to minute nerves and gan- glia. The small nerves which are found between the skin and the muscles of the back of the Frog, and which become appa- rent when the former is being stripped off, are extremely suita- ble for this purpose ; and if they be treated with strong acetic acid, a contraction of their tubes takes place, by which the axis cylinder is forced out from their cut extremities, so as to" be made more apparent than it can be in any other way. The "gelatinous" fibres are found in the greatest abundance in the Sympathetic nerves ; and their characters may be best studied in the smaller branches of that system. So, for the examination of 616 VERTEBRATED ANIMALS. the gauglionic vesicles, and of their relation to the nerve-tubes, it is better to take some minute ganglion as a whole (such as one of the Sympathetic ganglia of the frog, mouse, or other small animal), than to dissect the larger ganglionic masses, whose structure can only be successfully studied by such as are profi- cient in this kind of investigation. The nerves of the orbit of the eye of Fish, with the ophthalmic ganglion and its branches, which may be very readily got at in the Skate, and of which the components may be separated without much difficulty, form one of the most convenient objects for the demonstration of the principal forms of nerve-tissue, and especially for the connection of nerve-fibres and ganglionic corpuscles. No method of pre- serving the nerve-tissue has yet been devised, which makes it worth while to attempt to mount preparations for the sake of displaying its minute characters; but the general course of the nerve-tubes, and the disposition of the ganglionic vesicles, may be demonstrated in preparations preserved in weak spirit; and when the skin has been injected, the passage of the nerve-fibres to the papillae can sometimes be traced in vertical sections, mounted as opaque objects, and viewed by reflected light. The following method, recommended by Mr. J. Lockhart Clarke, for the examination of the structure of the Spinal Cord,1 would be equally applicable to that of other large ganglionic masses: — A perfectly fresh cord is to be hardened in strong spirit, so that extremely thin sections can be made with a very sharp knife ; and such sections, placed on slips of glass, are to be treated with a mixture of one part of acetic acid and three of spirit, which not only makes the fibrous portion more distinct, but also renders the vesicular portion more transparent. If it be desired to preserve such a section, it should be transferred, after maceration for an hour or two in the mixture of acetic acid and spirit, into pure spirit, in which it should be allowed to remain for about the same space of time ; from the spirit it should be transferred to oil of turpentine, which soon expels the spirit, and renders the section perfectly transparent, so that it can be examined with high magnifying powers ; and it may then be mounted in Canada balsam in the usual manner. 431. Circulation of the Blood. — One of the most interesting spectacles that the microscopist can enjoy, is that which is fur- nished by the circulation of the blood in the " capillary" blood- vessels, which distribute the fluid through the tissues it nourishes. This, of course, can only be observed in such parts of animal bodies, as are sufficiently thin and transparent to allow of the transmission of light through them without any disturbance of their ordinary structure; and the number of these is very limited. The web of the Frog's foot is perhaps the most suita- ble for ordinary purposes, more especially since this animal is to be easily obtained in almost every locality; and the following is 1 See his Memoir on that subject, in " Philos. Transact.," 1851. CIRCULATION OF BLOOD IN FROG'S FOOT. 617 the arrangement which the author has found most convenient for the purpose. A piece of thin cork is to be obtained, about 9 inches long and 3 inches wide (such pieces are prepared by the cork-cutters, as soles), and a hole about 3-8ths of an inch in diameter is to be cut at about the middle of its length, in such a position that, when the cork is secured upon the stage, this aperture may correspond with the axis of the microscope. The body of the frog is then to be folded in a piece of wet calico, one leg being left free, in such a manner as to confine its move- ments, bat not to press too tightly upon its body; and being then laid down near one end of the cork plate, the free leg is to be extended, so that the foot can be laid over the central aper- ture. The spreading out of the foot over the aperture is to be accomplished, either by passing pins through the edge of the web into the cork beneath, or by tying the ends of the toes by threads to pins stuck into the cork at a small distance from the aperture ; the former method is by far the least troublesome, and it may be doubted whether it is really the source of more suffer- ing to the animal than the latter is, the confinement being obviously that which is most felt. A few turns of tape, carried loosely around the calico bag, the projecting leg, and the cork, serve to prevent any sudden start; and when all is secure, the cork plate is to be laid down upon the stage of the microscope, where a few more turns of the tape will serve to keep it in place. The web being moistened with water (a precaution which should be repeated as often as the membrane exhibits the least appear- ance of dryness), and an adequate light being reflected through the web from the mirror, this wonderful spectacle is brought into view on the adjustment of the focus (a power of from 75 to 100 diameters being the most suitable for ordinary purposes), provided that no obstacle to the movement of the blood be pro- duced by undue pressure upon the body or leg of the animal. It will not unfrequently be found, however, that the current of blood is nearly or altogether stagnant for a time; this seems occasionally due to the animal's alarm at its new position, which weakens or suspends the action of its heart, the movement recommencing again after the lapse of a few minutes, although no change has been made in any of the external conditions. But if the movement should not renew itself, the tape which passes over the body should be slackened; and if this does not produce the desired effect, the calico envelope must also be loosened. When everything has once been properly adjusted, the animal will often lie for hours without moving, or will only give an occasional twitch. The movement of the blood will be distinctly seen by that of the corpuscles, which course after one another through the network of capillaries that intervenes be- tween the smallest arteries and the smallest veins; in those tubes that pass most directly from the veins to the arteries, the current is always in the same direction ; but in those which pass 618 VERTEBRATES ANIMALS. across between these, it may not unfrequently be seen that the direction of the movement changes from time to time. The larger vessels (Fig. 327), with which the capillaries are seen to FIG. 327. Capillary circulation in a portion of the web of a Frog's foot: 1, trunk of vein; 2, 2, its branches; 3, 3, pigment-cells. be connected, are almost always veins, as may be known from the direction of the flow of blood in them from the branches (2, 2) towards their trunks (1) ; the arteries, whose ultimate sub- divisions discharge themselves into the capillary network, are for the most part restricted to the immediate borders of the toes. When a power of 200 or 250 diameters is employed, the visible area is of course greatly reduced ; but the individual vessels and their contents are much more plainly seen ; and it may then be observed, that whilst the red corpuscles flow at a very rapid rate along the centre of each tube, the colorless corpuscles which are occasionally discernible, move slowly in the clear stream near its margin. 432. The circulation may also be displayed in the tongue of the Frog, by laying the animal down on its back, with its head close to the hole in the cork- plate, and, after securing the body in this position, drawing out the tongue with the forceps, and fixing it on the other side of the hole with pins. This method, however, is so much more distressing to the animal, that its employment seems scarcely justifiable for the mere purpose of display; and nothing but some anticipated benefit to science, can justify the laying open of the body of the living animal, for the purpose of CIRCULATION IN TADPOLE AND FISH. 619 examining the circulation of its lungs or mesentery. The tadpole of the Frog, when sufficiently young, furnishes a good display of the circulation in its tail ; and the difficulty of keeping it quiet during the observation may be overcome, by gradually mixing some hot water with that in which it is swimming, until it be- comes motionless ; this usually happens when it has been raised to a temperature between 100 and 110° ; and notwithstanding that the muscles of the body are throwTn into a state of spasmodic rigidity by this treatment, the heart continues to pulsate, and the circulation is maintained. The larva of the Water-Newt, when it can be obtained, furnishes a most beautiful display of the circu- lation, both in its external gills, and in its delicate feet. It may be enclosed in a large aquatic box or in a shallow cell, gentle pressure being made upon its body, so as to impede its move- ments, without stopping the heart's action. The circulation may also be seen in the tails of small fish, such as the Minnow or Stickle-back, by confining these animals in tubes, or in shallow cells, or in a large aquatic box ; but although the extreme trans- parency of these parts adapts them well for this purpose in one respect, yet the comparative scantiness of their bloodvessels pre- vents them from being as suitable as the Frog's web in another not less important particular. One of the most beautiful of all displays of the circulation, however, is that which may be seen upon the yolk-bag of young Fish (such as the trout) soon after they have been hatched ; and as it is their habit to remain almost entirely motionless at this stage of their existence, the observa- tion can be made with the greatest facility by means of the zoo- phyte-trough, provided that the subject of it can be obtained. Now that the artificial breeding of these fish is largely practised for the sake of stocking fish-ponds, there can seldom be much difficulty in procuring specimens at the proper period. The store of yolk which the yolk-bag supplies for the nutrition of the em- bryo, not being exhausted in the Fish (as it is in the bird) pre- viously to the hatching of the egg, this bag hangs down from the belly of the little creature on its emersion ; and continues to do so until its contents have been absorbed into the body, which does not happen for some little time afterwards. And the blood is distributed over it in copious streams, partly that it may draw into itself fresh nutritive material, and partly that it may be sub- jected to the aerating influence of the surrounding water. 433. Injected Preparations. — Next to the circulation of the blood in the living body, the varied distribution of the Capillaries in its* several organs, as shown by means of "injections" of coloring matter thrown into their principal vessels, is one of the most in- teresting subjects of microscopic examination. The art of making successful preparations of this kind, is one in which perfection can usually be attained only by long practice, and by attention to a great number of minute particulars; and better specimens may be obtained, therefore, from those who have made it a busi- ness to prepare them, than are likely to be prepared by amateurs 620 VERTEBRATED ANIMALS. for themselves. For this reason, no more than a general account of the process will be here offered ; the minute details which need to be attended to, in order to attain successful results, being readily accessible elsewhere to such as desire to put it in practice.1 The coloring matter which is altogether most suitable when only one set of vessels is to be injected, is Chinese vermilion. This, however, as commonly sold, contains numerous particles of far too large a size ; and it is necessary first to reduce it to a greater fineness by continued trituration in a mortar (an agate or a steel mortar is the best) with a small quantity of water, and then to get rid of the larger particles by a process of "levigation," ex- actly corresponding to that by which the particles of coarse sand, &c., are separated from the Diatomacese (p. 305). The fine powder thus obtained, ought not, when examined under a mag- nifying power of 200 diameters, to exhibit particles of any appre- ciable dimensions. The " size" or "gelatine" should be of a fine and pure quality, and should be of suificient strength to form a tolerably firm jelly when cold, whilst quite limpid when warm. It should be strained, whilst hot, through a piece of new flannel; and great care should be taken to preserve it free from dust, which may best be done by putting it into clean jars, covering its surface with a thin layer of alcohol. The proportion of levi- gated vermilion to be mixed with it for injection, is about 2 oz. to a pint; and this is to be stirred in the melted size, until the two are thoroughly incorporated, after which the mixture should be strained through muslin. The injection is thrown into the vessels by means of a brass syringe expressly constructed for the purpose, which has several jet-pipes of different sizes, adapted to the different dimensions of the vessels to be injected ; and these should either be furnished with a stop cock to prevent the return of the injection when the syringe is withdrawn, or a set of small corks of different sizes should be kept in readiness, with which they may be plugged. The pipe should be inserted into the cut end of the trunk which is to be injected, and should be tied therein by a silk thread. In injecting the vessels offish, mollusks, &c., the softness of the vessels renders them liable to break in the attempt to tie them ; and it is therefore better for the operator to satisfy himself with introducing a pipe as large as he can insert, and with passing it into the vessel as far as he can without violence. All the vessels from which the injection might escape, should be tied, and sometimes it is better to put a ligature round a part of the organ or tissue itself; thus, for example, when a portion of the intestinal tube is to be injected through its branch of the mesenteric artery, not only should ligatures be put round any divided vessels of the mesentery, but the cut ends of the intestinal tube should be firmly tied. The operation should either be performed when the body or organ is 1 See especially the article "Injection." in the " Micro^raphic Dictionary ;" Dr. Beale's treatise on "The Microscope, and its application to Clinical Medicine," Chap, viii; and M. Robin's work, " Du Microscope et des Injections." (Paris, 1 849.) (See also Appendix.) INJECTION OF BLOODVESSELS. 621 as fresli as possible, or after the expiry of sufficient time to allow the rigor-mortis to pass off, the presence of this being very inimical to the success of the injection. The part should be thoroughly warmed, by soaking in warm water for a time pro- portionate to its bulk; and the injection, the syringe, and the pipes should also have been subjected to a temperature sufficiently high to insure the free flow of the liquid. The force used in pressing down the piston should be very moderate at first ; but should be gradually increased as the vessels become filled ; and it is better to keep up a steady pressure for some time, than to attempt to distend them by a more powerful pressure, which will be certain to cause extravasation. This pressure should be maintained1 until the injection begins to flow from the large veins, and the tissue is thoroughly reddened ; and if one syringe- ful of injection after another be required for this purpose, the return of the injection should be prevented by stopping the nozzle of the jet-pipe when the syringe is removed for refilling. When the injection has been completed, any openings by which it can escape should be secured, and the preparation should then be placed for some hours in cold water, for the sake of causing the size to " set."2 434. Although no injections look so well by reflected light, as those which are made with vermilion, yet other coloring sub- stances may be advantageously employed for particular purposes. Thus the yellow chromate of lead, which is precipitated when a solution of acetate of lead is mixed with a solution of chromate of potass, is an extremely fine powder, which " runs" with great facility in an injection, and has the advantage of being very cheaply prepared. The best method of obtaining it, is to dis- solve 200 grains of acetate of lead and 105 grains of chromate of potass in separate quantities of water, to mix these, and then, after the subsidence of the precipitate, to pour off the super- natant fluid, so as to get rid of the acetate of potash which it contains, since this is apt to corrode the walls of the vessels, if the preparation be kept moist. The solutions should be mixed cold, and the precipitate should not be allowed to dry before being in- corporated with the size, four ounces of which will be the pro- portion appropriate to the quantity of the coloring-substance pro- duced by the above process. The same materials may be used in such a manner that the decomposition takes places within the vessels themselves, one of the solutions being thrown in first, and then the other ; and this process involves so little trouble or expense, that it may be considered the best for those who are 1 A simple mechanical arrangement for this purpose, by which the fatigue of main- taining this pressure with his hand is saved to the operator, is described in the " Micro- graphic Dictionary," p. 354. 2 The kidney of a sheep or pig is a very advantageous organ for the learner to practise on ; and he should first master the filling of the vessels from the arterial trunk alone, and then, when he has succeeded in this, he should fill the tubuli urinifrri with white injection, before sending colored injection into the renal artery. The entire systemic circulation of small animals, as mice, rats, frogs, &c., may be injected from the aorta ; and the pulmonary vessels from the pulmonary artery. 022 VERTEBRATED ANIMALS. novices in the operation, and who are desirous of perfecting themselves in the practice of the easier methods, before attempt- ing the more costly. By M. Doyere, who first devised this method, it was simply recommended to throw in saturated solu- tions of the two salts, one after the other; but Dr. Goadby, who has had much experience in the use of it, advises that gelatine should be employed, in the proportion of 2 oz. dissolved in 8 oz. of water, to 8 oz. of the saturated solutions of each salt. This method answers very well for preparations that are to be mounted dry ; but for such as are to be preserved in fluid, it is subject to the disadvantage of retaining in the vessels the solution of acetate of potash, which exerts a gradual corrosive action upon them. Dr. Goadby has met this objection, however, by suggest- ing the substitution of nitrate for acetate of lead ; the resulting nitrate of potash having rather a preservative than a corrosive action on the vessels. When it is desired to inject two or more sets of vessels (as the arteries, veins, and gland-ducts) of the same preparation, different coloring substances should be em- plo}^ed. For a ivhite injection, the carbonate of lead (prepared by mixing solutions of acetate of lead and carbonate of soda, and pouring off the supernatant liquid when the precipitate has fallen) is the best material. No blue injections can be much recom- mended, as they do not reflect light well, so that the vessels filled with them seem almost black ; the best is freshly-precipitated Prussian blue (formed by mixing solutions of persulphate of iron and ferrocyanide of potassium), which, to avoid the altera- tion of its color by the free alkali of the blood, should be triturated with its own weight of oxalic acid and a little water, and the mixture should then be combined with size, in the pro- portion of 146 grains of the former to 4 oz. of the latter. 435. Injected preparations may be preserved either dry or in fluid. The former method is well suited to sections of many solid organs, in which the disposi- n j\ FIG. 328. tion of the vessels does not sustain much alteration by drying ; for the colors of the vessels are displayed with greater brilliancy than by any other method, when such slices, after being well dried, are moist- ened with turpentine and mounted in Canada balsam. But for such an injection as that shown in Fig. 328, in which the form and dispo- sition of the intestinal villi would be completely altered by drying, it is indispensable that the prepara- tion should be mounted in fluid, in a cell deep enough to prevent any pressure on its surface. Either Goadby's solution or weak spirit answers the purpose very well. Villi of Small Intestine of Mo DISTRIBUTION OF CAPILLARIES. 623 436. A well-injected preparation should have its vessels com- pletely filled through every part; the particles of the coloring matter should be so closely compacted together, that they should not be distinguishable unless carefully looked for; and there should be no patches of pale uninjected tissue. Still, although the beauty of a specimen as a microscopic object is much im- paired by a deficiency in the filling of its vessels, yet to the ana- tomist the disposition of the vessels will be as apparent when they are only filled in part, as it is when they are fully distended; and imperfectly injected capillaries are better seen, when thin sections are mounted as transparent objects, than are such as have been completely filled. 437. A relation may generally be traced between the disposi- tion of the Capillary vessels, and the functions they are destined to subserve ; but that relation is obviously (so to speak) of a me- chanical kind ; the arrangement of the vessels not in any way determining the function, but merely administering to it, like the arrangement of water or gas-pipes in a manufactory. Thus in Fig. 329, A, we see that the capillaries of fatty tissue are dis- EIG. 329. Capillary network around Fat-cells, c Capillary network of Muscle. D Distribution of Capillaries in Mucous Membrane. Distribution of Capillary bloodyessels in Skin of Finger. posed in a network with rounded meshes, so as to distribute the blood among the fat-cells (§ 422) ; whilst at B we see the meshes enormously elongated, so as to permit muscular fibres to lie in them. Again, at c we observe the disposition of the capillaries around the orifices of the follicles of a mucous membrane ; whilst at D we see the looped arrangement which exists in the papillary surface of the skin, and which is. subservient to the nutrition of the epidermis and to the activity of the sensory nerves. 624 VERTEBRATED ANIMALS. FIG. 330. 438. In no part of the circulating apparatus, however, does the disposition of the capillaries present more points of interest, than it does in the Respiratory organs. In Fishes, the respiratory surface is formed by an outward extension into fringes of gills, each of which consists of an arch with straight laminee hanging down from it ; and every one of these laminae (Fig. 330) is furnished with a double row of leaflets, which is most minutely supplied with bloodvessels, their network (as seen at A) being so close, that its meshes (indicated by the dots in the figure) cover less space than the vessels themselves. The gills of Fish are not ciliated on their surface like those of Mollusks and of the larva of the Water-newt ; the necessity for such a mode of renew- ing the fluid in contact with them, TWO branchial processes of the Gili of the being superseded Jby the muscular Ed, showing the branchial lamellae:— A. apparatus with which the gill - portion of one of these processes enlarged, ^ • f^-w^-Up,! T^nt :n Dei 3CL l the respiratory surface is formed by the walls of an internal cavity, that of the lungs: these organs, however, are constructed on apian very different from that which they present in higher Yertebrata, the great extension of surface which is effected in the latter by the minute subdivision of the cavity, not being here necessary. In the Frog (for example) the cavity of each lung is undivided ; its walls, which are thin and FlG- 33L membranous at the lower part, there present a simple smooth ex- panse ; and it is only at the upper part, where the extensions of the tracheal cartilage form a network over the interior, that its surface is depressed into sacculi, whose lining is crowded with blood- vessels (Fig. 331). In this man- ner, a set of air-cells is formed in the thickness of the upper wall of the lung, which commu- nicates with the general cavity, and very much increases the sur- face over which the blood come§ into relation with the air; but each air-cell has a capillary network of its own, which lies on showing the capillary network of the la- Interior of upper part of Lung of Frog. LUNGS OF BIKDS AND MAMMALS. 625 one side against its wall, so as only to be exposed to the air on its free surface. In the elongated lung of the Snake, the same gene- ral arrangement prevails; but the cartilaginous reticulation of its upper part projects much further into the cavity, and encloses in its meshes (which are usually square, or nearly so) several layers of air-cells, which communicate, one through another, with the general cavity. The structure of the lungs of Birds presents us with an arrangement of a very different kind, the purpose of which is to expose a very large amount of capillary surface to the influence of the air. The entire mass of each may be considered as subdivided into an immense number of " lobules" or "lung- lets" (Fig. 332), each of which has its own bronchial tube (or FIG. 332. Interior structure of Lung of Fowl, as displayed by a section, A, passing in the direction of a bronchial tube, and by another section, B, cutting it across. subdivision of the windpipe), and its own system of bloodvessels, which have very little communication with those of other lobules. Each lobule has a central cavity, which closely resembles that of a Frog's lung in miniature ; having its walls strengthened by a network of cartilage derived from the bronchial tube, in the in- terstices of which are openings leading to sacculi in their sub- stance. But each of these cavities is surrounded by a solid plexus of bloodvessels, which does not seem to be covered by any limit- ing membrane, but which admits air from the central cavity freely between its meshes ; and thus its capillaries are in imme- diate relation with air on all sides, a provision that is obviously very favorable to the complete and rapid aeration of the blood they contain. In the lung of Man and Mammals, again, the plan of structure differs from the foregoing, though the general effect of it is the same. For the whole interior is divided up into minute air-cells, which freely communicate with each other, and with the ultimate ramifications of the air-tubes into which the trachea (windpipe) subdivides ; and the network of bloodvessels (Fig. 333) is so disposed in the partitions between these cavities, that the blood is exposed to the air on both sides. It has been calculated that the number of these air-cells grouped around the 40 626 VERTEBRATED ANIMALS. termination of each air-tube in man, is not less than 18,000 ; and that the total numher in the entire lungs is six hundred millions. 439. The following list of the parts of the bodies of Yerte- brata, of which Injected pre- FlG- 333- parations are most interesting as Microscopic objects, may be of service to those who may be inclined to apply themselves to their produc- tion. Alimentary Canal; Stomach, showing the orifices of the gastric follicles, and the rudimentary villi near the pylorus ; Small Intestine, showing the villi and the orifices of the follicles of Lieberkiihn, and at its lower Arrangement of the Capillaries of the air-cells of ^Qrf f>,o Potrovinn rrleinrlc! • the Human Lung. Pait tlie Cyeiiail glands , Large Intestine, showing the various glandular follicles: — Respiratory Organs; Lungs of Mammals, Birds, and Eeptiles ; Gills and Swimming-bladder of Fish: — G-landular Organs; Liver, Gall-bladder, Kidney, Paro- tid:— Generative Organs; Oviduct of Bird and Frog; Mamma- lian Placenta ; Uterine and Foetal Cotyledons of Ruminants : — Organs of Sense; Iris, Choroid, and Ciliary processes of Eye, Pupillary Membrane of foetus ; Papillae of Tongue ; Mucous Membrane of Nose; Papillae of Skin of finger: — Tegumentary Organs ; Skin of different parts, hairy and smooth, with vertical sections showing the vessels of the Hair-follicles, Sebaceous § lands, and Papillae; Matrix of nails, hoofs, &c. : — Tissues; 'ibrous, Muscular, Adipose, Sheath of Tendon. 440. Development. — The study of the Embryological develop- ment of Yertebrated animals has been pursued of late years with great zeal and success by the assistance of the Microscope; but as this is a department of inquiry which needs for its suc- cessful pursuit a thoroughly scientific culture, and is only likely to be taken up by a professed Physiologist, no good purpose seems likely to be served by here giving such an imperfect out- line of the process, as could alone be introduced into a work like the present; and the reader who may desire information upon it, will find no difficulty in obtaining this through systema- tic treatises on Physiology.1 1 The Author takes the liberty of referring to his " Principles of Comparative Physio- logy," 4th Ed. chap, xi, as containing a general view of the whole subject, with refer- ences to the principal sources of more detailed information. CHAPTER XIX. APPLICATIONS OF THE MICROSCOPE TO GEOLOGICAL INVESTIGATION. 441. THE utility of the Microscope is by no means limited to the determination of the structure and actions of the Organized beings at present living on the surface of the earth ; for a vast amount of information is afforded by its means to the geological inquirer, not only with regard to the minute characters of the many Vegetable and Animal remains that are entombed in the successive strata of which its crust is composed, but also with regard to the essential nature and composition of many of those strata themselves. We cannot have a better example of its value in both these respects, that that which is afforded by the results of microscopic examination of lignite or fossilized wood, and of ordinary coal, which there is every reason to regard as a product of the decay of wood. 442. Specimens of Fossilized Wood, in a state of more or less complete preservation, are found in numerous strata of very different ages, — more frequently, of course, in those whose materials were directly furnished by the dry land, and were deposited in its immediate proximity, than in those which were formed by the deposition of sediments at the bottom of a deep ocean. Generally speaking, it is only when the wood is found to have been penetrated by silex, that its organic structure is well preserved; but instances occur every now and then, in which penetration by carbonate of lime has proved equally favor- able. In either ease, transparent sections are needed for the full display of the organization ; but such sections, though made with great facility when lime was the fossilizing material, require much labor and skill when silex has to be dealt with. Occasion- ally, however, it has happened that the infiltration has filled the cavities of the cells and vessels, without consolidating their walls ; and as the latter have undergone decay without being replaced by any cementing material, the lignite, thus composed of the internal "casts" of the woody tissues, is very friable, its fibres separating from each other like those of asbestos ; and their laminae split asunder with a knife, or isolated fibres separated by rubbing down between the fingers, exhibit the characters of the woody structure extremely well, when mounted in Canada 628 MICROSCOPIC GEOLOGY. balsam. Generally speaking, the lignites of the Tertiary strata present a tolerably close resemblance to the wood of the existing period ; thus the ordinary structure of Dicotyledonous and Mono- cotyledonous stems, may be discovered in such lignites in the utmost perfection; and the peculiar modification presented by Coniferous wood, is also most distinctly exhibited (Fig. 171). As we descend, however, through the strata of the Secondary period, we more and more rarely meet with the ordinary dicoty- ledonous structure; and the lignites of the earliest deposits of these series are, almost universally, either gymnosperms or palms.1 Descending into the Palaeozoic series, we are presented in the vast Coal formations of our own and other countries, with an extraordinary proof of the prevalence of a most luxuriant vegetation in a comparatively early period of the world's history ; and the microscope lends the geologist essential assistance, not only in determining the nature of much of that vegetation, but also in demonstrating, what has been suspected on other grounds, that Coal itself is nothing else than a mass of decomposed vege- table matter, chiefly derived from the decay of Coniferous wood. The determination of the characters of the Ferns, Sigillarice, Lepidodendra, Calamites, and other kinds of vegetation whose forms are preserved in the shales and sandstones that are inter- posed between the strata of coal, must be chiefly based on their external characters ; since it is very seldom that any of the speci- mens present any such traces of minute internal structure, as can be subjected to microscopic elucidation. But notwithstand- ing the general absence of any definite form in the masses of de- composed wood of which Coal itself consists (these having ap- parently been reduced to a pulpy state by decay, before the process of consolidation by pressure, aided perhaps by heat, commenced), the traces of structure revealed by the Microscope, are sufficient not only to determine its vegetable origin, but, in some cases, to justify the Botanist in assigning the characters of the vegetation from which it must have been derived. Different specimens of Coal exhibit these structural characters in very different degrees of distinctness; but they uniformly indicate, with a clearness proportionate to their distinctness, that such vegetation must have been Coniferous in its nature, and that it probably approximated most nearly to that group of existing Conifers, to which the Araucarioe belong. These inferences are based upon the fact, that the woody structure consists of woody fibres without interposed vessels ; upon the presence of glandular dots on the woody fibres ; and upon the peculiar arrangement of these dots in two or more rows, alternating one with another (§§ 231, 238). 443. In examining £he structure of Coal, various methods may be followed. Of those kinds which have sufficient tenacity, thin sections may be made ; but the opacity of the substance requires 1 Under this head are included the Cycadece, along with the ordinary Conifera or pine and fir tribe. BOCKS FORMED OF MINUTE ORGANISMS. 629 that such sections should be ground extremely thin, before they become transparent ; and its friability renders this process one of great difficulty. Any section must either cross the woody tissue transversely, so that the appearance it presents will resemble that of Fig. 170 ; or it must traverse it vertically, in which case the fibrous structure will be brought into view, either as in Fig. 171, or as in Fig. 172 ; or it must pass in an intermediate direction. The following method, which would seem not only to be more simple, but also to give more satisfactory results, is recommended by the authors of the " Micrographic Dictionary" (p. 150): — " The coal is macerated for about a week in a solution of carbo- nate of potass ; at the end of that time, it is possible to cut tole- rably thin slices with a razor. These slices are then placed in a watch-glass with strong nitric acid, covered, and gently heated ; they soon turn brownish, then yellow, when the process must be arrested by dropping the whole into a saucer of cold water, or else the coal would be dissolved. The slices thus treated appear of a darkish amber color, very transparent, and exhibit the struc- ture, when existing, most clearly. The specimens are best pre- served in glycerine, in cells ; we find that spirit renders them opaque, and even Canada balsam has the same effect." When the coal is so friable, that no sections can be made of it by either of these methods, it may be ground to fine powder, and the par- ticles may then, after being mounted in Canada balsam, be sub- jected to microscopic examination; the results which this method affords, are by no means satisfactory in themselves ; but they will often enable the organic structure to be sufficiently deter- mined, by the comparison of the appearances presented by such fragments, with those which are more distinctly exhibited else- where. Valuable information may often be obtained, too, by treating the ash of an ordinary coal-fire in the same manner, or (still better) by burning to a white ash a specimen of coal that has been previously boiled in nitric acid, and then carefully mounting the ash in Canada balsam ; for mineral casts of vege- table cells and fibres may often be distinctly recognized in such ash ; and such casts are not unfrequently best afforded by samples of coal, in which the method of section is least successful in bringing to light the traces of organic structure, as is the case, for example, with the "anthracite" of Wales.1 444. Passing on now to the Animal kingdom, we shall first cite some parallel cases in which the essential nature of deposits that form a very important part of the Earth's crust, has been determined by the assistance of the microscope ; and shall then select a few examples of the most important contributions which it has afforded, to our acquaintance with types of Animal life long since extinct. It is an admitted rule in Geological science, that the past history of the Earth is to be interpreted, so far as may be found possible, by the study of the changes which are 1 See Prof. Quekett's Memoir on the Minute Structure of the Torbane-hill Mineral, in "Transact, of the Microsc. Societ." Ser. 2, vol. ii, p. 41, et seq. 630 MICROSCOPIC GEOLOGY. still going on. Thus, when we meet with an extensive stratum of fossilized Diatom.acece (§ 191) in what is now dry land, we can entertain no doubt that this siliceous deposit originally accumu- lated either at the bottom of a fresh-water lake, or beneath the waters of the ocean ; just as such deposits are formed at the pre- sent time, by the production and death of successive genera- tions of these bodies, whose indestructible casings accumulate in the lapse of ages, so as to form layers whose thickness is only limited by the time during which this process has been in action (§ 190). In like manner, when we meet with a limestone-rock entirely composed of the FlG- 334- calcareous shells of Fora- minifera, some of them entire, others broken up into minute particles, we interpret the phenomenon by the fact, that the dredg- ings obtained from certain parts of the ocean-bottom consists almost entirely of remains of existing Fora- minifera, in which entire shells, the animals of which may be yet alive, are mingled with the debris of others that have been reduced by the action of the waves to a fragmentary state.1 Now in the fine white mud which is brought up from almost every part of the sea-bottom of the Levant, where it forms a stratum that is con- tinually undergoing a slow but steady in- crease in thickness, the microscopic re- searches of Prof. "Wil- liamson2 have shown that not only are there multitudes of minute remains of living or- 1 Such a deposit, consisting chiefly of Orbitolites (§ 287) is at present in the act of formation on certain parts of the shores of Australia, as the Author is informed by Mr. J. Beete Jukes; thus affording the exact parallel to the stratum of Orbitolites (belonging, as the Author's investigations have led him to believe, to the very same species) that forms part of the " Calcaire Grossier" of the Paris basin. 2 " Memoirs of the Manchester Literary and Philosophical Society," vol. viii. Microscopic Organisms in Levant Mud : — A, D, siliceous spi- cules of Tethya; B, H, spicules of Geodia ; c, Sponge-spicule (unknown); E, calcareous spicule of Grantia / F, G, M, o, por- tions of calcareous skeleton of Echinodermata; H, I, calcareous spicule of Gorgonia ; K, L, N, siliceous spicules of Halichondria ; P, portion of prismatic layer of shell of Pinna. COMPOSITION OF MARINE DEPOSITS. 631 ganisms, both animal and vegetable, but that it is entirely or almost wholly composed of such remains. Among these were about 26 species of Diatom acese (siliceous), 8 species of Foramini- fera (calcareous), and a miscellaneous group of objects (Fig. 334), consisting of calcareous and siliceous spicules of Sponges and Gorgoniae, and of fragments of the calcareous skeletons of Echi- noderms and Mollusks. 445. ~Now almost exactly the same collection of forms, with the exception of the siliceous Diatomacese, is found in many parts of the " Calcaire Grossier" of the Paris basin, as well as in other extensive deposits of the same early tertiary period. And there is little doubt that a large proportion of the great Creta- ceous (chalk) formation has a like composition ; for many parts FIG 335. Microscopic Organisms in Chalk from Gravesend; a, 6, c, d, Textularia globulosa; c, «, «, Rotalia aspera ; /, Textularia aculeala; g, Plaimlaria hexas: h, Navicula. of it consist in great part of the minuter kinds of Foraminifera, whose shells are imbedded in a mass of apparently amorphous particles, many of which, nevertheless, present indications of being the worn fragments of similar shells, or of larger calcareous organisms. In the Chalk of some localities, Foraminifera consti- tute the principal part of the minute organisms which can be recognized with the microscope (Figs. 335, 336) ; in other in- stances, the disintegrated prisms of Pinna (§ 336) or other large shells of the like structure (as Inoceramus) constitute the great bulk ; whilst in other cases, again, the chief part is made up of the shells of Cytherina, a marine form of Entomostracous Crus- tacean (§ 367). Different specimens of Chalk vary greatly in the 632 MICROSCOPIC GEOLOGY. proportion which the distinctly organic remains bear to the amorphous particles, and which the different kinds of the former bear to each other ; and this is quite what might be anticipated, when we bear in mind the predominance of one or another tribe of animals or plants in the several parts of a large area. True Chalk seems to differ from the Levant Mud, in the small pro- portion which the siliceous remains of Diatomacese bear, in the former, to that which is mingled in the latter with the calcareous FIG. 336. Microscopic Organisms in Chalk from Meudon ; partly seen as opaque, and partly as transparent objects. shells of Foraminifera, &c. ; and it seems doubtful to what ex- tent they were present in the seas of that epoch. Such remains are found in abundance, however, forming marly strata which alternate with those of a chalky nature, in the South of Europe and the North of Africa (Fig. 101) ; and it is surmised by Prof. Ehrenberg, that the layers of flint which the British Chalk con- tains, have been derived by some metamorphic process from similar layers of siliceous Diatomacese which have disappeared. It is now certain, however, that the deposits referred to by Prof. Ehrenberg are of an age later than that of the great Chalk for- mation ; so that little support is furnished by their phenomena to his hypothesis. But whatever may have been the origin of the siliceous material, it may be stated as a fact beyond all question, that nodular flints and other analogous concretions (such as agates) may generally be considered as fossilized Sponges or Alcyonian Zoophytes; since not only are their external forms and their superficial markings often highly cha- ROCKS COMPOSED OF FORAMINIFERA. 633 racteristic of those organisms, but, when sections of them are made sufficiently thin to be transparent, a spongy texture may be most distinctly recognized in their interior.1 It is curious that many such sections contain well-preserved specimens of Xanthidia, which are Desmidiacese whose divided body is covered with long spinous projections, often cleft, and sometimes fur- nished with hooks at their extremities ; and we occasionally also find upon their surface, or even imbedded in their substance, Foraminiferous shells (especially Rotalioe), in which not only the substance of the shell has undergone silicification, but also that of the soft animal body, the shrunken form of which may be re- cognized in the dark carbonaceous hue imparted to the central portion of the silex which fills each chamber. 