Helmholtz, Hermann von Popular scientific lectures. J?rice 15 Cents. 4UMBOLDT LIBRARY. No. 24. to Trade. RETI:;, POPULAR IENTIFI ^*°5 Ar* AA ?T T^ PROF. H. HELMHOLTZ. _f-_J ' ' --1 ^W^^' J • * "" SQHMER ^-^-»7.-«T-m rtn-tT"TX TLTf\Qri1 ~f*f\ IOS. Ill M FR P'A>">S ARE AT PRESENT THE MOST POPULAR D PREFERRED BY THE LEADING ARTISTS. ER PIANOS ARE USED IN THE FOLLOWIK ; VOGT'S COSSRRVATORY OF K , -OF Mu«e.Y«d ™«Ng*RV deceived First Prize at Centennial Exhibition, TJg^lgf'oid7 1(882. ived First Prize at Exhibition, Montreal, Canada, i S O H IMC E «. & O O., NUFACTURERS OF GRAND, SQUARE AN ma 149, 1S1, 1&* ^ I™ V"** 14th ' J. FITZGEEAL3), PTIBLISHEB, 393 PEARL STREET, NEW YORK. I AN DON PHYSIC! CURB CONSTIPATION AND PILES. depend more closely on liie nature and on the peculiarities ot our percep- tions than is the case in other arts in which the nature of the material to be used and of the objects to be represented has a far greater influence. Yet even in those other branches of art, the especial mode of perception of that organ of sense by which the impression is taken xip is not without importance ; and a theoretical insight into its action, and into the principle of its methods cannot bo com- plete if this physiological element is not taken into account. Next to music this seems to predominate more particularly in painting, and this is the reason why I have chosen painting as the subject of my present lecture. The more immediate object of the painter is to produce in us by his palette a lively visual impression of the objects which he has endeavored to represent. The aim, in a certain sense, is to produce a kind of optical illusion ; not indeed that, like the birds who pecked at the painted granes of Apelles, we are to suppose we have present the real ob- jects themselves, and not a picture ; but in BO far that the artistic representation produces in us a conception of their objects as vivid and as powerful as if we had them actually before us. The study of what are called il- lusions of the senses is, however, a very prom- inent and important part of the physiology of the senses ; for just those cases in which external impressions evoke conceptions which are not in accordance witli reality are particularly instructive for discovering the laws of those means and processes by which normal perceptions originate. We must look upon artists as persons whose observa- tion of sensuous impressions is particularly vivid and accurate, and whose memory for these images is particularly true. That which long tradition has handed down to the men most gifted in this respect, and that which they have found by innumerable experiments in 'the most varied directions, as regards means and methods of representation, forms a series of important and significant facts, »vhich the physiologist, who has here to learn rom the artist, cannot afford to neglect. The tady of works of art will throw great light the question as to which elements and re- of our visual impressions are most iominant in determining our conception is seen, and what others are of lass ice. As far as lies within his power, will seek to foster the former at the :« latter. In this sense, then, a careful observation of the works of the great masters will bo ser\ u-c- able, not only to physiological optics, but also because the investigation of the laws of the perceptions and of the observations ot the senses will promote the theory of art. that is, the comprehension of its mode of action. We have not hero to do with a discussion of the ultimate objects and aims of art, but only with an examination of the action of the elementary means with which it works. The knowledge of the latter must, however, form an indispensable basis for the solution of the deeper questions, if we are to under- stand the problems which the artist has to solve, and the mode in which he attempts to attain his object. I need scarcely lay stress on the fact, fol- lowing as it does from what I have already said, that it is not my intention to furnish instructions according to which the artist is to work. I consider it a mistake to suppose that any kind of aesthetic lectures such an these can ever do so ; but it is a mistaka which those very frequently mako who have only practical objects in view. I. FORM. The painter seeks to produce mhispictnr* an image of external objects. The first aim of our investigation nmst be to ascertain \vhnt degree and what kind of similarity he can expect to attain, and what limits are assigned to him by the nature of his method. The uneducated observer usually requires nothing juore than an illusive resemblance to nature : the more this is obtained, the more does ho delight in the picture. An observer, on tho contrary, whose taste in works of art has been more finely educated, will, consciously or unconsciously, require something more, and something different. A faithful copy of crude nature he will at most regard as an ar- tistic feat. To satisfy him, he wi'l need ar- tistic selection, grouping, and even idealiza- tion of the objects represented. The human figures in a work of art must not bo the every-day figures, such as we see in photo- graphs ; they must have expression, and a characteristic development, and if possible beautiful forms, which have perhaps belonged to no living individuals, or indeed any in- dividuals which over have existed, but only to such a one as might exist, and as must exist, to produce a vivid perception of any particular aspect of human existence in it* complete and unhindered development. If, however, the artist is to produce an ar- tistic arrangement of only idealized types, whether of man or of natural objects, luunt not the picture be an actual, complete, and directly true delineation of that which wo\ild appear if it anywhere camo into being? Since the picture is on a plane surface, this faithful representation can of course only give a faithful perspective view of tho objects. Yet our eye, which in its optical properties is equivalent to a camera obscura, the well-known apparatus of the photogra- POPULAR SCIENTIFIC LECTURES. 007 pTicr, gives on the retina, which is its sensi- tive plate, only perspective views of the ex- tf rnal world ; these are stationary, like tbo c:iu\ving on a picture, as long as the stand- po:"t oi! tbe eye is iiot altered. And, in fact, ill >.e restrict cmrselves in the first place to tao 1'orui of tho object viewed, and disregard f"r Li:* present any consideration of color, I ;, a (Mrieot. perspective drawing we can pre- t»:ut t ..) tho o\c of an observer, who views it fr>'-u a correctly chosen point of view, tho frtia.- i'orms of the visual image as the inspec- tion of tho objects themselves would present to tii<- sf.iiio eye, when viewed from the corre- Kpoii'ihu,' point of view.' lint iipurt from the fact that any movement of (ho observer, whereby his eye changes its position, will produce displacements of the visual iantgo, different when he stands be- fore objects from those when he stands be- fore the image, I could speak of only one eye for which equality of impression is to be es- tablished. We however see the world with /ic<> eyes, which occupy somewhat different positions in space, and which therefore show two different perspective views of objects before us. This difference of the images of the two eyes forms ono of the most impor- tant means of estimating tho distance of ob- jects from our eye, and of estimating depth, and this is what is wanting to the painter, or even turns against him ; since in binocular vision the picture distinctly forces itself on otir perception as a plane surface. You must all have observed the wonderful vividness which the solid form of objects ac- quires when good stereoscopic images nro viewed in the stereoscope, a kind of vivid- ness in which either of the pictures is waiit- •ng when viewed without the stereoscope. Che illusion is most striking and instructive dth figures in simple line ; models of crys- ,als and the like, in which there is no other element of illusion. The reason cf this de- ception is, that looking with two eyes we view the world simultaneous!}' from somewhat different points of view, and thereby acquire two different perspective images. With the right eye we see somewhat more of the right side of objects before us, and also somewhat more of those behind it, than we do with the left eye ; and conversely we see with the left, more of the left side of an object, and of the background behind its left edges, and partially concealed by the edge. B\it a flat picture shows to the right eye absolutely tho pamo picture, and all objects represented upon it, as to the left eye. If, then, we make for each eye such a picture as that eye would perceive if itself looked at the object, and if both pictures are combined in the stereo- scope, so that each eyo sees its corresponding picture, then, as far as form is concerned, tho same impression is produced in the two eyes as the object itself produces. But if we look at a drawing or a pictxire with both eyes, wo just as easily recognize that it is a represen- tation on a plane surface, which is different from that which the actual object would show simultaneously to both ejes. Hence is (Tr.e the well-known increase in the vividness of a picture if it is looked at with only ono eye, and while quito stationary, through jv dark tube ; we thus exclude any comparison of its distance with that of adjacent objects in the room. For it must be observed th?.t as we use different pictures seen with the | imitate tho fine gradation and transitions of I light and shad.? on rounded surfaces, which! nro his chief jiunin.j of expressing their mod- 1 o.liing, with all their fino changes of curva- ture ; ho must take into account the exten- sion or restriction of tho sources of light, and tho mutual reflection of the surfaces ori oach other. While, the modifications of tho lighting on the surface of bodies themselves! is often dubious — for instance, an intaglio of a medal may, with a particular illumination, produce the impression of reliefs which aro only illuminated from the other side -doublo shadows, on tho contrary, are undoubted in- dications that the body which throws tho shadow is nearer the source of light than that which receives the shadow. This rulo is so completely without exception, that even in stereoscopic views a falsely placed double shadow may destroy or confuse the entire illusion. The various kinds of illumination r.ro not all equally favorable for obtaining the full effect of shadows. When the observer look'i at the objects in ihe name direction as that in which light fails upon them, he sees only their illuminated sides und nothing of the shadow ; the whole relief which the shadows could give then disappears. If the object is between the source of light and the observer he only sees the shadows. Hence we iiee.l lateral illumination for a picturesque shad- ing ; and over surfaces which, like those of plan 3 or hilly land, only present slightly moving figures, we require light which is al- most in the direction of the surface itself, for only such a one gives shadows. This is one of the reasons which makes illumination by the rising or tho setting sun so eifective. Tho forms of the landscape become more distinct. To this must also be added tho influence of color, and of aerial light, which we shall subsequently discuss Direct illumination from the sun, or from a name, makes tho shadows sharply denned and hard. Illumination from a very wide luminous surface?, such as ft cloudy sky, makes them confused, or destroys them al- together. Between these twu extremes there are transitions ; illumination by a portion of the sky, denned by a window, or by trees, etc., allows the shadows to bo more or less prominent according to the nature of the object. You must have seen of what impor- tance this is to photographers, who have to modify their light by all manner of screens and curtains in order to obtain well-mod- elled portraits. Of more importance for the representation of d pch than the elements hitherto enu- merated and which nro more or loss of local and accidental significance, is what is called arrial. perspective. By this we under- stand the optical action of tho light, which the illuminated masses of air, between the observer and distant objects, give. This; POPULAR scmrrriFic LTXTUIIES. 609 nrisos from a fine opacity in the atmos- phere, which never entirely disappears. If, in. ft transparent medium, there are fino transparent particles of varying density and varying refrangibility, in so far as they aro struck by it, they deflect the light passing through such a medium, partly by reflection and partly by refraction ; to use an optical expression, they scatter it in all directions. If the opaquo particles are sparsely distributed, so that a great part of the light can pass through them without being deflected, distant objects are seen in sharp, well-defined outlines throxigh such a medium, while at the same time a portion of the light which is deflected is distributed in the transparent medium as an opaque halo. Water rendered turbid by a few drops of milk shows this dispersion of the light and cloudiness very distinctly. The light in this case is deflected by the micro- Bcopic globules of butter which are suspended in the milk. In the ordinary air of our rooms this tur- bidity is very apparent when the room is closed, and a ray of sunlight is admitted through a narrow aperture. We see then some of these solar particles, large enough to be distinguished by the naked eye, while others form a fine homogeneous turbidity. But even the latter must consist mainly of suspended particles of organic substances, for, according to an observation of Tyndall, they can be burnt. If the flame of a spirit lamp is placed directly below the path of these rays, the air- rising from the flame stands out quite dark in the surrounding bright turbidity ; that is to say, the air ris- ing from the flame has been quite freed from dust. In the open air, besides dust nnd occasional smoke, we must often also take into account the turbidity arising from incipient aqueous deposits, where the tem- perature of moist air sinks so far that the water retained in it can no longer exist as invisible vapor. Part of the water settles then in the form of fine drops, as a kind of the very finest aqueous dust, and forms a finer or denser fog ; that is to say, cloud. The turbidity which forms in hot eunshino and dry air may arise, partly from dust which the ascending currents of warm air whirl about ; and partly from the irregular mix- ture of cold and warm layers of air of cliffer- eat densitj', as is seen in the tremulous mo- tion of the lower layers of air over surfaces irradiated by the sun. But science can as yet give no explanation ot' the turbidity in the higher regions of the atmosphere which produces the blue of the sky ; we do not know whether it arises from suspended par- ticles of foreign substances, or whether the molecules of air themselves may not act as turbid particles in the luminous ether. The color of the light reflected by the opaque particles mainly depends on their magnitude. When a block of wood floats on water, and by a succession of falling drops •we produco small wave-rings near it, these are repelled by the floating wood as if it were a solid wall. But in the long waves of the *ea a block of wood wotld bo rocked about without the waves being thereby ma- terially disturbed in their progress. Now light is well known to be an undulatory mo- tion of the ether which fills all space. Tho red and yellow rays have the longest waves, the blue and violet the shortest. Very fine particles, therefore, which disturb the uni- formity of the ether, will, accordingly, reflect the latter rays more markedly than the red and yellow rays. The light of turbid media is bluer, the finer are the opaque particles i while tho larger particles of uniform light reflect all colors, and therefore give a whitish turbidity. Of this kind is tho celestial blue, that is, tho color of the turbid atmosphere aV seen against dark cosmical space. The pure* and the more transparent the nir, the bluer is the sky. In like manner it is bluer and darker -when we ascend high rcountain.% partly because tho air at great heights is free* from turbidity, and partly because there in less air above us. But the same blue, which is seen against the dark celestial space, also occurs against dark terrestrial objects ; for instance, when a thick layer of illuminated air is between us and masses of deeply shaded or wooded hills. Tho same aerial light makes the sky blue, ns well cs tho mountains ; excepting that in tho forrart case it is pure, while in the latter it is mixei? with the light from objects behind ; an fact that bv a suitable grouping, position, and turn objects, by a suitable choice of the point o view, and by the mode of lighting, they learn to overcome the unfavorable conditions whicl are imposed on them in this respect. It might at first sight appear that of the reqiiisite tmth to nature of ft picture, sr much would remain that, seen from th proper point of view, it would at least produc the same distribution of light, color, and shadow in its field of view, and would pro duce in the interior of the eye exactly th same image on the retina as the Abject rep resented would do if we had it actually be fore us, and looked at it from a definite fixed point of view. It might seem to be ai object of pictorial skill to aim at prodxioing under the given limitations, the saim effcc as is produced by the object itself. If we proceed to examine whether, and ho\ far, painting can satisf y such a condition, w come upon diliiculties before which we shoulc perhaps shrink, if we did not know that the had been already overcome. Let us begin with the simplest case ; wit the quantitative relations between luminou intensities. If the artist is to imitate exnctl; the impression which the object produces or our eye, he ought to be able to dispose o brightness and dar!iness equal to that whic. nattire offers. But of this there can be n idea. Let mo givo a case in point. Le there ba, in a picture-gallery, a desert-Kcenc- in which a pjf>cession of Bedotiins, shroude in white, and of dark negroes, marche under the burning sunshine ; close to it bluish moonlight scene, where the moon i reflected in the water, and groups of trees, and human forms, are seen to be faintly in- dicated in the darkness. You know from experience that both pictures, if they are we; done, can produce with surprising vividiiei the representation of their objects ; and yt in both pictures the brightest parts are prc duced with the same white-lead, which i but slightly altered by admixtures ; whil the darkest parts are produced with the sain black. Both being hung on the same v/a share the same light, and the brightest t well as the darkest parts of the two source! differ as concerns the degree of their brigh ness. How is it, however, with the actual degroo of brightness represented ? Tho relatioi between the brightness of the sun's light an that of the moon was measured by Wollas ton, who compared their intensities wit' that of the light of candles of the same rn:» terial. He thus found that the luminosity o the sun is 800,000 times that of the brighter light of a full moon. An opaque body, which is lighted from an; source whatever, can. even in the most f;t vorable case, only emit as much light as fall upon it. Yet, from Lambert's observations even the whitest bodies only reflect abon two fifths of the incident light. The sun rays, which proceed parallel from the SUH whose diameter is 83,000 miles, when tht; reach us. are distributed uniformly over POPULAR SCIENTIFIC LECTUKES. 611 sphere 195 millions of miles in diameter. Its density r.ncl illuminating power is here only the one forty-thousandth of that with which it left the sun's surface ; and Lambert's •ember leads to the conclusion that even the brightest white surface on which the sun's rays fall vertically, has only the one hundred- thousandth part of the brightness of the sun's disk. The moon, however, is a gray body, whose mean brightness is only about one fifth of that of the purest white. And when the moon irradiates a body of the purest white on the earth, its brightness is only the hundred-thousandth part of the brightness of the moon itself ; hence the sun's disk is 80,000 million times brighter than a white which is irradiated by the full raoon. Now pictures which hang in a room are not lighted by the direct light of the sun, but by that which is reflected from the sky and clouds. I do not know of any direct measurements of the ordinary brightness of the light in a picture gallery, but estimates may be made from known data. With strong upper light and bright light from the clouds, the brightest white on a picture has probably l-20th of the brightness of white directly lighted by the sun ; it will generally be only l-40th, or even less. Hence the painter of the desert, even if he gives up the representation of the sun's disk, which is always very imperf ••*-., will have to represent the glaringly lighted gar- ments of his Bedouins with a white which, in the most favorable case, shows only the l-20th part of the brightness which corre- sponds to actual fact. If he could bring it, with its lighting unchanged, into the desert near the white there, it would seem like a dark gray. I found in fact, by an experi- ment, that lampblack, lighted by the sun, is not less than half as bright as Bhaded white in the brighter part of a room. On the picture of the moon, the same white •which has been used for depicting the Be- douins' garments must be used for repre- senting the moon's disk, and its reflection in the water ; although the real moon has only one fifth of this brightness, and its re- flection in v.'dter still less. Hence white garments in moonlight, or marble surfaces, even when the artist gives them a gray shade, will always be ten to twenty times as bright in his picture as they are in reality. On the other hand, the darkest black which the artist could apply would be scarcely sufficient to represent the real illumi- nation of a white object on which the moon shone. For even the deadest black coatings of lampblack, black velvet, when power- fully lighted appear gray, as we often enough know to our cost, when we wish to shut off superfluous light. I investigated a coating of lampblack, and found its brightness to be about one hundredth that of white paper. The brightest colors of a painter are only about one hundred times as bright as his darkest shades. The statements I have mado may perhaps appear exaggerated. But they depend upon measurements, and you can control them by well-known observations. According to Wol- laston, the light of the full moon is equal to that of a candle burning at a distance of 12 feet. You know that we cannot read by the light of the full mocn, though wej ian read at a distance of three or four feet frol a candle. Now assume that yon suddenly r ssed from a room in daylight to a vault per ctly daik, with the exception of the lighf :>f a single candle. Yoti would at first think ou were in absolute darkness, and at most you would only recognize the candle itself. In any case, you would not recognize the slightest traco of any objects at a distance of 12 feet from the candle. These, however, rre the objects whose ilhirnination is the same as that which the moonlight gives. You would only bo- come accustomed to the darkness after some time, and you would then find your way about without difficulty. If, now, you return to the daylight, v/hich before v/as perfectly comfortable, it will ap- pear so dazzling that you will perhaps liavo to close the eyes, and only bo able to gaxo round with a painful glare. You see thua that we are concerned here not with minute, but with colossal, differences. How now is it possible that, under such circumstances, we can imagine there is any similarity between the picture and reality ? Our discussion of what we did not see at first, but cou'.d afterward see in the vault, points to the most important element in the. solution ; it is the varying extent to which our senses aro deadened by light ; a process to which we can attach the same name, fa- tigue, as that for the corresponding one in the muscle. Any activity of our nervous sys- tem diminishes its power for the time being. The muscle is tired by work, the brain in lired by thinking, and by mental opera- tions ; the eye is tired by light, and the more so the more powerful the light. Fa- tigue makes it dull and insensitive to new im- pressions, so that it appreciates strong ones only moderately, and weak ones not at all. But now you see how different is the aim of the artist when these circumstances art taken into account. The eye of the truvellei in the desert, who is looking at the caravan, has been dulled to the last degree by the dazzling sunshine ; while that of the wan- derer by moonlight has been raised to the extreme of sensitiveness. The condition of one who is looking at a picture differs from both the above cases by possessing ^ certain mean degree of sensitiveness. Accordingly, the painter must endeavor to produce by hi* colors, 011 the moderately sensitive eye of the spectator, the same impression as that which the desert, on the ono hand, produces on the »>.-4.aoaea, and the moonlight, on the otuei hand, creates on the untired eye of its ob- server. Hence, along with the actual lu- minous phenomena of the outer world, the different physiological conditions of the ey« play a most important part in the work of the artist. What he has to give is not a rnerti f,12 SM:::;TI: o of the object, but a translation of his impression into another scale of sensi- tivenes.s, which belongs to a different degree of impressibility of the observing eye, in which the organ speaks a very different dia- lect in responding to the impressions of the onter world. In order to understand to what conclu- nious this leads, I must lirst of all ex- plain the law which Fechner discovered for the scale of sensitiveness of the eye, which is a particular case of the more general psycho-physical la to of the relations of the va- rious sensuous impressions to the irritations which produce them. This law may be ex- pressed as follows : Within very wide limits of iriyhtness, differences in the strength of liyht are •qu/itty distinct or appear equal in sensation, if fay form an equal fraction of the total quantity (f liyht compared. Thus, for instance, differ- jnces in intensity of one hundredth of the lolal amount can be recognized without great trouble with very different strengths of light, without exhibiting material differ- ences in the certainty and facility of the os- fcimate, whether the brightest daylight or the light of a good candle be used. The easiest method of producing accurately measurable differences in the brightness of two white surfaces, depends on the \ise of rapidly rotating disks. If a disk, like the adjacent one in Fig. 1, is made to rotate very rapidly (that is, 20 to 30 times in a second), it appears to the eye to be covered with three gray rings, as in Fig. 2. The reader must aowever, iigure to himself the gray of these rings, as it appears on the rotating disk of Fig 1, as a scarcely perceptible shade of the ground. When the rotation is r jud each rmg of the disk appears Illuminated, as if ill the light which fell itpon it had been uni- formly distributed over its entire surface. Those rings in which are the black bands, h«ve somewhat loss light than the quite vr.i.ie ones, nnd if the breadth of the marks is compared with the length of half the cir- cumference of the corresponding ring, wo got the fraction by which the intensity of the light in the white ground of the disk is dimin- ished in the ring in question. If the bands are all equally broad, as in Fig. 1, the inner rings appear darker than the outer ones, for in this latter case the same loss of light is distributed over a larger area than in the former. In this way extremely delicat* shades of brightness may bo obtained, and by this method, when the strength of the il- lumination varies, the brightness always di- minishes by the same proportion of its total value. Now it is found, in accordance with Fechner' s law, that the distinctness of the rings is nearly constant for very different strengths of light. We exclude, of course, the cases of too dazzling or of too dim a light. In both cases the finer distinction* can no longer be perceived by the aye. The case is quite different when for diffe» ent strengths of illumination we produce differences which always co-respond to the same quantity of light. If, for instance, we close the shutter of a room at daytime, so that it is quite dark, and now light it by a candle, we can discriminate without diffi- cult}' the shadows, such as that of the hand. thrown by the candle on a sheet of whitu paper. If, however, the shutters are again opened, so that daylight enters the room, for the same position of the hand we can no longer recognize the shadow, although ther« falls on that part of the white sheet, which ia not struck by this shadow, the same excesn of candle-light as upon the pails shaded by the hand. But this small quantity of light disappears in comparison with the newly ftdded daylight, provided that this strikes all parts of the white sheet uniformly. You set then that, while the difference between can- dle-light and darkness can be easily per- ceived, the equally great difference between daylight, on the one hand, and daylight plu« candle-light on the other, can be no longer recognized. This law is of gretit importance in discrim- inating between various degrees ci' bright- ness of natural objects. A. white body appear! white because it reflects a large traction, and a gray body appears gray because it reflects a small fraction, of incident light. For differ- ent intensities of illumination, the difference of brightness between the two will always correspond to the same fraction of their total brightness, and hence will be equally percep- tible to our eyes, provided wo do not ap- proach too near to the upper or the lowei limit of the brightness, for which Fechner' .v law no longer holds. Hence, on the whole the painter can produce what appears an equal difference for the spectator of his pio- ture, notwithstanding the varying strength oJ: POPULAR SCIENTIFIC LECTURES. 