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W. H. DALLIXGER, LL.D., F.R.S., &c. 

WITH TWESTY-osE PL.iTEH .ist> EKniT iirxiir.Kb w-o/i E.yvx.nixas 



All right! rtif^wnl 


Tbe use of the Microscope, both as an instrument of scientific research 
uid na a. means of affording pleasure and recreative instruction, has 
bccomeso widespread, and the instrument is nuw so frequently found 
in a.n expensive form capable of yielding in skilled hands good 
'iptical resultiS, that it is eminently desirable that a treatise should 
he within the reach of the student and the tiro alike, which would 
provide both with the elements of the theory and principles involved 
in the construction of the instrument itself, the nature of its latest 
appliances, and the proper conditions on which they can be em- 
ployed with the best results. Beyond this it should provide an 
outline of the latest and I>est modes of preparing, esamining, and 
mounting objects, and glance, with this purpose in view, at what is 
eisily jR'cesail lip fur the requirtments of tbe iiniiiteur in the entire 
■■rijaiiii' imd inorj;aiiie kiiif^dums. 

This ntrd liiis bei-n for many years met l>y tliiH lx)ok, and 
its six [ireo'tling editions have been an extremely gratifying ovidence 
of the inilustry and pi-udition of its Author. From the Ijeginning 
it opened the right path, and aflbrded excellent aid to the earnest 
aniiileiir and the careful student. 

Hut the Mici-ost'ope jn its very highest form has become — so far 
at least as objectives of the most perfect construction and greatest 
u>eful magnifying power are concerned- -so common that a much 
m<'Tf accurate account of tlie theoretical basis of the instrument 
itself and of the optical apparatus employed with it to obtain the 
best results with ' high powers ' is a want very widely felt. 

The Mlvances in the matlieniatical optics involved in the con- 
struction of the most perfect form of the present Microscope have 
lieeu lery rapid during the last twenty years ; and the progress in 
the [)rincipl<-s of pnictical construction and tbe a[>])licatiiin of theory 
liave, even since the last edition of this l)Ook was pu!)lislied, bi;en so 
marked as to produce a revolution in the instrument itself and in its 

au^ ni 



applictition. Tlio new diapensntion vas dimly indicated in the last 
edition ; but it hosi effected so radical a change in all that apper- 
taias to Microscopy that a thorough revision of the treatment of 
this treatise was required. The great principles involved in th© 
use of the new objecti^■es and the interpretation of the imagns pre- 
sented l>y their means, are distinct and unique ; and unless these h& 
clearly understood the intelligent use of the finest optical appliancea 
now produced by mathematical and practical optics cannot be 
hi*ouglit altout. They have not rendered the use of the instrument 
more difficult — they have rather simplified its employment, provided 
the o]>erator undrrstands tlie general nature and conditions oa 
which his Microscope should lie used. If the modern Microscope be, 
aa a nif clianical instrument with its accompanying optical apparatus, 
as goo<l as it can be, a critical image — a picture of the object baring 
the most delicately beautiful character — is attainable with 'low 
powers' and 'high powers' alike. Microscopisls are no longer 
divisible intd those who work with 'high powers' and those who 
work with 'low powei-s.' No one can work properly with either 
if lip dues not understantl the theory of their construction and the 
principles upon which to inteipret the results of their employment. 
If he is faTriiliar with these the employment of any range of magni- 
fying power is simply a question of cai*, experiment, and practice ; 
the principles applicable to the one are involved intheother. Thusi 
for example, a pi-oper understanding of the nature and mode of 
optical action of a ' sub-Stage condenser ' is as essential for the very 
finest rcsultti in the use of a 1-inch object-glass as in the use of a 
•2 mm. with N.A. 1-40 or the 2-^) mm. with N.A. I'GO, while it 
gives advantages not otherwise realisable if the right class of con- 
ilonser used in the right way be empluved with the older ,'nth inch 


But be efisenti&lly a cyclopedic work. This wm far more possible 
loMW man wheu Dr. Carpenter began his work than it was even 
vben he issued his last edition. But it ia practically impossible 
KW. It is with Microscopy as with every department of scientific 
nrk — we must depend upon the specialist for accurate knowledge. 

In the following pages I have been most generously aided. In 
DO department, not even that in which for twenty years I have 
b»«n specially at work, have I acted without the cordial interest, 
ta^gestion, and enlightenment afforded by kindred or similar workers. 
In every section experts have given me their unstinted help. 
To preserve the character of the book, however, and give it homo- 
geneity, it was essential that all should pass through one mind and 
be EO presented. My work for many years has familiarised me, 
more or less, with every department of Microscopy, and with the 
great majority of branches to which it is applied. I have therefore 
given a common form, for which I take the sole responsibility, 
to the entire treatise. The subject might have been carried over 
ten such volumes as this ; but we were of necessity limited as 
to space, and the specific aim has been to give such a condensed 
view of the whole range of subjects as would make this treatise 
St once a practical and a suggestive one. 

The first five chapters of the last edition are represented in this 
edition by seven chapters ; the whole matter of these seven chapters 
has been re- written, and two of them are on subjects not treated in 
»nv fiimier ediiion. These seven cliaptcrs represent the experience 
of ;i lifetime, ciiitiniipd and aided by the iidvice and }ivacticrtl help 
of some of the most exficrienced men in th<' world, and tliey may be 
Te:v\ l.y iinvoin' fiiiiiiHar with the use of algebraic symbols and tlie 
pnn-tici' of the rule cf tliri'i'. Tliey are not in any spjise abstruse, 
Hiiil tlifv are eveiywliere practical. 

In the second chapter, on The Principles and Theory of Vision 
witl[ the Conipound Jlicroseope, so much has been done during the twentv years by Dr. AliiiE, of .lena, that my firht desire was to 
induce hini tn summarise, for this trejitisc, the results of his twenty 
yctrs of unri-niitting and manellously productive labour. But the 
'tate of his health and his many obligations forbade this ; and at 
lenftii it became apparent that if this most desirable end were to 
fx- s<>eurpd, I must re-study with thi.s object all the luonoyrapii^ of 
tiiis autlior. I summarised iliem, not without anxiety ; but that was 
Bfjeedily renjoveil, for Dr. Auiik, with great generosity, consented to 
examine my results, and has been ^ooil enough to write (hat be has 
' read [my] clear expositions with the greatest interest ; ' and, after 
wonls which show his conlial fnendliness, he says ; 'I tind the wliole 
, . . much mor*' adequate to the puiposes of tlie book tliaii Isiiould 

viii PkEFACE 

have been able to write it. ... I feel the greatest satisfaction in 
Keeiiig my views represented in the book so extensively and inten- 

These words tire more than ^nerouB ; but I quote them ben 
in order tlmt the reader may be assured of the accuracy and 
eHicieiicy of t}ie account given in the following pages of the invalu- 
able (Ifiiionstrations, theories, and explanations presented by Dr. 
AuiiK on the optical principles and practice upon which the recent 
injpnivement i]\ the coiLstruction of microscopical lens systems has 
BO much depended. 

It will not be supposed that I implicitly coincide with every 
(tfitail. Dr. ABUif is too sincere a lover of independent judgment 
Ui fven desire this. But it was important that his views as such 
should be found in an 'accessible English form ; in that form I have 
Piideiivoured to present them ; and in the main there can be no 
doubt whatever that these teachings are absolutely incident with 
fdct ftn<l experience. In details, as may appear here and there in 
Ui<-s<5 ptRea, especially where it becijmes a question of practice, I may 
dilter iis t^i methiid, and even interpretation, from this distingnished 
inasti-r in Mathematical Optics. But our differences in no way affect 
the j,'r<;at princi|>lp8 he lias enunciated or the comprehensive theory 
of inicniseopicnl visiiin he has with such keen insight laid down. 

In prejiaring the remainder of the seven new chapters of this 
Iioiik I have sought nnd, without hesitancy, obtained advice and 
the advantage of the support of my own judgment and experienoe 
fnini many competent men of science, who have shown a sincere 
interest in my work and Jiave aided me in my endeavours. But, 
first un the list, I must place my friend Mr. E. M. Nelson. Our 
lines of experience with the Microscope have run parallel for maoy 


From Dr. lUriHius 1 tiNve rocpivnc) cxrdiiil niil in dotlittg with 
bU it{Mi<cui1 i>uliji-ct, tht! Rotifem ; and u> Mr. Auikrt Mini.tKL 1 uui 
undrr rcjiMl oliliukltmi tttr liui luwiftancn in n>j[nnl Ut iIr- A<:nrina. 

Mr. W, T. Scproi.K gave lae his mcuit welcome judgiu^iit Htid 
lulviirrv n^i^nrdiiiK my ohnptcr on Iktr)unting, nnd 1 i-ccrivi-d nlmt tlii^ 
Bi]|{)(eMioTit> uf Air. A. (JoLB witli niucli pIiM8UT« and a>lvittiU|{e'. 
I have n'cri%Td hi'ljt fmm Dr. A. IJiLr., of l>t>wninjH; (^>lli^, 
Oambridf^e, and (rom Frofeasor J. N. Lavolgv, o* Trinity College, 
CWinbridg* — from Imtli of whom special proceau« of pivparotton 
for histological vurk wnrv aunt. 

Mr. Prank Ckwi', *ith ciiftnictcristic ^n^rwwiy.aidi-d mmiuch 
by suggeations of special mid praclind value; aodMr. Joiix MAV-itt., 
Jan., th« pr««int S«cn^ry of the Koyal ^Cicroncapical tSocirty. luui 
beeD UDtiriiis in liia willin^neM to furnish i)ie aid which his jnfluetiee 
WAK «bl« to Ei<«urp. 

To Pn.feH«or W. Hickb, F.R.8., PrJiuripal of Firth CoUeje, 
6hetliel<l, I ant in<iebte(l for tlie revision of special sheets; ^ also I 
(>w«kokiiow!Mlgnii-nta U> Dr. HKSnv Cliffox Sobbv, K.R.S.. and to 
Dr. Ukovkm, as well as to othei-s, wliose «l£g;est)ons, advice, or cod- 
flrtnationof myjudgmraitahavn bcrn much MteemM] ; and praniinent 
WDOiigst thtne are ftufessor Alfrbd W. UesSKTT, llJic, and Professor 
F. Jy.WHRX Kkll, M.A., whom^ constant advico In thi'ir dn{iartn)entit 
u{ Biohicy 1 liavi' ri'L-rJtml thrciugliout ; wliilo in Iiliun>-i^ogi<«l 
Bubjncts 1 liav« been aided by tltn suggestions an<l «xp«ri«Dc« of 
ProfCMor J. ljttK.iit»o» UvtAMi, D.Kc. 

It will be obaervMt thitt every endeatvoar lias l>een niAd« to 
bring t«chiif iJinnKiny subJActadiscDtM-d in thiw hook into oonfunu Ely 
with till- most rtiw'nt kiiowledgi> of expert*. Many of tlie sections, 
in fftct, have hern wliiilly rrwritti-n and illtuAmted from new and 
ortKiual sctunt-ii : this luay he seen in the studious on tlie History 
as well US the Con Rt ruction nnd Vf! ot the MieroKoiK' and itci appJi- 
oncea, aa al^v in th»w «n Diatotnacen^. D«suiids, Saprophytes, 
itact«riii, Rfrttfrm, Acarina, nnd in U»« choptwn on Micniaoopiv 
Ceoloj-y and MintndoKV. To the same eod nineteen new plates 
havp Iweii prepai-dl and 300 additional woodcuts, many of which 
are aUo wt-w ; nnil for iJie UM- of the uwjnrity irf Ihohe which are 
not no, I am imlebtod to the Ivdilors an(i Sccroiary of ihn Koyal 
l^i<'r>i>c<>pii'Jil Society. 

Th*re cerlainly nevi-r was a time when the MicrMcope was to 
^nrrally ust^l n-s it now is. With lunny. lualiiuidy alalfd, it isiiiniply 
un inatrument eniployedffor elegant and instructive relaxation nnd 
nniiLsenicnt. Kor this thireran Im nnthitiKbut i-i'mii endation. hut it ih 
desirable that even this end should l>e tkiught intelligently. Tho social 
inllucnce u( iIh- Microacopniwaiiinstrunientemployed ttw recienticn 


and pleasure will be greater in proportion as a knowledge of th^ 
general principles on whicli the instrument is constructed are known, 
and as the principles of visual interpretation are understood. Thft- 
interests of these have beei) specially considered in the followiiig- 
pages ; but such an employment of the Microscope, if intelligently 
pursued, often leads to more or less of steady endeavour on the part 
of amateurs to understand the instrument and use it to a purpose 
in some special work, however modest. This is the reason of the- 
great increase of ' Clubs ' and Societies of various kinds, not only in 
Lomlim, and in the provinces, but throughout America ; and these 
are doing most valuable work. Their value consists not merely in the 
constant accumulation of new details concerning minute vegetable 
and animal life, and the minute details of larger forms, but in the 
constant improvement of the quality of the entire Microscope on its 
optical and meohaiiioal sides. It is largely to Amateur Microscopy 
that thedesire and iiio/ii-e for the great improvements in object-glasses 
and eye-pieces for the last twenty years are due. The men who have 
con)[iarcd the qualities of respective tenses, and have had specific ideas 
OS tJ) liow these could Iiecoine po^essed of still higher qualities, have 
been comparatively rarely those who have employed the Microscope 
for profession!!.! and educational purposes. They have the rather 
Hiui[)ly nsi'il employed in the execution of their professional work 
— the best with which the practical optician could supply them. 
It has been by amateur microscopists that the opticians have been 
incited tci the production of new and improved objectives. But it 
is the men who woi'k in our biological and me<iicftl schools that 
ultimalely reap the immense advantage— not only of greatly im- 
proved, but in the end of greatly cheapened, object-glasses. It is 
on this account to the advantage of all that the amateur micro- 
Hcopist should have within his reach a handbook dealing with the 
principles of his instrument and his subject. 



Pig, 1. M 6 diameters. Horizontal and tracsverBe section oE an orbltolite. 
Fig. 2. An imperfect or oncriticBl image of the minute hairs on the lining 
mcmbnne of tbe extremity of the pcoboscia of the blow-Sf ic fiIOdiamE.,takeD 
■itb a Zeiss apochromntic j^-incb objective of '96 N.A. x 3 projection e;e-piece ; 
Iihi it was illuoiinated b; a cone of small angle, viz. of 0-1 N.A., and illastiateB 
the noadvisability of small cones for illumination. 

Tha first obTions feature io the picture is the doubling of the haira which 
ire oat of focos ; but the important difference lies in the bright line with a 
dtrk edfie round the bairg which are piecisely in focus. This is a diffraction 
ctltct which is always present round the outlines of every object illuminated 
bf ■ eoue of inaalGcient angle. Experiment shows that this diffraction line 
ilmjE ceases to be visible wlien the aperture of the iUuminating cone is equal 
Io about two-tbirds the aperture of the objective used ; but it will become 
V^B distinctly apparent when the aperture of the cone is reduced less than 
Val( that of the objertivp. 

Fi^.-. .i. » 51(idiani>^. .\ correct orcriticil image of the minute hairs on the 
lining mtmbrane of tlie eitrtniityof the blo«-tly'> proboscis. In this piuture 
thttntuj hit* betn aiijuMe'l tor the long central iinir. It will be ob.serveii that 

tbL-- hair is very lin n) spinous ; it has nut tlie ring socket which is common 

t>i many hair- on inx-'Cts, but urons from a very delicalu meiiibrane. which in 
Ibe bsbain mount U transparent. This pliotoj^rapli was taken with a Zeiss 
•['■chtiimatio ; uf LI5 N.A. x'i projeciKin eye-piece. The illumination was 
(bat ii( a Ut>:<' ^oliil axial cone of ^.'1 N.A, froui an achromatic condenser. the 
M.urci- of liLihi l«ing frjcusaed on tht- object. 

Kif.'. 4. ^•^.■^■lion of cerebellum of a lainb, k 77 diams., by apochromatic 1-inch 
;( N..\. This preparation ivas courteously supplied to the present Editor by Dr. 
ii.ll. who--L- iiubedding and staining processes for llie.te tissues it beautifully 

Fi}f. 5. .^mphipleura poUuoida x ISilO diaras., by apochromatiii ^ r4 K.A. 
illuminaleil liy a very olilii|Ue pencil in one aiimuth alon>: the valve. 

Frg. 6. A hair of I'olyxenus laKorH>^, a wi'lt-knrnvn and exoollent teat 
o*iji-ct for medium powers x 4'.Wl dijiTii.'i. by apocltronialie \ ■'!."» N.A. 

I'ij". I- A small vessel in the bladder of a frug, prepared with nitrate of 
silver *lain, showing endotheliuni-cells, x 4II diaiu^., by Zei^.s .\, -J N.A. This 
..ljj<ct has been photographed for tlie pnri>o'ie <>( expt.sin;; the fallac.v which 
underlies the general I v accepted statement tliaL ' liNW-anyled ' glasses are the 
nii)-t suitable for histolo(;i«d purposes. The su[i[Hj«ition that it is so lias 
l*i.'n fijuodeil on (he fact tliat the penetr:Llion of a Iins varies inversely as its 
ajierture: therefore, it is .said, a ' low-an;;li'd' ^da-ss i» Io bo preferred to a 
uidr-angled one. l)ec;iUBe "deiith of focus.' whii li is siL]i|"jsed to enable one 
I'l -He into tissues i-' 'he end in view. 

OucarefuUy examining this figure it will be noticed that it i^ almost 


impuMHtlili; lo trace the oullinc of any parlicolnr endothelium -cell hecsiue i'* 
iiDBgu is confused wilh that of the lower aiile of the pipe. In a monocnl^"*^ 
microscopical ima^ea pcmpectite view does not exist ; it is better, therefore, t^-^ 
use a wide-angled lens, and so obtain a clear «iew of a tbia plane at onetiiu^^' 
imd educate the mind to appreciate solidit; by means of focal adjnsttnen*' 
It will be admitted that unless one approaches fig. 7 with a precoucpived ide^^ 
of nhAt an endothelium-ccll is like, the knowledge g&ined of it will be smaL^ 

Piir. N represents ihe same stnicEuTe. x ISS diams., by an apochromaticE^ 
i Gu S.A. Here onl; t)ie iipprr sarface of the pipe is seen, so that the out- 
fine iif the endothelium- eel In can be clearly Imced. The circular elastic tissue 
is also iliHplaycd. There is, moreover, an increased sharpness over the whole 
[lictiire, due to the i;rester aperture of the objective. 


Ki^- 1. Tbe innidc of a valve of Pleurosigma aoKolstnm, ahoning » 
' pOHtagu stamp ' fracture, k ITGO diatns., with an apochromatic j^ 1-4 N.A. bj 
Mr. T. V. Mmitli, and illustrating his view of the cature of the FleDrosiiniut 

Pig. 2, The onlsido of n valve of Pieuroeigroa angnlatum, showing a dif- 
ferent form of structure, x 1T5() diams., with an apochromatic ^^ ll N.A , by 
-Mr. T K Smith. TheMtwo plioto-micrognipllBdemonstratetheeiistence of «t 
li'ikst two liiyers in the unifiilatuni. 

Fi);. :<. CoscinodiKcaH asieromphalus, x 110 diams., with an apochromatic 
1-inch :l N.A. 

V\g. 4. A portion ci[ the prcecdine-, x 20O0 diams. to show tlto Incework 
inside the areolntion^. This lace work is believed lobe a perforated titructuie, 
as a frariiirt' pnssi->> through the markings. In the central areolation there 
are forty- six Bm]dler porfomticna surrounded by a crown of lifteea larger onea.' 
riiotOKTaphod wi'-h nn wpocliroumtio i 1-4 N.A. 

Kig. 5. Aulaiiodiscus Kittotiii, >: 270, by an apochromatic 1-inch -3 N.A. 

fit:. <l. A small portion in the centre of an Aulacodiscus Slurtii, » 2000, 
by un iipfx^hramntic ^ 14 N.A. itroodly speaking, the difference between the 
CoscinoiliM:! and tliu Aulocoiiisci lies in the fact that in the former tbe 
Kccond;iry structure is inside the primary, while in the latter it is exterior to it. 
Tills detinition, however, is not strictly accurate, as it is believed that the fine 
perforated ntructiire covers the entire valve, it being only optically bidden by 
Ihe primary structure. 

The whole of these demonstrations were photographed for the present 
Bilitor by his friend K, M. Nelson, Ksq., and have been reproduced from the 
nogiilivea by ft process of photo-printing. 


urrHOD OF rsiNO dibkct TaANSMiTTED light without the 


Pistes II. to V, are engraved from pbotogrspha, taken at Ibe request of 
the Editor by Mr. E. M. Nel»oii, from the arranged instmmeiKs. 

SEXL'AE. ues'sratiun OK voLVOX GLOBATOB. (After Coho) 

Fig. 1. Sphere of ^'otrox gtabaler at the epoch of sexual generation : a, 
Eperm-cell eontainiog cluster of anthcrozoids ; a^ sperm-cell showing side- 
view of discoidal clu.iter of antherozoids ; a', Bperm-cell whose cluster has 
broken up into its component antherozoids; a', sperm-eel! parti; emptied bj 
the escape of its antherozoids ; h h. flask-shaped genn-oells showing great 
increase in size without sal>divi8ion ; b', b', genn-cells with lai^ vacuoles in 
their interior ; ft", germ^cell whaie shape has changed to the globular. 

Fig. 2, Seiual celt, o, distinguishable from sterile cells, *, by its larger 

Fig. 3. Qerm-celi, with antheroids sniirming over its endocbrome. 

Fig. 4. Fertilised germ-cell, or oosphere, nith deiise envelope. 

Fig. 5. Sperm-cell, with its contained cluster of antberozoids, more 

Figs. 6, 7. Liberated antheroioids, with their flagelia. 



Fig. 1. Lynghija /rttuarii. I.ieb. x Ififi. 
Fig. 2- SinTiil'Kitt JeB3ii-ri. Kii. x 40<>. 
Fig. 3. Tolypothrir cirrhiita.C&tm. » 400. 
Fig. 4. Oteitlaritt imigui; Thw. x 400, 
Fig. 5. 0. FToliphii, Kti, x 400. 
Fig. 6. O. tetierrima, Klz. x 400. 
These figures are after Cooke. 



Fig, 1. Zygosperm of .ViBratferiat dntieuhta, T!rfb. (After Balft.) 

Fig. 2. iiifmariiim Brehiixmii. Men. (Af'er Cooke.) 

Fig. 3. Eiiattrvm pectinatum, IJrfeb. (After Ralfn.) 

Fig. 4. Zvpospenn of Stanra/tnim hirsvtum, Brt^b. {Aft;r Kalfs.) 

Fig. 5. S. graeile. Kalfs. (After Cooke.) 

Fig. fi. Xaathidivm amfi-atum, Ehrb. (After lUlf!.^ 

Fig. 7. Il%r«laria dura. Ktz, (After Cooke.) 

Fig. 8. Jl. dura, Ktz. x 400. (After Cooke.) 

Fig. 9. Scytnnema ruxtam, Brfib. x 400. (Alter Cooke.) 

Fig. 10. StauTottrvm hiriutum, Brfb. (After Cooke.) 




Ki)r. I. iHerasUriai cmx-melitfiiU, Shib. (After Cooke.) 

t"\ii. 2. Cloiteriiim t/taceum, Etirb. (After Cooke.) 

Y\R. i. DfiiKuiiiin Sitarl:ii. Ag. (After Cooke.) 

Fin. ^■ Pen'""' digitat. Ehrb. (After Cooke.) 

Ki^;, n. P. dijitiu. Khrb. (transversa view). 

V'nc. r>. Sinmt/rnia eondeiuata, Br£b. (After Cooke.) 

Ki^. T. DoridiHiH baciilunt, Br6b. (After Cooke.) 

Kin. N. OaaiUo:ggon BrebiuniiH. De Bary, conjugating. (After Cooke.) 



This i* a direct pliDti.-microtfraph, taken by Dr. R. ZeUs, as nmgniGed i900 
i1iB.iiii'1i'r:<. VVi.' <lii'ei:t ntleotion specially to it as giving evidence of the pre- 
hcni'i! (however originated) of the intercostal mark ings, which may be seen 
with cuniidcmble clearness on the right-hand aide of the midrib and in the 
iiiid'llc of the valvo. 


Thin plate bus a twofold puqioac. It is designed, first, to jastify the 
n|iiniiiim belil liy Dr. Iletiry van Keurck upon the ntructure of the valves ol 
iliiilimis, imd nlsii tn ^how how tne usual microscopical tests present them- 
wlvrs «lii-ii exaniintd with the new objectiTe with K.A. 1*60, lately constructed 
liy tlie h'irm of Zcisn. This objective is believed by Dr. van Heurck to realine 
wliat hi' coRKiilcrs (he highest results of pbotogmphtc optics, which in his 
JLi'll^iiuMit rnutd only be ^urpasiieci by linding a Dew immersion liquid of still 
liiybiT rGfractivi: inilei prcseiiliii); nil the necessary qualities, and which at the 
winiu time would not alfect the very dclicaie flint of which it is necessary to 
iiiiilti! ibi! front li'ns of thiaolijectivc. This medium be hopes may be some day 
TvaliKed. Unfurtanately. ii{>to this time, no indication permits as to foresee the 
iliscoviTy of the lirjoid desired. 

Thi^ following i» tlie way in which Dr. Henry van Heurck summarises bit 
hIcu,-. iijiuii ihf .-ti-iiitiirt of the vnlvi! :— 

1. The v.tlvu of diatoois ' U forrocd bv two membranea or thin i>Ute« and 


jgch, in biiBf, is the Tiew held bj Dr. ran Heurck en an interpret ntion oE 
niptwnt knowledge at the atiuctnre of the valre of the diatoms. We give 
Kr a dncriptioQ of the objects represented on the plate. 

Tigi. I, 2, 3. Ampkipteura prUneida, Euti, 1 and 2, valve resolved into 
(wrif. fig. 3 ■ 2000 diama. Fig. 1 x 3000 diama. Fig. 3, Valve resolved 
in uni at about 2300 diama. 
Fig. 4. Amphipleura Lindheimeri, Or,, x 2500 diams. 
Fig. G. Flevratijfma anaulatum, in faexagona, x (about) 10.000 dtams. 
Fig. 6. Idrm v 2000 diama., illnsory pearls which are formed by the angler 
I al tbe bezagonal cells when the focossing is not perfect. 

Fig. 7, The nineteenth bond of No^rt's test plate. This photo-micro- 
piph has been made except iooally with the apochroniatic ^ of 1-1 N.A. 
Ik lines being traced apon a cov^r in crown-glaes, the objective of N.A. 1-6 
cunot be uMd here. 

Fig. S. Surirtlla frmma, Bhib. x (about) 1000 diams. 

Fig. 9. Van HruTclda cratHtierrig, Btib. (Frnstnlja saxooica, Habb) x 200O 

All the photo- micrographs (except Bg. 7) have been done with the new ^- 
ack N.A. 1-60 of MM. ;Ceis8. 

Hwae micro-phot ograpba have been prodaced bj aunlight in a moDochro- 
DUic form, the special compeiuating eye-piece 12, and the Abbe condenser of 
NJ. 16 

Corers and slides in flint of l'T2 ; diatoms in a tnediom 2'4. 

We are boand, however, to note that the condenser uaed is not corrected in 
Uf mj ; its aberrations are enormous. Although the highest admiration mus1> 
be eipresaed for the skill exercised by Dr. van Henrck in these remarkable 
pboto-micrographa, andthehighe«testeem for his conrlcsy to the present Ediror 
in supplying them, it most not be forgotten that Dr. van Heurck was obliged 
lotmplojan imperfect condenser— a condenser absolutely ahcorrected— and 
illhoiqth we can testify to the high qnality and line corrections of ut least one 
of the lenses of N.A. 1-6, we are convinced that much of its real perfection 
in image-forming Is destroyed by uncorrected sub-stage illnmlnation. Upon 
the cotiections and large aplanatic area presented by the condenser and its 
citefol and efficient employment depends entirely the nnture of the image 
p^f■^^nted by the finest objective ever constructed ; and mh the perfection of the 
ohjpciive, with a high amplification and a grtat aiwrture, ia more nearly 
i|itniach«i. the niorp ilopcndent are we upon jiorfecl I'orrtelioii? in tlie coti- 
'I'-n'er to brinu out llie perfect image- forming power of the objective. No 
imagL' formed by such an objective as tliiit pcsses^ing N.A. l-liO ''jiii he coiisi- 
>itn-ii r« liable until a condenser corrected for all abberrat ions like the abjective 
ii.#lf is prcluce"! ; and so convinced are we of the possible value of tliis objec- 
liie that we trust it-s distinguished devisor atid maker may be soon ituhicod to 
prmliice Ibe condenser referred to. 

If. 1 lien, by the aid of the chemist we can discover media which wilt he 
of -iiHiciently Yiljth refractive index, and still tolerant ot or non-injurious to 
"Itanic twsuea iralnersed in it, a new line of investigation may be open to 
lii'tulop- and pathology.— \V. H. D. 


The specimens attached to the surface of a sea-weed are represented as 
■^rn under a Jth objective, with Lieberkiihn illumination: A. internal 
■lurface; B, external surface; C, front view, showing iucipient sub<Uvision, 



(Drawn from nature by Dr. Dallinger) 




The varione stajtes of the development of the nncleuB in two Mpropb^tic 
(irganitnis, as studied with recfnt homogeneouB and Bpochromatic objectives, 
Iwth in Ihe several at^es of iiraioii and genetic fiuion, indicating karyoki- 
naii. and proving, as ei«tahli>«hed in detail by the teit, that all the stepi in 
the cyclic changes of these unicellular forma are initiated in the nucleus befoie 
being participated in by the vchole body of the organism. (Diawn from nature 
by Dr. Dallinger.) 



Kig. 1. fTotrtiyria cain/raniiltita. 
Vig. 2. StephaHoeititi /Hckkprnii. 
Vig. S. .Velitvria riru/ent. 
Fig. 4. Pedalia* wirwui (side view). 
Fig. S. P. wtrHn (dorsal view, showing muscles). 
Kiti. fi. Cnprvi rvrhrrm (side view). 
Fig. 7, PAHmlina aeiili-nta (side view, corona eipanded). 
Fi^;. 8. lin\e of I'tilaUnii mimm. 

All these figures, save lig, 3, are reduced to scale from the beautdful plates 
in Ha<l»>n and Oo!!«'s JMi/rra. 



Fi^. I. Miluitiita iriiiiimliiia (a and b, lateral aspects). 

Fig. 2. Alreoiina Jtmcii (a, lateral aspect ; b, longitudinal lectioD). 

Fig. X. AttTorhiia Haiieota (a, lateral aspect ; *, portion of the test mora 
highly magnified, showing etruclure). 

Fig. t. Halifkytema Tamaaoniaii, showing the psendo-poIythalMnona 

Fig. 5. Ibid, (group of specimens m ntu). 

F'ig. G. Nai/lt>j/hragiuium aggluiinartt (a, lateral aspect; i, longitudinal 



n^ SS. Rutatia BecMtii. 

Flf. H. AifpAiiUfiMii £rMiitli(M,iiiiporiorUt«ralupoeti t, Infirlnr lat«nl 
ai|>Mt ; >\ p«ri)ifeenl wptct). 

tig. 3S> SimiiuiUlf Urrifa'a (I, Uicrnl Mpcot ; e. vcrtic*! •mUoo). 

FIk. 3^ roRlOB of Orbiloidet ntmimiliUfa. 


All ilic Ssma, WBMfit Of. 4, IIhIp XX.. are oopj*!! from pinle* <lrii«u liy 
Hf. A. D. UkhMl, F.L.S, &0. tiy the kind iJctmtsMuD of lh« ivpeoliTe 
vodetUa tbM publiubcd Ibcm. KI|[l 1 M C. I'latt: XVIII, and 1 tn :i. Plot* 
X IX.. lifT (mill * RrMkb OrlWiiI'lH-,' imbllolied bj th» Kay Socirty ; lljt. 7. PlnW 
XVIII..frun tliu* Juiinialuf Ibe LinnnanSod«ly ': 11^.4, Plate XIX., iiiirlfijc^. 
L.froni thv'Joutixil of ihc Royal Ul«TD*copi<ni) Society ':flg.&,ri<(c 


XIX. Kill fifflL 

nod^.Plalo XX-, frail tb« ■ Journal of th« Qa*)con Mlcm- 
V\g. 4, PlaU XX., is ilnwa after PUnteDbeq; by tho Editor. 



Vig, 1. ADaMmy of ytitltnit ttwtrfiraelut (male. dori^I ui«ot, • abcint fifty. 
I'Hw dcnal ponUiB of the ohitinoua OBo-*k«>lHtcin. an<t the Iai au'l muaclo* 
l«UcbsBderti« ili have lie«n r«iuar«d from iheabdumm, Tli« intemat oraaiu 
f*fe*bowB ptoitndins, a* lli«y lunaUyito kIicd the crcotuic it opened, u 
I tbonf-li Uicy wctt tuo laiyp lo b« contained in lli« i«ntiat «Ki-sktJulvti. hul 
^«f tlw UMOphagiu ■■ mxm ai tbc lop (the bnin bnving bom rvmovod). The 
^pcvmiuimlar gbmdii (brovn) lie nnvaeb itdcof tboi>Ki(baKiM. Tbc vcotrl- 
iloa In oolounxt pUik -, [art of it aod (hp wlioln of lh« cxca art comrod with 
I bodyicriiliil ti«iii«()-elloiT). Th« ImIm (white oliaitwl with bliiu) show at the 
'Mdea |j«titnidin|' from beneath ibe ti}iiii«ntaty oanal. 

Fl(;. 3. //onituilora tnafna ((«iiiale, lalornl aapeot, > aboal CO), Thcehitic 
M til* Mldo axA the fatty IImimi aitd intuvl** hara bMo romovvd. Allmcoluy 
, canal pink : occa of Uiu (istriculiu tpoitwl ; prevuiilrlcular glands browo ; 
'anperooxal |i;liuid wbitr; lU *e»olei fellow; ezpnlaocy rwicle. between 
•npetGoxal aiul niarioi. yny ; nvaiy and ovldncl* white (liwlod with blue and 
y«ikiw. 'rii«> icentt*! aad aiul pi"** *f° opm, and tlii> K^nlial »uck«n pro- 
Inidlnic. One niaiilla, whit«, ii Men between Ibe legs. 

Pi);, X Trffnmii¥i latvi (fciiialc. (loni*1 amoci, h al«Dt SS). Dortal 
iauMiltelRloa. fatty tlian*', and iobmIm T«movtd. Hame coIodth at Ixifiire. 
'Bniln (b«iwe«nn«v6nuieiilar|;land»)bluegipy, Man iliblM seen from abov* 
IwmI beliind.tbeirretTMioriiitisolMout ihatt. The trachea!, whfoh are pretest 
|In this iipcdea. are aeeo jiroocedlng t<i Ibclc (.liitniata In ilia aoMabola of the 

FIk. 4. Foiiial« ji:««iial oi^niit of Cepknu tefavnoMui (_ • about 30), YlfCl. 
^Cmtzal maiv. uridui'ii with tggt, i-aKinn mtii oTtpoiitor. 

Fie. 5. Tboaanwiof Aimaujpi-niViiLKiu ( i abi>ulSO). The Konital pUtM 
A tM tnoKlca and tandona which riiuve th«<iD, and i]i« ireniial auekeni, are 

IImm two SguiM are ledooed trDtn the original*. 
Fto. 0. Kympli (BCtiTS pufad •!■«•) of TVjrMwutau* hrrietHt ( m abont lOO) 
earrytaK ita caat <tor«al skiat). 


>. lljpo[jiaI (iiaTcUlDK) nymph of SiiuflyjiAiii AUHni (lenlral 
>> l<n). 


, . ■ t.. --'••■■ *- •■-■■' 

,'... .n..t'- r-' ■'■■'■ ■''- ■ 

r*l* * JBl!.!*! 