446. In examining Chalk or other similar mixed aggregations, whose component particles are easily separable from each other, it is desirable to separate, with as little trouble as possible, the larger and more definitely organized bodies, from the minute amorphous particles ; and the mode of doing this will depend upon whether we are operating upon the large or upon the small scale. If the former, a quantity of soft chalk should be rubbed to powder with water, by means of a soft brush ; and this wrater should then be proceeded with, according to the method of levi- gation already directed for separating the Diatomacese (§ 192). It will usually be found that the first deposits contain the larger Foraminifera, fragments of shell, &c., and that the smaller Fora- minifera and Sponge spicules fall next; the fine amorphous particles remaining diffused through the water after it has been standing for some time, so that they may be poured away. The organisms thus separated should be dried, and mounted in Canada balsam. If the smaller scale of preparation be preferred, as much chalk scraped fine as will lie on the point of a knife, is to be laid on a drop of water on the glass slide, and allowed to remain there for a few seconds ; the water, with any particles still floating on it, should then be removed ; and the sediment left on the glass should be dried and mounted in balsam. For examining the structure of flints, such chips as may be obtained with a hammer will commonly serve very well: a clear translu- cent flint being first selected, and the chips that are obtained being soaked for a short time in turpentine (which increases their transparency), those which show organic structure, whether Sponge tissue or Xanthidia, are to be selected and mounted in Canada balsam. The most perfect specimens of sponge-struc- ture, however, are only to be obtained by slicing and polishing, — a process which is best performed by the lapidary. 447. There are various other deposits, of less extent and importance than the great Chalk formation, which are, like it, composed in great part of microscopic organisms, chiefly minute 1 See Mr. Bowerbank's Memoirs in the " Transact, of the Geolog. Societ." 1840, and in the " Ann. of Nat. Hist." 1st Ser. vols. vii, x. 634 MICROSCOPIC GEOLOGY. Foraminifera ; and the presence of animals of this group may be recognized, by the assistance of this instrument, in sec- tions of calcareous rocks of various dates, whose chief materials seem to have been derived from Corals, Encrinite stems, or Molluscous shells. Thus in the "Crag" formation (tertiary) of the eastern coast of England, the greater portion of which is perceived by the unassisted eye to be composed of fragments of Shells, Corals (or rather Polyzoaria, § 325), and Echinodermata, the microscope enables us to discover Foraminifera, minute fragments of shells and corals, and spicules of Sponges ; the aggregate being such as is at present in process of formation on many parts of our shores, and having been, therefore, in all probability, a "littoral" formation, whilst the Chalk (with other formations chiefly consisting of Foraminifera) was deposited at the bottom of deeper waters. Many parts of the Oolitic (secon- dary) formation have an almost identical character, save that the forms of organic life give evidence of a different age; and in those portions which exhibit the "roe-stone" arrangement from which the rock derives its name (such as is beautifully displayed in many specimens of Bath-stone and Portland-stone), it is found by microscopic examination of transparent sections, that each rounded concretion is composed of a series of concentric spheres enclosing a central nucleus, which nucleus is often a Foramini- ferous shell. In the Carboniferous (palaeozoic) limestone, again, well-preserved specimens of Foraminifera present themselves; and there are certain bands of limestone of this epoch in Rus- sia, varying in thickness from fifteen inches to five feet, and frequently repeated through a vertical depth of two hundred feet, which are almost entirely composed of Foraminiferous shells belonging to a genus now extinct, the Fusulina. 448. It is not only, however, in the condition of organisms of microscopic size, that the Foraminifera have contributed iix an important degree to the formation of the solid crust of the earth ; for the Nummulitic limestone,1 — which forms a band, often 1800 miles in breadth, and frequently of enormous thickness, that may be traced from the Atlantic shores of Europe and Africa, through Western Asia to Northern India and China, and over vast areas of North America likewise, — is composed of an aggre- gation of larger bodies belonging to the same type ; the " matrix," or rock-substance, in which these are imbedded, being itself usually made up (as microscopic examination of their sections demonstrates) of the comminuted particles of similar organisms, and of smaller Foraminifera ; although it is sometimes composed (as in the British beds of London Clay which include Num- mulites) of accumulations of clayey or other inorganic particles. The structure of the Nummulite itself, as elucidated by micro- scopic examination, presents some extremely remarkable modi- fications of the ordinary Foraminiferous type. It is composed 1 The Pyramids of Egypt are made of this material. STRUCTURE OF NUMMULITE. 635 of a series of chambers, symmetrically disposed in a spiral round a centre, so that a section through its median plane would pre- sent very much the appearance of Fig. 209 ; but each whorl in- vests all the preceding whorls, so as to form a new layer over the entire surface of the disk ; and this layer is usually separated from that which it covers, by an intervening space, which is divided into smaller spaces of more or less regular form, by pro- longations from the partitions that divide the chambers of the central plane. These prolongations are very differently arranged in different species ; thus in some, as Nummulites distans, they keep their own separate course, tending towards the centre ; whilst in others, as N. Icevigata, they inosculate with each other, so as to divide the space that intervenes between one layer and another into an irregular network. Hence in a verti- cal section, such as that of which a part is shown in Fig. 337, we FIG. 337. Vertical Section of portion of Nummulites Icevigata:— a, margin of external whorl; 6, one of the outer row of chambers ; c, c, whorl invested by a ; d, one of the chambers of the fourth whorl from the margin; e, e', marginal portions of the enclosed whorls ; /, investing portion of outer whorl ; g, g, spaces left between the investing portions of successive whorls; h, h, sections of the partitions dividing these. see not only the succession of chambers along the central plane, each of them having its own roof and floor, and its own lateral par- titions dividing it from other chambers of the same whorl, but we also see the superposition of layers over the inner whorls ; so that any chamber d in a whorl that is surrounded by three others, is shut in above and below, not only by its proper shelly covering, but by three additional layers formed by the pro- longation of the shelly investments of the external whorls ; and in like manner, the innermost of the chambers here represented (that nearest e) is enclosed by nine layers above and below, in addition to that by which it is itself covered, these nine layers being extensions of the covering of the nine whorls that surround it. ISTotwithstanding that the inner chambers are thus so deeply buried in the mass of investing whorls, yet there is evidence that the segments of sarcode which they contained, were not cut off from communication with the exterior ; but that they may have retained their vitality to the last. The shell itself is almost every- where minutely porous, being penetrated by parallel tubuli, 636 MICROSCOPIC GEOLOGY. which pass from one surface to the other like those of dentine. These tubes are shown, as divided lengthways by a vertical sec- tion, in Fig. 338 (a, a) ; whilst the appearance they present when tL Portion of a thin section of .ZVwmmMWes Icevigata, taken in the direction of the preceding, highly magnified to show the minute structure of the shell : — a. a, portions of the ordinary shell-substance traversed by parallel tubuli ; b, b, portions forming the marginal wall, traversed by diverging and larger tubuli ; c, one of the chambers laid open; d, d, d, pillars of solid substance not perforated by tubuli. cut across in a horizontal section is shown in Fig. 339, the transparent shell: substance, a, a, a, being closely dotted with minute punctations which mark their orifices. In that portion of the shell, however, which forms the margin of each whorl (Fig. 338, 6, 5), the tubes are larger, and diverge from each other at greater intervals ; whilst at certain other points, d, c?, d, the shell-substance is not perforated by tubes, but is peculiarly dense in its texture, forming solid pillars which seem to strengthen the other parts. In Nummulites whose surfaces have been much exposed to attrition, it commonly happens that the pillars of the superficial layer, being harder than the ordinary shell-substance, and being consequently less worn down, are left as prominences ; the presence of which has often been accounted (but erroneously) as a specific character. The successive chambers of the same whorl communicate with each other by a passage left between the inner edge of the partition that separates them, and the margin of the preceding whorl that forms their inner boundary; this passage is sometimes a single large broad aperture, but is more commonly formed by the more or less complete coalescence of several separate perforations, as is seen in Fig. 337, b. Such marked differences in this respect are observable in the several parts of one and the same specimen, that it is obvious that very little account should be taken of differences in the form of aper- ture, as affording specific or generic distinctions among Fora- minifera of this type. But besides the foregoing means of com- munication, by which the segments of sarcode'included in the inner chambers were enabled to continue receiving supplies of nutriment, we meet in Nummulites with a remarkable develop- ment of that system of " interseptal" canals, one of the most characteristic examples of which among recent Foraminifera is ORBITOIDES. 637 FIG. 339. presented in Faujasina, as already described (§ 291). These canals are frequently found to be filled up in JSTummulites by the fossilizing material ; but a care- ful examination will generally disclose traces of them in the middle of the partitions that divide the chambers (Fig. 339, b, b)^ while from these may be seen to proceed the lateral branches (c, c) which, after bur- rowing (so to speak) in the walls of the chambers, enter them by large orifices (d). As the gene- ral distribution of this system of canals in the Nummulite is the same as that shown in Faujasina (Fig. 209), and as the canals, although smaller, are far more numerous, it is obvious that through its means the segments of sarcode occupying the cham- bers of the most internal walls could send their pseudopodial extensions at once to the exterior. Of all Foraminifera, the Nummulite is undoubtedly one of the most highly developed types ; and its extraordinary multiplication at the earliest part of the Tertiary period, is a very curious feature in the Earth's history. It is commonly considered that this type is now extinct ; but the Author, in common with Prof. Wil- liamson, is disposed to question whe- ther there is any essential difference between Nummulites and the existing genus Nonionina, which is very abun- dant in certain localities; since in many species of Nummulites, as in ISTonionina, the investing layers of the successive whorls are in immediate contact with those that have preceded them, instead of being separated, as in Fig. 337, by spaces prolonged from the cavities of the chambers. 449. The same Nummulitie lime- stone also contains, in certain locali- Seclion of oroides Prattu, parallel to , , ,, -r- , the surface ; traversing at a, a, the super- tieS (aS the SOUthwest OI _b ranee, north- ficjal layer, and at 6,6, the median layer. eastern India, &c.) a vast abundance of discoidal bodies termed Orbitoides, which are so similar to Nummulites as to have been taken for them, but which, while Portion of Horizontal Section of Num- mulite, showing the structure of the walls and of the septa of the chambers:— a, a, a, portion of the wall covering three chambers, the punctations of which are the orifice's of tubuli ; b, 6, septa between these chambers, containing canals which send out lateral branches, c, c, entering the chambers by larger orifices, one of which is seen at d. FlG. 340. 638 MICROSCOPIC GEOLOGY. still Foraminiferous, are formed upon a plan of structure altoge- ther different. On account of the minuteness of their parts, and the completeness of their fossilization, their structure can only be elucidated by sections thin enough to be examined by the microscope with transmitted light ; and it is consequently to the assistance afforded by this instrument, that we are indebted for our knowledge of the curious type of organization which it pre- sents. When one of these disks (which vary in size, in different species, from that of a four-penny piece to that of half a crown) is rubbed down so as to display its internal organization, two different kinds of structure are usually seen in it; one being composed of chambers of very definite form, quadrangular in some species, circular in others, arranged with a general but not constant regularity in concentric circles (Figs. 340, 341, 6, 5) ; the other, less transparent, being formed of minuter cells which have no such constancy of form, but which might almost be taken for the pieces of a dissected map (Figs. 340, 341, a, a). In the upper FIG. 341. Portions of the same section, more highly magnified : — a, superficial layer; 6, median layer. and lower walls of these last, minute punctations may be ob- served, which seem to be the orifices of connecting tubes whereby they are perforated. The relations of these two kinds of struc- ture to each other, are made evident by the examination of a vertical section (Fig. 342) ; which shows that the portion a, Figs. FIG. 342. Vertical Section of Orlritoides Prattii, showing the large central cell at a, and the median layer surrounding it, covered above and below by the superficial layers. 340, 341, forms the central plane, its concentric circles of cells being arranged round a large central cell a, as in Orbitolite (Fig. 206) ; whilst the cells of the portion b are irregularly superposed one upon the other, so as to form several layers, which are most numerous towards the centre of the disk, and thin away gradu- ally towards its margin. By the perforations in these layers, the pseudopodia proceeding from the central plane of chambers may have found their way direct to the surface, or at any rate would have been brought into connection with the segments lying nearer to it. No organisms precisely resembling the Orbitoides, ORIGIN OF ROCK-FORMATIONS. 639 are known to exist at present ; but there are some which differ from it so little, that a knowledge of their structure helps mate- rially to elucidate points, which would otherwise be rendered obscure in it through the changes induced by fossilization.1 450. The foregoing details, taken in addition to the facts of like nature that have been mentioned in previous parts of this work (as, for example, in § 294), will serve as examples of the essential importance of microscopic investigation, in determin- ing, on the one hand, the real character of various stratified deposits, and, on the other, in elucidating the nature of the organic remains which these may include. The former of these lines of inquiry has not yet attracted the attention which it deserves; since, as is very natural, the greater number of Micro- scopists are more attracted by those definite forms which they can distinctly recognize, than by the amorphous sediments which present no definite structural characters. Yet it is a question of extreme interest to the Geologist, to determine how far these had their origin in the disintegration of organic structures; and much light may often be thrown upon this question by careful microscopic analysis. Thus the author having been requested by Mr. Chas. Darwin, about twelve years since, to examine into the composition of the extensive calcareous deposit which covers the surface of the Pampas region of South America, and to compare it with that of the calcareous tufa still in process of formation along the coast of Chili, was able to state that their constituents were in all probability essentially the same, not- withstanding the difference in their mode of aggregation. For the Chilian tufa is obviously composed in great partof fragments of shells, distinguishable by the naked eye; the dense matrix in which these are imbedded is chiefly made up of minuter fragments, only distinguishable as such by the microscope; while through the midst of these is diffused an aggregation of amorphous particles, that present every appearance of having originated in the yet finer reduction of the same shells, either by attrition or by decomposition. In the Pampas deposit, on the other hand, the principal part was found to be composed of amorphous particles, so similar in aspect to those of the Chilian rock that their identity could scarcely be doubted ; and scattered at intervals through these were particles of shell, distinctly recognizable by the microscope, though invisible to the naked eye. Thus, although the evidence afforded by the larger frag- ments of shell was altogether wanting in the Pampas deposit, it could not be doubted that the materials of both were the same, those of the Pampean formation having been subject to greater comminution than those of the Chilian ; and this view served to 1 See the Author's Memoir on the Microscopic Structure of Nummulite, Orbitolite, and Orbitoides. in the "Quart. Journ. of the Geolog. Society," for Feb., 1850; and the ad- mirable " Description des Animaux Fossiles du Groupe Nummulitique de Tlnde," by MM. D'Archiac and Jules Haime. 640 MICROSCOPIC GEOLOGY. confirm, whilst it was itself confirmed by, the idea thought most probable on other grounds by Mr. Darwin, that the Pampean formation was slowly accumulated at the mouth of the former estuary of the Plata, and in the sea adjoining it.1 A similar line of inquiry has been of late systematically pursued by Mr. R. C. Sorby ; who has applied himself to the microscopic study of the composition of fresh- water marls and limestones, by ascertaining the characters and appearances of the minute particles into which shells resolve themselves by decay, and by estimating the relative proportions of the organic and inorganic ingredients of a rock, by delineating on paper (by means of the camera lucida) the outlines of the particles visible in thin sections, then cutting them out, and weighing the figures of each kind.2 451. It is obvious that, under ordinary circumstances, only the hard parts of the bodies of animals that have been entombed in the depths of the earth, are likely to be preserved ; but from these a vast amount of information may be drawn ; and the inspection of a microscopic fragment will often reveal, with the utmost certainty, the entire nature of the organism of which it formed part. In the examination of the minuter fossil Corals, and of those Polyzoaries (§ 325) which are commonly ranked with them, the assistance of the microscope is indispensable. Minute fragments of the "test" or "spines" of Echinodermata, and of all such Molluscous shells as present distinct appear- ances of structure (this being especially the case with the Brachio- poda, and with the families of Lamellibranchiate bivalves most nearly allied to them), may be unerringly identified by its means, when the ex^rnal form of these fragments would give no assist- ance whatever. In the study of the remarkable ancient group of Trilobites, not only does a microscopic examination of the casts which have been pre- FlQ- 343- served of the surface of their eyes (Fig. 343), serve to show the entire conformity in the structure of these organs to the "composite" type which is so remarkable a character- ^__ istic of the higher Articu- Eye of Troobtte. lata (§ 383), but it also brings to light certain peculiarities which help to determine the division of the great Crustacean series with which this group has most alliance.3 It is in the case of the Teeth, the Bones, and the Dermal skeleton of Vertebrated animals, however, that the value of Microscopic inquiry becomes most apparent; since the structure of these presents so many characteristics that are subject to well-marked variations in their several classes, orders, and families, that a knowledge of these 1 See Mr. C. Darwin's "Geological Observations on South America," p. 32. 2 See " Quart. Jonrn. of Geolog. Science,'1 1853, p. 344. 8 See Prof. Burmeister " On the Organization of the Trilobites," published by the Ray Society, p. 19. FOSSIL TEETH. 641 characters frequently enables the Microscopist to determine the nature of even the most fragmentary specimens, with a positive- ness which must appear altogether misplaced to such as have not studied the evidence. 452. It was in regard to Teeth, that the possibility of such de- terminations was first made clear by the laborious researches of Prof. Owen ;* and the following may be given as examples of their value : — A rock formation extends over many parts ot Russia, whose mineral characters might justify its being likened either to the Old or to the New Red Sandstone of this country, and whose position relatively to other strata is such, that there is great difficulty in obtaining evidence from the usual sources, as to its place in the series. Hence the only hope of settling this question (which was one of great practical importance, since, if the formation were new red, Coal might be expected to underlie it, whilst if old red, no reasonable hope of coal could be enter- tained) lay in the determination of the Organic remains which this stratum might yield ; but unfortunately these were few and fragmentary, consisting chiefly of teeth, which are seldom per- fectly preserved. From the gigantic size of these teeth, together with their form, it was at first inferred that they belonged to Saurian Reptiles, in which case the sandstone must have been considered as New Red ; but microscopic examination of their intimate structure unmistakably proved them to belong to a genus of Fishes (Dendrodus) which is exclusively Palaeozoic, and thus decided that the formation must be Old Red. So again, the microscopic examination of certain fragments of teeth found in a Sandstone of Warwickshire, disclosed a most remarkable type of tooth-structure (shown in Fig. 344), which was also ascertained to exist in certain teeth that had been discovered Fl«- 344. in the " keuper-sandstein" of Wirtemberg; and the identity or close resem- blance of the animals to which these teeth belonged having been thus establish- ed, it became almost cer- tain that the Warwickshire and Wirtemberg sand- stones were equivalent for- mations, a point of much geological importance. The next question arising out of this discovery, was the nature of the animal (provisionally termed La- byrinthodon, a name expressive of the most peculiar feature in its of Tooth of Labyrinthodon. 1 See his magnificent u Odontography." 41 642 MICROSCOPIC GEOLOGY. dental structure) to which these teeth belonged. They had been referred, from external characters merely, to the order of Saurian Reptiles ; but these characters were by no means conclusive ; and as the nearest approaches to their peculiar internal structure are presented by Fish-Lizards and Lizard-like Fish, it might be rea- sonably expected that the Labyrinthodon would combine with its reptilian characters an affinity to fish. This has been clearly proved to be the case, by the subsequent discovery of parts of its skeleton in which such characters are very obvious ; and by a very beautiful chain of reasoning, Prof. Owen succeeded in establishing a strong probability, that the Labyrinthodon was a gigantic Frog-like animal five or six feet long, with some pecu- liar affinities to Fishes, and a certain mixture also of Crocodilian characters ; and that it made the well-known foot-prints which have been brought to light, after an entombment whose duration can scarcely be conceived (much less estimated), in the Stourton quarries of Cheshire. 453. The more recent researches of Prof. Quekett on the minute structure of Bone,1 promise to be scarcely less fruitful in valuable results. From the average size and form of the " lacunse," their disposition in regard to each other and to the Haversian canals, and the number and course of the canaliculi, he feels assured that the nature of even a minute fragment of bone may be determined with a considerable approach to certainty; and the following examples, among many which might be cited, ap- pear to justify such assurance. Dr. Falconer, the distinguished investigator of the fossil remains of the Himalayan region, and the discoverer of the gigantic fossil Tortoise of the Sivalik Hills, having met with certain small bones about which he was doubt- ful, placed them in the hands of Prof. Quekett for minute exami- nation; and was informed, on microscopic evidence, that they might certainly be pronounced Reptilian, and probably belonged to an animal of the tortoise tribe ; and this determination was fully borne out by other evidence, which led Dr. Falconer to con- clude that they were toe bones of his great tortoise. Some frag- ments of bone were found, some years since, in a chalk-pit; which were considered by Prof. Owen to have formed part of the wing-bones of a long-winged sea-bird allied to the Albatross. This determination, founded solely on considerations derived from the very imperfectly preserved external forms of these frag- ments, was called in question by some other palaeontologists; who thought it more probable that these bones belonged to a large species of the extinct genus Pterodactylus, a flying-lizard, whose wing was extended upon a single immensely prolonged digit. No species of Pterodactyle, however, at all comparable to this in dimensions, was at that time known ; and the characters 1 See his Memoir on the " Comparative Structure of Bone," in the " Transac. of the Microsc. Societ." Ser. 1, vol. ii; and the "Catalogue of the Histological Museum of the Roy. Coll. of Surgeons," vol. ii. FOSSIL BONE. 643 furnished by the configuration of the bones, not being in any degree decisive, the question would have remained unsettled, had not an appeal been made to the Microscopic test. This appeal was so decisive, by showing that the minute structure of the bone in question corresponded exactly with that of Pterodactyle bone, and differed essentially from that of every known Bird, that no one who placed the least reliance upon that evidence could enter- tain the slightest doubt on the matter. By Prof. Owen, however, the validity of that evidence was questioned, and the bone was still maintained to be that of a bird ; until the question was finally set at rest, and the value of the microscopic test triumphantly confirmed, by the discovery of undoubted Pterodactyle bones of corresponding and even of greater dimensions, in the same and other chalk quarries.1 1 See Prof. Owen's Monograph on the British Fossil Reptiles of the Chalk Formation, p. 80, et scq. CHAPTER XX. INORGANIC OR MINERAL KINGDOM. — POLARIZATION. 454. ALTHOUGH by far the most numerous and most important applications of the Microscope, are those by which the structure and actions of Organized beings are made known to us, yet there are many Mineral substances which constitute both interesting and beautiful objects ; being remarkable either for the elegance of their forms, or for the beauty of their colors, or for both com- bined.* The natural forms of inorganic substances, when in any way symmetrical, are so in virtue of that peculiar arrangement of their particles which is termed crystallization ; and each sub- stance which crystallizes at all, does so after a certain type or plan, — the identity or difference of these types furnishing cha- racters of primary value to the Mineralogist. It does not follow, however, that the form of the crystal shall be constantly the same for each substance ; on the contrary, the same plan of crystallization may exhibit itself under a great variety of forms ; and the study of these, in such minute crystals as are appropriate subjects for observation by the microscope, is not only a very interesting application of its powers, but is capable of affording some valuable hints to the designer. This is particularly the case with crystals of Snow, which belong to the "hexagonal system," the basis of every figure being a hexagon of six rays ; for these rays " become incrusted with an endless variety of secondary formations of the same kind, some consisting of thin laminae alone, others of solid but translucent prisms heaped one upon another, and others gorgeously combining laminae and prisms in the richest profusion ;"! the angles by which these figures are bounded, being invariably 60° or 120°. Beautiful arborescent forms are not unfrequently produced by the peculiar mode of aggregation of individual crystals ; of this we have often an example on a large scale on a frosted window ; but micro- scopic crystallizations sometimes present the same curious phe- nomenon (Fig. 345). In the following list are enumerated some of the most interesting natural specimens, which the Mineral kingdom affords as microscopic objects ; these should be viewed by reflected light, under a very low power : — 1 See Mr. Glaisher's Memoir on "Snow-Crystals in 1855," with a number of beauti- ful figures, in " Q.uart. Journ. of Micros. Sci." vol. iii, p. HO. CRYSTALLIZATION OF SALTS. 645 Antimony, sulphuret. Asbestos. Aventurine. ditto, artificial. Copper, native. arseniate. malachite-ore. peacock-ore. pyrites (sulphuret). ruby-ore. Iron, ilvaite or Elba-ore. pyrites (sulphuret). Lapis lazuli. Lead, oxide (minium). sulphuret (galena). Silver, crystallized. Tin, crystallized. oxide. sulphuret. Zinc, crystallized. 455. The actual process of the Formation of Crystals may be watched under the microscope with the greatest facility ; all that is necessary being to lay on a slip of glass, previously warmed, a FIG. 345. saturated solution of the salt, and to incline the stage in a slight degree, so that the drop shall be thicker at its lower than at its upper edge. The crystallization will speedily begin at the upper edge, where the proportion ot liquid to solid is more speedily re- duced by evaporation, and will gradually extend downwards. It it should go on too slowly, or should cease altogether, whilst yet a large proportion of the liquid remains, the slide may be again warmed, and the part already soli- crystallized saver. dified may be redissolved; after which the process will recommence with increased rapidity. This interesting spectacle may be watched under any microscope ; and the works of Adams and others among the older observers, testify to the great interest which it had for them. It becomes far more striking, however, when the crystals, as they come into being, are made to stand out bright upon a dark ground, by the use of the spotted lens, the paraboloid, or any other form of black-ground illumination ; still more beautiful is the spectacle when the Polarizing apparatus is employed, so as to invest the crystals with the most gorgeous variety of hues. The following list specifies the salts and other mineral substances, whose crys- talline forms are most interesting. When these are viewed with polarized light, some of them exhibit a beautiful variety of colors of their own, whilst others require the interposition of the selenite plate for the development of color. Acetate of Copper. of Manganese. of Soda. of Zinc. Agate (transparent sections). Alum. Arragonite (transparent sections). Arseniate of Potass. Bicarbonate of Potass. Bichromate of Potass. Bichloride of Mercury. Boracic acid. 646 INORGANIC OR MINERAL KINGDOM. Borate of Ammonia. of Soda (borax). Carbonate of Lime (from urine of horse), of Potass, of Soda. Chlorate of Potass. Chloride of Barium, of Cobalt, of Sodium. Cholesterine. Chromate of Potass. Citric Acid. Cyanide of Mercury. Granite (transparent sections). Hypermanganate of Potass. Iodide of Potassium. of Quinine. Mannite. Murexide. Muriate of Ammonia. Nitrate of Ammonia, of Barytes. of Bismuth, of Copper, of Potass, of Soda, of Strontian. Nitrate of Uranium. Oxalic acid. Oxalate of Ammonia, of Chromium, of Lime, of Potass, of Soda. Oxalurate of Ammonia. Phosphate of Ammonia. Ammoniaco-Magnesian (tri- ple, of urine), of Soda. Prussiate of Potass (red). (yellow). Salicine. Sulphate of Ammonia, of Cadmium, of Copper. of Copper, ammoniated. of Iron, of Magnesia, of Potassa. of Soda, of Zinc. Tartaric Acid. Uric Acid. Urate of Ammonia, of Soda. 456. It not unfrequently happens that a remarkably beautiful specimen of crystallization developes itself, which the observer desires to keep for display. In order to do this successfully, it is necessary to exclude the air; and Mr. Warington recommends castor-oil as the best preservative. A small quantity of this should be poured on the crystallized surface, a gentle warmth applied, and a thin glass cover then laid upon the drop, and gradually pressed down ; and after the superfluous oil has been removed from the margin, a coat of marine glue or other varnish is to be applied. 457. Although most of the objects furnished by Vegetable and Animal structures, which are advantageously shown by Polarized light, have been already noticed in their appropriate places, it will be useful here to recapitulate the principal, with some additions. Vegetable. Cuticles, Hairs, and Scales, from Leaves (§§ 220, 246). Fibres of Cotton and Flax. Raphides (§ 230). Spiral cells and vessels (§§ 228, 232). Starch grains (§ 229). Wood, longitudinal sections of, mounted in balsam (§ 239). Animal. Fibres and Spicules of Sponges (§ 296). Polypidoms of Hydrozoa (§305). Polyzoaries (§ 330). Spicules of Gorgonia3 (§ 309). Tongues (Palates) of Gasteropoda (1 347). Scales of Fishes (§|408, 409). Sections of Hairs ($ 411). of Quills (§412). of Horns (§413). of Shells (§336). of Skin (§418). of Teeth (§§406,407). of Tendon, longitudinal. APPENDIX. , THE MICROSCOPE AS A MEANS OF DIAGNOSIS. APPENDIX BY THE EDITOR. THE MICROSCOPE AS A MEANS OF DIAGNOSIS. Value of the Microscope in the Diagnosis of Disease. — During the past few years the Microscope, in the hands of the physician, has become an indispensable auxiliary in the detection and diagnosis of disease. The anatomist in his researches into the structure and functions of various organs, and the physiologist in his at- tempts to unveil the mysterious phenomena of life, alike find in this instrument a valuable coadjutor. Indeed, the invention of the microscope has added to the already extensive list of the sciences another — Histology — full of importance and interest, constituting as it necessarily does, the basis of pathology. The results flowing from its application to medical inquiries are so important, that it has, at length, been assigned a place in the same category as the stethoscope, pleximeter, speculum, and other well-tried instruments employed in physical exploration. By its aid such an extensive acquaintance with the intimate structure of the tissues of the animal economy, both in health and disease, has been obtained, that the practitioner can now pursue his difficult profession, with far more accurate and rational views of the nature and pathology of the various affections which he is called upon to treat, than were enjoyed by his predecessors. Individual cases are constantly occurring where long-known and well-attested methods of investigation have signally failed to elicit the information necessary to a rational treatment. Upon many such cases the microscope casts a flood of light. We have but to glance over the rapidly increasing literature of our profes- sion, to discover many proofs of the obligations of practical and scientific medicine to the invention arid judicious application of the microscope. By its aid the impositions so frequently prac- tised upon the physician have been often detected. One of the most common of these attempts at deception consists in mixing various substances with urine, the patient pretending that he voided them by the urethra. Some of these, such as starch, flour, and sand, are readily detected by subjecting the fluid to 650 APPENDIX. microscopic examination ; and even where milk has heen added, it can be distinguished by the presence of oil-globules, from the so-called chylous urine, in which the fatty matter is found in a molecular state. In the same way have been ascertained the nature and origin of many strange and unusual substances dis- charged from the bowels. Drs. Bennett, Todd, Quekett, and others have placed upon record, numerous instances of the value of the microscope, in detecting impositions and establishing a certain diagnosis in obscure cases of disease. " Some years ago, I was summoned to see a Dispensary patient laboring under bronchitis, who was spitting florid blood. On examining the sputum with a microscope, I found that the colored blood-corpuscles were those of a bird. On my telling her that she had mixed a bird's blood with the expectoration, her astonishment was unbounded, and she confessed that she had done so for the purpose of imposition."1 The malignant or non-malignant character of certain sus- picious tumors has, on many occasions, been positively settled by recourse to the microscope, as the following example will show. " An eminent surgeon, in London, was treating a case of what he considered to be pharyngeal abscess. Before opening it, however, he scraped off a little of the matter on its surface with his nail, and took it to Mr. Quekett, who told me that on examin- ing it with a microscope, he found it to contain numerous can- cer-cells. The tumor was allowed to progress uninterruptedly ; and on the death of the individual, some months afterwards, the bones at the base of the cranium were found to be enlarged, from a cancerous growth."2 That medico-legal science has been greatly enriched and ren- dered far more certain in its results by the aid of the microscope, few persons will deny. The ends of justice have sometimes de- pended solely upon its power of detecting spermatozoa in cases of rape, of distinguishing between the stains of blood and those of colored fluids, or of pointing out the difference between human hair and that of animals. The microscope has afforded valuable assistance to the patho- logist, in disclosing the obscure processes by which changes or alterations in nutrition have gradually produced, in some of the most complex tissues of the body, the peculiar morbid condition known as fatty degeneration. In the hands of skilful observers, this instrument has taught us that apoplexy is not always de- pendent upon a plethoric or hypersemic state of the cerebral vessels, but is, in many instances, the result of altered nutrition affecting the structure of these vessels, impairing their strength and elasticity, and otherwise altering their properties and func- tions. A brief microscopic examination of the urine is not unfrequently sufiicient, as the laborious researches of Dr. John- 1 Bennett's Introduction to Clinical Medicine. 2 Bennett, op. cit. THE MICROSCOPE AS A MEANS OF DIAGNOSIS. 651 son have shown, to reveal, during life, the existence of this pecu- liar pathological condition in the kidney. But while, on the one hand, we urge upon the student the importance of the study of microscopy, and direct his attention, by way of proof, to the brilliant labors of the German School of Histologists ; on the other, we must caution him against its ex- clusive cultivation, to the neglect of the other established and reliable modes of investigation of which the intelligent physician avails himself, in the daily routine of business. " You must not suppose," writes Dr. Bennett,1 "that an additional method of gaining information implies abandonment of those, the utility of which has stood the test of experience. Men must learn the every-day use of their senses ; must know how to feel, hear, and see, in the same manner as they did before instruments were invented. We don't see the stars less clearly with our naked sight, because the telescope is necessary for an astronomer. Neither should a physician observe the symptoms of a disease less accurately because he examines the chest with a stethoscope, or a surgeon be less dexterous with the knife, because it is only by means of the microscope he can determine with exactitude the nature of a tumor." " We should learn to distinguish be- tween the mechanical means necessary for arriving at truths, and those powers of observation and mental processes which enable us to recognize, compare, and arrange the truths themselves. In short, rather endeavor to observe carefully and reason correctly on the facts presented to you, than waste your time in altering the fashion and improving the physical properties of the means by which facts are ascertained. At the same time, these are ab- solutely necessary; and perhaps no kind of knowledge has been so much advanced in modern times by the introduction of in- struments and physical means of investigation, as that of medi- cine. These enable the practitioner to extend the limits to which otherwise his senses would be limited. I do not say em- ploy one to the exclusion of the other, but be equally dexterous in the use of all. Do not endeavor to gain a miserable reputation as a microscopist, or as a stethoscopist ; but by the appropriate application of every instrument and means of research, seek to arrive at the most exact diagnosis and knowledge of disease, so as to earn for yourselves the title of enlightened medical practitioners." As with all other mechanical aids to the senses, the microscope, to be successfully applied in medicine, requires a degree of skill in its manipulation not to be acquired at once, but by repeated and persevering practice. Let the student, therefore, not be dis- couraged by the many failures and uncertainties which, to a greater or less extent, must necessarily accompany his early efforts. Let him remember that exact and accurate habits of observation are acquired slowly and almost insensibly, and that, in attempting to obtain proficiency in any practical art or science, 1 Introduction to Clinical Medicine. 652 APPENDIX. a methodical and systematic procedure is always requisite. So many unknown objects, so many strange and unusual forms, so many structural peculiarities are revealed to the eyes of the tyro in microscopy, that he is at once plunged into profound con- fusion, from which he can extricate himself only by adopting the most laborious and rigid system of observation. He should examine with the utmost care the physical appearance and cha- racter of the ultimate structures ; he should note the exact shape of the object, whether round or oval, globular or flat, &e. ; the peculiarities of its edge or border, whether fine and brilliantly illuminated, or dark and abrupt, whether smooth or rough, re- gular or irregular, serrated or beaded, &c. Peculiarities of color produced by strong and faint, and by reflected and transmitted light, should next claim his attention. The size of the object should, in all cases, be obtained by actual measurement, and all variations in diameter noted.1 The transparency must also be observed, — whether the body be opaque or diaphanous. If opaque, the degree of opacity must be stated, its causes, and the effects upon the transmission of the luminous rays. The superficial and deep-seated layers, and in the case of cellular and tubular bodies, the contents should also be investigated ; and lastly, the effects of various' reagents upon these physical pro- perties must be ascertained with the same care and patience. It will thus be seen that the successful application of the micro- scope to the diagnosis of disease, requires a very considerable acquaintance with the healthy appearance and structure of both the animal and vegetable tissues. Armed with this preliminary knowledge, the student will be surprised at the facility with which he will be enabled to distinguish from each other the various animal solids and fluids, different morbid products, the matters constituting food, &c., whether these be unchanged, or in a state of disintegration from the processes of mastication, digestion, ulceration, &c. In view, however, of the great difficulty experienced in demon- strating accurately the histological character of the healthy tissues, and the still greater difficulty of making out the charac- teristics of morbid growths, the student should exercise great caution and deliberation in pronouncing an opinion upon the nature of any morbid tissues examined by him. The student should early acquire the habit of recording all his observations in a note-book kept expressly for the purpose. He should exercise himself also in making drawings, as exact as possible, of all the objects he examines. Such a practice, though laborious, will leave upon his mind a more vivid and lasting im- pression of the various objects of his research, and gradually render him a very close and reliable observer. EXAMINATION OF THE NERVOUS SYSTEM. 