613 light in the gallery, provided he gives to his colors the same ratio of brightness as that •which actually exists. Foi, in fact, iii looking at natural objects, the absolute brightness in which they ap- pear to the eyo varies within very wide limits, according to the intensity of tho light and tho sensitiveness of tho eye. That which is constant is only the ratio of the brightness iu which surfaces of various depth of color appear to us when lighted to the same amount. But this ratio of bright- ness is for us the perception, from which wo form our judgment as to tho lighter or darker color of the bodies we see. Now this ratio can be imitated by the painter without restraint, and in conformity with nature, to evoke in us tho same conception as to the nature of the bodies seen. A truthful imita- tion in thas respect would be attained within the limits in which Fechner' s law holds, if the artist reproduced the fully lighted parts of the objects which he has to represent with pigments, which, with the samo light, were equal to the colors to be represented. This is approximately the case. On the whole, the painter chooses colored pigments which almost exactly reproduce the colors of the bodies represented, especially for objects of no great depth, such as portraits, and which are only darker in the shaded parts. Children begin to paint on this principle • they imitate one color by another ; and, in like manner also, nations in which painting has remained in a childish stage. Perfect artistic painting is • only reached when we have succeeded in imitating the action of light upon the eye, and not merely the pig- ments ; and only when we look at the object of pictorial representations from this point of view, will it be possible to understand the variations from nature which artists have to make in the choice of their scale cf color and of shade. These are, in the first case, due to the cir- cumstance that Fechner' s law only holds for mean degrees of brightness ; wuile, for a brightness which is too high or too low, ap- preciable divergences are met with. At both extremes of luminous intensity the eye is less sensitive for differences in light than is required by that law. With a very Btrong light it is dazzled ; that is, its inter- nal activity cannot keep pace with the ex- ternal excitations ; the nerves are too soon tired. Very bright objects appear almost always to be equally bright even wnen there are, in fact, material diff erences in their lu- minous intensity. The light at tho edge of the sun is only about half as bright as that at the centre, yet none of you will ha^e noticed that, if you have not looked through colored glasses, which reduce the brightness to a con- venient extent. With a weak light the eye is also less sensitive, but from the opposite reason. If a body is so feebly illuminated that we scarcely perceive it, we shall not bo able to perceive that its brightness is les- sened by a shadow by the one hundredth or even by a tenth. It follows from this, that, with moderate illumination, darker objects become more like the darkest objects, while with greater illumination brighter objects, become more like the brightest than should be the case in accordance with Fechner' s law, which holds for mean degrees of illumination. From this results, what, for painting, is an ex- tremely characteristic difference between the impression of very powerful and very feeble illumination. When painters wish to represent glowing sunshine, they make all objects almost equally bright, and thus produce with their moderately bright colors the impression which the sun's glow makes upon the daz- zled eye of the observer. If, on the contrary, they wish to represent moonshine, they only indicate the very brightest objects, particu- larly the reflection of moonlight on shining surfaces, and keep everything so dark as to be almost unrecognizable ; thafc is to say, they make . all dark objects more like the deepest dark which they can produce with their colors, than should be the case in ac- cordance with the true ratio of the luminos- ities. In both cases they express, by their gradation of the lights, the insenritiveneaa of the eye for differences of too brieht or too feeble lights. If they could emplo] the color of the dazzling brightness of full RUP shine, or of the actual dimness of moonlight, *hey would not need to represent the gradatrou of light in their picture other than it is in na- ture ; the picture would then make tho same impression on the eye as is produced by equal degrees of brightness of actual ob- jects. The alteration in the scale of shade which has been described is necessary be- cause the colors of the picture are seen iu the mean brightness of a moderately lighted room, for which Fechner' s law holds ; and therewith objects are to be represented whose brightness is beyond the limits of this law. We find that tho older masters, and pre- eminently Rembrandt, employ the same de- viation, which corresponds to that actually seen in moonlight landscapes ; and this in cases in which it is by no means wished to produce the impression of moonshine, or of a similar feeble light. The brightest parts of the objects are given in these pictures in bright, luminous yellowish colors ; but the shades toward the black are made very mark- ed, so that the darker objects are almost lost ia an impermeable darkness. But this dark- ness is covered with the yellowish haze of powerfully lighted aerial masses, so that, not- withstanding their darkness, these pictures give the impression of sunlight, nud tho very marked gradation of tho shadows, the contours of tho faces and figures, are made extremely prominent. Tho deviation from strict truth to nature is very remarkable in this shading, and yet these pictures give particularly bright and vivid aspects of the objects. Hence they are of particular inter- est for understanding the principles of pic- torial illumination. S14 POPULAR SCIENTIFIC LECTURES. In order to explain these actions we must, I think, consider that while Fechner's law is approximately correct for those mean lights which lire agreeable to the eye, the devia- tions which arc so marked, for too high or too low lights, are not without some influence in the region of the middle lights. We have to observe more closely in order to perceive this influence. It is found, in fact, that when the very finest differences of shade are reproduced on a rotating disk, they are only visible by a light which about corresponds to the illumination of a white paper on a bright day, which is lighted by the light of the sky, but is not diroctly struck by the sun. With such a light, shades of -y^ or T^5 of the total intensity can bo recognized. The light in which pictures are looked ut is, on the con- trary, much feebler ; and if we are to retain the same distinctness of the finest shadows and of the modelling of the contours which it produces, the gradations of shade in the picture must bo somewhat stronger than cor- responds to the exact luminous intensities. The darkest objects of the picture thereby become unnaturally dark, which is however not detrimental to the object of the artist if the attention of the observer is to be directed to the brighter parts. The great artistic effectiveness of this manner shows us that the chief emphasis is to be laid on imitating difference of brightness and not on absolute brightness ; and that the greatest differences in this latter respect can be borne without perceptible incongruity, if only their grada- tions are imitated with expression. m. COLOU. With these divergences in brightness are connected certain divergences in color, which, physiologically, are caused by the fact that th e scale of sensitiveness is different for different colors. The strength of the sensation produced by light of a particular color, and for a given intensity of light, de- pends altogether on the special reaction of that complex of nerves which are set in operation by the action of the light in ques- tion. Now all our sensations of color are admixtures of three simple sensations ; namely, of red, green, and violet, * which, by a not improbable supposition of Thomas Young, can be apprehended quite independ- ently of each other by three different systems of nerve-fibres. To this independence of the different sensations of color corresponds their independence in the gradation of in- tensity. Recent measurements! have shown that the sensitiveness of our eye for feeble shadows is greatest in the blue and least in the red. A difference of -j^ to -^-g of the intensity can be observed in the blue, and with an untired eye of -fa in the red ; or when the color is dimmed by being looked at for a long time, a difference of ^ to -V. Red, therefore, acts as a color toward whose shades the eye is relatively less sensitive than toward that of blue. In agreement with this, the impression of glare, as the intensity increases, is feebler in red than in blue. Ac- cording to an observation of Dovo, if a bio and a red paper be chosen which appear o equal brightness xinder a mean degree c. wnite light, as the light is made much dim- mer the blue appears brighter, and as tin light is much strengthened, the rod. I my- self have found that the same differences ar> seen, and even in a more striking manner, in the red and violet spectral colors, and, wheu their intensity is increased only moderately, by the same fraction for both. Now the impression of white is made up of the impressions which the individual spectral colors make on our eye. If we in- crease the brightness of white, the strength of the sensation for the red and yellow rays will relatively be more increased than that for the blue and violet. In bright white, therefore, the former will produce a rela- tively stronger impression than the latter ; in dull white the blue and bluish colors will have this effect. Very bright white appears therefore yellowish, and dull white appear* bluish. In our ordinary way of looking at the objects about us we are not so readily conscious of this ; for the direct comparison of colors of very different shade is difficult, and wo are accustomed to see in this altera- tion in the white the result of different illu- mination of one and the same white object, so that in judging pigment-colors we havo learned to eliminate the influence of bright- ness. ]f, however, to the painter is put the prob- lem of imitating, with faint colors, white ir- radiated by the sun, he can attain a hi^h de- gree of resemblance ; for by an admixture of yellow in his white he makes this color pre- ponderate just as it would preponderate in actual bright light, owing to the impression on the nerves. It is the same impression as that produced if we lock at a clouded land- scape through a yellow glass, and thereby give it the appearance of a sunny light. The artist will, on the contrary, give a bluish tint to moonlight, that is, a faint white ; for the colors on the picture must, as we have seen, be far brighter than the color to bo rep- resented. In moonshine scarcely any other color can be recognized than blue ; the blue starry sky or blue colors may still appear dis- tinctly colored, while yellow and red can only be seen as obscurations of the general bluish white or gray. I Vfill again remind you that theso changes of color would not bo necessary if the artist had at his disposal colors of the same bright- ness, or the same faintness, as are actually shown by the bodies irradiated by the sun or by the moon. The change of color, like the scale of shade, previously discussed, is a subjective action which the artist must represent objec- tively on his canvas, since moderately bright colors cannot produce them. We observe something quite similar in re-, gard to the phenomena of Contrast. By this term we understand cases in which the color or brightness of a surface appears changed by the proximity of a mass of another colox Aii sci:;:;i*ir:c LIXTUKES. ens or shade, and in such a manner that the original color appears darker by the proxim- ity of a brighter shade, and brighter by that of a darker shade ; while by a color of a different kind it tends toward the comple- mentary tint. The phenomena of contrast are very va- rious, and depend on different causes. One class, Chevreul's ftirrmlianwus Contrast, is inde- pendent of the motions of the eyes, and oc- curs with surfaces where there are very- slight differences in color and shade. This contrast appears both on the picture and in actual objects, and is well known to painters. Their mixtures of colors on the palette often appear quite different to what they are on the picture. The changes of color which are here met with are often very striking ; I will not, however, enter upon them, for they pro- duce no divergence between the picture and reality. The second class of phenomena of con- trast, and one which, for us, is mora impor- tant, is met with in changes of direction of the glance, and more especially between sur faces, in which there are great differences of shade and of color. As the eye glides over bright and dark, or colored objects and sur- fiuvs, the impression of each color changes, tor it is depicted on portions of the retina which directly before were struck by other colors and lights, and were therefore changed in their sensitiveness to an impression. This kind of contrast is therefore essentially de- pendent on movements of the eye, and has been called by Chevreul, " successive Contrast." We have already, seen that the retina is more sensitive in the dark to feeble light than it was before. 13y strong light, on the contrary, it is dulled, and is less sen.iitive to feeble lights which it had before perceived. This latter process is designated as " Fa- tigue" of the retina ; an exhaustion of the capability of the retina by its own activity, just as the muscles by their activity become tired. I must here remark that the fatigue of the retina by light does not necessarily extend to tho whole surface ; but when only a small portion of this membrane is struck by a minute, denned picture it can also be locally developed in this part only. You must all have observed the dark spots which move about in tho field of vision, when we have been looking for only a short time toward the setting sun, and which physiologists call negative after-images of the sun. They are duo to the fact that only those parts of the retina which are actually struck by the image of the sun in the eye, have become insensitive to a new impres- sion of light. If, with an eye which is thus locally tired, we look toward a uniformly bright surface, such as the sky, tho tired parts of the retina aro more feebly and more darkly affected than the other portions, so that the observer thinks he sees dark spots in the sky, which move about with his sight. We have then in juxtaposition, in the bright parts of the sky, the impression which these make upon tho untired parts of the retina, and in the dark spots their action on the tired portions. Objects, bright like the sun, produce negative after-images in tho most striking manner ; but with a little attention they may be seen even after much more moderate impressions of light. A longer time is required in order to develop such an impression, so that it may bo distinctly rec- ognized, and a definite point of the bright object must be fixed, without moving the eye, so that its image may be distinctly formed on tho retina, and only a limited por- tion of the retina be excited and tired, just as in producing sharp photographic portraits the object must bo stationary during the time of exposure in order that its image may not be displaced on the sensitive plate. The after-image in the eyo is, as it were, a photo- graph on the retina, which becomes visible owing to the altered sensitiveness toward fresh light, but only remains stationary for a short time ; it is longer, the more powerful and durable was the action of light If the object viewed was colored, for in- stance red paper, the after-image is of the complementary color on a gray ground ; in this case of a bluish green.* Rose-red paper, on the contrary, gives a pure green after- image, green a rose-red, blue a yellow, and yellow a blue. These phenomena, show that in the retina partial fatigue is possible for tha several colors. According to Thoinan Young's hypothesis of tho existence of three systems of fibres in the visual nerves,* of which one set perceives red whatever the kind of irritation, the second green, and the third violet, with green light, only thoso fibres of the retina which arc sensitive to green are powerfully excited and tired. If this same part of the retina is afterward il- luminated with white light, tho sensation of green is enfeebled, while that of red and vio- let is vivid and predominant ; their sum gives tho sensation of purplo, which mixed with tha unchanged white ground forms rose-red. In the ordinary way of looking at light and colored objects, we arj not accustomed to fix continuously 0110 and tho same point ; for following with the gazo the play of our at- tentiveness, we ara always turning it to new parts of the object as they happen to interest us. This way of looking, in which the eye is continually moving, an;l therefore the retinal image is also shifting about on tha retina, has moreover the advantage of avoid- ing disturbances of sight, which poworful and continuous after-images would bring with them. Yet here also, after-imago* are * In order to nee thin kind of imn','0 as distinctly a" poiuible. it ia desirable tx> avoid all movements of the eye. On n large sheet of dark gray pa|«er ft small bltick cross is draws;, the centre of which ia steadily viewed, and a quadrangular i-lieet of paper <>f that C' lor whose after- imitge is to be observed ia slid from the fide, co that one of its corners touches the CIOKS. The cheet is allowed to remain for a min- ute or two. the. rross being steadilv viewed, and it is then drawn suddenly away, without relaxing the view. In place of the sheet removed the after imag» appears tUcii on the dark ground. 616 POPULAR SCIENTIFIC LECTURES. not wanting ; only tbey are shadowy in tlieir contours, and of very short duration. If a red surface be laid upon a gray ground, and if we look from the red over the edge toward the gray, the edges of the gray will ueem as if struck by such an after-image of red, and will seem to be of a faint, bluish green. But as the after-image rapidly dis- appears it is mostly only those parts of the gray which nro nearest the red which show the change in a marked degree. This also is a phenomenon which is pro- duced more strongly by bright light and brilliant, saturated colors than by fainter light and duller colors. The artist, how- ever, works for the most part with the latter. He produces most of his tints by mixture ; «ach mixed pigment is, however, grayer and duller than the pure color of which it is mixed, and eveu the few pigments of a highly saturated shade which oil-painting can employ, are comparatively dark. The pigments employed in water-colors and col- ored chalks are again comparatively white. Hence such bright contrasts, as are observed in strongly colored and strongly lighted ob- jects in nature, cannot be expected from their representation in the picture. If, therefore, arith the pigments at his command, the artist vishes to reproduce the impression which objects give, as strikingly as possible, he _mst paint tho contrasts which they pro- duce. If the colors on the picture are as brilliant and luminous as in the actual ob- jects, tho contrasts in the former case would produce themselves as spontaneously as in the latter. Here, also, subjective phenomena of the eye must be objectively introduced into the picture, because the scale of color and of brightness is different upon the latter. With a little attention you will see that painters and draughtsmen generally make a plain, uniformly lighted nurface brighter, where it is close to a dark object, and darker, where it i:-? near a light object. You will find that uniform gray surfaces are given a yel- lowish tint at the edye where there is a back- ground of blue, and a rose-red tint where they impinge on green, provided that none of the light collected from the blue or green can fall upon the gray. Where the sun's rays passing through the green leafy shade of trees strike against the ground, they appear to the eye, tired with looking at the predomi- nant green, of a rose-red tint ; the whole daylight, entering through a slit, appears blue, compared with reddish-yellow candle- light. In this way they are represented by the painter, since the colors of his pictures are not bright enough to reproduce the con- trast without such help. To tiuj series of subjective phenomena, which artists are compelled to represent ob- jectively in their pictures, must be asso- ciated certain phenomena of iivadiation. By this is imderstood cases in which any bright object in the fieH spreads its light or color over the neighborhood. The phenomena are the more marked the brighter is the radi- ating object, and the halo is brightest ia the immediate neighborhood o> tlio bright o ject, but diminishes at a greater disumo These phenomena of irradiation nro :no striking around a very bright light on a d:u •] ground. If the view of the flame itself closed by a narrow dark object such as tl: finger, a bright, misty halo appears, whh i covers the whole neighborhood, und, at tl same time, any objects there may be in th dark part of the field of view are seen nioi distinctly. If the flame is partly screened Vi a ruler, this appears jagged where the flan ' projects beyond it. The luminosity in tb neighborhood of the flame is so intense thi its brightness can scarcely be distinguishe from that of the flame itself ; as is the c&f with all bright objects, the flame appeal magnified, and as if spreading over towar the adjacent dark objects. The causa of this phenomenon is quit similar to that of aerial pespective. It is du< to a diffusion of light which arises fron the passage of light through dull media, ex cepting that for the phenomena of aerial per spective the turbidity is to be sought in tht air in front of the eye, while for true phc nomena of irradiation it is to be sought in tht transparent media of the eye. When ever the healthiest human eye is examined \>\ powerful light, the best being a pencil o sunlight concentrated on the side by a con densing lens, it is seen that the sclerotici and crystalline lens are not perfectly clear If strongly illuminated, they both appeal whitish and as if rendered turbid by a tin mist. Both are, in fact, tissues of fibrour structure, and are not therefore so hoinogene ous as a pure liquid or a pure crystal. Ever inequality, however small, in the structure of a transparent body can, however, reflect some of the incident light — that is, can diffuse it in all directions.* The phenomena of irradiation alr>o oocm with moderate degrees of brightness. A darl; aperture in a sheet of paper illuminated b; the sun, or a small, dark object on a colored glass plate which is held against the clear sky, appear as if the color of the adjacen': surfaco were diffused over them. Hence the phenomena of irradiation an very similar to those which produce tin opacity of the air. The only essential differ- ence lies in this, that the opacity by lumin ous air is stronger before distant object which have a greater mass of air in front oi them than before near ones ; while irradia- tion in the eyes sheds its halo uniformly ovei near and over distant objects. Irradiation also belongs to the subjective phenomena of the eye which the artist, repre- sents objectively, because painted lights and painted sunlight are not bright enough t? produce a distinct irradiation in the eye cf the observer. The representation which the painter ha» * I cli*re?ard here tho view thfit irrndmtion in tlia ovu dependu on a diffusion of lh« excitation in the substance of th« nerve*, as this appears to me too hypothetic," I. Moreover, we are here concerned with, tho phc-noineua and not \vitli their cause. POPULAH SCIENTIFIC LECTURES. 