.. II... ■■'-"■' --'-^ ' 

i-.-. ■■ ",; 

-■: 1.I!-.: ,r^x^iata» -it 


i.. r. V*'*'" ■■ "■ 

,■!.- •.'"-' -■ ■ 

- 3 1: i-. .ln^ - Lie ^ul: :i cne 


n;, > 'i--'-- 

''lE'J. .n ai'US 

r / ■■ ■"■ '* ■' ' 
r/ - -•■" ■'"' '■■■■■ 




To be the owner of n well<chosen and admirably equipped imera- 
teepf, and even to have learnt the general purpose and relatioDS 
oS its parts and appliances, is by no means to be a master of the 
instrument, nor to be able to employ it to the full point of its 
efficiency even with moderate magnifying powers. It is an iostru- 
ment of precision, and both on its mechanical and optical sides 
fequires an intelligent understanding of principUa before the best 
opti«t results can be invariably obtained. 

We ni.iv be in a position, with equal facility, to Vtuy a high-class 
mierosmpp acid a;iss harp ; but the mere poKsession iiuikes 
lis nil more a «iiist«T of the instrument in the one case than the 
•illipr. An intfOliKent UTidcrstanding and experimental training are 
nprtlful to enable tbi' owner to use either instrument. In the case 
■ if the microsco]*, for the great majority of purposes to which it is 
iipplied in science, the amount of study and e.\ peri mental training 
ri"*iled is bv comparison incomparably less than in the case of the 
Timsical instrument. But the amount required is abaohitely essential, 
thf neglect ot it being the constant cause of loss of early enthusiasm 
.md not infre<|iient totjil failure. 

In the following pjiges we propose to treat the elementary 
jirinciples of the optics of tlie microscope in a practical manner, 
not merely laying down dogmatic statements, but endeavouring Ui 
•.(low tlie student how to demonstrate and comprehend the applica- 
fion of each general principle. But in doing this we are iMnuid to 
rt-niember a large section of the readers who will employ tlils ti-eatise, 
urid to wi treat the subji-ct that all the examples given, or that may 
'«- subse<juently required by the ordinary microscopist, may be 
worked out with no heavier demand upon mathematics than the 
• ■niployment of vulgar fractions and decimals. 

In like manner, although we shall again and again employ the 
tri't'uiometricid expression ' sine,' its use will not involve a mathe- 
matical knowledge of its meaning. The sines of angles may Iw found 


)>y publUli<Hl ublen. A taUe to quarter (logreea is given in Appendix 
A of tlii.i lK>ok, wliioh will, in the majority of caaew, suffice ; it is not- 
difBcult to tind such Ubles u8 may be iei|uirei:i. ' 

Of counio it IB luore tlinu desirable that (he iiiicroacopiKt lOioiild 
liuve ijockI matlieiiiAltcal tcuawledgo ; but tJietv are many men who 
d^re to utiuiin a useful knowledge of the principles of clenientnT'y 
optics who ai« witliout time or inclination, or both, to obtain the 
large nmthemuticaJ knowledge required. 

Xuw just as a man who is without a.ny McnnXo IcnowledKO of 
aatroQoray or mathematics niny find dine fromaeun-dialbyopplying 
the equaUon of time taken fmoi n, tablo in an «liDana«, so l)y tho 
use of a table of sines the microHCopist tnny txutch u««fol and ivliubh!- 
resolts, although he raiiy Imvr no plt^nr knowledge of trigonumetry, 
physical optics, nor the umthi-mnticnl proof of formulie. 

All micTOSPtilies u'lif^tJH-r tuni-U or rumpnund, in ordinniy us» 
depend for their niiignifyiujt ]>nwi>r uimn tli<- iLbility pni!ii'!».ied by 
lenxnt to rf/rnrt cir lytnd the lijilit wbich ]i;i«s<'ji through them. Re- 
fraction acts in nccorilanoi^ with thi- two followinff laws, \'\x. ;^ 

1. A r«y which in piissinK fr<nn a. rarn ininiium into ii deniwr 
medium mukea a cr-rliiin nngh- with tlip jitirmnl, i.i-. th/^ perpendicit- 
lar t»i the aurfacit or ]>lane at which the twii nifdin join, will, on 
entering tlie denser inrdiutu, nmVv a tmnl/er iiwjlr u'il/i iJie iwrmal, 
(-onversply, a. my pa^iti^' out in»i\ a. dt-nse medium into a rarer oiit^ 
making a ci-rtain aiih'te with the nuruiiit will, on etuergeitce from the 
densL> medium, make a (;ri^nter an^'le with the nonnuL 

The luy in one miHlium in i.-all<^d the I'ruttJpitl ray, and in tho 
other medium thc^ rrfritcfrd ray. 

The iuL-ident and refriictMl ray* are always in the Mim<i plane. 

S. Thn sine of Uie angle of incidence divided by the sine of tlm 
angle of rcfmetion is a caiistatit quantity for any two particulfu" 

AVIioii one of tlie media is air (accurately a i-acuum) the ratio of 
these sines b called the absolute refractive index i>f the mediutri. 
Ad every known medium h deniier than a vacuum it follovrii thut 
the an^ of the refracted i-ay in that tuedium will be lean than the 
angle of the incident ray in a, vacuum ; cmuMquently, the abaolute 
refractive index of any medium is fp«ater tiian unity. 

Further, tlie absolute refractive indei: fur any ]>articular substance 
will tiiSer according to tlie colour of the my of light employed. Tlie 
refraction is least for the red, and frreat«iit foi' tin- violet. The 
difference lietween these refractive ^aluea determines what is called 
the <{i«f>':r»ivf piit'fr of the suliHlnnce. 

This will 1>e understood by fig, 1. Let I C, a ray of li^ht travel- 
ling iu air, meet the surface A It of water at the {Kiint C. Through 
C draw N N' at right angles to the surface of the water A B. The 
line N W is called the ntirnmi to the surface A B. The my I C will 
not continue its patli tlirough the water in a straight line to <j ; but, 
because water is denser Won nir, it will be bent to B, that is 
towards N', The whol« ooursn of tlie ray will be I C R, of which 
the part I C is called the inciilntU ray, and K the r^raeUd ray. 



The Misle I C mnki>H with Uk- nnrninl N K , vul I C X, ia mlU-A th« 
on^ af inci'lrnf^ ; aiul Uiii angli' It C nuikw wjtli tite nonnul N" N, 
rit RON', in cuIIihI llw nni/ir of rf/rtiftion. 

ConvvrMtl/, if u rny R C, tmvt'tlin^ in wut<-r, ai«et tbe surface of 
- AB in tli« poiot C, it will not coiitiiiut* in n Ntmi;,')it line, but 
twUI be beiit to the |)oint I farlliirr iivray from N. Tliutt. wlien » 
mjr piMWH from n rarer t<> » fli'iiMir nivdiuui it b bent or if^fnicted 
[ifowarrft the Dormal, uid wlit-n it passeB out of a. dense tiie<liuiii into 
. nrer one it is l>erit or ix^ntct«d nimy from the norinat. 

Further, if tho shwted poi-tioii of Uie fipir« were gluss iustcnd of 
f '«r»(«r, t)i(* r«fr»<Tt«<l rayKO would lie beut still nearer N, and, 
[eonrerw)}', if the ray ftuatti out of glass into air, it would be 



/<i /A |.V' 

Pi-i. \.~~Tiit, n/tfOiiyn of lij^ht Tlie l4«t ot iiinM. 

iBore bent awnjr from the normal than if it liad passed out of water 
into air. 

The itngl« of incidence I C N is connected witlt the angle of nv 

^fraction It C N' {tui stated nbov#) by what i» known as Siiell's T^w of 

SioM. Tho constant rcUtion between the two sities for two upociGc 

KiMlia is called t<h« rr/'ni'-livf indrr of the medium, and is usually 

indicaUid in problems by the symbol ^. 

This law, stntnl with rpfrronce to the tigur*", would In? ; 


Bine RUN 

: ft = tlir refractive index of water. 

Id I C take any jioint, P, and from P draw PT perpendicular 
to N N'. Similarly in BC take any point, F, and draw FH per- 
peadieukr to N N . 

• i 


tlion, by 

)tuiy )i'>ii>''^ ni<i.V li^ t:ik«ii ill TC nu'l RC i( the [><>iiit8 li;ul biien 

t juJiciaijiily selected, we tiiiKbt luive (;re»ily u'liiplil^Ml the ahovu 

expreGsioD. Thus if >^0 t>ike two other puititn, K itiid K, such tlmt 

KC^ KC^ ami ilrav the perpendiculars tut before, we etioU hav« 


! 1^ ^ ana sin<; RC N' = ^ j?, and therefore ''^g = ;.. 
KC EC ^^ 


But M KC = EO l>y construction, we cftn write KC for EC 

.^*.= /I. K C is cancollpd, which lomvcs ^^ = ^ 

Aa jt can be pxprrinicntnlly i!ct«nnined for taiy two particular 
Di«dift, it follows thnt if nnp of tin- othirr tsnnii is known, then the 
remaining t>'rm cm l>n found. TIiuk, if fi nnd the angle of incidence 
are known, tlie xngic of rofrtction cnii bo found ; nnH if n and tlift 
ait^le of refraction nrc known, the angle of incidence can lie found. 
The unknown iiiiantit)* car ba found cither gMitnetrically or b; cal- 
culation wrhcn thi? othrr two terms ara jcivon. 

It will, of counwii, Ijc understood tlut, fur the same medium in 
every cnw, n rwl (uy would be bont or refracted less than a i-iolet 
ray. The vnlu© thurcfom of ^ for a rr^l ray will bo Iww than that of 
fi.' for n viiikt ray. As a jiracttiutl ilbi^trntion : The refractive in- 
rlcx fur a n'd niv in erown glass ts I'rilL'i ^ ji, and for a violrt ray 
is I'.WSS != fi\ till) diflercncL- bL-iii;; fx' — ji ='()|fi4. 

Tlie refnu'lirt- index for a red my in (lcii*rv tlint glnia is 1*7030 
^ /I, and for a Tioli-t ray is IT.iOl =: ^', tlm ilitteifriei' being ft' —ft 
= ■0471. 

Cunneriucntly th^re will lie a greatiT dilTf rcriice betwocn tlic Itmid- 
Lug of the tefruotivl ri-il and violet nivs in tlie cane nf dense Hint tlion 
In the caae of cn>wn ;:lniLi, tb« anglu of the incident ray with Ihn 
normal iM-ing Iho siiiai! in eilht-r casr. 

When- air f niori- correclly a vacuum) is not one of llifi media, 
Uien tJit- refnirtni- index is called the rrhtief refractive ind«x. 

The luiriiint Ut a fihw ntrfaet la always the |ier[ieudiculur to it ; 
the iwniial to n rphtrtcal tHrjW^ is the i^dius uf i-urvutui'e. The 
angle of tlie incident ray and the niiglo of the i*fract*?d ray are 
always measured with ihr. itonnnl, and not with the aurfaee. 

Tig. 3, <T, b, nIiowts the normaU A, il In both a |ilaue and a 
a]i3ierical surface, C D. 

Ill the ca»o of the «phmcal surface, H is tbecentre of eurvaturtvE F 


H the incident ray in air, FG the refracted ray in crown glass. Tha 
angle A F E is the angle of incidence, B F G the angle of refriictinn. 
Sine A F E divided by sine B F G is equal to the refractive index 
of air into crown glass, or, in other words, the absolute refractive 
index of crown glass, ft ; thus in this particular case : 

(Problem) I. : 

sin AFE 





This problem, however, is not actually needed by the reader of 
thb book, for a table of 

absolute refractive indices ^ 

is given in Appendix B. 

It will be clear from 
the above that when the 
refractive index, absolute 
or relative, of a ray from 
any first medioin ia given, 
the refractive index from 
the secKind to the first may 
be found. 

Thus, the absolute re- 
fractive index fi from air 
into glass being given as 

'- , find fi', the refractive 
index from gloss into air. 

(Problem) II. : 

, 1 1 2 

When the absolute ^^^ 
refractive indices of any 
two media are given, the 
relative refractive indices 
between the media can be 

Thus, the absolute re- 
fractive index fi of crown 
glass is 1'5, and the ab- 
solute refractive index n' 
of flint glass is 1 "C ; find the relative refractive index ft" from crown 
to flint. 

(Problem) III. r ^„ ^ / ^ IJ _ j.^gg 
fi 1'5 

The relative refractive index fi"' from flint to crown is determined 
by (problem) ii. : 

1 1 

Fio. 2.- 

-The nonnala to a plane »n3 a curved 


- = r.066 = -«^« 


Iw loutUy reflected friud tlie nariue C J} bMk Into tb* dsnMr 

A mmpla tUiulmCioii of thU U tiboivu in i-^. 3. It rvprnrntn 
t gUw of wiit«r Ao hold tlial l[i« aur&ue of the w«t«r i.i aIkivo the 
Tf v!n took o))lit|iiel)' frotii Imlow at this HUi-faoi-, it n]>p(!imi 
j[bter tliAn pulialiMl silver, aim! iiu object pUced Jn thi- wnti-r hiiai 
IhK uiTinr portjon of it brislitly rellei-twi. 

Th« iu;tlun oii ull h^hl iiwiduut on CD ia the denaer laiyliuni 
(lis. 2) at ati atiule ;;n.-;it«?r than the critical uiigle is jjreclacljr the 
mmi- in fact as )f C D were a silvered niinur. 

A critical aoifie <*n only exist in a dotmr mediura, for obviously 
bene can be iio critical angle in the rarer medium, since a my of 
any angle of uici<lence cwi eater. 

When the relative or absolute refractive index of Uie denser 
■inedium is given, the critical angle for tliat medium can lie found, 
"ihua : 'riie aleolute refractive index ot water is lS'i=fi ; find ita 
-critical angle $. 

(Problem) IV.: ^in 0= ' = . !,- = -75 ; 
ft 1 ■o.'J 
«=|8i= (found l.y table). 

So tho nine of tins critical utiftle is thi- rrviprocal of the n^rftclive 

The connection botwMin tbi- ]Ktth ot an incident my in a first 
allium and ita refracted my in a second ra«Uum is established l>y 
: fonuuhi 

rbrm ft is the nbaolnte retractive indox of the firttt mediura, fi the 
faiiKle of tliu incidient ray in it, /•' tlie abtolutc mfractive index ol 
thtt second niedittm, and «' the angle of the refractwl ray in it. 

Tlie angle f = ti" of the incident ray in tlie lirsl medium A F E 

{&g. 2) and /I ^ 1 f/ = ,,, the absolute refractive indices of botli the 

nedia, air and glass rtspcctivelyj being given, find f', the angle of 
Um nhtkcted ray in glass, 

(Problem) V. I : 

Sin -' __ J' sin » _ 1 x sin 4.'i<'_ 1 

*' = 2><'' (found by table). 

To put anotlu-r cnse. Suppose the angle 4>' =^ '28° (fig. 2, B FO) 
is given ; dnd^ tlu! refractive indices remniuing the same as before. 

(Prablem) V. 2 : 

u'ann' 1 -5 X sin 28* l'5x'741 .tunt 

f= 45" (found by table). 
Now, suppose the A side of C D (lig. 2) is crown g1a«a, /t = I'S, 
And the B aide of CD is Hint glass, /•' = l-G. Tho angle of the 
iui-ident ray A F E ^ = ■i.'i", find the angle of the ref iaote<i my ^' or 

IS ' 


Sin 9=' 




Vig. I. lri»t»mit j-ilmifii^im : ■ abual 40). 

Fig. S. Sympfi of «ia»f fp«<-br«. follr ^rom ( « aboot K). Hm < 
ellipse iriili ibe iimeniKi^ «et of Kska aturfaeil is [be cut urral dotMl 
abdomiiul skio. The oiber cvws of sctXef, belong to Ibe SDCccBdre iijMi|Jl» 

Fis. 3. Odc «r tbe KSlCii more bighl.T magnified. 


F%- 4. Bosimm aad gnai nptotial palpi, siih Ifaeii appendages of flhy ' 
\ftnt n^tutitrimm ( n »t»at I50V 

Fig. 5. SIfMa riintpttntlli (femaie. x about 135)- 


rL.\TE XS 
Claw of fiist leg ot !«ine »pecie«, being an or^an for boldiiigthe hair of tbe 


Pig. V. (iamant terrihiUt (male, x 30). A species found in mole^ 


Fig. 3. f 'reifa na liffem/nit (male, * aboat 95, apaiasiteof the cornmaat^ - 
Fi^. 4. Sarcoptei tratiri (the iich Diite, x aboat 150, adolt female). 

Path of light Timocou a PKtiM 9 

iir«m«nt it will bo fouiul Uwt G F iMLkw, with tlw iiormat &L 
F, an ftugl" 9' of '25^°, nnd for th« bla« niv an niiglo 9' of 26^". 

Il ultould be reni«iuberMl, Iiuwcvrt, that if tiw rrifracling angle 
of the priion iit Icnowo, there it no nmi^ty for tliix mnwuremeot, 
Lbeeituse it in alwuj-^ thu ililTi'reiice botweL'n t liia and the angle of 
tion befiiiv .IcttTniinwi, thus 50°— IMI" = ^jj*. 

<fe- <»*^ 




no. Sy— Diagntn o( derution ol InmiitoiU nf bf • prina, 

Tliis my E F now becomm tlie iiict<li.-nt my on th« nirfnco A C ; 
sd US thr Atigl« it mnlcee with the norma.! at V i* known, tmil as 
*iko ivfractive indices ranain the i«ini-, wo can, hy (problem) v. 3, 
Bml th*- nngl'ti of ri-frjicUon for etteh colour. 
If we tuke ted light : 

Sin ♦ = ^"^ *' = 11^"_^^ = 11_XJ13 = .732 . 
M I I 

^ the angle of refraction = i'" (found by t;ihle}. 
If we take blue light : 

Mn# =t . = — J. = —^- = -,,4 

It I 1 

^ tbo angle of rttfmction = 50)" (found by tablft). 

This dutp«rsion can now bo reproBontod in tlie diagnuu, seeing 
that it amounts to :i}°. 

tu optirs it IE convenient to use an expresdon to measum the 

dispersive power of diaplianons substnnces, which does not depend 

^va the refracting angtn of ths prism eniplo)mt. Further, In order 

it various solutnnc^'ji m&y be oompArM, their dispemve powers 

I all ntessurcd will) rrfon-nct^ to n cnrtAin solectod niy. (For this 

I the bisection of tho Doriodium linos is th« point in the 

' Bpectram often chosen.) 

In the crown and flint glassos mentioned on page 4 the dispemon 
the lines C and F, in the spectrum, rpforrod to the bisection 
' the sodium lim-s D, is as fgllows. Crown glass : — rrfrnctivo index 
bisection of lines D, l-.">l-fl = ^; litK- F, ir.i*395=/[ line C, 
ri>l&35 = fi,", Tlien thu dispersire power m 

fi-fC ^ 1:53395 - 1-51535 _ -OOSff _ (.,„„, 
■«-I ~i5179 -1 '":5lTa~" *■ 




Flo. 6. — Action ot a pair of priamB with their baeee in contact on 
p&tallel light. 

Fio. 9. — Action of a pair of priamB with their apiceB in contact on 
phiallel light. 

(■*, M «n» froH tlM etek kBd with a Ml tint 
■ the ntriov ftwmm wlach fcr www opdod pi 
lA loon w abovn m %k 11 umI IS. 
If w >ta^ tk* foor foOowiw Sgann. «e duU km 
action Of Irwm « %fat indoent on tbeir cnr&eeB. 
I tlut if » ndiant w pLiMd M the |)ri»d|Ml focus vt » 
: Imm, Uw nifi are mMtend i«imllel ; converMtjr, if parallel 
Ion aoonrrrghig inu,th«3rai« tiroa^ttoapHiUiqialfocDSOr 
Hl up"*' thp ^t'v^ 
n(, 1 1 aliowa that if a radiant b« placed Iq/ond Uie principal 



bcva of « conTsrging lens, the rays are brought to a focus beyond 
Ab principal focoa on th« other side of the lens. The nearer tiw 
ndwDt is to the principal focus, the farther away will be its coryuffole 
Jtemg from the other principal focos. In other words, there are two 
Mints in the axis bbcIi that if the object is one point it« focus will 
M the other ; these are reciprocal one to the other. These points, 

Fu. II.— Keonm, p1uk»«ounx, 
ud mimgiDB maniiciu lauM. 
^Fnta tliB ' PnnM <d Satnm.') 

Fia. 13.— Biconcave, pUno-ooncaTe, 
and diTflrging meniiciu leniea, 
(From the ' Forcei of Nahin.') 

^ focal diatancCB of which can always be calculated, are known as 

Should the ntdiant be at a distance from the principal focus equal 
"I the focal length of the lens (i.e. twice the focal lenyth from the 
Jem) then its conjugate will be at the same disbtiice from the focus 

F11'. 13.- \ nwliant at tha |irincipa1 focuR of a biconvex lens makea the refracted 
raj'H poralltit. 

Fio. U.— ,\ radiant placoO bcj-ond the principal focus caniwa rays lo converge 
beyond the principal tocns on the other Bide ol tlie ItnH. 

wi the other side of the lens {i.e. twice the focal length from the lens). 
Ill other words, when the object and its iimiKO are equidistant on 
either side of the lens, they are ettual to each other in M'=e, and 
VK toar times the focal length of the lens apart. 


This law forms a ready means of determining the focal length oP 
a lens. An object is placed in front of a lens and the distances 
between this object and the lens and a screen to receive the image- 
of the object are so adjusted that lA« tmo^ oflhs object becomes eqwU 
in sise to the object iUelf. The distaoce of the object from the screen 
divided by 4 gives the focal length of the lens. 

If a radiant be placed bettixen a lens and its principal focus, the 
rays on the other side of the lens are still divergent, and wilt never 
meet in a focus on that side. TliLs in seen in fig. 15 ; but if they are- 
traced bftckwarda, as in the dotted lines of fig. 15, they will then 

Fio. 15. — Rays diverge when a radient ia placed between a lens and its 
principal Focub. Fdcub of divergent rays is virtuaL 

meet in a point. This is called the virtiiai conjugate focus of the 
radiant. The principal focus of a concave (or diverging) lens is 
shown in fig. 16, It will be seen that the principal focus is not 
reo^but virtual.' Parallel tays falling on B concave lens are rendered 

spneaicAL aherration 

Pigs. 1" ami IS Hhow (lie i-efmcUiin of (*y» of iiioiiocbn>inikUo 
l%lit {xiralH Ut Um) lucis falling on » planooonrDx letis o{ crown 
gUw. Th«w figUKK illtuttHtfi : (I) LunKiludinul «plirri»l abemttiun 
bimI (2) the focal leiigth of o. pUjio-ooitvex Iviix mkI the poiiil troin 
wliicit it is iii(«iiureiJ. 

(1) In fvganl to the toniMtr it will b^ Men tlwt th« ImiKittulinal 
•plwncnl nl»MTali<m in gmatmi iu lig. 17, vlxn tht pai^lel my* 
o( lifiht f«ll upon the pUiNt iKirfaw, uid leut whw*, aa in fljt- 18, 
tlwy Aill upon llie Bplw-riciil surface. For spkmcal lAtrratioH t* Oia 



Pid. IT. — SidifftcU Kbtmiioii. 


iUUtntx <^ Uie /utttJt /ur nng my pitmttg through a Uit» /rwn the 
jirineipal/onu ^ihat Una. 

tThus in fiss. 17, 18, the spherical kl>«rnitian in W tor the rav 
R» n». um) F F" for thfl raj-s R' R', luwl tlw <!iflVriTncc liotwcMi 

7hprie»] alierrktion of Uw rm>8 R' R' kimJ tlutt of the mrs R* R* in 
r_FF, which isF-F". 

* J^iFFbliIPF' 

c • /• 

(2) Iu rirfpird to (ho focal length of a ptnno-eoiivex Iciis it niay 
be inoiileiitall)' noted t hut the focal length in fig. 17 iu twice the radiuii, 
uMw&ured trtati the vertex A, tluit in, A F. Rut in fig. It< it is twice 
t1>e nwHua iuea«ure<l from tlie point A : that is, the point Fiudistunt 
fnim the lenii t«'ice thenuliuK Irs* two-thirdsthc thictnrwiof thelena. 

It will he aeon, then, that the amount of Kjilirricat nljcmition is 
due to the thnp^ ot tlie lens, •ntl is I<--a»t in a biet>nifx ttnt, wlicn the 
nuliiof L-nn-atureitre in tlie proportion of 6:1, leAm lit mort cHrveJ 
tur/offjafff tht\ inriitfut light. Bat whmi th«< lens is turned round, 
(o Hint the oflier tide ^c«s the inci<lont %ht, tlic spherical aberration 
ratches a niaxinium. 

It wouhl be well for the Btudi-iit who dwirttt to liecome familiar 
with these facb;, without attempting any profound matliematicol 

Thus F F and F P" in (fig. 17). ?/= - .^ 
in (fig. 18) J/= - J 



grasp <if them, to dir&w such ft lens, and tnce the paths of tvo ran ui 

through it, one nenr the axis, the other near the edge ; then do tu ■?:. 

same with the lens reversed. .ii 

Fornmla for spherical abemttion : ' ; 



-whi'm /=priiK-ipal focal length; y = aemi-aperture ; ^ = nfr. 
inili'x ; und r, — r', radii. 

In iin «]ui-coiivex of crown, where p = ^, r ^ — ''t 


K/' =-; 


3 / 

111 u pliino roiivfx of crown, where 1^ = ^, — >"' = ct> , r = st 

^y' as — *. , ^. . Here parallel rays are incident oa the convex 

nHifiu'o. Hut whi'n parallel rays are incident on the plane surface, 

(( =3 '^ , f = Gc , — r' = \^ , r/= — i, • y', consequently the sphe- 

Hi'ut iilHTriitiiiii is four times as great (see Sgs. 17 and IS). 

Wlii'ii — c'sror, ami ;i^|-69, the plano-convex becomes the 

riiriii III' iiiiitiiiiuiii aU'iTntiou. 

In n I'limstil* biconvex lens, where —r'^Qr,B,nd fi ^ -, 

i J' ■= — ' , -'. , the piknillel rays being incident on the mora 

■'urvcd Hurtiici'. 

I''<ii'iiiii1u for liniliiig the principal focus F of a lens equivalent 
l<j t «ii (illicr li'iiaiMi whose foci urej,/' and their distance apart d: 
1 _ I 1 _ d 



This will be fully iimlrnttooH by th« aid of lig. 19. 

The white light, A A''',fnlliiigon tlie nori]>l)ornl portion of tbulenH, 
is so far (lisprnml or licicompowd that the violet raya are bmujitlit to 
a focus at C, ntid, croiuing tlioiv, diverge agoin and patA on utvunln 
F P ; whilst thn ivd mvn nir not brought to m focus until tliej- n-iu-h 

' point D, croMing iho divergent %-iolet mys at K K, The foci of 

I iDt«naodi«tc rnjr* of tlM- nfirctrum (indigo, l>ltir, green, jrellnw, 
fwid omngv) am intcrni<.-dinl« bptwei-ii thcKr t«<> oxti'eiiies. TIte 
ae»CI>, limiting tli« %'iolct and thr rvii. if t«nnMl the lanijilu.- 
Hit¥tl chromalif a/trrrnlion 'i/lAr. Un*. 

I( thp imagt' bo m«'ivpd upon n Kcrocn pineed at C, violet will 

Cnlomiiintc, i»ncl will U- lurroundcd by n priKinntic fringe in uhicli 
Of, green, ynltuw, i>r,iuu;e, niid ixid miiy he <li>>tingiii>hcd, tf, uii 
the otlwr liAmt, tlu: xcnNm be pWed nt O, tJM- imago wilt have a 


Pm. \9. — CSirouMtlc abonUion. 

ominanllyred tint. «nd will Itesummndad byaiteritsof oolouml 
^«a, i» iuv«rtMl ordL-r, furnictl b^ tlic other tuy* ot the spectrum 
whidi h&ve ra«t anil croMed. 

The line E £ joins th«^ points of Intersection between the red and 
Ute \'iotet rayii which nurlu tin- tHeanjiicu*, or the [H>int where the 
diKper&ion of the coloured niyii will be least. 

The a\:ul ray uiidcr|j;oe!i neither refnuittoit nor diiinenuon, knO 
Um tuittrer the nirs are to the axial the Ivna diHpenioii do tliey 
nodergo, Sitnilariy, when tJic refraoUon of the ruys is greateet at 
the |R'nj)herj' of u lens, there the diiperuon will be muBt. Heitue 
tile [M-rtiJieral portions uf uncorreutou leDne^ are fttop]w(t out, and 
the centre only uften um-d tliat the chromatic aberration may be 
ndaeed to a niiniiuum. 

Hanifeatly, therefore, the correction or neutrali-uttion of thi» 
ohnnuitic aberrattou, which U known in optics as iirlirviitati»m, ia a 
natter of the Bnt moment. Midtiplied colour foci between C aiut 
D (fig. 19) nutke a perfect optical image impomible. 

It is a qaestjou of intereat and int]>ortance to the niicros(x>]>Ltt to 
know \ow oeAroniatitm m obtaiitcd. 

In a prism the amount of diKpentiim or uni-qual Wudiiig of 
, R and V (fig. 5\ depeitda on twi> thingK : (1) the nature of the 
IriaKi uf wbidt tlie priain U compiHO-il, un<l (2) tbe refracting angle, 

If, for example, another prinn were taken, made of a difforvitt 
kind of glana, ptM^iesaiug only half the dijqierwve power of that in 
Um) figuns but Willi the angle H A C W, im m liiia case, the aepani- 



tion ol R uid V would oiilj- Ik) half a* (fftfit m that «flect<Hl by tho 
priMii in llie figure. 

TliiMi if aiiutlier priMii were mitde of the «<i»i^ materiiif a:i that 
miKumed iti lig. .'i, but willi only /<"(/' f/w r^'raflitiff awjU, vit :;.'i'', 
the diaperaiou belwt!eii R and ^' would abo b« but luilf (jiiit n-pro- 
hkuUmI. Also a prism htivitif; 50° uf refrftcling angle tfivei th« mudo 
Kinount of <lisper&ioD as that f rrmi a prism of 25^ <^ Mneting angle, 
but of twice its dispersive power. 

Under thmo conditions, wlien t>\w primi, rxnctly like another in 
lux^e and dispersive power, is placr<d close to it in an in%'<-rt<MJ 
position, the duipeniou of tho firet priKm in entirely neulraliaed by 
tlwt of the second iMcau^e it is procisniy t^ual in amount and 

opposite in puwer. 
This will be under- 
stood by a glance at 
fig. 20. Itul it will 
lie seen not oidy 
i!;disp«i>.ioti revened, 
but rofra«(ion hIim 
is neutralisGd, the 
•■iiHTKeiit my being 
[inrnlK'l to the in- 
cid<^iii my. Tliprcfiiro 
thfi<v)ualand inverted 
syitcm of prinras c«n 
lie of no poHHibte uho 
to th<* pitictical opti- 
cian ill the correc- 
tion of lens«« because 
the eonuMfifMier nnd (?(jy7yi!iw< of i-bvk am both essential to the 
oon«truction of oj>ticat iniitru incuts. Tht- dinpersion, ii) fact, must 
be dostxoyed witlii>itt neutralising t\\v rcrfraction.. 

SuppoM wit take a prinn with an angle i>[ -^0', composed of glass 
havtDg a oerttun (lisperaivt; power, and invert next it a pri^u of "Zh" 
wngle, oompoied of glut having twictt the dispersive power of the 
foriaer. DispcTKion will hi* niitiiif<-stly destroyed, because it is equnl 
lu amount and oppoail*? in tiatim- to that poaxeaaed by the pri«in of 
fiO* ; but the priiuu with an angle of 25* will not neutroliso all tho 
nfmotion elTeoted by thi; nrisin of 50°. 

Theee conditions plainly nuggi-^t the solution of tho problem, for 
part of the converpincL- is nmintaineil while the whole of tho 
dispersion is deatroyed. 

The spherical lenses which answer to tliajie prisms are ik crown 
biconvex, Kttiug into a flint pluno-conoare of double the dispersive 

It has been pointed out nliove that all the other colours lio in 
their proper order between the ray* R ni\d V (fig. 5). Let usseloct 
one, green, and represent it by U. Now {f Q lies midway l^etwoen 
K and V in the prism of 60° of angle, and also between R And V ia 
the prism of 35^ of uwie, its dispersion will aUo be neutrnlispd. 
This means that when Uie diapersion between the tlirec colours la 

Fin. ao.— B. 

' l''fn:ra ol Nnluro. J 



4o« kiud of Klaas is ]>ro|>orUoiiiiI ta their (tUperaion in tlie otiier, 
thill when any two «re defrayed tlie third is destroyed with Hiem. 
TliLi u II furl 11 III! I ely la not the ca^e in pi-ni-tti-e, because two kinds o£ 
.olan liBvin;; |>ropoi-liotint disper^oii powers cannot he obtained. 
Thia^ l>o«r«vi-r, i^ whut reiiUy hnppeiui. (.1 may lie miilniiy between 
R Kod V ill one kind of gla^ but in tlie other it may h'e, Cor 
inataiice, lauch nearer K, My a. third inst^iut of half the distjince 
-«i R from V, If now the disporsioii of R V lie destroyed, G will 
be left' outntAiulin^. If a different angle of prism be rhosen, ho that 
K tui<l U are neutralised, then V inuHt be left outstanding. 

Tliis want of proportion in the disperiion of the various colours 
of the speetrnm in two kinds of glass is t«mied the imttioHalilif of 
thu! tpeelrum, and the colour or colours left outstanding in a corrected 
fiooibination of lenscx is known as the ivi^tmUtry nfifctru«i. 

In some 8nbsi?(iu<-nt pages we shall hfive to cnll attention to the 
tUAnnfACtnre in Urnnany of kddmi new vitri^ius compounds by tlie 
-combination of which willi Suor Kpnr tht- seconilnry spccti'om has lieoti 
removed from tnicroscopo objrrtivrx, and nn ii/itxliromatie system of 
constriKtion haa liocn introduced. 

Mnuiwhile, we may n^mcmlxtr tliat it hu only been in i-ompa- 
rntively rco^nt limnt tlmt th«' construction of nchmnifttic object- 
_)^liiMiiii for mtcRnc[tp<-« liA-t hi-cn brought ab<>ut> but the grudual 
ealai^genient of apt^nre and thr gn-nti-r cuiiipli^t<-iir«s of tlio cor- 
rflctJoiu Kooii after tlio discovery of achnmuttixm rcnilered sensible 
.Mk jmperfi'ction in tJie pl^^f■>rlllance of tln-oi^ uiuIit certain 
eircamstanccfi, which hml prrviunjJy pawcl unnoticed, and Andrew 
Jt068 EDJule tlin important diocorrry that the use of cover-glass in 
moantinK minute objects inti-oduciil al>ernition. and that a very 
obrions iliffTirncii exist* in the prccinion of the image, according a* 
it is viewcl miVA ot wrilAnut « covering of thin ^oss, an olgoct- 
jhua which may he perfectly ndnpti-d to either gf thnce ccmlitioiu 
BUSSMiEililv drfective under the other. 

He aUo deviMid t}i(! miuinii of correcting this iTn>r, anil nubliKhc<l 
hiiidevicuin vol. li. of 'Tranxactionaof the ijucietyof Arts for ltJ37. 

Fig, 21 will illuntnLte the efli^et produced on the oorroctions of 
an objCot-glaxK by the interjiuution of a cover-glaxa txrtwcen tlie 
object mimI the (ibjootive. 

TIm niyi nuiialing from die object O in every direction fall upon 
"the oover-ghuM CC (/• ^ I'G). On tracing two definite rsyti, such 
an A and O B, it will be finmd ttiat they will be refracted to R 
and P (ihown by the dotted lines of thetiguru}. On their emergence 
into air tliey will be again refrartecl in a direction jMindltil to their 
tint pntli. ami uill enl*T the fnint lens of the objective at the 
pointH M and N. 

Kow an M R aod Ji P, produced, meet in T, it follows that, wo 
far an the i>hjective ia ooncenied, the rays M R, N P might have 
■diverged from tlie point T. 