653 FIG. 346. EXAMINATION OF THE NERVOUS SYSTEM. Brain. — As the nerve-fibres rapidly undergo change, the brain should be examined very soon after death, by depositing minute portions upon a perfectly clean slip of glass, and moistening them with serum or a weak sac- charine solution. If it is desired to examine the distribution and arrangement of nerve-fibres, the brain should be placed in a solu- tion of chromic acid; by the hardening thus produced it can be easily cut into thin slices by means of a Valentin's knife. The addition of water to a por- tion of white cerebral matter changes the natural appearance of these fibres, by separating the oily and albuminous contents of the tubular sheath. The oily matter collects into globules, giving a beaded appearance to the fibres. (Fig. 346, g, g'.} The nerve-tubes may be rendered very distinct by the addition of a dilute solution of caustic soda. Small portions of the meninges of the brain may be examined in the same manner. To examine the cerebral vessels, a thin section must be well washed, and subjected to gentle pressure. The vessels are thus deprived of their investing neurine, and may now be rendered more distinct by the dilute caustic soda. The corpora amylacea, or gritty particles found in the pineal gland and other parts of the brain (Fig. 347), must be separated from the nervous tissue for examination by repeated washing in water. Spinal Cord. — To examine the spinal cord with advantage, it should first be hardened in a solution of chromic acid, or in spirits of wine. The structure of thin sections is thus rendered quite conspicuous. The following method was employed by Mr. J. S. Clarke.1 " A perfectly fresh cord was hardened in spirits of wine, so that extremely thin sections, in various directions, could be made by means of a very sharp knife. A section so made was placed on a glass slide, and treated with a mixture composed of one part of acetic acid and three of spirits of wine, which not only makes the nerves and fibrous portion more distinct and conspicuous, 1 Philosophical Transactions, 185], Part ii. 654 APPENDIX. but renders also the gray substance much more transparent. The section was then covered with thin glass, and viewed first FIG. 347. The lower figure represents a choroid plexus with several small tumors at * * *, supposed at first to have been tubercular; they proved to consist of aggregations of concentric corpuscles, chole- sterine, and pure oil, united by areolar tissue ; the concentric corpuscles which are shown above the plexus are magnified 100 diameters. by reflected light with low magnifying powers, and then by transmitted light with higher ones. "According to the second method, the section is first mace- rated for an hour or two in the mixture of acetic acid and spirit. It is then removed into pure spirit, and allowed to remain there for about the same space of time. From the spirit it is trans- ferred to oil of turpentine, which expels the spirit in the form of opaque globules, and shortly (sometimes immediately) renders the section perfectly transparent. The preparation is then put up in Canada balsam, and covered with thin glass. By this means the nerve- fibrils and vessels become so beautifully distinct, that they may be clearly seen with the highest powers of the micro- scope. If the section be removed from the turpentine when it is only semi-trans- parent, we sometimes obtain a good view of the arrangement of the bloodvessels. This mode of preparation succeeds best in cold weather; for in summer, the cord, fresh when immersed in the spirit, re- mains more or less spongy, instead of be- coming firm and dense in the course of five or six days. The spirit should be diluted with an equal quantity of water during the first day, after which it should be used pure. Certain modifications of this mode of preparation EXAMINATION OF THE MUSCULAE SYSTEM. 655 may be sometimes employed witli advantage by a practised hand." Nerves. — The structure and arrangement of nerve-fibres are best studied in the mesentery of small animals, as the newt; though with a little care in manipulation they can be very well displayed in any part of the nervous system. Their ultimate distribution, however, presents greater difficulties. Phosphoric acid, and solutions of caustic soda, and iodine of different strengths, are of great use in rendering these fibres more dis- tinct. According to Dr. Waller, the tongue of the frog is best adapted for examining the arrangement of nerve-fibres in papillae. When the nerve-fibres are not quite fresh, or have been soaked in water, and where they have been stretched or subjected to pressure for some time, their structure wrill be found to have undergone certain peculiar changes, as complete conversion into fibrous tissue, fatty degeneration, &c. (Fig. 348.) EXAMINATION OF THE MUSCULAR SYSTEM. Muscular Fibre. — Sarcolemma. — Muscular fibre is of two kinds, — the striated, voluntary, or muscular fibre of animal life; and the unstriated, involuntary, or muscular fibre of organic life. The voluntary muscles of man and the lower animals furnish specimens of the striated fibre. They may be prepared for examination by cutting out a small slice from a muscle, sepa- rating the fibres with fine needles, and placing them upon a glass slide, and adding a drop or two of water. Muscles which have been boiled or hardened in chromic acid, corrosive sublimate, or spirits of wine, yield excellent sections for examination. The general anatomy of voluntary muscular fibre is well displayed in the thin slips of muscle lying just beneath the skin of small animals, as the frog; while the general arrangement and form of the fibres is well shown, according to Beale, in a transverse section of the pectoral muscle of a teal (Querquedula crecca), which has been put upon the stretch, and allowed to become perfectly dry. The ultimate fibrillae may be studied with advantage upon the muscular tissue of the eel and pig. The fibrillse are separated and rendered distinct by maceration in chromic acid. From the back of the throat, after a meal of meat, in the discharges of cholera patients and in vomited matters, admirable specimens, showing the transverse striae, may often be obtained. In examining the arrangement of the nuclei, solutions of caustic soda and acetic acid will be found very useful. In the tongue of the frog, as shown by Kolliker, and in the upper lip of the rat, according to Huxley, peculiar fibres, known as branched muscular fibres, may be found. To obtain specimens of these fibres, the tongue of a frog is boiled for a short time in water. A piece of the mucous membrane is then dissected away, 656 APPENDIX. and a very small portion of the submucous tissue cut from the edge of the tongue with a pair of sharp scissors. This is torn into fragments with very fine needles, and then placed in the field of a quarter-inch ohject-glass. If the tongue is boiled very long, the fibres become too brittle for separation and examination. The Crustacea, mollusca, and insecta present peculiarities in the structure of their voluntary muscular fibre which separate them in a marked manner from the higher divisions of the vertebrata. FIG. 349. FIG. 350. The involuntary, smooth, or non-striated muscular fibres, though appearing like flattened bands (Fig. 350), in reality, ac- cording to Kblliker, consist of elongated cells. They are found in various situations, as in the alimentary canal, the large and small arteries, veins and lymphatics, the trabecular tissue of the spleen, the uterus, bladder, and urethra, &c. "The contractile fibre-cells have been arranged in three classes : — 1. Short rounded or flattened cells, somewhat resembling epithelium. 2. Flattened bands, with fringed edges. 3. Long rounded or fusiform fibres, slightly wavy, and terminating at each end in a point. FIG. 351. " The first two varieties are obtained from the bloodvessels. The last form is met with in the intestinal canal, uterus, &c. These cells may be readily isolated by macerating small pieces of the muscular coat of the alimentary canal, &c., in dilute nitric acid, containing about 20 per cent, of strong acid. By a little EXAMINATION OF THE RESPIRATORY ORGANS. 657 tearing, with the aid of fine needles, separate cells may be readily obtained."1 (Fig. 350.) The sarcolemma is best seen in the long muscular fibres of the fin of the skate, by tearing them apart with delicate needles and spreading them out upon a piece of glass. In the heart the sarcolemma is so thin that it can scarcely be detected. Some observers doubt its existence (Fig. 351). EXAMINATION OF THE RESPIRATORY ORGANS. Healthy Lung. — The mucous membrane of the trachea and bronchial tubes, and the parenchymatous struc- FIG. 352. ture of the lung may be readily examined by cut- ting thin sections with a very sharp knife, moistening them with water and spreading them out upon slips of glass in the ordinary manner. To examine the ciliated epithelium, and the characteristic movements of the cilia, the air-passages should be scraped and the matters thus obtained softened with serum in- stead of water, and de- posited upon glass. The ciliary motion is well displayed in the branchiae £ \^ 11 XT, a, epithelium; 6, elastic trabeculse; c, membranous wall, of the mollusca, as the 'wilh fine elastic fil)r'e. oyster. The yellow elas- tic tissue of the lung is rendered quite distinct by the addition of acetic acid (Fig. 352). To examine the vascular tissue, the lung should be injected with a somewhat thick solution of transparent gelatine. This oozing through the walls of the vessels, fills and expands the air-cells so that their forms and arrangement can be easily de- tected, while the vessels are seen in their natural position, and apparently deprived of epithelium (Fig. 353). An excellent idea of the characteristic appearance of the tracheae of insects, may be obtained by separating with fine needles the viscera of a fly, placing them on a glass slide, and adding a few drops of water. 1 Beale. The Microscope, and its application to clinical medicine. 42 658 APPENDIX. Arrangement of the Capillaries of the air-cells of the Human Lung. Morbid Lung. — Diseased lung is examined in the manner de- scribed above. Emphysema- tous, tuberculous and pneu- monic lungs present points of the highest interest to the stu- dent of pathological histology. " A small fragment of tole- rably firm miliary tubercle, squeezed between glasses with a drop of water, and examined under a magnifying power of 250 diameters linear, presents a number of irregular shaped bodies, approaching a round, oval, or triangular form, vary- ing in their longest diame- ters from the l-4000th to the l-2000th of an inch. These are the so-called tubercle-corpuscles. They are composed of a distinct wall, containing generally three or more granules without any distinct nucleus, and are mixed with numerous granules and molecules, varying in size from a point scarcely measurable to the l-6000th of an inch in diameter (Fig. 354, a). If we add to these bodies a drop of weak acetic acid, all the corpuscles become more transparent, but are otherwise unchanged, and many of the granules disap- pear, as in Fig. 354. 6." ^ Yellow tubercle treated in the same manner ?<&•'§{$ ^»%P.Q presents similar corpuscles imbedded in a mole- ^JS't' ^P-^' cular and granular mass (Fig. 355). Sometimes •^ ® these corpuscles are observed to be larger and rounder, resembling in this respect those of scrofulous pus (Figs. 356, 357). Occasionally tuberculous matter will be seen to consist almost entirely of granules, and at other times of very minute molecules. FIG. 354. FIG. 3f>5. FIG. 356. FIG. 357. A thin section of the gray, semi-transparent granulation is very different in appearance from ordinary tubercle. The con- stituent elements, though more transparent, are less distinct (Fig. 358). In the cretaceous and calcareous forms of tubercle EXAMINATION OF THE RESPIRATORY ORGANS. 659 the corpuscles and granules are mixed with gritty saline parti- cles of an irregular form and size. Crystals of cholesterine are sometimes found in the cretaceous and cheesy varieties of tuber- cle (Fig. 359). If no crystals can be detected, a small quantity FIG. 358. FIG. 359. FIG. 360. Section of gray granulations after addition of acetic acid, showing air-vesicles filled with tubercles. of alcohol may be added to a portion of the tuberculous mass, and then evaporated. As the evaporation proceeds, the crystals will be fornied. Thin sections of calcareous lung present a granular appear- ance, in consequence of the close aggregation of the minute earthy particles. Fragments of the calcareous mass may be broken off, and examined with the low powers of the micro- scope, as in the case of opaque objects generally. A drop of acetic acid added to these fragments causes them to dissolve with effervescence, showing the presence of carbonate. If this solution be treated with excess of ammonia, phosphate of lime FIG. 361. Fio. 362. will be precipitated ; solution of the oxalate of ammonia will also detect the presence of lime. Irregular, black masses of pigmentary matter, consisting of ceedingly minute molecules, are also frequently found, mixed excee with tubercle (Fig. 360), giving the tissues a black or bluish 660 APPENDIX. tinge. As the tubercle becomes older, the pigmentary matter generally increases in quantity. It also varies in chemical com- position according to its situation. That obtained from the lungs and bronchial glands is pure charcoal and chemically indestructible ; that found in the peritoneum is destroyed by the action of alcohol and the mineral acids. Gulliver, Yogel, and Schroeder Van der Kolk affirm, that nucleated cells may be observed in miliary tubercle ; but both Lebert and Bennett deny it. (Figs. 361, 362.) It is oftentimes very difficult to distinguish tubercle from fibrinous exudations and from cancerous growth. " If we are asked," writes Prof. Bennett, "to determine what is positively tubercle, as distinguished from all other morbid products, we must answer, that deposition which is composed of the peculiar corpuscles and granules described arid figured in the preceding pages. From pus-corpuscles they are readily distinguished by the action of acetic acid, which in them causes no granular nucleus to appear. From plastic corpuscles they may be sepa- rated by their irregular form, smaller size, and the absence of primitive filaments. With the granular corpuscle they can scarcely ever be confounded, on account of its large size, brown- ish or blackish color, and nucleated or granular structure. The cells of cancer are large, transparent, and distinctly nucleated, and, consequently, easily distinguished from the small, iion- nucleated corpuscles of tubercle." "The only other structure likely to be confounded with tubercle is the reticulum of cancer, which not only presents a yellowish appearance closely resem- bling it, but is composed of nuclei and molecular matter result- ing from the disintegration of cancer-cells. But, as this reticu- lum. is always associated with cancerous formation, it may at once be distinguished by the cell-elements which accompany it. It should further be noticed that every form of exudation, at a certain period, presents a molecular and granular structure throughout, and that then it becomes impossible to determine its nature, unless it be associated with the more characteristic FIG. 364. FIG. 365. FIG. 366. Fig. 363, A, Tubercle corpuscle from lung; B, Pus corpuscles. Fig. 364. Plastic, or pyoid cor- puscles. Fig. 365. Granular corpuscles from cerebral softening. Fig, 366. Corpuscles in reticulum of cancer. elements distinctive of the simple, tubercular, or cancerous exudations." The accompanying figures will illustrate these distinctions. (Figs. 363, 364, 365, 366.) EXAMINATION OF THE RESPIRATORY ORGANS. 661 FIG. 367. Where a cavity is found in the lung, separate examinations should be made of its contents, the surface of the walls, and the subjacent tissue. In emphysema of the lungs, the investing membrane will be often found full of very small holes ; the vessels elongated, and the interspaces much enlarged; and the yellow elastic fibres stretched to such an extent as to be deprived of their elasticity. In the first stage of pneumonia, the vesicular walls contain here and there collections of minute granules. The epithelial cells are separated from the basement membrane ; and while the nuclei are unaltered, the cell- contents have become a little more granular. The minute capillaries are very much congested. As the engorge- ment proceeds, granules col- lect in greater quantities in the air-cells; while in the effused serum maybe detect- ed distinct blood-corpuscles, small nucleated cells, and exudation-corpuscles. These changes are shown in Fig. 36T. In the second or stage of red hepatization, the cavity of the air-vesicle is filled with cells about 0-018 of a milli- metre in size, of very varied shapes, and differing in respect to FIG. 368. FIG. 369. their contents. These are mixed with concrete albumen, oil- granules, and free fat. (Figs. 368, 369.) In the third stage — that of gray hepatization — large well- marked cells may be seen, containing granules and oil. Most 662 APPENDIX. of these cells are without nuclei. Occasionally they are found associated with large masses of pus-cells. The epithelium is wanting, and free fat-molecules and globules are abundant. (Fig. 370.)1 FIG. 370. FIG. 371. EXAMINATION OF THE GLANDULAR SYSTEM. Liver. — The relation to each other of the constituent elements of the liver, may be easily de- monstrated upon thin sections cut out of the fresh liver of a pig, with a Valentin's knife. The arrangement of the minute vessels are best seen in the injected liver of a frog. The larger vessels can be studied in thin sec- tions, from which the cells have been washed away by a stream of water, and dilute caustic soda afterwards ap- plied. Excellent illustrative specimens of the vessels may be obtained, by injecting the liver of some of the lower animals, as the frog, with two different colors ; throwing one into the portal vein, and the other into the hepatic artery or vein, so that the two shall meet in the capillaries. (Fig. 371.) The hepatic cells lie in the 1 See a valuable paper by Dr. Da Costa, of Philadelphia, on the Pathological Anatomy of Acute Pneumonia, in Amer. Journ. of Med. Sciences, Oct. 1855. Transverse section of a lobule of the human liver, highly magnified, and presenting to view the reticulated structure of the biliary tubes. In the centre of the figure is seen the hepatic vein cut across and several small branches terminat- ing in it. Where the injecting matter did not run freely, it is seen standing in dots along the course of the vessels. At the periphery are seen branches of the hepatic artery, vena por- tarum, and hepatic duct.— After Leidy. EXAMINATION OF THE GLANDULAR SYSTEM. 663 matrix or network formed by the union of the vascular and the condensed cellular tissues. They are of different sizes, though generally very small, and contain a few oil- globules. They may be obtained by scraping a freshly-cut surface; placed upon a slip of glass and moistened with a few drops of water, they are ready for examination. These cells undergo various changes in disease. Sometimes they are withered and shrunk ; sometimes filled with pale granules, as in diabetic cases, and at others, gorged with fat to such an extent as to obliterate the cell-walls, and give to the liver the appearance of ordinary adipose tissue. (Fig. 372.) FIG. 372. Kidney. — The general arrangement of the straight and con- voluted tubes, and the varying appearance of their epithelium, may be well shown upon thin sec- tions cut out of the cortical and medullary portions of the kidney, by means of a Valentin's knife. By scraping the freshly-cut sur- face, epithelial cells may be ob- tained, mixed, however, with Mai- pighian tufts and fragments of tubes. (Fig. 373.) In the convo- luted tubes the epithelium is thick and glandular ;, in the straight it presents a scaly appearance. Care must be taken not to con- found the tubes, as they bend in and out through the matrix, with circumscribed cysts. The kidney Of the mOUSe and Of many A, Portion of uriniferous tube lined by epithe- of the other rodentia, are better lium- B' EPithelial cells' hi^h'y magnified, c. -, . T n n . .. ,, Portion of tube from medullary substance, de- adapted for demonstrating the privedof epithelium. matrix than the human kidney. Indeed, its existence in the latter has been doubted by some, not- 664 APPENDIX. withstanding that Goodsir, Kolliker, and Johnson have described and figured it. "With care," says Mr. Beale, "I believe it may always be demonstrated in the healthy human kidney ; and in some specimens of fatty kidney, as well as in the small con- tracted kidney of chronic nephritis, it may very readily be seen by washing a thin section with a stream of water, in order to remove the epithelium and remaining portion of the tubes. The matrix appears to consist of very fine fibres, amongst which no indications of the yellow element can be detected. By the ad- dition of acetic acid it becomes more transparent, and a few granules are developed, but no other change is produced." In the uriniferous tubes of the frog and newt, ciliary motion is beautifully shown. The arrangement of the vessels of the Malpighian tufts can be studied with facility and success in the large tufts of the kidney of the horse. Sections of kidney are best kept in large thin glass-cells con- taining a solution of creasote or weak spirit and water. Morbid Kidney. — In disease the kidney oftentimes becomes quite opaque. This condition may result from a variety of causes ; as, hypertrophy of the matrix, deposition of fatty matter, and unusual accumulation of epithelium or oil-globules in the tubes. To examine such a kidney, therefore, the sections should be exceedingly thin. The matrix and the vessels, which are often much thickened in chronic nephritis, should be prepared for examination by the addition of acetic acid, or dilute caustic soda, and subsequent washing with clear water to remove the epithelium and granular matter from the tubes. Salivary Glands. — To examine the structure of the salivary glands, thin sections of the fresh gland should be treated first with acetic acid, and then with caustic soda in excess. The epithelium of the ultimate lobules is rendered distinct by soaking the section in acetic acid. In the ducts, large cells filled with oil-globules may sometimes be detected. Thymus and Thyroid G-lands. — Sections of the recent glands should, before being submitted to examination, be washed with water to clean away the soft and pulpy portions, which are apt to obscure the structure of the tissues. The relation of the lobules and other constituents is best shown in sections hardened with chromic acid ; though it must be borne in mind that this process alters the natural appearance of the cellular tissue. The mem- branous thyroid gland of small animals is well adapted for micro- scopic examination. Adipose Tissue. — To examine adipose tissue, small masses of fat-cells may be taken from the subcutaneous areolar tissue, and exposed to reflected light under a low power ; or thin sections, moistened with water, may be placed between two pieces of glass, and examined by means of transmitted light. The arrangement of vessels can only be demonstrated upon an injected prepara- tion. Small acicular crystals, of margaric acid and margarine, VASCULAR AND ABSORBENT SYSTEMS. 665 disposed in a stellate (Fig. 374, a) form, are sometimes observed, especially in specimens obtained from emaciated subjects. In disease, the fat- FIG. 374. cells are sometimes found degenerated, and containing a serous fluid, in which the nucleus is quite distinctly seen, amidst numerous granules. Sometimes the cell is shrivelled and irregular in form ; frequently it assumes an angular shape. Fatty degeneration consists in the con- version of healthy structures into true adipose tissue. The muscular system seems to be most liable to this change. Prior to the appearance of fatty degeneration in voluntary muscle, the transverse stride disappear. According to Mr. Quekett, the first trace of this disease is marked by a disturbance of the par- ticles of myoline, which appear as so many very minute granules scattered irregularly within the sarcolemma, leading one to sup- pose that the delicate cell around each particle had given way, thereby allowing the myoline to escape, and destroying all regu- larity both of the transverse and longitudinal markings. As the disease progresses, the myoline is replaced by minute highly- refracting globules of oil, until at last the whole sheath is full of them.1 The muscular fibres of the heart, and Especially those of the musculi pectinati, afford frequent instances of the change in question. Muscles which have been long disused, as in cases of paralysis, club foot, &c., exhibit this species of degeneration in a striking degree. "Fatty degeneration appears also to occur in osseous tissues, and indeed the disease termed Mollities ossium is of this nature. All bones so affected have thin walls, are always more or less soft, and contain an abundance of oil. I have examined the bones in several cases, and found that the disease first commences in the bone-cells, the cell itself becoming larger and larger, its canaliculi disappearing, and several of these cells uniting to form a cavity, in which oil-globules soon make their appearance, all the parts of the bone in the neighborhood of the cells becoming at the same time thin and transparent from the removal of the granules of earthy matter." (Quekett.) In the Lancet, for 1850, the student will find an interesting paper, by Mr. Canton, on the Arcus Senilis, produced by fatty degenera- tion of the cornea. In fatty degeneration of the kidney, the epithelial cells become filled with numerous oil-globules, to such an extent sometimes as to burst and be discharged in fragments, leaving the surface of the tubules in some places almost bare. 1 Lectures on Histology. 666 APPENDIX. FIG. 375. EXAMINATION OF THE VASCULAR AND ABSORBENT SYSTEMS. Vessels. — The examination of the minute vessels requires but little previous preparation. A piece of the pia mater, or the mesen- tery of a young child, or a small artery from which the cerebral neurine has been gently washed, may be placed for this purpose under the microscope. The epithelial cells of the lining mem- brane, and the contractile fibre-cells may be rendered distinct by the addition of acetic acid. To demonstrate the fibre-cells of the contractile coat, Dr. Beale recommends that an artery of moderate size, and not quite fresh, be slit up, and its lining membrane removed by careful scraping; the subjacent elastic tissue is then to be removed and torn to pieces with fine needles, and finally placed upon a glass slide and moistened with a few drops of water. The spindle-shaped or muscular fibre- cells are readily obtained from the renal veins. It is highly important that the student should make himself well acquainted with the healthy appearance of the mi- nute arteries of the brain, since they suf- fer remarkable changes in disease. In white softening of the brain, they un- dergo a sort of fatty degeneration, nu- merous minute oil-globules being aggre- gated together at short intervals along their walls. These oily masses are readily detected by their high refracting power. (Fig!> 375.) The vessels of the kidney are also, from the changes they suffer in disease, worthy of especial inves- tigation. To prepare them for examination it is only necessary to wash out from a thin section the epithelium of the renal tubes, and add a few drops of acetic acid to render them more distinct. The distinct arrangement of the nuclei of the circular and longi- tudinal fibres, and the greater thickness of their walls, will serve to distinguish the arteries from the veins. The coats of the latter are very thin. Thickening of the arterial coats of the corpora Malpighiana is well seen, according to Dr. Johnson, in the small, contracted drunkard's kidney.1 The normal thickness of the Malpighian artery is about one-fifth or one- sixth the diameter of the vessel; in this disease it is increased to one-third. Some observers have thought that they detected epithelial cells upon the external surface of the Malpighian vessels ; but the researches of Mr. Bowman negative this opinion. 1 Diseases of the Kidneys. SKIN, MUCOUS AND SEROUS MEMBRANES. 667 Plates and rings of bones, and atheromatous deposits, contain- ing oil-globules, granules, and crystals of cholesterine, are all FIG. 377. sometimes found in the larger ves- sels. (Fig. 376, 377.) Lymphatics. — It is very difficult to examine the minute structure of the lymphatic glands. The fluid which exudes from a freshly cut surface may be examined by mixing it with water and placing it between two pieces of glass. It is very important to distinguish between lymphatic cells on the one hand, and pus- globules and white corpuscles of the blood on the other ; not only as regards their appearance, but also in relation to the dif- ferent effects produced by reagents. EXAMINATION OF THE SKIN, MUCOUS AND SEROUS MEMBRANES, ETC. Skin. — A good view of all the structures composing the skin may be obtained by making a vertical section. In this manner the relation of the various layers, the arrangement of the hair- bulbs and sebaceous follicles, and the position and course of the sweat-ducts, may all be demonstrated. The following method of procedure is recorded by Dr. Beale, as quoted from Giraldis by Kblliker : " The skin should be perfectly fresh, and a piece about two inches square, or rather less, is to be stretched with the outer surface downwards upon a thick deal board by means of numerous pins. If the sudoriferous glands are to be included in the pre- paration care must be taken to leave sufficient of the cellular tissue adhering to the skin. The piece of skin is allowed to dry by exposure to the air. Several small pieces taken from various parts of the body may be pinned out on the same board, care being taken to attach a label to each. Specimens may be taken from the scalp, eyelids, chin, mamma, axilla, arm or leg, palm of the hand, tips of the fingers, scrotum, and sole of the foot. With these, the varying thickness of the epidermis and other peculiari- ties in the different regions may be demonstrated. 668 APPENDIX. " The portion of skin being quite dry, it is to be removed from the board, and, after cutting off the edge, several thin sections may be made by the aid of a very sharp knife through the whole thickness. In order to obtain a good specimen of the spiral por- tion of the sweat-ducts, the skin of the heel should be selected, and the section should be made parallel with the furrows, and in a slightly slanting direction, instead of at a right angle with the surface. " The sections may next be placed in a watch-glass, with a few drops of clean water, and in the course of a short time it will be found that they have again attained the original thickness of the skin, in consequence of the absorption of water. They may now be submitted to examination, and after selecting a satisfactory specimen, it may be mounted in weak spirit and water, Goadby's solution, or other preservative fluid ; or, the specimen may be washed in water, placed upon a slide, and allowed to dry slowly by spontaneous evaporation (when it will be found to have ad- hered tightly to the glass), and mounted in Canada balsam, with the usual precautions." Any opacity of the preparation may be removed by a weak solution of potash or caustic soda. Soaking in ether will dis- sipate the fat. The sweat-glands are made more distinct by soaking the tissue in a mixture of one part of nitric acid and two of water. Large flakes of cuticle may be obtained for examination by ex- posing a small piece of skin to a moist atmosphere for several days. The superficial cells of the cuticle are brought into view by scraping the surface of the skin with a knife. These cells are flattened and adherent, and present a scaly appearance. The deeper-seated epidermic cells are more or less round, and appear to rest upon a layer of very minute granules mixed with coloring matter. The deep cells are soluble in acetic acid ; the superficial are not. On the under surface of the cuticle are found a number of depressions,, which receive the tactile papillae of the cutisvera. The papillae may be studied either upon a vertical section made in the manner above described, or upon a section of the cutis itself, the cuticle having been first removed. From tneir large size, the papillae of the skin of the dog's foot are well adapted for examination. The papillary vessels are best seen in an injected specimen, while the nerves and " axis-corpuscles" are sometimes brought into view by the addition of acetic acid or a weak solution of caustic soda. The pigmentary cells are best seen in the skin of the negro, in that of some of the lower animals, and also in freckled surfaces. Cutaneous Eruptions, Growths, Ulcers, $c. — Corns, callosities, and condylomatous warts consist of condensed epidermic scales. In Yeruca achrocordon, the scales are collected around a central canal supplied with bloodvessels. Small cutaneous tumors are sometimes formed by the thickening of the subjacent areolar SKIN. MUCOUS AND SEROUS MEMBRANES. 669 FIG. 378. tissue. This appears to be the case in elephantiasis, where the hypertrophy is increased by the effusion of plastic lymph into the areolae and its subsequent organization. The squamous eruptions of the skin, — ichthyosis, pityriasis, psoriasis, — all consist of collections of epidermic scales. In pityriasis they are placed loosely together ; in psoriasis they are more aggregated ; and in ichthyosis very much condensed. A number of epidermic scales, arranged into the form of a capsule, constitute a favous crust. This cap- sule is lined by a mass of very fine granules, from which sprout crypto - gamic plants in the greatest abun- dance. Upon healthy granulating sur- faces may be seen pus-corpuscles, fibre-cells in various stages of deve- lopment, and newly-formed fibres. (Fig. 378.) In scrofulous and un- healthy sores, the broken-down pus bears some resemblance to tubercle-corpuscles. The epithelial ulcer or cancer, as it is commonly, but erro- neously called, generally commences on the lip as a small induration or wart, which soon softens in the centre, while the hardened edges extend over the cheek and chin. This softened matter consists of epithelial and fibre Fibrous 1issue formed from or fibro-plastic cells. (Fig. 381.) Sometimes the cells are large and flat, and contain numerous fat-molecules arid granules. (Fig. 380.) According to Bennett, the so-called Fibre-cells passing into fibres. FlG. 379. FlG. 380. FIG. 381. ip f"ii\w / mm Fig. 380. Altered epithelial cells, from ulcer of lip. Fig. 381. Epithelial and fibre-cells, from ulcer of lip. chimney-sweeps' cancer of the scrotum is essentially a similar formation, consisting externally of flattened epithelial scales passing into fibres ; and, deeper-seated, either groups of younger 670 APPENDIX. cells, or concentric layers of aggregated scales. The distinctions between these growths and true cancer will be pointed out here- after. Mucous Membrane. — The submucous areolar tissue maybe exa- mined upon a small piece cut from the under surface of the mu- cous membrane, and torn up with needles. Between the proper muscular coat of the small intestine and the basement-membrane, and in close apposition with the latter, Brucke has demonstrated a thin layer of pale muscular fibres, which is known as the muscu- lar layer of the mucous coat. The contractile fibre-cells of this coat are disposed in circular and longitudinal directions. The villi are best seen in a vertical section of the membrane. By washing off the epithelium, and adding a solution of acetic and nitric acid, composed of about one part of acid to four of water, the muscular fibres of the villi will be brought into view. The villi situated around intestinal ulcers are often found to be very much elongated. In the examination of such ulcers it is important in all cases to ascertain if the muscular coat has suffered from the ulcerative action or not. The presence or absence of non-striated fibres in the base of the ulcer will determine this point. Serous and Synovial Membranes. — Serous membranes consist mainly of condensed areolar tissue, containing an abundance of yellow elastic fibres. At the surface this areolar tissue is very dense; the deeper layers are less dense, and often contain fat- cells. Portions of recent membrane are generally necessary to demonstrate the delicate surface-cells. The fibres of the sub- basement tissue, and often the vessels and nerves, are well seen in the peritoneum of the mouse and other small animals. The vessels of the synovial membranes should be injected before exa- mination. In an injected specimen, the distribution of the vessels in the fringe-like processes which dip down into the joint, is dis- played to great advantage. In some cases of disease, as in ascites, and pleurisy of long standing, great alterations take place in the structure of the serous membrane, such as the deposition of a thick cellular layer over the whole surface. Cells of a similar character are also found in the fluid contained in the cavity. Epithelium. — Epithelium may be obtained for examination by scraping a mucous or serous surface with a sharp knife. It should then be placed upon a slip of glass, and moistened with water. Very delicate cells should be treated with serum, syrup, or a mixture of glycerine and water, in preference to pure water, as the rapidity of endosmose is checked, and the liability to rup- ture diminished. Acetic and nitric acids, tincture of iodine, and solutions of potash and soda of different strengths, are the most useful reagents in examining epithelium. Various kinds of epithelium are described by histologists. Scaly epithelium may be procured from the vagina, the mouth, &c. ; a modified form exists in the epidermis, in nails, and in hair. The vaginal epithelium consists of large, flat, ragged, and EXAMINATION OF THE EYE. 671 very irregular cells, folded over each other, and perhaps creased in different directions. The epithelial cells of the mouth have very distinct nuclei, which are m'ade to disappear under the action of acetic acid. Tessellated or pavement epithelium is "beautifully shown in the epidermis of the frog. It may be examined upon the choroid coat of the eye, the lining membrane of the heart, arteries, veins, and pelvis of the kidney, arid upon serous surfaces generally. Glandular or spheroidal epithelium consists of round cells, which occasionally become polyhedral from mutual pressure. The nucleus is usually well marked, and sometimes seems to be surrounded with numerous minute granules and oil-globules. This variety of epithelium is well shown in the sweat-glands, in the convoluted tubes of the kidney, in the follicles of the sto- mach, pancreas, liver, &c. Columnar, prismatic, or cylindrical epithelium, may be obtained from the gall-bladder, the ureters, the urethra, the intestinal villi, and the follicles of Lieberkiihn. Ciliated epithelium is found in the human body in the follow- ing situations : — On the surface of the ventricles of the brain, and on the choroid plexuses ; on the mucous membrane of the nose and its sinuses ; on the upper and posterior part of the soft palate, and in the Eustachian tube ; in the cavity of the tympa- num ; on the membrane lining the frontal and sphenoidal sinuses ; on the inner surface of the lachrymal sac and lachrymal canal ; on the mucous membrane of the larynx, trachea, and bronchial tubes ; upon the os uteri ; within the cavity of the uterus ; through- out the whole length of the Fallopian tubes, and upon their fim- briated extremities (Beale). For examination, this variety of epithelium may be obtained from the back part of a frog's mouth, or from the branchiae of an oyster or mussel. EXAMINATION OF THE EYE. The cornea is examined by dividing the ball transversely with a sharp knife, washing the anterior half, and removing the ciliary processes. It is then pinned out flat and allowed to dry ; thin sections are next made with very fine scissors ; these sections are moistened with water, and finally treated with acetic acid, in order to render the structures distinct. The elements composing the retina are most satisfactorily examined in microscopic sections made at right angles to the surface of the membrane, after maceration in dilute solution of chromic acid. Viewed in this manner, according to Prof. Good- sir, it exhibits from the peripheral to the central margin of a successful section a series of strata, which may be distinguished as the bacillary, white cellular, gray cellular, filamentary, and limitary layers. 672 APPENDIX. There are several methods of preparing the crystalline lens for examination. Minute portions of the recent lens may be mois- tened with water and placed under the microscope. The lens may be hardened in chromic acid, or soaked in oil for some time, and thick sections then made. To examine the fibres of the lens, the latter should be boiled, and the fibres torn off and sepa- rated with needles. In cases of cataract, the soft, pulpy, exter- nal portion of the lens will be seen to contain numerous oil- globules, consisting, according to Beale, chiefly of cholesterine dissolved in an oily fat. Larger globules of a different character are also observed. EXAMINATION OF THE HAKD TISSUES. Bone. — For the methods of cutting, grinding, and polishing thin sections of the hard tissues, the student is referred to the more elaborate works on the application of the microscope to clinical medicine, and to the chapter on this subject, in the pre- ceding pages, by the author. A thin section of bone, viewed by transmitted light and with a low power, presents numerous round or oval apertures. These are the orifices of the Haversian canals. In the flat bones these canals are radiating and parallel to the surfaces; in the long bones, they are parallel to the axis. They communicate with each other by transverse and oblique branches, and vary in size from about 1-1000 to 1-200". Under a higher power (200) the lacunae, bone-corpuscles, or bone-cells, and the canaliculi or calcigerous canals, become visi- ble, appearing like a number of dark spots with delicate lines radiating from their sides. Their dark appearance is due to the contained air, which is readily dissipated by immersion in oil of turpentine. They are oblong and flattened, and vary exceed- ingly in size. The laminated structure of the cartilage or osseous basis of bone is best seen by previously digesting sections in hydrochloric acid diluted with water (one part to twenty), in order to remove the inorganic matter. The laminse have a fibrous appearance, and are arranged either parallel to the surfaces of the bone, or in concentric layers around the Haversian canals. The nuclei of cartilage-cells may be rendered very distinct by boiling the cartilage for two or three minutes in water or a solu- tion of caustic soda. The different steps in the development of bone may be very well studied upon the long bones of young animals, and also upon the ossifying bones of the cranium. Thin sections of bone may be preserved in the dry state ; thick specimens should be mounted in thickened Canada bal- sam. Sections of cartilage are best kept in weak spirits of wine, or in weak solutions of creasote. Teeth. — Owing to the great hardness and brittleness of the EXAMINATION OF THE HARD TISSUES. 