617 to give of the lights and colors of his object I have described as a translation, and I have nrged that, as a general rule, it cannot give a 11 copy true in all its details. The altered scale of brightness which the artist must ap- ! ply in many cases is opposed to this. It is 'i not the colors of the objects, but the impres- 3 sion which they have given, or would give, ' which is to be imitated, so as to produce as * distinct and vivid a conception as possible '[of thoso objects. As the painter must ! jchange the scale of light and color in which he executes his picture, he only alters some- thing which is subject to manifold change ; according to the lighting, and the degree of !j ffatigue of the eye. He retains the more es- i sential, that is, tho gradations of brightness I and tint. Here present themselves a series '[ ; of phenomena which are occasioned by the I [manner in which the eye replies to an exter- I pal irritation ; and since they depend upon the intensity of this irritation they are not directly produced by the varied luminous in- tensity and colors of the picture. These ob- jective phenomena, which occur on looking at the object, would be wanting if the painter did not represent them objectively on his canvas. The fact that they are represented is particularly significant for tho kind of problem which is to be solved by a pictorial representation. Nov, in all translations, the individuality of the translator plays a part. In artistic productions many important points are left to the choice of the artist, which he can de- cide according to his individual taste, or ac- cording to the requirements of his subject. Within certain limits he can freely select the absolute brightness of his colors, as well as the strength of the shadows. Like Ilem- brandt, he may exaggerate them in order to obtain strong relief ; or he may diminish them, with Fra Angelico and his modern imi- tators, in order to soften earthly shadows in the representation of sacred objects. Like the Dutch school, he may represent the vary- ing light of the atmosphere, now bright and sunny, and now pale, or warm and cold, and thereby evoke in the observer moods which depend on the illumination and on the state of the weather ; or by means of undisturbed air he may cause his figures to stand out ob- jectively clear as it were, and uninfluenced by subjective impressions. By this means great variety is attained in what artists call " style" or" treatment," and indeed in their purely pictorial elements. IV. HABMONY OP COLOR. We here naturally raise the question : If, owing to the small quantity of light and saturation of his colors, the artist seeks, in all kinds of indirect ways, by imitating sub- jective impressions to attain resemblance to nature, as close as possible, but still imper- fect, would it not be more convenient to seek for means of obviating these evils ? Such there are indeed. Frescoes are some, times viewed in direct sunlight ; transpar- encies and paintings on glass can utilize far higher degrees of brightness, and far moro saturated colors ; in dioramas and in theatri- cal decorations wo may employ powerful ar- tificial light, and, if need be, the electrio light. But when I enumerate these branches of art, it will at once strike you that those works which we admire as the greatest mas- terpieces of painting, do not belong to this class ; but by far the larger number of tho great works of art are executed with tho com- paratively dull water or oil-colors, or at any rate for rooms with softened light. If higher artistic effects could bo attained with colors lighted by tho sun, we should un- doubtedly have pictures which took advan- tage of this. Fresco painting would have led to this ; or the experiments of Munich's celebrated optician Steinheil, which ho made as a matter of science, that is, to produce oil paintings which should be looked at in bright sunshine, would not be isolated. Experiment seems therefore to teach that moderation of light and of colors in pktures is ever advantageous, and we need only look at frescoes in direct sunlight, such as those of tho new Pinakothek in Munich, to learn in what this advantage consists. Their bright- ness is so great that we cannot look at them steadily for any length of time. And what in this case is so painful and so tiring to the eye, would also operate in a smaller degree if, in a picture, brilliant colors wero used, even locally and to p. moderate extent, which wero intended to represent bright sunlight, and a mass of light shed' over the picture. It is much easier to produce an accurate im- itation of tho feeble light of moonshine with artificial light in dioramas and theatra deco- rations. Wo may therefore designate truth to na- ture of ft beautiful picture us an ennobled fidelity to nature. Such a picture repro- duces all that is essential in the impression, and attains full vividness of conception, but without injury or tiring the eye by the nude lights of reality. The differences between art and nature are chiefly confined, as w have already seen, to those matters which w* can in reality only estimate in an uncertain manner, such as tho absolute intensities of light. That which is pleasant to the senses, the beneficial but not exhausting fatigue of our nerves, the feeling of comfort, corresponds in this case, as in others, to those conditions which are most favorable for perceiving the outer world, and which admit of tho finest discrimination and observation. It has been mentioned above that the dis- crimination of the finest shadows, and of the modelling which they express, is the most delicate under a certain moan brightness. I should liko to direct your attention to an- other point which has great importance in painting : I refer to our natural delight in colors, which has undoubtedly a great influ- ence upon our pleasure in the works of the painter. In its simplest expression, as pleas- ure in gaudy flowers, feathers, stones, in fire- works, and Bengal lights, this inclination has POPULAR SCIENTIFIC LECTURES. bnt little to do Tviih man's sense of art; It only appears nn the nat\;ral pleasure of the perceptive organism in the varying and mul- tifarious excitation of its various nerves, •which is necessary for its healthy continu- ance and productivity. But the thorough fitness in the construction of living organ- isms, whatever their origin, excludes the pos- sibility that in the majority of healthy indi- viduals an instinct should be developed or maintain itself •which did not nerve some definite purpose. We have not far to seek for the delight in light and in colors, and for the dread of dark- ness ; this coincides with the endeavor to Kee and to recognize surrounding objects. Darkness owes the greater part of the terror v,-hich it inspires to the fright of -what is un- known and cannot be recognized. A colored picture gives a far more accurate, richer, and easier conception than a similarly executed drawing, which only retains the contrasts of light and shade. A picture retains the lat- ter, but has in addition the material for dis- crimination which colors afford ; by which surfaces which appear equally bright in the drawing, owing to their different color, are now assigned to various objects, or again as alike in color are seen to be parts of the «amc, or of similar objects. In utilizing the relations thus naturally given, the artist, by means of prominent colors, can direct and enchain the attention of the observer upon the chief objects of the picture ; and by the variety of the garments he can discriminate the figures from each other, but complete each individual one in itself. Even the nat- ural pleasure in pure, strongly saturated colors finds its justification in this direction. The case is analogous to that in music, with the full, pure, well-sounding tones of a beau- tiful voice. Such a one is more expressive ; that is, even the smallest change of its pitch, or its quality- -any slight interruption, any tremulousness, any rising or falling in it — is at once more distinctly recognized by the hearer than could be the case with a less reg- ular sound ; and it seems also that the pow- erful excitation which it produces in the ear of the listener arouses trains of ideas and passions more strongly than does a feebler excitation of the same kind. A pure, fun- damental color bears to small admixtures the same relation as a dark ground on which the slightest shade of light is visible. Any of the ladies present will have known how sen- sitive clothes of uniform saturated shades are to dirt, in comparison with gray or gray- ish-brown materials. This also corresponds to the conclusions from Young's theory of colors. According to this theory, the percep- tion of each of the three fundamental colors •rises from the excitation of only one kind of sensitive fibres, while the two others are at rest, or at any rate are but feebly excited. A brilliant, pure color produces a powerful stimulus, and yet, at the same time, a great degree of sensitiveness to the admixture of other colors, in those systems of nerve-fibres which are at rest. The modelling of a e< ored surface mainly depends upon the reflc tion of light of other colors which falls up< them from without. It is more particular when the material glistens that the refle tions of the bright places are preferably the color of the incident light. In the dep of the folds, on the contrary, the colon surface reflects against itself, and therel makes its own color more saturated. A whi surface, on the contrary, of great brightnes produces a dazzling effect, and is thereby ij sensitive to slight degrees of shade. Stror colors thiis, by the powerful irritation whic they produce, can enchain the eye of the ol server, and yet be expressive for theslighte change of modelling or of illumination ; th is, they are expressive in the artistic sense II, on the other hand, wo coat too larp surfaces, they produce fatigue for the pron nent color, and a diminution in sensitivene toward it. This color then becomes mo gray, and on all surfaces of a different col the complementary tint appears, especial on gray or black surfaces. Hence thcrefo clothes, and more particularly curtain which are of too bright a single color, pr duce an unsatisfactory and fatiguing effee the clothes have moreover the disadvantaf for the wearer that they cover face and ban with the complementary color. Blue pr duces yellow, violet gives greenish yellow bright purple gives green, scarlet gives blu and, conversely, yellow gives blue, el There is another circumstance which t artist has to consider, that color is for him a important means of attracting the attentic of the observer. To bo able to do this must be sparing in *ho use of the pure colon otherwise they distract the attention, and t picture becomes glaring. It is necessary, o the other hand, to avoid a one-sided fatig of the eye by too prominent a color. This effected either by introducing the promine color to a moderate extent upon a du slightly colored ground, or by the juxtap sition of variously saturated colors, whi produce a certain equilibrium of irritation the eye, and, by the contrast in their af te images, strengthen and increase each otlu A green surface on which the green afte imago of a purple one falls, appears to be far purer green than without such an afte image. By fatigue toward purple, that toward red and violet, any admixture of the two colors in the green is enfeebled, wh this itself produces its full effect. In th way the sensation of green is purified fro any foreign admixture. Even the purest an most saturated green, which nature shows the prismatic spectrum, mav thus acquire high degree of saturation. We find thus th the other pairs of complementary color which we have mentioned, rnako each oth more brilliant by their contrast, while colo which are very similar are detrimental to ea other, and acquire a gray tint. These relations of the colors to each oth have manifestly a great influence on the c of joleasure which different combinatio POPULAR SCIENTIFIC LECTURES. 619 of ealors afford. Two colors may, without injury, be juxtaposed, which indeed are so similar as to look like varieties of the same color, produced by varying degrees of light and shade. Thus, upon scarlet the more shaded parts appear of a carmine, or on a straw-color they appear of a golden yellow. If we pass beyond these limits, we arrive at unpleasant combinations, such as carmine and orange, or orange and straw-yellow. The ^distance of the colors must then be increased, so as to create pleasing combinations once more. The complementary colors are those which are most distant from each other. When these are combined, such, for instance, as straw-color and ultramarine, or verdigris and purple, they have something insipid but crude; perhaps because we are prepared to expect the second color to appear as an after- image of the first, and it does not sufficiently appear to be a new and independent element in the compound. Hence, on the whole, combinations of those pairs are most pleas- ing in which the second color of the comple- mentary tint is near the first, though with a distinct difference. Thus, scarlet and green- ish blue are complementary. The combina- tion produced when the greenish blue is allowed to glide either into ultramarine, or yellowish green (sap green), is still more pleasing. In the latter case, the combina- tion tends toward yellow, and in the former, toward rose-red. Still more satisfactory com- 'binations are those of three tints which bring about equilibrium in the impression of color, and, notwithstanding the great body of color, avoid a one-sided fatigue of the eye, without falling into the baldness of complementary tints. To this belongs the combination which the Venetian masters used so much — red, green, and violet ; as well as Paul Vero- nese's purple, greenish blue, and yellow. The former triad corresponds approximately to the three fundamental colors, in so far as these can be prodticed by pigments ; the lat- ter gives i ence — is essentially based on the feeling! an easy, harmonic, vivid stream of our ceptions, v/hich, in spite of manifold chang flow toward a common object, bring to li;I laws hitherto concealed, and allow us to g;| in the deepest depths of sensation of ut all our thought and oui action, in tho greatest as well as .in tho least, is based on our c mfidenco ia the unchange- able order of n iture. and this confidence hua hitherto boon uio morn /justified, the deepei 122 POPULAR SCIENTIFIC LECTURES. we have penetrated into the interconnections of natural phenomena. And that the general laws, which we have found, also hold for the most distant vistas of spaco, has acquired wtrong actual confirmation during the past half-century. In the front rank of all, then, is the law of gravitation. The celestial bodies, as you all know, float and move in infinite space. Compared with the enormous distances be- tween them, each of TIS is but as a r--«un of dust. The nearest fixed stars, vie^ oJ even under the most powerful magnification, have no visible diameter ; and we may be sura that even our sun, looked at from the near- est fixed stars, would only appear as a single luminous point ; seeing that the masses of l lios • stars, in so far as they have been de- termined, have not been found to be materially different from that of the sun. But, notwithstanding these enormous dis- t-inof-s, there is an invisible tie between them which connects them together, and brings them in mutual interdependence. This is the force of gravitation, with which all heavy masses attract each other. We know this force as gravity, when it is operative between an earthly body and the mass of our earth. The force which causes a body to fall to the ground is none other than that which con- tinually compels the moon to accompany the earth in its path round the sun, and which keeps the earth itself from fleeing off into space, away from the sun. Yon may realize, by means of a simple mechanical model, the course of planetary motion. Fasten to the branch of * tree, at n Hufficient height, or to a rigid bar, fixed hori- jsontally in the wall, a sili cord, and at its end a small heavy body — for instance, a lead ball. If you allow this to hang at rest, it stretches the thread. This is the position of equilibri- um of the ball. To indicate this, and keep it visible, put in the place of the ball any other solid body — for instance, a large terres- trial globe on a stand. For this purpose the ball must bo pushed aside, but it presKea against tho globe, and, if taken away, it still tends to come back to it, because gravity im- pels it toward its position of equilibrium, which i.s in the centre of the sphere. And upon whatever wide it is drawn, the same thing always happens. This force, which drives the ball toward the globe, represents in our model the attraction which the earth «?xerts on the moon, or the sun on the plan- ets. After you have convinced yourselves of the accuracy of these facts, try to give the Ixall, when it is a little away from the globe, a slight throw in a lateral direction. If you have accurately hit the strength of the throw, the small ball will move round the large one in a circular path, and may retain thin motion for S3iu3 time ; just as the moon persists in its coursa round the earth, or the planets about the sun. Now, in our model, the cir- cles described by the lead ball will bo con- tinually narrower, because the opposing forces, tho resistance of the air, the rigidity of the thread, friction, cannot be eliminated, in this case, as they are excluded in the plan- etary system. If the path about tho attracting centre is exactly circular, tho attracting force always acts on the planets, or on the lead sphere, with equal strength. In this case, it is im- material according to what law the fore* would increase or diminish at other distance* from the centre in which the moving body does not come. If the original impulse ha* not been of the right strength in both case*, the paths will not be circular but elliptical, of the form of the curved line in Fig. 3. But these ellipses lie in both cases differently as regards the atiractiug centre, lu our model, the attracting force is stronger, the farther the lead sphere is removed from its position of equilibrium. Under these circumstances, the ellipse of the path has such a position in reference to the attracting centre, that this, is in th« centre, c, 01 the ellipse. For planets, on the contrary, tho attracting force is feebler the farther it is removed from the attracting body, and this is tho reason that an ellipsa in described, one of whose foci lies in th« contra of attraction. The two foci, a and b, aro two points which lio symmetrically toward tho ends of tho ellipse, and are characterized by tho property that the sum of their distances, am -f- bin, is the same t'rou* any given points. Kepler had found that the paths of th« planets are ellipses of thin kind ; and since, as tho above example shows, tho form ami position of th« orbit depend on tho law a"- cording to which the magnitude of the at- tracting force alters, Newton could deduce from the form of tho planetary oibits th* well-known law of tho force of gravitation, which attracts the planets to the sun, accord- ing to which this forc-j decreases with in- crease of distance as the square of that dis- tance. Terrestrial giavity must obey tliin law, and NoM'ton had tho wonderful self- denial to refrain from publishing his impor- tant discovery until it had acquired a direct eonnrmation ; this followed from the obser- POPULAR SCIENTIFIC; LECTURES. 823 vations, that the force which attracts the moon toward the earth, bears toward the gravity of a terrestrial body the ratio required by the above law. In the course of the eighteenth century tho power of mathematical analysis, and tho methods of astronomical observation, in- creased BO far that all the complicated actions, which take place between all tho planets, and all their satellites, in conse- quence of the mutual action of each upon t-ach,and which astronomers call disturbances - -disturbance, that is to say, of the simpler elliptical motions about the sun, which each one would produce if the others were absent — that all these could be theoretically pre- dicted from Newton' s law, and be accurately compared with what actually takes place in the heavens. The development of this theory of planetary motion in detail was, as has been said, the merit of Laplace. The agree- ment between this theory, which was de- veloped from the simple law of gravitation and the extremely complicated and manifold phenomena which follow therefrom, was BO complete and so accurate, as had never pre- viously been attained in any other branch of human knowledge. Emboldened by thia agreement, the next step was to conclude that where slight defects wer.e still constantly found, unknown causes must be at work. Thus, from Besscl's calculation of the dis- crepancy between the actual and the calculat- ed motion of Uranus, it was inferred that there must be another planet. The position of this planet was calculated by Leverrier and Adams, and thus Neptune, the most dis- tant of all known at that time, was discovered. But it was not merely in the region of the xttraction of our sun that the law of gravita- ion was found to hold. With regard to the led stars, it was found that double stars noved about each other in elliptical paths, jjid that therefore the same law of gravitation must hold for them as for our planetary sys- tem. The distance of some of them could be calculated. The nearest of them, a, in the constellation of the Centaur, is 1,039.600 miles farther from (he sun than the earth. Light, which has a velocity of 186,000 miles a second, which traverses the dis- tance from tli« sun to the earth in eight minutes, would take three years to travel from a Centauri lo us. The more deli- cate methods of modern astronomy havu made it possible to determine distances which light would take thirty-five years to traverse ; as, for instance, the Pole Star ; but the law of gravitation is seen to hold, ruling the motion of the double stars, at distances in the heavens which all the means we possess have hitherto utterly failed to measure. The knowledge of the law of gravitation has here also led to the discovery of new bodies, as in tho case of Neptune. Peters of Altona found, confirming therein a conjee- tare of BeKsel, that Sirius, the most brilliant of tho fixed Ktars, moves in an elliptical path about an invisible centre. This must have been due to an unseen companion, and wheo the excellent and powerful telescope of .tho University of Cambridge, in the United States, had been set up, this was discovered. It is not quite dark, but its light is so feeble that it can only be seen by the most perfect instruments. The mass of Sirius is found to be 13-76, and that of its satellite 6-71, timm the mass of the sun ; their mutual distance is equal to thirty-seven times the radius of th;» earth's orbit, and is therefore somewhat large? than the distance of Nf une from the sun. Another fixed star, P\ :yon, is in tho sani* case as Sirius, but its ; tellite has not y?i been discovered. You thus see that in gravitation we havo discovered a property common to an matte*, which is not confined to bodies in our syn- tem, but extends, as far in the celestial spac^ as our means of observation have hitherto been able to penetrate. Bat not merely is this universal property of'all mass shared by the most distant celes- tial bodies, as well as by terrestrial ones , but spectmm analysis has taught us that ft number of well-known terrestrial element^ are met with in the atmospheres of the fixed stars, and even of the nebuls9. You all Imow that a fine bright line ot light, seen through a glass prism, appears ;•* a colored band, red and yellow at one edge, blue and violet at the other, and green in tho middle. Such a colored image is called n spectrum — the rainbow is such a one, pro- duced by the refraction of light, though not exactly by a prism ; and it exhibits therefore the series of colors into which white sunlight can thus be decomposed. The formation of the prismatic spectrum depends on the fact that the sun's light, and that of most ignited bodies, is made up of various kinds of lights which appear of different colors to our eye*, and the rays of which are separated from each other when refracted by a prism. Now if a solid or a liquid is heated to such an extent that it becomes incandescent, the spectrum which its light gives is, like the rainbow, a broad colored band without any breaks, with the well-known series of colors, red, yellow, green, blue, and violet, and in no wise characteristic of tho nature of the body which emits the light. The case is different if the light is emitted by an ignited gas, or by an ignited vapor- that is, a substance vaporized by heat. The spectrum of such a body consists, then, of one or more, and sometimes even fr great number, of entirely distinct bright lines, whose, position and arrangement ia the spec- trum is characteristic for the substances of which the gas or vapor consists, ao that it can be ascertained, by means of spectrum analysis, what is tho chemical constitution of the ignited gaseous, body. Gaseous spectra of this kind are shown in the heavenly space by many nebulro ; for the most part they are spectra which show tho bright line of ignited hydrogen and oxygen, and along with it a lino which, as yet, has never been again found in the spectrum of any terrestrial el&- 634 POPULAR SCIENTIFIC LECTURES. FIG. 4. meat. Apart trom tho proof of two well- . known terrestrial elements, this discovery vas of the utmost importance, since it fur- nished the first unmistakable proof that the cosmical nobuke aro not, for the most part, small heaps of fine stars, but that the greater part of the light which they emit is really due to gaseous bodies. The gaseous spectra present a different ap- pearance when the gas is in front of an ignited solid whose temperature is far higher than that of the gas. The observer sees then a continuous spectrum of a solid, but traversed by tine dark lines, which are just visible in the places in which the gas alone, seen in front of a dark background, would show bright lines. Tho solar spectrum is of this kind, and also that of a great number of fixed stars. The dark lines of the solar spec- trum, originally discovered by Wollaston, •were first investigated and measured by Fraunhofer, and are hen."e known as Fraun- hofer's lines. Tar more powerful apparatus was after- ward nsed by Kirchhoff, and then by Angstrom, to push tho decomposition of light as far as possible. Fig. 4 represents an apparatus willi four prisms, constructed by Kteinheil for Kirchhotf. At the further end of the telescope B is a screen with a fine slit, representing a fine slice of light, which can be narrowed or widened by tho small screw, and by which tho light under investigation can be allowed to enter. It then passes through the telescope B, afterward through the four prisms, and finally through the tele- scope A, from which it reaches the eye of the observer. Figs. 5, (5, and 7 represent .smell portions of the solar spectrum as mapped by Kirchhoff, taken from the green, yellow, and golden-yelloMr, in which the chemical symbols below — Fe (iron), Ca (calcium \ Na (sodium \ Pb (lead) — and the affixed lines, indicate the positions in which the vapors of these metals when made incandescent, either in the flames or in the electrical spark, would show bright lines. The numbers above their, show how far these fractions of Kirchhoffa map of the whole system are apart from each other. Here, also, we see a predominance of iron lines. In the whole spectrum Kirch - boff found not less than 450. It follows from this, that the- solar atmos- phere contains an abundance of the vapon Fto. 5. Put. «. nf iron, which, by the way, Justifies us in concluding what nn enormously high temper- ature must prevail there. It shows, more- .<\ or, how our Figs. 5, C, and 7 indicate iron, calcium, and sodium, and also the pres- ence of hydrogen, of zinc, of copper, and of the metals of magnesia, alumina, baryta, and >th-r terrestrial elements. Lead, on the btLer hand, is wanting, as well as gold, silver, mercury, antimony, arsenic, and some others. The spectra of several fixed stars are simi- larly constituted ; they show systems of fine lines which can be identified with those of terrestrial elements. In the atmosphere of Aldebaran in Taurus there is, again, hydro' gen, iron, magnesium, calcium, sodium, and also mercury, antimony, and bismuth ; and, according to H. C. Vogel, there is in a Orionis the rare metal thallixim ; and so on. We cannot, indeed, say that we have ex- plained all spectra ; many fixed stars exhibit peculiarly banded spectra, probably belong- ing to gases whose molecules have not been completely resolved into their atoms by the high temperature. In the spectrum of the sun, also, are many lines which we cannot identify with those of terrestrial elements. It is possible that they may be due to sub stances unknown to us, it is also possible that they are produced by the excessively high temperature of the sun, ff.r transcend- ing anything we can produce. But this is certain, that the known terrestrial substances are widely diffused in space, and especially nitrogen, which constitutes the greater part nf onr atmosphere, and hydrogen, an element in water, which indeed is formed by its com- bustion. Both have been found in the irre solvable nebula?, and, from the inalterability of their shape, these must be musses of enormous dimensions and at an «n<>rimms distance. For this reason Hir W. jit>j-scuu POPULAII SC::::TTIFIC ^^^IOJL.^O. 625 considered that they did not belong to the system of our fixed stars, but were represent- atives of the manner in which other systems manifested themselves. Spectrum analj-sis has further taught us more about the sun, by which he is brought nearer to us, as it were, than could formerly have seemed possible. You know that the sun is an enormous sphere, whoso diameter is 112 times an great as that of the earth. We may consider what we see on its surface as a layer of incandescent vapor, which, to judge from the appearances of the sun-spots, has a depth of about 500 miles. This layer of vapor, which is continually radiating heat on the outside, and is certainly cooler than the inner masses of the sun, is, however, hotter than all our terrestrial flames — hotter even than the incandescent carbon points of the electrical arc, which represent the highest temperature attainable by terrestrial means. This can be deduced with certainty from Kirchhoffs law of the radiation of opaque bodies, from the greater luminous intensity of the sun. The older assumption, that the sun is a dark cool body, surrounded by u photosphere which only radiates heat anil light externally, contains a physical impossi- bility. Outside the opaque photosphere, the sun appears surrounded by a layer of transparent gases, which are hot enough to show in the spectrum bright colored lines, and are hence called the Chromosphere. They show tho bright lines of hydrogen, of sodium, of mag- nesium, and iron. In these layers of gas and of vapor about the sun enormous stormw occur, which are as much greater than those of our earth in extent and in velocity as the sun is greater than the earth. Currents of ignited hydrogen burst out several thousands of miles high, like gigantic jets or tongues of flame, with clouds of smoke above them.