Similarly, by tracing two of the less divergent ray« from they 

I will be made by the refraction of the cover-gtasa to appear as if 
they <Iiverged from X. Therefore, in consequence of the cover-glass, 
4be objective baa to deal with rays radiating afparenliyjrom tfffo du- 




titKt poinU, X aikI Y. If tliiTe wpre »<> cover-glass aJl tlie itiya wouU 
dirstrgo from O, und thon llic objertivp wouiii require to be |>erf««tly 
itplanatu: Tliis wonl (derivtHl from i! = privative, aud vXufdia, (<► 
wnnrt(!r,i.«. fn-cfroiu woniU'ritigor error) meRns, asused by ii|itii:iiuiSp 






Vta. Slv^Tke offset prodDHxl bj t. oom'-slwu on Ui« eorrnetioiu <it un 

thatoJl llie nija passing tlirou);h a leDB8yat«ni are lirnujjht to im irleiiti- 
cul oOTijugnle (ocua. as sbow-n in fig. 22. But as iiflfsited by tbe oo\'*r- 
glafis tbe iiinr^inul mya diverge, apparently, from a focus timrer t/m 
fSJfetivf than the central rays ; therefore the objective, to meet thia 
ponditiiin, inusi be wiwtt is called utKUr-rnrrrHnd ; a. conrlitioii pni- 
seiited in liy. 113, so :is lo fucua both thcAii points nt ancc. Here tho- 

r -F 

Fiu. 'la.— Aplnoatie sf ttem. 

Pia. 98.— Unddt.eorrecled iTitoiii. 

cur%-uture of the surface of the crown lens being incroasiil, thf flint 
plano-ooncavo is not sufficiently powerful to neutralise nil the 
aphericiLl ;il)crrfttion of the ciown, j\a a consequence tin? peripheral 
rays are brought to a fooua at K', while the central mys \u\i.>t on to 
y. This ia wbiil is uitwnt by ' under correction ' in an ohjecl-glass. 

In fig. 'H the reverse condition 

is prBsontod. for ths incident curvo 

of thecrown lens has lieeii flattened, 

while that of the Hint hns been 

deepened, which incre/iais the cor- 

-Owr-coinTleil ■ji.ttiii. rectivc power of the flint, and thus 

destroys the balance of the eoni- 

binntiOD in other directions. The rays pasoing through the periphery 

of tlie combination will ho brought to n focus F', while inc. central 

ntya will be focussed at !:'. Thu is what is known as ovtr-cormtion. 





An a/Jiirtalie olijreliet (Kg. J2} Oiin lie mntlf into an und^r- 
'torrtKted ubjiwtivn (tig. :i3) by citlirr (1) ■■oiwijiy ihn hitek t^tua i^ 
MiAtot tl u coinpo»aI la approaelt ihr/rotU l«tu. Th!a ia the<)ovice af 
Andrevr Hom, anil ix now eSccU-d ' hy incnn* of a spennl 'oiillnr ' 
jUTKDgranent, wliicli, by tin- nctioii uf a Mntw, uppn>xiinat«« or sopn- 
-nt«e the iiuit«bl» Imuei. But for thU n !ip«cial ilnvii!)) ix lui-dttl fur 
t«cb objective. (2) Tbo nutult win moi^over bi" *(*ur«l fry eaitnnij the 
vtfe-jtitce to nf^ronch the vfijecttve. This wf (.■ouriM! is acumipltibed 
.1^ the use of the ilR»w-tu)>e, unil luust be eniployerl with o1)i(«ti^■4!■ 
harbiff iS^d mouiitK. 

Cuuimj Ufve», ttint is, bmigin^ tliem ti>](Clher, whetJmr in the 
«l>jectiv« itavlf or in llie tutemscupe ns n wbule, by tihort«nin)( the 
4listanc« betw)>en the fcye-j>iec* and tlie objt-rtive tinilfrmrrrctii thf 
■tbftetiix, tlikt ia, gives poeitive aberration ; while the teparMian of 
tettrnt orfr-wrrwtM at givea ne^tive aberration. 

In uidng the wltaf torrfttum ' for a lou;,'er body or ii thicker 
«aver-glasa the colUr adjustment must be moved so us to cnuut the 

" : lenses of tl»e objedive to approaeA fJtf front lent, nhili? for n 
body or a lUlnner corer-gUas, the ndjustuit^nt must be 
moved so ma to iau»e their separation. 

Til etH-reetiwj fry Itttf length for a thiclcM' cover shorten t)ie tube, 
and for a thinner one lenf^hcn it. 

For the benefit of those who aim at work with lenses, Uiat is, 
as may be compassed with the aid of the uioijt eleinentary 
'natbematto, it may be well to indionto a simple metliod for tlie 
-ileductioii of the.Wi:t of phtwroMvex ran'/ biconeex IfHKt). 

In tig. IT the focus wt twice the radius measured from the vertex 
A, tlmt i.s, AF. But in li;;. 18 it is twice the radius measured 
fram the point A, tluit Is, the point 
F i« di.ilant from the lens twice the 
. nulin.t Ins two-thirda tJi« tbicknets 
' tilt- b-iui. 

Himilnrly, in fiff. 25, tke radiM 

^a Aieonrrj! Ifnt is measured from 
, the point A ; in other words, F is 

int from the lens the length of 

! ndius less one-fourth the thick- Flo. is.— The raiKos ei • toava 1mi<- 

m of that lens (neaily). 

Formula- rtliiting to ahieonwx Inis. — Where Pi8oiiefocaif,P'it« 
■eonjn^te^ K principal focua (solar focus, or that for a vei-y dittnnt 
objeetV It Ruliuii of curvature for one eurfncc, R' for tnc other 
y!, ft the refractive index of the niediam, then 




AIM'S if A is the distance of a focus from F, tlie principal foctis, 
' SccClupUrT. 


nnd B, the distance of its conjugate from F', the other prind] 
focus on the other side, then 

AB = FF'; 
or, in nik equiconvex lens, 


Ill an cquicouvcx lens of crown glass ^^ 1'5 if F^rftdiusof- 
curv'uturo. Itut in a plano-conrex lens of croim glass /i = l'5 if 
F ^ twice the nidius of curvature. 

Tm the iibove formula the thickness of the lens has been neglected^ 
111 thick lenses, however, its effect must not be disregarded, even if 
only approximate results are required. A very approximate det«niu- 
iiiitiou (if the principal focal length of an equiconvex lens nuiy be mads 
by subtracting from the result obtained by the foregoing formula 
onf> quarter of the thickness of the lens. 

Exnmjif. — Equiconvex lens of crown glass ^ = 1-5, r^i, thick- 
ness ^|. By above formula F = ^. Subtracting from tnis one- 
(lunrter of the thickness of the Ions we get F^-|^^as the distanott- 
between the focus and the surface of the lens. This is only -j^^inch 
fii:)ui the truth. Tf the lens were a sphere it would be accurate. 

Tn the case of a platio-convex lens the principal focus on the 
l■^^n^■l'\ sitlc is ('([unl to twice the radius as above, but on the piano 
side two-tliirtls of the thickne-ss of the lens must be subtracted 
fi-oin it. 

ExuiiipU: — ] n a hemispherical lens of crown gloss fi = 1 -5, radios 
^ .',, thickness^ .\, the principal focus on the convex side will be 
■ luc inch fnini the curved surface and on tlie plane aide | inch from 
till' pluue surface. 

Similarly, in an equiconcnve lens subtract from the princip*! 
focal li'tigtli, obtained by the above formula, half the thickness of 
the lens. In other wonts, measure the focal length from the centre 
of the lens. The focus is of course virtual. 

But a pi a nil -concave lens follons the plano-convex. The principal 



hat Ions, to which t)i« rays will he brought. It b evident that the 
T»y% would bo brought by th« lirst l«iis t<> a focua two inches behind' 
tlie wcmut if it wcrn not tlipre. I'his jioint, wliii'h ia nej^tive with 
n^;Mtl to thr second Ipds tiiust be tAkeii as t)i« value of p iu the 
fonnal*. Wo hnvp^ ihereforv : 

' + '-'■ 


V Ilitliprto nnr .itt4<ntion has i)cen coutiuetl, in studj'inK tlic action 

I of Irnivs, ti> tlu- nuinner in whicli thi^y act ujmii a butidlt? of {Htrallel 

I n^yt, ov iipiiii a ]H'[i(-il nf rnjrs iviaiog from s radiant |>oiia. Moie- 

I OTOT, W(! Iwvi- c(>nKi(trroH this ]x>int an situat«() in the line of axis 

H But thir surface of rvrry luiiiini>U8 body may be regarded ha conipre- 

■ hcndinf; an iiilinilv number of such points, from every one of which 

■ A pnncil of i!»yii j>ttwiTsU, to be refracted in it* pnssage through the 
I leus iic«.<o«xlin^ to chr InwK cminciAt«d. In tliis way a complet« 
I imam, i.e. pictum of tho object, will bo formed ui^oii e. suitable 
' sumce pincrd in Liu- porition of the focus. 

Then! nro two IcindK of image fonned by kna««, a rtnt inmge and 
n virtual imaf.'e. 

i, Thf /vrinalioH of a not imoffe means the production of il 
ineture by a Icili, or a. combination of IcnHcfl, whidi can bo thrxtwii 
upon m srreea ; such are tlie itnngps of ■ projection lantern »nd the 
ima^e proiluced by thecuiiii-ra upon the focussing glon. The ntanner 
in which this taki-H (>!«(.•<• will Im undentood by reference to tig. "JS, 
where A B in an »l)j«Tt [iIuchI kri/otitl P. the principal foca« of thtt 
Rplanatic maibinutiiin. Fn>in i^vcry point of A It nre i>iyB radiating 
At every ponible angle. l.ct A V and A II be two Buch mys 



Fmi. 96. — Tlw tannalion of s tml iiatip^ 

rtuliatinfc from the |ioint A. New if the refravti'in of these mys he 
tracrd. in the manner alnrtidv iiidicateil, thixiu^h ihe aphinalie com' 
tiin&tion, it will br fuoiKl that t\w rays which Wforc ininiersence 
were divdrKiii): are by the refinctioti of the combination on emer- 
gence rendpr«l fonversinj;. Thus the ray FO nieet^ HC at the 
point C. The point C la called the conJu;;»te fond point of A. an<l 
wherever there in a focal point theiv will lie hii tnisge. Therefore, 
at C, there will lie An iaiage of A. In the utme manner the ray» 


isKuin-; fn>iu evrr%- point along A B nay be traced, and wilt be foand' : 
TO hiive twi-h one its ivspectivt? cmjagate lying on C D, so the ooi- 
juirnf of B is at IX Henw il is at unce manifest that an inverUi ■■ 
"cMtijuirate iiuaj^ i>i tlw ol'jwt A B is formed at C D. Further, it - 
will U- imtio^ti th;it. aIi)iough the object ia straight, the image at it ■ 
is i-urvrtl tow;irvU tho Ifiis. 

If the olijtvi A B liad been cur\'e«l, so that it presented a conm 
iisjuvt to till* lens, then its c<>iijug!ite image C D wouUi have bMO - 
iiu>n> i-ur\t\l : but if AB hitd been slight! v concave towards thelen^ 
then its iMiijUiiiite H\iuld liave l>een straight. 

As bt-fon- stutixi the }<oint C hns been <let«rmined by tnong 
the ivfr»otii>n of two r.ivs' AFanil AH, through the lens. Anotte 
)iieihi>il is, however, often eiujiloyed. 

In overy leim therv is » ^loiiit which is called its optioal centn. 
Tlti> ]>(>ini is such th:it any r^ty. which in its refraction throu^ the 
lens imsst's ihroiii,'h this {x>iiit. will emej^ in n direction parallel to 
its jmth Ivfoif iimnerct-nce. Xnwaa leases for graphic and theoreti- 
cul purpiises nn' often :issuiueil to be tif insensible thickness, it has 
Invmne the prsu-tio- !■■ ilnw any ray passing through the optical 
iinitrt' of the lens n stntight line. Obviously, if the lens haasensible 
thii'kness the my onniiot be i-onsiilered a straight line, and in the 
iuii-i'ivu.v|>t', wliei-t- tlie It'nses are very thick in proportion to the 
leniftli of their fix'i, this metliod nill lead to much error. Of course, 
in tlii>st< iiises when* it ean U- taken ns a straight line, it saves the 
tnml'lt' of eonipiitiin; a seo»nd ray to intersect the first, as any ray 
iuUTSivting the siraifiht lini* will iletennine a conjugate focal point. 

In the HpiiT }virt of ti;;. lilJ the two mys, A F and A H, are 
tr.ii'iil tliiiiu^i) ilie lens to ih't«'nnine the point C, but in the lower 
]' lit of the 1i>;ure only the ray H K is traced, and the intersection of 
this my by tlie stniight line B D jNissing through the optical centrB 
(■iM'> tlie iKiiiit h. 

'2. All in^ai^■ is sjiiil to I.h' lirtiiiil when it cannot be receive<l on 
I) si'rei<ii. Fig. '27 shows how a virtual image is formed. The 
letters arv tlie sjini" as in the preceding tiirure, so as Ui show tlie 




SimllarlY, from evtrry point in A U there will be a cnrrcK{iot)(t)ng 
Ti'tinal point. Now if w« prmlnce F em) U IjackwArd* («w tbo 
'dott«d ImeH in the figure) wo shall find that thoyintorwciot tbo pojnb 
C. Aa the rnjrs F and H nro proditrJjr idpntic«l with mys wlitch 
would have diT«rged from the point C hnti it b«>eii an vntitjr, the 
Mtiual inukge therefora witl be an imngv vf a noii-existiiit picture 


The niethod uf dntwini; Uiin in exnctly cimitiir to that of (ho 

Cediiijj lijjur^ Ttie myit A F a:i<] A H arm tmcoil tbrouffh th« 
, and tii«ir prolotif^tioii hiickwardii (.see the dotted liiicK in the 
AfftK) ^L^ves ttie point C Also, an in tiw pniui^nx liKun.', any 
point of the picture >»n he found by tiucinx one rn^, such uh K ; 
then the int«rB«ction uf iti ba<:kwara ])roluiiKalion with a atniiglit 
Jiiie joining; B with the optical ceDln-, produoei], will j^ive D. 

Tlie points C and 13 are called the etriunl c<>tij"i/ati: /oci of A 

Fw. n.— Tlie Tomution of » ' virtoiU iiuii«o.' 

«ndB n^spwti^'cly. InmathemAticnl optics it appears as a negfttivo 
UttMatity which Ratihlies nn equation, and is a sort of tnetaphysico- 
BHitlieinfttical truth. In tliiscase the virtual image isconvax towarda 
-the lpn«. 

Vig. :i7illustrate8thenction of A simple microscope. The object 
itoelf ia not Mvn, but tlio picture presontnd to tlic eye i* an 
SDlaifCcd ftho^t of it. As somo eyp* win take in rays uf le»i div<-t^- 
«ac« Uian others, it might happen that the rays C K, C H, went too 
^ivrrgent for tbo olinervor's cye-Kight, in which cium tlm lens wouUI 
have to Im! withdrawn from the ubjeci. Kimilnrly. if the oliserver 
ware short - sighted, tbi- Inns must be plactid ncan-r the object to 
ntDdor the rnys mor^ divergent. Dr. Ahbe pointt cut' that the 
ffeDcroJIy ulopted notion of a 'linear amplitication nt n certain 
distance ' is, in fact, a very nwkward nnd irrational way of th^uing 
ih« ' oiDpli^ing power ' of a lL>ns or a hiui-systcm. 

< Journ. ItM-S. ral. i*. mr. il. p. US. 

26 elemesTary prisciples of microscopical optics 

III the formnln N =? the amplification of one and the si 

syst«ni varies with the length of /, or the 'distance of vision,'! 

an arbitrary- conventional value of I (i.e. 10 inches, or 250 iiiift)[^=~ 
must 1m> introduced in order to obtain comparable figures, IW^.— 
nctual 'linear amplification' of a Bystem is, of coarse, different mrj 
the case of a short-sborted eye, which projects the image at a d»^J- 
taiice of 100 mm., and a long-8ight«a one, which projects it it ^ 
1000 mm. Xevertheless, the ' amp/ifyinff potver ' of eotry »j/gttMi»'f 
nhraija the game /or /toth, because the ehort-nghtedatui the loHg-iigilfi j 
lAiseri-frs obtain the image of the tame object uiuier lie tanu vinm i 
anfflf, and consequently ike name real diameter of the retiniU imaft~ 1 
Tliat this is su will be seen from fig. 28, where the thick lines ihmr L 

Fjc}. Qfl. — TliE amplifying power of & 1eiu<- 



rliiniiuktcd \>y il^liriiiig tlic 10 iRchm merely us th(? <lUtiince from the 
ryo at whii-h tlif imii^ in mcmswred^ whether it Iw n ilistioot or tm 
iiKlistini-t ir^IlKl^ Fur if an itlKcrvcr, owin^j; U> tliu ncoommodatioii 
of hi* rj-r, ubuiijs a distinct iiiis^ at n distAnct! oE 10 foot, I may 
iii-v(trtJiirtoa.i Buuniv ii [ttiuic iit n di^tniKx; of lU iiirlim from the tm 
uti whicli ill*' ili^itiint iningu in virtuiilly iirojtvtM), Aiid iDcnsure tlM 
(lijuaetiT of tlint ]inijts:tion. Now this (fiaiiieU'r is stricUjr the same 
AM ihi- diAiiivU-j" *>f tbitt inutf^, w)ii<Ji nnotber oba-rver would really 
obtain with divliiM:: vixion nt thnt luinie disbiiicn of 10 inches. 

Thd uuly <liireTence ia Unit in thi- fcirmrr eusv m* inu*l tAkc tlio 
oentm of iho cireli-B of indiatindjiRKs instctiid of tht.- shnq> inmgi?- 
tiointd in (ht? hiltvr ca.-H-. If ih<' (-oiivmtiunnl leti;;i]i of /^ lU inchos 
IS intfrpretwl in tliiii "any (hh di.itaiici^ of j)roji->.-lion, indefH-iidriitly 
otdiMiiict vision) the iiliaui-dtl.v nt Icuat of u naX influenct! of tlio 
MKommoduliou on Qiv imnvr of a niici'osuope ia avoided. It Ih^odikx 
obvious that for Iou^-Mgbl«d utid for aliort-u^bted eyes the snuio X 
most indicate the aanie visual uuf;U of the enlariced objects, or tho 
Mune inagnitude of the retinal imaKev becuuse it indioates tlie sainu 
diameter of the projection at 10 iiiohe« dist^ioe. 

It wa8 1oii|; since puintc^d out by Ainiti, thni tin- introduction of 
ft drop of water betue«ii ibe front surface of tlio objective, aud 
either the object itself or its covering rI^^ would dimiuisb tlie tosx 
of light resglting from the poaiui^ of tlie rays from the object or ita 
CO*'ering glass into air, and then from air into the object- f-latts. But< 
it is obvious that when tlie rays ent«r tlie obJMt'gtAss from water 
instead of from sir, both ite refractive and it^ dispersive action will 
be greatly i-luuiiged, so as tu need an luiportjiitt constructive niorliU- 
catton to suit the uew cundittou. This modification seems never to 
have been -luceessfully effected by Anaci himself ; and his idea 
retnaloed unfmilful until it was taken up by Hartiuick and Nneliot, 
who showed that (he application of what is now kiiovnt as the 
t>nNi^«ii(im tyiltm to objectives of lugh power and targe apertiiro i& 
■tMnded with many advantages not otherwise attainnblr. For, Kt 
■Irwdy poiuiod out. the low of light iDcmtscs with the obliquity of 
tiie iitcinent mys ; so t]iat< when objectiv«« of very wide apcrtiirv 
are ased 'dry,' the advantages of its ittcrrasc are in grtiat dcgrov 
nullified by the rrfloction of a largo proportion of the rays falling 
^cry obli(|Uely upou th« peripheral portion of the front Imis. Wlinn, 
on the other hand, rays of the ume «>bli(iuity t-nt^r tJic piTiphcral 
portion of t!io l<tn» from water, the loss by rcflcrtion is gn-jitly 
raluntd, and tln^ bcnrlit dcHviihle from tho largi; njicrture is ])ro- 
portionnlly jiugtniMitf d. Again, thr ' immeruuD system ' allows iif a 
gmtter working diKtancv betwrcn thr o)iJi>ctivn and th<! object than 
IN otbi^vrisc: attuinublc with ttic same nxti-tit of apcirtun.- ; and lhi« 
u a gri'at advantaf^ in manipulation. Furtht-r, tlie observer is 
n!nd««d Inui d<^-n(I<'nt up<m tho cxactiicKs In the conxwtioii for the 
thiukncM of the covrrinu; ghiu, which it neirdiril where objectives q( 
largo Niicrturo oni uned 'dry'; for ua the amuunt of 'negative 
aht-rraltiin ' ia far amallcr when tbu rays wliioli emerge fruin tlie 
covering glami paM into water than wh<-a they ptuis into air, varia- 
tiimi in its thiotcnen produce a much Icax disturbing eOeL-t. Aud it 


is found practionlly that ' immenion ' objectivca can be conatruclwl 
witli magnifying [Ktoti-s BuQiciciitly high, and apertures sufficiently 
liirfie, for the niiyority of the ordinary purposes of scientific inreifa- 
giitioii, without liny necessity for eover-acljuBtment ; being originallf 
adapted to give the liest results with a covering gltus of suitabb 
thinness, and siuiiU dqiartures from this in either direction occosioa- 
in;; coiiiparativety little deteriorntion iit their performance. But 
iM-ynud nil thexe i-ensons for the superiority of the * immeniaa 
systnii ' is, as will l>e presently seen, the fact that it admits into iit 
lens u Ian;pr number of ' difi'rarfinn tjifelrn ' than can be posublf 
admitted by a lens workinK in nir ; and upon this depends the 
perfect ]>resentiitiiin of the image. 

T/i" innnfmion Kifulrm has still more recently been advanced upon 
by the application of a principle which lies at the root of the optiol 
intiTpn^tatioii of the images which modem lenses present, and 
which has giytiitly increased the value of the microscope as a scientific 
instrument. It is an improvement that primarily depends upon ■ 
correct tlienretical understanding of the principles of the construction 
of micrtisciipical lenses, and the interpretation of the manner is 
which t lie image is realised by the oliserver, Tlie ' late Mr. ToUat 
was the iirst to iidupt this system, as we point out 8ul>sequentlT ; 
but ir was to Pnifessiir Abl>ewe are indebted for its practical apfoi- 
oatioti, through wliom it is now known as the homogeneoiit gytitm. 
' Tlif idea nf i-eidising the various advaiita{,'es of snch ' a system by 
i-oristructing a certain clas.s of honjOii,'encoua objectives had.ProfesBor 
Alibe says,' 'for sonii' time presented itself to his mind.' "Tba 
niattJ-r assuiiii'd, liowiiver, suljseijupntly, a different shape in conse- 
ijui'rifo of a suggestion nuule bv Mr, John Ware Stephenson, . . . 
of London, who iiidep('n<lently iliscovered the principle of bomoge- 
ncfius immei-sioii.' ' 

Tills metliod eonsi>>ts of the ^placement of water between the 
covering jjkss of the mounteil object and the fnint surface of the 
object-glass by a li^juid having the same refr.ictive and disper^iive 


^fntnas lenMw. But in tliK iittw lusped in which the problnu watt 
pr««(>n(nt hy Mr. SU'j>h(!ii.ion it orrir*) wilh it now int«rest to Alibe, 
not only iw pniniiAiiig Ut largely (iupfiuin with t)w ' oonvction ccilUr,' 
bat aim to gnutly enUrgt! tin- 'nutnrtical n porta re,' and tlierefbra 
wears A grvAti^r ruiulviiig jxiwcr in thi^ (ihjrctivtt. 

One o( th<- tliffiuultiKi wn.t to linil n xuittUlc fluid to meet th6 
IHHWiiliiw UK U> rrtniclAfiix luid disptTxiim. Itul nftv.r n long neriea 
4)1 axporimentK Pn>[i'Hwi>r AbU- found that (til of crdnr wood so 
BMtljr onrra(pon<bi wicli crown ft\n.-a in thrsr rc-i|x>ct« thiit it served 
thp jHirpiBH) well. 

Tfu- rfiiult of AbWa oidcuUlioDK biis«d on Hr. S(«[>h<'niH>n'ii KUg- 
grstion wiis the conntruutiuu by CVtrl ileUa of n ,1^ lli with n X.A.' 
oi l-'JlH (if line ([uaJity, and ntiil Iiixlicr prouiiae, oixl ituLini[U<intly 
of a ijtli iind II I'^tli ill. objective of a like chunioler. 

It inny be well to note tluit Aiiiici suKK^itt^l the luie of oit 
itaimd of witt«r pnor to 18.'^Q, mid Mr. Wenlium u^iin rv%iv(.>d 
tbo sunKcttioii in ltS70.' But neither of th^si? in in eteti » n!tii(»t« 
•■tn«« nn antictF^iliou of the ' fionio;;i-neniis sysC«tu ' of li'iut-^ us wp 
now undentand it. The 'oil iimiK-mian ' in both tiiaUnces wjm an 
«xpe>lienl. Tlie principle on whioh the t-uoBt ruction carritid out by 
Pn>rnui(>r Ablw dc|ieiided wiis the 'optical' priui-iple tbitt a medium of 
high refractive power gives an mperturf j;reatly in «xoeu of the 
m»xiuiuMi( It^O") of » dry lens ; while Abbe'a expUnation, propounded 
in 1874, of the importmiit bwiring which the diffrtu-tion rfntrilt Imve 
on the fiHiDiition of the microscopic ican-^ makes the reaolviiiflr 
power of the object'ifUvw dependent upon the diffmotion [>eiicilx that 
mn tnknn up by it. 

All thi* wus unknown or uiutduiitted by thow who had previuu^dy 
miggcMed oil aa an imm^iion me«liuui, whi<.'h leavtv. tlia hoinoK<-'neou* 
>y»t«fu MH now employed wholly dependent upon tlie prindjiIeH 
BmuMtAteil by Abhe, artuug from the practical aum^Htion of Stephen' 
■on Mill nRsultinj;in the beautiful object-gUases of Abb^ and Zeisa. 

One of tlie esiential adrantages of this syMeni, beyond those 
■tated, is tliat by the suppreasion of spherical aberration in front of 
tin- olijfctives facilities ai-e aflbrded for oorrecliiig objectives of ({revt 
nuuwrii'iil aperture, b<)th in theory and pmctice, that reduce it to 
the level of the problem of correcting objectiven of Rio<lerate 'ftngle.' 
A» a t^■»ull, stimulated by the Dutnife^t advantage to be obtMn«d 
ami tlie wants of those engaged in actual research, Me%rs. Powell & 
LMlaod, of London, very soon made a y^th inch and a a\,th inch 
ubjcctivo on tlie homogeneous principle, with niiinericnl aperturcft 
ft»p(«tivety of I '36, and daring the year lt$85 produced lensesof an 
excellence impossible to any previous fiystou) of j^th inch, ^'^th inch, 

. ^,t)i inch power, having respectively numericnl apertures of 
i-50, while 1-52 is the theorvtical maximum. 

The use of a * correction cmllar' ts continued in the best English 
and German homogeneous objact-glaues, sspecially as aiding in tli» 
lleUoate corrections required to get the exact length -ivlation between 

I Th* nntnlug of tlu* txpTcMiod will In found on p. 18 Lul the sliotc ol Chajitot 
IL iniurt h« nuwtuUy mad. 

' U«ttM$ Mierv. Jaum. vol. ai. p. SOS. 



thp ohject-g]a.sB anil eyepiece. But this must also be aided ia 
t'liileavDuring to socure the most perfect ' critical images ' by & bodj- 
t.uVif provided with rack and pinion motion. Wben the two ■!• 
i^niliincl, if thp tiliject-glass is of perfect construction and of Utot 
fonn (upiiehnmmtic, ij.r.), rMults never before attainable can bent 
with comparative ease. And this, be it observed, does not in tin . 
least coniproniisp »ur Admission of th« perfect accuracy of tbs 
theoretical piinciple of the homogeneous system. 

With such evidence of advance in the optical canstructioQ of 
microscopes, ilependent apparently on such accessible conditions, tbs 
ijucKtion of what ia possible in the future of the instrument no donbt 
ohti'udes itself : that, however, cfi.n only be mnaiilered as haniig 
npplicittioii to the area of nur present knowle<lge and resources. It 
is impossilile to forecast the future ^^ncies which may be at tin 
<lispos;il of the prricticiil optician. To photogTuph atars in the im- 
measurable aiuplituiles of space, absolutely invisible to the hamiD 
eye, howi-vcr aided, was hardly within the pumew of the astroncmai 
of a (juiirter of a century ago ; that there maybe energies and 
nictlioilM discoverable by man that will open up possibilities to li» 
e.n^fei- stuilcnt of the minute in nature which will just as widely 
ovcrsti'p our present iiiethods of optical demonstnttion, there can b< 
little i-ciison to question. But it is no doubt true that with the in- 
strunicntK and meilia now at the disposal of the practical opticiaa 
no iiidetinite niid startling advance in microscopic optics is to be 
looked for. The ' atom ' is intinitely inaccessible with any conceiv- 
able application of all the resources within our reach. But optiesl 
improvi'inent of great value, bringing nature more and more nearij 
and accurately within our ken and reducing more and more certainly 
the inter]) retiition of the most dillicult textures and constmctiona in 
the niinuttwt accessible tissue to an extrl method, is certainly 
within our sight and reaclu It is not a small matter that the homo- 
gtuieous lenses were, in a comparatively short period of time, carried 
from II N. A. of r25 to Iv^O ; and this carried with it the eapacitv 

I hkVR jniit (leiaoriKtrMtiyl, nriiM'M f ntm the rf^tdtiai tpherUnl 
txttd eAroiHtitie nhrmitiat>*,i.\i«ft\}\t:T t»kfn orifpn inthettitnlofhomo- 
ynfity, atiMilulr prrcinon of etirvf,»t\d p'-rj'rft /■mlfrituj of Iki- ty»t«tn 
A\/irtuir» in n combination. TIiih oauHCJt tin- conu of rays proc-n<'<ltnf[ 
frnm thn obj<!ct U> unito, not in {wrfoct imagt- poinbi, but in ' lifclit 
KurfiKi'N iif ([Ttatt'r or Ic-S* i-xtwit firdt-s nf ili.swjiiition' -wliich 
liinils tli« distinct iH'si of minuti> (l*>(ailH. It is tbi- fnults cf the oi- 
jr'tipr tliat in ]>rin--tic« iin- aloni" iin[>ortii.iit, ixud with tli« crown and 
'flint gl»s« ciHiiiiionljr nt thi? ibti]touLl lA Iht- i>ptii'iiin tliAn^ nra two 
grCAt dmwbacirs to perfection, or mther tu im upproxitniitimi ti> i(. 

1. Tbc firrt arisM from the unt-iiual cotirai- <if tlic i/irjfrtinti in 
cniwn iind flint glass, aliv*dy describe'), which uiukt-K it iiii|HHt!iibl<! 
to unite iierfoctly, witli Ui* properties they [Hi<iapa8, a!l M« rolourtd 
*nj» III an iiHitye. Absolute &chnMn&tisra cannot by lh»?ir iiiMtnft Ixi 
MUined. tlw diaper^on at different parte of the ai)ectrum licin;; to 
««Ktly dinproporlional It has never 1>een jxiiuibl^ to unite more 
xtun two (lilTerent colours of the spectruui. Tlie rMt, in apjte of nil 
oflbrt, deviate uiid foi-ui ttie i^foiidart/ n/iwfrKin, leuviu;;, in thi- very 

. lenaea, circles of (lispersion not to be exclu(Ie<L 

2. The sei-uiid defect arise*) in the inipusHihility of cnrreotinK by 
tia of orchitary crown and flint glass the Bfh''rirat nlt^rntlion for 
' than "N'' colour, ff the spherical aberration l>e removed «ji 

tr afi may lie for tlie centre of the spectrum, tliere renuiiiu uiider- 
«orrectio» for the red, find over-correi^lion for the blue and violet 
imyv, prOKutin); a want of bnhtnce between the^/irouKi/ii; corrections 
fof the eeiilral aixl marginal xones of the objective. Alth»U{;li 

Ijierfect chroaialii- con-ectii-nH for the central raye may be effected, 
irinK iinafi^ of );r«at l>eiiuty, the chromatii; over-onrreotion for the 
(H-ripberal rnya with obli<iue illumination will show the borden of 
tlie ima;;e with diT.tinct chromatic frin»eti. 

To compeuMtte llieiso alien'fttionsin tlieeonMtruction of an objeot* 

M|[lais, what is needeil i« a vitreouH material applicable to optical 
purposes posHMcd of such properties that a relatively anialler re- 
imctive index could be united with a hij^her dispersive [luwer, or a 
lligher refractive index with a relatively lower dispersive power, 
Sy piuper combination of such materials if they bo provided with 
onliiuiry crowti and flint glass, to partly remove the chramatic and 
spherical alierrations independently of each other, and so to obey 
the conditions on which the removal of the chromatic difference 

^^epeniU, these aborrationii could bo compensated. 

All this wo* nopn and fully demonstrated and wt forth by Abbe 

' AS far back as 1>*7(!,' and Im pointed out that the furllier perfecting 
of the microscope in its dioptricjit working was dependent on the 
art of glaM nwlnsg : the production, that in to say, of Wtreoua com- 
poands pomoaring diHTcrcnt mlntions of refractive and dinpenuve 
power by mnuit of which the secondary spectrum could bo re- 

Por practical purpoww the matter was in abeyanoe until 1881, 

^Lnt aince that time L>r. Schott .ind Professor Abbe, with tlie activo 

t HaA<wa,A. W, BtrifK Aftrr iU vi*»eiuehaftUtlttn Apparalt a«f d«T Lun- 
• liUtnwIivtuiitn AumttlUng im Jakra 1970. 



cii-i>|ifi-ntioii iif the optionl workshops uf Zeiss, Imve undertaken &» 
l;il<oriou< mill [>roIi>ii^il investigation into t}ie improvement <f 
optii-ikl glLss. to which we have alluded : the result has been the pn>- 
itiK-tioii of ' cn>wii ' and ■ Hint ' ulass jM^isessing exactly the qualilw ■ 
fini'^lii'Hn us iuilisjteiiMible by Abbe. 

l>y i-hciiiioil. physii-.ll, and optical research tif a most laboriow 
ii.iliir>'. ;md (ly spectronii-tricolfcien-ationaof numerous experimental 
I'liNions Kysteinatii-aljy curried uut with a large variety of ch«tnicil 
cti'iiiciils, the relation between tlio vitreous products and thoT 
cliciiiicii! composition hits been more closely investigated. 

Ill tilt* i-rMwii anil Hint ghiss produee«l up to the time of Hixm - 
iuvt4.ti>pitioiis, the uiiifomiity of property arose from the relativelf 
Kiiiiiil inimlH'r of iiiaterials emplojevl. Aluminium and tballiDis, 
with silica, ;i1kali, lime, nml lead, formed the limit. By the use of 
mon- clii'niicnl clenwiils. esptvially phosphoric and boric acid astbt 
t'ssinit iiil constituents of fjlass duxes in the place of silica alone, flint 
anil I'lMwn j;l!»ss have Uh'ii pnxluced in which the dinprrg'wn in (*»■ 
liif-r- lit /mrlri <•/ ih-- nr-ctr'im in iii-itrfi/ jii-->porfloiiaf ; so that il 
:ii'hi-oin:it ii' I'oiiibiiiatioiis it is now a iiuestion of detail and practictt 
ii|iiii'^ lo climiiiiite aliiiitM entirely the seoondavy spectrum. Ontht 
i>ih<-i' li;ind. till' kinds of l;1iiss which can be used for optical purpOM 
h;i\c lin'M so iuiTe;iMil ill variety that, while the mean index ot K- 
fiiii'tioii is iHiiiKtant. CI ins id CM b!c vafiations can be given to the 
ilislHnsioii or to the rcinictivc inile.<L while the dispersion remuni 
ciiii-.iaiit. A hij-h index of refraction is no longer of necessity ao- 
c«tii|'iiiiii'il liy a hi^h dispersion in flint glass, but may be retained 
ill cri'W n f;l;iss w itii a low ilcpn-o of dis[iersioii. 