673 enamel, and its tendency to chip off, considerable difficulty is experienced in obtaining sections of teeth for examination. Sec- tions having been obtained, they may be moistened with water, turpentine, and Canada balsam, and examined by transmitted light with low powers. A tooth that has been macerated in strong acid for several days can be very readily cut in any direction. The dentinal tubules are microscopic canals, pursuing a waving and anastomosing course through the whole thickness of the dentine, from the wall of the pulp-cavity to the cement and enamel. According to Kolliker, each canal has a special wall, rather less in thickness than its diameter, which can only be ob- served in transverse sections, as a narrow yellowish ring sur- rounding the cavity ; in longitudinal sections, on the other hand, it is almost entirely invisible. These tubules may be isolated from each other by long maceration in acid, and subsequent soaking in dilute caustic soda or potash. The enamel consists of hexagonal or pentagonal prisms, one extremity of which is attached to Nasmyth's membrane, the other to the dentine. These prisms are very closely united, having no intervening substance between them. The development of the teeth may be studied in the lower animals, or in human embryos at different ages. Nails. — By maceration in water the nail maybe separated from the skin and thin sections made by means of a sharp knife. A nail consists of two layers, the upper or horny one forming the true nail, and marked with ridges on its lower surface, while the under soft one is continuous with the rete mucosum of the skin, from which it differs by the cells being elongated and ar- ranged perpendicularly. The horny layer is composed of flat- tened epidermic cells aggregated into laminae. The addition of caustic potash or soda in solution causes the nuclei to be de- veloped. Hair. — A white hair is best adapted for demonstrating the intimate structure of the shaft and other parts. For this purpose, solutions of soda and potash and strong sulphuric and acetic acids are found to be the most useful reagents. The cortex, which constitutes a large part of the shaft, presents upon its surface a great number of longitudinal strise or inter- rupted dark lines arid dots. Upon treating it with sulphuric acid at a gentle heat, as recommended by Kolliker, it is first changed into plates and fibres, varying in length and breadth, and afterwards converted into elliptical or spindle-shaped cells, which become flattened and angular from pressure. These cells contain elongated, dark-looking nuclei and pigment-granules, to which the color of the hair is due. The addition of caustic pot- ash or soda will isolate these pigment-granules, and sometimes cause them to exhibit molecular motion. The presence of small spaces containing air, and the unequal refraction of light by dif- 43 674 APPENDIX. ferent parts of the cells, give a striated or dotted appearance to the shaft. The medulla consists of numerous angular or rounded cells, containing granules or globules of fat, and arranged in one or more linear series. The cells of the cuticular coat are somewhat flattened, and quadrangular in form, the margins are black, and they are with- out a nucleus. The fibrous layer of the hair-follicle consists of an outer mem- brane composed of areolar tissue, having longitudinal fibres, with long, spindle-shaped nuclei, and an inner delicate membrane, consisting of transverse fibres, with long and narrow nuclei. The pulp of the hair consists of fibrous areolar tissue, containing nuclei and granules of fat, but no cells. EXAMINATION OF MORBID GROWTHS. Morbid growths are of common occurrence, and are found growing in different parts of the body ; externally upon the surface, and internally in the solid parenchyma of the viscera. Sometimes these growths, as in the case of fatty and certain fibrous tumors, bony exostoses, &c., consist of a simple hyper- trophy or rapid development of the tissues of the part in which they are located ; very frequently, again, they are highly complex in structure, and differ more or less from the surrounding tissues. It is exceedingly difficult to classify them, even for the purposes of study, since the microscopic characters of one run into and blend almost imperceptibly with those of another. Thus the so-called benign tumors present various shades of transition into the malignant forms. It will be seen, therefore, that the varie- ties of these growths are numerous. Sometimes the cutaneous epithelium undergoes an unusual development, constituting warts. The subcutan eous areolar tissue of the foot, leg, scrotum, and other parts, may be hypertrophied to such an extent as to occasion the most serious results. In making an examination of morbid growths, "the secretmn, if such exists on the surface of the tumor, should be first sepa- rately examined : secondly, the microscopical characters of the juice which exudes from the freshly-cut surface should be ascer- tained ; and, lastly, a thin section ought to be made, in order to determine the relation of the constituents of the tumor to each other, and especially the proportions in which the different ele- ments are present. Its connection with surrounding structures may be seen by examining a thin section, which should include a portion of the adjacent texture ; and these observations should be made first with low powers, and afterwards with a power of about two hundred diameters." (Beale.) The arrangement and direction of fibres ; the form, size, and contents of cells ; the presence or absence of nuclei, granules, EXAMINATION OF MORBID GROWTHS. 675 &c., and the effects of different reagents, should he carefully observed, and noted in a hook kept for that purpose. Cancer and Cancroid G-rowths. — " Cancer, or carcinoma, is a vascular, morbid production, characterized by a form of organic cell, which is peculiar, and never enters as a constituent in any normal tissue. It is usually deposited in the form of tumors, but occasionally as an infiltration, in any of the organs of the body, and the circumstances which give rise to its development are yet unknown to us. There are several varieties of cancer, and the physical ele- ments which ordinarily enter into their composition are as follow : 1. The characteristic cancer-cells, which are spherical, ovoid, irregularly polyhedral, and frequently exhibit caudate prolonga- tions. They average about the *02 mil. in diameter, and possess finely granular contents, with a round or oval nucleolated nu- cleus, as large or larger than a pus-corpuscle. Sometimes can- cer-cells are double the ordinary size, or more, and not unfre- quently contain several nuclei, or even other cells, constituting parent or endogenous cells. 2. Nuclei, which are spheroid or oval, and resemble those within cancer-cells. 3. Granules, and amorphous liquid or semi-solid matter. 4. Fusiform or fibro-plastic cells. These are liable to be con- founded with the characteristic cancer-cell, but usually may be distinguished by the smaller nucleus, and the disposition to elongate at opposite extremities, and pass from this condition into the form of bands or fibres. 5. Fibrous tissue ; most usually of the white variety, but not unfrequently mingled with elastic fibres. 6. Black pigment, in granules, or contained within cells. 7. Fat, in granules, globules, and in the form of adipose cells. 8. Vessels. The varieties of cancer are encephaloid, or medullary carci- noma, scirrhus, colloid, melanosis, and fungus heematodes. Encephaloid is that form in which the cancer-cell predominates over every other constituent. Occasionally, the cancer-cells exist in it to the exclusion of all other matters, except liquid, granules, and vessels. In scirrhus, fibrous matter predominates, and encloses the cancer-cells within the areolae. Colloid is composed of a fibrous stroma, with loculi, filled with a gelatinoid matter and cancer-cells. Melanotic cancer consists of any of the preceding forms, com- bined with black pigment. Fungus haematodes is a term applied to an unusually vascular form of cancer, or to any of the other varieties when they are ulcerated and liable to bleed.1 1 Prof. J. Leidy, in Gluge's Atlas of Histological Pathology, Amer. Ed. p. 69. 676 APPENDIX. The cancer-cells maybe demonstrated in the thick, opaque juice which exudes from the freshly-cut surface of a cancerous tumor, while the relation of the constituent elements of the mass are best shown upon a thin section, made with a Valentin's knife. The fluid portions containing the cells will be found, in the are- olsfi or interspaces formed by the crossing of the fibres. Both the fibres and cells vary much in appearance in different specimens. Sometimes the cells are round, sometimes elongated into fibres, and occasionally they present very irregular forms. The nuclei also differ considerably in numbers and size. Dr. Walshe divides cancerous tumors into three varieties, according as the viscous juice, fibrous, or cellular elements pre- dominate. Very great difficulty is often experienced in arriving at an accurate opinion as to the cancerous or non-cancerous nature of a tumor ; because there is no single element which can be re- garded as pathognomonic of true cancer. " Neither the cha- racter of the cells," says Dr. Beale, "nor the nature of the matrix, nor the arrangement of the elementary constituents, can separately determine the point, and it is only by carefully noting the collective appearances observed upon a microscopic exami- nation that we shall be enabled to decide." Enchondroma, epithelioma, certain fibrous tumors containing spindle-shaped cells, &c., resemble true cancer so strongly that they have been called cancroid growths. For the peculiarities of each of these varieties of tumor, the student is referred to the works of Lebert, Bennett, Wai she, and other writers upon this subject. In the following table, taken from Dr. Lionel Beale's work on the medical applications of the microscope, will be found enumerated the most important characters which dis- tinguish the true cancer-cells from those of cancroid tumors. ^ Cancerous. Cancroid. Cells not connected with the matrix Cells connected with the matrix, often in a regular manner, or forming laminae. forming distinct laminae. Cells differing much from each other Cells resembling each other in size in size and form. and general outline. Cells readily separable from each other. Cells often cohering by their edges, Cells not connected together at their which generally form straight lines, three margins, their edges seldom forming or four cells being frequently found straight lines. united together. Cells containing several smaller cells Cells usually containing one nucleus, in their interior often met with. Nuclei varying much in size and num- Nuclei not varying much in size in ber in different cells. different cells. Juice scraped from the cut surface Juice scraped from the cut surface, containing many cells floating freely in containing small collections of cells, the fluid, and not connected with each which are often connected with each other. other. Dr. Donaldson1 asserts positively that true cancer can be dis- 1 American Journal of Medical Sciences, January, 1853. EXAMINATION OF MORBID GROWTHS. 67T tinguislied from all other tissue, normal or pathological, by cer- tain clear and well-defined elements. If we "compare the physical characters of cancer with those of the simple tissues, such as the muscular, areolar, dartoric, osseous, &c., or with those of the compound, as the glandular, the synovial, the mu- cous, &c., the difference will be very apparent. Its greater or less firmness, its homogeneous fibrous aspect, with its lactescent infiltrated juice, are very characteristic. The presence of this peculiar fluid is, of itself, a point of differential diagnosis of great value; the microscope always detecting in it, when found, the presence of cancer-cells, &c. 'No matter what organ is the seat of the disease, this fluid can generally be scraped from the cut surface, or squeezed out by gentle pressure. It is particularly abundant in encephaloid, and frequently oozes out in drops, having a white cloudy FlG- 382- appearance, of the consistence of cream, and very much of its color, being slightly tinged with yellow. It may sometimes, on superficial inspection, be confounded with light-colored pus, which has, how- ever, with the yellow, a slightly greenish tinge. If, from the conditions of its for- mation, there can be any doubt, an appeal to the microscope will at once settle it by giving us the characteristic pus-globule" (FigS. 382, 383). Pus-Corpuscles. " The element of cancer consists of three parts, cell, nucleus, and nucleolus, all of which are peculiar to it. " In all the varieties of cancerous tissue, nuclei are to be found either enveloped by a cell, or floating free, generally more or less of both ; in some specimens there exist a large number of free nuclei with only an occasional cell. The form and appearance of these nuclei are the most constant and unvarying of all cancer elements. They are (Fig. 384, a) ovoid, or more or less round ; the latter are found more particularly when the eye or the lymphatic glands are the organs diseased. Sometimes (as in b) we find little pieces of the wall of the nuclei apparently nicked out, but evidently it is purely accidental, and the proper shape can easily be recognized. They have, ordinarily, in width, a diameter ot from l-100th of a millimetre, or (a millimetre being equal to •039th of an inch) of -0039th of an inch to l-66thof a millimetre; in one instance we met with one as wide as l-38th of a milli- metre; in length they measure from l-133d to l-100th of a milli- metre. Their contour is dark and well defined, with the interior containing very minute dark granulations; indeed, when the specimen is perfectly fresh, they have a homogeneous aspect, the granulations being so small as to give the appearance of a mere shading (Fig. 384, c) ; if the specimen is kept a day or two we find 678 APPENDIX. the interior filling up with large granulations (as in d). Within these nuclei, when they have not been obscured by granular or fatty degeneration, there is found, habitually, a small^bpdy, or nucleolus, averaging in diameter about l-500th of a millimetre. FIG. 384. Pus-corpuscles after acetic acid. Free cancer nuclei. These nucleoli have somewhat of a yellowish tinge, with a bril- liant centre and dark borders, refracting light like the fat-vesicles. We would call attention, particularly, to the peculiar brilliancy of the centres of these nucleoli, which, we think, is characteristic; it can almost invariably be noticed, if the focus is varied. Their large size, in proportion to the nuclei, should also be noticed, together with the great variableness of their position, sometimes being near the centre, and again in close contact with the walls (see Fig. 384, e). Ordinarily, in other elements, they are found almost constantly in the centre. Very frequently two or three nu- cleoli are found within the same nucleus. M. Eobin1 mentions the action of acetic acid upon cancer-nuclei and their nucleoli as differing from that on other elements, particularly epithelial ; it renders the nucleus gradually paler, together with the cell, de- stroying neither, but the nucleolus is perfectly untouched by it; whereas, in epithelial cells, where generally in those of the skin the nucleoli are wanting, the action of acetic acid destroys the cell, leaving the nucleus unaltered." The polygonal shape may be regarded as the typical form of the cancer-cell. "In hard firm tumors, particularly those of the breast and ovaries, the cells found are exceedingly irregular, some- times nearly triangular. (Fig. 385, /.) The ovoid or spherical are more frequently met with in soft or medullary cancer (Fig. 386, ^r), where there is but little pressure, although its juice appears often to be but one mass of cells. It is rare, however, that per- fectly round cells are met with, but very generally the angles are well rounded in those which appear to be derived directly from 1 MS. notes of his Cours de Histologie, 1850. EXAMINATION OF MORBID GROWTHS. 679 the polygonal form, the diameter of which is very variable, ordi- narily from l-75th to l-25th of a millimetre. One peculiarity of this, as of the other forms of cancer-cell, is the presence of the granulations of different sizes in their interior ; whereas, in epi- Fia. 385. thelial cells, the interior is generally, when fresh, of course, homogeneous. In cancer we find the three varieties of granula- tions given by M. Robin.1 First, the very fine black dots, found in all organic elements, and named by the French, very ap- propriately, poussiere organique. Secondly, the gray granulations, a form somewhat larger ; and, lastly, the fat granulations, distin- guished by the refraction of the light. This first variety of cells contains nuclei, having in their interior invariably one or more nucleoli, both of which retain the characteristic points described above. The large size of the nucleus, in proportion to the diameter of its cell, will at once strike the eye of the careful ob- server. The variable position, also, of the nucleus within the enclosure, appears to us to be peculiar to cancer ; in cells of other structures, the rule is to find the nucleus very nearly in the centre, except with fibro-plastic cells, where the nuclei appear to have a peculiar affinity for the walls. All the varieties of cancer-cells contain very frequently two or more nuclei ; whereas, the epi- thelial, more particularly those found on the surface of the body (where there is most danger of confusion and doubt), but rarely have more than one. Moreover, the cell of epithelium is much larger than that of cancer, yet the cancer-nucleus is twice as large 1 Tableaux d'Anatomie, &c. par Ch. Robin, Paris, 1851. 680 APPENDIX. as that of epithelium, as is also the nucleolus, compared with that found in epithelium." The caudated cells are of common occurrence in cancerous FIG 387. tumors ; in cancer of the bladder they are invariably present. (Fig. 387.) The fusiform cells are most frequently met with in cancer of the bones. (Fig. 388.) The concentric cancer-cell is FIG. 388. best seen, according to Robin, in cancer of the uterus, breast, and ovaries. (Fig. 389.) Examples of the compound or mother-cell EXAMINATION OF ANIMAL FLUIDS. 681 of cancer and the agglomerated nuclei are shown in Figs. 390, 391 (after Donaldson). FIG. 389. FIG. 391. FIG. 390. EXAMINATION OF ANIMAL FLUIDS. Blood. — To examine blood microscopically it is only necessary to press a small drop between two pieces of glass, until it is flat- tened out into a thin layer. A number of yellow, round, bi-concave disks will then be seen, varying in diameter from the l-5000th to the 1- 3000th of an inch, the average size being about l-4000th of an inch. These disks have a bright margin, and a dark centre ; or a bright centre and a dark margin, according to FIG. 392. FIG. 393. the focus in which they are placed. (Fig. 392.) Exposure to the atmosphere causes the edges of the corpuscles to lose their smooth outline, and become irregularly notched or serrated, and sometimes beaded. (Fig. 393 B.) The corpuscles vary in size in different animals. Thus in birds, fishes, and reptiles they are elliptical and flattened, and exhibit a distinct nucleus, which is generally oval. In the camel tribe they are elliptical and bi-convex. 682 APPENDIX. The red globules are diminished to half their size by prolonged maceration in serum. "Water renders them spherical and de- prives them of their color. In strong syrup they shrink very much, from the rapid exosmosis which takes place. Acetic acid renders the membrane of the corpuscle so trans- parent, as to be almost invisible, while nitric acid causes the out- line to become darker and thicker. Acid urine and the acid of the gastric juice produces a similar effect, as is seen in cases of hemorrhagic effusion into the stomach and bladder. The colorless or lymph-corpuscles of the blood are spherical, highly refractive, and vary in diameter from the l-2500th to the l-2000th of an inch. They have a granular appearance, which is lost by immersion in water, and are specifically lighter than the colored corpuscles. Within the cell-wall are numerous granules and molecules of different sizes, and one or more nuclei. (Fig. 394.) These nuclei are rendered distinct by acetic acid, while FIG. 394. Colorless corpuscles. Blood in Leucocythemia. the granules are dissolved. It has been estimated that the color- less corpuscles constitute about l-50th part of the corpuscular element of healthy blood. In enlargement of the spleen and lymphatic glands they increase in numbers, producing the con- dition called by Bennett, Leucocythemia or "white-cell blood." (Fig. 395.) In some dropsical and cancerous affections, a slight increase of these white globules has been observed. In certain extreme cases they equal the red corpuscles in number. Their nuclei have occasionally been seen quite naked by Yirchow and Bennett. Dr. Beale speaks of finding in the blood of some cholera cases, very large white cells, containing oil -globules col- lected together in one part, while the remainder of the cell was quite transparent. Where the blood is thickened from an excess of fibrine the colored corpuscles become caudate or flask-like in shape, and aggregate themselves in irregular masses, instead of in the form of rouleaux. (Fig. 396.) According to Funke and Kolliker yellowish crystals are some- times seen in the blood-corpuscles of the spleen of the dog, perch, and other animals. Oil-granules have also been observed in the blood, giving it a lactescent or creamy appearance. The EXAMINATION OF ANIMAL FLUIDS. 683 FIG. 396. microscopic changes which the blood undergoes in plethora, fever, and various other diseases, have yet to be accurately determined. Milk. — A microscopic exa- mination of milk reveals a number of spherical bodies, having dark, smooth, and well- defined, margins, and a trans- parent and highly refractive centre, and varying from a mere point up to the l-4000th or l-3000th of an inch in dia- meter. These bodies, consist of oil-globules invested with a covering of albumen, which prevents them from running together and forming larger globules when pressed between two pieces of glass. If this albuminous membrane be dis- solved with a little acetic acid or carbonate of soda, the oil is separated, and may be readily collected. Under this treatment the smaller globules may be made, by the slightest pressure, to run together and form larger ones. These globules collecting on the surface of milk, in virtue of their lighter specific gravity, constitute cream. An excess of ether effects the solution of the milk-globules, while water causes them to swell out. The colostrum or first milk of the human female is yellow in color, and contains many large cells filled with oil, and mingled with a number of compound granular bodies, which disappear about the fifth or sixth day after delivery. (Fig. 397.) ' In fresh and healthy milk, the globules are more or less uniform in size, and move freely in the surrounding fluid, showing no tendency to aggre- gate in masses. (Fig. 398^.) The microscope readily de- tects adulterations of milk. The addition of water causes the globules to be separated far- ther from each other. The presence of flour is determined by large starch-corpuscles, which strike a blue color with iodine. Gritty particles, soluble in the mineral acids, indicate the addition FIG. 397. 684 APPENDIX. of chalky matters. In milk to winch sheep's brains have been added, fine nerve-tubes will be seen in the field of the microscope mingled with oil-globules. A tendency on the part of the globules to collect in masses is an indication of acidity. Pus and blood- corpuscles are easily distinguished by their peculiar characteristics. The richness of milk is dependent upon the number of globules. In determining the quality of the milk of different animals the greatest care in manipulation is ne- cessary. Saliva. — A drop of saliva placed upon a glass slide and covered with a thin piece of glass, presents for examination epithelial scales from the buccal mucous membrane, salivary corpuscles, and numerous molecules and granules. The epithelial scales are flat plates, varying in length from the l-800th to the l-500th of an inch. They are generally oblong in shape, sometimes square, and occasionally very irregular. The edges are curled up and often adherent to those of other scales. With a magnifying power of 250 diameters linear, a round or oval nucleus may be seen in the substance of these scales, sur- rounded with a great number of molecules and granules. The addition of acetic acid renders the nucleus more distinct, at the same time increasing the transparency of the scale. Water pro- duces little or no effect. " The salivary corpuscles are colorless, spherical bodies, with smooth margins, varying in size from the l-3000th to the l-1800th of an inch in diameter. They contain a round nucleus, varying in size, but generally occupying a third of the cell ; and between this nucleus and the cell-wall are numerous molecules and granules, which communicate to the entire corpuscle a finely molecular aspect. The addition of water causes these bodies to swell out and enlarge from endosmosis ; while acetic acid some- what dissolves the cell-wall, and it becomes more transparent ; while the nucleus appears more distinct as a single, double, or tripartite body. Both water and acetic FIG- 399. acid also, produce coagulation of the ©•*"'! '181 albuminous matter contained in the fluid of the saliva, which assumes the form of molecular fibres, in which the corpuscles and epithelial scales become entangled, and present to the naked eye a white film."1 (Fig. 399.) The salivary corpuscles are accom- panied with a quantity of molecular and SaliTc7ieTand"gianueielthelial granular matter, which undergoes an increase in ulceration of the mucous 1 Dr. Bennett's Introduction to Clinical Medicine. m EXAMINATION OF ANIMAL FLUIDS. 685 membrane of the mouth. The debris of various articles taken as food are also often found in the saliva, rendering its study more difficult to the unpractised eye. Sputum. — To examine sputum it should first be placed in water, in order that any dense cretaceous or tubercular portions may be deposited at the bottom of the vessel, and thus separated from the lighter portions, which, on account of the confined air, will generally float upon the surface. The different constituent ele- ments may then be isolated by breaking up small detached masses with a glass rod, and spreading them out upon a glass slide. Parts presenting any peculiar appearances, as dark spots, fibrous tissue, &c., should be removed by means of broad-pointed forceps and scissors, and examined separately and with much care. The various matters which enter into the composition of sputum, and which complicate its study, are thus enumerated by Prof. Bennett. " 1st. All the tissues which enter into the composition of the lung, such as filamentous tissue, young and old epithelial cells, blood-corpuscles, &c. ' 2d. Mucus from the oesophagus, fauces, or mouth. 3d. Morbid growths, such as pus, pyoid and granular cells, tubercle-corpuscles, granules and amorphous molecular matter, pigmentary deposits of various forms, and parasitic vege- tations, which are occasionally found in the lining membrane of tubercular cavities. 4th. All the elements that enter into the composition of the food, whether animal or vegetable, which hang about the mouth or teeth, and which are often mingled with the sputum, such as pieces of bone or cartilage, muscular fasciculi, portions of esculent vegetables, as turnips, carrots, cab- bages, &c. ; or of grain, as barley, tapioca, sago, &c. ; or of bread and cakes ; or of fruit, as grapes, apples, oranges, &c." It has lately been asserted by Shroeder Van der Kolk, that fragments of pulmonary tissue may be detected in the sputum before the existence of ulceration can be revealed by physical exploration. This, however, is doubted by Prof. Bennett. A very common appearance in sputum under the microscope is represented by small masses of molecular and granular matter. In the sputum of phthisis small lumps of softened tubercle may often be found, mingled with purulent mucus, at the bottom of the vessel. They have a yellowish, cheesy appearance, and consist of small and somewhat transparent cells, round, oval, or triangular, in shape, and varying from the l-120th to the l-75th of a millimetre in their longest diameter. They are known as tubercle-corpuscles, contain a number of granules, and are sur- rounded by free granular matter and oil-globules. (Fig. 400.) In the sputum which accompanies the cretaceous or calcareous transformation of tubercle in the lungs, small, hard or gritty, and white masses will be found, consisting of amorphous aggre- gations of phosphate and carbonate of lime mixed with some 686 APPENDIX. animal matter. (Fig. 401.) Pus and blood-corpuscles are often observed in the sputum, and occasionally crystals of choleste- rine. FIG. 400. FIG. 401. >%>%r% ^y^;§illl: FIG. 402. The sputum of acute pneumonia often contains minute fib rinous casts of the bronchial tubes, together with large cells filled with oil-globules, and numerous finely granu- lar cells, similar to pus-globules. (Fig. 402.) The thickened sputum expectorated in the morn- ing on first rising, consists of epithelial cells pressed more or less closely together, and varying somewhat both in shape and size. The dark color is due to the numerous carbonaceous granules contained in the cells. Mucus. — The gelatinous material known as mucus, presents different appearances, according to the pecu- liar cell-structures and pigmentary matters which it contains. It possesses no essential morphological element, the so-called mucus-corpuscles being in all probability merely pus-cells, or modifications of epithelium. According to Prof. Bennett, irritation of a mucous surface causes the exuda- tion which is poured out to be transformed into pus-corpuscles by mixing with the gelatinous secretion. The thick white gela- tinous mucus secreted by the lining-membrane of the os uteri contains numerous epithelial cells, while gonorrhoea! or catarrhal mucus abounds in pus-cells. The viscid albuminous substance in which these cells are contained manifests a marked tendency to coagulate in the form of minute fibres, and constitutes the most characteristic element of mucus. An increase of the cell- elements, and a diminution of viscidity, are indications of disease ; while an increase of the albuminous matter, and a decrease in the number of cells, are marks of a more healthy mucus. Pus. — Normal pus, placed between two glasses and examined with a magnifying power of two hundred diameters, exhibits numerous granular corpuscles floating in a clear fluid, called liquor puris. These corpuscles are larger than blood-globules, have a smooth margin and a finely tuberculated surface, and vary in diameter from the l-1300th to the l-1000th of an inch. Many of them contain a round or oval nucleus, which is rendered more distinct on the addition of water, and is liberated in the EXAMINATION OF ANIMAL FLUIDS. 687 form of granules, having a central dark spot, by the addition ot strong acetic acid, which dissolves the cell-wall. (Fig. 403.) In FIG. 404. '' scrofulous pus and in various unhealthy discharges, these corpus- cles lose their globular form, and are found surrounded with numerous molecules and granules. (Fig. 404.) In gangrenous and ichorous pus they are mixed with broken-down blood-glo- bules, remains of tissue, &c. Dr. Beale makes the following judicious remarks with regard to these pus-corpuscles. "The cells above referred to have been considered as characteristic of pus, and much trouble was taken, in the earlier days of microscopical research, to assign definite characters to them, by which they might be distinguished from the so-called mucus-corpuscle, and other cells which they much resemble. Such a distinction, however, cannot be made, for, in the first place, cells may be obtained which present various stages, apparently intermediate between an ordinary epithelial cell, and a pus-globule ; secondly, cells agreeing in their micro- scopical characters with the pus-globule, are not unfrequently formed on the surface of a mucous membrane, without its func- tion being seriously impaired, and certainly without the occur- rence of those preliminary changes which usually precede the formation of pus ; and, thirdly, cells are found in the lymph, in the blood, in the lymphatic glands, in the serous fluid in the in- terior of cysts, and in many other situations, which in their size,. form, and general appearance so much resemble the globules found in true pus, that it is quite impossible to assign characters by which they may be distinguished. The figures of these cells, as they appear before and after treatment by acetic acid, often could not be distinguished from the figures of pus-cells, treated in a similar manner, given by the same authors. " Cases occur in which it appears almost useless to attempt to decide as to the presence or absence of pus, if only a few glo- bules are to be found (nor do I think that if such were possible, it would be of any advantage), because no characters by which the globules can be distinguished individually have been laid down. " At the same time it must not be supposed that the diagnosis of pus is a matter of secondary importance ; and all that is in- tended in introducing these observations is to impress upon the student the importance of not stating that pus has been found in 688 APPENDIX. any particular locality, or in any particular fluid, merely because a few cells having all the characters of a pus-globule have been observed. To say that 'pus had been found in the blood,' or that 'the casts of the uriniferous tubes contained pus,' would lead to a very different inference from that derived from the statement that ' cells having all the characters of pus-globules had been found in the blood,' or that the 'casts of the tubes con- tained cells resembling those of pus.' The former will be true in extremely few cases ; the latter in a vast number that fall under the observation of every practitioner. If, however, we find a considerable number of globules under the field of the microscope, of nearly uniform size, agreeing in general charac- ters with the pus-globule, and upon the addition of acetic acid exhibiting the characteristic reaction, we shall seldom be wrong in inferring that they are really pus-cells." Faeces. — In the faeces may be found, says Prof. Bennott, "1st. All the parts which compose the structure of the walls of the alimentary canal ; 2d. All kinds of morbid products ; and 3d. All the elements which enter into the composition of food." In severe attacks of dysentery, epithelial scales, membranous flakes, shreds of fibrous matter, pus-globules, and blood-corpus- cles are all observed in the discharges, mingled sometimes with crystals of the triple phosphate, and occasionally with numerous torulse and sarcinse. In ulceration of the intestines, pus-globules may readily be detected upon the surface of the faecal masses. Perfectly formed pus and blood-globules originate low down in the rectum, near the anus; broken-down globules originate higher up in the bowel. The white flocculi composing the stools of cholera patients consist of epithelial cells imbedded in mucus. Sometimes the sheaths of the villi are also found, together with free nuclei. In the stools of typhus and other fevers of a low type great quantities of crystallized phosphates and carbonates are found.' Dr. Farre, Prof. Bennett, and others, have observed collections of confervoid growths in the matters discharged from the bowels. Urine. — The student should early accustom himself to the examination of the various constituents of healthy urine. He cannot be too familiar with the different appearances presented by different specimens of this important secretion. Sometimes the urine will appear utterly structureless or homogeneous, offering absolutely nothing for examination. At other times, even the well-educated eye, will with difficulty identify the various and dissimilar objects crowded together in the field of the microscope, and which have accidentally found their way into the urine, or been introduced by the patient for purposes of deception. Starch-granules, woody fibres, hair, fragments of wool, cotton, feathers, &c., have all in their turn, and very fre- quently, been mistaken for some of the ordinary matters de- posited by the urine. "We take from Dr. Beale's work on the EXAMINATION OF ANIMAL FLUIDS. 689 Microscope, the following table of the most common extraneous matters constantly met with in urinary deposits. Fragments of human hair. Fragments of tea-leaves, or separated Cats' hair. spiral vessels and cellular tissue. Hair from blankets. Fibres of coniferous or other wood Portions of feathers. swept off the floor. Fibres of worsted of various colors. Particles of sand. Fibres of cotton of various colors. Oily matter in distinct globules, aris- Fibres of flax. ing from the use of an oiled cathe- Potato starch. ter, or from the accidental presence Rice starch. of milk or butter. Wheat starch, bread crumbs. Great care, therefore, should be exercised in the collection and preparation of urine for microscopic examination. It is a very good practical rule to examine a portion of the urine an hour or two after it is voided ; and another portion after standing a day, or in some instances two days. Occasionally it will be necessary to institute the examination immediately upon the passage of the secretion, as in those instances where there is a strong and rapid tendency to decomposition upon exposure to the atmo- sphere, or where this change has already taken place in the bladder. Urine containing lithic acid or oxalate of lime should be allowed to stand for some time in order that it may be de- posited. Perfectly healthy urine, after standing for about twelve hours, exhibits a slight cloudy deposition, consisting of epithelial scales, some crystals of the triple phosphate, and granular fragments of the urate of ammonia. Urinary deposits are best obtained for examination by pouring the fluid into a tall wine-glass, or wide-mouthed bottle, capable of containing several ounces, and allowing it to stand for a few hours. The clear supernatant liquid should then be poured off, and the thick turbid under-stratum of urine, containing the deposit, emptied into a small test-tube. The sediment will thus be obtained in a small bulk, and can readily be examined by being deposited upon a glass slide after the liquid portions have been removed by means of a pipette. Large crystals of uric acid require an inch object-glass to be seen distinctly; when very small a fourth of an inch glass is necessary, as is the case also with the octohedral crystals of the oxalate of lime, epithelium, casts of the uriniferous tubes, &c. The eighth of an inch object-glass is best adapted for the exami- nation of spermatozoa. In examining the deposits small portions should be brought into the field of the microscope at a time, and in this careful manner the different constituents, and their relation to each other, may first be studied to considerable advantage with the lower magnifying powers. Afterwards the nature and structure of each object may be more minutely investigated by subjecting it to the higher glasses. Sometimes it will be found advisable 44 690 APPENDIX. to examine the deposit in various fluids, such as water, mucilage, spirit, Canada balsam, turpentine, &c. Occasionally it is neces- sary to resort to certain chemical reagents, before the deposit can be examined satisfactorily. Thus, in some amorphous sediments containing lithic acid alone, or combined with alkaline bases, the familiar rhomboidal crystals cannot be detected until the mass be first dissolved with potash, and then treated with excess of acetic acid. ORGANIC CONSTITUENTS OF URINARY DEPOSITS. In healthy urine the mucus settles towards the bottom, as a flocculent, transparent, and somewhat bulky cloud. In this trans- parent substance the microscope detects FIG. 405. merely a few granular cells, larger than blood-corpuscles, of very delicate struc- ture and surrounded by a few minute granules. In disease the mucous de- position often loses its transparency, becoming viscid and thick from the addition of various kinds of epithelium. The peculiar appearance of the epithe- lial cells will indicate the part of the genito-urinary mucous membrane from which the mucus was secreted. (Fig. 405.) In some diseases of the bladder, as cystitis, a thick, glairy, and gela- tinous deposit will often appear, simulating inspissated mucus. This is pus chemically changed by contact with the carbonate ot ammonia generated in the de- composition of urea by the al- kaline mucus. Very minute octohedral crystals of oxalate of lime are sometimes found like dark square-shaped specks imbedded in the mucus. A power of two hundred is gene- rally sufficient to distinguish them. Fragments of cotton- fibre, hair, &c., are sometimes found incrusted with these crys- tals. The epithelium found in the urine differs in different speci- mens, according to the part of the urinary apparatus from which it is derived. In the con- voluted portion of the tubuli uriniferi it is glandular, and forms a thick layer upon the basement membrane. (Fig. 406.) Mucous corpuscles and epithelium. FlG. 406. A, Portion of a secreting canal from cortical portion. B, Epithelium gland cells, magnified 700 times, n, Portion of a canal from medullary sub- stance of kidney. ORGANIC CONSTITUENTS OF URINARY DEPOSITS. 691 In the straight portion of the tubes it is flatter, and more like the scaly variety. In the pelvis of the kidney, it is tessellated or pavement-like, consisting of thin, flat scales united at their edges. In the ureter it is columnar or cylindrical in shape, having a large and distinct nucleus. In the fundus of the bladder colum- nar epithelium is found mixed with large oval cells ; flattened cells having a distinct nucleus and nucleolus abound in the trigone. Fia. 407. FIG. 408. Mucus, pus, blood, and epithelium from leucorrhcea. In the mucous follicles columnar epithelium is found; on the sur- face between them, the scaly variety. The columnar variety pre- vails in the posterior part of the urethra ; anteriorly it becomes scaly. The vaginal epithelium found in the urine of females, is also of the scaly variety. (Fig. 407.) Occasionally casts, consisting of moulds of the uriniferous tubes, are observed in the urine. They furnish valuable aids in arriving at a correct diagnosis as to the pathological changes which may be going on in the kidney. They consist mainly of oily granules, or epithelial cells abundantly supplied with these granules. (Fig. 408 A.) Pro£ Bennett divides them into two distinct varie- ties, namely, — " 1st. Fibrin- ous or exudation casts, which are most commonly found in the urine at critical periods of acute inflammations, especially in scarlatina, small-pox, pneu- monia, &c. 2d. Casts, with oily granules, indicative of chronic disease, and especially of Bright's disease. (Fig. 4094.) At the 692 APPENDIX. FIG. 409. same time, it should be understood that they may be more or less associated together, and that the rule is not invariable." Dr. Beale divides them into three classes according to their diameter; namely, 1st. Casts of medium diameter, about the l-700th of an inch. These contain granular matter with epithelial debris, oil-globules, and occasionally blood and pus-corpuscles. In the urine of a cholera patient, Dr. Beale once detected dumb- bell and octohedral crystals of oxalate of lime in one of these casts. 2d. Casts of con- siderable diameter, about the l-500th of an inch. These are transparent and have a smooth, glistening, or waxy appearance. Sometimes they are granular. 3d. Casts of small diameter about the 1-1 000th of an inch. According to Dr. Johnson these originate in cases in which there is no tendency on the part of the epithelium to desquamate, as in non-desquamative ne- phritis. Spermatozoa are some- times found in the urine when examined soon after it has been passed. They can be seen with a power of two hundred diameters, though in demonstrating them it is better to employ a power of four hundred diameters. (Fig. FIG. 411. FIG. 410. Tube containing an homogeneous cast. 411.) The presence of these bodies in the urine must not be regarded as a sign of spermatorrhoea, unless accompanied with the symp- ORGANIC CONSTITUENTS OF URINARY DEPOSITS. 693 toms of that disease. In urine, which has heen allowed to stand for some time, vibriones and certain forms of vegetable fungi or torulse are gradually developed. With a power of two hundred diameters, vibriones, looking like minute lines, may be seen writhing about in the mucus. They are always to be found in decomposing urine, and are sometimes generated in the bladder. The species of fungi vary in different specimens of urine, and appear after different intervals of time. Dr. Hassall, however, considers the different species to be merely the successive stages of development of the same fungus ; the stage of development being dependent upon the degree of acidity of the urine, and the length of time in which it has been exposed to the air. In acid urine, containing nitrogenous matter, and exposed to the atmo- sphere, a peculiar fungus is generated known as the penicilium glaucum, — the same fungus which is developed in lactic acid fermentation. Dr. Hassall has shown that in urine containing even very minute traces of sugar, a peculiar fungus is de- veloped, which may be regarded as the charac- teristic test of the presence of sugar, since it is found in no other condition of urine. In diabetic urine torulse are often very rapidly developed. (Fig. 412.) The fat-cells found in the urine are usually epi- thelial cells loaded with oil. According to Dr. Beale, fatty matter may occur in the urine in three conditions. 1st. " As distinct and separate globules, resembling those which are produced by intimately mixing oil with water by the aid of mucilage, &c. ^ (Fig. 413.) When fatty matter occurs in this state only in urine, its presence is usually accidental. 2d. In the form of globules enclosed within a cell-wall, or in casts. 