* These structures could formerly only be viewed at the time of a total eclipse of the sun, forming what were called the rose-red protuberances. We now possess a method, devised by MM. Jansen and Lockyer, by which they may at any time be seen by the aid of the spectroscope. 9 On the other hand, there vre individual darker parts on the sun's surface, what are called sun-spots, which were seen as long ago as by Galileo. They are funnel-shaped, the sides of the funnel are not so dark as the deepest part, the core. Fig. 8 represents such a spot according to Padre Secchi, as seen under powerful magnification. Theii diameter is often more than many tens oJ thousands of miles, so that two or three earths could lie in one of them. These spots may stand for woeks or months, slowly chang ing, before they are again resolved, and mean' while several rotations of the sun may takv place. Sometimes, however, there are very rapid changes in them. That the core if il.'cpor than the edge of the surrounding penumbr.i follows from their respective di» placements as they come near the edge, arid uro therefore seen in a very oblique direction. ii«'lC UiUTUllES. FIG. If I Fig. 9 represents in A to E the different aspects of such a spot as it cornea iiear the odgc of the sun. Just on the edge of these spots there are spectroscopic indications of the most violent motion, and in their vicinity there are often large protuberances ; they show compara- tively often a rotatory motion. They may bo considered to he places where the cooler gases from the outer layers of the sun's at- mosphere sink down, and perhaps produce local superficial coolings of the sun's mass. To understand the origin of these phenomena, it must be remembered that the gases, as they rise from the hot body of the sun, are charged with vapors of difficultly volatile • According to H. C. Vogel's observations In Both- kamp, to a heizht of 70,000 miles. Tho spcctro«c«pic displacement of the lines showed yeUvitiex of IB to n mile* in a second ; and, according to Lockyer, of even 87 to 48 miles. metals, whlcn expand as they n«cencf, and partly by their expansion, aud partly bv radiation into space, must become cooled. At the same time, they deposit their morn difficultly volatile constituents as fog or cloud. This cooling can only, of course, ba regarded as comparative ; their temperature is probably, even then, higher than any tem- .pe.rature attainable on the earth. If now tho tipper layers, freed trom the heavier vapors, sink down, there will be a space over th« sun's body which is free from cloud. They appear then as depressions, because about them are layers of ignited vapors a ) much aa 500 miles in height. Violent storms cannot fail to occur in th« sun's atmosphere, because it is cooled on the outside, and the coolest and comparatively densest and heaviest parts come to lio over (he hotter and lighter ones. This is the reason w>>v we have frequent, and at tim«Q POPULAR SCIENTIFIC LECTURES. fV37 •ndden and violent, movements in the earth's fctmosphere, becanse this is heated from tho ground made hot by the sun and is cooled above. With the far more colossal magnitude mid temperature of the sun, its meteorologi- cal processes are on a far larger scale, and are fur more violent. We will now pass to the question of the permanence of the present condition of our wystein. For a long time the view was pretty generally held that, in its chief features at uny rate, it was unchangeable. This opinion was based mainly on the conclusions at which Laplace had arrived as the final results of his long and laborious investigations, of the influence of planetary disturbances. By disturbances of the planetary motion astron- omers understand, as I have already men- tioned, those deviations from the purely elliptical motion which are due to the attrac- tion of various planets and satellites upon each other. The attraction of the sun, as by far the largest body of our system, is indeed the chief and preponderating force which produces the motion of the planets. If it alone were operative, each of the planets would move continuously in a constant ellipse whose axes would retain the same direction and the same magnitude, making the revolu- tions always in the same length of time. J;ut, in point of fact, in addition to the at- traction of the sun there are the attractions of all other planets, which, though umall, yet, in long periods of time, do effect slow changes in the plane, the direction, and the magnitude of the axes of its elliptical orbit. It has been asked whether these attractions in the orbit of the planet could go BO far as to cause two adjacent planets to encounter each other, KO that individual ones fall into the sun. Laplace was able to reply that this could not be the case ; that all alterations in the planetary orbits produced by this kind of disturbance must periodically increase and decrease, and again revert to a mean con- dition. But it must not be forgotten that this result of Laplace's investigation!! only applies to disturbances due to the reciprocal attraction of planets upon each other, and on the assumption that no forces of other kinds have any influence on their motions. On our earth we cannot produce such an everlasting motion as that of the planets seems to be ; for resisting forces are contin- ually being opposed to all movements of ter- restrial bodies. The best known of these nre what we call friction, resistance of the air, and inelastic impact. Henco the fundamental law of mechanics, according to which every motion of a body on which no force acts goes on in a straight line forever with unchanged velocity, never holds fully. Even if wo eliminate the influence of gravity in n ball, for example, which rolls on a plane surface, we see it go on for a while, and the farther the smoother is tho path ; but at tho same time wo hear tho rolling bail xaake a clattering sound — that is, it produces waves of Bound in the surrounding bodieg ; there in friction even on tho smoothest sur- face ; this set:i tho surrounding air in vibra- tion, and imuarts to it some of its own motion. Thus it happens that its velocity is continually less and less until it finally ceases. In like manner, even the most care- fully constructed wheel which plays upon fine points, once made to turn, goes on for a quarter of an hour, or even more, but then stops. For there is always some friction on tho axles, and in addition there is the resist- ance of the air, which resistance is mainly due to that of the particles of air against each other, due to their friction against tha wheel. If we could once set n body in rotation, and ki^ep it from falling, without its being supported by another body, and if we could transfer the whole arrangement to an abso- lute vacuum, it would continue to move for- ever with undiminished velocity. This case, which cannot be realized on terrestrial bodies, is apparently met with in tho planeU with their satellites. They appear to move in tho perfectly vacuous cosmical space, without contact with any body which could produce friction, and hence their motioij seems to be one which never diminishes. You see, however, that the justification of this conclusion depends on the question whether cosmical space is really quito vacu- ous. Is there nowhero any friction in th*» motion of the planets ? From the progress which the knowledge of nature has made since tho timo of Laplaco, we must now answer both questions in the negative. Celestial space is not absolutely vacuous. In the first place, it i < filled by that continu- ous medium the agitation of which consti- tutes light and radiant heat, and which phys- icists know as th3 luminifertms ether. In the second place, large and small fragments of heavy mutter, from the size of huge stone* to that of dust, are still everywhere) scat- tered ; at any rate, in those parts of Hpaca which our earth traverses. The existence of the luminiferous ether cannot be considered doubtful. That light and radiant heat are due to a motion whiub spreads in all directions has been sufficiently proved. For the transference of such a motion through space there must bo some- thing which can be moved. Indeed, from the magnitude of the action of this motion, or from that which the science of mechanic.! calls its i:t.« iui:a, we may indeed assign certain limits for tho density of this medium. Such a calculation has been made by Sir W. Thom- son, the celebrated Glasgow physicist. II* lias found that the density may possibly ba far less than that of the air in tho most per- fect exhaustion obtainable by A good air- pump ; but that the mass of the cthor can- not _bo_abjspluteVy_t:>iiil to x.oro. A voinui't * This calculation would, however, lost it* bnm-a if Mnxwcii'H hvpoi he-is wore cmitlrinrd. nixonling Jx» which li^ht tU-iiuida on electrical aiul raa^ueticAlcNi • tUuuouj, 138 POPULAR SCIENTIFIC MATURES. equal to that of tho earth cannot, contain less than 2775 pounds of luminous ether.* The phenomena in celestial space are in conformity with this. Just as a heavy stone flung through the air shows scarcely any in- fluence of the resistance of the air, while a light feather is appreciably hindered ; in like manner tho medium which fills space is far too attenuated for any diminution to have been perceived in tins notion o£ lLo planets since the time in which we possess astronomical observations of their path. It is different with the smaller bodies of our system. Encke in particular has Bhown, with reference to the well-known small comet which bears his name, that it circulates round the sun in ever-diminishing orbits and in ever shorter periods of revolu- tion. Its motion is similar to that of the circular pendulum which wo have men- tioned, and which, having its velocity grad- ually delayed by the resistance of the inr, de- scribes circles about its centre of attraction, which continually become smaller and smaller. The reason for this phenomenon is the following : The force which offers a re- sistance to the attraction of the sun on all comets and planets, and which prevents them from getting continually nearer to the sun, is what is called the centrifugal force — that is, the tendency to continue their motion in a straight line in the direction of their path. As'the force of their motion diminishes, they yield by a corresponding amount to the at- traction of tho sun, and get nearer to it. If the resistance continues, they will continue to get nearer the sun until they fall into it. Encke' s comet is no doubt in this condition. But tho resistance whoso presence in spaco is hereby indicated, must act, and has long continued to act, in the same manner on tho far larger masses of the planets. Tho presence of partly fine and partly coarse heavy masses diffused in cosinical space is more distinctly revealed by tho phe- nomena of asteroids and of meteorites. Wo know now that these are bodies which ranged about in cosmical space, before they caiue within the region of our terrestrial atmos- phere. In the more strongly resisting medi- um which this atmosphere offers they are delayed in their motion, and at the same time are heated by the corresponding friction. Many of them may still find an escape from the terrestrial atmosphere, and continue their path through space with an altered and retarded motion. Others fall to the earth ; the larger ones as meteorites, while tho Hmaller ones arc probably resolved into dust by the heat, and iw such fall without being Keen. According to A2"\.»r.der Ilerschel'-s estimate, we may figure shooting-stars a.-< be- ing on an average of tho samo»>i/e as paving- Ktuncs. Their incandescence mostly occurs in the higher and most attenuated regions of the atmosphere, eighteen miles and more above the surface of tho earth. As they move in spaco under tho influence of tho name laws as the planets and comets, they possess a planetary velocity of from eighteen to forty miles In a second. By this, aTeo, wo observe that they aro in fact steUe caiientv, falling stars, as they have long been culled by poets. This enormous velocity with which they" enter our atmosphere is undoubtedly t)i 3 cnuso of their becoming heated. You nil know that friction heats the bodies rubbed. Every match that we ignite, every badly greased coach-wheel, every auger which we work in hard wood, teaches this. The air, like solid bodies, not only becomes heated by friction, but also by the work consumed in its compression. One of tho most im- portant results of modern physics, the actual proof of which is mainly due to tho English- man Joule, is that, in such a case, tho heat developed is exactly proportional to the work expended. If, like tho mechanicians, wo measure tho work done by tho weight which would be necessary to produce it, mul- tiplied by the height from which it must fall, Joule has shown that tho work, produced by a given weight of water falling through .1 height of 425 metres, would be just sufficient to raise tho same weight of water through ono degree Centigrade. The equivalent i:i work of a velocity of eighteen to twenty -four miles in a second may bo easily calculated from known mechanical laws ; and this, transformed into heat, would bo sufficient to raiso the temperature of a picco of r.iettorio iron to 900,000 to 2,500,000 degrees Cen- tigrade, provided that all tho heat were re- tained by the iron, and did not, us it un- doubtedly does, mainly pass into tLo air. This calculation shows, at any rate, that tho velocity of the shooting-stars is perfectly ad- equate to raiso them to tho most violent in- candescence. Tho temperatures attainable by terrestrial means scarcely exceed 2000 de- grees. In fact, the outer crusts of meteoric stones generally show truces of incipient fusion ; and in cases in which observers ex- amined with sufficient promptitude tho stones which had fallen they found 1 hem hot on tho surface, while the interior of detached pieces seemed to show tho intense cold of cosmical space. To the individual observer who casually looks toward tho starry bky the meteorites appear as u rare and exceptional phenome- non. If, however, they are continuously observed, they aro seen with tolerable regxi- larity, especially toward morning, when they usually fall. But a single observer only views but u. small part of the atmosphere ; and if they are calculated for the entire sur- face of the earth, it results that about seien and a half millions fall every day. In our re- gions of space, they are somewhat sparse and distant from each other. According to Alex- ander Herschel's estimates, each Ktono is, on an average, at a distance of 450 miles from its neighbors. But tho earth moves through 18 miles every second, and has a diameter of 7820 miles, and therefore sweeps through 87'» millions of cubic miles of space every second, and curries with it whatever stones aro con- tained therein. LIuny groups are irregularly distributed .OPULAU sci:::rnnc LIXTURES. CS9 in space, being probably thoso vliicli hnvo ftlready undergone disturbances by planets. There are also denser swarms which move in regular elliptical orbits, cutting the earth's orbit in definite places, and therefore always occur on particular days of the year. Thus the 10th of Augiist of each year is remarkable, and every thirty-three years the splendid fire- works of* the 12th to the 14th of November repeats itself for a few years. It is remark- able that certain comets accompany the paths of these swarnis, and give rise to the suppo- sition that the comets gradually split up into meteoric swarms. This is an important process. What tho earth does is dona by the other planets, and in a far higher degree by the sun, toward which all the smaller bodies of our system must fall ; those, therefore, that are more subject to the influence of tho resisting medi- um, and which must fall the more rapidly, the smaller they are. The earth and tho planets have for millions of years been sweep- ing together the loose masses in space, and they hold fast what they have once attracted. But it follows from this that the earth and the planets were once smaller than they are now, wl that more mass wrw diffused in Bpaco ; and if we follow out this considera- tion it takes us back to a state of things in which, perhaps, all the mass now accumulat- ed in the sun and in the planets, wandered loosely diffused in space. If wo consider, further, that the small masses oi! meteorites as they now fall, have perhaps been formed by the gradual aggregation of fine dust, wo BOO ourselves led to a primitive condition of fine nebulous masses. From this point of view, that tho fall of ehooting-stars and of meteorites is perhaps only a small survival of a process which onco built up worlds, it assumes far greater sig- nificance. This would be a supposition of which \vo might admit the possibility, but which could not perhaps claim any great degree of prob- ability, if we did not find that our prede- cessors, starting from quite different con- siderations, had arrived at the same hypoth- ecis. You know that a considerable number of planets rotate around tho sun besides tho eight larger ones, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Nep- tune ; in the interval between Mara and Ju- piter there circulate, as far as we know, 156 small planets or planetoids. Moons also rotate about the larger planets — that is, about the Earth iind the four most distant ones, Jupiter, Saturn, Uranus, and Neptune ; and lastly tho Sun, and at any rate tho larger planets, rotate about their own axes. Now, in tho first place, it in remarkable that all the pianos of rotation of the planets and of their satellites, as well as the equatorial planes of these planets, do not vary much from each other, and that ill those planes all the rotation is in tho same direction. Tho only considerable exceptions known are tho moons of Uranus, whoso plane i.* almost ut right angles to the planes of the larger plan- ets. It must at the same time be remarked that the coincidence, in tho direction of these planes, is on tho whole greater, the longer are the bodies and the larger the paths in question ; while in the smaller bodies, and for the smaller paths, especially for tho rotations of the planets about their own axes, considerable divergences occur. Thus tho planes of all the planets, with the exception of Mercury and of the small ones between Mars and Jupiter, differ at most by three degrees from the path of the Earth. The equatorial plane of tho Sun deviates by only seven and a half degrees, that of Jupiter only half as much. The equatorial piano of the Earth deviates, it is true, to the extent of twenty-throe and a half degrees, and that of Mars by twenty-eight and a half degrees, and the separate paths of the small planet's satel- lites differ still more. But in these paths they all move direct, all in the same direction about tho sun, and, as far as can be ascertained, also about their own axes, like the earth— that is, from west to east. If they had originated independently of each other, and had come together, any direction of the planes for each individual one would have been equally probable ; a reverse direction of tho orbit would have been just as probable as a direct one ; decidedly elliptical paths would have been as probable as the almost circular ones which we meet with in all the bodies we have named. There is, in fact, a complete irregularity in the comets and meteoric Gwarms, which we have much reason for con- sidering to be formations which have only accidentally come within the sphere of tha sun's attraction. Tho number of coincidences in the orbits of tho planets and their satellites is too great to be ascribed to accident. We must in- quire for the reason of this coincidence, and this can only be sought in a primitive con- nection of the entire mass. Now, we are ac- quainted with forces and processes which condense an originally diffused mass, but none which could drive into space such largo masses, as the planets, in the condition we now find them. Moreover, if they had be- come detached from the common mass, at a placo much nearer the sun, they ought to have a markedly elliptical orbit. We must assume, accordingly, that this mass i:i its primitive condition extended at least to tho orbit of the outermost planats. These were the essential features of th> considerations which led Kant and Laplaco to their hypothesis. In their view our sys- tem was originally a chaotic ball of nebulou.i matter, of which originally, when it extended to the path of the most distant planet, many billions of cubio miles could contain scarcely a gramme of mass. This ball, when it had become detached from tho nebulous balls of fue adjacent fixed rtars, possessed it slow movement of rotatiof It became condensed under the influence 1 lhe reciprocal attrac- tion of its parts ; an \ the degree in which it condensed, tho rr motion increased, 830 POPULAR SCIENTIFIC LECTURES. and formed it into a flat disk. From time to time masics at the circumference of this disk became detached under the influence of the increasing centrifugal force ; that •which be- came detached formed again into n rotating nebulous mass, which either simply con- densed and formed a planet, or during this condensation again repelled masses from the periphery, which became satellites, or in one case, that of Saturn, remained as a coherent ring. In another case, the mass which sep- arated from the outside of the chief ball, di- vided into many parts, detached from each other, and furnished the swarms of small planets between Mars and Jupiter. Our more recent experience as to the na- ture of star showers teaches us that this pro- cess of the condensation of loosely diffused masses to form larger bodies is by no means complete, but still goes on, though the traces are slight. The form in which it now ap- pears is altered by the fact that meanwhile the gaseous or dust like mass diffused in space had united under the influence of the force of attraction, and of the force of crys- tallization of their constituents, to larger pieces than originally existed. The showers of stars, as examples now tak- ing pluce of the process which formed the heavenly bodies, are important from another point of view. They develop light and heat ; and that directs us to a third series of con- siderations, which leads again to the same goal. All life and all motion on our earth is, with few exceptions, kept up by a single force, that of the sun's rays, which bring to xts light and heat. They warm the air of the hot zones, this becomes lighter and ascends, while the colder air flows toward the poles. Thus is formed the great circulation of the passage - winds. Local differences of tem- perature over land and sea, plains and moun- tains, disturb the uniformity of this great motion, and produce for us the capricious change of winds. Warm aqueous vapors ascend v/ith the warm air, become condensed into clouds, and fall ia the cooler zones, and upon tho snowy tops of the mountains, as rain and as snow. The water collects in brooks, in rivers, moistens the plains, and makes life possible ; crumbles the stones, carries their fragments along, and thus works at the geological transformation of the earth's surface. It is only under the influence, of the sun's rays that the variegated covering of plants of the earth grows ; and while they grow, they accumulate in their structure organic matter, which partly serves tho whole animal kingdom as food, and serves man more particularly as fuel. Coals and lignites, the sources of pcwer of our steam-engines, are remains of primitive plants, the ancient production of the sun's rays. Need we wonder if, to our forefathers of the Aryan race in India and Persia, the sun appeared as the fittest symbol of the Deity ? They were right in regarding it as the giver of all life— as the ultimate source of almost »11 that has happened on earth. But whence does tho sun acquire thla force ? It radiates forth a more intense light than can bo attained with any terrestrial means. It yields as much heat as if 1500 pounds of coal were burned every hour upon each square foot of its surface. Of tho heat which thus issues from it, tho small fraction which enters our atmosphere furnishes a great mechanical force. Every steam-engine teaches us that heat can produce such force. The sun, in fact, drives on earth a kind of steam - engine whose performances are far greater than those of artificially constructerl machines. The circulation of water in the atmosphere raises, as has been said, tho water evaporated from the warm tropical seas to the mountain heights ; it is, as it were, a water-raising engine of the most magnifi- cent kind, with whose power no artificial machine can bo even distantly compared. I have previously explained the mechanical equivalent of heat. Calculated by that standard, the work which the sun produces by its radiation is equal to the constant ex- ertion of 7000 horse-power lor each square foot of the sun's surface. For a long time experience had impressed on our mechanicians that a working force cannot be produced from nothing ; that it can only be taken from the stores which na- ture possesses ; which are strictly limited, and which cannot be increased at pleasure — whether it be taken from the rushing water or from tho wind ; whether from the layers of coal, or from men and from animals, which cannot work without the consumption of food. Modern physics has attempted to prove the universality of this experience, to show that it applies to the great whole of all natural processes, and is independent of the special interests of man. These have been generalized and comprehended in the all- ruling natural law of the Conservation of Force. No natural process, and no series of natural processes, can be found, however manifold may be the changes which take place among them, by which a motive force can be continuously produced without a corresponding consumption. Just as tho human race finds on earth but a limited sup- ply of motive forces, capable of producing work, which it can utilize but not increase, so also must this be the case in the great whole of nature. The universe has its definite store of force, which works in it \inder ever- varying l<>nas ; in indestructible, not to be i ; ; •. n -JIM •< I, everlasting and unchangeable like mutter itself. It seems as if Goethe had an idea of this when he makes the earth-spirit speak of himself as the representative of natural force. In the current* of life, in the tempests of motion, lu tho ii.-rvor • f nrt. in the fire, in the storm, liithcr and thilher, 'Her und under. Wend 1 and wander. Jiinh and the grave, LtanltlaM ocean, Where the restless wave Undulates ever Under and over. Their seething utrifo POPULAR SCIENTIFIC LECTURES. 