The pi-iii'tical conseciuence of this is that l>oth the imperfoctionii 
iniilicuable frinii an oliji'ctive constnictM of ordinary crown and 
(liiil iiliisj. i-.iii I-. iiiiii h'U--- h'-ni, •liiiiiiinffd, and the seconds^ 
s}iii-li'«iii aniiiiUcil ; it is ri'nioved and reduced to a residue of 
cliriniiatism of a lerliarA- chaificter, while the chromatic difference 
of aiihrricid nl-ecTittiun can be eliminate or completely corrected 


1, Uwse obJrctivcK mtut «pparentlv coinninnd 3 high price. But, 
jiTtrn Biioli nn object-glow —«- hi cli is tJio production of a thurougbl/ 
omprrtonl pnicticAl optician — and its stlvuitagn, tliroretiol and 
ctical, arc gn-at. 

I. Tk' ap^rtur- of ihf tAj'fCtiv^ can h* tUitutd lo il» /uli eHent. 
In tiM hmt of tlio older objpct-gla8S4M ftt least oiui-tenth of tliv 
Avikitabk Bpcrtun^ vtm ntrlMK ; th« inalioiinblo defact in the coii- 
vrrci-noo 01 tli" rayn provontod a pn>p«>r combined notion of the 
oiilcniinst lonc and tlio o-ntnil parts of tbo «p«frtur«, and tliereforw 
br IIkkhi o1)i<H?tiv(H it biiK nnvnr bncn jiOMiblf to rralitu: th<! amount ' 
ot iwolt'int; piiupr indicnti-d by iliror)- with > f[iv<'n a|K>rtui«. Dut 
in a wclI-riiniitrui'tiKl ajncbroinittic nbjcctivR tlio Ki-ROitdnry 6peo- 
tmin lH*in;i Timir>;-4Hl, anil tbr KpIxTical alwrmtion bi-ing uniformly 
vorrrvXrA tor (liHVri--iit parts of tlin spectrum -tticro is a prnctically 
licrfi-ct focal coiiccntrttirin of tli<- riiys in ttwt iniiigc. 

2. Inrrrfitr i-J Hin-inil'iixHiJ }ii,iirf l-if iifUim ii/' f/>iviii>/i/ fntttirutlfd 
rye-/fifc<ii is also u nunl imjxirtaiit foatiin! of (ibjivtivi-a <jf tliix cla«a, 
TIm randt of Ibis is Unit firvat ma^nifyiiix jiciwim- rnii bo nbloined 
by objectives of n^ljititely lar|;e focal Ic-ngtlis. W'l; biivr iilwnye 
m'linlaiued tlie utility of high eve-pincijig uinii^r proper conditions, 
nnd vriih &uilable aperlureH luiil (iiu- corrections in the obj^ijtivc; 
the |i4iy»ical brigtitJi«H, wo learn from Ablie, in every ciiae di>p(fnds 
only upon the aperture and tlio totid uiai^nifying power ; and it in 
o( nu moment in what way tlie littter is pn«Iticed— bj* iiiflunK of focnl 
l«liKlh iif the objective, lenfftli of tube, luid focal leiifith of eye-jiiecp. 
But Ii« Itaa further shown uit' (bat with the beiit objeclivrsof tJiu 
<>1<I eonxtruelion, aiid with tarffc iipPrlun-s, the limits of a eomjilrtrly 
MLltifiu^f try cleamMS of iraaa^i are rnichtHJ when tin- rfu/w r-junjilili ca- 
tion ia four- tosix-fold; that ia, when the t4)talnia|iiufyinKpow<Toftho 
objective and eye-piece to^tJier is four to aix times an tjn-at as that 
obtained with file objeirtive when ustvX by ilself »h a iiia;;iiifying 
leiiB. On Uie olher Imnd. with apoehroiualic objectives the atrailubk 
Niipnr-iiniplificatiun— even wilh the )-rmtent upertureti— is at Inuit 
twelve- to (ifteen-fold. and considi-nibly biifher with medium and low 

\ Afhrt'iiuilijim tottdira tifmonf nn irlifal iKiinl iri l/u^»^ iibjeeliet». 
Tlif inngCB are practically free frooi colour over tlie entire area. 
TliiH is of great %'alue in photo •micrography. Tlie convction errors 
of the oiiliiiiiry achrouiatic syslenu ore much more powerful ua 
disturbing infiuences tlinn in orilinary observation with the eye, 

4. In spite of tlie removal of tno si^condary spectrum certain 
enjnor deviiitiona of a tertiary nature i-eniaiiied, and arc inevitable 
in all o1>i(>cti\'cs of great aperture in whicb tbo front lenif cannot be 
aiailc ncbiwnatic by itsolf. With oivlinary achromatic objective!, 
from thp properties of the gla-s used, the amount of thiR is very «ii- 
f«|u*l ill the central and )>oripberal parts, but in the npochrnmntio 
objccC-glaas it is approximately cuhkUiiiI /or alt parU </ iJir. oj^itiivj, 

■ RxotfiliiiK «lu« rmoliitimi in eStvUil hj li|[ht at rilr>ni<- i<Mi^<iUj, If llw 
— tsi—ml Mao of Ifao objtctire is conecUd aloua, &nd tlutofUy Ifcflnipluyt'lf si that 
liMil mimII; (AuJ iwiotntiini umy l>« ■(v<inipliah«l. 

' ■ On llw Rvlstioa or Apcrloic \o Povur,' Jomm. BJt.8. UM, p. HOa. 




iukI thfTffvr* it aUow* »f eorreetion hy tht ei/t-pieet, a specUl con* 
ttruolt<m pouesBing t^at Itat ojifoititf diffecMices of inngnif^'itig 
pownr (or (liferent colours. Tho C'^O'pi^ce is so coiisiructra as to 
oontpletclf tieeuro the dwIri-H rrault, nnd, lu ve have eUMd aboT<% 
itouei free (rom colour aiv olitAiniHl. 

6. Th« eloMtifieation tifOf •■i/''7<u^a for this Dj-stom of objoctivos 
hu be«n estAblished )>/ Ablir, and <l«pands on tho increAse in th« 
tol«) RUft^iifyinf; pnwor of Uin luirroccopo obtained by moans of tho 
eye-piece ai comjxtrml with that girnn hy tho obj^ctivo alone. Tho 
fiuniber which dwiotos how ninny tiin"i an pyo-pioco incrcasM (ho 
tiiftK"'fyin^ power of tho ohjcrtivp, whrn hh.'iI with n given liody- 
t.ulie, ^vfA t\w proper mMuinre of tho oy»-pwce nmignification, and at 
the Huno timo th« Sgnrp-i for mtionnl numcrntian.' 

From tiw'ir propprtic* thcrco ore known oa 'compensating oy*- 

TImi following is A fair typical iH^lnctinn of the objc>i:tiTos and 
oro-pi(v<<g ftiniishiHl from tJm workshops of Curl Zcikk, of J«na, on 
tliis important Ky>t<Tni, vie : 

A/Mfirvpuitie OhjertirfK 

A|i0U» r«Ml BncHrti Bialraltnt 



Waicr InuaMnion 
llomogeni.'Oiu Ininu^rsiva 












1 inali. 

ti " 

rt •■ 

Vamftiuatiag Kyt-jiieet*/or BnfflM ftoJirt 
S 4 B 1> IB S; 

It ia of interest lo note thnl Mi'mni, Powell and Lisiland, on 
receiving the q>eoiHl ;:U(i8 (i-oiu <ii'nniiny prorccited im mediately to 
the piodaction of a ,',-iuL'h iibjcctivc with uonipeusatinft cyn-piMHi 
OQ H formula devised by 3Ir. T. P»wrll, wliich were HuppoHi'il to 
be apochromatic. The workniaivtliip wim of thi hiyL tUss for wliinh, 
In the manufacture of teiise<t, tluit finn liav<- Ix-como dint inf;u is lied ; 
and this objective has, together with thosi> KiilMr(|ucntly producnl, 
brought out Admirably the (juuUty of lliir work; for we tiow know 
that tho i>erfoct apochroniatic objeetivo rcquircn Attorile 1i-iim!« in 
ita combinations to obtain the tieedful cnrn^ctions. But witJiout 
the tUD of thoto this tinu cMoe so nmr thi' curlier Mpochroinatic 
objectives of Zeiss, for visual purpoaeA, that it wns not easy lo diacoi-er 
tlwir deficiency. This was <)ue entiivly (« tierfccl workinauHhtp. 
The mae firm have sinro produceil » n- nmr k able lens on tlie nanio 
system, having a. N.A. of l-.iO, with u power of ,'„th o( »n inch. 
Objeot-glASSCS are also now made by otlicr imtkors, Englidi, Euiii- 

< ' Oa Iin]ir<nr«aiuiuU of Uw UienMuopg with Um »h1 of iww \daiia at vnticol elut' 
(Abbe), /oam. a.MS. IMT, f.ii H*t^. is 



pe&n, and American, known as ' apochromatic ' ; but we doubt, in 
the majority of cases, if the apochromatism is attained, for it has 
only recently been made known that, in addition to the special glass 
used in their construction, the Abbe apochromatic systems had in- 
serted also the fluorite letuea referred to above, which materially 
affects the result. It is remarkable, however, how near some makers 
have brought their results to those of Abbe without the advantages 
of the fluorite lens. "With this material employed In their con- 
struction Messrs. Powell and Lealand are making beautiful object- 
glasses. We have recently used a new |-in. made by them, to which 
-we have seen no successful rival. 





We ure now prepared to mter upon the npplicutiuii of thu ojiUcaF 
priudjitw which nave beoii explained uid illiutratetl in llie turceoing 
pof^ to tJi« cen'^niefi/in of mirrwiro/it*. Those are disUn^uihed 
ua nmfilf and fomjxinnd. nnch kind having its peculiar ttdvMntagm 
tu the student of natiiiv. Their •■Mvntial difference oonuita !d this 
tliat in the former, the mys of light which enter the eye of the 
obaerver proce<«d ilirectly from tlu* ohjct itself, after having been 
»ul)jectcd only t<> n rhnngo in their onune, as wo liuve slvnvn by 
Ug. 36, which fully rx|ilnins thf iictinn nf the simple lena ; wliilHt in 
the compound miornKonpe nn I'nUrmicd iniiu/r of the ol^ect is formed 
by one lenis, wliicli imiigr in mnpiitiixl tii tho ahmmt by anotfanr. 
Its if he were vie.wiii;£ thp oly<s;t itnelf. tit tlje fomjiouml micru- 
aoojM! not less than two lemses iimii bo employed : rinc to fonii the 
enUrK'ed iuia^ nf tlie object, immediately over wlucb it is phiced, 
und hence caIImI tho ohjft-giatt ; whilst llio other ugnin inagnitieii 
that iutage, and, being int«rposed between it und the eye of the 
observer, ia called tho rye-glriAr. A perfect objeet-glaaa, ma we have 
Aoeu, tiiusi. coiiai»t of n cnmhiiuition of lenses, anil the eye-gbus is 
best combineil witli another hrns interposed between it^lf and the 
object-glass, the two togetlir^ fnrnihig what is termed an ryr-jiir.r'. 
The compound microwoix' must he the euhjeet of eareful und dp- 
l»iled TOHJtideration ; hut it. must \u- remembered that the shorter 
the focUH of tile >iimpl« magnifying Ion», the smaller muat be tlic 
diutwtvr of the splii-re i>f wliie.h it forms |iart ; and, unless it& 
aperture be pmportioniitt'ly reduceit, the distinctneu of the iinngi> 
will be destroyi^d by the nnliericnl and rhmmatic aberrations neces- 
sarily resulting from ila high curvature. Yet notwithstanding tliu 
loss of light and other drnwhttcks attendant on the use of sinxle 
lenses of high |»ower, they proved of great valao to the older micro- 
Nctrpisti (aiming whom Lee u went iciek should 1>e specinlly naiut<d), on 
account of their freedinn fn>m the errors to which the compound 
microscope of the uld cini*tnieti(m wua neooBsarily subject ; and llm 
amount of excellent work done by means nf them i^iirprises everyone 
who studiea the hisbiry of niioroacopic imiuiry. An important im- 
provement on tile single lens was intmduciHl by Dr. NVoIlaston, who 
devised the dfuhlrl, still known by his name, which consi»<t8 of two 
plano-convvx Irnscs, wlto-K? foeal lengths nn- in the proportion of oni.' 
to throe or nearly *>■, having their convex sides directed towards 



Um cyo, and Ui« kns <>f dtortwt focnl length iM«r<uit tlio olijoct. In 
I>T. \S'ollK.'.t4>n's original combinAtinii no |)frrf(tniU-<l iliaphragni (at 
' atoj) ') VTM inUirpOMd, aiid the dutAncc wtwtum the latnitcK was left 
to bo ilctcnDinod bjr oxperiinont in nicl) ciwr, A grrAt improrem^nt 
w»« Nnb«M|ui>ntly nrndi?. however, V-y thv intrtxluetiou of n 'rtitp' 
betwMD tbu lotiKVii, nnd hv tho division cif thn powvr of thr Kiiinll«r 
Ir-tui butwMD two (tvpocinlly when a very nhort focu« in nniuir<Nl). m 
ma to fonn a tripUl, a» limt saggesbNl hv Mr. Hutlnnd.' When 
comliinNtionx of thin ktnil nrn w«[| const ructi^l, bolli tJin Kph^ricn] 
And tiu! chromntic abrrriitinnK Arc so mui-li rtx)uoir<l thnt th^ nngle 
of kp4;rtDro tn»y ho connidcmblv mlnrgrd without much Mcritico of 
diitinctn<9a ; niid li4-ncrr f»rn]), »ivn v(^ low powers, sucb 'doublete' 
aiwl ' trijilcU ' iir« fur KUpcrior to singlo l«nsw. Tbew com bin at ions 
took Ihi- pUce of sitiglo Inues Among microsoopist* (ia this country 
at luutt), whu wi-ru prosecuting miniit« investigations tn Anatomy 
ktui pliyBiolof/ [trior to iiiv \as% improvvmeDts ctfoct«d in the com- 
fMKind micnMoiipo h>' tJic ticlinitnntiiuition of itA obj«ct-g)it»>n!. 

Another (orni of .liaip)*- inugnitiiT, posatcKsing ccrtnin lulvnnlngea 
over the otdinary duubtcf-vinxcx l<-n>, in that oomnioidy known by 
tht^ name ot the * Cnddington ' Irns.' Thf^ tint idrn of it wiw givi^n 
by Dr. Wnlhutoik, wlio propoM-d to np]>ly turn plano-convex or hrini* 
aphnical leiufji by tlicir pliino kiiIi', with a ' stop ' intitrposcd, tltv 
oenlr&l apertun- of which Hhouhl lit- niuiil to one-fifth of thi> focnl 
Ico^tli. The grat advAntAgr of Kurh » tens in, tJiat the alJiijae 
pMicibt paai^ like the iN-iitntl ones, at right anglcM to tlie MirfiicL', m> 
lluit tbcy lire Init littJi* isubjttct toalH'rmtion. The idtn wak further 
iiapnived upon by Sir I>. l(i-ewst«r, who pointed out thnt the .fnme 
end would hv much lietter answered by taking a sphere of fthua. 'ind 
grinding n d<H-p groovp in itn (Njuntorinl pnrt, whi^-h jJiould Ik; then 
filk-d with opmguo mntter, ko qx to limit tlir centrnl ntierlure. Sucb 
* b-ns gi%'<!s a large tiehl of view, mliiiitK a coniidiM-niile nmoiint of 
light, iiiid LI e(|ually good in ntl directionn ; tiut its p(>wt-r of clelinitioD 
ia bv i>» ni««n* (■<|ual t'> thitt of iin nchroniiitic len«, or even of a 
■doubh't. Thin furni i« i-hii-lly useful, iherft'ori-, ii» n hnnd-ningnilii^r, 
in whtL-ti neither high pnwer nor perfect definition is nH|uired, it* 
prculiar c|ualitiiui rendering it siiiii-rior t<i an ordinary hiui for the 
cl&os of object* for which n luiiid-inagnirn'r of lui^lium power ts 
required, M.iny of tlie ningnitimi sohl ;is ■ C'o<ld>ii;,'ti>ii l«naea, 
Iiowever, are not really portion.t of splieivii, but iirt- munufautured 
put «( ordinary doublC'Convcs lenw*. «nJ ;ir« therefore deHtitut« of 
Uia specia] Mlvantngeti of tlie niil 'Coddin)^>in.' The 'Staoliope' 
lens soniewhat rtoenibles the pn«nling in appenmuce, but differa 
bom it cneDtiolly in pntpertien. It is notliiug moi-e than a doubl«- 
enovex lenti, havtOK two surfooeti of unei[ual curvatures, Nepurated 
from eoeh other by « consideralil« thiekneM of ghtst, the di>.1nnee 
of tlte two surfnceH from each otlier lieing so a<^ustod ihaX when lite 
luost convex in turned luwarils the eye minute objects plnced ou the 

< TT»iuaetiomBflhtSeeiMjio/ArU.iB\.i&t. 

* HiU muu^ bonver, I< nxMI iuappropniite. ■iuon Ur. Codditigtaiii nTithvr huh, 
Mr evir dkinwd to be. Clia inTimlor o( Aa taoAx uf ooiutnMtion Y^ wbldi tius loo* 



otli«r HUifue ahall be in tlie focus at the Iimis. Thin ia an t!ii»y mode 
of iipftlyiti;; k riitlier high iDaf,'iiifvi>iK powi-r to acales o£ bulL(;r8i«s' 
win^, uiid other »imilai' flnt nnd luiiiute i>bjoct«, whk^h will niodiljr 
Mtlliere Ui th« <iurfiK« of the gla«a ; tLiid it ahi> siTves to (l(^L(>ct tho 

SntH-'iiov of the InrgpT niiiinalcuk->i or of cry^tAk in mitiutv (Inijin of 
uid, til exliibit thi> 'eoU' in pikste or vinegm- ^'l-. A noaifiMl 
fonu of tlici ' r^tAiili<i[>(> ' lens, iti wliich the ^urfnoe remote from the 
eve in jtliiiie instciul of convex, has been brouj{ht out in FraDUit umlcr 
tJie name of *St4Uiho«copp,' f^iid lins been especially applied tu ihn 
enlMfpenient of ininuto piHnivs phntoginptied on i(a plivue surfacn in 
Uifl focus of itK convex xurfaci!. A good ' Slauhoscope,' inuKiufying 
from 100 to I'lO dijimetcni, in n voiy convenient form of liand- 
uugniller foi' t)ie rccngiiition of <lintaniK, infusoriA, etc., all tliut irt 
required being to plnccr n niinutc dixip of the liquid to bo exantined 
ou the piano mifnco of thp Iraiii oncl then to hold it up to tht) 
light. But nil linnd Icimni vw have jfflt wen will compare witli the 
8teiidieil 'loiipv' of nx nud t«n diunelera mods by Zeiss, and 
It«ielmrt's porkrt loupM. 

For the «rdiniir>' pnrpo«rs of mieroMopic dtssoction nngff^ leti»^ 
of from 3 indues to 1 indi fooiui oiiKwirr very well. Hut when higher 
powers arc miuirt^d, and vrhi-ti tlio ui>n of nven the lower powers is- 
conttnuod for nny Irngtii of time., grtmt advantage it derived from 
the ftniploytncnt of acliniiniitic conibinationK, now inadn expressly 
for thin puqxvie by K-vi-ral optit^iiuiK. T\u: combinations' 
givn much nioiv li|;ht tJmn single li-nsrs, with tuucIi Imttmlclinitton,. 
a very (Int field, longnr working ilislnnpo (wliich is very important 
in minute diwcction), nnd, ns a eoiiKi-qiience, grrnier ' focivl depth ' 
or ■ ponetmtton,' i.n. a clearer vit^w of those imrtji of the object 
which Ho aboi'f^ or Inflow the exnot Utcal plane. And only thoso 
who linve carried on a ])■«<:<• of miuntit nnil diDiciilt diiuicctiou 
tliriiiigh KuvemI i^onMcutivt' hount can appn-ciatct the advantage in 
cwmfort imil m dimininhed /ati'jue of r>/r which ix gnjncil by the 
substitution of one of thme achminatic condii nations for a Kingle 
leiiK of <»|uivalent focus, tiven wtirrtt the umi of the 
fonntsr rvveulii no detiiil that ix not ilixcrrniblt! by the 

Although nut Mrictly its potiition, it iit convenient 
lien* to refer to wlitit i.i Known as tho ' Briicke lens ' ; 
it in much usi-d on the Conliueiit. but Aovs not »p- 
p&u' in any Etigli.ih ti-eatise we have neon. It lias 
two loiiara for tJie objective, and a conciiie eye leiiB, 
It is illuHtnttetl in fi);. 29, 

To reineily the inconvenience of the lens being t«o 
ctosf to the rtbjeut in all hut low powers. Chnrletc 
Chevalier, in his 'Manuel du Micrngraplie ' (H^SO), 
proposed to place above a doublet a eimcAve acliru- 
matic lell^. the di^tnnce of wbieh could Im> varied at; 
^ensure. The eflectof thi a combination is to incivaso the ninj^nifying 

Kwer and lengUien the focus. Thus arranged, this iustruiueut will 
the most jKtweiful of all fiinple microscopes and the 8pace 
«Tiukbl« for s«lpeLa, needles, d:c. will be niucli greater than 

Fi... au— 'J'lic 
Urilcko leus. 




tvitli n doubiM alone, llx* furllier the con en t«- lirits urcmovetl bxtm 
tfao ktt«r, th« grtMUie will l.<o lti« umplidralion.' 

This cMntMnstioii, npplJMl to lenses far cxMnining tite pye and 
•kin, ullims tliir uiw of doubtet& which lenvc n coiiiiiderftble diotancD 
•Luvr tlio oIn(>ct, ftiid it is this ici«n wliicli lias goremed tlte coti- 
atructiwi of tli<r ltnick« tciis. 

' Tbu l«ns hu » Tory long focus, and Uie ronstruction is that of 
tliA Gntil<4> tittcscopo as applied to ojicm-glassps, but thp aniplilicatiou 
ai the obji>rtiv« in much grcatpr than tlint usually obtained inopera- 
iImmm. The focus isnboat^cin., and thopoivor three to eight times. 
lUttorpoweris obt<une<l bv Icngtliening thir tu>>e, bj- which lueauii 
yOtit distnnco bvtvren the two lenses is much cnlM-geH, and the 
iiplifiaKtion inrn-ai^nl u-ithoat inoonvmiMitly modifying the focus.' 
Tliia lens mii}- >h- iis«d in piftoo of the body of o, compound 
■uicro«cii[ie, wht-n it ix tloiired to <tieisoct or to find smnll objocts, or 
it OKU bo ndajitt-d to a aiiople microwopo or IcnS'holder, with froiu 
3 to 8 an. Ix-lvm-ii the object anil objveliro. 

Compootld microscope. — The ctinigioutid microscope, in its most 
fiimplo Uma, ccm.vistJ! of only two l<'nMii, tlm ohjrct'jtnM and tlie 
^*]ff-^aaM. Tbu fijriiicr reocivcs the lijilit-mys direct from the object 
aght into ivna proximity to it, and furm.t nn rtilarKi'd but. inni^ed 
and myrastf iuugo itt ngmtti'rili.stiini-i-ou the other kuIc ; whiliitthu 
littter reoeJVM lii« ntys which ntv diver^iii){ t'ratn litis imnge, as if 
they prooeed«d from an object nclunlly occupying itt position and 
mmrjged to ita diuiensiunx, and bringit thmo to tlie eye, ko altering 
I'tlwir course as to make that imng*' apjicar far lurtcer to llic eyv, pre- 
cb6lj an iu the niiM) of tho ximpic laicroseope. It is obvious that, 
in the use of tlio very suno lenses, a cunsidemblo variety of ina^cnify- 
ing power may be obtained by merely altering their iiosilion in n^gnrd 
ti>eucli otlierand to the object. For if tboryi-.gliua Iw mrried farther 
from tjie object kIasx. whiUt tbeobjeot \i approxijnatt-il ni^nrer to llio 
latter, the imagi? ■*, ill hn fomivd at a gnMter distance fmin it, and itii 
" aensionswillninHnitienllybeMmmentod; wltikt,uutlieothcrhund, 
the eye-glass ))e biou;(ht nearer to the object-glu», and the objt-ct 
njrUier from i(, tlt« distjuice of the image will be a much 
kller multiple of tti« distanoe of the object, and its <limensioiu 
projioniiinat«ly dimini»heii. WeUmllbereitftersee that this mode of 
Varying (lie magmt'yiiiK power of euiujHiuiid inicnaGO]>e^ may be 
turned to good account in more than one mode ; but tliere are limits 
to the use which can be ad\'autag«ously mad« of it. Tlie am]ili- 
licatioii may also be varied by altering the magnifying power of 
tho eye-glass: but here, too, (here are relative limits to th«; in- 
Iwoaee : since defects of tho object -glit«s which are not |icrccj>tibte 
'wb«n itsin)ag« is hut moderately eidarge^l aixt brought into injurioua 
prominencn when the iui|ierfect image is amplilied to a much graiter 
Wt«nt. Hence, in pro;iorlion as tho object glass appi-oachca more 
■ly a perfect coml>ination lioes it Mlmit of higher eyt-pieces. In 
''pncUce, and with tho old achromatic system, it is gcnerully found 

> Ur<l<m. C\ Traitf rfn Utrntrope et dn Ii^ittliont, Sud «d. 8to, pp. SB, ii. 

46 visios wrrn ttie compousd aocRoscopE 

much Twttw to vary tliQ poww by omplnyitig rjliject-gltSBM of dif- 
ferent foci, nit objoct-gliMB (if loiuf focus fonning uii imnge which 
is itot at mnny tiint's the <listAn<Hi of thr objnot fruni tlie oUtor stdo 
of tlip \f:nf, and which, thi-rofoiT, in not of Dituiy titiit^s it» ilimen.-don ', 
wliilEt nn objnct-glnss of thort focus rcc^uin-H thut t)ic objiiot elioulc) 
h« im ncnrly niiiiroximntcil to it tlial tho ilisturiot! of tho imn^ iit iv 
much tiiffhf^r iiiulti|)!(! of tbi- objttct, iukI ila diiueiuions an- pmpor- 
tionnblylnrjftT. In whiitftvw mudo iiirri'a^ml anipliliuitioii inny Ih? 
nbtAiiii<il two things must always r&iult fi-oiii tlie ehanjit- : tin- jim- 
j>orti'>n of thi! nurfnoe of the object of whii.-biiM iiiia;;»i.-aii lie furiixHl 
lEitut hr^ dimintHbiHl. mid the <|uanlity of li>;ht b|>re;id ovpr thnt 
iiUBgn niuKt 1* jinjpjrtioiiably loscieued. But. iis we have atated, tliU 
in d«poii(ient on tlio iipeiture and tolul iiiA[{iiifyinf; jiower, aud uul 
upon (anioiiK^l "llier tliiiifjs) Uie focal 1eiL<^tL of the eye-piece. 

We Hbull fiioilitutc the coiiiprehetiaioii by tbo student of tho 
]>rinciple!i of the iiio<lerii form of a. ooiupound microscope by meaiia 
uf lij;- 30. Ill tliia ti{-iii« the optical portioo, that is. the objective and 
^^■piece, nre di'awn to the full nhe, but the distAnce between tlieae 
luw, froui tlie exigoncieH of s|)ac«. been much curtailed. A low- 
povrer objective has been specially chosen for Kimplicitr, and a com- 
peiisatiti<; eyepiece {viHi Chnptcr V.) has been introduced to show 
its form And mode of Action. 

Tlie objective is a copy of nn old Rom 1-inch of ISSfi. Tho 
incident front (that is, the lens on which the incident bcnms from 
tho object Rnt strike) is a cnnvei of lon^ nulius ; the incident sur- 
face of the Bint lens of the tmck combination in a concave of very 
lonff reditu, being in fact alnut twenty inches. 

The object V han only rnvA drawn from one sid« in order that 
» clearer perception of the junh of the rays may be seen. This pair 
0( nyn ptuscs from the ahmw (object) Ihrough the combination of 
l«nH-« forming; the objedivc, giving nn ini'erlr/i rcnl ime^ at A It. 
Thi» imajfc, in fact, hft8 a convfx curve towards the cyp-picco : this 
i« a pottitioii that will tnid to iiKrreaisn thi; cui-\'a1iiiv of the virtual 
{maga C I) giien by tho cyi'-iiiccp-. T/ifn rai/M an? di-awn through 
thft fiye-picce, whicli givis u magnilied virtiinl imnse of the real 
imo^ from the objrctive, in order to nuggest thatwitli theeyn-picoo 
tbcrn in • cotiimmcejnpnt, tut it wi-re, rfc iioro, the invi-rtj-d imagu 

JA B) nt tin? diaphmgm of the i-ye-pii-t^i! bring thi' subjud of still 
urthcr and often gn-at magtiilication. 

In addition to the two Ichspb nf which the (^impound inicnwoopo 
may be ooouidrnHl tJ^i ■■HMrntiidly c»n.1l^t, it was Boon (ound nemlful 
to introduce auollirr lenn, or n cmiibinution of leiuies, btitween tho 
obj«ct-g^laM and tlit? imii;;<* fi>rmtHl by it, tlio purj<oso of this being 
to ohanjiie the couisi' of (h<( rays in such a nimuier that the ttiiKKO 
may be formed of dimeuMons not too gi-eat for the whole of it to 
come within tin; i-angv of the eye-glaits. As it thus allowH more of 
the object to be aw?n at once, it has been colled Uie ji(td</Ut»g ; but 
it is now usually considered as belonging to the ocular end of tha 
i»6truuiei]t, the fi/fi/fiim and tlie jiefii-sftfu* beini* together teruied 
tiie et/':-//ieiyr, or ocular. Various forms of this eye-piece ha\'e been 
proposed by dilTerent opticians, iinil one or another will be preferred. 

Fto, 80- — Path of a ray of light throagh li modent combination of lenKB Tor 
Gompotiiid microflcope- 

42 viaios wrrn rnR compoitsd MicnafcoPK 

accurding to tin- purpos'- for whidi it may be ro<iuircd. Tlint wtiii-li, 
until tho coiiKtruction of the coinpmsntion ryo-pii-cpis by Abbe, wns- 
consideml tJir tnont ndvnnUigrous to pm])loy with nclii-oinntic object- 
gUssoe, to till! pcrfiimiuncc of wliioli it i» tlcsircii togi^v tho greatest 
poesiblo rflVct, wji» tcrmwl tlip llui/tjli'iiuin, hiiving beon miployed 
by HuyghuiiK for bis tt-li-scufK-i;, ntthnugh without thn knowlcagp of 
all the iitlriinta^es which iu lirst const i-ut'tiun ii>nd«rii it cnpiiblo of 
kfTording. This cye-pincc-, with »thnrs, will bu vonKidiMXHl in di^tjiil 
ill tin; chapttT (v.) given in ]iurt to their vouHiclRrulion ; hut this 
«yc-pi<'CP L'uitiiistti of two jiluno-coiin-x Iwisir*, with tliPir ])laii(! HidoH 
towiirtlH th« i-ye. A '»toji' or diauhnigui, B B, In- plii«rd 
bwtweeii lilt-' two leiiHeH, in tlip visuiil fot'UB of thi* t-ye-ghxss, which 
in, of ooune, tku jKuitioii wlit-n-iii the iiu^i^ of tin- object will bo 
formed by the ray^ brau^lit iiito convergeiu-e by their pHssagu 
tJirough the Beild-gUuH. Huygheua deviawl this arriin;4eiui'iit merely 
to diminish the apbericul aliemition ; but it was subsi'iiut^ntty shown 
by Uoscovich tlbiit tbi> diruiuatiu dingjcmion v/a^ uUo in gi-uit purl 
oorrectod by it. With tJie upocliroiuutic lenses of the litgheat and 
best quality, (see Chapter V.) uu amount of obtttinableeye-piet-iug, if 
it be of the ' com pen sat ion ' form, cau bti>»k down the image. Tim 
editor has iried in Miin to break down tho imago furmed by » 
31 min., a 12 miii-, a, G mui., and x 4 mm., nil dry npochrontatics by 
Zeiss, aD<l especially with a \i\i by Powell and Lcalnnd. It is, how- 
ever, a matter of moment and interest to note tliKt with 'jtmd objeo- 
tivee of tho ordinary achromatic construction of larKo N. A. tho com- 
peosating eye-pieces give Wtter results than Huygbenian. 

But of tlio old form of achromatic object-glass it is true of the 
uiaiority tluit they will not liear high eye-piecing. * B ' is a con- 
v«uieut and useful eye-piece. For \'iL-wing Urge flat objects, such aa 
tran*ver«je sections of wood or of ectiin us- spines, uudor low luagni' 
^ng powers, the eye-piece known as KrHnfr't nmy be employed, 
but there is little advantage to be gained. Thin coiistniction will 
be fully de»;ril>ed iu Cliapter V. A flat. wcU-illuniinated field of as 
much M foui-Uion inches in diameter may thus bo obtaint^d with very 
littlo loM of light ; but, on thx other hand, there is a very 
serious falling off of defining power, which R-nrWs tho Kollncr eye- 
piece unsuitable for objects presenting iuitiut<r »tnictuml tlctails ; 
and it w an additional objection that tho smnllest sprck <ir Km^mr 
upon tho surface of the licld-glnss is inadr so unpleasantly obvious 
that tlic most caroful cleansing of that surhce is rcijuired m^ry 
time that this oyc-pioce is used. Hmco it is blotter litti'd for tho 
occasional display of objects of tho charai^lcr already s|)ccili«d than 
for the Rcientilic requirements of tlic working microscopist. 

A iotid eyo-picoo made on the prinrijile of tin? ' 8tauho]io ' hiin 
Is Koniotimcs usc<l in plncc of the oiilinary Huyghcniiui, when high 
magnifying power is rajuircd for tt-sling tlit- pirrforin.'xnce of objec- 
tives. TIk' lower surface, whir:h Irn* tho les-ii-r conv^-sity, serves as 
ft 'lif^Ul-gtaHt' ; whilst the imago formitl by lliix in majEoiliiHl by the 
highly convex upper surfact! to which the i-ye is ajiplicd, the advan- 
tage supposed tu be derivt^d from this (■an.vt ruction lying in the 
abolition of the place surfaces of llie two lenacs of the ordiii&ry eye- 




\A 'pMitivo' or Itiiin»(lvD's «^}'i;>pi«oc~in wMch t{i« lit-lil- 
k'btiM! caov<!X itido ix luniod jipwitras, is pincecl so tnucli iititrirr 
tbn i.',v<!-}tlAu tluit the image formi^d by the ol>j«ctive lies bolow 
inrttaul at alxivn it- ~wiu formvrljr used for tlio purpuae of niioro- 
mctrv, A <lividrd glau biding litt«n in tliu cxncl ]>liui« occupieil by 
(fan iin«^, sn thul its »ca\t! aiid th» itnitgc iiru boUi lungniliMl 
t«grtl»i-r by tht! kiuui* iiiUrriKMrtl lietwn-n tbMn mi<1 tlio eye. Tho 
iBiiiiO end, however, inuy buourriulilyititaiiiL-d with the Huyt;h«!niiia 
«ye-|>iece tJuil no eMentlal ndriintttgt^ in gaintd by the ustt of that 
of ItaiuMleR. th* field ot whi<^h i» distiiiL't only in ila cimtrt-. 

Aperture in microscopic objectives and the principles of micro- 
icopio vision. — It in aoir of the utmost inonu-nt t!ml we s}i<iu1d 
u»di-n>tHiiil citarly llie lunuiins nod impnrtntK-i- of 'upcrtui'e ' in 
luicronoupic ubj^vtivtu, und by tluit nieniiK lie Ivd to a percviitioit of 
the priaciples uf the iiiOKt n^cent and only niiioiuU theory oi inicro- 
Hopto TioMXii. Within the In-tt twenty yt-am Uiis untirc subjeirt 
bas undei;f(otie u rix<>roui iind i>xhnuxtive rc-itive*tig»tiori by ono 
ot tJie iDoat oompeteiit and niustirrlr nintJieuuttiail und priii'ticiil 
' (^iticiana iti the world, Profmnor Afibi^ of Jenit ; and, an a nn»uli, 
. sone of th^ jud;,iiioiila uui) opiuions, lu wi^Il n* what were sujipusml 
to be estabtislied truttiH, depauliug ujipuri'iitly upon the aiiujilest 
principles, and not believed to be open to uliaiiKe, ha*e been ftliowu 
to be nbtwlutely without fouiulatioa ; while priiu-iptea hitherto ([uite 
unknown and (uuiuBpei-te<l bare been shown to operate and (o rent 
on clearly deinoustrable mathematical and physical bantu. The 
Rfiull has lieeii « complete revolution of what weit^ held to he 
fundamental princi]>l<^ of uiioroKoopit: opticn and tlio theory of 
vinioii with miera!tc»]>io obJMt-glaaBes. 