3d. In the so-called ' chylous urine,' the fatty matter is suspended in an exceedingly minute state of division." FIG. 414. FIG. 413. FIG. 412. Blood-globules generally form a brownish-red granular sedi- ment at the bottom of the vessel. If the urine has been standing long, the globules will appear very much changed in shape. Acid urine containing blood has a smoky hue, the globules 694 APPENDIX. FIG. 415. appearing of a dark brown color. Where the liquid has a neutral, or slightly alkaline reaction, the globules are red. Pus-globules are also occasionally found in the urine, more or less changed in shape, according to the length of time they have remained in the liquid. After long soaking they completely disintegrate. De- posits of pus are often accompanied with crystals of the triple phosphate. This is especially the case when the pus is derived from the bladder. Large and small organic globules, exudation cells, spherical cells con- taining nuclei and granular matter, &c., are also found in the urine. For descriptions of these bodies, the stu- dent is referred to the work of Dr. Golding Bird, on Urinary Deposits. INORGANIC CONSTITUENTS OF URINARY DEPOSITS. Other varieties lAihic or Uric acid is one of the most common urinary deposits. In color it varies from a light fawn to a deep orange-red, the usual hue of the crystals being yellow. Some- times they are almost colorless. They assume a great variety of forms, the most common and most characteris- tic of which is the rhomboidal, Fig. 413, — the form usually generated when the lithates are decomposed by means of an acid. Fig. 414 re- presents the peculiar forms of uric acid found in the urine of patients laboring under acute and scarla- tinous dropsy. Sometimes the crys- tals are square and lozenge-shaped. are seen to consist of adhering masses and FIG. 416. 0 00 $§)CQ 0 SM flat scales, with transverse and longitudinal markings. Some INORGANIC CONSTITUENTS OF URINE. 695 times they are six-sided, resembling crystals of cystine, but dis- tinguishable from the latter, by two of their sides being longer than the others. Occasionally they appear as truncated or rounded columns. Cystine crystallizes in flat, hexagonal plates, with irregularly hexagonal markings on their surface. Sometimes radiating lines pass from an opaque centre to the margins. (Fig. 416.) A deposit of cystine may be readily distinguished from that of the pale urates, which it resembles in appearance, by not dis- solving when heated. Oxalate of Lime occurs in oc- tohedral crystals, having one axis shorter than the other two. It is important to remember that these crystals difler in ap- pearance, according to the posi- tion in which they are viewed, as shown in Figs. 417, 418. When large they may be seen with the FIG. 418. FIG. 417. Various forms of oxalate of lime. unassisted eye as minute glistening points imbedded in the sedi- ment. Where crystals of the oxalate of lime are associated with and obscured by the pale lithates, the addition of a drop or two of solution of potash, by dissolving the latter, will render the former more apparent. Occasionally oxalate of lime assumes the form FIG. 419. of dumb-bells. Dr. Golding Bird considers these ^ ^ to consist of oxalurate of lime, on account of lp their polarizing influence upon light. (Fig. 419.) ^ ^ The dumb-bell crystals are probably compound, & ' * iK ^ appearing to be formed of collections of minute o (if) ^® acicular crystals. They are generally accom- 5^ ^ panied by the octohedral form, and, according to Dumb-beii crystals. Dr. Beale, their appearance is sometimes pre- t ceded and succeeded by the presence of the circular, oval, and less regular forms of crystals. They are formed in the kidney, 696 APPENDIX. having been met with in the uriniferous tubes after death, and in fibrinous casts of those tubes. Phosphate of lime and lithic acid also assume the dumb-bell form ; but the solubility in pot- ash, and the different refracting power of these latter crystals, distinguish them from those of the oxalate. FIG. 420. Lithates constitute the so-called "lateritious deposits" in urine. According to Heintz these buff-colored or deep red sediments consist mainly of lithate of soda mixed with small portions of the lithates of ammonia and lime, and a trace of the lithate of magnesia. Under the microscope they appear as minute granules in different states of aggregation. (Fig. 420.) The lithates of ammonia and soda sometimes occur in spherical masses, adher- ing to thin films of the phosphates. The ammoniacal lithate occasionally assumes a stellate form; Prof. Bennett has seen it arranged in such a manner as strongly to resemble in appearance an organic membrane. The triple phosphate, or ammonio-phosphate, of magnesia, occurs in the form of triangular prisms, occasionally truncated, FJG. 421. FIG. 422. and sometimes with terminal facets. (Fig. 421.) If the urine be very ammoniacal, they present a star-like or foliaceous appear- ance (Fig. 422). Carbonate of lime is sometimes associated with the earthy phosphates in human urine, but rarely in a crystalline form. It generally appears in small round masses, or as an amorphous powder. INORGANIC CONSTITUENTS OF URINE. 697 Urinary deposits may be preserved either in the dry way, in Canada balsam, turpentine, oil, and similar fluids, or in aqueous solutions. Only large crystals of the oxalate of lime, lithic acid, and some of the phosphates and lithates, can be preserved in the dry way. Dr. Beale gives the following directions for the pre- servation of urinary deposits: "After the crystals have been allowed to collect at the bottom of a conical glass vessel, the clear supernatant fluid is to be poured off, and the crystals are to be washed with a little dilute alcohol, or with a very weak solu- tion of acetic acid. When the process of washing has been re- peated two or three times, a small quantity of the deposit is to be transferred by means of a pipette to a glass slide, and the greater part of the fluid soaked up with a small piece of blotting paper. The crystals are next to be spread a little over the glass with the aid of a fine needle, in order to separate the individual crystals from each other; and the slide is to be placed in a warm place, or in the sun, until quite dry; but care must be taken that the drying is not carried on too rapidly, and that too great a degree of heat is not employed. A narrow rim of paper, or card- board, is next to be gummed on the slide so as to include the crystals in a sort of shallow cell; and lastly, the glass cover is to be put on and kept in its place either by anointing the edges with a little gum-water, or by pasting it down with narrow strips of paper, which may be variously arranged and ornamented accord- ing to taste. "If the crystals of lithic acid are to be mounted in Canada balsam, they should be carefully dried first, as above directed, and afterwards over sulphuric acid, and then moistened with a small drop of spirits of turpentine. The slide is now to be slightly warmed, in order to volatilize the greater part of the turpentine, and a drop of Canada balsam is to be dropped upon the preparation from the end of a wire, which may be readily effected by holding the wire with the balsam over the lamp or hot brass plate for a minute or two in order to soften it. The slide is next to be held over a lamp, in order to keep the balsam fluid until any air-bubbles which may be present have collected into one spot on the surface of the liquid balsam, an operation which is expedited by gently moving the slide from side to side. The air-bubbles may now be removed by touching them with a fine-pointed wire. Lastly, the glass cover is to be taken up with a pair of forceps, slightly warmed over a lamp, and one edge is allowed to touch the balsam. The surface is permitted to fall gradually upon the balsam, so that it is wetted by it regularly, and only by very slow degrees, for otherwise air-bubbles would yet be included in the preparation. The glass slide with the pre- paration may now be set aside to cool." There are many substances, however, which cannot be pre- served to advantage in Canada balsam, or by the dry method. Such'are epithelium, casts, torulee, conferva, fat-cells, pus, mucus, &c. Such substances should be placed in shallow glass-cells, 698 APPENDIX. and covered with aqueous solutions, varying in character and strength to suit the specimen. The best preservative fluids are weak spirit, glycerine diluted with water, solutions of gelatine, creasote, naphtha, &c. The gelatine solution answers very well for the preservation of dumb-bell crystals of oxalate of lime, while the creasote and naphtha solutions are better adapted for the preservation of epithelium, tubular casts, &c. Crystals of the triple phosphate are best kept in aqueous solutions of ammonia ; for cystine dilute acetic acid answers very well. Vomited matters consist of articles of food variously altered by the digestive processes, epithelium and mucus from the mouth, fauces, pharynx, oesophagus, and stomach, gastric juice, bile, and the various matters generated in disease. As different portions of vomit contain different ingredients, small portions taken from points considerably separated, should successively be subjected to examination. The various transitions which alimentary sub- stances undergo in the stomach, must often necessarily render the determination of the exact composition of the vomited matters a point of extreme difficulty. Starch-granules are often met with in abundance, but some- times so changed as to require the addition of tincture of iodine to detect them. Fig. 423 represents the appearance of starch corpuscles after partial digestion in the FIG. 423. stomach. The epithelium is also frequently found to be more or less altered from en- dosmosis, and partial digestion. Yibriones and various species of torulse are also ob- served in vomit. The sarcina ventriculi, a pe- culiar fungus discovered by Mr. Goodsir, in matters ejected from the stomach, has also been observed in the faeces, in the urine, and in an abscess of the lung. The fluid of waterbrash consists mainly of epithelial scales and small oil-globules. In the rice-water vomit of cholera patients numerous flocculi of epithelial cells are found. The coffee-ground vomit appears to consist mainly of the color- ing matter of the blood reduced to a finely granular state and mingled with disintegrated blood-corpuscles. The sedi- ment deposited by the black-vomit of yellow fever upon stand- ing, in all probability is chiefly composed of blood-globules in various stages of disintegration. The epithelial cells of this fluid " vary in respect to their abundance, size, and shape, and while stated by some to have presented themselves in all the specimens examined, they have, in some instances, been found wanting. Of the six specimens reported upon by Dr. Leidy, two were deficient in this particular. The size and shape of these cells, as observed by Dr. Riddell, have already been referred to. In the hands of Dr. Michel, the scaly, columnar, and spheroidal, have, at different times, been plainly made out with their nuclei SEROUS AND DROPSICAL FLUIDS. 699 FIG. 424. and nucleoli, but in very different proportions — the scaly or la- mellar cells being always most numerous."1 Uterine and Vaginal Discharges present quite different charac- ters in different specimens. The examination should be insti- tuted as soon after they are collected as possible, and without the addition of water, as this may affect the natural appearance of the constituent elements. Epithelial cells and blood-globules in varying quantities compose the menstrual discharge. In leu- corrhcea many of the epithelial cells are filled with oil-granules, and mingled to a greater or less extent with pus- corpuscles. Blood-globules are also observed very much altered in shape. (See Fig. 407.) In cancer of the uterus the microscopic exa- mination of the discharges, becomes highly important in arriving at an accurate diagnosis. Cancer-cells, in such cases, may often be detected in the discharges. When they are broken down or considerably altered in form, not a little difficulty will be experienced in assigning to them their true value. The student should be careful, also, not to confound the columnar epithelium of the ureter, with the spindle-shaped cancer-cells. Fig. 424 represents the microscopic appearances of some cancerous juice squeezed from the uterus ; that to the left is the natural appearance, the other after the addition of acetic acid. FIG. 425. SEROUS AND DROPSICAL FLUIDS. The sedimentary matters should be collected from serous fluids, and examined in the same way as urinary deposits. An examination of the freshly effused fluid of ascites reveals only a few cells floating in a clear liquid. In chronic as- cites, however, numerous gra- nular and spherical cells, mostly non-nucleated and va- rying in size, are observed. The sediment consists of deli- cate fibres, interlaced, and having cells in the meshes or interstices, together with plates of cholesterine. Occa- sionally blood and pus-cor- puscles may also be detected. The hydrocelic fluid consists of some delicate cells, and oil- globules, and occasionally some spermatozoa and plates of chole- sterine. 1 Dr. R. La Roche on the Nature and Composition of Black Vomit. Med. Sciences, April, 1854. Arner. Jour, of 700 APPENDIX. Cells, oil-globules, free granular matter, and occasionally blood- corpuscles and crystals of cholesterine are the principal constitu- ents of the deposit obtained from ovarian fluid. Sometimes masses of gelatinous or colloid matter are mixed with these ele- ments. Minute fibres are sometimes observed crossing each other in various directions, and contain in their meshes thus formed, a transparent jelly filled with round or oval corpuscles. (See Fig. 424.) The cells are either small, transparent, granular, and non-nucleated, or large, opaque, and filled with oil-globules. Fig. 376 represents fatty granules, mixed with plates of choles- terine from an ovarian tumor, after Prof. Bennett. INJECTIONS. In studying the vascularity of tissues, injected specimens are of great utility. The student should give some attention, there- fore, to the practice of injecting the different organized structures which he may desire to examine. Dr. Beale, in his admirable little work on the application of the microscope to clinical medi- cine— a work of which we have availed ourselves freely in the construction of this chapter — gives the following practical direc- tions as to the time, mode, &c., of making injections. " Generally, it may be remarked that we should not attempt to inject while the rigor mortis lasts. Many days may in some cases with advantage be allowed to elapse, particularly if the weather is cold, while in warm weather we are compelled to in- ject soon after death. As a general rule, the more delicate the tissue, and the thinner the vessels, the sooner should the injection be performed. Many of the lower animals, annelids, mollusca, &c., and fishes, should be injected soon after death. In making minute injections of the brain, only a short time should be allowed to elapse after the death of the animal, before the injection is commenced. Injections of the alimentary canal of the higher animals should be performed early — not more than a day or two after. " Minute injections of the papillae of skin, particularly of the fingers and toes, cannot be successfully made until the cuticle has become somewhat softened by allowing the preparation to remain in a damp cloth, or to soak in water, for some days. In these situations the yessels are strong, and in their ordinary state, the injection will not traverse them, in consequence of the cuticle preventing their gradual distension by the injecting fluid. A similar plan must be followed in making injections of the tongue, and other parts where the epithelial covering is unusually dense, and firmly adherent to the vascular surface beneath. "If the subject be a small animal, it is better to take out part of the sternum, and fix the pipe in the aorta. If only part of an animal is to be injected, the largest artery supplying the part should be selected, and all the other open vessels may be tied or stopped with the small forceps. INJECTIONS OF VEINS, ETC. 701 " A small portion of intestine can be injected by cutting out the corresponding portion of mesentery attached to it ; and after searching for a large vessel, all the others may be tied, together with the open ends of the alimentary tube." " A pipe of some- what smaller diameter than the vessel should be selected, and an opening may then be made in the vessel of sufficient size to admit the pipe, which can now be inserted. The needle, charged with thread or silk, is then carefully passed round the vessel, the thread seized with forceps, and the needle withdrawn over the thread. This operation is sufficiently simple where the vessel is large and strong ; but where thin and easily torn, it requires great care. The thread is now tied tightly round the vessel close to the extremity of the pipe, and then attached to the two projecting wires, to prevent the possibility of slipping. "In injecting from veins a similar method is pursued, taking care to choose a vein in which the valves are not numerous, or in which they are altogether absent. The portal vein can be reached by opening the abdominal cavity, care being taken not to tear any of the branches below the point where the pipe is inserted. " Greater care is required to fix the pipe in the vessels of fish, in consequence of their being so readily torn. Excellent injec- tions of fish may frequently be made as follows : The tail is cut off with a sharp knife at a short distance posterior to the anus, and if the cut surface be examined the ventral artery may be easily found situated immediately beneath the bodies of the ver- tebrae. A pipe is carefully introduced and pushed down some distance, so as to prevent the injection from coming out, or the end of the vessel may sometimes be separated from the surround- ing parts and tied in the usual way. By this simple proceeding capital injections can often be made very easily. "Minute injections of the branchiae of some of the mollusca may often be made by very carefully placing the pipe in the largest vessel that can be found, and slowly injecting. The ex- treme delicacy of the vessels prevents any attempt being made to tie them to the pipe, and, of course, much injection will be lost. From the large size of the vessels, however, much will run into the capillaries. In this way I have easily succeeded in in- jecting the branchiae of the Pinna ingens, and fresh-water mussel (Anodon), both of which form beautiful microscopical objects. "In order to inject the smaller gasteropods (slugs, snails, &e.), we must pursue a different method. In the muscular foot of these are situated many large lacunae, or cavities, which commu- nicate with the vascular system, or, in fact, form the vessels which are distributed to this organ. If the injection can be forced into any of these lacunae, it may be made to traverse the whole vascular system. To introduce the pipe a small hole is made obliquely in the foot, taking care not to force the instru- ment too far. A small pipe is next inserted, and when the pre- 702 APPENDIX. paration is warm enough the injection of size and vermilion is very slowly and carefully forced in, and the progress which is made can be seen by observing the vessels distributed to the respiratory organs. When a sufficient quantity of injection has been introduced, the pipe may be withdrawn, and the hole plugged with a piece of wood cut to the proper size, to prevent the injection again escaping before the size has had time to set. "The vascular system of insects may sometimes be partially injected by forcing the injection into the abdominal cavity, from which it finds entrance into the dorsal vessel, and from thence is distributed to various parts of the body. The injection in the cavity of the abdomen is then allowed to escape. "It is very important that the size and vermilion, or other in- jection which is to be thrown into the vessels, should be thoroughly mixed and well strained before being used. The coloring matter, properly powdered, should be placed in a small earthenware mortar, and the melted size or other fluid carefully added by degrees, the whole being constantly stirred until well mixed. When the proper color has been obtained, the whole must be strained through muslin, or through a fine perforated strainer, into another vessel, which should be kept warm. The injection should be well stirred with a wooden stick previous to filling the syringe. " We can judge of the intensity of the color by removing a drop of the solution with a stirring-rod, and allowing it to fall on a white plate so as to form a thin stratum, which should have a pretty deep color. It is always better to have too large a quan- tity of the coloring matter rather than too little. " Of vermilion, about two ounces will be sufficient for a pint of size ; but it is better in all cases to regulate the quantity by examining the intensity of the color in the manner just men- tioned. "When the preparation is warmed through, the injection pro- perly strained, and the pipe fixed in the vessel, we may proceed carefully to inject, taking care that the injection is kept at a pro- per temperature, by allowing it to remain in the warm water- bath during the operation. " The air should be first withdrawn from the upper part of the vessel by means of the syringe, after which the stop-cock is turned off and left attached to the pipe. The syringe is then disconnected, and after being washed out once or twice with warm water, is nearly filled with injection, which must be well stirred up immediately before it is taken. The syringe should not be quite filled, in order that the air in the pipe may be made to rise into the syringe through the injection, by the ascent of the piston, before any of the latter is forced into the vessel. The end of the syringe is then to be pressed firmly into the upper part of the stop-cock with a slightly screwing movement. " The piston is now very gently forced down by the thumb, ETC. 703 until the syringe has been nearly emptied, when the stop-cock must be turned off, and the syringe refilled with warm injection as before. " Care must always be taken to keep the syringe in the inclined position, so that any air which may be in it, may remain in the upper part ; and for the same reason, all the injection should not be forced out, for fear of the enclosed air entering the vessels, in which case all chance of obtaining a successful injection would be destroyed. " After a certain quantity of fluid has been injected, it will be necessary to use a greater amount of force, wrhich, however, must be increased very gradually, and should only be sufficient to de- press the piston very slowly. If too great force be employed, extravasation will be produced before the capillaries are half filled. Gentle and very gradually-increased pressure, kept up for a considerable time, will cause the minute vessels to become slowly distended without giving way to any great extent. At the same time it must be borne in mind that extravasation fre- quently occurs at various points in a successful injection ; but the longer this event can be kept off, the more likely are we to succeed. "When the injection begins to flow from the large veins mixed with the blood contained in these vessels, and the surface of the injected preparation looks of a red color, and has a some- what velvety appearance, we may infer that the injection has been completed. This occurs at different periods. Sometimes the first or second syringeful causes a general redness of surface, while in other instances a considerable time will elapse before more than a slight blush appears. As a general rule, it is better to proceed slowrly and cautiously, and to use as little force as pos- sible, which should not be more than sufficient to produce an observable depression on the piston. Many minute injections will require an hour or more to complete. " When the injecting is completed, and all the openings by which any of the injection could escape during cooling are closed, the preparation should be placed in cold water, and allowed to remain until the size has set, which will require twelve hours or more in hot weather." In the Medical Examiner of Philadelphia, for December, 1849, Dr. P. B. Goddard details the following method of making minute ethereal injections, originally employed by him. "For the purpose of making such an injection, the anatomist must provide himself with a small and good syringe ; some ver- milion, very finely ground in oil ; a glass-stoppered bottle, and some sulphuric ether. The prepared vermilion paint must be put into the ground-stoppered bottle, and about twenty or thirty times its bulk of sulphuric acid added ; the stopper must then be put in its place, and the whole well shaken. This forms the material of the injection. Let the anatomist now procure the organ to be 704 APPENDIX. injected (say a sheep's kidney, which is very difficult to inject in any other way, and forms an excellent criterion of success), and fix his pipe in the artery, leaving the vein open. Having given his material a good shake, let him pour it into a cup, and fill the syringe. Now inject with a slow, gradual, and moderate pressure. At first, the matter will return by the vein, colored, hut in a few moments this will cease, and nothing will appear except the clear ether, which will distil freely from the patulous vein. This must he watched, and when it ceases, the injection is complete. The kidney is now to be placed in warm water of 120° Fahrenheit, for a quarter of an hour, to drive off the ether, when it may be sliced and dried, or preserved in alcohol, Goadby's solution, or any other antiseptic fluid. For glands, as the kidney, liver, &c., it is better to dry and mount the sections in Canada balsam, but for membranous preparations, stomach, intestine, &c., the plan of mounting in a cell filled with antiseptic solution is preferable." MICROSCOPES QF AMERICAN MANUFACTURE. Microscopes of great excellence are manufactured in this country. These, from their comparative cheapness, and the facility with which they can be procured, offer inducements to students and others to procure them at home, and thus save time to themselves, at the same time that they stimulate the manufac- turers to make increased efforts to attain even greater excellence. A brief description of some of the best will enable the reader to form some comparative estimate of their value. Mr. CHARLES A. SPENCER, of Canastota, New York, has manu- factured a microscope of great excellence, the objectives of which will bear comparison with the best of foreign construction. His common angle of aperture for £ inch objectives is 135° ; for J inch, 170°, and for T'z and Jff inch, 176°. This is believed to be the largest angle ever given to an object-glass, and for sharpness of definition and power of penetration, they are unexcelled by any of foreign make. " To Mr. Spencer is due the credit of having first resolved, with lenses of his own construction, the fine markings on the Navicula Spencerii and Grammatophora Subtilissima : these minute shells have since been adopted by microscopists as test-objects for the highest powers. The Navicula Spencerii, will exhibit one set of lines with Mr. Spencer's Jth-inch object-glass : both sets with the Jth-inch. The Grammatophora Subtilissima is a good test for a j^th or y^th. Of several microscopes made by Mr. Spencer, two or three only will be here noticed. His first-class or best instrument is mounted on trunnions, and embraces all the acknowledged im- provements, in form and stage, whereby the greatest steadiness and freedom from tremor are secured. The price of this instru- MICROSCOPES OF AMERICAN MAKE. 705 ment, with all the accessories and full sets of object-glasses, will approach $350. (Fig. 426.) FIG. 426. Spencer's Trunnion Microscope. The second-class instruments, complete as to ohject-glasses and accessories, but mounted less expensively, cost from $200 to $250. A very efficient microscope, is one known as the "Pritchard form :" this instrument has been somewhat modified by Mr. Spencer, and where a less expensive instrument than either of the others is desired, this one will be found a good working in- strument, and available for all purposes of anatomical study. The cost of this form, with object-glasses as high as the Jth, with the usual accessories, is from $125 to $150. Mr. Spencer also makes some simpler forms of instruments, 45 706 APPENDIX. FiG. 427. and yet very efficient working ones, with objectives as high as ^th, the price of which does not exceed $75. 1 Mr. JAS. W. QUEEN, of Philadelphia, has prepared one of the most convenient and portable microscopes for the use of students that has been made. It is all contained in a mahogany box, six inches square and eleven inches high, with a small drawer within for objects, forceps, &c. The stand is of cast iron, with two up- rights supporting the stage and body of the instrument; between the uprights is an axis upon which the whole upper part of the instrument turns, enabling it to take a horizontal or vertical position, or any intermediate angle most convenient for observa- tion. The movable part, consisting of the stage, the body, and bar containing the adjustment, is fixed to the axis, on which it readily turns. There is an inner tube for increasing power by extending the distance between the eye and object-glass. Within the quadrangular bar, to the top of which is attached the body of the instrument, is the fine adjustment for foci ; the milled head on the top of the bar operates upon the spring within, and carries the body and object-glass from, or to, the object being observed, with the greatest facility. The stage upon which objects are placed to be viewed is made double, the upper portion be- ing movable by a small lever which produces rectilinear or eccentric motion, thus giving great facilities to the microsco- pist for viewing any portion of the object he may desire; the slide containing the ob- ject is kept in place by two spring clips attached to the stage ; the under portion of the stage carries a revolving plate to which the polarizing prism is attached when in use; a diaphragm with various aper- tures is beneath the stage; the mirror is made to slide up and down on the tube that supports it, and can be turned to any angle for light that may be desired; a condensing lens for opaque objects, mounted with a jointed arm, is attached to the collar through which the body passes. Two eye-pieces and two object-glasses are fur- nished with the instrument, also the polarizing prisms. The lowest power is about 50 diameters, and that can be increased to 400 or 500. A drawing-prism can also be furnished when de- sired. (Fig. 427.) Messrs. J. & W. GRUNOW, of New Haven, Connecticut, have in- 1 See Hassall's Microscopic Anatomy, edited by H. Vanarsdale, M.D. J. W. Queen's Microscope. MICROSCOPES OF AMERICAN MAKE. 707 vented an admirable Student's Microscope, which commends itself to all who desire efficiency, cheapness, and portability. The accompanying cut explains its appearance and mode of action. It is mounted on a tripod base, with uprights of japanned cast iron. It has a quick and a slow movement, with draw-tube and a stage 3 by 4 inches, movable by a lever. This movement the Messrs. G. have nearly perfected, and at the suggestion of Dr. Goddard, of this city, an eminent microscopist, they have so arranged it that the stage follows the hand instead of taking the opposite direction, as is usually the case in other instruments having the lever stage. (Fig. 428.) FIG. 428. J. & W. Grunow's Student's Microscope. Messrs. Grunow have also improved their stand in the mode of adaptation of accessory apparatus, especially of that which is attached below the stage; this is held in place by a bayonet '08 APPENDIX. catch, and when once centred it may be detached and reattached with the greatest facility, without requiring a readjustment. Fig. 429 represents a smaller and less expensive form of stu- dent's microscope, made by the Messrs. Grunow, and possessing the same objectives as that represented in Fig. 428, but with a non- FiG 429. movable stage. Messrs. Grunow also manufacture a first class microscope, with brass mountings and all the accessories. (Fig. 430.) Their instruments have given great satisfaction wherever they have been used. THE INVERTED MICROSCOPE OF DR. J. LAWRENCE SMITH. This instrument was invented and brought to the notice of the Societe de Biologie of Paris, in 1850, by Dr. Smith, for whom SMITH'S INVERTED MICROSCOPE. '09 the first one was constructed by Nachet, of Paris. Its design is to enable the chemist to carry on observations under the micro- scope without the risk of injuring his objectives, or at least having his view obscured, by the fumes arising from liquids experi- mented upon. This has always been a serious difficulty in FIG. 430. micro-chemical research, and, together with the difficulty of .manipulating in the limited space between the object-glass and the stage, has hitherto prevented this branch of scientific inquiry from being fully illustrated. " The only way," says Dr. Smith, " by which these difficulties can be surmounted, is by putting the object-glass beneath the stage and the object above it, with an optical arrangement of such a nature as to permit observa- tion." It was with this view that M. Chevalier made a chemical 10 APPENDIX. support, to go with his general instrument, and so constructed it, that it could be inverted so as to have the stage above, and the objective below the object; but every one who has used it knows how awkward it is for manipulation, although exceed- ingly ingenious. Dr. Smith, impressed with these difficulties, was led to the construction of the instrument, which, in this country, bears his name. It was important for the arrangement in question, so to have the relative position of the stage and eye-piece, that the eye, while on a level with the latter, could readily see the former, and guide the required manipulation. The most important part of the instrument is a four-sided prism, Fig. 430, with the angles ab, be, cd, and da, respectively 55°, FIG. 431. 107J°, 52J°, 145°, the angles being of such dimensions that a ray of light passing into the prism in the direction J£a,and perpendicular to the upper sur- face of the prism, after undergo- ing total reflection from the in- ner surfaces (on both of which the light strikes at an angle much less than forty-five de- grees), will pass out perpendi- cular to the surface connected with the body of the instru- ment. It will be readily seen how a ray of light, entering the object-glass, descends into the prism, and passes out of it up- wards, through the eye-glass, the tube of which is inclined to the perpendicular 35°. The other parts of the instrument will be understood by looking at the figure. (Fig. 431.) The illumina- tion of the object is effected by a prism, instead of a mirror. In examining an object with this microscope, the object is arranged in the ordinary way ; when liquid, it is placed in a watch-glass, or such glass cells as are convenient to use. In employing reagents, they can be added and watched immediately, for it is readily seen how the eye guides the manipulation on the stage and looks into the instrument almost at one and the same time. In observing with high powers, as the object-glass is beneath the glass supporting the object, and as the glass is usually of a certain thickness, the method of observation must be changed. For all powers resorted to in chemical examination this difficulty never occurs, and in using high powers it is easily obviated. When the object is already mounted and dry, the thin glass can be readily turned downwards ; but where it is moist, as for in- SMITH'S INVERTED MICROSCOPE. 711 stance, in examining fresh Desmidice&ndDiatomacece, the following plan is resorted to, namely : to use a cell, made of a thin piece of brass or glass, perforated with a hole, about a half an inch in diameter ; it is best to give the hole a considerable bevel in one direction, as it facilitates the cleaning of it. Over the small end FIG. 432. of the hole a piece of thin glass is stuck, with balsam or other cement. "When used, the object to be examined is placed within, and a cover of thin glass placed above. When brass is used to make the cell, it may be as thin as the twentieth of an inch. Dr. Smith remarks that for all observation with high powers, the Inverted Microscope is decidedly superior to the ordinary forms of mounting, for in the latter case, when an object-glass of a iVth or iVth inch focus is used, the focus is too short to admit of the use of cells ; whereas, in the inverted form, as the object is looked at from beneath, the cell may be as thick as one pleases. Another thing connected with this class of observations, is that the Diatomacese and Desmidise can be observed to much greater advantage from beneath than from above, for reasons that will be obvious to persons accustomed to observe these classes of objects. Another advantage possessed by this instrument calculated to extend its use for general purposes, is its great capacity for every variety of illumination, without sacrificing the ease and freedom 712 APPENDIX. from fatigue belonging to the use of this form of miscroscope ; for when placed on a table, rather higher than the one commonly used, and a foot or two from the edge, the observer can recline on his arms, and observe for hours without the slightest sensa- tion of fatigue.1 Messrs. J. & "W. Grunow, New Haven, Conn., are the manufacturers of this instrument. DR. J. LAWRENCE SMITH'S GONIOMETER AND MICROMETER. Professor Smith has also invented a Goniometer for measur- ing the angles of crystals under the microscope. It is also com- bined with a Micrometer. The following is a description of the instrument with the method of using. (Figs. 433-4.) E is the upper end of the draw-tube of the microscope, with the ring k soldered to it. Over this ring k, screws another ring F, which serves as a support and as a centre to the graduated circle Z>, which freely, but without shaking, revolves upon the same. Into the bore of the ring F, fits by its lower conical end A, the tube 6r, which is held in it by a screw-ring y, that prevents its being taken out. Into the tube 6r, which also has a free revolving movement, fits the positive eye-piece a, d being the field-lens, s the eye-lens. The slide b 6, on opposite sides of 6r, admits of the micrometer with its mounting B, B being introduced into 6r, and the gradua- tion being brought into the field of the eye-piece. FIG. 433. 0 is an index, attached to G- by the screw c ; it may be taken off, when the apparatus is not used as a goniometer. 1 See American Journal of Science and Arts, for September, 1852, in which the entire paper of Prof. Smith will be found. USE OF THE GONIOMETER. 713 USE OF THE GONIOMETER. Bring the object into focus near the centre of the field of the micrometer, apply your finger to the knob K, and revolve the micrometer, till the lines of its graduation are parallel to one FIG. 434. side of the angle to be measured. Revolve then separately the graduated circle, till zero is brought to agree with the point of the index C. Then revolve again the micrometer by the knob K, until the graduation lines are parallel to the other side of the angle to be measured, when the index C will show the value of this angle. (The references are the same in both cuts.) The graduated lines of the micrometer are generally ^Jo of the American inch apart. But their relative value as micrometer, with the different object-glasses and eye-pieces, must be ascer- tained by a glass stage-micrometer, and recorded in a table. INDEX. Aberration, chromatic, 73, 74. spherical, 71, 72. Acalephs, see Medusas. Acarida, 581, 582. Achyla prolifera, 315, 316. Acineta-form of Vorticella, 419, 420. Achnanthes, 297. Achromatic Condenser, 131-133, 136 ; use of, 172, 173. Achromatic correction, 41 ; principle of, 73, 74 ; application of, 74-76. Actiniform Zoophytes, 474-476. Actinocydus, 289, 290. Actinophrys, 407-409. Adipose tissue, 607, 608, 664. Adjustment of Focus, 86, 87; uses of, 162- 165. of Object-glass, for thickness of cover, 76, 77 ; mode of making, 165-168. Agates, sponge-remains in, 633. Air-bubbles, microscopic appearances of, 184. Alcyonian Zoophytes, 472-474. Alcyonidium, 496, 497. ALGJE, higher, microscopic structure of, 328-333 (see Protophyta). Amaroucium, structure of, 501, 502. Amici's Prism for oblique illumination, 137. Amoeba, 405-407. Anacharis alsinastrum, rotation in, 364, 365. Anagallis, petal of, spiral vessels in, 396. Angular aperture of object-glasses, 71-73, 76; real value of, 188-192. Animalcule-Cage, 148, 168. ANIMALCULES, early researches on, 38 ; Ehrenberg's observations on, 47-49, 411- 424 ; Dujardin's researches on, 49 ; see Infusoria, Rhizopoda, and Rotifera. Animals, Distinction of, from Plants, 247, 248,409,410. ANNELIDA, 535 ; marine, circulation in, 535, 536 ; metamorphoses of, 536, 537; lumi- nosity of, 537 ; fresh- water, 537, 538. Annular Condenser, Shadbolt's, 139, 140. ANNULOSA, 529 ; see Entozoa, Turbellaria, and Annelida. Anomia, structure of shell of, 512. Antenna3 of Insects, 564, 565. Anthers, structure of, 396, 397. Antheridia of Cryptogamia, discovery of, 45, 46; see Antherozoids. Antherozoids, of Vaucheria, 314 ; ofSphas- roplea, 318 ; of Characeas, 325, 326 ; of Fuci, 330 ; of Florideae, 331 ; of Mar- chantia, 347 ; of Mosses, 349 ; of Ferns, 355, 356. Aphides, non-sexual reproduction of, 580, 581. Anthozoa. 457, 472-476. Apple, cuticle of, 389, 390. Apus, 544. Aquatic Box, 148, 169. ARACHNIDA, microscopic forms of, 581, 582 ; eyes of, 582 ; respiratory organs of, 582, 583 ; feet of, 583 ; spinning apparatus of, 583. Arachnoidiscus, 290, 291. Archegonia, of Marchantia, 344-347 ; of Mosses, 349 ; of Ferns, 356-358. Arcus Senilis, 665. Areolar tissue, 603. Argulus, 547, 548. Artemia, 545. Ascaris, 531, 532. Asci of Lichens, 334 ; of Fungi, 335. Ascidians, compound, 50,500 ; social, 502— 505. Asphalte, use of, 207. Aspidium, fructification of, 353-355. Aspidisca-form of Trichoda, 421. Asteriada, skeleton of, 482, 483 ; metamor- phoses of, 486-488. Asteroida, 472-474. Auditory vesicles of Mollusks, 527, 528 ; development of, 525. Avicularia of Polyzoa, 498, 499. Azure blue butterfly, scales of, 195, 558. Bacillaria paradoxa, 296 ; movements of, 287, 288. Bailey, Prof., his diatomaceous tests, 199. Balani, metamorphoses of, 548-550. Bafk, structure of, 384, 385. Barnacles, metamorphoses of, 548-550. Bat, hair of, 195, 595,596. Battledoor scale of Polyommatus, 195, 558. Batrachospermece, 321, 322. Bee, eyes of, 561 ; proboscis of, 567 ; wings of, 574 ; sting of, 578. Berg-mehl, 303, 304. Beroe, 470, 471. Biddulphia, 299 ; markings on, 281 ; self- division of, 284. Binocular Microscopes, 114-117. Bird, Dr. Golding, on preparation of Zoo- phytes, 474. 716 INDEX. BIRDS, bone of, 587 ; feathers of, 597, 598 ; blood of, 599, 600 ; lungs of, 624-626. Bird's-head processes of Poly/oa, 498, 499. Black-Japan varnish, 219. Black-Ground Illuminators, 138-140, 174, 175. Blenny, viviparous, scales of, 591. Blood-Disks of Vertebrata, 183, 598-601 ; mode of examining and preserving, 601, 602 ; circulation of, mode of examining, 616-619, 691. Bloodvessels, injected preparations of, 619-623; disposition of, in different parts, 623-626 ; morbid, 666. Bone, structure of, 183, 584-587 ; mode of making sections of, 207-211, 587. Bones, fossil, examination of, 207-643. Botrytis bassiana, 336. Botryllians, 502. Bowerbankia, 50, 496, 500. Brachionus, 427, 429, 434. Brachiopoda, structure of shell of, 513-515. Branchiopoda, 542-545. Branchipus, 545. Brooke, Mr., his Object-glass holder, 127. Brunswick-black, use of, 218. Buccinum, tongue of, 521 ; egg-capsules of, 522 ; development of, 523. Bugs and their allies, 648 ; wings of, 676. Busula avicularia, 498, 499. BuTPs-Eye Condenser, 143 ; use of, 178. Busk, Mr. G., on Volvox, 260, 265. Butterflies, see Lepidoptera. Cabinets for microscopic objects, 243. Cactus, raphides of, 371. Calycajithus, stem of, 385. Camera Lucida, 125, 126 ; use of, in Mi- crometry, 126. CampanularidcB, 463-465. Cgmpylodiscus, 292. Canada balsam, use of, as cement, 208, 209, 219, 220 ; mounting of objects in, 224-232. Canal-system of Foraminifera, 446, 447. Canaliculi of bone, 585-587. Cancer cells, 675. Capillaries, circulation in, 616-619 ; injec- tion of, 619-622; distribution of, 623-626. Carp, scales of, 592. Cartilage, structure of, 608,609. Cells for mounting objects, of cement, 235; of thin glass, 236 ; of plate-glass, 237 ; shallow, 237 ; deep and built up, 238-240 ; mounting objects in, 240-242. Cells, Animal, nature of, 404, 405. Cells, Vegetable, 42 ; nature of, 249 ; rotation in, 363-367 ; thickening deposits in, 367, 368 ; spiral deposits in, 369 ; starch-grains in, 370, 371 ; raphides in, 371 . Cellular Tissue, ordinary form of, 360-362; stellate, 362 ; cubical, 363 ; dimensions of, 363 ; component cells of, 363. Cement-Cells, mode of making, 235, 236. Cementum of Teeth, 589-591. CEPHALOPODS, shell of, 516, 517 ; chromato- phores of, 528. Ceramiacece, 331. Chatophoracea, 321, 322. Chalk, Foraminifera, &c., of, 631-633. Characece, 322, 323; rotation of fluid in, 47, 324; multiplication of, by zoospores, 324; sexual apparatus of, 325, 326. Cherry-stone, cells of, 368. Chemical Reagents, use of, in Microscopic research, 211-213. Chilodon, teeth of, 415 ; self-division of, 418. Chirodota, calcareous skeleton of, 486. Chloride of Calcium, as mounting medium, 233. Choroid, pigment of, 605. Chromatic Aberration, 71,72; means of re ducing and correcting, 73, 74. Chromatophores of Cephalopods, 527. Cidaris, spines of, 481. Ciliary action, nature of, 424, 425 ; on gills of Mollusks, 527. Ciliated epithelium, 606, 607. Circulation of Blood in Vertebrata, 616- 619; in Insects, 569, 570; alternating, in Tunicata, 504, 505. Cirrhipeds, metamorphoses of, 548-550. Clavellinidce, 503. Clarke, Mr. J. L., his mode of preparing sections of Spinal Cord, 616. Cleanliness, importance of, to Microscope, 158-160 ; in mounting objects, 242. Closterium, movement of fluid in, 267,268; duplicative subdivision of, 268,269; mul- tiplication of, by gonidia, 275 ; conjuga- tion of, 274. Clypeaster, spines of, 481. Coal, nature of, 627-629. Coarse-adjustment, 86-88; uses of, 162,163. Cockchafer, cellular integument of, 556 ; eyes of, 562 ; antenna of, 565 ; spiracle of larva of, 572. Cockle of v/heat, 531, 532. Coddington lens, 80. Cohn, Dr., his account of various states of Protococcus, 253-259 ; his researches on Infusoria, 425, note; on reproduction of Rotifera, 430, 431. Coleoptera, mouth of, 565-567. Collection of objects, general directions for, 243-245. Collomia, spiral fibres of, 368-370. Colostrum, 683. Columella of Mosses, 351. Comatula, metamorphosis of, 490. Compound Microscope, principle of, 81-85; various forms of, 96-117. Compressorium, 150-152; use of, 168. Concave lenses, refraction by, 69. Conceptacles of Marchantia, 345, 346. Condenser, Achromatic, 131-133, 136 ; use of, 172, 173. Annular, 139. for Opaque Objects, ordinary, 144; bull's-eye, 144; mode of using, 177-179. Confervacece, 317; self-division of, 318; zoospores of, 318; sexual reproduction of, 318, 319. Coniferce, peculiar woody fibre of, 37?; ab- sence of ducts in, 375,382; fossil, 382, 628, 629. Conjugation of Palmoglffia, 252; of Des- midfaeeffi, 274-276 ; of Diatomaceae, 284- 286 ; of Conjugateae, 319, 320 ; (supposed) of Actinophrys, 408. INDEX. 