631 Heaving M>«1 weaving The ehaogM of life. At the whirling loom of time unawcd, I work the living mantle of God. Let ns return to the special question which concerns us here : Whence does the sun de- rive this enormous store of force which it nends out ? On earth the processes of combustion arc the most abundant source of heat. Does the sun's heat originate in a process of this kind ? To this question we can reply with a complete and decided negative, for we now know that the sun contains the terrestrial elements with which we arc acquainted. Let us select from among them the two, which, for the smallest mass, produce the greatest amount of heat when they combine ; let us assume that the sun consists of hydro- gen and oxygen, mixed in the proportion in which they would unite to form water. Tho mass of the sun is known, and also the quan- tity of heat produced by the union of known weights of oxygen and hydrogen. Calcula- tion shows that under the above supposition, the heat resulting from their combustion would be siifficient to keep up the radiation of heat from the sun for 3021 years. That, it is true, is a long time, but even profane history teaches that the sun has lighted and warmed us for 3000 years, and geology puts it beyond doubt that this period must be ex- tended to millions of years. Known chemical forces are tHus so com- pletely inadequate, even on the most favor- able assumption, to explain the production of heat which takes place in the sun, that we must quite drop this hypothesis. We must seek for forces of far greater magnitude, and these we can only find in cosmical attraction. We have already seen that the comparatively small masses of shooting-stars and meteorites can produce extraordinarily large amounts of heat when their cosmical velocities are arrested by our atmosphere. Now the force which has pro- duced these great velocities is gravitation. We know of this force as one acting on the surface of our planet when it appears as ter- restrial gravity. We know that a weight raised from the earth can drive our clocks, and that in like manner the gravity of the water rushing down from tho mountains works our mills. If a weight falls from a height and strikes the ground its mass loses, indeed, the visible motion which it had as a whole— in fact, however, this motion is not lost ; it is trans- ferred to the smallest elementary particles of the mass, and this invisible vibration of the molecules is the motion of heat. Visible motion is transformed by impact into the motion of heat. That which holds in this respect for grav- ity, holds also for gravitation. A heavy mass, of whatever kind, which is suspended in space separated from another heavy moss, represents a force capable of work. For both masses attract each other, and, if un- restrained by centrifugal force, they move toward each other under the influence of this attraction ; this takes place with ever- increasing velocity ; and if this velocity is finally destroyed, whether this be suddenly, by collision, or gradually, by the fnction of movable parts, it develops the corresponding quantity of the motion of heat, the amount of which can be calculated from the equiva- lence, previously established, between heat and mechanical work. Now we may assume with great probability that very many more meteors fall upon tht) sun than upon the earth, and with greater velocity, too, and therefore give more heat. Yet the hypothesis, that the entire amount of the sun's heat which is? continually lost by radiation, is made up by tha fall of meteors, a hypothesis which was propounded by Mayer, and has been favorably adopted by several other physicists, is open, according to Sir W. Thomson's investigations, to ob- jection ; for, assuming it to hold, the mass of the sun should increase so rapidly that the consequences would have shown theuiselven in the accelerated motion of the planets. The entire loss of heat from the sun cannot at all events be produced in this way ; at tha most a portion, which, however, may not be inconsiderable. If, now, there is no present manifestation of force sufficient to cover the expenditure of the sun's heat, the min must originally have had a storo of heat which it gradually gives out. Bat whenca this store? W« know that the cosmical forces alone oould have produced it. And here the hypothesis, previously discussed an to the origin of the sun, comes to our aid. If the mass of the sun had been once diffused in cosmical space, and had then been condensed — that is, had fallen together under the influence of celes- tial gravity— if then the resultant motion had been destroyed by friction and impact, with the production of heat, the new world produced by such condensation must have acquired a store of heat not only of consider- able, but even of colossal, magnitude. Calculation shows that, assuming the thermal capacity of the sun to be the same as that »f water, the temperature might be raised to 28,000, 000 of degrees, if this quan- tity of heat could over have been present in the sun at one time. This cannot be assumed, for such an increase of temperature would offer the greatest hindrance to con- densation. It is probable rather that a great part of this heat, which was produced by condensation, began to radiate into space before this condensation was complete. But the heat which the nun could have previously developed by its condensation, would have been sufficient to cover its present expendi- ture for not less than 22,000,000 of years of the past. Aad the sun is by no moans so dense as it may become. Spectrum analysis demon* strates the presence of largo masses of iron and of other known constituents of the rocks. The pressure which endeavors to condense the interior is about 800 times us great as 632 v«~ • » tfcat In thft rontro of t!io p.irth ; :-'vl r.>t the density of tho si;n, o\vir.g pro'iunty to if.,s enormous tcmpuratr.ro, is 1 -isr. Ibuu a quarter of the mean density of th« oar;h. Wo may therefore assume vitlx groat prob- ability thut the nun will still continue in its condensation, oven if it only uttaiuod (hj density of the earth — though it will prob- ably become far denser in the interior owin^ to the enormous pressure— this would develop Fn. 10. fresh quantities of heat, which would be sufficient to maintai a for an additional 17,- 000,000 of yeara the same intensity of sun- shine as that which is now the source of all terrestrial life. The smaller bodies of our system might be- come less hot than the sun, because the at- traction of the fresh masses would be feebler. A body like the earth might, if even we put its thermal capacity as high as that of water, become heated to even 9000 degrees, to more than our names can produce. The smaller bodies must cool more rapidly as long as they are still liquid. The increase in tem- perature, with the depth, is shown in bore- holes and in mines. The existence of hot wells and of volcanic eruptions shows that ill the interior of the earth there is a very high temperature, which can scarcely be any- thing than a remnant of the high temperature which prevailed at the time of its production. At any rate, the attempts to discover for the internal heat of the earth a more recent origin in chemical processes, have hitherto rostix] on very arbitrary assumptions ; and, iiwiii;»arc'd with the general uniform distribu- tion of the internal hefct, are somewhat iri- uutucieiit. On the other hand, considering the huge masses of Jupiter, of Saturn, of Uranus, and of Neptune, their small density, as well as that of the eun, is surprising, while the (smaller planets and the moon approximate to the density of the earth. We are here re- minded of the higher initial temperature, and the slower cooling, which characterizes large masses.* Tho moon, on tho contrary, ex- hibits formations on its surface which are * Mr. Zix-i'ix-r concl di-» from photometric meaa- nrenientB, which, however, iiewl confirmation, that Jupiter tlill possesses a Julit of its owu. ~~"'*~**O i T"V "r* •- f>T7»C . .A/ J*< JLi v* 1 L IvISS. strikingly suggestive of volcanic craters, and t'.unt to a former state of ignition of our sut- • Iili.0. The mode of its rotation, moreover, that it, always turns tho same side to ward the i:;t:-th, i.-i a peculiarity which might Inive been produced by the friction of a fluid. At pres- ent no truce of such a one can be perceived. You see, thus, by what various paths we are constantly led to the same primitive con- ditions. The hypothesis of Kant and La- place is seen to bo one of the happiest ideas in science, which at first astounds us, and then connects us in all directions with other discoveries, by which the conclusions are confirmed until we have confidence in them. In this case another circumstance has con- tributed—that is, the observation that this process of transformation, which the theory in question presupposes, goes on still, though on a smaller scale, seeing that all stages of that process can still be found to exist. For, as we have already seen, the larger bodies which are already formed go on in- creasing with the development of heat, by the attraction of the meteoric masses already diffused in space. Even now the smaller bodies are slowly drawn toward the sun by the resistance in space. We still find in the firmament of fixed stars, according to Sir J. Herschel's newest catalogue, over 5000 nebu- lous spots, of which those whose light is sufficiently strong give for the most part a colored spectrum of fine bright lines, as they appear in the spectra of the ignited gases. The nebulas are partly rounded structures, which are called planetary nelmlm 'Fig. 101 ; sometimes wholly irregular in form, as the large nebula in Orion, represented in Fig. 11 ; they are partly annular, as in the figur. , in Fig. 12, from the Canes Venatici. They arc fo-r the most part feebly luminous over their whole surface, while the fixed stars only ap- pear as luminous points. In many nebulas small stars can be seen, as in Jb'i^s. 13 and 14, from Sagittarius and Aurigo. More stars are continually being discovered in them, the better are tho tele- scopes used in their analysis. Thus, before the discovery of spectrum analysis, Sir W. Herschel's former view might be regarded »s the most probable, that that which we see i,a be nebula) are only heaps of very fine stars, of other Milky Ways. Now, however, sp&o truni analysis has shown a gas spectrum in many nebulas which contains stars, while* actual heaps of stars show the continuous spectrum of ignited solid bodies. Nebulas have in general three distinctly recognizable lines, one of which, in the blue, belongs to hydrogen, a second in bluish-green to nitro- gen,* while the third, between the two, is of unknown origin. Fig. 15 shows such a spectrum of a small but bright nebula in the Dragon. Traces of other bright lines are seen along with them, and sometimes also, as in Fig. 15, traces of a continuous spectrum ; all of which, however, are too feeble to admit of accurate investigation. It must be ob- served here that the light of very feeble ob- jects which give a continuous spectrum aro POPULAR SCIENTIFIC LECTURES. 11. Fio. 15. POPULAR SCIENTIFIC LECTUHES. .. . . - oistribnted by the spectroscope over a large surface, and are therefore greatly enfeebled or even extinguished, while the undecompos- able light of bright gas lines remains am decomposed, and hence can still be seen. In any case, the decomposition of tho light of the nebulae shows that by far tho greater part of their luminous surface is due to ignited gases, of which hydrogen forms i\ prominent constituent. In the planetary UiiiKsori, tho spherical or discoidal, it might be supposed that the gaseous mass had attained u con- dition of equilibrium ; but most other neb- ulas exhibit highly irregular forms, which by no means correspond to such a condition. AS, however, their shape has either not at all altered, or not appreciably, since they have been known and observed, they must cither have very little mass, or they must be of colossal size and distance. The former does not appear very probable, because small masses very soon give out their heat, and hence we are left to the second alternative, that they are ot huge dimensions and dis- tances. The same conclusion had been orig- inally drawn by Sir W. Herschel, on the assumption that tho nebulre were heaps of stars. With those nebulae which, besides tho lines of gases, also show the continuous spectrum of ignited denser bodies, are connected spots which art partly irresolvable and partly re- solvable into heaps of stars, which only show the light of the latter kind. The countless luminous stars of the heav- enly firmament, whose number increases with each newer and more perfect telescope, associate themselves with this primitive con- dition of tho worlds as they are formed. They are like our sun in magnitude, in lumi- nosity, and on the whole also in the chemi- cal condition of their surface, although there may bo differences in the quantity of indi- vidual elements. But we find also in space a third stadium, that of extinct suns ; and for this also there are actual evidences. In the first place, there are, in the course of history, pretty fre- quent examples of the appearance of new stars. In 1572 Tycho Brahc observed euch a one, which, though gradually burning paler, Kio. 14. was visible for two years, stood still like a fixed star, and finally reverted to tho dark- ness from which it had so suddenly emerged. Tho largest of them all seems to have been that observed by Kepler in the year 1604, which was brighter than a star of the first magnitude, and was observed from Septem- ber 27th, 1604, until March, 1G06. The reason of its luminosity was probably the collision with a smaller world. In a more recent case, in which on May 12th, 186f>, a email star of tho tenth magnitude in the Corona suddenly burst out to ono of tho second magnitude, spectrum analysis showed that it was an outburst of ignited hydrogen which produced tho light. This was only luminous for twelve days. In other cases obscure heavenly bodies have discovered themselves by their attrac- tion on adjacent bright stars, and the mo- tions of the latter thereby produced. Such an influence is observed in Sirius and Procy- on. By means of a new refracting telescope Messrs. Alvan Clarke and Pond, of Cam- bridge, U. S., have discovered in the case of Sirius a scarcely visible star, which has but little luminosity, but is almost seven times as heavy as the sun, has about half the mass of Sirius, and whoso distance from Sirius is about equal to that of Neptune from the sun. The satellite of Procyon has not yet been seen ; it appears to be quite dark. Thus there are extinct suns. The fact that there arc such lends now weight to the reasons which permit us to conclude that otur sun also is a body which slowly gives out its store of heat, and thus will some time becom* extinct. Tho term of 17,000,000 years which I have given may perhaps become considerably pro- longed by tho gradual abatement of radia- tion, by the new accretion of falling meteors, and by still greater condensation than that •which I have assumed in that calculation. But wo know of no natural process which could spare our sun the fate which has man- ifestly fallen upon other suns. This is * thought which we only reluctantly admit ; it seems to us an insult to the beneficent Crea- tive Power which we otherwise find at work in organisms and especially in living ones. But wo must reconcile ourselves to th« POPULAR SCIENTIFIC LECTURES. 685 thought that, however we may consider our- selves to bo the centre and final object of creation, we are but as dust on the earth ; which again is but a speck of dust in the im- mensity of space ; and the previous duration of our race, even if we follow it far beyond our written history, into the era of the lake dwellings or of the mammoth, is but an in- stant compared with the primeval times of our planet, when living beings existed upon it, whose strange and unearthly remains still gaze at us from their ancient tombs ; and far more does the duration of our race sink into insignificance compared with the enormous periods during which worlds have been in process of formation, and will still contimie to form when our sun is extinguished, and our earth is either solidified hi cold or is united with the ignited central body of our system. But who knows whether the first living in- habitants of the warm sea on the young world, whom wo ought perhaps to honor as our ancestors, would not have regarded our present cooler condition with as much horror as we look on a world without a sun ? Con- sidering the wonderful adaptability to the conditions of life which all organisms pos- sess, who knows to what degree of perfection our posterity will have been developed in 17,000,000 of years, and whether our fossilized bones will not perhaps seem to them as monstrous as those of the Ichthyosaurus now do ; and whether they, adjusted for a more sensitive state of equilibrium, will not consider the extremes of temperature, within which we now exist, to be just as violent and destructive as those of the older geological times appear to us ? Yea, even if sun and earth should solidify and become motionless, •who could say what new worlds would not be ready to develop life ? Meteoric stones sometimes contain hydrocarbons ; the light of the heads of comets exhibits a spectrum which is most like that of the electrical light in gases containing hydrogen and carbon. But carbon is the element, which is charac- teristic of organic compounds, from which living bodies are built up. Who knows whether these bodies, which everywhere •warm through space, do not scatter germs of life wherever there is a new world, which has become capable of giving a dwelling- place to organic bodies'. And this life we 'mi'*'"t perhaps consider as allied to ours in its primitive germ, however different might be the form which it would assume in udapt- ilig itself to its new dwelling-place. However this may be, that which most arouses our moral feelings at the thought of a future, though possibly very remote, cessa- tion of all living creation on the earth, is more particularly the question whether all this life is not an aimless sport, which will ultimately fall a prey to destruction by brute force ? Under the light of Darwin's great thonght we begin to see that not only pleas- ure and joy, but also pain, struggle, and death, are the powerful means by which na- ture has built up her liner and more perfect forms of life. And we men know more par- ticularly that in our intelligence, our civic order, and our morality we are living on the inheritance which our forefathers have gained for us, and that which we acquire in the same way will in like manner ennoble the life of our posterity. Thus the individ- ual, who works for the ideal objects of hu- manity, even if in a modest position, and in a limited sphere of activity, may bear with- out fear the thought that the thread of his own consciousness will one day break. But even men of such free and large order of minds ns Lessing and David Strauss could not reconcile themselves to the thought of a final destruction of the living race, and with it of all the fruits of all past generations. As jret we know of no fact, which can bo established by scientific observation, which would show that the finer and complex forms of vital motion could exist otherwise than ia the dense material of organic life ; that it can propagate itself as the sound-movement of a string can leave its originally narrow and fixed home and diffuse itself in the air, keep- ing all the time its pitch, and the most deli- cate shade of its color-tint ; and that, when it meets another string attuned to it, starts this again or excites a flame ready to sing to the same tone. The flame even, which, of all processes in inanimate nature, is the closest typo of life, may become extinct, but the heat which it produces continues to exist — indestructible, imperishable, as an invisible motion, now agitating the molecules of pon- derable matter, and then radiating into boundless space as the vibration of an ether. Even there it retains the characteristic pecu- liarities of its origin, and it reveals its his- tory to the inquirer who questions it by the spectroscope. United afresh, these rays may ignite a new flame, and thus, as it were, ac- quire a new bodily existence. Just as the flame remains the same in ap- pearance, and continues to exist with the same form and structure, although it draws every minute fresh combustible vapor, and fresh oxygen from the air, into the vortex of its ascending current ; and just as the wave goes on in unaltered form, and is yet being reconstructed every moment from freah par- ticles of water, so also in the living being, it ia. not the definite niiiss of substance, which now constitutes the body, to which the con^ tinuance of the individual is attached. For the material of the body, like that of the flame, is subject to continuous and compara- tively rapid change — a change the more rapid, the livelier the activity of the organ* in question. Home constituents are renewed from day to day, some from month to month, and others only after years. That which con- tinues to exist us a particular individual is like the flume and the wave — only the form of motion which continually attracts fresh matter into its vortex and expels the old. The observer with a deaf ear only recognizes the vibration of sound as long as it is visible and can be f«-lt, bound up with heavy matter. Axe our sensvn, in reference to life, liko the nan deaf ear In this respect? ADDENDUM. THE foregoing sentences on this pago gave rise to a controversial attack by Mr. J. C. F. Zoellner, in his book " On the Nature of the Comets," on Sir W. Thomson, on which I took occasion to express myself briefly in the pre- face to the second part of the German transla- tion of the " Handbook of Theoretical Phy- sics," by Thomson and Tait. I give here the passage in question : M I will mention here a further objection. i!t refers o the question as to the possibility that organic germs may occur in meteoric stones, and be conveyed to the celestial bodies which have been cooled. In his opening Address at the Meeting of the British Association in Edinburgh, in August, 1871, Sir W. Thomson had described this aa ' not unscientific.' Here also, if there is an error, I must confess that I also am a culprit. I had mentioned the same view as a possible mode of explaining the transmission cf organ- isms through space, even a little before Sir W. Thomson, in a lecture delivered in the spring of the same year at Heidelberg and Cologne, but not published. I cannot object if any one considers this hypothesis to be in a high, or even in the highest, degree im- probable. But to me it seems a perfectly correct scientific procedure, that when all our attempts fail in producing organisms from inanimate matter, we may inquire whether life has ever originated at all or not, and whether its germs have not been trans- ported from one world to another, and have developed themselves wherever they found a favorable soil. " Mr. Zoellner' s so-called physical objec- tions are but of very small weight. He re- calls the history of meteoric stone, and adds (p. xxvi. ) : 'If, therefore, that meteoric stones covered with organisms had escaped with a whole skin in the smash-up of its mother-body, and had not shared the general rise of temperature, it must necessarily have first passed through the atmosphere of the earth before it could deliver itself of its • organisms for the purpose of peopling the earth.' ••Now, in the first place, we know from repeated observations that the larger mete- oric stones only become heated in their out- side layer during their fall through the at- mosphere, while the interior is cold, or even very cold. Hence all germs which there might be in the crevices would be safe from combustion in the earth's atmosphere. But even those germs which were collected on the surface when they reached the -highest and most attenuated layer of the atmosphere would long before have been blown away by the powerful draught of air, before the stone reached the denser parts of the gaseous mass, where the compression would be sufficient to produce an appreciable heat. And, on the other hand, as far as the impact of two bodies is concerned, as Thomson assumes, POPULAR SCIENTIFIC LECTURES. the lirst consequences would be powerful mechanical motions, and only in the degree in which this would bo destroyed by friction would heat be produced. Wo do not knovr whether that would last for hours, for da\ s. or for weeks. The fragments, which at tin.- first moment were scattered with planetary velocity, might escape without any disi-n- gagement of heat. I consider it even not improbable that a stone, or shower of stones, flying through the higher regions of the at- mosphere cf a celestial body, carries with it a mass of air which contains unburm d germs. " As I have already remarked, I am not in- clined to suggest that all these possibilitisH are probabilities. They are questions th« existence and signification of which wo must remember, in order that if the case arise thc-v may be solved by actual observations or by conclusions therefrom." ON THOUGHT IN MEDICINE. A» ADDRESS DELIVERED AUGUST 2, 1877, ON THS ANNIVERSARY OP THE FOUNDATION OF THE INSTITUTE FOB THE rDUCATTON OP ABMY 8UEGEON8. IT is now thirty-five years since, on the 24 of August, I stood on the rostrum in the Hall of this Institute, before another such audi- ence as this, and read a paper on the opera- tion of Venal Tumors. I was then a pupil of this Institution, and was just at the end of my studies. I had never seen a tumor cut, and the subject-matter of my lecture was merely compiled from books; but book knowl- edge played at that time a far wider and a far more influential part in medicine than we are at present disposed to assign to it. It was a period of fermentation, of the fight be- tween learned tradition and the new spirit of natural science, which would have no more of tradition, but wished to depend upon in- dividual experience. The authorities at that time judged more favorably of my essay tlran I did myself, and I still possess the books which were awarded to rue as the prize. The recollections which crowd in upon me on this occasion have brought vividly before my mind a picture of the then condition of our science, of our endeavors and of our hopes, and have led mo to compare the paat state of things with that into which it has developed. Much indeed has been accom- plished. Although all that we hoped for has not been fulfilled, and many things have turned out differently from what we wished, yet wo have gained much for which we could not hare dared to hope. Just as the history of the world has made one of its few giant steps before our eyes, so also has our science ; hence an old student, like myself, scarcely recognizes the somewhat matronly aspect of Dame Medicine, when he accidentally coinee POPULAR SCIENTIFIC LECTURES. 087 again in relation to fcer, so vigorous nnrt so capable of growth has she become in the fountain of youth of the natural sciences. I may, perhaps, retain the impression ot this antagonism, more freshly than those of my contemporaries whom I have the honor to see assembled before me ; and who, hav- ing remained permanently connected with science and practice, have been less struck and less surprised by great changes, taking place as they do by slow steps. This must be my excuse for speaking to you about the metamorphosis which has taken place in medicine during this period, and with the results of whose development you are better acqiminted than I am. I should like the im- pression of this development and of its causes not to be quite lost on the younger of my hearers. They have no special incentive for consulting the literature of that period ; they would meet with principles which ap- pear as if written in a lost tongue, so that it is by no means easy for us to transfer our- selves into the mode of thought of a peiiod which is so far behind us. The course of development of medicine is an instructive lesson on the true principles of scientific in- quiry, and the positive part of this lesson has, perhaps, in no previous time been so impressively taught as in the last generation. The task falls to me, of teaching that branch of the natural sciences which has to make the widest generalizations, and has to discuss the meaning of fundamental ideas ; and which has, on that account, been not unfitly termed Natural Philosophy by the English-speaking peoples. Hence it does not fall too far out of the range of my official duties and of my own studies, if I attempt to discourse here of the principles of scientific method, in reference to the sciences of ex- perience. As regards my acquaintance with the tone of thought of the older medicine, indepen- dently of the general obligation, incumbent on every educated physician, of understanding the literature of his science and the direction as well as the conditions of its progress, there was in my case a special incentive. In my first professorship at Konigsberg, from the year 1849 to 1856, I had to lecture each win- ter on general pathology — that is, on that part of the subject which contains the gen- eral theoretical conceptions of the nature of disease, and of the principles of its treatment. General pathology was regarded by our elders as the fairest blossom of medical sci- ence. But in fact, that which formed its essence possesses only historical interest for the disciples of modern natural science. Many of my predecessors have broken a lance for the scientific defence of this essence, and more especially Eenle and Lotz. The latter, whose starting-point was also medi- cine, had, in his general pathology and ther- apeutics, arranged it very thoroughly and methodically and with great critical acumen. My own original inclination was toward physics ; external circumstances compelled mo to commence the study of medicine. which was made possible tome by the liberal arrangements of this Institution. It had, however, been the custom of a former timo to combine the study of medicine with that of the natural sciences, and whatever in thin was compulsory I must consider fortunate ; not merely that I entered medicine at a timo in which any one who was even moderately at homo in physical considerations found a fruitful virgin soil for cultivation ; but 1 consider the study of medicine to have been that training which preached more impress- ively and more convincingly than any other could have done, the everlasting principles of all scientific work ; principles which are so simple and yet are ever forgotten again ; so clear and yet always hidden by a deceptive veil. Perhaps only he can appreciate the im- mense importance and the fearful practical scope of the problems of medical theory, who has watched the fading eye of approaching death, and witnessed the distracted grief of affection, and who has asked himself the solemn questions, Has all been done which could bo done to ward off the dread event? Have all the resources and all the means which science has accumulated become ex- hausted ? Provided that he remains undisturbed in his study, the purely theoretical inquirer may smile with calm contempt when, for a time, vanity and conceit seek to swell themselves in science and stir up a commotion. Or he may consider ancient prejudices to be in- teresting and pardonable, as remains of poetic romance, or of youthful enthusiasm. To one who has to contend with the hostile forces of fact, indifference and romance dis^. appear ; that which he knows and can do, is exposed to severe tests ; he can only use the hard and clear light of facts, and must give> up the notion of lulling himself in agreeable illusions. 