One of tlie foremost errors relates to tli« mode in which inicro- 
I MOfiio tmuffes are formed. It was uMumed that their formation 
\ tocdc pUoo ou ortlinary dioptric principles. As the tianier* or the 
' teleMope fiivmed images, so it was assamed tJat the image in the 
ioompouud micraseoiw was brought about. Tlie delicate and coiu- 

filex structure of an insect's acaie or of a diatom were beliered to 
orm tl»-ir iimmes aocortliiig to tJie same precise dioptric laws by 
which tlie image of tlie moon or Mars is foi-med in the telescope. 
, Hence it was taken lor granted that every function of tlie micro- 
' s«^ was determined by tJie geometrically ti-oceable relations of the 
refracted rays of light. 

Wiih tli« teleKXqie what is known as its 'aperture ' is siiDply 
I Bstimated by the diameter of the object'gUss. Itut n cIosa cxnmi- 
&tttjon iti theory and practice of the conditions of vision with micro- 
scopic objectives shows that such an estiniato of apirture is wholly 
wrong in principle. The front Irns of » j'j-in. objective tnay b«? no 
I more than the j'^th of an inch in di«mrti-r, while a Sin. objective 
may Iwve a diameter of half an inch. Vet it 'm tll<^ nmoller lens that 
has by far tlte larger 'aportnre.' 

light is dixpei-gied from ovenr point on the Jturfaee of an objeet 
in kU directioiM up to 1(*0*. Only an extn-snely narrow pencil of 
this can be raooivcd by the human cw, a large wuicil of light 
emanating from the object being lost on iiicli sicle of wliat the eye 


reoeivM. The apparent problem uf prncticiil optiirx is to be at 
bjr toMuu of lenMo^ to gatlief up iiiid brin^' to ii focus iia many of < 
unadinitted mj-H as possible. liieK«ienil maniwr in ^^bicll lei)§e8 
act in doing ihU we have eodeavoured iu un elomoutary manner to 

80011 after ocUroniatic object-glosaea were tirsl made, Dr. Ooring 
found tlint the markings on Bpeciol ubjects^-such as the sailes of 
the win^ of in^^ecU — could lie seen by some objec-t-giassos, while 
with others, although tlie magnifying power was equal, it wns im- 
possible to ilisceiii them. Id every «ieo tlie greater 'angle 'was 
ahown to possess the gi"eatei' ' resolving ' or delineating power ; anil 
this led to the important conclusion that power of ' resolution ' in n 
lens was depcndnit upon ' nngiiUr aperture.' 

This, howf ver. was nt « time when only ' dry ' obj«ctivm were 
in une : tin-: iuinier^ion and homogeneous systems, ns we uae tiiom, 

' iinliiiown. 

(as wc sIiilII KuliEciiuently sw), oven with objoctivrai employed 

r vith nil', the niigle of the mdinnt pencil did not utVonI 11 true 
■vatBplUWan ; wlii^n iinnieraion objective-s were iTitro>liice<l — cilijcctiveii 
in which wiit*T in- etMlnr oil replacixl th" nir betwwn the i>bjf(;. 
liv« and the upiMT stirfiicp of the covci- of the mouuted tibjiKtt- 
tln- nut- ii{ 'tiii/lrf oi Jipi'rturc bccftinein tin- utmcist. degrei- iiiisldUflintf; 
for diflt-ri-nt mtilin with ditiVrrnt rcfnictii-e indices were emplcywl, 
and tint •iiir//r lit till- riidiinit priicil wiuf nuppcieed ni)t oidy to iulioit 
iif n c'oinpnriiMin of two npfrlun'S in tint muiic meiliuui, but alw> In 
lie a alamlard 'if i-imjpuri-%iin whi-n the- ni<ilia wrrtt tlifli-Teiit. It 
wiw, in shgrt, bclievwl that nn angle of IW in air reinrwiiUHl a 
Urgii I'xeeiw iif aperture in comparison with 96° in waljir and 82° in 
baUam or oil, (tcnoting, in reality, what wo* IjoHcvlhI to i>c tint 
ni'cn'/iium apttrtiint of any kind of Objective, which could not, it 
wan lield, be (tscenled, but oidy e(|uall«l, by ll*0° in water »r oil; 
in other wunLi, tJiut a radiant pencil haj> exactly tJie latme value, 
when the nnt/lrn are equal, no matter wbnt the refractive index o£ 
the uiedium ihrou;(h which tin- pencil might l>c piutsin.t;. 

But tu u tlioniuifh plivsicvil and miitbeinalio&l study of tJie (|ueti- 
tioti such an lluit in wliit'li Priifniator Abho engaged, it eoon became 
apparent that even in llie «amu medium the otdy exact method of 
^Otnpariaon for objective when the fundamendd phenomena of 
optica (which the i)ldfr dpticiuns had disn-gnrdeil) were t.-ikfri inH* 
account — was not a ccimjuiriwn liy the angl'-s of the radiant pencils 
only, but a oompariMm by tlicir sim-s ; while, when the me<lia are 
dilTerent, the indices i>f tlicMi' mnliji would be found to form an 
oitsential factor in the pnililem : for nn nni^le uf 180" in i>ir is eijual 
to 96° in water or S'J " in oil ; hcncn tliren unities uiif{ht all have the 
same munber i^f di^grees aixl yet denote dillerent values, according 
aa they were in mr, water, or oil. 

Tlius tliere mi^ht be liirgi- iliverRence of n/rerfiirf in two or 
more c-asea while the umjU won idcnticiLl, iliiU frum this the grcate-Kt 
confusion was not only [ussible but wun reidiiied. 

A solutiou of tlie dilBculty wax (il8 we have indicated above) 
discovered by Professor Ablje : and it is to Mr, Frank Cri»p'» 

now 'ArEnmiE' is obtained 45 

Ifldd eipmition of Abbo'n vlnbonitt! nioniigmphi thnt the Eiigli.ih 
studCTit ia iromrntely in(Ul)trtL' 

The dc6nitJon i>f ' npcrtun? ' in ttn Ii-gitimitU' teiiiH.' of *»p«ning' 
a «)iown l>y AhXx: to b<' ohtnini'd wticn we cunipKn^ the diftini-trr vt 
ihn {>encil f-mtr^nt iram tlio«>bjt«liv(' witli tin- f<A-«l Icn^'tli of tlmt 

It will be tlivirftbln U> «xpliiin siimi'whut innro in detail how thiii 
cnitdiHion t* (trrivwi nt, an pven in Profiwsor Alibi'"* najHTrs. 

Tnking in the first csxm? » ninglr-V'-a^ tiiicrtuicopi:, tni? number of 
nya ailmitlm) within i»ii- inericliiinal pfauie of tlic li^ns i^vjih'tilty in- 
cntnwi OM thn (liiinictfrr of th«l«nii (idl<>th«rcircuiDKLanc<'s rt-iniiiiiin^ 
ihi- HUni'), fiir in the miiToncopp wo \vkve at till' luu'k of tin- Irn» tho 
trawr: rin'ain^tJini.'M as niv in front in tlitr csisi- of iIm- tolracupr-. Th«i 
Inrgnr or nniiDiTr iiunibt-r of i>mi-'rf[t:-iit my* will thrrtrftiri- In? proprrly 
mnMmrnd by tli« cicur <liaiuet«r ; ami, lu no niyn cuti rmrrg/- thnt 
hnvi^ not first tn^n admitud, tJiJa must alau )p^'e the mMuiura of tho 
lulmitt^^ ntyn. 

Suppofo novr tbnt tho fockl lengths of the Ictmes comparMl iirc 
not thi' ■uiiw— what, then, is the proper ueasore uf tiiv rays nd- 
mittM] I 

If the two lenses hare equal openings but different focal tenths, 
tliey tnniiuiit the mme number of rays to equal area^ of an image 
at m definite distance, because they would admit the sarne number if 
itn object were substituted fr>r the inia;^ — -that is, if the It-iit were 
iiw-d US a telescope- objective. But as the focal len^'ths &re different, 
llir amplification of Uie iinaRca ia different alnu, mid cqu*! arenH of 
thrxc imaf^w correspond to different areas of the object from which 
tin- rays are eoni.-cti.-iI. Tlierefore llie hifflier jiowcr lens, vrilJi the 
naiDD i>pi'nin;f m tJie lower power, will admit a ywilT number of 
rnyn in nil from tlie same object, because it mluiita the eame nuutber 
iw the latter from a nnalUr portion of the object. Tlios, if the focal 
lengths of tw» lenseB are as 2 ; I. and the first amjilifies X diameterv, 
the lecimd will amplify '1 X with the iame diuance of the inio^, so 
tluit tjM! r»ys which are collected U- a gircn field of 1 mm. diametei- 

ot tba lnM((e ar« admitted /rmn a field of „ mm. in tlie iLrat case 

and of ^r-^ mm. in the Mwoitd. Inasmuch aa the ' opening' of tlie 

objective ia estimated by the diameter {and not bv the area), tJie 
higlwr-power letia admits ri^'i''-'- as many rays as tho lower power, 
bawause It admits the mime number from a licld vi An//* tho diunetWi 
and tn general the wlmiMion of luyK with the namtt opening bat 
diff<-reiit powers mii»t lio irt the inverse ratio of tin- fitcal lengths. 

In the case of tho ninglc tcn^ therefore, it* aptirtum must bo 
delei'mined liy 'Ac ratio tiriwtfn the ttlrar njiruinff nttil lh« j'oaU 
Irm/ih, in order to define the same thing as is denoted in tlie teleseope 
by the ahfilule opening 

t 'Od Uib Bilimiitiou o( Apcrtnrs in Urn Microjrope' [Abbot, Jon™. B.if.S. 
Mr. IL rol. i, $)•• : * Nnl«n ou Ap*rtuii>, HirrnacDplml Viilim. aiul Ilia Vklnc of widA- 
M^lnl IniiiMruiu OI>)c«tl>ct,' ibi'J. 30U; ' Thu Apcrton- ot UionnHmr* ObjwtiTM,' 
Emgluk JVrrAii Hie 



Conhiilcr now thi- nmipimuil ol>i<!oliv«! — thi- uiiMt imporUint cnsci 
til the iiiicn>«c<ipc. Wlmt is lli<! oriviiiii;; of thin cumpoiiitt! M'stvtn 1 
We inii»t nilhcn- In tb« dianiHvr iii Uie iu)>tiitt«<l cniii' nt tlint plnno 
wli^rp it hnn its iihiinatf uiivtitiijiin ralue, whlob is <)Lvu)u»ly the 
tliAinrrttrr of tlii; pi-iii--il ut its eiiirrK^'ncs from the syiAxan, or, pnicti- 
CJilIy. thn elrar, rjf^lirv diaiiielrr !•/ f/i<^ linri Irni. Tlin iMiicrgmt 
pencil from u niionisfopo-olyeclive tuni verging to u n'Jntirt'Iy <liiit«nt 
focus lui» itH rftVH iippiMxiuiiklrly [Mii^lleJ, anil the coiiiHticnuE an 
onco intim »tiuiUr lo tlmse of llie T^IeHtope-objecUv*^ on the sjdr of 
tho objwt, Tlmiliaiiielerof tbi? f ii\ei'i;eiil pencil, u-hellitrr it Mm-rfpui 
from a single lens nr from ii cniiipo«it<"'j"stMii, mu&t ihtrn^fon- iilway* 
hnvn the iuua« fti};iiilicfLtiou. Tlu- iiitluoiice of tlie [Mwor <iti fm-iil 
Ivngtb tibo remains tlip wime ji« in the cn>* nf ihe sinj-le li-ns. An 
obJMtive with a foonl Icnglb pi]iinl to bnlf that of uDotlier hi1ou1», 
witlk tlie Bum<? liiienr opening, twin.' ah mnny ruyn n» the latter, 
because the »roptificHtion of the iningc at on« nii<] the »inie <li&lurioiJ 
w (loubleil, iind the same niimlicr of rnya prinspqiiently are arlmitted 
by the hif-her power from n lielil of jmlf th« <lin.iHeter. Atid this 
will hold good wlietlier tbc iin'^lium arouml the object is the riiho 
in the CH80 of l>otb objective* oi- iliHerent ; f<ir an imnieraion sybtem 
nnd n dry Hyst^iii always give the mime amplilicalion wbeii tbe focal 
len^'tb is the uitno. 

Thus we (irrivc nt tiw gmutral propoutioii for nil kinds of oljec- 
tivea First, when the power i* the wime, tlu- ndmission of r»y» 
varies with the iltiimetj-r of tJie ]n>ni^il nt it* einergeiico. Secondly, 
when ihe powi-rx arc diH'ereiit f hi- Mij/ir mtmi^on requires diftrtnt 
opening* in tbe proportion of the focnl lengtlis, or, conversely, with 
ibe urrtir ojirniiig tlir. tulmunSon ia in iiirrrtf profiiirlion to the focal 
length — tlint is tin' objcctivi; whieb has the wider pencil relatively 
to it^ focnl length hnK the Ini^-r iipi-rture. 

Thus we Riie that, just an in tbe iiOeJieope tho absolute diatuetur 
of the iibji'i't-;(la!Mt ilefincs tbe aiHTtiirp, so in the nticroscoiie th<i 
ratio lictwren the utilini^l iliiiinelt-r <if the back lens ntul tJi9 focal 
length of ibc objiH-'tive diTlintut its iipcrtui-c 

This ili'ttniiion is dearly a duHiiiiton of anrrturo in its primary 
and "inly Ir';;itinmle iiM-ainiiig hh 'npi^ning' — tliat is, the capacity of 
tbe objcH-tive for admiltitig rays from (hi- oiijcct tinil tiviiiMnitliii;; 
them to tbe image ; and it at once ki>Ivc>s the dithculty which has 
always la-eii inv'ulveil in the conaidenition of the apertures of 
iiami'nuon objeL'tives, 

So long as the angles were taken as tho jimpcr expression of 
n)}ertuiv, it was ditbcult for those who were not well versed in 
optical niatte™ to avoid reganling an angle of ISO" in «ir as the 
uiuximum aperture that any objective could attain. Hence, water- 
iininenion objectives of 96' aud oil-iunneraion objectives of SS* 
well- lookixl up"iu afl being of much Itnf aiwrtim- than a drv objective 
of ISO', wliiUl, in fact, they are all njiinf. that is, they all transmit 
the same myti from tho object t<i tlie image. Theirfoi-e, ISO" in 
water and 180' in oil are uneijual, iind nri* iiuieh larger aper- 
tures than the 1)50* whicli is the maximuu tluit Uii: air objective can 



If WB Cfinipara a Henc« of ilry and oil-imiaersioii olijeotivM, and, 
eooimenciiig with vorv nriiUI nir-«nglra, prof;rw8 up to 1H0° air- 
«i)i;l«. thru tiikiiig nn i)tl>itnni<>rKiiin of li'l" aim pntgrasing Mf^in to 
180° uil'iMi^lf, tiM> ratio of np^niDg to power pmgraasM coittinuallr 
nlfu, and ■llnino iu miixiiDtiin, not in the ouo ot tbf nir-aiitjle of 
IriO" (wlinri it w r%nct\y P(|uivnl«nt to tli« uil-Mtgl« o£ &i')f but ia 
gmttnt al ill* nil-ai)gl«i of 180'. 

If w(i oKMinM- thp objwtivirsi in hflvc tJie Hini»i iwwor throtigliout, 
w«i got ml of one of tlw factorN "f tlin mtio, iiml we tinve only to 
conipnn' thn duuDc4cn of the cmsracDt bmmsi and can reprettont 
ttirii- n'liitiiMM by di*pninK. Pig. SrilliutnMM fir* casMof different 
api>rturvit of j-iD. «t>iccti%'m— 
vit tltour of tlry oljjoi^tivi'ii of 
60", »7=, nnd 180' air.anjrlc, a 
wnlrr-imtufRiion ttf 180" »*iit<w- 
ontflf*. nnil an oiliinmi-rn'cm of 
\K(y oil'Htl;:!*-. TIm! itllliT (foltwl 
circUw ill the two Inllin' ca.iiw nre 
of tlM- Hinic HIM as lliat corre- 
vpundin^' to tli<! 1^0* air-iinjjl'*. 

A ilry olijectivo of the full 
fnuximuin air-augle of 1^0° is 
only «bl(- (wh«th«r the first sur- 
face is pUueormDcnve)luutiliM 
n diameter of back lens ei|iial to 
twion Um: fiicn) length, while ntt 
imnM-rKiiiii leiis of iiveit only tO0° 
(in fC^njM) ni|uir«ii and ntiliM!* n 
titrj/rr diantcter, Le. it t* alile 
to tnuixuitt mora rays from tlio 
object to tho imafie than ANy 
dry abii-ctiv« U onpabl« of tranii- 
inittinji- Whf'iiever the oiiKle of 
on iuiDi«rsioii le«a einwdt Iwive 
tli« cntiuil uncle for the iiuuer- 
uoR-duiil, i-e. 9fi" for wiil.-r or 
82* for oil, itd ai)erture ii in ex- 
cept of that of n drv objeciive of 

Hitvtiif! «f>ttled th« principle, 
it wiu ntill nirrniinry. liowev«r, 
to Hnd a j>r<if>«r notalMin for cotn- 
imriii;; aperturea Tlw a^trono. 
in«r can conijutre tl>c iipertures 

ot his variotiA teleAco]ie8 hy simply exprenaing then) in incfara ; Lnt 
thifii8ol>vio4islynotavail.thletot!ivinioratii.'upii>t,who has todcul witli 
tlie mfui of two viirying qunntitie*. 

Prof««or AWw h*re a-piin oonf'-mwl a boon Upon micreMtqdstsbjr 
Uu. dtsrovi-ry (in ISiX imlependcntly cimlirtued by Profouor Helm- 
boltx shortly afterwards) lh*t * gfrnOTwl relation existed b0tw<!«n tho 
pencil oduii'lted into tho front <S tbo objective and tluit emerging 

Xnuflai) JUMtiiR 



s M* t4Uui(lr. 

Xiamtal Xpmxan 

Wlnilinl Anrtan 

Fio. 81.— 1t«UiTB<liuiu4on al tho (nUI- 
Md) hmck tniH4 <tt tjrnuai itrjr Bnil 
knlMffBloii objecli¥t-* of ilie a^mv 
pownr (II treoi ui nir-aiiicla ul W to 
u oil-anxle uf l(W', 


visioK WITH TnR rojrpf«-xD MicnawoPE 

from the bnck of tlio objective, so tlint tlio ratio of the Spini-diAiiie(«r 
of the emergent pencil to the focnl length of thp objeictive eonlil be 
fl)cpt«»eit by the une of hulf the nngle of Hpertuiv (») ' multiplied 
by thn refractive index of thp tnedium (n) in front <^ tlie nbjecti^'v, 
or M sin w (n being I'O for nir, 133 for water, and !-■'■ for oil or 

V (/• u* 

Pio. m— Illaiilnlinil of the Uw of c<in*w|U'>nn' fcii alilioiillo aritnintt. 

Let O and O* (lie. 32) be the cwnjugnte apliinalio foci of a wide- 
•ngled BJ-Btetn; u, U tlie angles of inclination of any lirt, m.y» nilmitt«d 

' In llip oriRiiuil tniuliitlrai iif tlir Mwn uf Prc(p*K,r Abhe from Uvnuon inla 
KugUili tlui Urriuiui nuUiviniitinl •jiuIhiIii li»m bcpii rrlaiuod. Iii tlio ■niiiniitrT of 

yX Ol OliJMIln » ti ilu * ^ I -« K -VJ = M. 

> ilii ■■ I4ii-I7t= ■i9=SJi. Dlobjaelln, 

AbiuIv >|iatuH of 

9il /I " /S \jr 35'.-' w/ H = /.5 

«ri»*.T jj =/-5 \l. aft/ix n -f-3 «'=■>!? In (Iu(, 

~^ ^ ; - — ^ . irliLcli If riuIt*- 

\«tX to the uifukf 

»X o» twJ'awrc*' Inn*" l-O" ■'<=■*'■ 



trrD.AI^4MMU)rara«Ba(«c(nMi>iBU)i.fl»n>irtUip«a*(Eu|{Utlt). AlmXJl.uid 

A1ib»1| UmwIm «ld (ttanMintnUou* prownUJ In tlt» lolloirini riie« llin K>litor has 
— mttf Mt fntiSod In aUctuiB tins, vipwullj •» the <jitiuuj fMm «! njmbol ob- 





UfC mdiniit. nnd h*. U* t1i« nnglm of the same rays on tbeir 
' ; Uwn «-c ishiill hnvr always 

sin IT* : NJn u* : : sin U : sin ti ; 

sin V* sin «• 


^ const. ^ e 

that is, th« *iu«8 of ihc' nugl<<s of tho conjagnto rays on both sides 
iA an ai'ltnalK system always yield one an<t llic satn« (|U')lletkt f, 
whMtcvor mys nitty be coiisidiTcd, so long iis the same systeiu and 
tlie ssoio foci ore in qneetion. 

Tliia propOHition h'>U\& good for cvv>Ty aimn^mont of media, and 
refrtctjng surfoces tJiat may go ti> the c<>in]i'>Kitii>ii <if the syst^tii. and 
for every ptKition of object and imngi?. It is the Uw ujioti whk-h ile* 
penats the d«'linr«tion of an imago by moans n( wiilc-nngU-d jjeticils. 

When, then, the values in nny given ciue-ii of the exproosion 
n sin » (which ik known as tlie'numerKmlnpertunt' and expreasod \>J 
N.A.) luut be<rn ascertained, the obJMtives am instantly compared as 
regards their api-rtun?, and, moreover, as 1^(1" in air is eqaal to l-O 
(since n = I'O and the sine of half 180° or 90* = 1-0), wo see with 
equal reiulinrvs whethtT th<^ apitrtiiiv of tlie objective is smaller or 
InWT (hnn that corresponilin;u: to 1K0° in air. 

Thou, Mipposo we desire to compare the mlativc aprrture of three 

Uiiu in «rar Unirvraitim, and is thonmeUjr andnvtooil Mnnngit XlaUenHy mvn. 
But lA IhoM nnaomattoMil !•> nutiiucuiUoitl tonnula coulDalon a^he ttuiiy itr'ttn 
ham OiD jaitaiMMilion of diflormt qmbol* mvuiinic prvciiwljr til* Mma UiiDg. Ta 
maul lk» pnnJbU mfVMily af Umu' ihSu fiiotnol* i* IiuutImI with ds kD«iai|NUi)in|[ 
il>W.i"" w iBuMnte tha Mmtit; of ■ n rin u ' with ' ft tin f-' 

Tha aUidaDi who Iim iDMl«nil SooD'a Ijiw of Sinct^ fliMi and UlnalniUd on )>. S 
(flff, I), win bjra iilkQcvnl the flipmAt on p. M undBntaud UiaDtuainji uid import. 
wm- el lliu riprewion ' N.A.' ^Dfiwcloal Bpntvirs) nnd at Ibc nuno lime will {rup 
vhprrin it dill*p* imm 'aiipiW apcvtnn' (f.r-). H* will al-Kt jfoirriv* hcrw tt oomo* 
%n pu* ilutui iinpilw>|»itiU«ol TO'iu S«u in «|niTBli!iit to nu rniKolu apertorv oF 
tad' IB nir. 

In Uic fleorclliD njqxtr hamiipharlokl Icm nipreKntn llii*/riin( of * liomORpncoa* 
(BUwmiMi obJxcliTii. 11 i« uqipowd to \m tocattA au lui oUji'ti in <^>iiiUrt aitti tii« 
loTO liUe of * iv>r(r.(lkM, Botixmi Uia plana front iit Uid Ivit* uiil the nppprtDiluv 
<rf lln covtv-clau i* a dnip of oil at <«du-1KKii). wbiMo Tslractiii: iii-lri Ik 1 n, Ihuiik 
tUuh klftnfnl with llii« fxntr-fftaas and th* fmnt Innic 

It ii nndoatcad Ihftl no *up ia naad. and UuL then) i* nothing brtwDou Iho otjoft 
■ad Ok (rant Ian* of tJu> nwdaivNr. 

In Uiia laat Ott udt AB u^ normal iy. S. fl|t. 3); on llio Mt-hnnd tide tlieto 
k s my wtiiek nukn aii aeigln ol 36° vith uio nonud ui (Imh iuolu^ into nil on th* 
lilhi-liaod aida of tbr nonnal. By 8nctr> hmnala tlb BJ — 

11 t'U 

«' - «P (bom Tabb t.) 
Tfcwfiiiii Iba njr OB *m«ovin|[ from Ih* under mHaM nl Ik* ooMr-glaaa will nolca 
■Otan^flfWrnm llienaimal. 

TbB douad linaa abow tlm pith at Llin ray whrrt ihr Gennan fymb«U an atail. 
n lin ■ ■• n* tin ti*; 

■in u*- 

. a «in >i _ I'B >i_-*T3 


- -m. 

■ 00° (Irom Tabln I.) 
Iha *ino ol biLLt th> 
hf tha w fawtii* inilfi lit thn n>«di4nn. 

VwmHmJ nyrrlmrf, tfasNfov. I* Iha *ino <il hjdt the aitgithir aprrlurt nmltipliad 

' It wfl) bo obatrrcd OM lbs rajm paanlae Ihioneli tlw oil e( ivd^t alitor (ha IroM 
liHi viUraal lalrartioa; thi* indua In the tort tlmt llio utcuUa in wliich Ilia raja Uo 
tosnlUac an ol tha muo ndncUvv iada, i.e. the; uw boiDOfviM'Din. 


oIgecUv«!«, one a dry ubjeotive, tlie Kecoiiil a wnl«r-tmin«rdon, nii'l 
the thini an oil-jtimwrsiau. Thme wuuM \m entnpnerti on thn 
■nguliir n|)erturi> view mt, nay, 71* air-an^fle, 85" wjiti-r-nnglc, nn»l 
|1K° Intlsuu-BTl^le : HO that u calcillatjuu must be vrnrlccd out to 
Arrive Hi it tluo nppreciuUun of theocluul ri'laliun iH'twiH^n thrm. 
Applyinf. tiowcver. "tiuiiieripar aperture, wliicli jfivi-* -6(1 for tlurdry 
objectiif-, -itOfor ibt- waKT-imiueruonifini} ]30forili<- nil-imnifrsiiwi, 
their relative apertures atv iimiiedbitely apprei-iated.and It uxwni, for 
tuKlJirice, tluit llie iipertore of tlie water- iiii mem ion is somewhat liiot 
than that of a dry objective tif IW.and Utal ihe ajicrtun! uf tJic 
otl<iniinenuou exceeds tbnt of the latter by 30 per i-i<iit. 

When these considerations hnvc l^een Appreciated, tlie ndvantA^ 
pocMwed by immention in compEirison with dry objectivm in no 
lODgW obscured. Inslivul of tliiii ndvantuge con^istin^; merely in 
iocKoaed working dif-inncc <>r nbsencp of correction- col lur. it i» mh-d 
that a wide-angleil imim^rsion objective hnit a Ini'ger apei-tm-e tlioit 
a dry objective of tin- tiiaxiniuin angle of IftO^ ; so that for any <if 
the |>urposee for whieh apritiirp i» <!esired ao iminenuon mu»t 
necensorily be prefcmfl lo a dry objectivr. 

1. There ex irti then a definite ratio hetween the linear o|)emng 
and the foctd tenj^tii of a syxti-ni, wliieh niust bo entirely indepen- 
dent of the coni]i"viiticm niid armn;b^mcnt of the system, and solely 
detenuiiied by llii- jil>i)vi?-iiirntioni.Hl npcrturc ix|uivaleni o! th« 
admitted cone i.f rays. When the <H]uivnlenl. is tlie same we have 
always the siim? jir<>iHirtioii <>f oiwning to fi>pn,l li-nirtli, trhatover may 
be the particuUir nrninji^'mcnt of refnicting medin in the sy^lfrm, 

2. If the (ibjectivm whose npertures are ci'iiij'ered work in tbi- 
tame medium, nnd admit angles of, say, GO", W. If^O", their npt-r- 
ttireu are not iii the ratio.% of tlnee nunibcrs, but are as -AO, '70, and 
rO. The 180°, for instaiici-, ilors not ropresenl llim- times thenptn-- 
ture of the 60", but <irii-r only. 

3. If lh<- obj<'ctive'i work in i/ifhrfnl media, as air and oil, the 
latter may hnvp an a]lc!rtun^ excPMling that of a dry objectivw of 
180' anjfK-. Fitr witJi the dry ol(jectiv<' tbp reftuctiie index (») and 
the sine of half the maxiulUIii an|.'le (ii) Uitli ^ 1, bo that » aui u 
^ 1 also, whilst with the iuimeniion objriTtive ii is greater than 1 (say 
1'5 for oil), and the iiii;{l«u may therefcni- lie niucli lex^* tbau in the 
COM of llie dry objective, and yet tho valui-of the expiiisaioii n sin •* 
(Lc. the aperture) may be Rreiiier t!mn t 0. 

The two latt«r deilui-tioiiii are so tlinrclly opposed to what was 
■ooepUxl by tlie older opticians and niicroacopistw that a closer if 
brief uonstdent ion of some of the puinbt which bear upon tliisbt^neh 
of Uie wubjecl may here l»e servieeably summarised. 

Take, first, the case of the medium lieing the namp. 

Differeuoe of aperture involves a diflWn^nt fiwintity nf light aA- 
mittMl to the objective provided all other cireuni.t lances are eqiml. 
HcncotheqnestJonof aperture leads to the eontiidi-nition of the;)Ai»ft>- 
i«w<r*oaf equivalent of different apertures or ufwrlurc anglm. It is 
not tA the essence of the problem, but it aiTords an additional iUti»- 
tratioH of numerical awiture. nnd is thus of gr<«t scn-ico in itn 
expositton. It is manifest that aperture oaunot bv based ou qoantity 


rji light iHoan - iai>r(: lijcKt cnn nlvrsys \» ubtainecl in K\u-- inume by 
throwing luum npoo Uui otijnrt l>ut n'>incrMUN;in thuiimount of tUu- 
miiuttion cnn make a dry Iran oqud inpcrrfonniMM!ML*ref(«nU.-i}H?rtur« 
tA A wwle-nnjilod imiuitnton li-n-v. 

Tti<! po[)ulnr notinn of ii pi-iicil of li^'ht inny bn UluslratMl by 
fig- •t^i whii-h imumes tliat Ui'ti; is rxiual intiMiiity of emission in 
>ll dimctiuiiK, and ifamt tbe intensity of a (lortioii i>f tJin jienuil taken 
chMO to tbft ]>fTp<'n<Ii<!u1tir in i(l«-nliciil with thnt »f onolhcr portion 
flfrqunl nii^ubir«xt<-iuion, but more n-moviil frooi Uie pi'^iAiidicuUr. 
On this vievf, Iht^n-fun-, the ijniHtily n/H^ht (■ontainMl in any ^iven 
pcnciU mity b« coiuiian^l by siiiijity coinjiitnng th« f^ontent^ uf the 
afilitl cones. 

VTben, however, a/i«rf unt is considerei), iind the valuen of n tan h 
lira workMl out fur imrticuliir macs, they are aana to dilTer from 

Pio. n.— DUsnuD thowinit 4ncnnMu iafereaM m la tbo tatMuil)! ol pmitted n,j*. 

those obtained by eatliaitting in the alxtw ninniicr the amount of 
light IQ the solid cones, and sodio perplexity natumtly nv'utm from 
the aupposilinn that tliemtrojiurvof thnnpfrtumof th« obji-ctiv<r does 
not cMTrapond to that of the quantity of light which it ndinits. 

All this arlMiS from lht> niistniccn owtuinption that a luminous 
pennl is properly re|nvji«iiile<l by fig. 33, 

In tlw Ia«I mntuiy { lU) Unuguct' aud lAnibcrt' vstAbli&hiH) 
the important fnvt that 
with any mirfawi of "ni- 
jfiirm railialian tho iiilen- 
sity (>f the emittt^l rtit/n ia 
not the MiDU) in all dirrc- 
tjiins. The powrr of rmlt- 
tion and tin; inti-njnty of 
tbo TmvA (i.e. the ijuaiiLity 
•rf ligot cmAiuitiiig fn>in 
a givMi Mirfocr -clement 
within u mii<! of given 
narrow uiglt;) variea in 

the prc.«.rti<.n of the «.- p„,m._ti„ toM».!(, «t -nilil^ T.r.Un«llU 
•Ml' of tbi- angle of oblt<tUi- maiv in <U1 ajrvotjniu. 

ty undor which Ui« ray is 

' iu)itt«dj in other words, in the proporlioti of the enaiaf of tiie anglo 
of deflection from ibe periiendicular to ihu lutmnoua suiiooe onaer 


■ TraiU J'Opti^ue lur fa Gnulatiim dt ta Lumiirv, ITSO. 
* Pfto4dm<(nn, ITW. 




whiL'li till' ray is sent. ouL Tlio rays are wore inteuBc- iu [in ip iri Inn 
ns <lify nre itiolined to the Hurfiioo which otnittt thttiu, su tli^it u j«iuat 
viirii>H ill jinvjHirticin as it is taken clase in nr ia reuiavi<il iruiu tU 
pcqicndioulur. A pencil is not, therrfon;, corrwrtJy n-preseiited 
tig. 33, but l)_v tig. 34, the denEtty of the rays decreasing cuiitinBO" 
tsMBi the vertic^ to the horizontal. 

Owini' to the diftereTit eniis-sion in different directioua. the qu 
titles of light emitted by an elcnieiit in the same mtidiuiii in i 
of diffpnmt uigle such as w and «■', fig. 35. tiro nut in ihe rati 
of thi; solid cones, as would lie the coix with ^uilI einia 
buL in the ratio <if the squurea of llie sines i>f the senii-aiigloa. sot 
the Miuiii-es of the sines of thesenii-iingleiifonKtitute tliO true mo 
of ih« (|uuntily of light contnined in any KciHd [>encil, 

Wt«3i, tlit-refore, tlie niediuin is the same, it is aeew that tlinw = 

Piif» So. — Tlio mietiuiLl Eiui»(ion of luya- 

is no oontrndiction betwt«n the measure of the aperture of aa w- i 
id that of tlie quantity of lijrlit admitted by it. 




I : ous ur), CiiRscqtKinU}', if tlw nMUtnption yren true, fc uinst 
ppenr to be brightt-r tlian a, and ih** ^>Iwr« would show increasing 
i^taan from tho Cfntro to th« circumferenoe. Close to the 
mrgin Um incmukn ou^lit to W vcrj rnpid, And th«< l)nghtn««s n 
largri multiple at that nt the cimtn!. 

This, nn i» wfll known, ■« not tlio case, tho projection i4 the 
apturrr showing in^u&I bri^htoMs. Tho ({unntity of light, llierefore. 
fiaittod iittm b vithia m frivnn Rtnjtll 
ootid cmo k' in nn obli<iuo direction 
tntist be l<-!(!( (liiin thiit wtncli is, cmiU 
tnl fnioD fi within nn itiiinl solid conn 
u in n p<rr]ii-n(li(.'uW dirrction, and thn 
inti-nidly of tlit* nirs deortiiuin in 
th'- projMirtJOD of 1 ; ooa w wh«n the 
tib1i'|uity w Incrauee. 