717 Contractile vesicle of Volvox,260; of Amoe- ba, 405 ; of Actinophrys,407 ; of Infuso- ria, 417, 418. Convex lenses, refraction by, 65-69 ; for- mation of images by, 69. Coquilla nut, cells of, 368. Coral, red, 473 ; stony, 473, 474. Corallines (zoophytic), 38,49 ; true, 331,332. Cordylophora, 461, 462. Cork, 380. Corpuscles of Blood, 598-601. Corti, Abbe, on rotation in Chara, 47. Corynida, 461, 462. Coscinoditcus, 289, 290. Cosmarium, self-division of, 269; conjuga- tion of, 274. Crab, shell-structure of, 550, 551 ; meta- morphoses of, 551, 552. Cricket, gastric teeth of, 569 ; sounds pro- duced by, 575. Crinoidea, skeleton of, 482. Crusta petrosa of Teeth, 589-591. CRUSTACEA, 539 ; lower forms of, 539-541; entomostracous, 541-546; suctorial, 547, 548; cirrhiped, 548-550; decapod, shell of, 550, 551; metamorphoses of, 551, 552. CRYPTOGAMIA, microscopic study of, 43-46 ; movements of, 44 ; sexuality of, 45, 46, 359. (See Protophyta, Alga, Mosses, &c.) Crystals of snow, 644. Crystallization, Microscopic, 645. Ctenoid scales of Fish, 594. Curculionidcs, 556-559. Cutaneous eruptions, 668. Cuticle of Equisetacese, 358; of Flowering Plants, 388-391. Cuttle-fish, shell of, 516, 517. Cycloid scales of Fish, 592. Cyclops, 542, 543 ; fertility of, 546. Cydippe, 470, 471. Cypris, 541, 542. Cyprcea, structure of shell of, 515. Cystic Entozoa, 530. Cysticercus, 530. Cythere, 542. Dalrymple, Mr., on Notommata, 429. Dalyell, Sir J. G., on development of Me- dusae, 468-470. Daphnia, 544 ; ephippial eggs of, 546, 547. Darwin, Mr., on Pampean formation, 639, 640. Deane's Gelatine, 232-234. . Decapod Crustacea, shell of, 550, 551 ; me- tamorphoses of, 551, 552. Defining power of object-glasses, 187, 188. Demodexfolliculorum, 582. Dentine of Teeth, 588, 589. Dendrodus, teeth of, 641. Dermestes, hair of, 195, 559. Desmidiacece, general structure of, 265-267 ; movement of fluid in, 267, 268 ; duplica- tive subdivision of, 268-270; formation of gonidia by, 270-273 ; organization and multiplication of varieties in, 273,274; conjugation of, 274-276; collection of, 276, 277. Deutzia, stellate hairs of, 390, 391. Development, Von Baer's law of, 52. Animal, application of Mi- croscope to, 50-57. Development, Vegetable, application of Mi- croscope to, 42-47, Diamond-beetle, 556-560 ; foot of, 576. Diaphragm-Plate, 130, 131. Diatoma, 298. Diatomacea, vegetable nature of, 277; co- hesion of frustules of, 278, 279 ; siliceous envelope of, 277-280 ; markings of, 280- 283 ; self-division of, 283, 284 ; gonidia of, 284; conjugation of, 284-286; limits of species of, 286, 287, 301 ; movements of, 287, 288 ; classification of, 288-301 ; general habits of, 301-303 ; fossilized de- posits of, 303, 304, 629-633 ; collection of, 304-306 ; mounting of, 306, 307 ; their value as tests, 195-199. Dicotyledonous Stems, structure of, 378- 386. Dictyochalix, 453. Didymoprium, self-division of, 269 ; conju- gation of, 274, 275. Diffraction of Light, errors arising from, 181, 182. Dipping-Tubes, 152. Dissecting Microscope, Quekett's, 94-96. Smith and Beck's, 103-105. Dissection, microscopic, 201-203. Distoma, 532, 533. Dog, foot of, epidermis of, 606. Doris, spicular skeleton of, 516. Dotted Ducts, 374. Doublet, microscopic, 79. Draw-Tube, 118, 119. Dropsical fluids, examination of, 699. Dry-mounting of objects, 221-225. Ducts of Plants, 374,375. Dujardin, M., on Rotifera, 432-435. Duplicative subdivision, of Palmoglaea, 252; of Protococcus, 255 ; of Desmidiaceas, 268-270; of Diatomaceae, 283, 284; of Confervaceae, 318,319; of Protozoa, 407- 409 ; of Infusoria, 418. Dusideia, skeleton of, 455. Dytiscus, foot of, 577, 578. Eagle Ray, teeth of, 589. Echinida, shell of, 477, 478; ambulacral disks of, 479 ; spines of, 479, 480 ; pedi- cellariae of, 481, 482 ; teeth of, 482 ; me- tamorphosis of, 488-491. ECHINODERMATA, skeleton of, 477-486; metamorphoses of, 486-491. Educational Microscopes, 93-97, 104, note. Educational Value of Microscope, 57-64. Eel, scales of, 591, 592. Eels, of Paste and Vinegar, 531. Eggs of Insects, 579,580. Egg-shell, fibrous structure of, 602. Ehrenberg, Prof., his researches on Ani- malcules, 47-49, 411-424, 428-435; on Polycystina, 449-452. Elaters of Marchantia, 349. Elementary Tissues : see Tissues. Elytra, of Beetles, 575. Embryo, vegetable, development of, 400. Enamel of Teeth, 589-591. Encrinites, 482. Encysting process of Infusoria, 419-422. Endochrome of vegetable cell, 249. 718 INDEX. Enterobryus, 338-340. Entomoslracous Crustacea, 541 ; their clas- sification, 541-545; reproduction of, 545- 547. Entozoa, 529; simplest form of, 530, 531 ; cystic, 530; nematoid, 531,532; trema- tode, 532, 533. Ephemera, larva of, 555-569, 573. Epidermis of Animals, structure of, 604, 605. of Plants, nature of, 391. Epithelium, 606, 607; ciliated, 607, 608. Equisetacece, 358 ; cuticle of, 358 ; spores of, 359. Erector, 119, 120. Ether injection, 703. Euryale, skeleton of, 482. Eye, examination of, 671. Eyes, care of, 158. Eyes, of Mollusks, 526, 527; of Insects, 561-564. Eye-piece, 82; Huyghenian, 83, 84 ; Rams- den's, 84, 85; Achromatic, 85; Menis- cus, 85; Micrometric, 122-124, 713. Fallacies of Microscopists, 39-41, 180-186. Falconer, Dr., on bones of fossil Tortoise, 642. Fat-cells, 607, 608 ; capillaries of, 623, 624. Fatty degeneration, 665. Faujasina, 439 ; canal-system of, 446, 447. Feathers, structure of, 597, 598. Feet of Insects, 576, 578. FERNS, 352, 353 ; fructification of, 353, 354 ; spores of, 354, 355 ; prothallium of, 355 ; antheridia of, 355, 356; archegonia of, 356, 357; generation and development of, 357, 358. Fertilization of ovule, in flowering plants, 399-401. Fibre-cells of anthers, 396, 397. Fibres, Muscular, 611-614. Nervous, 614-616. Fibrillae of Muscle, structure of, 612, 613. Fibrous tissues, 602-604. Field's Compound Microscope, 97-99. Simple Microscope, 93, 94. Filiferous capsules of Zoophytes, 474-476. Finder, 129, 130. Fine Adjustment, 86, 87; uses of, 163, Io4« FISHES, bone of, 587; teeth of, 588 ; scales of, 591-593; blood of, 598-600; circula- tion in, 619; gills of, 624. Fishing-Tubes, 132. Flatness of field of object-glasses, 189. Flint, organic structure in, 633 ; examina- tion of, 634. fbruietf, 331. Floscularians,432, 433. Flowers, small, as microscopic objects, 394, 395. Fluid, mounting objects in, 234-242. Fluke, 532, 533. Flustra, 49, 492-497. Fly, number of objects furnished by, 553 ; tongue of, 566, 567; spiracle of, 572; wing of, 574 ; foot of, 576, 577. Focal Adjustment, 86, 87; precautions in making, 162-165 ; errors arising from im- perfection of, 183, 184. Foraminifera, 48, 436; their relation to Rhizopoda, 437; their general structure, 437-440; peculiar forms of, 440-445 ; re- lation of, 10 Sponges, 446; canal-system of, 446, 447 ; collection and mounting of, 446-449 ; fossil deposits of, 629-634. Forceps, 153 ; stage, 146. Fossil Bone, 642. Diatomace®, 303, 304, 629-633. Fossil Foraminifera, 629-639. ' Polycystma, 451,452. Sponges, 632. Teeth, 641. Wood. 388, 627-629. Fowl, lung of, 625. Frog, blood of, 599, 600 ; circulation in web of, 616-618 ; in tongue of, 618, 619 ; lung of, 624. Fucacece, 328, 329; sexual apparatus of, 328-331 ; development of, 331. Fungi, simplest forms of, 334, 335; .in bodies of living animals, 335-340; in sub- stance, or on surface of plants, 340-343 ; higher forms of, 343, 344. Fusulina, 634. Furze, Mr., his arrangement for illumina lion, 143. Gad-flies, ovipositor of, 579. Gaillonella, 299. Gairdner's Simple Microscope, 92, 93. Gall-flies, ovipositor of, 579. Ganoid scales of Fish, 593. GASTEROPODA, structure of shell of, 515, 516 ; tongues of, 517-521 ; development of, 521-525 ; organs of sense of, 526-528. Gastric teeth of Mollusks, 521 ; of Insects, 569. Gelatine, Deane's. 232-234. Gelatinous nerve-fibres, 614, 615. Geology, applications of Microscope to, 627. Geranium-pe\a\, peculiar cells of, 395. Gills, of Mollusks, ciliary motion on, 526 ; of Fishes, distribution of vessels in, 624; of Water-newt, circulation in, 629 ; ciliary movement in, 624. Gillett's Condenser, 132. Glands, structure of, 609-611. Glass Slides, 214. Thin, 214-217. Glue, liquid, use of, 218. marine, use of, 219-221. Glycerine, use of, in mounting objects, 232. Gnats, Iarva3 of, 569, 570. Goadby's Solution, 233. Gold-size, use of, 218. Goniometer, 124. Gomphonema, 294, 295. Gonidia, multiplication by, in Desmidiacece, 270-272 ; in Diatomaceee, 284 ; in Hydro- dictyon, 300 ; in Chara, 325 ; in Florideae, 331 ; in Lichens, 333. Gorgonia, spicules of, 474. Gosse, Mr., on Melicerta, 433; on thread- cells of Zoophytes, 474-476. Grammatophora, 298; its use as test, 199. Grantia, structure of, 309-317. Grasses, silicified cuticle of, 390, 391. Gregarina, 530, 531. Grinding and Polishing of Sections, 207- 211. INDEX. 719 Gromia, 437, 438. Growing-Slide, 147. Grubb's Prism for oblique illumination, 137. Guano, Diatomaceae of, 304-306. Hairs, of Mammals, 183; structure of, 593- 596; mode of mounting, 597; of Insects, 559-561. Hairs, of vegetable cuticles, 390 ; rotation of fluid in, 365, 366. Halichondria, spicules of, 454. Harvest-bug, 582. Haversian Canals of bone, 585. Heliopelta, 290, 291. Haustellate Mouth, 567, 568. Henfrey, Mr., on development of pollen- grains, 396. HepaticcB, 344 ; see Marchantia. Herapathite, 140. Hepworth, Mr., on feet of Insects, 577. Highley's Hospital Microscope, 99, 100. Jlippocrepian Polyzoa, 497-500. Histology, 54. Hcematococcus , its relations to Protococcus, 254. sanguineus, 308. Hogg, Mr., on development of Lymnaeus, 523. Holland, Mr., his triplet, 80. Holcthurida, skeletons of, 485, 486. Hoofs, structure of, 598. Hooker, Dr. Jas.,on Antarctic Diatomaceae, 301, 302. Horns, structure of, 598. Huxley, Mr., on Rotifera, 430, 431,435; on Noctiluca, 472; on blood of Annelida, 536. Huyghenian eye-piece, 82-84. Hydatina, 434 ; reproduction of, 431. Hydra, discovery of, 37; structure of, 457- 461. Hydrodictyon, 316, 317. Hydrozoa, 461-470. Hymenoptera, proboscis of, 566, 567 ; wings of, 574 ; stings and ovipositors of, 578- 579. Jackson, Mr., his eye-piece micrometer, 122, 123. Jelly-fish, development of, 467-470. Ice-plant, cuticle of, 390. Ichneumonidce, ovipositor of, 578. Illumination of opaque objects, 176-180; of transparent objects, 172-175. Illuminators, Black-ground, 138-140, 174, 175. Oblique, 135-138, 173, 174. White Cloud, 134, 173. Images, formation of, by convex lenses, 69. Indicator, 124. Indusium of Ferns, 352-354. Inflection of Light, errors arising from, 181, 182. Infundibulate Polyzoa, 497-500. Infusorial Earths, 303-306, 629. INFUSORIA, 48, 411, 412; structure of, 413- 417; movements of, 415, 416; repro- duction of, 418-423; peculiar forms of, 423, 424. Injections of bloodvessels, mode of making, 619-622 ; mode of mounting, 622, 623, 700. Inorganic substances as objects, 644. INSECTS, great number of objects furnished by, 553, 554; microscopic forms of, 554, 555; antennae of, 564, 565; circulation of blood in, 569, 570; eggs of, 579-581 ; eyes of, 561-564; feet of, 576-578 ; gastric teeth of, 569 ; hairs of, 559-561 ; integu- ment of, 555, 556 ; mouth of, 565-568 ; ovipositors of, 578, 579 ; scales of, 556- 561 ; spiracles of, 572-574 ; stings of, 578, 579 ; tracheae of, 570-574 ; wings of, 574-576. Iris, structure of leaf of, 392-394. Islhmia, 298'; markings on, 280, 281 ; self- division of, 283. Itch-Acarus, 587. Integument of Insects, structure of, 555, 556. Kidneys, structure of, 610; morbid, 664. Labelling of objects, 242, 243. Labyrinthodon, tooth of, 641. Lacunae of bone, 585-588. Laguncula, 492-495, 500. Lamps, microscope, 154-157. Lealand, Mr., his preparations of muscular fibre, 195, 614. Leaves, structure of, 392-394; mode of examining, 394. Leech, 538; teeth of, 538. Legg, Mr., on collection of Foraminifera, 447-449. Lepidoptera, scales of, 556-561 ; proboscis of, 567, 568; wings of, 574, 575; eggs of, 564. LepidosteuK, bony scales of, 586, 587, 593. Lepisma, scales of, 559. Lepralia, 492-497. Lever of Contact, 216. Levant Mud, microscopic organisms of, 630-633. Lever-Stage, 128, 707. Lichens, 333, 334. Licmophora, 295, 296. Lieberkiihn, speculum of, 146; mode of using, 179, 180. Light, suitable for Microscope, 154-157 ; position of, 157 ; arrangement of, for transparent objects, 169-176 ; for opaque objects, 177-180. Liquid Glue, use of, 218. Lister, Mr., his improvements in Achro- matic lenses, 75, 76. Liver, structure of, 609; morbid, 663. Lungs of Reptiles, 624 ; of Birds, 624, 625; of Mammals, 626 ; morbid, 658. Lymnceus, development of, 521, 522. Magnetic Stage, 130. Magnifying power, mode of determining, 199,200; augmentation of, 161; of dif- ferent objectives, 192-196. Mahogany, section of, 383. MAMMALS, bone of, 587; teeth of, 589-591 ; hairs, &c., of, 593-598; blood of, 598- 602; lungs of, 625. 720 Man, hair of, 595, 596. Mandibulate mouth of Insects, 565-567. Marchantia, general structure, of, 344, 345; stomata of, 345, 346 ; conceptacles of, 346 ; sexual apparatus of, 347, 348. MargaritacecB, shells of. 506-508, 509. Marine Glue, uses of, 219-221. Mastogloia, 299-301. Medullary Rays, 381-384. Medusa, development of, from Zoophytes, 52, 53, 467-470. Megatherium, teeth of, 590. Melicertians, 432-434. Meloseira, 299 ; self-division of, 284 ; con- jugation of, 286. Menelaus, scale of, 557. Meniscus eye-piece, 85. Meridian, 296, 297. Metamorphoses of Animals, 50 ; of Cirrhi- peds, 50, 548-550; of Crustacea, 52,551, 552 ; of Echinoderms, 52, 486-491 ; of Infusoria, 419-422 ; of Mollusks, 51, 504, 505 ; of Annelids, 52, 536, 537. Micrometer, Cobweb, 120-122; eye-piece, 122-124. Micrometry, by Micrometer, 120-124; by Camera Lucida, 126. MICROSCOPE, early history of, 35-41 ; later history of, 41-57; educa- tional uses of, 57-64. optical principles of construc- tion of, 77-85 ; mechanical principles of construction of, 86-90. Compound, 81-85, 96, 97. Field's, 97-99. Highley's, 99, 100. Nachet's, 100-102. Ditto, binocular, 114-117. Powell and Lealand's, 110- 112. Ross's, 108-110. Smith and Beck's large, 112- 114. Ditto dissecting, 103-105. Ditto student's, 102, 103. Ditto educational, 104, note. Warington's universal, 105- 108. Simple, 77-81, 90. Field's, 93, 94. Gairdner's, 92, 93. Grunow's, 706. Queen's, 706. Quekett's, 94-96. Ross's, 90-92. Spencer's, 704. support required for, 154 ; care of, 158-160; general arrangement of, 160; for transparent objects, 168- 172 ; for opaque objects, 175-177. Microscopic Dissections, 201-203. Microscopists, fallacies of, 39-41. Microtome, 203, 204. Mildew, fungous vegetation of, 340, 341 Milk, 683. Millon's test for albuminous substances, 213. Mineral Objects, 645. Minnow, circulation in, 619. Molecular movement, 185, 186. MOLLUSCA, shells of, 506-517 ; tongues of 517-521 ; development of, 521-525 ; ciliary motion on gills of, 526 ; organs of sense of, 527, 528. Monocotyledonous Stems, structure of, 376-378. Morbid growths, examination of, 674. Morpho Menelaus, scale of, 194, 557. MOSSES, structure of, 348, 349 ; sexual ap- paratus of, 347-350; urns of, 350, 351 ; peristome of, 351 : development of spores of, 352. Moths ; see Lepidoptera. Mould, fungous vegetation of, 340, 341. Mounting of objects ; see Objects. Mounting-Plate. 21*9. Mouse, Ear of, cartilage of, 608. Mouth of Insects, 565-568. Mucous Membranes, structure of, 604 ; capillaries of, 623 ; ulcers of. 670. Miiller, O. F., his researches on Animal- cules, 38. Miiller, Prof., his researches on Echino- derm-larvae, 486-491. Muscardine, or silk-worm disease, 44, 45, 336. Muscular fibre, structure of, 611-613 ; value of, as test-object, 195 ; mode of examining and preparing, 613, 614, 655; capillaries of, 623, 624. Musk-deer, hair of, 595. Mussel, ciliary action on gills of, 526. Mya, structure of hinge-tooth of, 509. Mycelium of Fungi, 340-343. Myliobates, teeth of, 589. Nachet's Microscope, 100-102 ; his Bino- cular Microscope, 114-117 ; his prism for oblique illumination, 136, 137. Nacre, structure of, 509-511. Nais, 537, 538. Nassula, teeth of, 415. Naviculce, 293, 294 ; movements of, 287. Needles for dissection, mode of mounting, 203. Nematoid Entozoa, 531, 532. Nepenthes, spiral vessels of, 374. Nervous Tissue, structure of, 614, 615, 653; mode of examining, 615, 616, 653. Newt, circulation in larva of, 619. Nicol-Prism, 140. Nobert's Test, 196, 197. Noctiluca, 471, 472. NostochacecB, 312, 313. Notommala, 429-435. Nucleus of Vegetable cells, 251, 365'; of Infusoria, 418. Nudibranchs, development of, 521 , 522. Nummulite, structure of, 446, 634-637. Nuphar lutea, parenchyma of, 362. Object-Finder, 129, 130. Object-glasses, achromatic, principle of, 73, 74; construction of, 74-76; adjustment of, for covering of object, 76, 77, 165- 168; defining power of, 187, 188; pene- trating power of, 188 ; resolving power of, 188, 189; flatness of field of, 189; comparative value of, 18S-192 ; different powers of, 193-198; tests for, 193-199; INDEX. 721 determination of magnifying power of, 199, 200. Object-glass holder, Mr. Brooke's, 127. Object-Marker, 127, 128. Objects, mode of mounting, dry, 221-224 ; in Canada balsam, 224-232; in preserva- tive fluids, 232-242; see Opaque and Transparent Objects. Oblique Illuminators, 135-138, 173, 174. Ocelli of compound eyes of Insects, 561. (Edogonium, zoospore of, 318 ; sexual re- production of, 319. Oil-globules, microscopic appearances of, 184. Oleander, cuticle of, 389. Oncidium, spiral cells of, 368. Onion, raphides of, 371. Oolite, structure of, 634. Opaque Objects, arrangement of Micro- scope for, 175-177 ; various modes of illu- minating, 177-180 ; modes of mounting, 121-124. Opercula of Mosses, 350. Ophiocoma, 482. Opkiurida, skeleton of, 482. 483. Ophrydina, 423. Orbitoides, structure of, 637-639. Orbitolite, structure and development of, 440-445. Ornithorhyncus, hair of, 595. Oscillator iacece, 311-313. Ovipositors, 578, 579. Ovules of Phanerogamia, 399; fertilization of, 48, 399, 400 ; mode of studying, 400, 401. Owen, Prof., on Fossil Teeth, 641, 642 ; on Fossil Bone, 642, 643. Oxytricha-form of Trichoda, 421, 422. Pigment-cells, 605. Pith, structure of, 378, 379. Pinna, structure of shell of, 506-508. Pinnularia, 294. Placoid scales of Fish, 593. Planaria, 533-535. Plants, distinction of from Animals, 247, 248,409,410. Plantago, rotation in hairs of, 366. Pleurosigma, 294 ; nature of markings on, 281, 282 ; value of, as tests, 195-198. Pluteus-larva of Echinus, 488-490. Pneumonia, pathological condition in, 661. Podura, scale of, 195, 556, 557. Polarization, objects suitable for, 645. Polarizing Apparatus, 140-143. Pollen-grains, development of, 397; struc- ture and markings of, 194, 397-399. Polycystina, nature of, 449, 450 ; distribu- tion of, 450-452. Polygastrica ; see Infusoria. Polyommatus argus, scale of, 557, 558. Polypes ; see Hydra and Zoophytes. Polypodium, fructification of, 353. POLYZOA, 49, 492 ; general structure of, 492- 496 ; molluscan nature of, 496 ; classifi- cation of, 497-500. Potato-disease, 342. Powell and Lealand's Microscope, 110-112 ; their Achromatic Condenser, 133. Preservative Liquids, 232-234. Primordial utricle, 248. Prismatic shell-substance, Prisms, Dujardin's, 133 ce, 506-508. ,131,173; Nachet's, Palm, stem of, 377, 378. Palmellacea, 307-309. Palmoglcea macrococca, life-history of, 251- 253. Pampean deposit, organic composition of, 639. Papilla? of skin, structure of, 605-615 ; of tongue, 615. Parabolic Illuminator, 139, 140. Paramecium, 413, 414 ; contractile vesicle of, 417; multiplication of, 418. Parasitic fungi, 335-443. Pearls, structure of, 509-511. Pecari, hair of, 595. Pecten, eyes of, 527 ; tentacles of, 528. Pediastrum, multiplication and develop- ment of, 270-273. Pedicellarice of Echinoderms, 481, 482. Penetrating power of object-glasses, 188. Pentacrinus, skeleton of, 482 ; larval, 491. Peristome of Mosses, 350, 351. Perophora, 502-505. Petals of Flowers, structure of, 395, 396. Pettenkofer's test, 212. PHANEROGAMIA , elementary tissues of, 360- 376 (see Tissues of Plants); stems and roots of, 376-388 ; cuticle and leaves of, 388-394 ; flowers of, 394-401 ; seeds of, 401-403. Phytelephas, cells of, 368. Phyllopoda, 544. 136; Amici's, 137; Grubb's, 137; pola- rizing, 140, 141. Proboscis of Bee, 566, 567; of Butterfly, 567, 568 ; of Fly, 566, 567. Proteus, 405. Protococcus pluvialis, life-history of, 253- 257 ; various forms of, 257, 258 ; con- ditions influencing changes of, 258, 259. Protoplasm of vegetable cell, 248. PROTOPHYTA, general characters of, 247- 249. PROTOZOA, 404-409 ; their relations to Pro- tophyta, 247, 248, 409, 410. Pseudopodia, of Rhizopods, 407-411. Pterodactyle, bone of, 642. Puccinia, 341. Purpura, development of, 521-525. Pycnogonidce, 539-541. Quekett, Mr., his Dissecting Microscope, 94-96 ; his Indicator, 124 ; on Structure of Bone, 642, 643. Querquedula crecca, 655. Rainey, Mr., his moderator, 156. Ramsden's eye-piece, 84, 85. Raphides, vegetable, 371, 372. Reade, Rev. J. B., on black-ground illumi- nation, 138. Reagents, Chemical, use of, in Microscopic research, 211-213. Red Corpuscles of Blood, 599, 600. Red Snow, 307. Refraction, laws of, 65, 66 ; by convex lenses, 65-69 ; by concave and meniscus lenses, 69. 46 722 INDEX. Reindeer, hair of, 595. REPTILES, bone of, 587 ; teeth of, 591 ; scales of, 593 ; blood of, 598-600 ; lungs of, 624. Resolving power of object-glasses, 187-189. Rete Mucosum, 604. Reticulated Ducts, 374. Rhinoceros, horn of, 598. RHIZOPODA, 47, 410,411. Rhubarb, raphides of, 371. Rhynconellidce, structure of shell of, 513-515. Rice-Paper, 362. Rocheafalcata, 390. Rocks, structure of, 639. Roots, structure of, 386 ; mode of making sections of, 386-388. Rosalina, 439. Ross, Mr., on adjustment of object-glass, 76, 77. his Compound Microscope, 108- 110; his Achromatic Con- denser, 131-133; his Simple Microscope, 90-92 ; his lever of contact, 216 ; his side re- flector, 145 ; his eye-piece Micrometer, 122. Rotation of fluid in Chara, 324 ; in cells of Phanerogamia, 363-367. Rotifer, anatomy of, 428-430 : reproduction of, 431 ; desiccation of, 432; occurrence of, in leaf-cells of Sphagnum, 426. ROTIFERA, 57, 412 ; general structure of, 426-430; reproduction of, 430-432; desic- cation of, 432 ^classification of, 432-434. Rush, stellate parenchyma of, 362. Rust, of Corn, 341. Sable, hair of, 595. Saliva, 684. Sarcode, 405-409. Sarcolemma, 655. Saw-Jlies, ovipositor of, 578. Scalariform due's of Ferns, 352, 353, 374. Scales, of cuticle of Plants, 390. of Fish, 591-593. of Insects, 556-561 ; their use as test-objects, 194. of Reptiles and Mammals, 594. Schacht on fertilization of ovule, 399-401. Schleiden, Prof., his researches, 42; on fertilization of ovule, 399-401. Schultz, Prof., his experiments on develop- ment of Infusoria, 422, 423. Schultz's solution, 212. Schwann, doctrines of, 56. Scissors for microscopic dissection, 201-203 ; for cutting thin sections. 204. Sclerogen, deposit of, on walls of cells, 367. Sea- Anemone, 474-476. Sealing-wax varnish, 219. Section-Instrument, 205-207. Sections, thin, mode of making, of soft sub- stances, 204, 205 ; of substances of me- dium hardness, 205-207; of hard sub- stances, 207-211 ; of Wood, 386-388; of Echinus-spines, 483, 484 ; of Bones and Teeth, 587; of Hairs, 597. Seeds, microscopic characters of, 401-403. Segmentation of Yolk-mass, 521. SeFenite-Plate, 142. Self-division of cells; see Duplicative Sub- division. Serous membranes, structure of, 604. Sertularidce, 463-465. Shadbolt, Mr., on Arachnoidiscus, 290. his Annular condenser, 139 ; his Turn-table, 236. Shark, teeth of, 588, 589 ; scales, &c., of, 593. Shell, of Crustacea, 550, 551 ; of Echinida, 477. 478 ; of Mollusca, 506-517. Side- Reflector, Ross's, 145. Siliceous Cuticles, 358, 391. Silk-worm disease, 336. Simple Microscope, principle of, 77-81 ; various forms of, 90-96. Siphonacece, 313-317. Skin, structure of, 604,605 ; papillae of, 605- 615. Slider-forceps, 226. Slides, glass, 214. wooden, 221, 222. Slug, rudimentary shell of, 516. Smith's, Prof. J. L., inverted microscope, 710; goniometer and micrometer, 712. Smith, Prof. W., on Diatomaceae, 301, 302. Smith and Beck's Dissecting Microscope, 103-105; their Large Compound Micro- scope, 112-114 ; their Achromatic Con- denser, 133; their Student's Microscope, 102, 103; their Educational Microscopes, 89. Snake, lung of, 624. Snow-crystals, 644. Sole, skin and scales of, 591, 592. Sollitt's Achromatic Condenser, 136. Sorby, Mr., his microscopic examination of rocks, 640. Spatangus, spines of, 481. Speculum-disk for drawing, 125. Spencer, Mr., his object-glass, 51, note; his microscopes, 704. Spermatia of Lichens, 334 ; of Fungi, 344. Spermogonia of Lichens, 333 ; of Fungi, 344. Sphacelaria, 328. Sphagnum, peculiar cells of, 348, 349 ; oc- currence of Rotifer in leaf-cells of, 426. Sphceria, development of, within animals, 338. Sphceroplea, sexual reproduction of, 318. Sphcerosira volvox, 264. Spherical Aberration, 70; means of re- ducing and correcting, 71, 72. Spicules of Sponges, 453-455 ; of Alcyonian Zoophytes, 472-474 ; of Tunicated Mol- lusks, 500 ; of Doris, 516. Spiders, eyes of, 582 ; respiratory organs of, 583 ; feet of, 583 ; spinning apparatus of, 583. Spinal Cord, mode of preparing sections of, 616. Spines of Echinus, &c., 479, 480; mode of making sections of, 482-484. Spinning-apparatus of Spiders, 583. Spiracles of Insects, 571-573. Spiral Vessels, 373, 374. SPONGES, their structure, 452, 453 ; ciliary action in, 453 ; skeleton of, 453-455 ; re- production of, 455 ; examination of, 455 ; fossil, 633. INDEX. 723 Sporangia, of Desmidiaceae, 274, 275 ; of Diatomacerc, 284-286 ; of Fuci, 330. Spores of Hepaiicae, 347; of Mosses, 351, 352; of Ferns, 355; of Equisetaceae, 358. Spotted Lens, 138. Spring-press, 227. Spirogyra, 241. Sputum, 685. Squirrel, hair of, 594, 595. Stage, lever, 128, 129; magnetic, 130. movements of, 109, 110. Stage-Forceps, 146. Stage-Plate, glass, 147. Stanhope lens, 80, 81. Star-Anise, cells of seed-coat of, 367, 368. Starch Granules in cells, 370, 371; appear- ance of, by polarized light, 370, 371. Staurastrum, prominences of, 265, 266 ; self-division of, 269 ; varieties of, 273. Stein, Dr., his researches on Infusoria, 419, 420. Stellate cells of Rush, 362; of water-lily, 362 Stemmata of Insects, 563. Stems, Endogenous, structure of, 376-378. Exogenous, structure and develop- ment of, 378-386. Mode of making sections of, 386- 388. Stentor, 423. Stephanoceros, Eichornii, 433. Stickleback, circulation in, 619. Stigmata of Insects, 571-573. Stings, structure of, 578, 579. Stomata of Marchantia, 345, 346 ; of Flow- ering Plants, 391, 392. Suctorial Crustacea, 547, 548. Suminski, Count, on development of Ferns, 46. Surirella, 293; conjugation in, 284. Swarming, of Desmidiacea?, 270. Synapta, calcareous skeleton of, 485, 486. Tadpole, pigment-cells of, 604, 605 ; mode ol viewing circulation in, 619. Tania, 529,530 Tardigrada, 435 ; desiccation of, 432. Teeth, of Echinida, 482 ; of Mollusks, 517- 521; of Leech. 538; of Vertebrata, struc- ture of, 588-591; fossil, 641, 642; mode of making sections of, 207-211, 587. Terebdla, circulation and respiration in, 535, 536. Terebratula, structure of shell of, 513-515. Test-Bottles, 212. Test-Liquids, 212,213. Test- Objects, 192-199. Tetraspores of Florideae, 331, 332. Thalassicolla, 445. Thaumantias, 467. Thecae of Fungi, 334, 335 ; of Ferns, 352, 353 Thin Glass, 214-216. Thompson, Mr. J. V., on Polyzoa, 492; on metamorphosis of Cirrhipeds, 548-550; on metamorphoses of Crustacea, 551, 552. Thread-cells of Zoophytes, 474-476. Thread, glutinous, of spider's web, 582, 583. Thrush, fungous vegetation of, 340. Thwaites's fluid for Algre, 232. Thymus and thyroid gland, 664. Ticks, 581. Tinea favosa, fungous vegetation of, 340. Tissues, Elementary, of Animals, micro- scopic study of, 53-55, 584 ; independent life of, 56, 57; see Blood, Bone, Capilla- ries, Cartilage, Epidermis, Epithelium, Fat, Feathers, Fibrous Tissues, Glands, Hair, Horn, Mucous Membranes, Muscle, Nervous Tissues, Pigment-cells, Scales, Serous Membranes, Teeth. Tissues, Elementary, of Plants, 360 ; eel- lular, 360-363 ; varieties of, 367-371 ; woody, 372 ; fibro-vascular, 372 ; vascu- lar, 373, 374 ; vasiform, 374, 375 ; dis- section of, 375, 376. Tongues of Gasteropods, 517-520; of In- sects, 565-567. Torula cerevisice, 334, 335. Tracheae of Insects, 570-572 ; mode of pre- paring, 573, 574. Tradescantia, rotation in hairs of, 365, 366. Transparent Objects, arrangement of Mi- croscope for, 168-172; various modes of illuminating, 172-175. Trematode Enlozoa, 532, 533. Trembley, his researches on the Hydra, 37. Triceralium, 292; markings on, 281. Trychoda lynceus, bristles of, 415 ; meta- morphoses of, 420-422. Trilobite, eye of, 640. Triplet, Microscopic, 80. Trout, circulation in young, 619. Tubercle, microscopic appearance of, 658. TubularidcB, 462, 463, 465. Tubules, nervous, 614, 615. TUNICATA, Compound, general organiza- tion of, 500-502; different types of, 502- 504 ; alternating circulation in, 504, 505 ; development of, 505. Turbellaria, 533-535. Turn-table, Shadbolt's, 236. Vlvacece, 309-311. Unionidce, shells of, 508-512. Uredo, 342. Urinary deposits, 688. Urns of Mosses, 350, 351. Vacuoles, microscopic appearances of, 185. Valentin's knife, 205. Vallisneria spiralis, rotation in, 363, 364. Varnishes useful to Microscopists, 216-219. Vasiform Tissue, 374, 375. Vaucher, his researches on Confervas, 38. Vaucheria, 313; zoospores of, 314, 315; sexual reproduction of, 315, 316. Vegetable Organization, general nature of, 248; see Plants. Vegetable Cell, nature of, 248, 249. Vegetable Ivory, 368. VERTEBRATA, elementary structure of, 585- 616 (see Tissues); blood of, 598-601; circulation in, 616-619; development of, 626. Vesicles of Nervous tissue, 614, 615. Vibracula of Polyzoa, 498, 499. Villi of intestine, injections of, 622. Vine disease, 342. Volvox, structure of, 259, 260; develop- ment of, 261-265. 724 INDEX. Vomited matters, 698. Vomit, black, 698. Von Baer's law of development, 52. Vorticella, 416-423; encysting process in, 419, 420. Warington's Universal Microscope, 105- 108. Water-vascular system, of Rotifera, 429, 430 ; of Entozoa, 530. Wenham, Mr., his Parabolic Speculum, 139. his observations on rotation, 364-367. Whalebone, structure of, 598. Wheel- Animalcules ; see Rotifera. White-Cloud Illuminator, 134, 173. White Corpuscles of blood, 598-601. White Fibrous tissue, 603. Williamson, Prof., on Volvox, 261-265. Wings of Insects, 574-576. Winter-eggs of Rotifera, 431 ; of Hydra, 61 ; of Entomostraca, 546. Wood, of Exogenous stems, 378-384. Woody Fibre, 372 ; glandular, of 'Conifer*, 373. Xanthidia of Flints, 633. Yeast, a vegetable substance, 45 ; produc- tion of, 45, 334, 335. Yellow fibrous tissue, 603, 604. Yellow Water-lily, parenchyma of, 362. Yucca, stomata of, 391, 392. Zoophyte-Trough, 149, 150. ZOOPHYTES, 457; Hydraform, 461-465; preparation of, for microscope, 465-467; development of Medusae from, 52, 53. 467-470 ; Alcyonian, 472-474 ; Actini- form, 474-476. Zoospores, formation of, by Ulvaceae, 310, 311 ; by Vaucheria, 314, 315 ; by Achlya, 315,316; by Confervaceffi, 318, 319 ; "by Cha3tophoracea3, 321 ; by Fucaceae, 330, 331. THE END. O. LEA/S (LATE LEA & BLANCHARD'S) OF MEDICAL AND SUEGICAL PUBLICATIONS. In asking the attention of the profession to the works contained in the following pages, the publisher would state that no pains are spared to secure a continuance of the confidence earned for the publications of the house by their careful selection and accuracy and finish of execution. It will be observed that the prices during the last four years have not been advanced in anything like proportion to the increased cost of manufacture, and there is no pro- bability of a decrease of cost that will warrant a reduction during the coming season. The printed prices are those at which books can generally be supplied by booksellers throughout the United States, who can readily procure for their customers any works not kept in stock. 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LEA'S PUBLICATIONS — (Dictionaries). fiUNGLISON (ROBLEY}, M.D., Professor of Institutes of Medicine in Jefferson Medical College, Philadelphia. MEDICAL LEXICON; A DICTIONARY OF MEDICAL SCIENCE: Con- taining a concise explanation of the various Subjects and Terms of Anatomy, Physiology, Pathology, Hygiene, Therapeutics, Pharmacology, Pharmacy, Surgery, Obstetrics, Medical Jurisprudence, and Dentistry. Notices of Climate and of Mineral Waters ; Formulae for Officinal, Empirical, and Dietetic Preparations; with the Accentuation and Etymology of the Terms, and .the French and other Synonymes ; so as to constitute a French as well as English Medical Lexicon. Thoroughly Revised, and very greatly Modified and Augmented. In one very large and handsome royal octavo volume of 1048 double-columned pages, in small type; strongly done up in extra cloth, $6 00; leather, raised bands, $6 75. 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By enlarging the size of the volume to a royal octavo, and by the employment of a small but clear type, on extra fine paper, the additions have been incorporated without materially increasing the bulk of the volume, and the matter of two or three ordinary octavos has been compressed into the space of one not unhandy for consultation and reference. It is undoubtedly the most complete and useful It would be a work of supererogation to bestow a word of praise upon this Lexicon. We can only wonder at the labor expended, for whenever we refer •(o its pages for information we are seldom disap- pointed in finding all we desire, whether it be in ac- centuation, etymology, or definition of terms. — New York Medical Journal, November, 1865. It would be mere waste of words in us to express our admiration of a work which is so universally and deservedly appreciated. The most admirable work of its kind in the English language. As a book of reference it is invaluable to the medical practi- tioner, and in every instance that we have turned over its pages for information we have been charmed by the clearness of language and the accuracy of detail with which each abounds We can most cor- dially and confidently commend it to our readers. — Glasgow Medical Journal, January, 1866. A work to which there is no equal in the English language. — Edinburgh Medical Journal. It is something more than a dictionary, and some- thing less than an encyclopaedia. This edition of the well-known work is a great improvement on its pre- decessors. The book is one of the very few of which it may be said with truth that every medical man should possess it. — London Medical Times, Aug. 26, 1865. Few works of the class exhibit a grander monument of patient research and of scientific lore. The extent of the sale of this lexicon is sufficient to testify to its usefulness, and to the great service conferred by Dr. Robley Dunglison on the profession, and indeed on others, by an inducement for him to seize the moment of excited curiosity to inform him- self on any subject, and which is otherwise too often allowed to pass unimproved.— Si. Louis Med. and Surg. Journal. WANNER ( THOMAS HA WKES), M- D. A MANUAL OF CLINICAL MEDICINE AND PHYSICAL DIAG- NOSIS. Third American, from the second enlarged and revised English edition. To which is added The Code of Ethics of the American Medical Association. In one hand- some volume 12mo. (Preparing for early publication.) This work, after undergoing a very thorough revision at the hands of the author, may now be expected to appear shortly. The title scarcely affords a proper idea of the range of subjects em- braced in the volume, as it contains not only very full details of diagnostic symptoms properly classified, but also a large amount of information on matters of every day practical importance, not usually touched upon in the systematic works, or scattered through many different volumes. ^PUBLICATIONS — (Anatomy). (HENRY], F.R.S., » Lecturer on Anatomy at St. George's Hospital, London. ANATOMY, DESCRIPTIVE AND SURGICAL. The Drawings by H. V. CARTER, M. D., late Demonstrator on Anatomy at St. George's Hospital; the Dissec- tions jointly by the AUTHOR and DR.- CARTER. Second American, from the second revised and improved London edition. In one magnificent imperial octavo volume, of over 800 pages, with 388 large and elaborate engravings on wood. Price in extra cloth, $6 00 j leather, raised bands, $7 00. The author has endeavored in this work to cover a more extended range of subjects than is cus- tomary in the ordinary text-books, by giving not only the details necessary for the student, but also the application of those details in the practice of medicine and surgery, thus rendering it both a guide for the learner, and an admirable work of reference for the active practitioner. The en- gravings form a special feature in the work, many of them being the size of nature, nearly all original, and having the names of the various parts printed on the body of the cut, in place of figures of reference, with descriptions at the foot. They thus form a complete and splendid series, which will greatly assist the student in obtaining a clear idea of Anatomy, and will also serve to refresh the memory of those who may find in the exigencies of practice the necessity of recalling the details of the dissecting room ; while combining, as it does, a complete Atlas of Anatomy, with a thorough treatise on systematic, descriptive, and applied Anatomy, the work will be found of essential use to all physicians who receive students in. their offices, relieving both preceptor and pupil of much labor in laying the groundwork of a thorough medical education. Notwithstanding its exceedingly low price, the work will be found, in every detail of mechanical execution, one of the handsomest that has yet been offered to the American profession ; while the careful scrutiny of a competent anatomist has relieved it of whatever typographical errors existed in the English edition. and with scarce a reference to the printed text. The Thns it is that book after book makes the labor of the student easier than before, and since we have seen Blanchard & Lea's new edition of Gray's Ana- tomy, certainly the finest work of the kind now ex- tant, we would fain hope that the bugbear of medical students will lose half^s, horrors, and this necessary foundation of physiological science will be much fa- .diluted and advanced. — N.'O. Med. Ntws. The various points illustrated are marked directly on the structure; that is, whether it be muscle, pro- cess, artery, nerve, valve, etc. etc. — we say each point ?s distinctly marked by lettered engravings, so that the student perceives at once each point described as readily as if pointed out on the subject by the de- monstrator. Most of the illustrations are thus ren- dered exceedingly satisfactory, and to the physician they serve to refresh the memory with great readiness j go unrewarded. — Nashville Mtd. and Surg, Journal. surgical application of the various regions is also pre- sented with force and clearness, impressing upon the student at each step of his research all the important relations of the structure demonstrated. — Cincinnati Lancet. This is, we believe, the handsomest book on Ana- tomy as yet published in our language, and bids fair to become in a short time THE standard text-book of our colleges and studies. Students and practitioners will alike appreciate this book. We predict for it a bright career, and are fully prepared to endorse the statement of the London Lancet, that "We are not acquainted with any work in any language which can take equal rank with the one before us." Paper, printing, binding, all are excellent, and we feel that a grateful profession will not allow the publishers to •QUITE (HENRY H.), M.D., , and JJORNER ( WILLIAM E.}, M.D., Prof, of Surgery in the Univ. of Penna., &c. Late Prof, of Anatomy in the Univ. of Penna., F THE SAME AUTHOR. PRINCIPLES OF COMPARATIVE PHYSIOLOGY. New Ameri- can, from the Fourth and Revised London Edition. In one large and handsome octavo volume, with over three hundred beautiful illustrations. Pp. 752. Extra cloth, $5 00. As a complete and condensed treatise on its extended and important subject, this work becomes a necessity to students of natural science, while the very low price at which it is offered places it within the reach of all. £>Y THE SAME AUTHOR. THE MICROSCOPE AND ITS REVELATIONS. With an Appen, dix containing the Applications of the Microscope to Clinical Medicine, &c. By F. Gr. SMITH M. D. Illustrated by four hundred and thirty-four beautiful engravings on wood. In one large and very handsome octavo volume, of 724 pages, extra cloth, $5 25. (ROBERT B.}, M.D. F.R.S., and £0 WMAN ( W.), F.R.S. THE PHYSIOLOGICAL ANATOMY AND PHYSIOLOGY OF MAN. With about three hundred large and beautiful illustrations on wood. Complete in one large octavo volume of 950 pages, extra cloth. Price $4 75. The names of Todd and Bowman have long been familiar to the student of physiology. In this work we have the ripe experience of these laborious physi- ologists on every branch of this science. They gave each subject the most thorough and critical examina- tion before making it a matter of record. Thus, while they advanced tardily, apparently, in their publica- tion, the work thus issued was a complete exponent of the science of physiology at the time of its final appearance. We can, therefore, recommend this work as one of the most reliable which the student or practitioner can consult relating to physiology.— .V. Y. Journal of Medicine. To it the rising generation of medical men will owe, in great measure, a familiar acquaintance with all the chief truths respecting the healthy structure and working of the frames which are to form the subject of their care. The possession of such know- ledge will do more to make sound and able practi- tioners than anything else. — British and Foreign Medico-Chiriirgical Review. (WILLIAM SENHOUSE), M.D., A MANUAL OF PHYSIOLOGY. A new American from the third and improved London edition. With two hundred illustrations. In one large and hand- some royal 12mo. volume. Pp. 586. Extra cloth, $2 25 ; leather, $2 75. By the use of a fine and clear type, a very large amount of matter has been condensed into a comparatively small volume, and at its exceedingly low price it will be found a most desirable manual for students or for gentlemen desirous to refresh their knowledge of modern physiology. It is at once convenient in size, comprehensive in design, and concise in statement, and altogether well adapted for the purpose designed. — St. Louis Med. and Surg. Journal. The physiological reader will find it a most excel- lent guide in the study of physiology in its most ad- vanced and perfect form. The author has shown, himself capable of giving details sufficiently ample in a condensed and concentrated shape, on a science in which it is necessary at once to be correct and not lengthened.— Edinburgh Med. and Surg. Journal. 10 HENRY C. LEA'S PUBLICATIONS — (Physiology). J^ALTON (J. C.