1 rejoice, therefore, that I can once more- address an assembly consisting almost ex- clusively of medical men who have gone through the same school. Medicine was once the intellectual home in which I grew up, and even the emigrant best understands and is best understood by his native land. If I am called upon to designate in one word the fundamental error of that former time, I should be inclined to say that it pur- sued a false ideal of science in a one-sided and erroneous reverence for the deductive method. Medicine, it is true, was not the only science which was involved in this error but in no other science have the consequences, been so glaring, or have so hindered progress, as in medicine. The history of this science claims, therefore, a special interest in the history of the development of the human mind. None other is, perhaps, more fitted to show that a true criticism of the sources of cognition is also practically an exceedingly important object of true philosophy The proud word of Hippocrates, " Godlike is the physician who is a philosopher," served, as it were, as a banner oi the old. deductive 1-38 POPULAR SCIENTIFIC LECTURES. mpdicino. \Vo may admit Ibis if only \ve once agree what wo nre to understand as a philosopher. Fur thrt ancients, philosophy embraced all theoretical knowledge ; their philosophers pursued mathematics, physics, astronomy, uatur.il history, in close connection with true philosophical or metaphysical considerations. li', therefore, we are to understand the med- ical philosopher of Hippocrates to be a man who has a perfected insight into the causal connection of natural processes, we shall in fact bo able to say with Hippocrates, Such a one can give help like a god. Understood in this sense, the aphorism describes in three words the ideal which our science has to strive after. But who can allege that it will ever attain this ideal ? But those disciples of medicine who thought themselves divine even in their own lifetime, and who wished to impose them- selves upon others as such, were not inclined to postpone their hopes for so long a period. The requirements for the QiAuanijto; were con- siderably moderated. Every adherent of any given cosmological system, in which, for well or ill, facts must be made to correspond with reality, felt himself to be a philosopher. The philosophers of that time knew little more of the laws of nature than the unlearned layman ; but the stress of their endeavors was laid upon thinking, upon the logical con- sequence and completeness of the system. It is not difficult to understand how in peri- ods of youthf ul development, such a one-sided overestimate of thought could be arrived at. The superiority of man over animals, of the scholar over the barbarian, depends upon thinking ; sensation, feeling, perception, on the contrary, he shares with his lower fellow- creatures, and in acuteness of the senses many of these are even superior to him. That man strives to develop his thinking faculty to the utmost is a problem on the solution of which the feeling of his own dig- nity, as well as of his own practical power, depends ; and it is n natural error to have considered unimportant the dowry of mental capacities which nature had given to animals, and to have believed that thought could bo liberated from its natural basis, observation, and perception, to begin its Icarian flight of metaphysical speculation. It is, in fact, no easy problem to ascertain completely the origins of our knowledge. An enormous amount is transmitted by speech and writing. This power which man pos- sesses of gathering together the stores of knowledge of generations, is the chief reason of his superiority over the animal, who is re- stricted to an inherited blind instinct and to its individual experience. But all transmit . ted knowledge is handed on already formed ; whence the reporter has derived it, or how much criticism he has bestowed upon it, can seldom bo made out, especially if the tra- lition has been handed down through sev- eral generations. We must admit it all upon good faith ; we cannot arrive at the source ; and when many generations have contented themselves with such knowledge, havo bi ought no criticism to bear upon it ; have, iadi-ed, gradually added all kinds of small alterations, which ultimately grew up to large ones — after all this, strange things aro often reported and believed under the authority of primeval wisdom. A curious case of this kind is the history of the circula- tion of the blood, of which we shall still have to speak. But another kind of tradition by speech, which long remained undetected, is even still more confusing for one who reflects upon the origin of knowledge. Speech cannot readily develop names for classes of objects or for classes of processes, if we have not been accustomed very often to mention together the corresponding individuals, things, and separate cases, and to assert what there is in common about them. They must, therefore, possess many points in com- mon. Or if we, reflecting scientifically upon this, select some of these characteristics, and collate them to fcrm a definition, the com- mon possession of these selected characteris- tics must necessitate that in the given cases a great number of other characteristics are to be regularly met with ; there must be a nat- ural connection between the first and tba last-named characteristics. If, for instance, we assign the name of mammals to those ani- mals which, when young, are suckled by their mothers, we can assert further, in reference to them, that they aro all warm- blooded animals, born alive, that they have a spinal column but no quadrate bone, breathe through lungs, have separate divisions of the heart, etc. Hence the fact, that in the speech of an intelligent observing people a certain class of things are included in one name, indicates that these things or cases fall under a common natural relationship ; by this alone a host of experiences are transmit- ted from preceding generations without this appearing to be the case. "The adult, moreover, when he begins to re- flect upon the origin of his knowledge, is in possession of a huge mass of every-day ex- periences, which in great part reach back to the obscurity of his first childhood. Every- thing individual has long been forgotten, but the similar traces which the daily repetition of similar cases has left in his memory haw deeply engraved themselves. And since only that which is in conformity with law is always repeated with regularity, these deeply impressed remains of all previous concep- tions are just the conceptions of what is con- formable to law in the things and processes. Thus man, when he begins to reflect, finds that he possesses a wide range of acquire- ments of which he knows not whence they came, which he has possessed as long ns ho can remember. We need not refer even to the possibility of inheritance by procreation. The conceptions which he has formed, which his mother tongue has transmitted, assert themselves ns regulative powers, even in the objective world of fact, and as he does not know that he or his forefathers havo de- POPULAR SCIENTIFIC LECTURES. veloport these conceptions from the tilings themselves, the world of facts seems to him, like his conceptions, to bo governed by in- tdlcctual forces. Wo recognize this psycho- logical anthropomorphism, from the Ideas of Plato, to the immanent dialectic of the cos- micul process of Hegel, and to the uncon- scious will of Schopenhauer Natural science, which informer times was virtually identical with medicine, followed the path of philosophy : the. deductive method seemed to be capable of doing every- thing. Socrates, it is true, had developed the inductive conception in the most instruc- tive manner. But the best which he accom- plished remained virtually misnr^r- «".><:. I will not lead you through the motley con- fusion of pathological theories which, accord- ing to the varying inclination of their authors, sprouted up in consequence of this or the other increase of natural knowledge, and were mostly put forth by physicians, who obtained fame and renown as great observers and empirics, independently of their theories. Then canio the less gifted pupils, who copied their master, exaggerated his theory, made it more one-sided and more logical, without re- gard to any discordance with nature. The more rigid tho system, the fewer and the, more thorough were the methods to which the healing art was restricted. The more tho schools were driven into a comer by the in- crease in actual knowledge, the more did they depend upon the ancient authorities, and tho more intolerant were they against innova- tion. The great reformer of anatomy, Vesa- lius, was cited before the theological faculty of Salamanca ; Servetua was burned at Geneva along with his book, in which ho do- scribed the circulation of the lungs ; and tho Paris faculty prohibited the teaching of Har- vey's doctrine of the circulation of the blood in its lecture-rooms. At tho same time the bases of tho systems from which these schools started were mostly views on. natural science which it would have Iwen quite right to utilize within a narrow circle. What was not right was tho delusion that it was more scientific to refer ail diseases to one kind of explanation, than to several. What was called the solidar pathology want- ed to deduce even-thing from the altered mechanism of tho solid parts, especially from their altered tension ; from the stridwn and iaxum. from tone and want of tone, and after- ward from strained or relaxed nerves and from obstructions in tho vessels. Humoral pathology was only acquainted with altera- tions in mixture. The four cardinal fluids, representatives of tho classical four elements, blood, phlegm, black and yellow gall ; with others, the acrimonies or dyscrasies, which had to be expelled by sweating and purging ; in the beginning of our modern epoch, the acids and alkalies or the alchemistic spirits, und the occult qualities of the substances assimilated -all these were the elements of this chemistry. Along with these were found all kinds of physiological conceptions, some of which contained remarkable- forcshadow- ings, such AS the f/t^vrnv Vtp/iov, the inherent vital force of Hippocrates, which is kept up by nutritive substances, this again boils i:\ the stomach and is the source of all motion ; here the thread is begun to be spun which subsequently led a physician to the law of the conservation of force. On the othff hand, the jrrriyta which is half spirit and half air, which can be driven from the lungs into the arteries and fills them, has produced much contusion. The? fact that air is generally found in the arteries of dead bodies, which indeed only penetrates in tho moment- iu which the vessels are cut, led the ancients to the belief that air is also present in the arte- ries during life. The Areins only remainosl then in which blood could circulate. It was believed to be formed in the liver, to mov.-i from there to the heart, and through tho veins to the organs. Any careful observation of the operation of blood-letting must hav« taught that, in the veins, it comes from tbo periphery, and flows toward tho heart. But this false theory had become so mixed up with the explanation of fever and of inflammation, that it acquired the authority of a dogma, which it v/as dangerous to attack. Yet tho essential and fundamental error c£ this system was, and still continued to be, the false kind of logical conclusion to which it wa.s supposed to lead : the conception that it must be possible to build a complete system which would embrace all forms of disease, and their cure, upon any one such simple explanation. Complete knowledge of the causal connection of one class of phe- nomena gives finally a logical coherent sys- tem. There i.s no prouder edifice of tho most exact thought than modern astronomy, de- duced even to the minutest of its small dis- turbances, from Newton's law of gravitation. But Newton had been preceded by Kepler, who had by induction collated all the facts ; and the astronomers have never believed that Newton's force excluded the simultaneous! action of other forces. They have been con- tinually on the watch to see whether friction, resisting media, r.nd swarms of meteors have not also some influence. The older philoso- phers and physicians believed they couM deduce, before they had settled their general principles by induction. They forgot that u deduction can have no more certainty than the principle from which it is deduced ; and that each new induction must in tho first place bo a new test, by experience, of its own bases. That, a conclusion is deduced by tho strictest logical method from an uncertain premiss does not give it a hair's breadth of certainty or of value. One chamet eristic of tho schools which built up their system on such hypotheses, which they assumed as dogmas, is the in- tolerance of expression which I have already partially mentioned. One who works upon a well-ascertained foundation may readily ad- mit an error ; he loses, by so doing, nothing more than that in which ho erred. If, how- ever, the starting-point has boen placed upon a hypothesis, which either appears guaranteed friO POPULAR SCIENTIFIC LECTURES. by authority, or is only chosen because it agrees with that which it is wished to believe true, any crack may then hopulesxly destroy tho whole fabric of conviction. The con- vinced disciples must therefore claim for each individual part of such a fabric the same de- gree of infallibility ; for the anatomy of Hip- pocrates just as much as for fever crises ; every opponent must only appear then as stupid or depraved, and the dispute will thus, according to old precedent, bo so much the more passionate and personal, the more un- certain is the basis which is defended. We have frequent opportunities of confirming these general rules in the schools of dogmatic deductive medicine. They turned their in- tolerance partly against each other, and partly against the eclectics who found various ex- planations for various forms of disease. This method, which in its essence is completely justified, had, in the eyes of systematists, the defect of being illogical. And yet the great- est physicians and observers, Hippocrates at the head, Arfitceus, Galen, Sydenham, and lioerhaave, had becomo eclectics, or at any r:Ue very lax systematists. About tho time when we seniors com- menced the study of medicine, it was still t;nuer tho influence of the important dis- coveries which Albrecht von jgnJUer had made on tlie excitability of nerves ; "and which ha had placed in connection with tho vitalistic theory of the nature of life. Haller had ob- served tho excitability in the nerves and muscles of amputated members. The roost surprising tiling to him was, that the most varied external actions, mechanical, chemical, thermal, to which electrical ones were subse- quently added, had always the same result — namely, that they produced muscular con- traction. They were only quantitatively distinguished as regards their action on the organism, that is, only by the strength of the otritation ; he designate:! thorn by the com- mon name of stimulus ; he called the altered condition of the nerve the excitation, and its capacity of responding to a stimulus the ex- citability, which was lost at death. This en- tire condition of things, which physically speaking asserts no more than that the nerves, as concerns the changes which take place in them after excitation, are in an ex- ceedingly unstable state of equilibrium ; this was looked upon as the fundamental property of animal life, and was unhesitatingly trans- ferred to the other organs and tissues of the body, for which there was no similar justifi- cation. It was believed that none of them were active of themselves, but must receive an impulse by a stimulus from without ; air and nourishment were considered to be the normal stimuli. The kind of activity seemed, on the contrary, to be conditioned by tha specific energy of tho organ, under the influ- ence of the vital force. Increase or diminu- tion of the excitability was the category un- der which the whole of tho acute diseases were referred, and from which indications were taken as to whether tho treatment should be lowering or stimulating. The rigid one-sidetlness and the unrelenting logic witi which Robert Brown had once worked out this system was broken, but it always fur- nished the leading points of view. Tho vital force had formerly lodged 03 ethereal spirit, as a Pnouma in tho arteries ; it had then with Paracelsus acquired the form of an Archens, a kind of useful Kobold, or indwelling alchemist, and had acquired its clearest scientific position as " soul of life," anima inscia, in Georg Ernst Stahl, who, in the first half of the last centuryT^ffBs pro- fessor of chemistry and pathology in Halle. Stahl had a clear and acute mind, which is informing and stimulating, from the way in which he states the proper question, even in those cases in which he decides against our present views. Ho it is who established tho first comprehensive system of chemistry, that of phlogiston. If wo translate bin phlogiston into latent heat, the theoretical bases of his system passed essentially into the system of Lavoisier ; Stahl did not then know oxygen, which occasioned some false hypotheses ; for instance, on the negative gravity of phlogiston. Stahl's " soul of liio" is, on the whole;, constructed on the pattern on which the pietistic communities of that period represented to themselves the sinful human soul ; it is subject to errors and pas- sions, to sloth, fear, impatience, sorrow, in- discretion, despair. The physician rrmst first appease it, or then incite it, or punish it, and compel it to repent. And the way in which, at the same time, he established tho necessity of the physical and vital actions was well thought out. The soul of life gov- erns the body, and only acts by means of tho physico-chemical forces of the substances assimilated. But it has tho power to bind arid to loose these forces, to allow them full play or to restrain them. After death the re- strained forces become free, and evoke putre- faction and decomposition. For the refuta- tion of this hypothesis of binding and loos- ing, it was necessary to discover the law of the conservation of force. The second half of tho previous century was too much possessed by tho principles of rationalism to recognize openly Stahl's " soul of life." It was presented more scientifically as vital force, Vis vitalis, while in the main it retained its functions, and under the name of" nature's healing power" it played a prom- inent part in the treatment of diseases. The doctrine of vital force entered into tho pathological system of changes in irritability. The attempt was made to separate the direct actions of the virus which produce disease, in HO far as they depended on tho play of blind natural forces, tho symptomnta mvrbi, from those which brought on the reaction of vital force, the symptomala readioni.i. The latter were principally seen in inflammation nnd in fever. It was the function of the phy- sician to observe tho strength of thif. reaction, and to stimulate or moderate it according to circumstances. The treatment of fever seemod at that time to be the chief point ; to be that part of POPULAR SCIENTIFIC LECTURES. medicine -which ImcT a real scientific founda- tion, uml in which the local trp.it mont fell comparatively into tho background. The therapeutics of febrile diseases had thereby become very monotonous, although the means indicated by theory were still abundant- ly used, and especially blood-letting, which since that time has almost been entirely abandoned. Therapeutics became still inoro impoverished as tho younger and more criti- cal generation grew up, and tested tho assumptions of that which was considered to be scientific. Among the younger generation were many who, in despair as to their sci- ence, hud almost entirely given tip therapeu- tics, or on principle had grasped at an em- piricism such as Rademacher then taught, which regarded any expectation of a scientific explanation as a vain hope. AVhat wo learned at that time were only the ruins of the older dogmatiam.bat their doubt- ful features soon manifested themselves. The vitalistic physician considered that the essential part of the vital processes did not depend upon natural forces, which, doing their work with blind necessity and accord- ing to a fixed law, determined the result. What these forces could do appeared quite subordinate, and scarcely worthy of a minute study. He thought that he had to deal with a soul-like being, to which a thinker, a phi- losopher, and an intelligent man must be op- posed. May I elucidate this by a few out- lines ? At this time auscultation and percussion of the organs of the chest were being regu- larly practised in tho clinical wards. But I have often heard it maintained that they were a coarse mechanical means of investiga- tion which a physician with a clear mental vision did not need ; and it indeed lowered and debased the patient, who was anyhow a human being, by treating him as a machine. To feel the pulse seemed the most direct method of learning tho mode of action of tho vital force, and it was practised, therefore, as by far the most important means of inves- tigation. To count with a repeater was quite usual, but seemed to the old gentlemen as a method not quite in good taste. Thero was, as yet, no idea of measuring temperature in cii.sos of disease. In reference to thu ophthalmoscope, a celebrated surgical col- league said to me that ho would never use the instrument, it wad too dangerous to admit crude light into diseased eyes ; another said tho mirror might be useful for physicians with bad eyes ; his, however, were good, and he did not need it. A professor of physiology of that time, cel- ebrated for his literary activity, and noted as an orator and intelligent man, had a dispute on tho images in the e>o with his colleague tho physicist. Tho hitter challenged the physiologist to visit him and witness the ex- periment. The physiologist, however, re- fused his request with indignation ; alleging that a physiologist had nothing to do with experiments ; they were of no good but for th« physicist. Another aged and learned pro- fessor of ni<>rf'.pr>Ti!icR, \riio occupied hlnisefit mu.-ii v.vJi tilt; reorganization of the uni- • versities, was urgent with mo to divide phys- iology, in order to restore the good old time ; that I myself should lecture on the r«ally in- tellectual part, and should hand over the lower experimental part to a colleague whom he regarded as good enough for tho purpose. Ho quite gave ruo up when I said tiiat I my- self considered experiments to bo the true basis of science. I mention those points, which I myself have experienced, to elucidate the feeling of tho older schools, and indeed of the most illustrious representatives of medical science, in reference to the progressive set of ideas of tho natural sciences ; in literature these ideaf. naturally found feebler expression, for'ihe old gentlemen wero cautious and worldly wise. You will understand how great a hin- drance to progress such a feeling on the part of infiue itial and respected men must hivo been. The medical education of that timo was based mainly on the study of books ; there were still lectures, which were restrict- ed to mere dictation ; for experiments arid demonstrations in the laboratory the pro- vision made was sometimes good and some- times the reverse ; there were 110 physiologi- cal and physical laboratories in which tho student himself might go to work. Liebig's great deed, the foundation of the chemical laboratory, was complete, us far as chemistry w*as concerned, but his example had not been imitated elsewhere. Yet medicine possessed in anatomical dissections a great means of education for independent observation, which is wanting in tho other faculties, nud to which 1 am disposed to attach great weight. Microscopic demonstrations were isolated and infrequent in the lectures. Microscopic instruments wero costly and scarce. I cams into possession of one by having spent my autumn vacation in 1841 in the Charite, pros- trated by typhoid fever ; as pupil, 1 was nursed without expense, and on my recovery I found myself in possession of the savings of my small resources.. The instrument was not beautiful, yet I was able to recognize by its means the prolongations of the ganglionic cells in the inveitenrata, which I described in my dissertation, e.rid to investigate tho vibrions in my research on putrefaction and fermentation. Any of my fellow-students who wished to make experiments had to do so at tho cost of his pocket-money. OHO thing we learned thereby, which tho younger generation does not, perhaps, learn KO veil in thelaboratoric-M — that is, to consider in all directions tho i ways and means of attaining th;-; riul, and to exhaust nil possibilities in th« consideration, until a practicable path was found. "\Vo had, it is true, an almost uncultivated field beforo us, in which almost every stroke of tho spade might produeu remunerative- results. It WHS one man more especially who aroused our enthusiasm for work in tho right direction— th&t is, Johannes Millie?, tho phys- 642 POPULAR SCIENTIFIC LECTUPtES. lologlst. In &ic theoretical vtewg ho I'.ivored the vitalistic hypothesis, but in the most essential points he was ft natxiral philoso- phor, firm and immovable ; for him, nil theo- ries were but hypotheses, which luvd to be tested by facts, and about which facts could alone decide. Even the views upon thosa points which most easily crystallize into dog- mas, on the mode of activity of the vital force and the activity of the conscious soul, he tried continually to define more precisely, to prove or to refute by means of facts. And, although the art of anatomical inves- tigation was most familiar to him, and he therefore recurred most willingly to this, yet he worked himself into the chemical and physical methods which were more foreign to him. Ho furnished the proof that fibxiue is dissolved in blood ; he experimented on thox propagation of sound in such mechanisms as are found in the drum of the ear ; he treated the action of the eye as an optician. His most important performance for the physi- ology of the nervous system, as well as for the theory of cognition, was the actual definite establishment of the doctrine of the specific energies of the nerves. In reference to the separation of the nerves of motor and sensi- ble energy, he showed how to make the ex- perimental proof of Bell's law of the roots of the spinal cord so as to be free from errors ; and in regard to the sensible energies he not only established the general law, but carried out a great number of separate investigations, to eliminate objections, and to refute false indications and evasions. That which hitherto had been imagined from the data of overy-day experience, and which had been sought to be expressed in a vague manner, in which the true was mixed up with the false ; or which had just been established for indi- vidual branches, such as by Dr. Young for the theory of colors, or by Sir Charles Bell for ftie motor nerves, that emerged from Mailer's hands in a state of classical perfec- tion— a scientific achievement whose value I am inclined to consider as equal to that of the discovery of the law of gravitation. His scientific tendency, and more especially his example, were continued in his pupils. We had been preceded by Schwann, Henle, Ileichert, Peters, llemak ; I met as fellow- students E. Du Bois-Beymond, Virchow, Briicke, Ludwig, Traube, J. Meyer, Lieber- kiihn, Hallmann ; we were succeeded by A. von Gracfe, W. Busch, Max Bchultze, A. Schneider. Microscopic and pathological anatomy, the study of organic types, physiology, experi- mental pathology and therapeutics, ophthal- mology, developed themselves in Germany •tinder the influence of this powerful impulse £ar beyond the standard of rival adjacent countries. This was helped by the labors of those of similar tendencies among Miiller's contemporaries, among whom the three brothers Weber of Leipzig must first of all be mentioned, who have built solid foundations in the mechanism of the circulation, of tne muscles, of the joints, and of the ear. The attack was made wherever a way cor bo perceived of understanding one of tho vital processes ; it was assumed that they could bo understood, and success justilkil this assumption. A delicate and copious technical apparatus has been developed in the methods of microscopy, of physiological chemistry, and of vivisection ; the latter greatly facilitated more particularly by tho use of ansesthetic ether and of the paralyzing curara, by which a number of deep problems became open to attack, which to our genera- tion seemed hopeless. The thermometer, tho ophthalmoscope, the auricular speculum, the laryngoscope, nervous irritation on the liv- ing body, opened out to tho physician possi- bilities of delicate and yet certain diagnosis where there seemed to be absolute darkness. The continually increasing number of proved parasitical organisms substitute tangible objects for mystical entities, and teach liin surgeon to forestall the fearfully subtle dis- eases of decomposition. But do not think, gentlemen, that the struggle is at an end. As long as there are people of such astounding conceit ns to imagine that they can effect, by a few clevt-r strokes, that which man can otherwise only hope to achieve by toilsome labor, hypothe- ses will bo started which, propounded as dog- mas, at once promise to solve all riddles. And as long as there are people who believe implicitly in that which they -\\ish to be true, so long will the hypotheses of the former find credence. Both classes will certainly not die out, and to the latter the majority will always belong. There are two characteristics more particu- larly which metaphysical systems have always possessed. ]n the first place man is always desirous cf feeling himself to be a being of a higher order, far beyond the stand- ard of the rest of nature ; this wish is satis- fied by the spiiitualiste. On the other hand, he would like to believe that by his thought he was unrestrained lord of the world, and of course by his thinking with those concep- tions, to the development of which he has attained ; this is attempted to be satisfied by the materialists. But one who, like the physician, has actively to face natural forces which bring about weal or woe, is also under tho obliga- tion of seeking for a knowledge of the truth, and of the truth only ; without considering whether, what ho finds, is pleasant in one way or the other. His aim is one which is firmly settled ; for him the success of fac1« is alone finally decisive. He must endesiM-r to ascertain beforehand, what will bo tin- result of his attack if he pursues this or thin course. In order to acquire this foreknowl- edge of what is coming, but of what has not been settled by observations, no other " method is possible than that of endeavoring to arrive at the laws of facts by observations ; and we can only learn them by induction, by the careful selection, collation, and observa- tion of those cases which fall Tinder the law. When we fancy that we have arrived at a POPULAR SCIENTIFIC LECTURES. law, the business of deduction comnifncrr,. It is then our duty to develop the con«f>- quences of our law as completely as may be, but in the iirst place only to apply to them the test of experience, so far as they can be tested, and then to decide by this test whether tho law holds, and to what extent. This is a test which really never ceases: The truo natural philosopher reflects at each new phenomenon, whether the best estab- lished Liws of the best known forces may not experience a change ; it can of course only be a question of a change which does not contradict the wkolo store of our previously collected experiences. It never thus attains unconditional truth, but such a high degree of probability that it is practically equal to cer- tainty. The metaphysicians may amuse them- selves at this ; we will take their mocking to heart when they are in a position to do bet- ter, or even as well. The old words of Soc- rates, the prime master of inductive defi- nitions, in reference to them are just as fresh as they were two thousand years ago : " They imagined they knew what they did not know, and he at any rate had the advantage of rwyt pretending to know what he did not know." And again, ho was surprised at its not being clear to them that it is not possible for men to discover such things ; since even those who most prided themselves on the speeches made on the matter, did not agree among themselves, but behaved to each other like madmen (ro,;; [iaiv<>fj.ivoi$ OMO'U?). Socrate:> calls them rotJS/^yzTT-oix^oi'oi'vra?. Schopen- hauer calls himself fi Mont Blanc, by the side of a mole-heap, when he compares him- self with a natural philosopher. The pupils admire these big words and try to imitate the master. In speaking against the empty manufacture of hypotheses, do not by any means suppose that I wish to diminish the real value of orig- inal thoughts. The first discovery of a new law is the discovery of n similarity which has hitherto been concealed in the conrso of natural processes. It is a manifestation of that which our forefather* in a serious sense described as " wit ;" it is of the same quality as the highest performances of artistic per- ception in the discovery of new types ot ex- pression. It is something which cannot bo forced, and which cannot be acquired by any known method. Hence all tlioso aspire after it who wish to pass as tho favored children of genius. It seems, too, so easy, s:> free from trouble, to get by sudden mental flashes an unattainable advantage over our contem- poraries. Tho true artist and the true in- quirer know.} that great works can only ba produced by hard work. Tho proof that the ideas formed do not merely scrape together superficial resemblances, but are produced by a quick glance into tho connection of tha whole, can only bo acquired when these ideas are completely developed — that is, for a newly discovered natural law, only by its agreement with facts. This estimate must by no means bo regarded as depending on external success, but tho success ia here closely connected with the depth and com- pleteness of the preliminary perceptions. To find superficial resemblances is easy ; it is amusing in society, and witty thoughts soon procure for their author tho name of a clever man. Among the great number of such ideas, there must bo some which are ultimately found to be partially or wholly correct ; it would be a stroke of skill always to guess falsely. In such a happy chance a man can loudly claim his priority for tho dis- covery ; if otherwise, a lucky oblivion con- ceals tha false conclusions. Tho adherents of such a process are glad to certify the value of a first thought. Conscientious workers who are shy at bringing their thoughts before the public before they have tested them in all directions, solved all doubts, and have firmly established the proof, these are at a decided disadvantage. To settle the present kind of questions of priority, only by tho date of their first publication, and without considering the ripeness of the research, has seriously favored this mischief. In the " type case" of the printer all tha wisdom of the world is contained which has been or can be discovered ; it is only requi- site to know how the letters are to be arranged. So also, in the hundreds of books and pam- phlets which are every year published about ether, the structure of atoms, the theory of perception, as well as on tne nature of the asthenic fever and carcinoma, ail the most refined shades of possible hypotheses are ex- hausted, and among these there must neces- sarily be many fragments of the correct theory. But who knows how to find them '! I insist upon this in order to make clear to you that all this literature, of untried and unconfirmed hypotheses, has no value in the progress of science. On the contrary, tho few sound ideas which they maj- contain are concealed by the rubbish of tho rest ; and one who wants to publish something really new — facts — sees himself open to the danger of countless claims of priority, unless he is prepared to waste time and power in reading beforehand a quantity of absolutely useless books, and to destroy his readers' patience by a multitude of useless quotations. Our generation has had to suffer under the tyranny of spiritualistic metaphysics ; tha newer generation will probably have to guard against that of the materialistic hypotheses. Kant's rejection of the claims of pure thought has gradually made some impression, bul Kant allowed oneway of escape, it was as clear to him as to Socrates that all metaphysi- cal systems which up to that time hurl been propounded were tissues of false conclusions. Jlis " Kritik der reinen Vernuiift ' is n con- tinual sermon against the use of tho category of thought beyond the limits of possible ex- perience. But geometry seemed to him to do something which metaphysics was stn vine; after ; and hence geometrical axioms, which he looked upon as d priori principles antooe- deut to all experience, ho held to be ^ivpn by transcendental intuition, or as the inherent form ul. all external intuition. Since thai POPULAR SCIENTIFIC LECTU11ES. time, pare d priori intuition has "been the anchoring-ground of metaphysicians. It is even more convenient than pure thought, be- cause everything can be heaped on it without going into chains of reasoning, which might bo capable of proof or of refutation The nativistic theory of perception of the senses is the expression of this theory in physiology. All mathematicians united to fight against any attempt to resolve the intuitions into their natural elements ; whether the so-called pure or the empirical, the axioms of geome- try, the principles of mechanics, or the per- cetions of vision. For this reason, there- fore, the mathematical investigations of Lobatschewsky, Gauss, and Eiemann on the alterations which are logically possible in the axioms of geometry ; and the proof that the axiomr are principles which are to be confirm- ed or perhaps even refuted by experience, and can accordingly be acquired from experience — these I consider to be very important steps. That all metaphysical sects get into a rage about this must not lead you astray, for these investigations lay the axe at the bases of ap- parently the firmest supports which their claims still possess. Against those investiga- tors who endeavor to eliminate from among the perceptions of the senses, whatever there JZifly be of the actions of memory, and of the repetition of similar impressions, which oc- cur in memory ; whatever, in short, is a mat- ter of experience, against them it is attempt- ed to raise a party cry that they are spiritual- ists. As if memory, experience, and custom were not also facts, whose laws are to be sought, and which are not to be explained away because they cannot be glibly referred to reflex actions, and to the complex of the prolongation of ganglionic cells, and of the connection of nerve-fibres in the brain. Indeed, however self-evident, and however important the principle may appear to be, that natural science has to seek for the laws of facts, this principle is nevertheless often forgotten. In recognizing the law found, as a force which rules the processes in nature, we conceive it objectively asa/orce, and such a reference of individual cases to a force which under given conditions produces a definite result, that we designate as a causal explanation of phenomena. We cannot always refer to the forces of atoms ; we speak of a refractive force, of electro-motive and of electro-dynamic force. But do not forget the given conditions and the given result. If these cannot be given, the explanation at- tempted is merely a modest confession of ignorance, and then it is decidedly better to confess this openly. If any process in vegetation is referred to forces in the cells, without a closer definition of the conditions among which, and of the direction in which, they work, this can at most assert that the more remote parts of the organism are without influence ; but it would be difficult to confirm this with certainty in more than a few cases. In like manner, the originally definite sense which Johannes Muller gave to the rtlea of reflex action, is gradually evaporated into thin, that when an impression has been made on any part of the nervous system, and an action occurs in any other part, this is supposed to have been ex- plained by saying that it is a reflex action. Much may be imposed upon the irr3solvablo complexity of the nerve-fibres of the brain. But the resemblance to the qualitates occultce of ancient medicine is very suspicious. From the entire chain of my argument it follows that v/hat I have said against meta- physics is not intended against philosophy. But metaphysicians have always tried to plume themselves on being philosophers, and philosophical amateurs have mostly taken an interest in the high-flying speculations of the metaphysicians, by which they hope in ft short time, and at no great trouble, to learn the whole of what is worth knowing. On another occasion I compared the relation- ship of metaphysics to philosophy with that of astrology to astronomy. The former had the most exciting interest for the public at large, and especially for the fashionable world, and turned its alleged connoisseiirs into influential persons. Astronomy, on the contrary, although it had become the ideal of scientific research, had to be content with a small number of quietly working disciples. In like manner, philosophy, if it gives up metaphysics, still possesses a wide and im- portant field, the knowledge of mental and spiritual processes and their laws. Just as the anatomist, when ho has reached the lim- its of microscopic vision, must try to gain an insight into the action of his optical in- strument, in like manner every scientific in- quirer must study minutely the chief instru- ment of his research as to its capabilities. The groping of the medical schools for the last two thousand years is, among other things, an illustration of the harm of errone- ous views in this respect. And the physician, the statesman, the jurist, the clergyman, and the teacher, ought to be able to build upon a knowledge of physical processes if they wish to acquire a true scientific basis for their practical activity. But the true science of philosophy has had, perhaps, to suffer more from the evil mental habits and the false ideals of metaphysics than even medicine itself. One word of warning. I should not like you to think that my statements are influ- enced by personal irritation. I need not ex- plain that one who has such opinions as I have laid before j-ou, who impresses on his pupils, whenever he can, the principle that " a metaphysical conclusion is either .1 false conclusion or a concealed experimental con- clusion," that he is not exactly beloved by the votaries of metaphysics or of intuitive conceptions. Metaphysicians, like ail those who cannot give any decisive reasons to their opponents, are usually not very polite in their controversy ; one's own success may approximately be estimated from the increas- ing want of politeness in the replies. Pcri:L.v:: m:::;- My own researches lia% e led me more than other disciples of the school of natural sci- ence into controversial regions ; and the ex- pressions of metaphysical discontent havo perhaps concerned me even moro than my friends, as many of you aro doubtless aware. In order, therefore, to leave my own per- sonal opinions quite on one side, I have allowed two unsuspected warrantors to speak for me — Socrates and Kant— both of whom were certain that all metaphysical systems established up to their time were full of empty false conclusions, and who guarded them- selves against adding any new ones. In order to show that the matter has not changed, either in the last two thousand years or in the last one hundred years, let me conclude with a sentence of ono who was unfortunately too soon taken away from us, Frederick Albert Lange, the author of the " History of Materialism." In his posthu- mous " Logical Studies," which he wrote in anticipation of his approaching end, he gives the following picture, which struck me be- cause it would hold just as well in reference to solidar or humoral pathologists, or any other of the old dogmatic schools of medicine. Lange says : The Hegelian ascribes to the Herbartian a less perfect knowledge than to himself, and conversely ; but neither hesi- tates to consider the knowledge of the other to be higher compared with that of tho em- piricist, and to recognize in it at any rate an approximation to the only true knowledge. It is seen, also, that here no regard is paid to the validity of the proof, and that a mere statement in the form of a deduction from tho entirety of a system is recognized as " apodictic knowledge." Let us, then, throw no stones at our old medical predecessors, who in dark ages, and with but slight preliminary knowledge, fell into precisely the same errors as the great in- telligences of what wishes to be thought tho illuminated nineteenth century. They did no worse than their predecessors, except that the nonsense of their method was more promi- nent in the matter of natural science. Let us work on. In this work of true intelligence physicians are called upon to play a promi- nent part. Among those who are continually called upon actively to preserve and apply their knowledge of nature, you are thosa who begin with the best mental preparation, and are acquainted with the most varied regions of natural phenomena. In order, finally, to conclude our consulta- tion on the condition of Dame Medicine cor- rectly with the epikrisis, I think wo have every reason to be content with the success of the treatment which the school of natural science has applied, arid we can only recom- mend the younger generation to continue the same therapeutics. or. ON ACADEMIC FREEDOM IN GERMAN UNIVERSITIES. INAUGURAL ADDRESS AS RECTOR C? THE FEED- KEICK WILLIAM UNIVERSITY OF BERLIN. Ul> LIVEIIED OCTOBER 15, 1877. IN entering upon tho honorable office to •which the confidence of my colleagues has called me, my first duty is once more openly to express my thanks to those who have thus honored me by their confidence. I have tho mo7»o reason to appreciate it highly, as it was conferred upon me, notwithstanding that I havo been but few years among you, and notwithstanding that I belong to a branch of natural science which has come within the circle of university instruction in some sense as a foreign element ; which has necessitated many changes in the old order of university teaching, and which will, perhaps, necessi- tate other changes. It is indeed just in that branch (Physics) which I represent, and which forms the theoretical basis of all other branches of natural science, that tho particu- lar characteristics of their methods are most definitely pronounced. I have already been several times in the position of having to propose alterations in the previous regula- tions of the university, and I have always had the pleasure of meeting with the ready assistance of my colleagues in the faculty, and of the Senate. That you have made me the director of the business of this university for this yeai is a proof that you regard me as no thoughtless innovator. For, in fact, however the objects, the methods, the more immediate aims of investigations in the nat- ural sciences may differ externally from those of tho mental sciences, and however foreign their results, and however remote their interest may often appear, to those who are accustomed only to the direct manifesta- tions and products of mental activity, there is in reality, as I have endeavored to show in my discourse as Rector at Heidelberg, tho closest connection in the essentials of scien- tific methods, as well as in the ultimate aims of both classes of the sciences. Even if most of tho objects of investigation of the natural sciences are not directly connected with tho interests of the mind, it cannot, on the othei hand, be forgotten that the power of true scientific method stands out in the natural sciences far more prominently— that tho real is far more sharply separated from tho un- real, by the incorruptible criticism of facts, than is the case with the more complex prob- lems of mental science. And not merely the development of this new side of scientific activity, which waa almost unknown to antiquity, but also tho influence of many political, social, and even international relationships make themselves felt, and require to be taken into account. The circle of our students has had to be in- creased ; a changed national life makes other demand.) upon those who aro leaving ; the sciences become moro und more specialized POPULAR SCIENTIFIC LECTURES. anil divided ; exclusive of the libraries, larger and moro varied appliances for study are required. "Wo can scarcely foresee what fresh demands and what new problems wo may have to meet in the more immediate future. On the other hand, the German universi- ties have conquered a position of honor not confined to their fatherland ; the eyes of the civilized world are upon them. Scholars speaking the most different languages crowd toward them, even from the farthest parts of the earth. Such a position would be easily lost by a false step, but would be difficult to regain. Under these circumstances it is our duty to get a clear understanding of the reason 'for the previous prosperity of our universities ; we must try to find what is the feature in their arrangements which we must seek to retain as a precious jewel, and where, on the contrary, we may give way when changes are required. I consider myself fey no means entitled to give a final opinion OR this mat- ter. The point of view of any single individ- ual is restricted ; representatives of other sciences will be able to contribute some- thing. But I think that ft final result can only be arrived at when each one becomes clear as to the state of things as seen from his point of view. The European universities of the Middle Ago had their origin as free private unions of their students, who came together under the influence of celebrated teachers, and themselves arranged their own affairs. In recognition of the public advantage of these unions they soon obtained from the state privileges; and honorable rights, especially that of an independent jurisdiction, and the right of granting academic degrees. The students of that time were mostly men of mature years, who frequented the university more immediately for their own instruction, and without any direct practical object ; but younger men soon began to be sent, who, for the most part, were placed under the superin- tendence of the older members. The sepa- rate universities split again into closer eco- nomic unions, under the name of " Nations," "Bursaries," "Colleges," whose older mem- bers, the seniors, governed the common affairs of each such union, and also met together for regulating the common affairs of the uni- versity. In the courtyard of the University cf Bologna are still to be seen the coats-of- firms, and lists of members and seniors, of many such nations in ancient times. Tho older graduated members were regarded as permanent life members of such unions, and they retained the right of voting, as is still the case in the College of Doctors in the Uni- versity of Vienna, and in the Colleges of Ox- ford and of Cambridge, or was until recently. Such a free confederation of independent men, in which teachers as well as taught were brought together by no other interest than that of love of science ; SOIMO by the desire of discovering the treasure of mental culturo which antiotiity had bequeathed others endeavoring to kindle in a new gen- eration the ideal enthusiasm which had ani- mated their lives. Such was tho origin of universities, based, in the conception, and in the plan of their organization, upon the most perfect freedom. But we must not think here of freedom of teaching in the modern sense. The majority was usually very in- tolerant of divergent opinions. Not unfre- quently the adherents of the minority were compelled to quit the university in a body. This was not restricted to those cases in which the Church intermeddled, and whero political or metaphysical propositions were in question. Even the medical faculties — that of Paris, the most celebrated of all at the head — allowed no divergence from that which they regarded as the teaching of Hip- pocrates. Any one who used the medicines of the Arabians or who believed in the circu- lation of the blood was expelled. The change, in the universities to their present constitution, was caused mainly by the fact that the state granted to them material help, but required, on the other hand, the right of co-operating in their man- agement. The course of this development was different in different European countries, partly owing to divergent political conditions and partly to that of national character. Until lately it might have been said that the least change has taken place in the old English universities, Oxford and Cambridge. Their great endowments, the political feeling of the English for the retention of existing rights, had excluded almost all change, even in directions in which such change was urgently required. Until of late both uni- versities had in great measure retained their character as schools for the clergy, formerly of the Roman and now of the Anglican Church, whose instruction laymen might also share in so far as it could serve the general education of the mind ; they were subjected to such ft control and mode of life as was foimerly considered to be good for young priests. They lived, as they still live, in colleges, Tinder the superintendence of a number of older graduate members (Fellows) of the college ; in other respocts in the style and habits of the well-to-do classes in Eng- land. The range and the method of tho instruc- tign is a more highly developed gyinuasial instruction ; though in its limitation to what is afterward required in the examination, and in (ho minute study of the contents of pre- S'-nbed text-books, it is more like the Rope- titoria win. 'i are here and there held in our universities. Tho acquirements of the stu- dents are controlled by searching examina- tions for academical degrees, in which very special knowledge is required, though only for limited regions. By such examinations the academical degrees are acquired. While the English universities give but little for the endowment of tho positions of approved scientific teachers, and do not log- ically apply even that little for this object, tUe^ have aiothe** arranfjeiaeni which is ap- POPULAR SCIENTIFIC LECTURES. 