Ail thtrQ in ou« und the uins 
tumlinni ch» number of oonveyed 
hy » )>encil iind th« photometnc*! 
juutiiy of liRht are proportioiuvl, this 
em of Lambert, eeCahUahed for 
^laiQ than a ceuturjr. ia sufficient of 
iUelf to overthrow the very baaii of 
the angular expreKuun of apertun?, 
and to prove tJwtv when we are dealing 
with one and the «ini« iiMtftuni only, 
the itn^i! is not the >uffi«wnt «xpi«s- 
an, bat tliat it is the »iti^ of the lemi- 
wbich niuat bo token. 
We may ptuat now to tlw cane of t)i« 
^ineili* being <i^fhvni, as air and oil, 
Mul coiuporin;; the ajterture of a dry 
objective of 180^ with that of aiioil-iiu- 
mersion objective of 1 00 '. the valued of 
M sin H (or the ' numerical ' aperture) 
give 1-0 for the former and I'li for the 
latter, which in theroforo represeat«d to 
have a lary^ a^wrture tluui a dry ob- 
jective of ti» gtvatem posaible angle. 

In tliis ntae also coonderable per- 
plexity has ariaen. It has been assumed 
that the total amount of light emitted 
fhiin a riidinut [joint untler a ;;it'e)i 
lixed il turn illation inuat tie the Miuie, 
^hether the pcnnt is in air, water, or oil, and that that heinvc so, 
^lielSCK* admitted by tlie dry objective must itT)rriicnt n maximum 
Quantity of ligh»> a ' whole ' which canmit bi! oxwi-«le<l, but only 
Kjuall'^I, hy a water- or oil-immersion objoctive, Th<^ numerical 
upertare nol.ttion f,*ivitig dgures in exoeta of TO (which reprenenU 
j IW in air) is consequently supposed to bo clearly fm>neous and 
[misleiuling. Here the whole difliculty lit-s in tin- nli*>lute!y false 
bumptton that there is identity of nuliation in dilti^icut Bivdia. 

Fill. U^-Kncnia ol a brisht 
ii|)1iiittcal objoct •antuiB 


, ■. ..•'.:»':'.::• t'siiiililiilied. l>r distin^.'ui^hed K-gcnrch, tbe pn^ 

. -. \1:-. -■ •■ - ■ I'-iiiMifiii of nlxKh' — inreguiJ tohent Aswa 

-■■ .v.- h, •{if'-n III iii'-^itr, but vnripK in the raM 

-■.-;■» ■: ',:•-. Tvfrmivf indicfs, so tliat tiie whole emittii 

' s^-fj.-.'-rit iu«-;it of n self-luminoiiB body is iocreuM 

.- ■■ : 1 ■- when ihis bmlv is bi-ouj^ht from air intoi 

■ ... " r^i~i.:ivr index «. If a glowing body at a cob 

■ - ■ . -- --.:. '.* .■» Iwr >if iron, could be immersed in I 

••;-i.."-.? ;:. irx !:i ^-ueh a way that the surface wen 

s. k ■ ■ ",>.- :_*ii;-jii'., and the eye of the obaerrer ini' 

. ■ ~ i-, " ■ ..t ,'; ■,;::''iis. the body would beseenJr^ifti 

- ■ :•: T- ;>,r;:-:i 4>f 1* : 4 than it appeare<l in sir 

s-.'T"^ ^ rvdiv.i'-u in air is indeed /'« than cIm 

~ ■■ - ,- -1. ■ ^i:.i - :^ wa;*T or oil, as the squares of the 

■- .- :.-..,.. -.ii .i* ll\ i::. and 2-^5. 

- >i ■ -■- ■-■■*. ■/' 'i;/Af eniitteil from nn object 

. ..i.....i ■ . s ■ • s-.i-.ti^uiwi by the angle of &a 

1 : ■ :•; -11 ;ii.: ■. ■■ r ,-l:. it lie meiisured in any vaj hf 

- .:.i ■ ■■ ■■■ "-'.<• : ;a::::y dopends under all circnu- 

' '■ . ■ .. -1. ■'■'- •'ini-onglf nnd Ihn Tr/mcU» 

•■■ :.■■'.'/ ■■'■■' .-■■ ■'■-■■' i.» '"HiiiioHx, nnd isexpresaed 

.1- •.■r> !■;■" !■ ^" "i.e Sijuare of the 'uumertal 



".■ "i. .■ i'Ji- «• iii;.i ■,;■■ ;sT.T;.t-_:' T. ■■{ the quantity of Ugktit 

.'.■■" ii. ■.■.■i;.jil.-i-- !i:':' r- ii. ■.-" -a-'Jc. tK-' expression of a/frtiire.* 

"•\ -, .,:■ ;-'W [.rt]i:.r<-", :, !■: -i: ■:■' '.•• :inoilier point. It*»ia 

■■ ^-r■. ■ ■- ;.!y i;. ;;-,:::.■. >-~' ■;!". "hat the superiority of im- 

■■-■■■.- ■-.■; .".■. .-.■■■;. I (.? .■.-.■.eikJ to the case of tbo 

■ ■ - ■. — ', ■...■;;. ■.•l!^s■.:. -.■.■.^r'^'i ■.■.■■*»'fs, 

'■ '■ ',-■:' ::. .-i:r. sfcv ■*,■■. is sh'twn in fig. 37, a diy 

■-.;■. ' ■■,■ .•.-t:-^T:- vi'.'. ^,-: s'r^.■; :■■ i-lt:;:; it. If, however, tl*' 

ifetw^ U ri lu'-tj . V- :■ f,- ^'. i; -.< rw V-neer pos,>il)Ie for Si) Wk* 




A dry objective vna thvrvtnrv suppiiHitl to W [dueeil at a (liaad- 

r«i)ta(;e when u^ied uikiu tmlKiiii-mDUtituI objects, itti »iM-i-ltire being 

nippoaed to be • cut dovr n ' by the bnlmtm, ami tJn- lutvatituge of the 

piinuoeraiftii obje>.'li^'l> wna (.'oiiiiidirtvil Ui r^t on the f»ot that it 

Btorsil, in the caae of thi- Imlsiini uiountMl object, the siiiue coTuli- 

tiooB ne subsUled in tlie aim- of tlii> (Irt-inount*-!! objei-t^ nltowing as 

rlkrgv (but no larger) an ajx-rluiw to be obiuiiied with the fonnar 

tobj«ct MS i^i oblAinect hy tlie <ir)' objective ti-itli the Utter. 

Tlw error here lies in the aasutnpttoii of the identity of mdiutioo 
in Kir and liAlsain. IE there were iii fact Buy sudi identity, th« 

170' IN A IK 

fia. SH. 

conclosion nborc rwfivred to would, of oourso, be correct, for if i« 
fi];. 37 the iti'r (N*tii'il <if I TO' vom identioHl witii tho talimm pencil of 
170^ (bIi»w» by till- <li>llril littr* in ti),'. Si*), tlwiro would imttiuuirily 

1 1w a nrlutive liiu »f H^hc in the Inttrr qomh'. in coTi!tet]uei)Ce of ho 
tnaeli of tin- pi^ticil bi'tii^ rrdccti-il back at thv coviT-^lawi. 

Wbi^t, huwvvi-r, thv incrcMO of rnilintion with the incnvise in 
tfae refnuttivi? itidi'x »f tb« Birdium in i-rcngniKiii, ttic iiiixtnk<! of the 

fpr«c«(liD^ view t» opjinviiitrrd. Tim 170° in iiti- of fig. 37 ia not 
eqtuU to, bat much Ii-tu tliivn, tlnj 170° in hcUnm of li)f. 'Ai*. und not- 
Witkitkinliug thiit a ){n-iit part uf thr Litter doe^ imt rwich the 

firaportioii of Ihc iif^ct<Lf44 of tlicir n4ij, tl follow* U^jtl jf mi* il^^kj^iialr tlic rEbrliUH 1^ 
' n nn u for «■ nn ^\. ih? »x*u ft Wwtirclr^ A irill bv Ut thMkrvA *)f tlio ciroh^ M &« the 
I anArcQi Iho rtuLat oi A i* lf> lliu muojv of Lho rAdiUM of il, or oa ih tui if)' it to 
I (if an u')<. 

pr^fOI-ttftial la 

(n'stn, fb)* 

fniVM a. 



pfopm-tional to 

(^ am Pj i 

Fn. A !■— Tlw Mik* of two «t,{«Mtt«a af the amg imirrTliiil JUTcrsil i|vrlnni. 

Thn Mivbnt «ill otiwnvn tUitt lli' mliiin fi B ix ImIcc that of the nidlat of A: 
' •(RiM^dmll* llip mm* of B will 1w four tunn Kt ^mit, iw tlml of A : wliirli mWHiii 
ttul. linm dm nuiDnricD.) a,\»t\n,n\ nl (lia okjnMliw B I* Iviw u gml w Uut o( Ih* 
obJMliiv A. iU illnminkting pomt will be lonr UmM M )[rwl. 


o1>joctiv<i in conKM]u«nco of total n^d<wtiuIl, yrl llw r«inain(l«r (S0°) 
which dneit n^ach it i» th« cuuct o^aivalent i>{ tlie uir-^encil ut %. 37, 
ihtt two nir-pendlii uf ITO* bniiiji; lu nil rmiio^U jdraiicul. 

The immrnuon olijeolive, llif^rofori', vhiob U able to reodre tits 
wbolo bcUtim pencil of 170" (iluttM] lines tn fi);. 38), talces up t» 
yrtattr quuntlty o[ tigbt tluin lh« aiV pencil of fig. 37, and 80 not 
merflr equals the dry objectivo but surpa^Aea it. 

Let it be ai>e<;ially noted tliat in dealing with the quautity of 
light id cuiiiiection wiUi Rp«tui«, iUb ide» lia« ool been that we haie 
boon cngHged with what in in tiny sense raaeiititil, but to remove « 
diffieulty felt by ninny. It must be clear to a11 tliAl if ti grwter 
Aperture aignided nothing nioretlinn n gnsiter quantity of light, that 
is to say, if tliero wore no siicb tj-fcijie differenoa of the rays wliicb 
can be utilised by different npei'ttiivg, ns wo have denidnstrated 
above, the whole question would be of (|<iite &uboidiiiate interest. 

Another subject requiring »ionie further ektcidAtinn hero is tJia 
'different angulnr dintriliutiim of tlio niys in iliffereut media.' Tho 
MSeaoe of the idfji of 'iiperturii ' is iv/ii/iV'- ojirniny. Kowcvw 
deftMd, it« signiliennoc ran only br npprecinted by taking into 
aoeoont tho iiiMgv-fenning p<-iicil rmrrgml ivom thi> objtrti^-e, und 
t3te change in ita diAni(<t«r c<>i»H'i)Uf nt upiiii the ndnnKsion of difTrrent 
con«£ of light. This dtnniHci- iiflrtrdK n ^-isibIl' indication of tho 
MWjuAt <>f rutin (not mere (pumtity of light phutonietrically, which 
cnn bit nuulily varied) which nm williirlfd to a given nnsi of tho 
imAgV, nn<t which luUKt haVR I>rfn gnthcnnl in by the \vn* from tbo 
coqjiigiito »r«R of the object. If tin; ilinineterof the emergent pencil 
is aeon to be tocreiufed, whilst tho nmp I iti cation of the imngennd tha 
Awal len^h oro unchanged, it in cloir thnt thi> objectivi^ must hnva 
ndmittml mow rnyn from everj' element of tho object because it hu 
colloctod more to even- element of nn cciunlly rnliirijrH iimige. Mani- 
foctly wognt iinneciiriitcmcrtMireof whntii!n'/«nWf(/int«anobji'Ctiv« 
by being able tn rntimnte whiit it r-tnitt. It is, phyxii-uDy impomblo 
that a tiysteui «f lininfx slmuhl iMuit more lifiht than it has tnken in, 

Hencii 'api'rtun- ' niejin.s the grentiTorTencapacity of objcotivcx 
for f{ntlicrin;;-in rny^i from hnniiii>u« objectx. 

When ihe iidniitt'iil pencil is in thn Binin niMlium, wtt *ee tho 
additional purlions uf the .solid mno from the r.idiant, which corro- 
ipond to lilt! iidiliticinid piniion.^of thp enUr^'ing npcning. But if in 
any other case (e.g. whpre the metUum is difl'ercnt) ivi- see tJiat n 
certJiin Wilid wimv A, fn>m a ntdliini is transmittpil thruugh a certain 
opening, «, and that another solid cone uf rays, B, cannot be truna> 
mitted through the same ojiitiung, 't. but requires a wider one, ff, 
wbilst all other circumsfaunw. excei)t thow of the nulianl, have 
remuned the aame, we ran oidy conclude that the j^ncil B must 
oontMD raya which ni-e not eontainetl in A. even if the admitted cone 
ia not increaaed in uiiw. For the ad<lii)onal portion (a — o) of the 
wid«r opening, S oonveya rays to the ima^e which are certainly not 
conveyed by the smaller opening «. Prrmi the radiant only can this 
surplus come, and tie poncd B which n-iiuirea thi- additional opi^ning 
mvst ombroce more mys, ^i<eN i/ it shmilil Hot br ot'</r*aler aHyU. 

A given objective may, in fact, collect the rays from a iiidiunt in 




^1^r aIuimC to the eniin^ fieinui]itici«, find it theu ntiibes • defioitn 
o|«iiiu|( ■loubl« its focal Iliii^Ii. But when the radiant is in batsuin 
<«it)iout Mijr ol)i«r nlcvrnliun), tbeMim^ i>))cn)ii;'is Mentu be utilised 
by the raya which oi* within a tLiimlt«r toiio ol not more than 82*, 
and ntya wbirb *i* uut^de tliis<^<>ni> i-etjuire a surplus o])enitig wliidi 
» n^vfr nxiuii-ed for niya in air. 

Thia liui<l3 ;;ood u-hotlipr iboro be rofivrtlon or no refraclion at 
tliir front Mirfitce of the »yslr^iii ; tlie (litlereiice ia baaed solely on t]iQ 
<li(1t-r«'nre of tli« mctliuni, Cotis«((ueutlj wo arrive at the ooQCluaioii 
tbai the sulid nnio of i*i' in Iwlsara eiubmc«8 the same r«iys which, 
in kir, are eiulimc«(l bv tho wholp homisphore. and evory wider l-ou« 
in tmlaani ext^cding trie i^'i' conveys more rnyEfrom the object than 
Hpe adinitteil by tho wliolr heniispbcre of radintiou in wr. 

Il followis ihrivfon-, thnt Ihe furiq rayit which in air ^ra spread 
over tht> whole bomisphTe nin ch'sol together or oonpresud in 
l"il-L»m within « nnrrown' ":onicnl Kpaco of ll^nroiind tlip perpen- 
'iifuUr, and all DtVK which tritvcl in balmm ontKido this cunc con- 
6titut^ « tiirpfuA of nr«- r-ig*, which nro nevwr mrt with in air— that 

is. Of not rtnillnl nArn t/f /J'Jfi't in lit rtiV. Thn toM uliji'h tuk«il 

|ila«t in ihi? I*tl<T earn- t^an ««vcr Ix- dimprnwitrd f»r by incrMuw of 
illumination Immaum) tho roy* which ure lost nrc '/i//rwn/ ra^s 
ph>->iicfllly to tlwco obtaiucd by nnyillutniDation, however intrnnr, in 
H iDeiliuiD lik« air. 

In the paprr of ProfMMor Abl>c th«ro is an i^alionitn uid carrful 
eloridntion of ihiH chnngi- inth«iu)guInrdi«tribut)on of thnnulinting 
light when tlie niedium IscbangMl ; but to Mr. C'rispV pnpt-ron lltv 
same Jiobject, giving an expo«ition and simplili cation of AblioKdi'- 
raonatrfttion, the norico will look with the utmost pmlit.' Tin- 
fotlawing extract will giv« cU-amnw and iMnphoKis to thv nbovft 
(leda«tiouB of Abbe : — 

' If we tabe tho now of r^/mfli/iH, then ocfl of the most fuuda- 
mental of optical ]>Hiicipl<-« hhons that the mum mys which ui nir 
occupy the wfatile liemisphero 

arc cotiipr««sed in a tDediuoi of ojt aa^- g' 

higher t«fisctive index within 
a so&ller angl<>, rix. twico tlM> 
critical angle. If in Hg. 311 
the object is iltunnnat4xl by nn 
incident cono of my» of nmrty 
S2* witliin the »Ji^c, and thw 
slide ImK air abor« in thi- firat 

mtr and oil in tho Kccond, it \n obviotin tJint tlin xiimn rny which is 
inci'knt on thn obi<'ct at ni-nrly 41° will always emerge in air at nn 
angli< nf wnrly 90° (a'), and in oil nt nrarly -tl" (n"), an that the 
■Blue rayfi which in air an- <'X]Nin(lcil avrr iitr. whole bmniiiphere are 
enoipre*."sl into 82' in oil, and, then-fore, mys lo^yond ^'2" in oil n-|iri'S»iil Bitqilun raya in exciiw of tllosi- found in the uir- 

■ If. on the otbiT hand, the caso of <liffi-netittn ia considered, then 
^ Fmitnltofer's law nhown ihat the Hinio diffracted beams which in air 


Fill, S!i,— <'fim]Mimll-, I 
light myi. In tail diiti 


■i.m ut 




occupy tlic whiilf hftnisphi-ri- (tip. 40) nrc in oil coinprcMod witliiit. 
nil uiifcle oi ^-^ niund tlic direct Ucsua (tig. 4 1 ), w> that them in i-ooiik 
for ivdditJDiiiil iHHtiiis.' 

The u»t-i|iiiikl oiiuii-ulent of equal niigltw bccomm, thtmton, a de- 

Fio. Ill,— JiifTrn.'l'-ii ln'"i II an. 

Fl-'- 41- — DifTiiicLoi) b«u£iie iii (lil. 

iiiaiistrutcd trutii— a irutlmliicti iKcitpnblcof i*sp(!riQicntul proof by 
every owut-r of u fair niicniacopp. 

Aoyoiia poMi'ssiiig n dry oliject-gliiss of itn .ijiei'ttin; of 170*, for 
U2uuuple, uuiy n-iiilily do so. In thi.s t-usp, •', it, lig. 41', will rcproaetit 

Flo. a. 

the pencil radiating from an object in air, and cupable of hiixig 
taken up hy that objective. Tliis pencil, on its etnei'gence from the- 
bncic IwiK of the cvimbiDatiuii, will present a diameter wjmewliiit less 
than twii'p the fociil leiiKth of the objeetive preipiiteil in fig, 43, 
Bnt let the object lie now plm-ed in Cnnnfla luil'^iiu nnil 
eorered in the usual way ; the nu-jU of the pencil, by 
ibe ;fi*atw reimctive power of the uiediuin, will In- de- 
iiionatrably reduwd to 80°. as sliown iti fig. 14. But it 
will be found, on examination of the 'wryritf pencil' 
from the back lens, that tliiK jiencil oce ti pi i« exactly the 
Fia, *3. )*aine dianiHerlliK- -13)aal>efoi'e. The medium in which 
the object i» has not, nf eourae, altered the /wtcrof tlm- 
objective ; and siiice the diameter of the emergent pencil i« the sanu? 
in Imtti cases, the ratio of •opening' to fiwal length, which in tlie 
aperture, is the same also, Hfiiee it is §een in the ninipleKt way 
that dijitrrnt angles in medi« of different refractive inmce* mny 

170* IN AIR 



Fro. U. 

denote etpiat apertufv*, and etfual ttnyitt in diflereiit niediA dcnotiv- 
d^trnfU apcrtnr'ji. 

That 'immemon' uhjeetive« Diay have ^eater aiterturtTS tlian. 
the maximum atljtinable by a dry objective is capable of viguftUy 
simple proof by accessible experiment. 

If an idl.iintiierniou objective of 122" IviWm anjfle be t^iken, and 
KoilKitniiutiid that the whole aperture in filled with the incident rays,, 
ami if we use lirst an object mounted In air, we really find that wo 




oajccrm air 



-^h^J— ' a COKRT CI ABB 

Tvi. 4S.— Dbcnm illtulnilinK Jiflcrxuc* nt minxinE 
|«nsn ■illwut wid willi balMm. 

haveft'/ryobject-KUusofniMfl}: l)40°ntigulnrapprtniv. Tliis te readily 
•Miiby lif;. *5. By tlipM-ntngrmnitprcsi-ntml in tho tignre tli« cover- 
'-[lus IS pnn.-ticiiUy tlm 

St surface iif tbe ul>- V 

wc'tive, for tlie (roiit \ 

'Jiftnt. tlie itiim«rsi<»i \ 

Bai'L »ikI tbi- cover- 
are All iionio- 
aus And ai the 
sune refractive index, 
*nd cons«|ueHlly ibey 
form n front lei)H of 
extra til ickneBB. When 
tbo olv^ct is cinsp lo 
the coTor-glAss th« pencil nulintini; from it will be wvj nearly 160^ 
ftnd tbf (imergent pencil will be t,vvn to utilise ao much of tlie buck 
Icnn of tbo comMnation ns is equnl t^ twice the focal length of tlie 
ol>jecli\-C ad ihown in the iHiwr cii-cle of fig, 46. 

If nnn- we nin Oannda balsatn beneath the corer-glam so as W 

liininrrse th(> object, tlis pendl tnkeii up by the objective is no longer 

'iW, but only IL'l'"; bntin spit«af tltat tbedinniet«r 

of tb»«nHf]gont pencil is lat^ tJian it was wbm the 

' <ol tho pencil wa«l?0'in air, iind in rppresentnl 

tho ootcr circle in fig. 41). !n both theae caMS 

Jm pow«'r i» idciitiotl ; it follown, thercforr, that the 

grenler diameter of the emergent pencil from the 

back of tho combination denotes the greater n/x-rci/w 

of tlie immorcioin objective over that of the dry one, 

iltJioagh it poeRCMcd an ar^yft of 180°. From this eacape is ImpOR- 

Blble, and it in for tliis i-eason tliat optician* make the bitck Unnea of 

' eir immersion object-glasses larger tlmn those of dry oon of the 

1 power. 

Many further illuKtrntioiis might ho p\'cn, but nonf affonling 

Fjnater mcility thnn tho following, t'\t. -. ' Select a goifl siM-cimen off 

VjimplrifJrurti prJIvcitta nnd OHO obli<|i]e illumi- 

' lutHtn, bringing out clearly the striation, 

'On rpinoWng tlie eyc-piecc, placing tlte 
pupil on the itir-imnge of the ilintnm, and 
looking down oii the leiu:, the direct incident 
b««iD will bo KO^n nmcrgiug a* a bright spot, 
tRod exactly (>ppiHit4- and rloi* la thr margin 
a faint bluinh ligtit («v fig. 17). If now a 
nnistl pioct- ))f papi-r i.i ]>lacMt on the biick lens 
of tho obji-ctivn HO AH to JQst cover up the blue 
light, ami tho oyo-picce is rt-placifl, tJiP diatom 
is atill vi.iiblc. but all tlic strialion which was 
iBiagrd by tho blui- marginal light luia entirely 
■ppearetL Tln> latter must therefore coii- 
; of nnage-fortiiing rayn.' 

Enougli luw thus \m-n lulvnncnl to enable the rtinh-ntof eren 
tfao etcioentaiy princi|tlcii of tnoderii object-glMs conntructinn to 

rio. 14. 

Fki, *T.— Bm* n( luim 

\ntop»^ wImtii ,1. fielitt' 
rida liH )i«n nanl- 
i.iL rfifFwlug ipatal 
LI a>i>b apot oppulte. 


demoDstrAte for himself ttiat immersion lenses not only possess at^ 
excess of aperture over dry lenses, but that the rays so in excess an P 
image -forming. ■** 

The refractive indices of (cedar) oil, wat*r, and air are respaa--)'' 
lively ir)2, 1-33, and 10. 'Angular aperture' claimed that tta j"" 
awjlm of the admitted pencils to lenses of these three constmctioBi 5*^ 
expressed equal ' apertures,' But this is a fallacy, now so palpafali^ |-= 
but which has exerted an influence so deterrent on the progrcH f- 
of the construction of our higher object-glasses and condeDser^ I 
that its final disappearance as an unjustified assumption which had j^ 
crept into the area of theoretical and practical optics, unverified \n ; 
facts and devoid of the wedding garment of deduction, is a triumftt ^ 
which will make the name of Abbe long and gratefully remem- ^ 
bered. = 

The principle npon which iaorease of numerical apertnrs giTM 
increEued adTanta^s to aa object-gflaas manifestly needs carefal 
study and elucidation. We have but to refer to the best work dons 
by those who have employed the microscope to any scientific purpose 
for the past fifty years to discover that there has been an admission, 
which has steadily strengthened, that by enlargement of aperture an 
increase in the efficiency of the objective, when weU made, wa» 
inevitable. During the last twenty -five years this has been especial^ 
manifest. To increase the aperture of an objective under the name 
of greater ' angle ' has been the special aim of the optician and the 
coustAnt and increasing desire of all workers with moderate and 
high powers. 

The true explanation of this is quite independent of any con- 
sideration of apertures in excess of tbe maximum in air, and indeed 
of the whole question of immersion objectives. The old view that 
all high and excellent results depended on the angle at which the 
light emerged from the object, involving some assumed property of 
a special kind in the obliquity as such, has been most tenaciously 
held ; but it is an j: in the pmlilem which hn.s not only never b<*n 

' AS«!J! • AS seen OP SO VAITE 


inienMi:u]>(! wiw il«t«rnuned by tbe f.'eoinetrinlljr traooablu n^laliflIls 
4)1 the refraded mj« of li}*ht, 

A proloiv^ courwe irf aUe nud exh&u&iive exjwrinirnts con- 
ducted by Abbe shuu-ed lluit, wliiUt tbe old view liekl good iu 
wrtain cases capable of defitiile \'«rificuiiou, yet UmI tli» \a»t 
tnujnrity ot objects, and especially tiiose with whiah tbe hi^lMtst 
i|QalttieH of an objective are called into operation, the prodticliun of 
the mictxwKopir image is wholly and alisolutely dependent, uol uimti 
the ol>li(|uity of tbe rays lo lAt objfft, an it liiid l>eeii w Untin una aa 
stoutly luaiiitAiiiccI, but upon their obliquity lo lite itxi* <^' lite mii'rii- 

Such connut oljgecta as require only a few degrees of apei-iuro 

tu disclose them m« d«peiidpnt on 'isliadoa- effects'; but when 

«xtTeiucly minute and delicate structures aro to be disclosed siuuU 

KperturBS are absolutely useless and mero increase of obliquity of 

{leucil OS such is powerl«»s to nlt«r tli« result. It can be effected 

*mly by incrcaswl numerical iipnrtura, ahiiwing that the greater 

liliquity of tbe mys int'ident on, or remitted tnm, the object is not, 

1 csnnot be, (i/iitfljtai rlrvincnt in the superior optical perform- 

OCT of gmalT njH^rture. If it were so, nil (he results of incinised 

Fapetture woulil Ix* Keciire<l by iiti-linituj Uir nhjrrt to the axis of the 

taicriMctipe ; but it iruiy )>e ii-iidily texti'H that when n )(ivcn object 

^cannot be 'resolviil,' or it« utructum dclim*nt<-d, byimobjuctive with 

I apttrture of 80° in thi^ oniinnrj- position, but ma Im resolved in 

tbe onlinnry nusitiun Ity an <>bj<<ctiro with an ajKrrttin! of &0°, no 

indinatiot* of At oHjtict to tlie nxis of tlie instniincnt will enable tho 

^objective of 80° to do tho work easily done by oive of 90*. Thia 

>y be bested by anyone p ouKsai ng tltc imdrnments. 

Aa a inatter of fact, thin «o-rnll(Kl Init iwiiginiiry ' angular grip ' 

is sreatec in a wide-nuglecl dry Irns than in finct of HO' ImlHim-angle, 

iid it is csrtainly cut down more mid n)ore whf^n witti one and tbe 

P«aine objective pmpamtjons nr«^ obRrrviil in water, Wsnm, and 

cetlar oil euccewively. If now the anKl'-x '/""l angleji are effective 

in rtntf way, mmflltiug must hf h,tt by cbiingp i>f iingle in this diive- 

kiirin, and sumethtng ought to be gnincil by cbangn in the if verse 

Vdirvction, otlier conditionx ir-mnintiig thn Hnnic. It i« ntvcllcH to 

V/ that nil e\piTieni-e nnd the eiilit« counu^ of pmuf iind reiuioning 

given nln\Ti ni-e ilinnietrii-ully ojijKWHnl Ut such coituIuHioiis. 

SimiUrly it will \w nuinifeil that the oonoeptiim that *soli<l 
vision * or perspective irffet-'t in a microscopic imoMe in one of the 
eopseqaeoctw of oblique 'lingular' illainiiialion is equully iuvalitU 
I( anwHHK that the dilfcrvnt pcrB[iective ^'ii-ws of a preuurulion 
exainineat with the miorascope, which correspond to the diflbrent 
obUquitin, |>nMlaou the wtnie effects as if tliey were seeu aqxiiately 
by iliir>Tmit ey«ii, an in the oust- with the binocular microseope. In 
roality, whenever we have the advauta;;e of Kiltd vi«o», owing to a 
dlfl4>rviit [lenpcctive projection of different inuigca, in the microscope 
or otlierwiMt, tliis in H>lely because the different iuiugea are seen by 
tfiffrrrHt eyes. Ill niicronciipic vision tliere is no dtflerence of ftro- 
iMtion connected with difffiftil obliquities ; in the binocukr micro- 
Ewope then is a divunitj- of images which are depicted by pencils of 

visios wrrn titk rostporsp microscope 

different obliquities ftt ttic oltjccl, which i§ a r^rlaiH tindot pftrspre- 
tive ctiffer«noe ; but the nbovp nnd other obwrvatiiniA und oxjinrinif nt« 
show that even heiv there is wwontinl divergence from ttie conditii>n> 
of ordin&ry vision. 

It is thus plain tlint wheni^ver Aperture is eRective indrliiKmtion 
the mode in which it hccomns «» in nnt by nicnns of lli« obliquity of 
ibn ny» to the object ; while it hm nirendy lieen shown that 
increase of light, nlwnys cnnt:omitnnt vith tlie a*e of innnemion 
objoctircH, is b r^lntivr ndvnntA;^., but no part of the evpUiuition <>t 
the superior nation of th« tombinntion of lensrs, Atfffe is demon - 
strnbly not the tnio \m*ii< for (hi? c-oinjinrison of objwtivoa ; it fuiU 
in regiird to npfrtiirc in K''n''r"l. so fur iis it bus reUliou to opiMiinK ; 
it fniU equnlly im rr^'ni-il to ihr iuiinlM>r of rnyn Bnd lh(> quuiittty of 
lii^fat ndniiltnl to ihi- Hystciin of lrn»i^ ; wbili* ilifnihire in re^itrd to 
the delinraitini^' pi'Wrr of objcctivpa is <?vi"rywhcrp sern and admitted. 

At tliir ■•nuie tiiii» it is. plnin that the eaanf of increasHi jMurer of 
pcrfornmnci^ in th« obj<M.'li^i.' is dii-cftly connected with the )arp;er 
oprninif or 'ntiorturp' of the imniersii)ii and boin<igi>neous systems. 
In othi>r worcfs, it bt^comwi i-lear thiil Kmii'thiiuf is admitted into t)i« 
■objectives witJi ;(nMiti-r ii[M«rluit^ which (■Dnlriliutcs (o tli© formation 
of Jui imiij;^, Huch Oa objectives of lencr iiprrture rannot form 
brctituiR their ' Openings ' or 'ai>erlures' c«nnot oflmit that 'some- 

What tbia in Iwcoines explicable by tlie ratearche* of Abbe. It 
ill di'jnonatrated that inicroaoo|>ic vision in siii ffirwriu. There is, 
nnd ain lie, «o oonipftrison lietwwn tuiiTawo]>ic nnd mftcroscopic 
vision. The iniajcex of minute objty:t!( are ti»t delineated microscopi- 
cally by in<wn>i of the ordinary laws of mfrru-lion ; they are not 
•liojilrii-nl H'^ultA. but dejiend entirely on tbo laws of d^ffmction. 
These romn witJiiit the wiope of and di-manM.rntc the undulntorv 
theory of light, an(t in\'«lve a cluiructt^ri-itic ('ban|."e which material 
particIcK or fine Aruotunil detail?, in |iro|Kjrti<in to their minuteiiesH. 
rffcct in transmitted raVB of lij^iiU Tht- chnn^fi- iv-nsiKts generallv 
in tJin bruikiaj; ujj of an incident riiy into n aroiip of rays with 
lar^ angular iliiijierBiun within tlie range of which pcriotlic alterna- 
tion* i)f dark Jiud tinht trccur. 

If a piece of wine be held in a strong beam of divrrgent light 8i> 
tliat it.1 (ihadow fall upon a white Burface, the »h(idow will not be 
«harp and black, but surrounded by luniinoun frintr<« having the 
colours of the sjiectruni, and the centre, wheru the bluck nhadow of 
the wire should be^isa IniDinous line, a><if th*: wire were trantpnivnt. 
Tlua plieiiomenaii, as is gmierally known, is due to the infi/Hion of 
the (liveriiing rays on eillier side of tlie wire. The inflected rays in 
passing over one edge of the wire meet the rays inflected by the 
other ed^e and 'interfere.' producing alternate increase and diminii- 
tJOB of amplitnde of oscillation or tmdutatory intensity, and giving 
rise to coloured fringiM if whito light is userl, an<l if homogi-neouo 
Hffht be employed giving origin toidternate bands of light und dark, 
the centre always being luminous. 

Again, if a disc perforated witli a very small hole in the centre 
te Ima in a pencil of diverging liglit, those undulations which pus 



<lirei-tly through tlx' uporttire inicrfore iritli thosp poMiiig obli()uei]y 
mt till' nige i>f the ditc niul produ*:^, «I oertMtn (lUtance:^, a dark 
spot, Ht iitl>rr ilUtiinci--K iixTrsKnl UrightiiPM, on thnt [lerl of tite 
^uwlow irhirh i% ojipoKiti? thr^ tiiirrtui^ in tlip disc ; so tlint light is 
supplniitrd liv'tnrkncKsnml iliirkii>->« chaiigcH] to light, b}'tho discord 
■or coDoonl of thr luminouK wftvo«. 

Itidvpondentlv »f nil i-xiK'ntiicnt, the Unit prindploe of nnduUtory 
optics lew) to Uif-si^ cxpi'riiiii'iital oonrluniimK. Tho litwi; of rc«ti- 
■liiMnr prapOftBtioii of thi* luniiiKiUK niyx i>f n-llnction nnd I't^iiictioii 
atvt not ail— I'll'. Inwu. Tbi-y nri.iti fratn, and dnjH'iiil upon, n c<!rtnin 
v-rtiiliirH Wtwci^ti the wavi'-li'ti^'llia unit the ulixoltitf^ (litiitriiMonM of 
thi^ ol)jixTt« by which tliv wiitiim are inttircvptul, or redoct4!d, or 

Takiiijj; na iUuHtrativi> th«? wavirt <rf /otinJ, an afouiifie Khadnw in 
only pnxlui'nl if tin- nbsim-li- l>« iiiaiiy tiiui'ii ft'^-'^tfr ilian Ilit- li-nytli 
■ of the KOUnd'K'avys. If tlip oliataolf is n'du<'t'<l, tlis wnvi-* \ia,ss cum- 
plett^y round U <iinl there is iki shiulow, i.r if llio noti-s nn* of higluir 
pitch. Ml that the wflvpa am ;i«luced. a stimlli-r cibsUicle thiin before 
will produce lh« shadow. In ihe tiuie of lijiht there u™ tuiuilar 
j>h<!iioineiiii. If the ohnraclfs I» the jiusiui;^ of )ii;ht be large in 
uoniparisoti with the wnve-kni^h.i, shadow ctTeuta nnult 1 hut if ihe 
lineM* dimeitsions of object a art- reduced to tmall mutlijilfx ttt tlie 
wKre-l«nKlIi> "f H;;ht, all ahiid»W!i ur diniilar effects of nrilidity muHt 
oeaite. As in tlie insiancM ;'twii abo^ e, U^'tit and lUrk, or luaxiina 
and mininia of luintnusily, iiil«i'<.-han^e tiieir n»ruiul |>OMtioiui by 
tiamioiiy or iH.iharnioiiy of luminouti wavMt. 