}, M.D., Professor of Physiology in the College of Physicians and Surgeons, New York, &c. A TREATISE ON HUMAN PHYSIOLOGY, Designed for the use of Students and Practitioners of Medicine. Third edition, revised, with nearly three hun- dred illustrations on wood. In one very beautiful octavo volume, of 700 pages, extra cloth, $5 25 ; leather, $6 25. " In the present edition of this work the general plan and arrangement of the two former ones are retained. The improvements and additions which have been introduced consist in the incor- poration into the text of certain new facts and discoveries, relating mainly to details, which have made their appearance within the last three years." — Author'1 s Prefafa. The rapid demand for another edition of this work sufficiently shows that the author has suc- ceeded in his efforts to produce a text-book of standard and permanent value, embodying within a moderate compass all that is definitively and positively known within the domain of Human Physiology. His high reputation as an original observer and investigator is a guarantee that in again revising it he has introduced whatever is necessary to render it thoroughly on a level with the advanced science of the day, and this has been accomplished without unduly increasing the size of the volume. No exertion has been spared to maintain the standard of typographical execution which has rendered this work admittedly one of the handsomest volumes as yet produced in this country. We believe we fully recognize the value of Draper and Dunglison, Carpenter and Kirkes, and Todd and Bowman, and yet we unhesitatingly place Dalton at the head of the list, for qualities already enumerated. In the important feature of illustration, Dalton's work is without a peer, either in adaptedness to the text, simplicity and graphicness of design, or elegance of artistic execution. — Chicago Med. Examiner. In calling attention to the recent publication of the third edition of this book, it will only be necessary to say that it retains all the merits and essentially the same plan of the two former editions, with which every American student of medicine is undoubtedly familiar. The distinguished author has added to the text all the important discoveries in experimental physiology and embryology which have appeared during the last three years. — Boston Med. and Surg. Journal, June 30, 1864. The arrangement of the work is excellent. The facts and theories put forward in it are brought up to the present time. Indeed, it may be looked upon as presenting the latest views of physiologists in a con- densed form, written in a clear, distinct manner, and in a style which makes it not only a book of study- to the student, or of reference to the medical practi- tioner, but a book which may be taken up and read with both pleasure and profit at any time. — Canada I-fed. Journal, October, 1864. In Dr. Dalton's excellent treatise we have one of the latest contributions of our American brethren to medical science, and its popularity may be estimated by the fact that this, the second edition, follows upon the first with the short interval of two years. The author has succeeded in giving his readers an exceed- ingly accurate and at the same time most readable re-?um6 of the present condition of physiological science ; and, moreover, he has not been content with mere compilation, but has ably investigated the func- tions of the body for himself, many of the original experiments and observations being of the greatest value. — London Med. Review. This work, recognized as a standard text-book bj the medical schools, and with which the members of the profession are so familiar, demands, bat a brief notice. Its popularity is attested by the rapidity with which former editions have been exhausted. — Chicago Med. Journal, April, 1864. To the student of physiology, no work as yet pub- lished could be more satisfactory as a guide, not only to a correct knowledge of the physiological subjects embraced in its limits, but, what is of far greater importance, it will prove the best teacher of the modes of investigation by which that knowledge can he acquired, and, if necessary, tested. — The Columbus Review of Mad and Surgery. Until within a very recent date, American works on physiology were almost entirely unknown in Ea- ,rope, a circumstance solely due to the fact of their being little else than crude compilations of European works. Within the last few years, however, a great change has taken place for the better, and onr friends on the other side of the Atlantic can now boast of possessing manuals equalled by lew and excelled by none of our own. In Dr. Dalton's treatise we are glad to find a valuable addition to physiological lite- rature. With pleasure we have observed throughout the volume proof of the author not being a mere compiler of the ideas of others, hut an active laborer in the field of science.— The, Brit, aud For. Mvdico- Chirurgioal Review. JT)UNGLISON (ROBLEY], M.D., Professor of Institutes of Medicine in Jefferson Medical College, Philadelphia. HUMAN PHYSIOLOGY. Eighth edition. Thoroughly revised and extensively modified and enlarged, with five hundred and thirty-two illustrations. In two large and handsomely printed octavo volumes of about 1500 pages, extra cloth. $7 00. TEHMANN (C. G.} PHYSIOLOGICAL CHEMISTRY. Translated from the second edi- tion by GEORGE E. DAY, M. D., F. R. S., Ac., edited by R. E. ROGERS, M. D., Professor of Chemistry in the Medical Department of the University of Pennsylvania, with illustrations selected from Funke's Atlas of Physiological Chemistry, and an Appendix of plates. Com- plete in two large and handsome octavo volumes, containing 1200 pages, with nearly two hundred illustrations, extra cloth. $6 00. jgr THE SAME AUTHOR. MANUAL OF CHEMICAL PHYSIOLOGY. Translated from the German, with Notes and Additions, by J. CHE STOW MORRIS, M. D., with an Introductory Essay on Vital Force, by Professor SAMUEL JACKSON, M. D., of the University of Pennsyl- vania. With illustrations on wood. In one very handsome octavo volume of 336 pages, extra cloth. $2 25. HENRY C. LEA'S PUBLICATIONS — (Chemistry}. 11 ~£>RANDE ( WM. T.}, D. C. L., and BAYLOR (ALFRED S.), M.D., F.R.$ CHEMISTRY. In one handsome 8vo. vol. (Neiv and revised edition preparing,} A most comprehensive and compact volume. Its information is recent, and is conveyed in clear lan- guage. Its index of sixty closely-printed columns shows with what care new discoveries have been added to well-known facts. — The Chemical News. THE HANDBOOK IN CHEMISTRY OF THE STUDENT. — For clearness of language, accuracy of description, extent of information, and freedom from pedantry and mysticism, uo other text-book comes into coin- petition with it.— The Lancet. The authors set out with the definite purpose of writing a book which shall be intelligible to any educated man. Thus conceived, and worked out in the most sturdy, common-sense method, this book gives in the clearest and most summary method possible all the facts aud doctrines of c*hemistry.— Medical Times. the cleare«t, and most practical that can be put it the hands of the student. — Edinburgh Ned. Journal. It Abounds in innumerable interesting facts not to be found elsewhere; and froru the masterly manner in which every subject is handled, with its pleasing mode of describing even the dryest details, it cannot fail to prove acceptable, not only to those for whom it is intended, but to the profession at large. — Canada Lancet. We have for a long time felt that the preparation of a proper chemical text-book for students would be time better spent than in the invention of a novel system of classification or the discovery of half a dozen new elements ending in ium. We believe this want has at last been satisfied in the book now before us, which has been prepared expressly for medical students by two of the most experienced teachers of the science in England. — Boston Med. and Surgical We can cordially recommend this work as one of i Journal. ~>0 WMAN (JOHN E.),M. D. PRACTICAL HANDBOOK OF MEDICAL CHEMISTRY. Edited by C. L. BLOXAM, Professor of Practical Chemistry in King's College, London. Fourth American, from the fourth and revised English Edition. In one neat volume, royal 12mo., pp. 351, with numerous illustrations, extra cloth. $2 25. The fourth edition of this invaluable text-book of Medical Chemistry was published in England in Octo- ber of the last year. The Editor has brought down the Handbook to that date, introducing, as far as was compatible with the necessary conciseness of such a work, all the valuable discoveries in the science which have corne to light since the previous edition was printed. The work is indispensable to every student of medicine or enlightened practitioner. It is printed in clear type, and the illustrations are numerous and intelligible. — Boston Med. and Surg. Journal. T>Y THE SAME AUTHOR. — The medical student and practitioner have already appreciated properly this small manual, in which instruction for the examination and analysis of the urine, blood aud other animal products, both healthy and morbid, are accurately given. The directions for the detection of poisons in organic mixtures and in the tissues are exceedingly well exposed in a con- cise and simple manner. This fourth edition has been thoroughly revised by the editor, and brought up to the present state of practical medical chemistry. — Pacific Med. and Surg. Journal. INTRODUCTION TO PRACTICAL CHEMISTRY, INCLUDING ANALYSIS. Fourth American, from the fifth and revised London edition. With numer- ous illustrations. In one neat vol., royal 12mo., extra cloth. $2 25. (Just ready ) One of the most complete manuals that has for a I very judiciously simplified his subjects and illustra- long time been given to the medical student. — ! tions as much as possible, and presents all of the Athenaum. details pertaining to chemical analysis, and other We regard it as realizing almost everything to be i Portions difficult for beginners to comprehend, in desired in an introduction to Practical Chemistry, i 8uch a clear aud distiuct manner as to remove all It is by far the best adapted for the Chemical student doubt °\ difficulty. Thus a subject which is usually of any that has yet fallen in our way.— British and \ regarded by students as almost beyond their com- Foreign Medico-Chiruraical Review. I prehension, is rendered very easy of acquisition. , , . .,, • Several Valuable tables, a, glossary, etc., all combiue The best introductory work on the subject with | to d th k pecuiiaerly aflapted to the wants which we are acquainted.— Edinburgh Monthly Jour. \ nf .„„,,. Qn,i oa „„„{, TO/> iw..i»..*JI it ^V.*T« ™ ! This little treatise, or manual, is designed espe- cially for beginners. With this view the author has of such; and as such we commend it to Western Lancet. •The QHAHAM (THOMAS), F.R.S. THE ELEMENTS OF INORGANIC. CHEMISTRY, including the Applications of the Science in the Arts. New and much enlarged edition, by HENRY WATTS and ROBERT BRIDGES, M. D. Complete in one large and handsome octavo volume, of over 800 very large pages, with two hundred and thirty-two wood-cuts, extra cloth. $5 50. Part II., completing the work from p. 431 to end, with Index, Title Matter, Ac., may be had separate, cloth backs and paper sides. Price $3 00. From Prof. E. N. Horsford, Harvard College. It has, in its earlier and less perfect editions, been familiar to me, and the excellence of its plan and the clearness and completeness of its discussions, have long been my admiration. No reader of English works on this science can afford to be without this edition of Prof. Graham's .Elements.— SilUman's Journal, March, 1858. From Prof. Wolcott Gibbs, N. Y. Free Academy. The work is an admirable one in all respects, and its republication here cannot fail to exert a positive influence upon the progress of science in this country. 12 HENRY C. LEA'S PUBLICATIONS — {Chemistry and Pharmacy). [GEORGE], Ph.D. A MANUAL OF ELEMENTARY CHEMISTRY; Theoretical and Practical. With one hundred and ninety-seven illustrations. Edited by ROBERT BRIDGES, M. D. In one large royal 12ino. volume, of 600 pages, extra cloth, $2 00 ; leather, £2 50. We know of no treatise in the language so well calculated to aid the student iu becoming familiar with the numerous facts in the intrinsic science on which it treats, or one better calculated as a text- book for those attending Chemical lectures. * * * * The best text-book on Chemistry that has issued from our press. — American Medical Journal. We again most cheerfully recommend it as the best text-book for students in attendance upon Chem- ical lectures that we have yet examined. — III. and Ind. Med. and Surg. Journal. A first-rate work upon a first-rate subject.— Stf. Louis Med. and Surg. Journal. No manual of Chemistry which we have met comes so near meeting the wants of the beginner.— Western Journal of Medicine and Surgery. We know of none within the same limits which has higher claims to our confidence as a college class- book, both for accuracy of detail and scientific ar- rangement.— Augusta Medical Journal. We know of no text-book on chemistry that we would sooner recommend to the student than this edition of Prof. Fownes' work. — Montreal Medical Chronicle. A new and revised edition of one of the best elemen- tary works on chemistry accessible to the American and English student. — N. Y. Journal of Medical ami Collateral Science. We unhesitatingly recommend it to medical stu- dents.— N. W. Med. and Surg. Journal. This is a most excellent text-book for class instruc- tion in chfemistry, whether for schools or colleges.— Sillimaris Journal. ABEL AND BLOXAM'S HANDBOOK OF CHEMIS- TRY, Theoretical, Practical, and Technical.! With a recommendatory Preface, by Dr. HOFFMAN. In one large octavo volume of 662 pages, with illus- trations, extra cloth, $4 50. GARDNER'S MEDICAL CHEMISTRY, for the Use of Students, and the Profession. In one royal 12mo. volume, with wood-cuts; pp. 396, extra cloth, $1 00. KNAPP'S TECHNOLOGY ; or Chemistry Applied to the Arts, and to Manufactures. Edited, with numerous notes and additions, by Dr. EDMUM- RONALS, and Dr. TUOHAS RICHARDSON. With Amer- ican additions, by Prof. WALTER R. JOHNSON. la two very handsome octavo volumes, containing about 1000 pages, and 500 wood engravings, extra cloth, $6 00. PARRISH (ED WARD], Professor of Materia Medica in the Philadelphia College of Pharmacy. A TREATISE ON PHARMACY. Designed as a Text-Book for the Student, and as a Guide for the Physician and Pharmaceutist. With many Formulae and Prescriptions. Third Edition, greatly improved. In one handsome octavo volume, of 850 pages, with several hundred illustrations, extra cloth. $5 00. The rapid progress made in the science and art of Pharmacy, and the many changes in the last edition of the Pharmacopoeia have required a very thorough revision of this work to render it worthy the continued confidence with which it has heretofore been favored. In effecting this, many portions have been condensed, and every effort has been made to avoid increasing unduly the bulk of the volume, yet, notwithstanding this, it will be found enlarged by about one hundred and fifty pages. The author's aim has been to present in a clear and compendious manner every- thing of value to the prescriber and dispenser of medicines, and the work, it is hoped, will be found more than ever a complete book of reference and text-book, indispensable to all who desire to keep on a level with the advance of knowledge connected with their profession. The immense amount of practical information condensed in its pages may be estimated from the fact that the Index contains about 4700 items. Under the head of Acids there are 312 refer- ences; under Emplastrum, 36; Extracts, 159; Lozenges, 25; Mixtures, 55; Pills, 56; Syrups, 131 ; Tinctures, 138 ; Unguentum, 57, &c. We have examined this large volume with a good deal of care, and find that the author has completely exhausted the subject upon which he treats ; a more complete work, we think, it would be impossible to find. To the student of pharmacy the work is indis- pensable ; indeed, so far as we know, it is the only one of its kind in existence, and even to the physician or medical student who can spare five dollars to pur- chase it, we feel sure the practical information he will obtain will more than compensate him for the outlay. — Canada Med. Journal, Nov. 1864. The medical student and the practising physician will find the volume of inestimable worth for study and reference. — San Francisco Med. Press, July, 1864. When we say that this book is in some respects the best which has been published on the subject in the English language for a great many years, we do not wish it to be understood as very extravagant praise. In truth, it is not so much the best as the only book.— The London Chemical News. An attempt to furnish anything like an analysis of Parrish's very valuable and elaborate Treatise on Practical Pharmacy would require more space than we have at our disposal. This, however, is not so much a matter of regret, inasmuch as it would be difficult to think of any point,1 however minute and apparently trivial, connected with the manipulation of pharinaceutic substances or appliances which has not been clearly and carefully discussed in this vol- ume. Want of space prevents our enlarging further on this valuable work, and we must conclude by a simple expression of our hearty appreciation of its merits.— Dublin Quarterly Jour, of Medical Science, August, 1S64. We have in this able and elaborate work a fair ex- position of pharmaceutical science as it exists in the United States ; and it shows that our transatlantic friends have given the subject most elaborate con- sideration, and have brought their art to a degree of perfection which, we believe, is scarcely to be sur- passed anywhere. The book is, of course, of more direct value to the medicine maker than to the physi- cian ; yet Mr. PARRISH has not failed to introduce matter in which the prescriber is quite as much interested as the componnder of remedies. In con- clusion, we can only express our high opinion of the value of this work as a guide to the pharmaceutist, and in many respects to the physician, not only in America, but in other parts of the world.— British Med. Journal, Nov. 12th, 1S64. The former editions have been sufficiently long before the medical public to render the merits of the work well known. It is certainly one of the most complete and valuable works on practical pharmacy to which the student, the practitioner, or the apothe- cary can have access. — Chicago Medical Examiner, March, 1864. HENRY C. LEA'S PUBLICATIONS — (Mat.Med.and Therapeutics}. 13 /GRIFFITH (ROBERT E.), M.D. A UNIVERSAL FORMULARY, Containing the Methods of Pre- paring and Administering Officinal and other Medicines. The whole adapted to Physicians and Pharmaceutists. Second edition, thoroughly revised, with nume'rous additions, by ROBERT P. THOMAS, M.D., Professor of Materia Medica in the Philadelphia College of Pharmacy. In one large and handsome octavo volume of 650 pages, double-columns. Extra cloth, $4 00 ; leather, $5 00. In this volume, the Formulary proper occupies over 400 double-column pages, and contains about 5000 formulas, among which, besides those strictly medical, will be found numerous valuable receipts for the preparation of essences, perfumes, inks, soaps, varnishes, &e. Ac. In addition to this, the work contains a vast amount of information indispensable for daily reference by the prac- tising physician and apothecary, embracing Tables of Weights and Measures, Specific Gravity, Tem'perature for Pharmaceutical Operations, Hydrometrical Equivalents, Specific Gravities of some of the Preparations of the Pharmacopoeias, Relation between different Thermometrieal Scale?, Explanation of Abbreviations used in Formulae, Vocabulary of Words used in Prescriptions, Ob- servations on the Management of the Sick Room, Doses of Medicines, Rules for the Administration of Medicines, Management of Convalescence and Relapses, Dietetic Preparations not included in the Formulary, List of Incompatibles, Posological Table, Table of Pharmaceutical Names which differ in the Pharmacopoeias, Officinal Preparations and Directions, and Poisons. Three complete and extended Indexes render the work especially adapted for immediate consul- tation. One, of DISEASES AND THEIR REMEDIES, presents under the head of each disease the remedial agents which have been usefully exhibited in it, with reference to the formulae containing them — while another of PHARMACEUTICAL and BOTANICAL NAMES, and a very thorough GENERAL INDEX afford the means of obtaining at once any information desired. The Formulary itself is arranged alphabetically, under the heads of the leading constituents of the prescriptions. This is one of the most useful books for the prac- tising physician which has been, issued from the press of late years, containing a vast variety of formulas for the safe and convenient administration of medi- cines, all arranged upon scientific and rational prin- ciples, with the quantities stated in full, without signs or abbreviations. — Memphis Med. Recorder. We know of none in our language, or any other, so comprehensive in its details. — London Lancet. One of the most complete works of the kind in any language. — Edinburgh Med. Journal. We are not cognizant of the existence of a parallel work. — London Med. Gazette. &TILLE (ALFRED], M.D., Professor of Theory and Practice of Medicine in the University of Penna. THERAPEUTICS AND MATERIA MEDICA; a Systematic Treatise on the Action and Uses of Medicinal Agents, including their Description and History. Second edition, revised and enlarged. In two large and handsome octavo volumes, of 1592 pages. Extra cloth, $10 00; leather, raised bands, $12 00. Dr. Stille's splendid work on therapeutics and ma- I We have placed first on the list Dr. Stille's great teria medica.— London Med. Times,* April 8, 1865. j work on therapeutics.— Edinburgh Med. Journ., 1865. (BENJAMIN], M.D. THE MEDICAL FORMULARY: being a Collection of Prescriptions derived from the writings and practice of many of the most eminent physicians of America and Europe. Together with the usual Dietetic Preparations and Antidotes for Poisons. To which is added an Appendix, on the Endermic use of Medicines, and on the use of Ether and Chloroform. The whole accompanied with-a few brief Pharmaceutic and Medical Ob- servation's. Eleventh edition, carefully revised and much extended by ROBERT P. THOMAS, M. D., Professor of Materia Medica in the Philadelphia College of Pharmacy. In one volume 8vo., of about 350 pages. $3 00. We endorse the favorable opinion which the book has so long established for itself, and take this occa- sion to commend it to our readers as one of the con- venient handbooks of the office and library.— Cin- cinnati Lancet, Feb. 1864. The work has long been before the profession, and its merits are well known. The present edition con- tains many valuable additions, and will be found to be an exceedingly convenient and useful volume for reference by the medical practitioner. — Chicago Medical Examiner, March, 1864. The work is now so well known, and has been so frequently noticed in this Journal as the successive editions appeared, that it is sufficient, on the present occasion, to state that the editor has introduced into the eleventh edition a large amount of new matter, derived from the current medical and pharmaceutical works, as well as a number of valuable prescriptions furnished from private sources. A very comprehen- sive and extremely useful index has also been sup- plied, which facilitates reference to the particular article the' preseriber may wish to administer; and the language of the Formalary has been made to cor- respond with the nomenclature of the new national Pharmacopoeia.— Am. Jour. Med. Sciences, Jan. 1864. TlUNGLISON (ROBLEY], M.D., "^ Professor of Institutes of Medicine in Jefferson Medical College, Philadelphia. GENERAL THERAPEUTICS AND MATERIA MEDICA; adapted for a Medical Text-Book. With Indexes of Remedies and of Diseases and their Remedies. Sixth edition, revised and improved. With one hundred and ninety-three illustrations. In two large and handsomely printed octavo vols. of about 1100 pages, extra cloth. $6 50. T>r THE SAME AUTHOR. NEW REMEDIES, WITH FORMULAE FOR THEIR PREPARA- TION AND ADMINISTRATION. Seventh edition, with extensive additions. In ojie very large octavo volume of 770 pages, extra cloth. $4 00. 14 HENRY C. LEA'S PUBLICATIONS — (Mat. Ned. and Therapeutics). JJEREIRA (JONATHAN], M.D., F.R.S. and L.S. MATERIA MEDICA AND THERAPEUTICS; being an Abridg- ment of the late Dr. Pereira's Elements of Materia Medica, arranged in conformity with the British Pharmacopoeia, arfd adapted to the use of Medical Practitioners, Chemists and Druggists, Medical and Pharmaceutical Students, . Only 592 pages, while Pereira's. original volumes included 2000, and yet the results of many years' ad- ditional research in pharmacology and therapeutics are embodied in the new edition. Uuqwestionnb]y Dr. Farre has conferred a great benefit upon medical students and practitioners. And in both respects we think he has acted very judiciously. And the work is now condensed — brought fully into accordance wiih the pharmacological opinions in vogue, and can be used with great advantage as a handbook fwr exami- nations.— The Lancet, December, 1S65. S1ARSON (JOSEPH], M.D., Professor of Materia Medica and Pharmacy in the University of Pennsylvania, *OKITANSKY (CARL], M.D., Curator of the Imperial Pathological Museum, and Professor at the University of Vienna. A MANUAL OF PATHOLOGICAL ANATOMY. Translated by W. E. SWAINE, EDWARD SIEVEKING, C. H. MOOIIE, and G. E. DAY. Four volumes octavo, bound in two, of about 1200 pages, extra cloth. $7 50. GLUGE'S ATLAS OP PATHOLOGICAL HISTOLOGY. Translated, with Notes and Additions, by JOSEPH LEIDY, M. D. In one volume, very large imperial quarto, with 320 copper-plate figures, plain and colored, extra cloth. $i 00. SIMON'S GENERAL PATHOLOGY, as conducive to the Establishment of Rational Principles for the Prevention and Cure of Disease. In one octavo volume of 212 pages, extra cloth. $1 25. WILLIAMS (CHARLES J. B.}, M.D., Professor of Clinical Medicine in University College, London. PRINCIPLES OF MEDICINE. An Elementary View of the Causes, Nature, Treatment, Diagnosis, and Prognosis of Disease ; with brief remarks on Hygienics, or the preservation of health. A new American, from the third and revised London edition. In one octavo volume of about 500 pages, extra cloth. $3 50. The unequivocal favor with which this work has j A text-book to which no other in our language is been received by the profession, both in Europe and America, is one among the many gratifying evidences which might be adduced as going to show that there is a steady progress taking place in the science as well as in the art of medicine. — St. Louis Med. and Surg. Journal. No work has ever achieved or maintained a more deserved reputation. — Virginia Med. and Surg. Journal. One of the best works on the subject of which it treats in our language. It has already commended itself to the high regard of the profession ; and we may well say that we know of no single volume that will afford the source of so thorough a drilling in the principles of practice as this. • Students and practitioners should make themselves intimately familiar with its teachings — they will find their labor and study most amply repaid. — Cincinnati Med. Observer. There is no work in medical literature which can fill the place of this one. It is the Primer of the- young practitioner, the Koran of the scientific one. — Stethoscope. comparable. — Charleston Med. Journal. The lengthened analysis we have given of Dr. Wil- liams's Principles of Medicine will, we trust, clearly prove to our readers his perfect competency for the task he has undertaken — that of imparting to the student, as well as to the more experienced practi- tioner, a knowledge of those general principles of pathology on which alone a correct practice can be founded. The absolute necessity of such a work must be evident to all who pretend to more than mere empii'icism. We must conclude by again ex- pressing our high sense of the immense benefit which Dr. Williams has conferred on medicine by the pub- lication .of this work. We are certain that in the present state of our knowledge his Principles of Medi- cine could not possibly be surpassed. While we regret the loss which many of the rising generation of practitioners have sustained by his resignation o the Chair at University College, it is comforting to feel that his writings must long continue to exert a powerful influence on the practice of that profession for the improvement of which he has so assiduously and successfully labored, and in which he holds so distinguished a position. — London Jour, of Medicine 16 HENRY C. LEA'S PUBLICATIONS — (Practice of Medicine). TjlLINT (A USTIN), M. D., J- Professor of the Principles and Practice of Medicine in Bellenue Med. College, N. Y. A TREATISE ON THE PRINCIPLES AND PRACTICE OF MEDICINE ; designed for the use of Students and Practitioners of Medicine. Second edition, revised and enlarged. In one large and closely printed octavo volume of nearly 1000 pages; handsome extra cloth, $6 50; or strongly bound in leather, with raised bands, $750. (Now Ready.) From the Preface to the Second Edition. Four months after the publication of this treatise, the author was notified that a second edition was called for. The speedy exhaustion of the first edition, unexpected in view of its large size, naturally intensified the desire to make the work still more acceptable to practitioners and students of Medicine; and, notwithstanding the brief period allowed for a revision, additions have been made which, it is believed, will enhance the practical utility of the volume. The por- tion treating of Pyaemia has been rewritten; three affections, omitted in the first edition, have been introduced, viz., Pertussis, General Cerebral Paralysis, and Polyuria; Epidemic Cholera has been considered at greater length; the thermometric phenomena of disease have received taller consideration, and, in connection with many affections, there has been added new matter, much of which relates to special therapeutics. We consider the book, in all its essentials, as the best adapted to the student of any of our numerous text-books on this subject. — N. T. Medical Journal, Jan. 1867. Its terse conciseness fully redeems it from being ranked among heavy and common-place works, while the unmistakable way in which Dr. Flint gives his own views is quite refreshing, and far from common. It is a book of enormous research ; the writer is evi- dently a man of observation and large experience ; his views are practically sonnd and theoretically moderate, and we have no hesitation in commending his magnum opus to oar readers —Dublin Medical Press and Circular, May 16, 1866. We are happy to think that this object is- very suc- cessfully attained in the work before us. Solid, com- pact, yet clear, it adequately represents the present condition of practical medicine as it could be done only by a physician who unites to scientific culture a large and well-digested experience in the clinical study of medicine. — Am. Journal Medical Sciences, July, 1866. In the plan of the work and the treatment of indi- vidual subjects' there is a freshness and an originality which make it worthy of the study of practitioners as well as students. It is, indeed, an admirable book, and highly creditable to American medicine. For clearness and conciseness in style, for careful reason- ing upon what is known, for lucid distinction between what we know and what we do not know, between what nature does in disease and what the physician can do and should, for richness in good clinical ob- servation, for independence of statement and opinion on great points of practice, and for general sagacity and good judgment, the work is most meritorious. It is singularly rich in good qualities, and free from faults.— London Lancet, June 23, 1866. We have no hesitation in expressing a favorable opinion of it.— London Medical Times and Gazette, June 23, 1866 For terseness, simplicity and distinctness of style, comprehensiveness of scope, and cool, impartial and well-balanced judgment in the examination of dis- puted questions, it has no superior — we are tempted than those of any medical book we know of, and yoir get their meaning at .a glance. Every sentence gives you a distinct idea. — Pacific Medical and Surgical Journal, June, 1866. A book of inestimable value, as the recorded expe- rience of one of the clearest and best educated minds ever devoted to the theory and practice of medicine. Dr. Flint's THEORY AND PBACTICE OF MEDICINB will be eagerly perused by all our readers — will be re- garded as the BIBI,E of practical medicine. — Buffalo Med. and Surg. Journal, May, 1866. In following out such a plan Dr. Flint has suc- ceeded most admirably, and gives to his readers a work that is not only very readable, interesting, and concise, but in every respect calculated to meet the requirements of professional men of every class. The student has presented to him, in the plainest possible manner, the symptoms of disease, the prin- ciples which should guide him in its treatment, and the difficulties which have to be surmounted in order to arrive at a correct diagnosis. The practitioner, besides having such aids, has offered to him the eon- elusion which the experience of the professor has enabled him to arrive at in reference to the relative merits of different therapeutical agents, and different methods of treatment. This new work, as a whole, will add not a little to the well-earned reputation of Prof. Flint as a medical writer and teacher. The number of years in which he has been engaged in the active duties of his profession, both in public and private life, have given him an amount of experi- ence which has eminently fitted him for the produc- tion of a work which must necessarily extend over such a wide range of subjects. We cannot see how it can fail to meet with universal favor. — N. Y. Mcc£. Record, April 2, 1866. The Practice of Medicine of Prof. Flint is, un- doubtedly, a most excellent work, and is much better suited to the special needs of the American student and practitioner than any other accessible fro them. We predict for the book a very great, and, as we be- lieve, well deserved popularity. — Cincinnati Jrmr- nal of Medicine, March, 1866. Contains all that has recently been added to our knowledge of this department of medicine. — Detv&it to say, no equal. Its sentences are brief — shorter Review, April, 1866. T)UNGLISON, FORBES, TWEE DIE, AND CONOLLY. THE CYCLOPAEDIA OF PRACTICAL MEDICINE: comprising Treatises on the Nature and Treatment of Diseases, Materia Mediea and Therapeutics, Diseases of Women and Children, Medical Jurisprudence, Ac. &e. In four large super-roya! octavo volumes, of 3254 double-columned pages, strongly and handsomely bound. $15. *%* This work contains no less than four hundred and eighteen distinct treatises, contributed by sixty-eight distinguished physicians. The most complete work on practical medicine extant, or at least in our language. — Buffalo Medical and Surgical Journal. For reference, it is above all price to every practi- tioner.— Western Lancet. One of the most valuable medical publications of the day. As a work of reference it is invaluable.— Western Journal of Medicine and Surgery. It has been to us, both as learner and teacher, a work for ready and frequent reference, one in which modern English medicine is exhibited in the most ad- vantageous light. — Medical Examiner. ~DARLOW (GEORGE H.), M.D. A MANUAL OF THE PRACTICE OF MEDICINE. With Additions by D. F. CONDIE, M.D., author of "A Practical Treatise on Diseases of Children," ., '•*-* Professor of Practice of Medicine in Jefferson Medical College, Philadelphia. ELEMENTS OF MEDICINE; a Compendious View of Pathology and Therapeutics, or the History and Treatment of Diseases. Second edition, revised. In one large and handsome octavo volume, of 750 pages, extra cloth. $4 00. B ARCLAY (A. W.), M. D. A MANUAL OF MEDICAL DIAGNOSIS; being an Analysis of the Signs and Symptoms of Disease. Third American from the second and revised London edition. In one neat octavo volume of 451 pages, extra cloth. $3 50. utility. — London I The book should be in the hands of every practical man. — Dublin Med. Press. A work of immense practical Med. Times and Gazette. JJARTSHORNE (HENRY], M.D. PRACTICE OF MEDI- In one handsome royal 12mo. ESSENTIALS OF THE PRINCIPLES AND CINE. A handy-book for Students and Practitioners, volume. (Preparing.} -^ALLEMAND AND WILSON. A PRACTICAL TREATISE ON THE CAUSES, SYMPTOMS, AND TREATMENT OF SPERMATORRHOEA. By M. LALLEMAND. Translated and edited by HENRY J. McDouGALL. Fifth American edition. To which is added ON DISEASES OF THE yESICULJE SEMINALES, AND THEIR ASSOCIATED ORGANS. With special reference to the Morbid Secretions of the Prostatic and Urethral Mucous Membrane. By MARRIS WILSON, M.D. In one neat octavo volume, of about 400 pp., extra cloth, $2 75. ONDON SOCIETY OF MEDICAL OBSERVATION. WHAT TO OBSERVE AT THE BEDSIDE AND AFTER DEATH IN MEDICAL CASES. Published under the authority of the London Society for Medical Observation. A new American, from the second and revised London edition. In one very handsome volume, royal 12mo., extra cloth. $1 00. LAYCOCK'S LECTURES ON THE PRINCIPLES AND METHODS OF MEDICAL OBSERVATION AND RE- SEARCH. For the use of advanced students and junior practitioners. In one very neat royal 12mo. volume, extra cloth. $1 00. HOLLAND'S MEDICAL NOTES AND REFLEC- TIONS. From the third and enlarged .English edi- tion. In one handsome octavo volume of about 500 pages, extra cloth. $3 50. 18 HENRY C. LEA'S PUBLICATIONS — (Practice of Medicine). TjlLINT (AUSTIN], M.D., -*- Professor of the Principles and Practice of Medicine in BfUevue Hospital Med. College, N. Y. A PRACTICAL TREATISE ON THE PHYSICAL EXPLORA- TION OF THE CHEST AND THE DIAGNOSIS OF DISEASES AFFECTING THE RESPIRATORY ORGANS. Second and revised edition. In one handsome octavo volume of 595 pages, extra cloth, $4 50. (Now ready.) During the ten years which have elapsed since the preparation of the first edition of this work* much has been added to our knowledge of its subject.' The position of the author has been such as to keep him necessarily familiar with every step of progress, and to enable him to test the im- portance of all investigations. He has revised the work thoroughly, and it may therefore be regarded as entirely on a level with the most advanced condition of its important topic. The chapter on Phthisis is replete with interest; I which pervades his whole work lends an additional and his remarks on the diagnosis, especially in the | force to its thoroughly practical character, which early stages, are remarkable for their acumen and great practical value. Dr. Flint's style is clear and elegant, and the' tone of freshness and originality cannot fail to obtain for it a place as a standard wor on diseases of the respiratory system. — Lun'don Lancet, Jan. 19, 1867. Y THE SAME A UTHOR. B A PRACTICAL TREATISE ON THE DIAGNOSIS, PATHOLOGY, AND TREATMENT OF DISEASES OF THE HEART. In one neat octavo volume of nearly 500 pages, with a plate ; extra cloth, $3 50. We question the fact of any recent American author ] servers. The work of Dr Flint, which has received in our profession being more extensively known, or j this short notice at our hands, in connection with his more deservedly esteemed in this country than Dr. other volume, whose title we have placed at the head Flint. We willingly acknowledge his success, more particularly in the volume on diseases of the heart, in of our observations, may be regarded as constituting a complete guide to the diagnosis of diseases of the making an extended personal clinical study available j chest; and for this purpose we have much pleasur for purposes of illustration, in connection with cases and every confidence in recommending them.— Brit. which have been reported by other trustworthy ob- and For. Med.-Chir. Review. BLAKISTON ON CERTAIN DISEASES OF THE CHEST. In one volume octavo. $1 25. BUCKLER ON FIBRO-BRONCHITIS AND RHEU- MATIC PNEUMONIA. In one octavo vol., extra cloth, pp. 150. $125. FISKE FUND PRIZE ESSAYS.— LEE ON THE EF- FECTS OF CLIMATE ON TUBERCULOUS DIS- EASE. AND WARREN ON THE INFLUENCE OF PREGNANCY ON THE DEVELOPMENT OF TU- BERCLES. Together in one neat octavo volume, extra cloth, $1 00. HUGHES' CLINICAL INTRODUCTION TO AUS- CULTATION AND OTHER MODES OF PHYSICAL DIAGNOSIS. Second edition. One volume royal 12mo., extra cloth, pp. 304 $1 2.5. WALSHE'S PRACTICAL TREATISE ON DISEASES OF THE LUNGS. Third American, from the third revised and ranch enlarged London edition. In one neat octavo volume of nearly 500 pages, extra cloth. Price $3 00. WALSHE'S PRACTICAL TREATISE ON THE DIS- EASES OF THE HEART AND GREAT VESSELS. Third American, from the third revised and much enlarged London edition. In one handsome octavo volume of 420 pages, extra cloth. $3 00. flHAMBERS (T. K.}, M. D., Consulting Physician to St. Mary's Hospital, London, &c. THE INDIGESTIONS; or, Diseases of the Digestive Organs Functionally Treated. In one handsome octavo volume. (Just Ready.) As a practical work, illustrated by a large number of cases, and detailing the experience of its distinguished author on so important and intractable a class of diseases, this volume can scarcely fail to command the immediate attention of the profession. CONTENTS. CHAP. I. Introduction. II. Indigestion of Various Foods. III. Habits of Social Life Leading to Indigestion. IV. Abdominal Pains. V. Vomiting. VI. Flatulence. VII. Diarrhoea^ VIII. Constipation and Costiveness. IX. Nervous Diseases Connected with Indigestion — Analysis. JftRINTON ( WILLIAM}, M. D., F. R. S. LECTURES ON THE DISEASES OF THE STOMACH; with an Introduction on its Anatomy and Physiology. From the second and enlarged London edi- tion. With illustrations on wood. In one handsome octavo volume of about 300 page.=, extra cloth. $3 25. (Just issued.) Nowhere can be found a more full, accurate, plain, and instructive history of these diseases, or more ra- tional views respecting their pathology and therapeu- tics.— Am. Joitrn. of the Med. Sciences, April, 1865. The first edition of this work became, immediately after its publication, a standard authority on the dis- The most complete work in our language upon the diagnosis and treatment of these puzzling and impor- tant diseases. — Boston Med. and Surg. Journal, Nov. 1865. These lectures comprise a brief but condensed and quite perfect account of what is at proserit known eases, functional and organic, of the primal organ of I concerning diseases of the stomach. The anatomy, the human machine. It is unnecessary here to repeat j physiology, symptoms, and treatment are so pre- the praise which we formerly bestowed on the book when it was a debutant, soliciting professional favor. —Brit, and For. Med.-Chir. Review, April, 1865. sented as to make the work a very instructive and popular one with practitioners of medicine. — Buffalo Med. and Surg. Journal, Dec. 1S65. H ABERSHON (S. 0.}, M.D. PATHOLOGICAL AND PRACTICAL OBSERVATIONS ON DIS- EASES OF THE ALIMENTARY CANAL, (ESOPHAGUS, STOMACH, CAECUM, AND INTESTINES. With illustrations on wood, extra cloth. $2 50. In one handsome octavo volume of 312 HENRY C. LEA'S PUBLICATIONS— (Practice of Medicine). 19 ftUMSTEAD (FREEMAN J.}, M.D., -D Lecturer on Mater ia Medica and Venereal Diseases at the Col. of Phys. and Sura., New York, &c. THE PATHOLOGY AND TREATMENT OF VENEREAL DIS- EASES. Including the results of recent investigations upon the subject. A new and re- vised edition, with illustrations. In one large and handsome octavo volume of 640 pages, extra cloth, $5 00. (Lately Issued.) During the short time which has elapsed since the appearance of this work, it has assumed the position of a recognized authority on the subject wherever the language is spoken, and its transla- tion into Italian shows that its reputation is not confined to our own tongue. The singular clear- ness with which the modern doctrines of venereal diseases are set forth renders it admirably adapted to the student, while the fulness of its practical details and directions as to treatment makes it indispensable to the practitioner. The few notices subjoined will show the very high position universally accorded to it by the medical press of both hemispheres. Well known as one of the best authorities of the I our approval and praise as the second edition of Dr. 7-psent day on the subject. — British and For. Med.- Bumstead's work. — Glasgow Med. Journal, Oct. 1864. We know of 'no treatise in any language which is its equal in point of completeness and practical sim- plicity.— Boston Medical and Surgical Journal, Jan. 30, 1S64. The book is one which every practitioner should p Chirurg. Review, April, 1866. A regular store-house of special information. — London Lancet, Feb. 24, 1866. A remarkably clear and full systematic treatise on the whole subject. — Lond. Med. Times and Gazette. The best, completes!, fullest monograph on this | have in his possession, and, we may further say, the subject in our language.— British American Journal, j °**V book UPOU the subject which he should acknow- andS^y^rnaL ^'^ libn"*-J'ac^ *«*' \ Sa^ca!To^nal, Jaly'l^ ^ The best work with which we are acquainted, and We have no doubt that it will supersede in America erery other treatise on Venereal. — San Francisco Med, Press, Oct. 1S64. the most convenient hand-book for the busy practi- tioner — Cincinnati Lancet, July, 1864. -Brit, and Foreign Med.