647 parently of great promise for scientific study, but which has hitherto not effected much ; that is the institution of Fellowships. Those who have passed the best examinations are elected as Fellows of their college, whero they have a home, and along with this, a re- spectable income, so that they can devote the whole of their leisure to scientific pur- suits. Both Oxford and Cambridge have each more than 500 such fellowships. The Fellows may, but need not act as tutors for the students. They need not even live ia the university town, but may spend their stipends where they like, and ia many cases may retain the fellowships for aa indefinite period. With some exceptions, they only lose it in case they marry, or aro elected to certain offices. They are tho real successors of the old corporation of students, by and for which the university was founded and endowed. But however beautiful this plan may seem, and notwithstanding tho enormous sums devoted to it, in the opinion of all unprejudiced Englishmen it does but little for science ; manifestly because most of these young men, although they are the pick of the students, and in the most favorable conditions possible for scientific work, have in their student-career not come sufficiently in contact with the living spirit of inquiry, to work on afterward on their own account, and with their own enthusiasm. In certain respects the English universities do a great deal. They bring up their stu- dents as cultivated men, who are expected not to break through the restrictions of their political and ecclesiastical party, and, in fact, do not thus break through. In two re- spects we might well endeavor to imitate them. In the first place, together with a lively feeling for tha beauty and youthful freshness of antiquity, they develop in a high degree a sense for delicacy and precision ia writing which shows itself in the way in which they handle their mother-tongue. I fear that one of the weakest sides in the in- struction of German youth is in this direc- tion, la the second place the English uni- versities, like their schools, take greater care of the bodily health of their students. They live and work in airy, spacious buildings, surrounded by lawns and groves of trees ; they tind much of their pleasure in games which excite a passionate rivalry in the develop- ment of bodily energy and skill, and which in this respect are far more efficacious than onr gymnastic and fencing exercises. It must not be forgotten that, the more young men are cut off from fresh air and from the opportunity of vigorous exercise, the more induced will they be to seek an apparent re- freshment in the misuse of tobacco and of intoxicating drinks. It must also be admit- ted that the English universities accustom their students to energetic and accurate work, and keep them up to the habits of ed- ucated society. The moral effect of the more rigorous control is said to bo rather illusory. The Scotch universities and some smaller English foundations of more recent origin — University College and King's College in London, and Owens College in Manchester — are constituted more on the German and Dutch model. The development of French universities has been quite different, and indeed almost in the opposite direction. In accordance with the tendency of the French to throw overboard everything of historic develop- ment to suit some rationalistic theory, their faculties have logically become purely insti- tutes for instruction — special schools, with' definite regulations for the course of instruc- tion, developed and quite distinct from those institutions which are to further the progress of science, such as the College de France, the Jardin des Plantes, and the Ecole des Etudes Superieures. The faculties are entirely sep- arated from one another, even when they are in the same town. The course of study ia definitely prescribed, and is controlled by frequent examinations. French teaching is confined to that which is clearly established, and transmits this in a well arranged, well- worked-out manner, which is easily intelli- gible, and does not excite doubt nor the necessity for deeper inquiry. The teacher? need only possess good receptive talents. Thus in France it is looked upon as a false step when a young man of promising talent takes a professorship in a faculty in the prov- inces. The method of instruction in Fiance is well adapted to give pupils, of even mod- erate capacity, sufficient knowledge ior the routine of their calling. They have no choice between different teachers, and they swear in verba inayistri ; this gives a happy self-satisfaction and freedom from doubts. If the teacher has been "well chosen, this is sufficient in ordinary cases, ia which the pupil does what ho has seen his tc-acher do. It is only unusual cases that test how much actual insight and judgment the pupil has acquired. Tho French people are moreover gifted, vivacious, and ambitious, and this corrects many defects la their system of teaching. A special feature in tho organization of French universities consists in the fact that the position of the teacher is quite indepen- dent of tho favor of his hearers ; the pupila who belong to his faculty are generally com- pelled to attend his lectures, and the far from inconsiderable fees which they pay flow into the chest of tho Minister of Education ; the regular salaries of the university profess- ors are defrayed from this source ; the state gives but an insignificant contribution toward the maintenance of the university. When, therefore, the teacher has no real pleasure in teaching, or is not ambitions of having a number of pupils, he very sooa becomes in- , different to the success of his teaching, and is inclined to take things easily. Outside the lecture-rooms, tho French students live without control, find associate with young rncn of other callings, without any special e.spri/ de corp.v or common feeling. Tho development of the German universities differs characteristically from these two ex- POPULAK SCIENTIFIC LECTU11ES. tremcs. Too poor in tKeir own possessions not to bo compelled, with increasing de- mands for the means of instruction, eagerly to accept tlio help of tho state, and too weak to resist encroachments upon their ancient rights in times in which modern states at- tempt to consolidate themselves, the German universities have had to submit themselves to tho controlling influence of tho state. Owing to thin latter circumstance the decision in all important university matters has in principle been transferred to the state, and in times of religions or political excitement this supreme power has occasionally been unscrupulously exerted. But in most cases the states which were working out their own independence were favorably disposed tow- ard the universities ; they required intel- ligent officials, and the fame of their coun- try's university conferred a certain lustre upon the government. The ruling officials were, moreover, for the most part students of the university ; they remained attached to it. It is very remarkable how among wars and political changes in the states fighting with the decaying empire for the consolida- tion of their young sovereignties, while almost all other privileged orders were de- stroyed, the universities of Germany saved a far greater nucleus of their internal freedom and of the most valuable side of this free- dom, than in conscientious, conservative England, and than in France with its wild chase after freedom. We have retained the old conception of students, as that of young men responsible to themselves, striving after science of their own free will, a,nd to whom it is left to arrange their own plan of studies as they think best. If attendance on particular lec- tures was enjoined for certain callings — what are called " compulsory lectures" — these regulations were net made by the university, but by the state, which was afterward to ad- mit candidates to these callings. At the $jur.o time the students had, and still have, perfect freedom to migrate from one German viniversity to another, from Dorpat to Zurich, from "Vienna lo Gratz ; and in each university they had freo choice among the teachers of the same Bubject, without reference to their position as ordinary or extraordinary pro- fessors or as private docents. The students are, in fact, free to acquire any part of their instruction from books ; it is highly desir- able that the works of great men of past times should form an essential part of study. Outside the university there is no control over tho proceedings of the students, so long as they do not come in collision with the guardians of public order. Beyond these cases the only control to which they are sub- ject is that of their colleagues, which pre- vents them from doing anything which is repugnant to tho feeling of honor of their own body. The universities of the Middle Ages formed definite close corporations, with their own jurisdiction, which extended to the right over life and death of their own mem- bers. AB they lived lor the most part on foreign soil, it was necessary to have their own jurisdiction, partly to protect the mem- bers from tho caprices of foreign judges, partly to keep up that degree of respect and order, within tho society, which wns neces- sary to secure tho continuation of the rights of hospitality on a foreign soil ; and partly, again, to settle disputes among the members. In modern times the remains of this aca- demic jurisdiction have bjr degrees been completely transferred to the ordinary courts, or will be so transferred ; but it is still nec- essary to maintain certain restrictions on a union of strong and spirited young men, which guarantee the peace of their fellow- ctudents and that of the citizens. In cases of collision this is the object of the disciplin- ary power of the university authorities. This object, however, must be mainly attained by the sense of honor of the students ; and it must be considered fortunate that German students have retained a vivid sense of cor- porate union, and of what is intimately con- nected therewith, a requirement of honorable behavior in the individual. I am by no means prepared to defend every individual regulation in the Codex of Students' Honor ; there are many Middle Age remains a7cong them which were better swept away ; but that can only be done by the students thorn- selves. For most foreigners the uncontrolled free- dom of Gentian students is a subject of PS- tonishment ; the more so as it is usually Borne obvious excrescences of this freedom which first meet their ej'es ; they are minblo to understand how young men can be so left to themselves without the greatest detriment. The German looks back to his student life na to his golden age ; our literature and our pootry are fall of expressions of this feeling. Nothing of this kind is but even faintly sug- gested in the literature of other European peoples. The German student alone has this perfect joy in the time, in which, in the first delight in. youthful responsibility, and freed more immediately from having to work for extraneous interests, he can devote him- self to the task of striving after the best i:nd noblest which the human race has hitherto been able to attain in knowledge and in spec- .ulation, closely joined in friendly rivalry with a large body of associates of similar aspirations, and in daily mental iiitcrcoursa with teachers from whom he learns some- thing of the workings of tho thoughts of -in- dependent minds. When I think of my own student, life, and of the impression which a man like Johannes Muller, tho physiologist, made upon us, I Ttrmit place a very high value upon this lat- ter point. Any one who has onco como in contact with one or more men of the first rank must have had his whole mental stand- ard altered for tho rest of his life. Such in- tercourse is, moreover, the most interesting that life can offer. You, my younger friends, have received in this freedom of the German students a costly and valuable inheritance of preceding g«n- POPULAR SCIENTIFIC LECTURES. 049 erations. Keep it — and hand it on to coming races, purified and ennobled if possible. You have to maintain it, by each, in his place, taking care that the body of German students is worthy of the confidence which has hitherto accorded such a measure of free- dom. But freedom necessarily implies re- fiponsibility. It is as injurious a present for weak, as it is valuable for strong characters. Do not wonder if parents and statesmen sometimes urge that a more rigid system of supervision and control, like that of the English, shall be introduced even among us. There is no doubt that, by such a system, many a one would be saved who is ruined by freedom. But the state and the nation is best served by those who can bear freedom, and have shown that they know how to work and to struggle, from their own force and insight and from their own interest in sci- ence. My having previously dwelt on tho influ- ence of mental intercourse with distinguished men leads me to discuss another point in which German universities are distinguished from the English and French ones. It is that we start with tho object of having in- struction given, if possible, only by teachers who have proved their own power of advanc- ing science. This also is a point in respect to which the English and French often ex- press their surprise. They lay more weight than the Germans on what is called tho " talent for teaching" — that is, the power of explaining the subjects of instruction in ft well-arranged and clear manner, and, if possible, with eloquence, and so as to enter- tain and to fix the attention. Lectures of eloquent orators at the College de France, Jardin des Plantes, as well as in Oxford and Cambridge, are often the centres of the ele- gant and the educated world. In Germany we are not only indifferent to, but even dis- trustful of, oratorical ornament, and often enough are more negligent than we should be of the outer forms of the lecture. There can be no doubt that a good lecture can bo followed with far less exertion than a bad one ; that the matter of the first can be more certainly and completely apprehended ; that n well-arranged explanation, which develops the salient points and the divisions of the subject, and which bring* it, as it were, almost intuitively before us, can impart more- information in the same time than one which has the opposite qualities. I am by no means prepared to defend what is, frequently, our too great contempt for form in speech and in writing. It cannot also bo doubted that many original men, who have done con- siderable scientific work, have often an un- couth, heavy, and hesitating delivery. Yet I have not infrequently seen that such teach- ers had crowded lecture-rooms, while empty- headed orators excited astonishment in the first lecture, fatigue in tho second, and were deserted in the third. Any one who desires to give his hearers a perfect conviction of the truth of his principles must, first of all. know from his own experience how conviction is acquired and how not. He must have known how to acquire conviction where no prede- cessor had been before him — that is, ho must have worked at tho confines of human knowl- edge and have conquered for it new regions. A teacher who retails convictions which aro foreign to him, is sufficient for those pupils who depend upon authority as the source of their knowledge, but not for such as require bases for their conviction which extend to \/ the very bottom. You will see that this in an honorable con- fidence which the nation reposes in you. Definite courses and specified teachers are not prescribed to you. You aro regarded as men whose unfettered conviction is to be gained ; who know how to distinguish what is essential from what is only apparent ; who can no longer be appeased by an appeal to any authority, and who no longer let them- selves be so appeased. Care is also always taken that you yourselves should penetrate to the sources of knowledge in so far as these consist in books and monuments, or in ex- periments, and in the observation of natural objects and processes. Even the smaller German universities have their own libraries, collections of casts, and the like. And in the establishment of labora- tories for chemistry, microscopy, physiology, and physics, Germany has preceded all other* European countries, who are now beginning* to emulate her In our own university we may in the next few weeks expect the open- ing of two new institutions devoted to in- struction in natural science. The free conviction of the student can only bo acquired when freedom of expression is guaranteed to the teacher's own conviction — the liberty of teaddng. This has not always been insured, either in Germany or in tho adjacent countries. In times of political and ecclesiastical struggle tho ruling parties havo often enough allowed themselves to encroach ; this has always been regarded by the Ger- man nation as an attack upon their sanctu- ary. The advanced political freedom of the new German Empire has brought a cure for this. At this moment, the most extreme con- sequences of materialistic metaphysics, the boldest speculations upon the basis of Dar- win's theory of evolution, may be taught in German universities with as little restraint as the most extreme deification of papal infalli- bility. As in the tribune of European parlia- ments it is forbidden to suspect motives or indulge in abuse of the personal qualities of our opponents, so also is any incitement to such acts as are legally forbidden. But there is no obstacle to the discussion of a scientific question in a scientific spirit. In English and French universities there is less idea of liberty of teaching in this sense. Even in the College do France the lectures of a man of Benan' s scientific importance and earnest- ness are forbidden. I have to speak of another aspect of our liberty of teaching. That is, the extended sense in which German universities hare ad- 650 POPULAR SCIENTIFIC LECTURES. mlttoil toachr-rs. In the original meaning of the word, a doctor is a " teacher," or one whose c:vi)a(%ity as teacher is recognized. In the universities of the Middle Ages any doc- tor who found pupils could set up as teacher. In course of time the practical signification of the title was changed. Most of those who sought the title did not intend to act as teach- ers, but only needed it as an official recogni- tion of their scientific training. Only in Ger- many are there any remains of this ancient right. In accordance with the altered mean- ing of the title of doctor, and the minuter specialization of the subjects of instruction, a special proof of more profound scientific proficiency, in the particular branch in which they wish to habilitate, is required from those doctors who desire to exercise the right of teaching. In most German universi- ties, moreover, the legal status of these habil- itated doctors as teachers is exa»tly the same as that of the ordinary professors. In a few places they are subject to some slight restric- tions, which, however, have scarcely any practical effect. The senior teachers of the university, especially the ordinary professors, have this amount of favor, that, on the one hand, in those branches in which special ap- paratus is needed for instruction, they can more freely dispose of the means belonging to the stiite ; while on the other it falls to them to hold the examinations in the faculty, and, as a matter of fact, often also the state examination. This naturally exerts a certain pressure on the weaker minds among the students. The influence of examinations is, however, often exaggerated. In the frequent migrations of our students, a great number of examinations are held in which the candi- dates have never attended the lectures of the examiners. On no feature of our university arrange- ments do foreigners express their astonish- ment so much as about the position of pri- vate docents. They are surprised, and even envious, that we have such a number of young men who, without salary, for the most part with insignificant incomes from fees, and with very uncertain prospects for the future, devote themselves to strenuous scien- tific work. And, judging us from the point of view of basely practical interests, they are equally sin-prised that the faculties so readily admit young men who at any moment may change from assistants to competitors ; and further, that only in the most exceptional cases is anything ever heard of unworthy means of competition in what is a matter of some delicacy. The appointment to vacate professorships, like the admission of private docents, rests, though not unconditionally, and not in the \ / last resort, with the faculty, that is with the body of ordinary professors. These form, in German universities, that residuum of former colleges of doctors to which the rights of the old corporations have been transferred. They form as it were a select committee of the graduates of a former epoch, but estab- lished with the co-operation of the Govern- ment. The usual form for the nomlnatT'n of new ordinary professors is that tl.»> hu-uitv proposes three candidates to Government ior its choice ; where the Government, Lowtvi-r, does not consider itself restricted to tho candidates proposed. Excepting in times of heated party conflict it is very unusual for the proposals of tho faculty to be passed over. If there is not a very obvious reason for hesitation it is always u serious personal re- sponsibility for the executive officials to elect, in opposition to the proposals of com- petent judges, a teacher vho has publicly to prove his capacity before large circles. Tho professors have, however, the strong- est motives for securing to the faculty the best teachers. The most essential condition for being able to work with pleasure at the preparation of lectures is the consciousness of having not too smuJ. a number of intelli- gent listeners ; moreover, a considerable fraction of the income of many teachers de- pends upon tho number of their hearers. Each one must wish that his faculty, as a whole, shall attract as numerous and as in- telligent a body of students as possible. That, however, can only be attained by choosing as many able teachers, whether professors or docents, as possible. On the other hand, a professor's attempt to stimu- late his hearers to vigorous and independent. research can only be successful when it i« supported by his colleagues ; besides this, working with distinguished colleagues makes life in university circles interesting, instruc- tive, and stimulating. A facility must have greatly sunk, it must not only have lost its sense of dignity, but also even the most or- dinary worldly prudence, if other motives could preponderate over these ; and such u faculty would soon ruin itself. With regard to the spectre of rivalry among university teachers with which it is some- times attempted to frighten public opinion, there can be none such if the students and their teachers are of the right kind. In the first place, it is only in large universities that there are two to teach one and the same branch ; and even if there is no difference iij the officirl definition of the subject, there will be a difference in the scientific tenden- cies of the teachers ; they will be able to di- vide the work in such a manner that each has that side which ho most completely masters. Two distinguished teachers who are thus complementary to jach other, form then so strong a centre of attraction for tho students that both suffer no loss of hearers, though they may have to share among themselves a number of the less zealous ones. Tho disagreeable effects jf rivalry will be feared by a teacher who does not feel quite certain in his scientific position. This can have no considerable influence oa the official decisions of the faculty whcu it is only a question of one, or of a small number, of tho voters. The predominance of a distinct scientific school in a faculty may bccoruo more injuri- ous than such personal interests. When tho POPULAR SCIENTIFIC LECTURES. school h-i~, s •,i.i:itifiv\'i!!y outlived itself, stu- dent-t \viil pr Voabiy migrate by degrees to other universities. This may extend over a long period, and the faculty in question will suffer during that time. We so<> bi> -it how strenuously tho \iniversi- ties under this system have sought to attract the scientific ability of Germany when wo consider how many pioneers have remained outside the universities. The answer to such an inquiry is given in the not infrequent jest or sneer that all wisdom in Germany is pr»- I'essorinl wisdom. If we look nt England, we see men like Humphry Davy, Faraday, Mill, GiMto, who have had no connection with English universities. If, oa the other hand, we d;3vlu :fc from the list of Garman men of science those who, like David Strauss, havo been driven away by Government for ecclesi- astical or for political reasons, and those who, as members of learned academies, had tha right to deliver lectures in the universities, as Alexander and Wilhelm voa Hujaholdt, Leopold von Bach, an I others, the rest will only form a small fraction of tho number of the men of equal scientific standing who have been at work in the universities ; while the same calculation made for England would give exactly tho opposite result. I have often wondered that tho Royal Institu- tion of London, a private society, which provides for its members and others short courses of lectures on the Progress of Natural Science, should have been able to retain per- manently the services of men of such scien- tific importance as Humphry Davy and Faraday. It was no question of great emolu- ments ; the jo men were manifestly attracted by a select public consisting of men and women of independent mental culture. In Germany tho universities are unmistakably tho institutions which exert the most power- ful attraction on the taught. But it is clear that this attraction depends on the teacher's hope that he will not oiily find in the uni- versity a body of pupils enthusiastic and ac- customed to work, but such also as devote themselves to the formation of an indepen- dent conviction. It is only with such stu- dents that the intelligence of tho teacher bears any further fruit. The entire organization of our universi- ties is thus permeated by this respect for a free independent conviction, which is more Htrongly impressed ou the Germans than on their Aryan kindred of the Celtic and Romanic branches, in whom practical politi- cal motives havo greater weight. They are able, and as it would seem with perfect con- scientiousness, to restrain the inquiring mind from tho investigation of those princi- ples which appear to them to be beyond the rango of discussion, as forming the founda- tion of their political, social, and religious organization ; they think themselves quite justified in not allowing their youth to look beyond the boundary which they themselves are not disposed to overstep. If, therefore, any region of questions is to be considered as outside tho range of discus- sion, however remote and restricted it may be, and however good may bo the intention, the pupils must bo kept in the prescribed path, and teachers must bo appointed who do not rebel against authority. Wo can then, however, only speak of freo conviction in a very limited sense. You see how different was the plan of our forefathers. However violently they may at limes have interfered with individual result." of scientific inquiry, they never wished to pull it up by tho roots. An opinion which was not based upon independent conviction appeared to them of no value. In their hearts they never lost faith that freedom alpn.3 could cure the errors of frecdom7lmd"a riper knowledge the ercOTS^Fwlialris- uni'ipe. Tho same spirit which overthrew the yoke of the Church of Rome also organized the Ger- man universities. But any institution based upon freedom must also be able to calculate on the judg- ment and reasonableness of those to whom freedom is granted. Apart from the points which have been previously discussed, where the students themselves are left to decide on the courso of their studies and to select their teachers, the above considerations show how the students react upon their teachers. To produce a good course of lec- tures is a labor which is renewed every term. New matter is continually being added which necessitates a reconsideration and rearrange- ment of tho old from fresh points of view. The teacher would soon bo dispirited in his work if he could not count upon the zeal and the interest of bis hearers. The estimate •which he places on his task will depend on how far he is followed by the appreciation of u sufficient number of, at any rate, his more intelligent hearers. The iifnux of hearers to the lectures of a teacher has no slight influ- ence upon his iiiiue and promotion, and, therefore, upon the composition of the body of teachers. In all these respects, it is assumed that the general public opinion among the students cannot go permanently wrong. The majority of them — who are, as it were, the representatives of the general opinion — must come to ws with a sufliciently logically trained judgment, with a sufficient habit of mental exertion, with a tact Kufiiciently developed on the best models, to be able to discriminate truth from the bab- bling appearance of truth. Among the stu- dents are to be found (Lose intelligent heads who will l)e the ijifiitnl leaders of the next generation, and v. ho, perhaps, in a few years, will direct to themselves the eyes of the world. Occasional errors in youthful nnd excitable Hpirits naturally occur ; but, on the whole, \vo may bo pretty sure that they will soon set themselves right. Thus prepared, they have hitherto been sent to r.s by the gymnasiums. It would be very dangerous for the universities if large numbers of students frequented them, who were less developed in the above respects. 652 POPULAR SCIENTIFIC LECTURES. The general self-respect of the students must nut bo allowed to sink. If tbnt -were the OIIKO, tho dangers of academic, freedom would choke its blessings. It must therefore not bo L--I