It iH then liv ineuna of tlidrnction phenoniena that Abbe is 
miabled to OK|>laiii the forinatiou of the iinaKea of trhjectit cmituiniiig 
ddicnt« striie or structure. >»it ri-iiuiriuK larx" ui)erlure* for thi.*ir 
conplete or approiiiuuite delinMtion. In the itiii^reslH of thin ux- 
pcoilioit we tuuai liere for a iiiomeiit diveifje on sliKhlly [Wrsutinl 
(pvuDiU. It box b**ii the ;*ood fortune of the present inlilor to obtain 
the ctiarteotis conMiit of Ur. Abbe lo read au<l critioiie the whoht 
o( the preeent cliftpt*r ; however caivful and eametit a Htudent of 
snch complex ami •>ri;.'inal work »a Dr. Abbe liu Aoaa and reoordetl 
^ in Uennan an<l Ku^linh during itie laat twenty years or uiore, it in 
~ kposBiUe to be wholly a:tti»lieii with the noai sjmpathtTtic and 
neere dewre to give smh woi-k a popular form uiilesa it should 
have been perut^ed aii<l accepted by lite author. Th*. Abbe has read 
tlie entire chapter, aud, with many f^nerous words besiiles, relieve« 
tlu^ inlitor in h)§ con»ci<ntsnw>8 of great responsibility by saying that 
ht! distinctly approves of the 'lively interest and care whicli (the 
premnt editor) haa bc«towed on the oxpoHtion of his (Dr. Abbe's) 
vif^ws,' and that he feels ' the gnmtesC satiEfuctinn in seeing (hi&) 
virw-n rcpmiented ... so cxt«nM¥ely and int«n»iv<i|y.' 

But bo;,-oni) thin, an original worker like l>r, Ahbc would almost 

inevitably lind, in the course of yi-nrn, rcnsoji for slight vn-bal and 

.other mofv seriouK modifications of his infcrrnci-t, ex pin nations, and 

rviewK ; and thn Kditor hns great satinfftction in being nblo to put 

thcM' inntificntionii wlif^re they occur, with th'' approval of I>r. Ahbo. 

In th« expositions of Dr. Abbe's views on the diSraztion theory 

64 Tisios wrrii the comfoitsd aucBoscopE 

of microtwopic vLuun givcm up lo thit tiuic, it Iium lieftn usiwl In 
Nt«t« thnt he hflA nml tati^lit tbiit thu Diici'ixtci>pic imn^'n coiiKisU nt 
dm tuurriiTijKuirit iinaifeji, «nch liuviitK n diKtincl duirocto' fu w<!ll ■ 
HK n (lifli-rLiit origin, au<1 cn]M(bl« of beinj; iicparot«<l and i-xiuninifd * 
apHTt from micIi utiirr. Tlu- one oallecl tin.- ' iib!ii>q>tiun huiige ' Lt » 
HiDiUituilc (if the iibjccl itiH'lf. ftii inutee of tlif lunin uuilJneH of tbo 
Inrtfcr iiirU ; Imt l>y tlie otAer imsKe m1 mirtu^"* airucturtw, Btriatit>n, 
ADcl (l.'licaW fomplcsit^ of detail v/ionf d^menU lie *a vlo*f UMjrtker 
<w lo areitnuin diffinetum jAenoinena can aJone bo foiiueil. beiruuiw 
these taulii ntil \jv (fDometrloally itnufced. So tbnt id case of jui 
object with lines closer than tbe ,j>^„ of nn inch opart, tho iraofe 
eeeii by the eye is formed, not simply by tlie central dioptric beam, 
but by the johit action of that and the superimposed diffraction 
imagea, and their exa*:t union in the upper focal plane of the objective. 

The fint of Chme vas held to be a nrtnilivfi inuige, representjiif; 
gcouietricnlly tlie constituent part« of tlio object ; hut tbe second 
Iras considcrtxl n. iMit'tluv. iinitge because it delineates structure, the 
parts of which nppenr sclf-lumiuous on acciiuut <>f the ilittVaction 
pheconienn which they tviuse. It was this 'diffraction image' that 
was said t<i lx> the instrument of what has so long been known as the 
* reselling ' power of leiison. 

Hut Dr. Abtx', aith the full light of further investigation and 
expcrif-ncc, Akhh not hi^Kitato to modify this explanation. H« says : 
'I no lunger maintain in principle the distinction between the 
*'ab»ornliiin JmnEt'" (or dirrct dioptrical image) and the "diffraction 
image' nor do I hold that the microscnpical itnap? of an object 
consisU of two superimposed images of iliff^irnt nrii/in or different 
mode of production. 

'This dijitinclioii. which, in fact, I made in my fiintpuporof 1873, 
arose from the limited experimental character of my tirrt ivsearchcs 
and tho want of a more exhaustive thcoi-ctical conudenition at tliat 
period, f wiu not then able to observe in the microacopo the dif- 
fmction pffivt prrHlucetl by rclativnly course objects Iwcause my 
exjK:rimcnts were not made with objectives nf Kutliciently long focus ; 
hence it apptuin^l that, t^oarse objcets; (or the outtiiirt of objects 
containing tinir structural cli^il.t) wem dupictcil by the directly 
Lmnsini tied beam (if lijiht wih-Iy, witliout tlie co-operation of dilTractca 

' My view* on llii.s subject have undervfone important nxHliiica- 
tionn. Theoretical considerations luive 1«I mo to the conclusion 
that there must always bo tJie wanv. conditions of thi- delineation n» 
hntff a» thf oltjfeU arfi drpirted hy mean* «/ traiumittni or rrJleeUd 
Iv/ht, whether the objects are of coarse or very fine structure^ 
Further experiments with a Inrgn niicrascope, having an objectiw 
of about twelve inches focnl length, have enabled me to lustually 
obwerve the diffraotioti i-ffeci. and its influence on the image, viewing 
gratings of nut more than forty lines i>er inch.' 

I DifFnction eltict* mnj b« obMRwl wtllinnt i micniicepe : tbujr eaa bo aanljr 
d()nUHUilialo>l br obiervlii^ t. \ianf-9miRB tluouub > liuim podiet'luiiiUEUichiiif at m 
Sun )isiiM win bliuil. TluK can 1m duuu niudilv h)' |i1iicinif til? nyt clo« to tbe lincu 



*Uy pwB e nt vie<vrs riaj' \>t! tlttix tx|>rMsdl : With coimo object« 
iho diRrnt'tci] fttetit ofT) rn^K (>r!(>ii(;ii>X to lix itinidcnt mj' or pencil 
»i<e atl ci'iiliiii^l wilJiiii a tvry narrow iiHyulnr tpitc* aroumi tifil 
iwiilml niff, aiul <lo uot apjimr H('|>an>U<il I'mni I his rxr'-pt with 
i>t>j«»tit'a> «>f vvry lowg fivuH. Tin? fA(>/« of nuoti n niim>w iliOruntioii 
fH-m'il i.i C"m»tt(ui'iit1y ;ilwuj-» lulmilU-il to Oiu olyni-tivn ((i^rfAirrwilh 
llie (limit (iiici<l«iill )huiiii, utiat«v^r miiv In- lilt- ilin-i-lion <tf iiici- 
<leiins axial or 'ibli<|ue. ALi.-i>r(IiiiK ^^ t''*i projKkiilioii ot n. 72 (I) 
ilip iniAp' is in this I'luie NtHctly iiiiiiiliir to the object, i.*!. Ui« HK.>ct 
is the mmti m if we luul a diret-t doliiif^alioti liy the incidttnt fluni-* 
of li^ht simie, ahiI as if the iaia^ did not d<^pe»d at all u[ioii Uid 
dtflVnctiv« ivctioti of the oUJMrt, 

' If w« haTo n preparutiim like m diatom— a relatively eamo 
o1>J«ct, including Siw iitructural detAiU—or another prqiantiou ooii- 
tHtning coarse elenientH anil fine one« in jiixta|)osition, the total 
(MITractioii niTect may he itepnrnl«(i (theoretically nixl piw.>tiually) 
into two partn : (I) thai which depend* on, or corro«poiiiU with, tJie 
coanw oI>J<>ct (rt,g. the outlines of th« diatom) or to l)ie coarse 
t>lciii«nl« ; am) <2| thnt dr^ndiiijf upon, or rrstiilting from, the tine 
atriKtuml detail or t]>i'niinuti> Homrntji, Thcfor'-i.'oiii^oonHderation 
Mp|tliee to ( I ) : thi« cnnititiient part of tlx^ totnl diirraction pencil 
of the prnparation whioh in aihnittifl to tli<i (ibjpclivo r^mplrtrltf, 
ttid'>[>'^<lently of th<> limiting aclion of t.lin h'li.t (ijiniin^^, and hence 
the •K>ii'et|H>iidiii|; prvrts of tlir ■ilijcrt (oiitlini-H .Iv.) iiri- depictisl as 
if thnm worn a diirct di-lim'nliim, i.i-. in pi'rf<'(a similnrity— even 
with tow npcrtarm. Those diirr.tolod myn within the whule diirrar- 
tio4i pencil which are due to the miiiulf ch^incnt* nro stntngly 
(|pllecte<l from t)ie incident lnwiiiii t*) whioh tliey bvloiig.' '^ 

Ai--c"'rdi(ig to the Ir«g or groat^rr apiitiiiv of the ohji'Hiri; and 
tl>c axial or obti<iup incidence trf th« ilbiininntini! pem-il or cone. 
Mm pari of the total 'litTmctioii peoril will \y Kiibjcct In a niore or 
IcM in«)niplet4« iviInii«»ion to tin- olyectivi*, ami the corns |>oii ding 
itmge will lliQrefore Ehow thi^ rharai-ti-nslic tnicm of thn difl'raction 
iniaf^, that is to say, cJiwiKe of lupi^nt with dilVerent apnrtun^K and 
<l)fremnt illumination, itUiiinilnrity to the real Ktructure, anil so 
forth. Thii«wr« have i>mrtii^iil/^. iu most cbsi-h, a compo«iUon of 
the micrcHicopical iniiij.-t% ■■(insi*titig of twct KujiprimiiascKl iiiiaftcu of 
HifTorent Itehaviour, Hut th« diH'i'rent* in not to be considtirM) one 
o( frrinciiilf, «o far an tin- prn-iartiim of (lie tninK" i-i conci-rncd ; fur 
it nepcnos lolcly upon tin; dilti-rciit 11 ntfulur ex predion of thedillrac- 
tion fiina WJUjltinB from conrsy uni) fmni exlrv luely line elemt-uta.' 

RnmininK, thf-n, our i Hunt rati ijiii of dillVaiition phenninenn oa 
■pplieil to the thi-orj- of iniero5copiu vii.ion. we would point out that 
l*Thnp» thi- loiwt »Tvii'i'!ii>lo illustriition for our purpose is a pliit« 
iif z\nK)i i-utiil with tiiii- j»inilk-l linei. If the flame of a candle l)e ni 
ploceil tluil itH imnye may bu ateii throu;;h the centre of th« plate, thia 

' IicUsr Irom Dr. Abbf . 

■ TIm* it niiin-KnL OiaI tioUj 1^10 ' itbi«or[rlioii tmaff* ' ao^I the 'djfTrwtian lni«t^* 
4f* iHtv bcdd to lio r^uiillj of difTmrlinn nntrin ; tiut, vKilnl a h^rii of >iniiU Afuirltira 
wadtd uirvtlw lotiiHt «ilJi (uliity. tl uuulJ U- )iowcrlr« to ratviil Uio l<U*r beniuo 
nf it* iHiiUt^ Okporit]' ti> Rstliur in tl>« ilmiiicly dvllvHad lUflnwUon raf* (llU to tb* 
■ilaialw Dlonitni*. 

• V 




diitral imaRn will l>e clear and immlourM, hnt it will he flunkM on 
vither siiln liy n i-ow nf TOlonrw) iip4><!tni of t.lin flnnii- wliifh nrr faiiit«r 
and more ilini ii.i tlipy rurcilp from t\\c n-ntrf : fij{, -(?* illiistrntp* tliLt, 
A Biniiliir phi'iionii'iiini nmy hi- jirtidiicjiil liy liust scnttered 
over & /,'liu3 plrtlp mill !•)■ "tlur oliji-cln wIiom- slriu'turr contnins very 
IDJTiuto [inrliolttt, tlie Ityht JUilfi-rinK 'i cbiinictfriHtic c]iiiti)^i in pnn*- 

in^ tliniiigli Bui-li (ilijpL-ts, that t^hnnge 
viiii-Msting iti tliL- l.'ivukin)^ u].i of n 
juimll"! Ihtiiiii of li^liL into u j;riiii{i of 
riiy« iliverfjiiig with widi- niiKl''-i "nil 
'"■ '"' f( inning a rL-j^'iiIiir st-i-it-s o( nuixiinii iind 

miniinn of intensity "f UkIii dm- t*) ililU-n-iico of jih'iiH--of vihmtion. 
Ill ihr: imnii- w«y in ihti niiunracoiie ihe diirntcl.ion jx-iicit orifpn- 
iitiii|t frmn n bt«iu incident uiwin, for insljinee, a iliiLlum iipjmnrK 
MX n fun iif inuhiled mys, iliiTciisiiif! In iiilensity Sb llit-y are furtln^r 
n-ninvpcl fn>m the diivi'liunof llie incident benui tr;insinitle(l lhn>a};h 
Lh(! ntriii'tun>, the intorfeiMn-e of the prlinnry witvf« jiving a nuiulier 
of xiii?(^>».%iv(> inuxinia of li^'lit with dMik intirrMpaces. 

With (IiiyliKht iltnininntion if h dinplira^ni uprning ho iiilerpoBcd 
lic-twci'ii tlie mirror .in*! n pUto 'if rulc<l litiPM plimil upon the ttluKe, 
tJi<! a[>|H'uriiiii'« sliiiwn in li^. 4!) will Ixi ohn'crvi'd ot the back of th« 

olijwtivo on ri'infiV'ing the ey«-j>it*e iind 
I'Mtkingdowii tlio tuljcof the uiicroBixi|)e. 
Thfi iiniitrrvl circlo i* nn ininge of the ili;i- 
]>hrii(.in OI)oninj; proilurnd hy the direct^ 
ho-cidleil 110)1 -ditTnictcd niya, while thoKe 
on I'ithei' »iile are tlie dilViitction initiscjt 
ppiihwwl Iiy the riiys whioh are bent off 
fi-oni the incident priicil. In hu)nogen&- 
oni light the ccntrul nnd Intcntl iiDagM 
ngrro in si^p mid form, hut in Vfhit* light 
tho diHrnctivl iniugcs (iro nulinlly drawn 
out with Ihe aaU'T edges n-d and the 
inniT Idiip (the rovtrrw of tho ordinary 
vi>r.'trmii), foniiiiip. in fiu-t. ri-pidar spcc- 
tni, the dinlance sejuimtinK njich of which VBricn invrrwly m the 
H<«ene88 of the liiio», being, for iiisinmc, with thn ainio objoctjve 
twice M far apart when the liniv aw twiir as vhine. 

'Hie formation of the niitiu»ffifiiiid image is cxplnintil hy the 
fact that (he rayft collected nl thi* hack iif the objcctivi', depicting 
there the direct and spectrid imagcn of the sourcn of light, n-iich in 
their further course the plane which in conjtigiitt.' to the ohjert, nnd 
jrive riie there to an interference phenomenon (owing to iheconnec- 
tians of the undulations), tliiit iiiterfen'nce cfli'ci giving lli« idlimnt* 
ima^ which is oti»erved by the eve piet-e, aU'l which ihen-fora 
depend* etAf'ntinlly on tho number and diHtributiuii of the dilTmcttil 
bcfiijis wliieh entpr the objeclivo. 

It would exit'cd the limiti^ and iht* object of llii^ handhootc to 
attempt a thc-orclicnl demonstration of the 9U':1ii:n of diffraction 
Kpcctra in fornniig th» images of line structuie and i^trintion so as 
to nlfonl ' rceolutioi),' Those who desire to pursue this part of tlie 

Pin t9, 



agl>j«ct m»y <lo no nrnut profitublj' I'jr thi-v «tu<ly of (he on!j' book in 
our Innguiigr thnt <U'-»U exhiia stive ty with ttic theory of modem 
microHntpiflal "ptic-s, vit. tlio tranxUtioii of Nacgeli nnd Sihwciidener'a 
' SIkraKciilH- in Tho«>rj- nnd Pmctici-,' tntnslnt^d nnd pUrfd within 
tl>ri roncl) of KDKli*li mioriDBnopi.ita b}' tlin joint la1x>iir of ^Ir. Frnnk 
Criup otiil Mr. John Mnyal), juti. Thti <-\-[H^rimcnt*] proof of the 
difTmction tlicorj- o( niicro»(y>pio riKiuii lic» within tlic rnngo of our 


no. so. — ]>inf«tion gmiaf . 



purpott) and tUei followiug exprinioiits irill KUtlico to »liow tho^w} who 
poaw MO tb« iostrumeots, and dnirotJie cvidr'nco, that to tho octioe 
oC difihwtioa epeotra w« am indebted for microscopical ddincAtion. 

The first axperiment shows tbiit with, for inKtonco, tho central 
bean, or any one of the s]iectra) beams alone, only the contour of 

I olyect is tKvn, tlio addiliou of at least one ditlr»ction spectrum 
: «8a«nii«l U) th« visibility of tho structure. 

Fl;;. fiO iihon'a iho nppeikTance presented by no object composed 
of wide And nari'ow liiwH ruled on gloss wlion viewed in the ordinary 
wny with tlie <-}'e-|>i(.H.v in pUti-e, and lig. 51 the ^pearauce prosent'd 
M iho l»ck of tiiv ubjfi-tive when toe oy«-piQoe is removed, the 

Via. G3, Fki. t>. 

■pectra l»inK r-iii^ied on either "idc of the central (whito) imngc, and 
at rijKht an^jka to the din-ction of the lines ; in (vccordance with theory, 
I tli*y an- farlhtr iipart for the line lines than for the wide nnea. 

If now, by a diaphmjtm nt tho back of tlie objective, like S^ 52, 
*e cover up all the diflrnct ion-spectra, allowing only thr- dirccti mya 
lt> reach Uie image, tho object will a]>pear to bo wholly deprived of 



linn tlrtAils, oii]y the Outline remaining, iiml «-vi-rv ili<tuic-ntiuQ 
i)f minute struclure diaappeanng jusl Ha if the mii-tiucopc liiul «ud- 
tlenJv lout, ila optical power {see &g. 53). 

"iUii illufltratea « cAne of the lAliteratioH «f stnic-ture hj obstruct- 
ing the punse of the dilTractioii-speotfa bi the eyo-pi«oe. 

TtieMCona experiment shows how the uppeftrance of fine structnra 
tnuy be oreaUd by maDipulAting tJie iipM'tm. 



Pin. U. 

Tf * (tiuphra^n auch lu that shown in Hg. 54 is plucn] nt the hAck 
of th« objeftivi', K() a.1 to cm «if each alternate one i>( thi- upppr row 
of spoctra in fij,-- RO, lliat row will obt ioufily become idouticAl with 
th<nower one, and if tbp theory holHiigiiod, wesliiiuUl linil thn image 
iif tbi' upper line* identical with that of the Inwer. (tn rtiplaciD^ 
the oye-pieco wi* kp* th.-it it in so : th« uppfi' set nf linfs are tloublea 
ID nmubor, a iii-w line ap|)eariiiK in lh(- cimtn? "t thi- .ipnce between 
Qftch of the oil) (upper) <>ti«i, und uppi^r anil Inwi-r sl-i.s having beconis 
to all appeonuiw laenlical (lifj. fi'i). 

In tno laiiio way, if we stop olTall hut the outn'spectra, a8inl!)[. 
56, tho HnrK are apparently again doublerl, nud are xoen as in fig. 57, 


Flo. so. 

Fill. 57. 

A case of apparent creation of structure similar in priDciple to 
the f«reg:oiiig, though more striking, is aftbrded by a network of 
sqiutres, suub as fig. 58, having aidta /xiiW/r/ to the page, which gives 
toe .spectra ahown in 6g. 5'i, consisting of vertical rows for tho 
horiionlal lines and hnriiontal rows for ihi.- verticitl ones. But it 
b iwdily leen that twg diagonal rows of spectra exist at ri^bt 



Utgtes to tlw two diaj^iinis of the 8(|UArv«, just as would ah»e from 
aetB of lin«fi in tht^ Hirvctjou of the diagoiutls. 'o that if the tlieory 
bolds good w« ought to find, on obBtracCiug aU the uther speotnt and 

7w. SH. 


allowing only tiiu diii:(oiial ona to juwx u> th<! ryc-picec, that ilto 
verticttl and liorixmit'il lines faaro <li.-ui]>jic!aml, iiiul two iwv aeta of 
line* at riykl angfen U> l/i« dwgonaU in thvir placti. 

Pin. nu. 


On inserliii^ the diaphntKiu, flg. 60, nnil rpplHcinjf the cjo-pioc^ 
we find, in the pUwo of the old network, the owt sliovrn in fig. 61, 


Fid. 03. 

tho squnm bnn;t, however, mnAllcr in the proportion <if 1 ■.■J'2,»a 
tbp)' Khoutd \k ill Kxnct ■ooonl«ii«c with thmry. 

An ytyect such ,» PleHrwiymn an^lntuvt, which gfrw kIx dif- 



^notion spectra jtrrang^ ii« Jit fig. 02, sliduld, AocordiiijK to thnory, 
«bow ninmitigs in a. hexiwonal nmuiffciiinnt. Fur thi-m will W oiut 
Mt of liiirK nl. right Ml^M to 6 iir, luiotlirr snt lit right oiulta to 
<■(!_/) nml II thiril nt riffht iin((l™ Uii/nii. Th<-si' thnv srIh <$ liiii^M 
will DJiviiiiisly pniilucn tliii a]i|H>iinuici' sJmwn in Gr. 63. 

A gnavt'vari^ty «f othrr n]>|M-iiniiici-K iimy Im pnidm^tv] vfilli this 
iwRio lUTiiiijfcim-nt iif R}M?clrn. Any tw.i iidjiiu'iil ijiwitnl with lh« 
crntra) Ih-uai (ns 'jc«) will furai (-rjuiliiti'i-a] trUuKl"! jiimI ffive 
hexiut<'t3iil iiiiirkintfs. Or by .it<i|i]iiii;; <'ff nil liut iff e {nv ft <Vy"j we 
iijiuiit Jiiivi? the .ijii'ctiii ill till-- foriii of eijiiiliitf i-al triiui^tloB : hut iw 
thev Biw tmw furtlii^r U)iiii't, the M(ltw of tin? ti'i;kn;;ii-s in tlii? Iwo 
caoec beiiiK lut ^ 3 il, the ht^xii^MiiH will be mimlliL-r tiii<l thi«e times 
as nuuei-ous. Tbcir sidea will iilau be arniuyed at a difieretit ttn-'le 
t<> those of the fimt set. The licxaf^Diii) may also be entiivly 
obliterated by aiUntttiiig only llie spectra g e or g/or lif, Ac. when 
new lines will appear parallel al right angleu or ohlitjnely inclined 
to tbo median line. 

By varying the combinations of the iipcctra, therrforo, different 
flguresof varying sixe aud po«ilioniiarcproducc<l,alluf which cannot 
of course n-prewent r.lie true htntcturo. 

In practice, indeed, it has bw*n proved that if the iMwition and 
relative intt-nsity "! the spectra, ut found in any |>articular cam-, Im 
giv«n,what tlid resultant imngo will he can W rmi IniI i.y ntathcmn- 
tical cftluubitiiinK whiilly, anil with an exactn*!w ili il. imy even to 
.■Minic Rxlent tmnncirnd the results of prcriout observation nii thn 
actual image of the object wlirise spwtm formed tlifi oiatheniaticuin's 

If /'. iiitffuf'ituiii be illuminated by central light transmitted 
from an achromatic condenser, and exuniiiied by means of a homo- 
geneous lens of largo aperture, Mr. tStephenson points out' that 
uiidi-r grdinary conditions it would show, on withdrawing thcevo- 
nii-cft and looking down the tidio, one bright eentml light from tho 
lanip with six e(|tiidiKtant surrounding dilFraction spectra, produced 
Tiy the linen (' if, indeed, linos tliey be ') in tho object itself- Hut let 
u dtoii mad" (if hWlc paper, which 'ntitWy exchidesth«icc4iti«lbeaiD 
of light, be placi-il at tJie hack of the objective and close to tlio po»- 
tc-rior leiu ; in the rtop let six murginal openings be niade through 
which tlip dilFnu-tion spectra may pa*s. On cxaminingthe imago wo 
find that in li«u of tho ordinary hexagonal markings tho valvo 
appears of a iH-iiiitiful blue colour on a blark ground, and covprod 
wilh circular spots, clearly defined, and admitting of the use of deep 

Tills ia precisely what we IcJim from Alibe that the diflraction 
theory iiivolvtw. In support of thi», thft pIiilnNnpliieal faculty of tbo 
Uiiiveniity of J<'na hail pronosftd as n, nucilioii tit tho mathematical 
students the olTcct pnnlucrtl in Itie micfiscoiin by (hest? interfvrenoe 
phenomeiia. Oni* iirobli-m wan that of tJie apfieiifanci' pnxluend by 
six equidistant npectr.i in a circle ; they corri?spond |)r<>ci»ely with 
tho spectra of I', uui/uliitum, lut accnatible tn ua with our present 
■lumcrioal ai^erture ; and the dtt^TMn of the diffraction iimige, do- 
■ Joum. IL3t.S. ToL I. Vm. p. ISO, 



<Iuc<i] from theory, lA v'tint xpcctm of Uic gn-cit po&ilbu niii3 iiiUm- 
nt,v nt tlw proposed (Utn shoulil pvt) u waa in % 61. Hut whtit 

^ Jrvmx quiUf 118 niUL-li to thi- purpose w, tJwt Dr. i^i» liiis pn>du<;ecl a 
fine phntiigmph ol i", attifulatum, givi-n in Ptul« X., wllere it will 
be snin thiit tho detoilu aihuwn in %■ <>1 iippenr. 

Lut it tir clearly iiii<li>rstocKl tliiit llii* ilwi* not pret«iut (o be an 
inWr^tmtAliiin of the miirkin^'K of Uh- dintuni ; it iii nnly lidil l>jr 
Abb«i tobvnn nocurute imli^'Ution by utlculntion of nbal iinit^'e tlie 

l^v«p iliOinction qwotr* ahould producv. Au opiiml ^'Iiwa uiid 
JD)?di» for 'tnountitig 'nti<l 'iiuiiieniiAn ' ot immenaely greitwr refrao- 
livHand dinpiTsivo indioeit— at present wholly iiiacceftsibli? In us — 

LfittLst, be oktiUrnds, be found aiul t.'ni|>l(>yed lirfore >(/' tlip dilfniction 
iliectnt of /'. itaiji^ittum <.-uuld be luliuittMl lo fonu iu dW'Iutc and 

i-oiiiplele "dilTrnction inin(,"p' ; but from»urli Hpoctmiis thoubj^ctivo 

i-mploveal c«n Hillnil, it it. n)nint«ini-d by Abbn iJiiit thr mntlic- 

mntidan can i»ocumtHr «hovf wlmt the imnt^'will W. In Uipciiscof 

/'. amjuiitlum tiirory indicntnl ihn it/itiml, but not n'--ri>*ar\tif ih* 

Mtntetitml oxixti-ncc ' of xnialli-r mnrkings, nhowit in tig. 64, brtwM-n 

IhccirciilnrspolK, T!ii»»' lii»d mil bi-i-n l>rfi>rr «p->'ii by olMW'rvrrii ; nnd 

^tliff mfttbnnaticiiin whi> miwli- tlu- cuk-ulnticin (I)r. Fichbom) bad tu'vrr 

TO th* dJAtom nndi-r thp niicrra-M-'opi- ; but wbi-n 31r. Sit-pbt-nion 

■ ri^-t"!tnmii»Hl the obji-ci — .itoppinj; out tlm ■■cntral bi-ani nn atwve 

Mi-ficriU-d and nllowiiiK the six s|)C(.'tra only to ]iaax bf >aw tlie 

' iniitll innricingsi, nnil nltiiwi'd tbi-ni ut ii nieiHiiiK of the Koyal Micro- 

'xtiiiicnl Society to luiiny enrwrt* who wert- tben?. Tliry wpi* Diuall 

anil fnint, and n"i<ioubt punr'lyoi>tiad;and, wre learn fri>i 11 experiment, 

tuajr rviulily vjicnp.' oliserviition : but by careful invtsiigiilion tbey 

' Co*/. Abbo't ifowt note, p|i. 78 ef lofi 

~. ^ ■■■;■:;-; zRT C-,'SIP"'V"SI' MICROSCOPE 

- - -- ■ ■ ■— -T:-r *s ilwy ftre capable of being demw 

::!-. i:^tb«>iDBTioian. > 

.: -,;-*-■ ^■v=Ti;pt)<>ii> nnd with all other couddoH 

~ " ?■-■!■" :'.ci>>."iiTea*vii* objectives of greatest ua^ 

■ > TiZ ;.;. -J* ihe real stractore of the DM 
' • --. "^^"t iwiTTi thai •iifiiiiilar Btructnres *3 

■ »■ i(.._~. vhrn the 'Itfierenee (d that 
- ~ . '-^ iz.i .> 'LV^-rseiv riiiiitar utrvettire* nM 

- • :- -. •\i---.r liiffmctive iinngies are dim 

• . - ■.■;.•—.- it_.'tre »ns«"t'rs point for point h) tki 

« - ^- 1. ■. - T^r/.Tv i-.^aMes a snfe inference to !■ 

.:.~ ■: •:..: ..l;iei-i : but the diffractiffli ot 

»-~ : -■; >-r:;.;::rv siand in no direct reUtiai 

1.. . .:> r.-'l "i necessity couformable tt 

- . -- : ->-•.■.■■•■., i^iinute structural details «• 

■ •-.-. ■ c^' :L.T-.r',u;iy or ili'tpirically, and can- 

-,' "-"•riii*. hut only as *iyni <t 

■ - - . . - >,-. ;• :ri^ j>anii'les composing th» 

.~ . .. -a:- "y ''r inferred from the in»^ 

.:a :■■: ;rf«:-.oe in the oVgect of Bui 

' :■• * . -.I'.^.-f :r.e.-]Cv'iiifditlraction phoio- 

. - ■ - ■., .i-^r^r.i-r.umlnTof diffracted™}* 

■■■,■. -V riiiiilitrili/ betatcn Ai 
r;. . ■ ~r. .r^TVi' — 

. ■■ i-r- : ■--* i-',v:ids always ontheM* 

-• • •:: : „-^ -,* ■■".■r.-fi-it-nt to emit. 

; ^ ■ - :■: r-L.:.cSr. :-f i.-^Juf-il corjyutde"' 

-■ I- ivt-.r^ ■..>.-..:L\-il to that of ftf^ 

--.■■■ 5- .. ,■; . s .v^:c '■ iwkinwnnd, and theofj 

k)K>«« umi ctMor WB4 I* * «Mi^«*M» kid &**rh' ubif^nii di^i*'. 

■Uhi^svA 1^ f^w tW «^<^ haMjpbere. proved tbi 


c<>nipiit«, wlii<;h would give an pxnctly §iraiiai- dilfnictioii fiui, but 
l^ahraptlr lirolirn olTnt t)w> limit of tli^ftperture. Theorj- show s that 
thmiul->JtBpisl (ittjoct which couid yi«Id such a pMrticulsr dilTractioii 
efli-ct muni (nithout cati«idvring oth*r dilleiviices) be j/rraier in 
hftdlh Ituui lUKithor oiM ndding the full continuous dissipation of 
lis-ht ; in oth'T words llic nctnnl thread, so inconceivably fine, 
lii-tiitiiiinjic Ui iIm^ Itiict«nuiu has produce<l a 'dilTmction «ff<Mit' 
Lthnni^h the mlcnii>cnpe, rmultiDg in ihc nppc4irnR«e of a tliread 
p%kioli is tJw- ' liiflmctioii ituuge.' But this latter is greater in width 
thnti the itctDa) thmtd or pmtoplnsuuc tibro wouUf bo could it be 
Kopn din-rtly witlxnit the Mia of niHraction. 

(2) Wlit-iwrvcr a poriinn "f the total dilTmction (an appertainiog 
to a }^rpn si nictun- ix l<i«t, tlm itnngn will lie mom or Icm inmrnptrli 
tfi'l •liiuiniUar frtrtii thn oVyr-ct ; nnd in gi-ncrid tho diKKilnilnrity 
rill Iw th(* gnratitr the xinnllcr thi'fr&ctintiof light ndiiiittt^l. With 
Ktmciures of KYtiry kind (n-gutiir nnd imvutar) tho iinngti will Iom 
Wicm nn^i mors t)i« indicMtioiiK of th« minaln* delotlx, us the prri* 
plivml (morn (iell<>Gt«<l) niyx of the diflnction pencil Arts vaort: and 
wore exclude*]. Theiina^' tht-nhi^coinexthatof u (/u/mtiftitructurr, 
namely, of ono the wAo/- a/ irhow iliff'nfinl Amnu would (if it 
pbynudly exiatetl) tw repreaenUid Iiy the MdViMv/dim-actedbeuiu of 
the Mmoture in question. 

At tliia |ioii)t it ia suitable to point out that Dr. Abbe em- 
phasiws to the pr«iient editor the iiiijiortance of interpreliiix the 
* iiitei-costal points' ^liown by 5Ii-. Slej>heiitio)i in /', ittujulatiim 
(tig. G4) as not a rev«-Ution of fol xtructuiv. 'Tlie fuot is llmt tho 
inaj^, which U olitained by 9>ti)ppiii;{ olV tlio diii-cl beuiu, nill bo 
titort tiianmUar froiu the real Ktructtirc tlutu the [Witmry iumge. 
T( hiu already b<«ii sliuwii lli;it the ilinx-'tly tFaiiainitted ray in a 
coiuiitueiit and niont esHrntial port of the total dilTractiou pencil 
KppertiiiiiiiiK lo the Uruclun- ; it is the ccntnil mnjcimmn of this 
pencil. If this be Klupiied utF a greater iiait of tlie loUil diffi-aiiioii 
peni-ii iH tont tlianotherwiHe, and the image, coiiBe(|iiently. in a more in- 
•.-oui|>lete one, aud therefore more diadmilar than the oitliiiary image. 

' Th" interfM nfthe Keptrimrnt in question is oowaequeiilly oomtlued 
tti two points, vbt.— 

t. * It is lui exetuplillcntioti of Ui« general pro|K>sitioii that tho 
tnnv object atforda (litlereiit iinwj** If ditTerent poi'tiom of tJie total 
diflmction fan are a<Iiiiitt«d to the objective. 

ii. ' The image in question shows to the observer what would bo 
th(> true tuif>-fi \tt that ntruoture which will uplit up an incident lieain 
of light into tix isoUted maxima of second order of rfjuni intonsity, 
M)>pn>3Ming totally the (centi^l) masiuium of tlic tint order, as 
Hi;. 65 ; a strnclure of such a particular and unusual (lifTractiun etTcct 
is theoretically possible, although it nmy be probably iin^xmsible to 
nvlisn it practically. Mr. Ktephenson's vxperinicnl shows, in fact, 
till- true projection of the hypothetical structure. 

(3) ' As long i» the clement* of a utructuro arc larjie multiples of 
ihc wavelength of lighl, the breaking up of the ray* liy diffmctioa 
is confinm) to «initiler iind Bnualtf^rRn^'U-i^ ; that is, all diU'racted rays 
of perceptible intensity will bi; contpri»c«l within a narrow oo«e 

■3ttP''m3- :MtatmBigs. 

lETi^a 1*- ms'TCUG c-i» 

ttn-rs- Tn.iy » i* oimc on. xa^ 
-Cirrr Ji -le out- * r m-*- 

■:■■ tn-r^ 


hi:nr lixarstiUKr a: 

xi=.a& ^ait f ui rffc. liw iiiffmiiwl 
vr^:s ail t* wmoGHC in ai 
-■'T^ a wm— ■ IT numiL i 
.^•mvT or *»-•■ »-tir^ ini'tiMi o( ' 
I'ii'ir TM«raiMi difinuoinB &■ ' 

-■■Hill nt niBBkiitfiL if i^My '^'i* **' , 
upsium ic nuui. «aimVD"«sn-lai^ ' 
Tn<r« "XSMT BBiuoBfttKBXi ma oQ** 

I in thin CHtn cnndnol to th« proporUon l>otire«ai the brij{ltt «iid dii- 
iM^ tnl«nipiu«ii nE tbe KtrinUon luid to the a[^wainDC« of the eon- 
toam of thtr utriir. 