-Ctururg. Review, Jan., '60. j has bp'eu left Uttaoticed._Z)«6i/n Quarterly Journal We have not met with any which so highly merits ; of Medical Science, August, 1864. ffULLERIER (A.}, a^d ftUMSTEAD (FREEMAN J.}, Surgeon to the Hbpital du Midi. ' F* Lecturer on Venereal Diseases at the College of Physicians and Surgeons, N. Y. AN ATLAS OF VENEREAL DISEASES. Translated and edited by FREEMAN J. BDMSTEAD. To make a large imperial 4to. volume, with plates containing about 150 figures, beautifully colored, many of them the size of life. (Preparing.) In charge of the celebrated Hopital du Midi, where M. Ricord gained his immense experience, M. CulLerier is known as one of the most profound syphilographers of the present day. This work presents the results of his observations and reflections on the whole round of venereal acci- dents and affections, and is illustrated with a complete series of colored plates, more minute and extensive than anything of the kind that has yet been laid before the profession. The translator and editor, Dr. Bumstead, is so well known in this country as an authority on the subject, and as a clear and elegant writer, that his connection with the work is sufficient guarantee that its value will be increased in passing through his hands. TflCORD (P.), M.D. LETTERS ON SYPHILIS. Translated by W. P. LATTIMOSE, M. D. In one neat octavo volume, of 270 pages, extra cloth, $2 00. B UDD (GEORGE], M.D. ON DISEASES OF THE LIVER. Third American, from the third and enlarged London edition. In one very handsome octavo volume, extra cloth, with four beautifully colored plates, and numerous wood-cuts, pp. 500. $4 00. ^A ROCHE (12.), M.D. YELLOW FEVER, considered in its Historical, Pathological, Etio- logical, and Therapeutical Relations. Including a Sketch of the Disease as it has occurred in Philadelphia from 1699 to 1854, with an examination of the connections between it and the fevers known under the same name in other parts of temperate as well as in tropical regions. In two large and handsome octavo volumes, of nearly 1500 pages, extra cloth, $700. >Y THE SAME AUTHOR. PNEUMONIA ; its Supposed Connection, Pathological, and Etiological, with Autumnal Fevers, including an Inquiry into the Existence and Morbid Agency of Malaria. In one handsome octavo volume, extra cloth, of 500 pages. $3 00. TYONS.(ROBERT D.), K. C. C. A TREATISE ON FEVER; or, Selections from a Course of Lectures on Fever. Being part of a Course of Theory and Practice of Medicine. In one neat octavo volume, of 36£ pages, extra cloth. $2 25. CLYMER ON FEVERS ; THEIR DIAGNOSIS, PA- I TODD'S CLINICAL LECTURES ON CERTAIN ACT'TE 20 HENRY C. LEA'S PUBLICATIONS — (Practice of Medicine). ROBERTS ( WILLIAM], M. D., -*-•' Lecturer on Medicine in the Manchester School of Medicine, &c. A PRACTICAL* TREATISE ON URINARY AND RENAL DIS- EASES, including Urinary Deposits. Illustrated by numerous cases and engravings. In one very handsome octavo volume of 516 pp., extra cloth. $4 50. (Now Ready.) The want has for some time heen felt of a work which should render accessible to the American profession in a compendious and convenient form, the results of the numerous and important researches which have of late years elucidated the pathology of Urinary and Renal Diseases. It has been the aim of the author in the present volume to set forth in a form divested of undue technicality, the practical condition of the subject in its most advanced stage of progress. In endeavoring to accomplish this, he has refrained from crowding the volume with minute chemical .and physiological details, which would unfit it for its object of affording to the* physician a guide in his daily practice, and to the student a condensed and intelligible compendium of all that is practically important on the subject. To aid in this, numerous cases and illustrations have been introduced throughout the work. . In carrying out this design, he has not only made good use of his own practical knowledge, but has brought together from various sources a vast amount of information, some of which is not generally pos- sessed by the profession iu this country. We must now bring our notice of this book to a close, re- gretting only that we are obliged to resist the temp- tation of giving further extracts from it. Dr. Roberts has already on several occasions placed before the profession the results of researches made by him on various points connected with the urine, and had thus led us to expect from him something good — in which expectation we have been by no means disappointed. The book is, beyond question, the most comprehen- Bird on Urinary Deposits," beinarfor the present out of print, gentlemen will find in the sive work on urinary and renal diseases, considered ia their strictly practical aspect, that we possess in the English language. — British Medical Journal, Dec. 9, 1865. We have read this book with much satisfaction. It will take its place beside the best treatises in pur language upon urinary pathology and therapeutics. Not the least of its merits is that the author, unlike some other book-makers, is contented to withhold much that he is well qualified to discuss in order to impart to his volume such a strictly practical charac- ter as cannot fail to render it popular among British, readers. — London Med. Times and Gazette, March 17, 1866. above work a trustworthy substitute. MORLAND ON THE MORBID EFFECTS OF THE RETENTION IN THE BLOOD OF THE ELE- MENTS OF THE URINARY SECRETION. In one small octavo volume, 83 pages, extra cloth. 75 cents. BLOOD AND URINE (MANUALS ON). By J. W. GRIFFITH, G. 0. REESE, and A. MARKWICK. One volume, royal 12mo., extra cloth, with plates, pp. 460. $1 25. T)ANIEL H. TUKE,M.D., -*-S Visiting Medical Officer to the York •k Retreat. J1UCKNILL (J. C.^M.D., and -*-•' Med, Superintendent of the Devon Lunatic Asylum.-*-^ Visiting Medical Officer ti A MANUAL OF PSYCHOLOGICAL MEDICINE; containing the History, Nosology, Description, Statistics, Diagnosis, Pathology, and Treatment of In- sanity. With a Plate. In one handsome octavo volume, of 536 pages, extra cloth. £4 25. A work alike characterized by great classical ele- essays, in complete and logical treatment, and the gance and a careful and judicious discrimination on clear, practical manner in which their subjects are the diagnosis, pathology and treatment of this dread- discussed. They will be cited as authority wherever ful malady. — Va. Med. and Surff. Journal. the language is used, and will, no doubt, be exten- We do not know where anything can be found in sively translated. — Amer. Journal of Insanity. the literature of the specialty to compare with these HARRISON'S ESSAY TOWARDS A CORRECT THEORY OF THE NERVOUS SYSTEM. In one octavo volume of 292 pp. $1 50. \ SOLLY ON THE HUMAN BRAIN; its Structure, Physiology, and Diseases. From the Second and much enlarged London edition. In one octavo volume of 500 pages, with 120 wood-cuts; extra cloth. $2 50. S MITH (EDWARD], M.D. CONSUMPTION; ITS EARLY AND REMEDIABLE STAGES. one neat octavo volume of 254 pages, extra cloth. $2 25. (H. H.}, M.D. ASTHMA; its Pathology, Causes, Consequences, and Treatment. one volume, octavo, extra cloth. $2 50. In In (D. J).), M.D. DIPHTHERIA ; its Nature and Treatment, with an account of the His- tory of its Prevalence in various Countries. Second and revised edition. In one neat royal 12mo. volume, extra cloth. $1 25. (Just issued.) TONES (C. HANDFIELD), M. D., Physician to St. Mary's Hospital, &c. CLINICAL OBSERVATIONS ON FUNCTIONAL NERVOUS DISORDERS. In one handsome octavo volume of 348 pages, extra cloth, $3 25. (Now Ready.} The wide scope of the treatise, and its practical character, as illustrated by the large number of cases reported in detail by the author, can hardly fail to render it exceedingly valuable to the profession. HENRY C. LEA'S PUBLICATIONS— (Diseases of the Skin). 21 (ERASMUS), F.R.S., ON DISEASES OF THE SKIN. The sixth American, from the fifth and enlarged English edition. In one large octavo volume of nearly 700 pages, extra cloth. $4 50. Also— A SERIES OF PLATES ILLUSTRATING "WILSON ON DIS- EASES OF THE SKIN;" consisting of twenty beautifully executed plates, of which thir- teen are exquisitely colored, presenting the Normal Anatomy and Pathology of the Skin, and embracing accurate representations of about one hundred varieties of disease, most of them the size of nature. Price, in extra cloth, $5 50. Also, the T^xt and Plates, bound in one handsome volume, extra cloth. Price $9 50. This classical work has for twenty years occupied the position of the leading authority on cuta- neous diseases in the English language, and the industry of the author keeps it on a level with the advance of science, in the frequent revisions which it receives at his hands. The large size of the volume enables him to enter thoroughly into detail on all the subjects embraced in it, while its very moderate price places it within the reach of every one interested in this department of practice. Such a "work as the one before us is a most capital and acceptable help. Mr. Wilson has long been held as high authority in this department of medicine, and his book on diseases of the skin has long been re garde d as one of the best text-books extant on the subject. The present edition is carefully prepared, and brought up in its revision to the present time. In this edition we have also included the beautiful series of plates illustrative of the text, and in the last edi- tion published separately. There are twenty of these plates, nearly all of them colored to nature, and ex- hibiting with great fidelity the various groups of diseases treated of in the body of the work. — Cin- cinnati Lancet, June, 1863. No one treating skin diseases should be without a copy of this standard work. — Canada Lancet. August, 1863. Y THE SAME AUTHOR. We can safely recommend it to the profession as the best work on the subject now in existence in the English language. — Medical Times and Gazette. Mr. Wilson's volume is an excellent digest of the actual amount of knowledge of cutaneous diseases ; it includes almost every fact or opinion of importance connected with the anatomy and pathology of the skin. — British and Foreign "Medical Review. These plates are very accurate, and are executed with an elegance and taste which ar e highly Artist wh . creditable to the artistic skill of the American artist who executed them. — St. Louis Med. Journal. The drawings are very perfect, and the finish and coloring artistic and correct ; the volume is an indis- peiiiable companion to the book it illustrates and completes. — Charleston Medical Journal. THE STUDENT'S BOOK OF CUTANEOUS MEDICINE and DIS- EASES OF THE SKIN. In one very handsome royal 12mo. volume. $3 50. (Now Ready.} This new class-book will be admirably adapted to I Thoroughly practical in the best sense.— Brit. Med. the necessities of students. — Lancet. Y THE SAME AUTHOR. Journal. HEALTHY SKIN; a Popular Treatise on the Skin and Hair, their Preservation and Management. One vol. 12mo., pp. 291, with illustrations, cloth. $1 00 A ELIGAN (J. MOORE], M.D., M.R.I.A., A PRACTICAL TREATISE ON DISEASES OF THE SKIN. Fifth American, from the second and enlarged Dublin edition by T. W. Belcher, M. D. In one neat royal 12mo. volume of 462 pages, extra cloth. $2 25. (Now Ready.) Y THE SAME AUTHOR. ATLAS OF CUTANEOUS DISEASES. In one beautiful quarto volume, with exquisitely colored plates, .), M. D., Late Professor of Obstetrics, &c. in Jefferson Medical College, Philadelphia. WOMAN: HER DISEASES AND THEIR REMEDIES. A Series of Lectures to his Class. Fourth and Improved edition. In one large and beautifully printed octavo volume of over 700 pages, extra cloth, $5 00; leather, $6 00. That this work has been thoroughly appreciated mend with great pleasure a much improved edition by the profession of this country as well us of Europe, i of a work in which we saw little room for improve- is fully attested by the fact of its having reached its ' ment. — NetthtiU* Medical Journal. fourth edition in a period of less than twelve years. We greet this new edition of Dr. METGS' work on Its value has been much enhanced by many impor- woma~ with much pleasnre and commend it to the tant additions, and it contains a fund of useful in- profes8ion, especially to the younger members, who formation, conveyed in an easy and delightful style, receive much valuable instruction from its Every topic discussed by the author is rendered so -s conveved in a pleasing style. The teaching plain as to be readily understood by every student : throughout the work reflects the highest credit npoS and, for our own part, we consider it not only one of . the h|ad and heart of the autkor.-CWcatfo Medical the most readable of books, but one or priceless value journal to the practitioner engaged in the practice of those diseases peculiar to females.— N, Am. Med.-Chir. Re- view. We read the book and find him more — an original The rules of the art here described, the obstetrical opinions here expressed, the general directions and advice given and suggested, are, beyond any cavil, unexceptionably sagacious and prudent. They are thinker, an eloquent expounder, And a thorough founded on a large practice, have been tested by a practitioner. The book is but twelve years old, but i long experience, and come from lips to whose teach- it has been so much appreciated by the profession ing thousands have listened for many years, and that edition after edition has been demanded, and I never without profit. — Charleston Med. Journal and now the fourth is on the table by us. We recom- , Review. B Y THE SAME A UTHOR. ON THE NATURE, SIGNS, AND TREATMENT OF CHILDBED FEVER. In a Series of Letters addressed to the Students of his Class. In one handsome octavo volume of 365 pages, extra cloth. $2 00. SJHURCHILL (FLEETWOOD}, M. D., M. R. L A. ON THE DISEASES OF WOMEN; including those of Pregnancy and Childbed. A new American edition, revised by the Author. With Notes and Additions, by D. FRANCIS CONDIE, M. D., author of " A Practical Treatise on the Diseases of Chil- dren."' With numerous illustrations. In one large and handsome octavo volume of 768 pa,ges, extra cloth, $4 00 ; leather, $5 00. From the Author"1 s Preface. In reviewing this edition, at the request of my Amer-icnn publishers, I have inserted several new sections and chapters, and I have added, I believe, all the information we have derived from recent researches; in addition to which the publishers have been fortunate enough to secure the services of an able and highly esteemed editor in Dr. Condie. present day. To Dr. CHURCHILL, then, are the pro- fession deeply indebted for supplying them with so great a desideratum — the achievement of which de- servedly entitles his name, already intimately asso- ciated with the diseases of women, to rank very high as an authority upon this subject. We would briefly characterize it as one of the most useful which has issued from the press for many years. To all it bears its own recommendation ; and will be found to be invaluable to the student as a text-book, no less than as a compendious work of reference to the qualified practitioner. — Glasgow Med. Journal. As an epitome of all that is known in this depart- ment of medicine, the book before us is perhaps the fullest and most valuable in the English language. — D'/Uin Medical Press. It was left for Dr CHCRCHTLL to gather the scat- tered facts from rtieir various sources, and reduce (hem to a general system. This he has done with a masterly hand in the volume now before us; in which, to the results of his own extensive observa- tion, he has added the views of all British and for- eign writers of any note ; thus giving us in a com- plete form, all that is known upon this subject at the JJ Y THE SAME A UTHOR. ' ESSAYS ON THE PUERPERAL FEYER, AND OTHER DIS- EASES PECULIAR TO WOMEN. Selected from the writings of British Authors previ- ous to the close of the Eighteenth Century. In one neat octavo volume of about 450 pages, extra cloth. $2 50. THOMAS (T. GAILLARD},M.D., J- Professor of Obstetrics, &c. in the College of Physicians and Surgeons, N. Y., Ac. A COMPLETE PRACTICAL TREATISE ON THE DISEASES OF FEMALES. In one large and handsome octavo volume, with illustrations. (Preparing.) jDROWN (ISAAC BAKER], M. D. ON SOME DISEASES OF WOMEN ADMITTING OF SURGICAL TREATMENT. With handsome illustrations. One volume 8vo., extra cloth, pp. 276. $1 60. ASHWELL'S PRACTICAL TREATISE ON THE DIS- EASES PECULIAR TO WOMEN. Illustrated by Cases derived from Hospital and Private Practice. Third American, from the Third and revised Lon- don edition. In 'one octavo volume, extra cloth, of 528 pages. $3 60. RIGBY ON THE CONSTITUTIONAL TREATMENT OF FEMALE DISEASES. In one neat royal 12mo. volume, extra cloth, of about 2oO pages. $1 00. DEWEES'S TREATISE ON THE DISEASES OF FE- MALES. With illustrations. Eleventh Edition, with the Author's last improvements and correc- tions. In one octavo volume of 536 pages, with. plates, extra cloth, $3 00. COLOMBAT DE L'ISERE ON THE DISEASES OF FEMALES. Translated by C. D. MEIQS, M. D. Se- cond edition. In one vol. Svo, extra cloth, with. numerous wood-cuts, pp. 720. $3 75. 24 HENRY C. LEA'S -PUBLICATIONS — (Diseases of Women). TTODGE (HUGH L.), M.D. ON DISEASES PECULIAR TO WOMEN; including Displacements of the Uterus. With original illustrations. In one beautifully printed octavo volume of nearly 500 pages, extra cloth. $3 75. ' CONTENTS. PART I. DISEASES OP IRRITATION. — CHAPTER I. Nervous Irritation, and its consequences. II. Irritable Uterus — Complications. III. Local Symptoms of Irritable Uterus. IV. Local Symptoms of Irritable Uterus. V. General Symptoms of Irritable Diseases. VI. General Symptoms of Irritable Uterus — Reflex Influences of Cerebral and Spinal Irritation. VII. Pro- gress and Terminations of Irritable Uterus. VIII. Causes and Pathology of Irritable Diseases. IX. Treatment of Irritable Uterus— Removal or Palliation of the Cause. X. Treatment of Irritable Uterus — to Diminish or Destroy the Morbid Irritability. XI. Treatment of Irritable Uterus — modified by Menstrual Disorders and Inflammations. XII. Treatment of Irritable Uterus Complicated with Secondary and Sympathetic Symptoms. PART II. DISPLACEMENTS OF THE UTERUS. — CHAPTER I. Displacement of the Uterus. II. Causes and Symptoms of Displacement of the Uterus. III. Diagnosis of Displacement of the Uterus. IV. Treatment of Displacement of the Uterus. V. Treatment, continued — Internal Supporters. VI. Treatment, continued — Lever Pessaries. VII. Treatment, continued. VIIJ. Treatment of Complications of Displacements. PART III. DISEASES OP SEDATION. — CHAPTER I. General and Local Sedation. II. Sedation of Uterus. III. Diagnosis and Treatment. (CHARLES), M.D. LECTURES ON THE DISEASES OF WOMEN. Second American, from the second London edition. In one neat octavo volume of about 500 pages, extra cloth. $3 25. We have thus embodied, in this series of leqtures, one of the most valuable treatises on the diseases of the female sexual system unconnected with gestation, in our language, aad one which cannot fail, from the lucid manner in which the various subjects have been treated, and the careful discrimination used in dealing only with facts, to recommend the volume to the careful study of every practitioner, as affording his safest guides to practice within our knowledge. We have seldom perused a work of a more thoroughly practical character than the one before us. Every page teems with the most truthful and ace urafe infor- mation, and we certainly do not know of any other work from which the physician, in active practice, can more readily obtain advice of the soundest cha- racter upon the peculiar diseases which have been made the subject of elucidation.— British Am. Mtd. Journal. We return the author our grateful thanks for the vast amount of instruction he has afforded us. His valuable treatise needs no eulogy on' our part. His graphic diction and truthful pictures of disease all speak for themselves. — Medico-Chirurg. Review. Most justly esteemed a .standard work It bears evidence of having been carefully revised, and is well worthy of the fame it has already obtained. — Dub. Med. Quar. Jour. >F THE SAME AUTHOR. AN ENQUIRY INTO TI^E PATHOLOGICAL IMPORTANCE OP ULCERATION OF THE OS UTERI. In one neat octavo volume, extra cloth. $1 25. gTMPSON (SIR JAMES F.), M.D. CLINICAL LECTURES ON THE DISEASES OF WOMEN. With numerous illustrations. In one handsome octave volume of over 500 pages, extra cloth. $4. The principal topics embraced in the Lectures are Vesico-Vaginal Fistula, Cancer of the Uterus, Treatment of Carcinoma by Caustics, Dysmenorrhoea, Amenorrhoea, Closures, Contractions, &c., of the Vagina, Vulvitis, Causes of Death after Surgical Operations, Surgical Fever, Phlegniiisia Dolens, Coccyodinia, Pelvic Cellulitis, Pelvic Hasmatoma, Spurious Pregnancy, Ovarian Dropsy, Ovariotomy, Cranioclasm, Diseases of the Fallopian Tubes, Puerperal Mania, Sub Involution and Super-Involution of the Uterus, ENNET (HENRY), M.D. A PRACTICAL TREATISE ON INFLAMMATION OF THE UTERUS, ITS CERVIX AND APPENDAGES, and on its connection with Uterine Dis- ease. Sixth American, from the fourth and revised English edition. In one octavo volume of about 500 pages, extra cloth. $375. (Recently Issued.) From the Author's Preface. During the past two years, this revision of former labors has been my principal occupation, and in its present state the work may be considered to embody the matured experience of the many years I have devoted to the study of uterine disease. Indeed, the entire volume is so replete with infor- mation, to all appearance so perfect in its details, that we could scarcely have thought another page or para- graph was required for the full description of all that is now known with regard to the diseases under con- thor. To speak of it except in terms of the highest approval would be impossible, and we gladly avail .ourselves of the present opportunity to recommend it in the most unqualified manner to the profession. —Dublin Med. Press. sideration if we had not been so informed by the au- THE SAME AUTHOR. A REVIEW OF THE PRESENT STATE OF UTERINE PATHO- LOGY. In one small octavo volume, extra cloth. 50 cents. JU^KI c. .LEA'S PUBLICATIONS — (Midwifery). 25 jff ODGE (HUGH L.}, M.D., Late Professor of Midwifery, &c. in the University of Pennsylvania, &c. THE PRINCIPLES AND PRACTICE OF OBSTETRICS. Illus- trated with large lithographic plates containing one hundred and fifty-nine figures from original photographs, and with numerous wood-cuts. In one large and beautifully printed quarto volume of 550 double-columned pages, strongly bound in extra cloth, $14. (Late- l FROM THE AUTHOR'S PREFACE. "Influenced by these motives, the author has, in this volume, endeavored to present not simply his own opinions, but also those of the most distinguished authorities in the profession; so that it may be considered a digest of the theory and practice of Obstetrics at the present period." In carrying out this design, the ample space afforded by the quarto form has enabled the author to enter thoroughly into all details, and in combining the results of his long experience and study with the teachings of other distinguished authors, he cannot fail to afford to the practitioner what- ever counsel and assistance may be required in doubtful cases and emergencies. A distinguishing feature of the work is the profuseness of its illustrations. The lithographic plates are all original, and, to insure their accuracy, they have been copied from photographs taken expressly for the purpose. Besides these, a very full series of engravings on wood will be found scattered through the text, so that all the details given by the author are amply elucidated by the illustrations. It may be added that no pains or expense have been spared to render the mechanical execution of the work in every respect worthy of the character and value of the teachings it contains. *$(.* Specimens of the plates and letterpress will be forwarded to any address free by mail on receipt of six cents in postage stamps. The work of Dr. Hodge is something more than a simple pr^entation of his' particular views in the de- partment OT Obstetrics ; it is something more than an ordinary treatise on midwifery ; it is, in fact, a cyclo- paedia of midwifery. He has aimed to embody in a single volnme the whole science and art of Obstetrics. An elaborate text is combined with accurate and va- ried pictorial illustrations, so that no fact or principle is left unstated or unexplained. — Am. Med. Times, Sept. 8, 1S64. We should like to analyze the remainder of this excellent work, but already has this review extended beyond our limited space. We cannot conclude this notice without referring to the excellent finish of the work. In typography it is not to be excelled; the paper is superior to what is usually afforded by our American cousins, quite equal to the best of English books. The engravings and lithographs are most beautifully executed. The work recommends itself for its originality, and is in every way a most valu- able addition to those on the subject of obstetrics. — Canada Med. Journal, .Oct. 1S64. It is very large, profusely and elegantly illustrated, and is fitted to take its place near the works of great obstetricians. Of the American works on the subject it is decidedly the best.— Edinb. Med. Jour., Dec. '6-1. We have examined Professor Hodge's work with great satisfaction ; every topic is elaborated most fully. The views of the author are comprehensive, and concisely stated. The rules of practice are judi- cious, and will enable the practitioner to meet every emergency of obstetric complication with confidence. — Chicago Med. Journal, Aug. 1864. More time than we have had at our disposal since we received the great work of Dr. Hodge is necessary to do it justice. It is undoubtedly by far the most original, complete, and carefully composed treatise on the principles and practice of Obstetrics which has ever been issued from the American press. — Pacifie Med. and Stirg. Journal, July, 1864. We have read Dr. Hodge's book with great plea- sure, and have much satisfaction in expressing our commendation of it as a whole. It is certainly highly instructive, and in the main, we believe, correct. The great attention which'the author has devoted to the mechanism of parturition, taken along with the con- clusions at which he has arrived, point, we think, conclusively to the fact that, in Britain at least, the doctrines of Naegele have been too blindly received. —Glasgow Ned. Journal, Oct. 1864. MONTGOMERY (W. F.), M.D., Professor of Midwifery in the King's and Qtteen's College of Physicians in Ireland. AN EXPOSITION OF THE SIGNS AND SYMPTOMS OF PREG- NANCY. With some other Papers on Subjects connected with Midwifery. From the second and enlarged English edition. With two exquisite colored plates, and numerous wood-cuts. In one very handsome octavo volume of nearly 600 pages, extra cloth. $3 75. MILLER (HENRY], M.D., Professor of Obstetrics and Diseases of Women and Children in the University of Louisville. PRINCIPLES AND PRACTICE OF OBSTETRICS, &c.; including the Treatment of Chronic Inflammation of the Cervix and Body of the Uterus considered as a frequent cause of Abortion. With about one hundred illustrations on wood. In one very handsome octavo volume of over 600 pages, extra cloth. $3 75. TYLER SMITH ON PARTURITION, AND THE PRIN- CIPLES AND PRACTICE OF OBSTETRICS, lu one royal 12mo. volume, extra cloth, of 400 pau^s. $150. RIGBY'S SYSTEM OF MIDWIFERY. With Notes and Additional Illustrations. Second American edition. One volnme octavo, extra cloth, 422 pages. $2 50. DEWEES'S COMPREHENSIVE SYSTEM OF MID- WIFERY. Illustrated by occasional cases and many engravings. Twelfth edition, with the au- thor's lust improvements and corrections. In one octavo volume, extra cloth, of 600 pages. $3 50. 26 HENRY C. LEA'S PUBLICATIONS — (Midwifery). PAMSBOTSAM (FRANCIS H.}, M.D. THE PRINCIPLES AND PRACTICE OF OBSTETRIC MEDI- CINE AND SURGERY, in reference to the Process of Parturition. A new and enlarged edition, thoroughly revised by the author. With additions by W. V. KEATING, M. D., Professor of Obstetrics, - sonably hope that henceforth the subject of this trea- tise will cease to be among the opprobria of medical science. — London Medical Review. nograph that has ever appeared on the anatomy and diseases of the ear, and one of the most valuable con- tributions to the art and science of surgeryiu the nineteenth century. — N. Am. Med.-Chirurg. Review. T A URENCE (JOHN Z.}, F. R. C. S., and Jlf'OON (ROBERT (7.), Editor of the Ophthalmic Review, &c. House Surgeon to the Southwark Oph- thalmic Hospital, &c. A HANDY-BOOK OF OPHTHALMIC SURGERY, for the use of Practitioners. With numerous illustrations. In one very handsome octavo volume, eatra .cloth. $2 50. (Now Ready.} (FROM THE AUTHORS' PREFACE.) "In writing these pnges it has been our aim to bring the principles and practice of modern ophthalmic surgery within a small compass, to supply the wants of the busy practitioner, who may have neither time nor opportunity to read the innumerable contributions that ophthalmic surgery and science have received within the last fifteen years. " In describing symptoms, we have limited ourselves to those which are essential for the recog- nition of disease; in describing operations, &c., to those details which are essential for its treat- ment.'1 TONES (T. WHARTON), F.R.S., Professor of Ophthalmic Med. and Surg. in University College, London. THE PRINCIPLES AND PRACTICE OF OPHTHALMIC MEDI- CINE AND SURGERY. With one hundred and seventeen illustrations. Third and re- vised American, with Additions from the second London edition. In one handsome octavo volume of 455 pages, extra cloth. $3 25. We know of no work which contains the same amount I It is an excellent practical treatise on the medical of information in the same compass; we especially re- | and surgical diseases of the eye, and is well adapted commend the book to the American physician and me- I to the wants both of the student and practitioner. — dical student.— San Francisco Med. Press. \ Chicago Med. Examiner. M 'ACKENZIE tW.), M.D., Surgeon Oculist in Scotland in ordinary to her Majesty, &c. PRACTICAL TREATISE ON DISEASES AND INJURIES OF THE EYE- To which is prefixed an Anatomical Introduction explanatory of a Horizontal Section of the Human Eyeball, by THOMAS WHARTON JONES, F. R. S. From the fourth revised and enlarged London edition. With Notes and Additions by ADDINELL HEWSON, M. D., Surgeon to Wills Hospital, &c. .fee. In one very large and handsome octavo volume of 1027 pages, extra cloth, with plates and numerous wood-cuts. $(> 50. BORLAND (W. TV.), M.D. DISEASES OF THE URINARY ORGANS; a Compendium of their Diagnosis, Pathology, and Treatment. With illustrations. In one large and handsome octavo volume 6f about 600 pages, extra cloth. $3 50. Taken as a whole, we can recommend Dr Morland's I of every medical or surgical practitioner. — Brit, and compendium as a very desirable addition to the library \ For. Med.-Chir. Review, April, 1859. BRETON (T. /.) ON THE DISEASES, INJURIES, AND MALFORMATIONS OF THE RECTUM AND ANUS; with remarks on Habitual Constipation. Second American, from the fourth and enlarged London edition. With handsome illustrations. In one very beautifully printed octavo volume of about 300 pages. $3 25. (Just Issued.) We can recommend this volume of Mr Ashton's in the strongest terms, as containing all the latest details of the pathology and treatment of diseases connected with the rectum. — Canada Med. Journ., March, 1866. This is a new and carefully revised edition of one of the most valuable special treatises that the phy- sician and surgeon can have in his library. — Chicago Medical Examiner, Jan. 1866. The short period which has elapsed since the ap- pearance of the former American reprint, and the numerous editions published in England, are the best arguments we can offer of the merits, and of the use- lessness of any commendation on our part of a book already so favorably known to our readers. — Boston Med. and Surg. Journal, Jan. 2-3, 1866. HENRY C. LEA'S PUBLICATIONS — (Medical Jurisprudence, &c.). 31 HTAYLOR (ALFRED S.), M.D., Lecturer on Med. Jurisp. and Chemistry in Guy's Hospital. MEDICAL JURISPRUDENCE. Sixth American, from the eighth and revised London edition. With Notes and References to American Decisions, by CLE- MENT B. PENROSE, of the Philadelphia Bar. In one large octavo volume of 776 pages, extra cloth, $4 50 ; leather, $5 50. (Now Ready.) Considerable additions have been made by the editor to this edition, comprising some important sections from the author's larger work, " The Principles and Practice of Medical Jurisprudence," as well as references to American law and practice. The notes of the former editor, Dr. Harts- home, have likewise been retained, and the whole is presented as fully worthy to maintain the distinguished position which the work has acquired as a leading text-book and authority on the subject. We have the more pleasure in expressing our hea,rty coincidence with the general verdict of the two professions, medical and legal, in favor of this admirable treatise, which, like the one just men- tioned, although printed in the manual form, is really the most elaborate work on the subject that our lite- rature possesses, and will unquestionably hold its ground as the standard of medical jurisprudence in this country so long as it shall be kept by its author so completely up to the mark asit now is. — The Brit- ish and Foreign Medico-Chirurgical Review. Without materially increasing the bulk of this most admirable work, we have a new edition brought close up to the present day, with old errors removed and very many new discoveries added. This is a work well worthy the high position of its author, and a fair representative and exponent of the state of foren- sic medicine in this country, second to none, we ven- ture to say, in the world. To attain this every chapter has undergone a close revision, and many new cases and observations have been added; at the same time no extensive changes have been made because un- called for. It would be a waste of time to attempt any description of this work, which must have found its way to the bookshelf of almost every practitioner in the kingdom ; those who have it not should pos- sess it forthwith. There is no more useful work of reference on this or any subject. — London Medical Review. Taylor's Medical Jurisprudence has been the text- book in our colleges for years, and the present edi- tion, with the valuable additions made by the Ameri- can editor, reuder it the most standard work of the day, on the peculiar province of medicine on which it treats. The American editor, Dr. Hartshorne, has done his duty to the text, and, upon the whole, we cannot but consider this volume the best and richest treatise on medical jurisprudence in our language. — »Brit. Am. Med. Journal. The presentation to the profession of a new and im- pi^oved edition of this well-known and deservedly popular work cannot be looked upon otherwise than as a subject of congratulation. The book has many merits. It is brief, it is comprehensive ; it treats in a clear and satisfactory manner upon a large number of medico-legal subjects, the most interesting and im- portant that can be presented to the attention of the physician, and the completeness of the work is en- hanced, especially to the American reader, by the appropriate though pot very copious notes and re- ferences to recent American cases, by Dr. Hartshorne. — Chicago Med. Jour. , We need hardly say that this work is quite beyond the pale of criticism, and that all we have to do is to congratulate the profession on having its contents again laid before them, in 18(51, in a thoroughly re- vised condition. — British Med. Journal. TKTINSLOW (FORBES), M.D., D.C.L., ON OBSCURE DISEASES OF THE BRAIN AND DISORDERS OF THE MIND; their incipient Symptoms, Pathology, Diagnosis, Treatment, and Pro- phylaxis. Second American, from the third and revised English edition. In one handsome octavo volume of nearly 600 pages, extra cloth. $4 25. (Just Issiied.) SUMMARY OF CONTENTS. CHAPTER I. Introduction — II. Morbid Phenomena of Intelligence — III. Premonitory Symp- toms of Insanity — IV. Confessions of Patients after Recovery — V. State of the Mind during Recovery — VI. Anomalous and Masked Affections of the Mind — VII. Stage of Consciousness— VIII. Stage of Exaltation — IX. Stage of Mental Depression — X. Stage of Aberration — XI. Im- pairment of Mind — XII. Morbid Phenomena of Attention — XIII. Morbid Phenomena of Memory — XIV. Acute Disorders of Memory — XV. Chronic Affections of Memory — XVI. Perversion and Exaltation of Memory — XVII. Psychology and Pathology of Memory — XVIII. Morbid Pheno- mena of Motion — XIX. Morbid Phenomena of Speech — XX. Morbid Phenomena of Sensation — XXI. Morbid Phenomena of the Special Senses — XXII. Morbid Phenomena of Vision, Hearing, Taste, Touch, and Smell— XXIII. Morbid Phenomena of Sleep and Dreaming— XXIV. Morbid Phenomena of Organic and Nutritive Life — XXV. General Principles of Pathology, Diagnosis, Treatment, and Prophylaxis. Of the merits of Dr. Winslow's treatise the profes- sion has sutliciently judged. It has taken its place in the front rank of the works upon the special depart- ment of practical medicine to which it periains. — Cincinnati Journal of Medicine, March, 1866. It is an interesting volume that will amply repay for a careful perusal by all intelligent readers. — Chicago Med. Examiner, Feb. 1866. A work which, like the present, will largely aid the practitioner in recognizing and arresting the first insidious advances of cerebral and mental disease, is one of immense practical value, and demands earnest attention and diligent study on the part of all who have embraced the medical profession, and have thereby undertaken responsibilities in which the welfare and happiness of individuals and families are largely involved. We shall therefore close this brief and necessarily very imperfect notice of Dr. Winslow's great aud classical work by expressing our conviction that it is long since so important and beautifully written a volume has issued from the British medical press. The details of the manage- ment of confirmed cases of insanity more nearly in- terest those who have made mental diseases their special study; but Dr. Winslow's masterly exposi- tion of the early symptoms, and hi.s graphic descrip- tions of tlio, insidious advances of incipient insanity, toge; her with his judicious observations on the treat- ment ol' disorders of the mind, should, we repeat, be carefully studied by all who have undertaken the responsibilities of medical practice. — Dublin Medical Press. It is the most Interesting as well as valuable book that we have seen for a long time. It is truly fasci- nating.— Am. Jour. Med. Sciences. Dr. Winslow's work will undoubtedly occupy an unique position in the medico-psychological litera- ture of this country. — London Med. Review. 32 HENRY C. LEA'S PUBLICATIONS. INDEX TO CATALOGUE. Abel and Bloxam's Handbook of Chemistry Allen's Dissector and Practical Anatomist American Journal of the Medical Sciences Anatomical Atla.s, by Smith and Homer Ashton on the Rectum and Anus . *. Ashwell on Diseases of Females . Blakiston on the Chest Brinton on the Stomach .... Barclay's Medical Diagnosis .... Barlow's Practice of Medicine Barwell on the Joints Beale on the Laws of Health Beunet (Henry) on Diseases of the Uterus . Bennet's Review of Uterine Pathology Bowman's (John E.) Practical Chemistry . Bowman's (John E.) Medical Chemistry Brande & Taylor's Chemistry Brodie's Clinical Lectures on Surgery . , Brown on the Surgical Diseases of Women . Buckler on Bronchitis Bucknill and Tuke on Insanity Budd on Diseases of the Liver Bumstead on Venereal ..... Bumstead and Cullerier's Atlas of Venereal Di Carpenter's Human Physiology . Carpenter's Comparative Physiology . Carpenter on the Microscope Carpenter on the Use and Abuse of Alcohol Carson's Synopsis of Materia Medica . Chambers on the Indigestions Christison and Griffith's Dispensatory Churchill's System of Midwifery . Churchill on Diseases of Females Churchill on Diseases of Children Churchill on Puerperal Fever Clymer on Fevers . . Colombat de 1'Isere on Females, by Wcigs . Condie on Diseases of Children . Cooper's (B. B.) Lectures on Surgery . Cooper (Sir A. P.) on the Testis, &c. . Cullerier's Atlas of Venereal Diseases .- Curling on Diseases of the Testis . Cyclopedia of Practical Medicine . Dalton's Human Physiology ... De Jongh on Cod-Liver Oil .... Dewees's System of Midwifery Dewees on Diseases of Females . Dewees on Diseases of Children . . . Dickson's Practice of Medicine Druitt's Modern Surgery .... Dunglison's Medical Dictionary . Dunglison's Human Physiology . Dunglison on New Eemedies . Dunglison's Therapeutics and Materia Medica Ellis's Medical Formulary, by Thomas Erichsen's System of Surgery Erichsen on Nervous Injuries Fergussou's Operative Surgery Flint on Respiratory Organs .... Flint on the Heart ...... Flint's Practice of Medicine .... Fownes's Elementary Chemistry . Gardner's Medical Chemistry Gibson's Surgery ...... Gluge^s Pathological Histology, by Lcidy . Graham's Elements of Chemistry . Gray's Anatomy Griffith's (R. E.) Universal Formulary . Griffith's (J. W.) Manual on the Blood, £c. . Gross on Urinary Organs .... Gross on Foreign Bodies in Air-Passages Gross's Principles and Practice of Surgery . Gross's Pathological Anatomy Hartshorne's Essentials of Medicine . Habershon on Alimentary Ca,nal . Hamilton on Dislocations and Fractures Harrison on the Nervous System . Hoblyn's Medical Dictionary Hodge on Women ...... Hodge's Obstetrics ..'.... Hodge's Practical Dissections Holland's Medical Notes and Reflections Homer's Anatomy and Histology PAGE . 12 7 1 '. 30 . 23 . IS | . 18 | . 17 i . 16 . 29 ! • H| . 24 i . 24 ! . 11 . 11 . 11 . 29 . 23 . 18 . 20 . 19 . 19 !. 19 14 14 1.8 14 26 23 22 23 19 23 | 22 29 29 19 29 16 10 14 2.) 23 22 17 29 5 10 13 13 13 28 2S 28 18 IS 16 12 12 29 Ifi 11 13 20 27 27 27 LJ 17 IS 29 20 5 24 25 8 17 7 Hughes on Auscultation and Percussion Hillier's Handbook of Skin Diseases . Jones's (T. W.) Ophthalmic Medicine and Surg. Jones and Sieveking's Pathological Anatomy Jones (C. Handfield) on Nervous Disorders . Kirkes' Physiology Knapp's Chemical Technology Lallemand and Wilson on Spermatorrhoea . La Roche on Yellow Fever .... La Roche ou Pneumonia, &c. Laurence and Moon's Ophthalmic Surgery . Laycock on Medical Observation . Lehmann's Physiological Chemistry, 2 vola. Lehmann's Chemical Physiology . Ludlow's Manual of Examinations Lyons on Fever Maclise's Surgical Anatomy .... Malgaigne's Operative Surgery, by Brittan . Markwick's Examination of Urine Mayue's Dispensatory and Formulary Mackenzie on Diseases of the Eye Medical News and Library . Meigs's Obstetrics, the Science and the Ai't . Meigs's- Letters on Diseases of Women Meigs on Puerperal Fever .... Miller's System of Obstetrics Miller's Practice of Surgery .... Miller's Principles of Surgery Montgomery on Pregnancy .... Morland on Urinary Organs .... Morland on Uremia ..... Neill and Smith's Compendium of Med. Science Neligan's Atlas of Diseases of the Skin Neligan on Diseases of the Skin. . Prize Essays on Consumption • . Parrish's Practical Pharmacy Peaslee's Human Histology .... Pirrie's System of Surgery .... Pereira's Mat. Medica and Thera Quaiu and Sharpey's Anatomy, by Leidy Roberts on Urinary Diseases .... Ramsbotham on Parturition .... Reese on Blood and Urine .... Ricord's Letters on Syphilis, by Lattimore . Rigby on Female Diseases .... Rigby's Midwifery Rokitansky's Pathological Anatomy . Royle's Materia Medica and Therapeutics . Sargent's Minor Surgery . . . . Sharpey and Quain's Anatomy, by Leidy . Simon's General Pathology .... Simpson on Females Skey's Operative Surgery .... Slade on Diphtheria Smith (H. H.) and Homer's Anatomical Atlas Smith (Tyler) on Parturition Smith (Edward) on Consumption . Solly on Anatomy and Diseases of the Brain StillS 's Therapeutics Salter on Asthma Tanner's Manual of Clinical Medicine . Taylor's Medical Jurisprudence . Thomas on Diseases of Females . Todd and Bowman's Physiological Anatomy Todd on Acute Diseases Toynbee on the Ear Walshe on the Lungs Walshe on the Heart Watson's Practice of Physic .... West on Diseases of Females West on Diseases of Children West on Ulceration of Os Uteri . What to Observe in Medical Cases Williams's Principles of Medicine Wilson's Human Anatomy .... Wilson's Dissector . . . . . . Wilson on Diseases of the Skin . Wilson's Plates on Diseases of the Skin Wilson's Handbook of Cutaneous Medicine Wilson on Healthy Skin . . . Wilson on Spermatorrhoea .... Winslow on Brain and Mind PAWE . IS . 21 20 RETURN TO the circulation desk of any University of California Library or to the NORTHERN REGIONAL LIBRARY FACILITY Bldg. 400, Richmond Field Station University of California Richmond, CA 94804-4698 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS • 2-month loans may be renewed by calling (510)642-6753 • 1 -year loans may be recharged by bringing books to NRLF • Renewals and recharges may be made 4 days prior to due date DUE AS STAMPED BELOW MAR 1 6 2009 DD20 6M 9-03