'If nottnorr tlisn the »i(l two rn^'K of the total cUffraotioQ fan nr» 
ailtnitlrrd, Uui diiric nnd tliii light ititrrvnls «re aJmiy* Hbowti of 
nfipmxiiiiJitrly n/unl hrradlh, i-vi-n if tlti- nvil pivtpnrtioti of tiolJi 
intrr^'iiUiliircrsintii-li from 1:1; nnd tlii! ilark ami bright eiri:^ kIjow 
nlwnyn yntjiinlltf incrcaiun^ mid decrctuJDg brightness; in ollwr 
wonu, wuiit of dUliiict ciinlours. 

■ This ]ilwiHHiii-tHJtt uliiiws ibii ty|iiciil picture of every rejfular 
iitri*tioa tor th« d<M>ivtioii nf vhioli i)c>t mon: thnn two iliflVftction 
rayii on be uliSt'ueiJ. For cxuiiiiili', AiH/'hiyJcura /rf/ufiilii, or any 
otber Ktriittioii which In iifar to iW iiuiit <i[ n-colutimi fortbc'>ptictLl 
syntviii in n^, and, ih.')'i'fin-r, awn with i>bli<|ue light, brings only 
j'W! diflViu-'tod bouiu into tilt? objective. 

iL ' Wheiii'ver » structure (jt»-es rise to u dilfniolitm tun of ci>n- 
luderabte aiiKuliir ext^imion, tJio ob3(fr%'at[oii with n c<;ntriil incidnnt 
\itaua or axial li^bt niay lose » k''^'*"' "'' 'unitller iiortioii of tho 
w-bot« dillWti^d tiifht if tlif tLtigubr exi^uusiou of ilie Uu i'^xUiikU to 
thv nperturv of tliv objei-live iu vse. But ubli<|ut- illuminatiiiii mujit 
always involve a loss, and tbu loss is uul cui)liti«(l to the «xt«miil 
(penpliemi) mys of (be dilTraction pe»dl (as U the ci»ae in oentnl 
light), but the jMi'tion /'>#' will tnore nnd moi'e £<xt«ud to o:i<! full 
half of tiie whole wbeii tbo ubliijuity is ^.tuduttlly iucreii^ed to th« 
utmost limit, so iJiat ibe direct b«4iiii toiti'lien tbe ed^e ai the a]i«r' 
tore, !t follows tliat the imajfcs which aro obtained nith otitU/u^ 
lisbt wfill ulnaj-B lje tiici>iiiplet« and not awtiMr lu u t;H>ni<!tri«al 
])mj.:ctioii of the itbjeot ; ami KCuenUly (tbou|;h not alwiiy») lumi! (li«- 
uniilar llutii thosi- by ot-ntral light ia rogaixl to ibe uiiiiuC«i' details. 

' Strict ly similar images uuinot be ex|)erted, except with a ci-Hlntl 
illumination «itli :i narrow incident peui-il, lievause tbin is th» 
iieeessary i.-ondition fiir the possible admission of the whole of ihe 
diffm«t«d light.' 

Lei it lie noted that tbeae principles of tbe diifractiou theory of 
micTOfcoopical virion relate to Htrui-tures of all kinds, whatever uiny 
\» their physical and geouielrimt comjiosition. Irre^lnrstnictures, 
iuiUted elements of any sliape. equally produce dilTractioii ell'ects, 
•>bn>rv<Nl eitlier l>y tnuwmitted or reflected light, and lieing either 
tmoKpnrent. seinitraitsparent, or opaque. 

The vnlae of 'i = n an u indiontcs the numbt-r of rays which 
«n objective ran admit ; the nj)erture equivalent measurwt the very 
MWMice of microscopical pcrfomiance. ll measures the degroo in 
which ft given objective is compptent to exhibit a trac, roniplftf 
di-lincution of stnirti;r»« of given tninntene*s, and con%-crRcly ibn 
proportion «f a in different obj<vliveH is the exact mi-nsum of th«' 
dilfciTTit </'y'*' iif BtinuUiiftf of BtructonJ dotni til which tbcy i-an 
reach, either wilJi pi-rfect similarity of the itni^ or with an ifjual 
degn-n of inciinipliTtciiesA of tbe image, provided tJiat tlie purely 
(tiofitricnl conditions an- the same. 

' Revolving ' jiowor is thuH a special function of ancrture. T\w 
lijnit of viAiblo nepgiratiau in dclicatv Ktruuturo an<i atnatiou is 



determined by the fact ihat no resolution can Ije elTficied unless nt 
lout two diflVtvciion penrih are adruhted. and the adnns^ion of these 
wehavouen is absolnlely dependent on thenpertur<- of thcobjective. 
The rule giv(>n liy Professor Abbe for determining the grestest 
nombei- of lintw per inch which ran be resolved by oblique light will 
be found (inking nny j{iven colour ns a Iwinis) to lie equnl to (Wc 
th*' nurntrr of ttiiiiulalioiis in im iii<-h innlliiitifii 6y (A' numtrietJ 

To thoKC who havo studied this subject it will he seen that the 
'numwHcft! niiwliipo ' hore takes the place of what wmi formerly th(? 
' «iuo of half the angle of Aperture ' ; And it hn« the elTcct of giving 
the propcwiUon a broiiilci- generality. By using the ' sinn of half 
tlic angfn of aperture,' the proposition is only tnm with tlm ndditiim 
thHl tlx! ninnbcr of undulntions be calculated fnim lh« wavr-lcn^th 
within tilt! iii)L-cinl uiediuni to which the angle of aiirrtun- ri^liilcs. 

lu intrmluoiiig the numericnl npm-lurc instead <if the sinn of the 
iin^li!. the liittfr {the nine) is inerrnKcd in the priipi)rtiim of 1 ; n 
(■> Htiindiiig fur the indirx of the nirdiuni), nnd thnt has the luime 
effect as iucfMainj; the iithm- factor the iiunihcr of undulatioiut. 

WTiat the colour pniployiii Khoiild he is only cn[Mil)li?of individual 
<let«rin)iuitJou, nincu tlii> cnjiocity tor upprceiatiikg light varlea with 
dilTerent indivi<lualit. 

If. for iiuituncf, we Uilce 'IS/i in tlie nolnr spectnim aa being 
sutticiently luuiinoUK for iiKioii, we find the niiixiniuni — so far as 
seeing is concerned— to l»e 1 IH.OOO to the inch (the oliject, in this 
(uue, lieing in ttir) ; but rh thi- non -luminuuii chemical my* i-eniain 
in the field after the departuri- of thf lUihle spectrum, a iilioto- 
grn]>hic iiuoge of line-s much cloj-er together might be pniduceu. 


Fio. M. 

Till* important nuliject can scarcely be considered oonptote, evert 
in outline, vrithout a brief consideration, in their combined relationB, 
ol ApertUTCt in oxocas of 180* in air and the special function ihesei 
aperture* posKH. 

1. SuppoKo Miy object compo«ed of minute elemental in if;;ular 
arrongemont, such as « dintnu) valve ; and, w confine the consitlera 
tioii to the uuwt uuipU case, suppose it illuminated by « narrow 


9xin\ pencil irf incul^nt rnys. If diii nlijccl U <itisi.'r\-wl in nir, tli« 
nidiniiiii] friim every point of tlio uhjei-t is, in c<>nsiH|ucnc[! of tlio 
ditfVn^-tii'n cirect, conip>v«] of an axjil p«ucil S, fiR. 6G (tin- ilimct 
«oRtinuntioii of ih« incident rajra), and a number uf b«it-oir pcndlK, 
^ir ^ti • • ' ^irrotinding H.' 

If, DOW, inntciu) of iiir, tlie objert in {nimeraed in n innlium <if 
grmUr ivfrnctive index, n, it r««ulu from Fmunliofer'!! foniiutii that 
the sine of the angle of rleRection oi rlie tii«t, Gecond, , . . Ix'tit-ulT 

Pia. ST. 

I IS r'vhi>v7 in the exact pi^jxirtiim of n, nml the angle i* «•- 
laUo— thnt >i(,thewhaln fan <rf the ■lit)rn<;t«d mys is coi'trirl'il 
ia coinwmwin with its cxti^iuiun in nir. Fig. 67 will rq>rc8ctnt the 
taat ol th« snmn object in oil. 

If now any <lry obji^ctivr (with a gii'i-n angular scmi-nprrttirn «) 
u cafMble <if gntti>trintf-in frtiin air the tint, nr the fint and nr^ctrnd, 
diffraction braims on cvi-ry side of tin- axinl pencil, another objctiti%-n 
with a more di-n>R front medium of thc^ n-froctivn index, », will \m: 
CJlpftbU^<>f admitting, from witbtn tlnMlruiK' medium, exact ty t]iei"i»>« 
liMUu (no mon^ «u(l no leta), if ita amjular semi-apin'turt^ V, is lf*i 
thui H in till! jirttjtortion : 

8)n r : sin II ^ 1 : fi, 
W . 

n Htn V = sin n, 

all other circuaMtancea— object an<l illumiiisticm—remainioK the 

For example, n diatom for which \\\fi tlistaiics of the Ktriie \t 0-6 fi, 
wiU giro tbojifnif bent ■off beniii of gn-en light (* = -53 ji) in niV under 
an anglu of 66-r>«. Tliis will bt? just admitted by a dry objectivo of 
1S3° autfvlar apertun\ In haUim (n = Tft) the same pencil will 
be defected by 37*.'f° only, and wi>uld be adiuitled, therefore, by «n 
objective of nut morv Uian 75" Imlsain-ao^'le. Again, if the dintAUce 
of tbo line* nhould bo greater, aa 1*2^, the *eevu'l deflected beam 

■ Id ip. M, or, uid n 8, «id tt, tit mippMMd M bo idnntiMl witli Uio mrlM**. 
but •» ilnwu U « (liglit lacIiaMMui lo Uuia (ot ttM parpow ol cIcuomb ia ttw dia- 




would Iw «iiiitlrii in air umli-r nn nngit- «i 66'5% lint tn liihmn tlio 
tkird would ultniii Ihesnnii" oljliijuitj. WliiUt now ibodryobjt^utivo 
of 133" air-angle cannot lulmit tmirt! than iht! Ivro firnt lUflractioi ' 
boams on eavh sidi' of Itin axis, the inimoniiuu of 133° lialsaui-aiiK! 
is cajiable oj adtuUiin;; tmin lutl.iiiui thivt! on fucb aide under «xautl 
UieHanie iUaininatiun. ' 

It foUowH, Uierefitre, that a balBiun-aiiglt! of TS* denotes Uie same 
ftperturo »* the larger atranKle of I3:i'. ajid a baltain-MiKle of 133* 
a much s^rwiter ii]wrlure tliiin an aii-angle of Uie name imuiber of 
degrei's, and in ;^-uc-ral two uperlures of differeiil ol>i<!t;tive8 muhl " 
equal ii tiie luiieti of the acnii-angleft are in lUo invei-se ratio of tbi 
refractivo index of the luediuiu to wiucb lliey relate— or, which is tbe 
same thing, if the product of the refractive index multiplied bj- sine 
of the Angalar semi -aperture (» aiii m) yields the soiue value for both, 
Le. if thoy ai-e of tbe awne numerical ajierlure. 

2. Suppose the same object tu be ohaefved by a dry objective 
of u given air-angle, ut fint in air uncovered, and then in balsam 
protected by a cover-glaaa. The tlret case would be represented by 


flg. 66, and the second by Sg. fiS- As we Iwi-e seen, the group of 
diflVnctcd b«ains from the (Ajjcct in Imlsaiii is cnntrtcUfl in com- 
paTLRon to that in air in the ratio of the refnicli\'G indox. But 

> Tlia loltoirlng uo the Mtuiil ftuglis loprauintcil Ed tlie dingnuoi, ric: 

(Btriai •• 9-3 fi. tuto- length \-' 35 n, mcdiam ut h = 1.) 
ri, - n 3U" 

8, = 1]0= II' 
S, - W 811' 

9, - IMI" 0'. 

fSliia — 14 M. wtre-leuEtli \ o SS u. meiliuiu ImIbiiu n •• I'S.) 
8, = lU" Vf 

s, - 00' a' 

8, - H> ae* 

S» » ao^ u'. 

imy oiuEcnvKs 

Hccimlin^ U» tb« law <tf rffnu:ti4>n, ttiia (Cfuuiii on IMMWg to air bjT 
tho pliUM> KtiriiUM! (if lli« u>v(.'riii}(-)[1uiei, U ryrratl out -tJle sine* o( 
ihoMiglnt liritk;! (imiiNirMl -in \\\v ratio of thi'Xiimr n-fTnirtivi< index. 
Ci>niu-<|M<Mitly till' vanuuii (Uirnu-'tion (ii-iioilit, tliu firxl, ><xs)n<t, . . , 
nn «vcry niilt-, iifttT iJirir iritnsmiiuuin into air, hnvu pJiAclly tJie 
KanM» obliijuity wliii'li tliey hiive iu tlie owe uf <lin!ct emimtn in 
mV from an uiuMveresI i)lij«s:t. 

If iHiw any dry uKji^ctive of. wty, 133" iitr-uiiKl" i» cii[i«lil« nf 
mlmiltiii;; a nrliiiri iiuiiilier of tWite [feiiciU fnilu the uncuvr^nl 
(iKjrv^L, it will :iiltntt t'Xticlly Ihe miini^ poiiciU fn>ia tlie buUnm- 
imiuiitod olfji'i't. Tlie <'i>iil ructMl coiie in Imlskin of TR^ nn^uUr 
a[i«rture MiibnuM!8 nil ruy§ which Are emitted in air witbin a cont* 
of 133". 

TI»o aperture "f an otijoclivn in nwt, tlicreforc, cut duwu l)y 
nutuiiting thi« o)>ji'ct in n di'iMp nici'iiuTii, for no i-ny which could be 
taken in fi-om iho uncovcixn! ohJM-t in lf>«l by the luvlsnm'muuiiting. 

3. A coinnHriKoii of tigu. I>l>, <!7. nnd Ot! will nhow thnt n cono of 
82" within tbn hciUnin iniiliiiin mnbmctt* aH tho ditl'iticted rays 
which *n> cinitlril from tlir objrct in air or tmniiniitl^Hl from lial»am 
to air. This, bowcvvr, is not the totality of my* whii.'h nro I'liiitted 
ill tbn balanm. Tho forniuin of PrnuniiofiT shows thiit Hw iiiiiiil>«<r 
<if thf <ll>ittriil twainK Ik i/tvitlrr in IkiIwiiii thiin in nir in the Hamc 
mtionath'! n-fruclivi- index. 

A sInK'tun- the dtntAiun' of whotn elcinents oiiunhi i-'i ft emit* In 
halaun <rix distinct U-itma on <!atb aitln of the ilimnt biiiui, but In nir 
tmly,^*!^ (sc« dgs. CO, 67, Jvnd 6l*); tlin lifth nnd si\th arc completely 
lo«t in nir. Adry objrotivrof iinnn;i;iilnrt>iirrturccIiBr'ly approticbing 
ISU° will not I'vrn tiike in the fourth di'tWlitl Ixutiii, its tmx in de- 
flect«<l nt nn nngli- of S0°. But any iuiuiersiim-^liuw of n ludsnin- 
»n:>lc nlighlly i-xciiiling t^l!" will tiilti' in the fourth, and if the 
ImUam-nnKle nIiouIiI rxims] 112° it will l.-ike in thr tiftli Ix-nni nlno. 
[irovialifl till' olijivl is in linlanni, and in optical continuity with the 
frvint of the Icn*. 

Thtitt, iLUnin. it issrrn (lu won l)efore shown by the pun-ly dioptric 
cnrlh'Hl) thiit an imnwruon objcivtive of biilHani-anKl<-' exceeding 82" 
lioH > widi-r »)H>rtun- tJiAn any dry ubjeelive of inaiiiuuni ungln can 
have, for it in (!n|uil>lr of )-athi-rinK in from obj«cU in u dfosi- nK^lium 
r»ys which arc not acceciiiblu to an iiir-anf;le of ]W\ 

It is, tJi«ii, by the abui'C fiki:t« and re.uionin;', placed bnyond all 
dis|>ut« — 

1. Tliat a wiil«'aDh'lMl 'immersion ' or 'hoinogvoeous' objective 
]iow(nu(s an a|H-rlure in excc«s of IKO" 'angular aperture * in Atr; 

2. Tlutl ttw- ifi'Ciit value of tliis-always manifest practically— is 
fully itcL'oant«i! for and explnintHl by the ai^raelion theory of mivro- 
HXijiic vision ; and 

X Tlial ' drj' ' obji-olivw, so far as n-jpints the pitrfoct deliiirntiou 
'l/ t'rrj/ minttn *trio'l*4rr», can only lif ccmsideri'd as n-jirfSTnting ati 
ili>|iei?f<rt phiu« of ron»trui7tion. WIkmi miuli* by tbf \fnt banils, 
willi every jir<«caiition and care miploytsl tu wriiri' tlm Ih-bI. |K>Ksibl« 
corrfy^tiond, their ilcfi-rtA do not Vm in thtr dircotion nf tin* pniaen- 
tatioii of false or evni [MirLially erroneuua and distorted nniij^M, 


Thi'ir ilnfncta nra iJurlr inovitalile incapacity to npcti up <lctAilii in 
rtrupturi? thiit can h« (iiwUmi-fl with rulatlv« enae \iy ide inritiiriein 
I'nJnun oil itnmorsion, lUiil (wjx-ciiitly iin 'apocliroJiiiitic' objcrtivo trf 

5 rout a]K!rLun-, of tlin nil rc^c-oliiif; lUflfniclion lieams avlurM by the 
ry lenn of ('niiiviiltTtit [lowt-r. 

Willi [jry u)>ji-ciiveB Hfilimiliil ir'StillH linv^ boiM) attaiiiM lx)tli in 
l<>w o.u<l liit;ii pi>w(T uoi'k ; liut nil tlii^ Iutt4.-r ih l>ifiii;*ailviiiu-e(l iijiou 
liy ri-vinioii Willi 1<■Il)u.^'« [if •^(>atcr aperture in a Hirikiti;; luumivr. 
For {wi'iity yeiki-H wi! Iiav« liucu aixitiK <^i' ^"^f- Enjjlisli inicrotcopo 
tii'iJciMV t» c-iiliir;,'e the '»nj;!c' of our objectives, and ouiployiiig 
them froiii a ^-iucli to a n'„'i<"^'i focus. We have teen them 
adviince from dry to water iininersion. n.tid from this t^) oil : frum 
^^■iiich, a I'^-iiidi, and a ^^^-iiich of N.A. 0-il."i cnch. and re- 
sportively to water immersions of N.A. 1'04 nnH then to 'oil 
iinmeriioiis ' or liontogeneous lenses of N.A. I'-IS for the n'^-inch 
iiiid ,'n-iiich respectively, and ultimately hy a j'^-inch with N.A. 
of I.W; and from that we h/ive pmgressed to tlie apochnmintic 
ohjnctivfis with coinjir-iiKatin^ eyp-piecM, 

Now thn olijr-ctiii'M wilji which thi- imrliiM- work done hy the 
prewtnt editor niiiJ liis coIlfiiKU", I>r. Drj^ndnli', was cffrctwl— to 
wliii'h ntluxion i» mniln oidy as brinj;; thr iuMnnrtr with which we 
hiiv*- most practical famitiTirity— am still in our jHM.ii-s.sitm ; what 
was rc'vi'nli-'l l>y them fiflcen, twelve, or ten viMirs npj wi- can 
I'xartly itjmiiI t<i dity ; ami in the jji-neral fi-ntures of tlii> work in 
tin- liroiid ch.T.nn'ti-ri»tio5 of the lite historiefi of thi- saprnpliylio 
orgaiiUmn, minute ax they arc, revision with t)l>je<^tiv™ of N.A. l-rtl) 
and other Icn*™ of thr ljc*t EnElish and (irnmin nmkers, n-vi>als no 
]Misilivcori-or, i-vi-n in the niinnlritlfit ihc ilMuiU then diin'ovcriil and 
di-lincaU-d. Itut the lai^-r li-nse* nf gnsit api'rl.ure and liriuitiful 
corrertionB lia>'e iijKsnetl up ulriirlurr nlisolntrly itmiailile btj'orf. 

TliuM, for osample, n minute oval orKanism from tin- nn'nnt') ^ 
the r,aan^^ "f *n inch in lonft diametM- waa known t«> [KinaMa * 
distinct nucleuK ; the Ion;; diameter of this wiia from the ,int]i to 
the I'jtli of the diameter of tJie whole body of the or;|;anisiii. In the 
ob«ervatioiis of twi to lifteen yeara since the cyulic clian;^ of tJie 
entire or^^'anism were clearly visible and conabintly observed ; but 
of tlie niifl'-u» notbin;; could be made out save that it a}i}itarffl to 
share the chntij^s in self-division and ({enetio TeprmluctioD, tntfifttof 
by the organism as a whole. Itut by lenses of N.A. I'SO and the 
finest apoeln'omatic objectives of Zeiss, especially a most l*eautifully 
corrected 'i mm. and 2 mm., cirticture of a remarkable kind has 
been demonstrated in the nucleus, and it has been sliown that the of the great cyclic change* take* place in the mi^-lrtt*, and 
i* then Rhiirrd in by the organigm as a whole. In short, wn have 
(liscovrired as much concerning the ' ina^rcessible ' nucleus — which 
inaT 1)e not more than, say, a twelfth of the long ilinmeter of the 
wlicilc nrgnniKm — hy mtvins of tmivr fmrrrt, hut f/rrnfrr apn'tufrg, aa 
wn wire able to (ind cimccrning the wmipletc IcKly of the saprophyte 
with dry ohjei-livi-n. 

Hut in spiti- of these fufta there is a certain I'lnm of even In's'' 
jxiwiT work iu biol(>gy from which tlic dry Icnx can never be dw- 



mined. It muit ii|w»ys bean iiidi§pen»al)Ie iustruuK'ni. in n Inrgi* 

pttrt of the work done in the Htudy of the life hinKiry of ndivu 

livitig orgHni«ni8 ; and wliAt«>i-r Accessories in reMuircli <ui such 

■al^^eets be eiii]>loj*ed, the main jmlli of nccurnt« mid well <x>nvlflit4;d 

^aownry mxM lie by ultimate nnd coi)aecuti\-e inference ti> tlie 

dun^n of thr liviny orgnni«in. Uut we c;tnnot wrilh any certainty 

ilii this with pithrr a vrntci' immersion or a homofneneoufl objei-tive. 

With an octirr organism untler iitrestij^tion, we desire, an far as 

pnLTtirablo, to limit thn area of il« cxcuniioiiH ; a cover-gliisfl of nut 

inore than four-tRnthit or a <)aart«r of an inch in diameter ia tarfp- 

#iioukIi when iihjwtivps n 5V inch to a j'« inch are used. 

or when the m-rnt S mm. objitctivr with 37 oyc-pipcP is employed. 

To havr oil or wati-r on the top of thr <«vrr, Imtwin^i it and th« 

boot lens of thii objrctivo combinntinn, ix, with nlmoHt inevitable 

wrtainty, notiiicr oi- later, in fulUiwiiig th«ciliji»ot witli counter nwve- 

nwatu 4if the sUtftf, to nuu'ti thl^ ttlgn of thi- cover, and <viu*« tho oi) 

ar water uborv to mingh' hy oapillunty witJi tin- mi)iutj'dr(>pof fluid 

Bader obacTvatioii, and thus t<> involve th<- whoti; in mtn.-'tropbri. . 

To do tl»e main work of studyiiift coimpcutivelv tlic life history 
"t«ukno"«-n itrjpiniims, dry ohjpctivps will ami mu.^t In- used; but 
in rII caieH »utb wi.rk must In- Bupph-niMilMl hy Ihir unrof "bjectivps 
•rfgreat a|>"--rturt*. Tlit- drlaiU and ri^lutimia of ininutp stnictum 
nun be alu<)ip<l in oiii* lii'ld. and their (;i-iiet'id origin mid sii{ueiTc;pij 
ill tncrtbcr. T)i« Uttir will he ' contitiuous.' t)ie furim-r wdl b'r 
fmployed aa neceaaity indicates. The diffmction tlieury of micro- 
<^<ii: tUion doea not invalidate, hut in realii)', under detinuhle 
oniditiona, direcia thi" omploymt^nt of ' iinrrou- " njiiTturi's. All 
<)(1)en(Is on tli* rninutenetu uf niicroiioopic d<<tail. The law has been 
enniiciated abovr : the minuter the dimensions <tt the structural 
fH>enta, the wider m«*( be the aT>ertur« : the latj^er tlie dirtaits of 
alliiiuit« atnietuK!, tJie iiarruwer the aperture that will mnjpw. Tliis 
ii lni« in regard to ubjecta of *\ cr}' kind : there ia no variation in 
th» tDnditiona of mionwcopiegU delineation. 

TH« mtn eiigaK^d in nucraioojnoal reaeiarL-li have dilTereut aims, 
Oir, the Kani<- worker at different times differs in the object puraucd. 
'lltiBialt* structure' is not the uh'' consideivition of tlie niicro- 
Kojidtt ; be often, ils indicated above, has to take a compreheiLsive 
new of ihe whole object or objects of his research, ajiart from tho 
most otnoplex and delicate details. 

I( in fully to liuppoNe that because great apertures have Iteen 
|*a\'nil theuretically and pviU'tically tn hp able to open out minule 
rtnicture no pei-fecliy, thercfciie thvif is no ntiiion (/V(jv for small 
i|ierlures. Low am pi ill cation cannot tvmW distinctly visible de- 
lailt l«yond a certain limit of minutonc^M^, and wide npcilui-ea 
Miuiol be uliliHed unless tliero i» a concurrent linrjir ainplitication 
of the iiua^^e which ia (iomp*1*nt to exhibit to the eyv thn uninllest 
diii>cuAioti» which ar* 1^ optical law within the rraeh and gnwp vi 
nidi an aperture. 

In the satnv way great aniplitication will he»H if we have 
•mail aperturoH which dulinentr details of dimi-najons only caiHiblo 
of being distinctly sci-n in an hna^'e of much lower 11 mpli fixation. 



It will be '«mpty auipliticniion,' IifcatiKe thno U nothing in tbn 
uimfte whii;h requires bo ii)uch pnwi^r for ili}>tiiict. n-cojuinitioii. If 
Uie yaierr be (Utficicqit, aperture will uut nvnil ; if tlif ii{H-rtun- be 
waiitii));. uothing io gntncM hy high pourfir. If tlii> jinjiulur ikpt-rturv 
<if ( 1 1« microscope iK such thnt tho aelincAtion at fitin liiii«, whiiMt 
)tit«n>pac(« lire one mirmn,' in juKt puimiWc, il in fruitlfss U)H>ur tii 
increase the Am|>)ificntinii )jov<>ml wlmt wt; knnw to 1w mOit'it'ut fur 
tbeir obaei'vation. Wp piitciitiiiHy diHirrmtiatc whiit wi- ur« [io<*'Vr- 

Thus it mny bo infermcl fnnn \.\ic ilitliiictiiin theory, u »uob, tliJit 
vrideaperturo Mhoulil iicccim[i»nv hi;,'h niiipliticntioii, wi<l niixlnrutK 
npm-ture be thp nccoiuixinimcuit <jf l<iw or iii<i()piiit<^ luiiplitiriitioii. 

The circuinKtiti]c<« «ii which what lius Ixn-n uilieil * pt-netratioii ' 
in object! vee i« ddprnilciit will In- nhcirlJy oi'iiMili-red ; ' it way bn 
stAtnl here thitt thi^irr iinil I'xfmrii-iice ulike shuwtliit 'penetruUori ' 
is rt-xluoetl witi) incri^iiAiiifc uperture uivlei' oni! iitiil th<! Haiti« uinpli* 
tiiution. As wtt hiivu iiidioiitMl, thi-ru are mimy tubjuctn u{ Htudy 
mijl n«)sin;h prvM-MilMl tu the biulof^isl for which b<< iiveda ux much 
' MMKitrutiim ' lis (BiHaible, TliJa i» iilwiiy-> lUe case whew tlie recog- 
nition of Bulid foruifi— iiB the iiifuaoria, (or exauipltf— is iiuportaiit. 
A (iiir vision of diH'ereiit pUmes at onct- U rwiuinnj.* lodeed tlui 
X're;n*r part of all iiiorpbol(>;,'iojit work i» of this kind : hei«, tlien, 
ill the woitin of Abhe, 'n proper econuluy of aperture tti of eqiuO 
im]:<ortiince with economy of jxiwer/' 

Wheiiovei' the depth of the oliject or objecUi under (ibsen'stioii ■« 
not very rejiirici*<l,aiid for tJio purjmseHof obw^r\'atioii wi- nH|uirf depth 
dinieiision, low mid moderate powei-s uul&t be useil ; • anil no jircntor 
*perture nhould therefore Ite used than is requiriHl for llie etfcctive- 
new of these powem — an excesa in such a cjise is u rejil danmsv.'* 

Moreover, in biolonical work -ooiuluni appliciition »f the inxtni- 
nient u> varied object« — lenses of motipi-ate a|H'rture an<l suitAlJv 
power fucilitAte certainty of action ami ouiH-rve labour. InoriMMi 
of aperture involves a diniinifihed workin-; diiitunce in the objcntitx^ 
and it Ls inseparftblo from a rapid increase of si-niiiliility of the 
obJoctiviM for flight deviatiuitij from ihe uonditiuns of pi-i^ect cor- 
rection. If it be not ttfrmfiry t<i encounter the possibK- dithcuttjtw 
tlic-Rc thinKK involve, to do ho is to lone valuable luontent*. Thwso 
dilKcultii'K, of cnurw, are diminished by the use of liomogeneous, and 
(!«|iecially apithniniatic objfctivf's, but e»'en with these thoy arc 
not, in pmclice, ilone nwiiy where the beht i-eault^ lire souifht. 

Emjitiit} tlf Jull nii'rlurr miiltthlr lo Ihr ix/ufi- umil. This ix tilt!i 
pniclioul maxim Uiught in eflivt by llio Abbe theory of microKCupic 

It luui been suyjtested that all objectives lie uiade of relatively 
wiilti aiH-rturm, unil thai they Im- "stopped down' by diaphrsgma 
when the work of ' lower aperluvps ' hus to be done. But this in 

> k micron i* u •• ,^,n min. Vidf Jaunt. H.31.S. lnn)4, ii|i. aot luid SU; and 
aVu(u(w.vo1. xiivlii. |i|i. 391, 941. > S«i p.Ha. 

*Abbe'ii rapluiktinit nt the raaacin of Uu nan-itertouiaiiin |irrveptic>D at Uwm i< 
gilranfHapp. B3 »( tq.). 

* ' 'rtw Relation rf Apertare to Paint,' Journ. B.X.S. mtrit* ii. vol U, p. 804. 






not A Miggi^tioti thiit ciiniiiK'iiclK iisclf tn the workiii<; iMolo^iHt If 
tbrtv werp no otiH-r dcfwU in «uch n method, the fact tJuit tlie 
wnrlcin^ dixtnnon roHWinx UDkltrinl would \>e fatal ; and we ouy 
fie}y lulojit the xtiit«iiirnt of Ablic,' that '*cientitlc work willi the 
Diicnncopi- will iilwiiy s rrt^ain*, not only high power objectives of 
ibe widnt nttaiiulilf! *p('rtur«, but nlso cnrpftilly tinulied lower 
poweni of snuill bihI vf^rj- inotii-riitr nporloivji.' 

We coiiipli-ti* thin Jvctioii with a Inhlr of nvmer'trnl nprrlur**, 
which will \w fouiiil on tlic fotlowiiiK iwigf. Ak nirtvuly ttAt«d. the 
r«uj|viii;( powi>ni i\n- pxiiotly pro[MirtioniiI t*> thl^nummlud iipfiiturw. 
»iid the exjirt^siim* for tliis UtU-r will nHow the rcHilvinK iMiwwrof 
ditferetit ol>jr«tivRt to Ix^ coin[)arwl, not only if the inialium bp the 
MUM' ii) eaoli. but al^to if it In* (lilTi-rRnt. 

Thf first (Nihiniii f{i»eH the nunimcnl apirrturvH from ■<0 to I'.W. 
The »«?ond, third, hihI fourtli, tin- air-, wjil«r-, nn<l "il' {or babuim-) 
KngteH of iipertun?, n.rn-spoiidiii;; to every '02 (if N.A. from 17° air- 
Mi^« (o 1(*0' baUaiii-iiii^le. Tlit- tlieoi-eli<.-Hl rwwJWjJK po**"' '" 
liii«!!i lu tbe tnoli is shown in the bixtli column ; tJie line E of tin* 
iip««truii) about the middle of the pi-en (X = 05269^), bein;; taken. 

The ouluimi giviiij; 'iliuininatiiiK jHiwer,' we havealready aei-n, 
ia of lees impoitance ; while it must be boine in mind iii usin;c 
the column of ' penetrating ])owei' ' tlial several data besiden 

— ^ to nwko up the total depth of vision with the iui«r<iiicope, 

P«netn»tiiig Power in Objectives.— Int^Uij^bility and M!<iuftn«i. 
tnair<' thitri ciistoni, sugyo-t llir cojikI deration <.if this subject nt thiM 
point. Tbe trufl meaning mmI real niluo of ' [1e]>lh of fiicus.' or whrn. 
is known «a 'penetrating p«iwer,' follows logically upon the alxive 

That qtutiCy in «i obj«vtirc which wns EUpi>aaH(l to f-ndow it 
with a cnpncitv of visiual mng« in a wrtiral dirn-tion, that i-s in the 
ilirrctiim of III*" a\ix ■>f viHioii, hns Wfw c/ittnd "pfni-tration,' it 
heinR KUppoKxl that by thin ' proprrty ' parts of the objnct not in 
tbn fooal plane L-oulil \»' nHicially pn-xi-ntiil, m> aa, to unable their 
perspective and otlier rotutians with what lim priMiitely in the fooal 
phu>e \*i be cleurly tnicnl out. 

Conc<:ming the niunnrr in which thi.i quality of the objeL'ttv^ 
operated, there have U-en nuiM divers* opituons ; indeed, the wholo 
matter wa.i involved in oliscurity. The remurkable innight and 
lennting of Prufesxir Ablie liav«, liowever, found fur thin imjjurtant 
■object a sound Miientitic Ixutiii. 

The deliutstion of iieiul objecttt by a system of leuHes 'u, by 
virtue of the niont general lawH of optical delineation, duliject to a 
pecular diitproporttou in ainpliticvitiou. The linear auipHl^eation of 
the (/'7//A-dime»aion is, when both the c>bjeet and the image are in 
the same medium (air), found to be always etjual \ti tlie tqnart of 
the lioeur aiiipliti<nktioii of the dimenaiona at nght angles to the 
optical axis ; but if the object be in a more highly refracting mnlinni 
than air, it is equal to thin square di%'i<led by the refractive index 
4>f tlie medium. In proportion to the lateral iimplificntion then is 

> * TIm BcfaUoa ol Apcrtara to Fovei,' loitris. li.JI.S. toriu ii. lul. ii. p. SOe. - 


I itlS> 

Iffl^V S.A. TABLE Etc. 8$ 







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=i- Q 

o >« C _■ ~ ^ ~ 


I 4 

■ -S '-" 

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_ -, r- _ .» _^ = 

= Sl='bi 


S'ti'o fi.S S-^ °'S ~ =o"£ 







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t g X 9f TT s £ sfi^s ^« 5 X 9 £ sa ^s 3 s SE:i 

fS -?.^ — 
















£ 7 

a DUO o qqd □ 0004^00 oooooooOLl 

§ i 

««rH oa<D t-s »^«e « ^ O et B £■« lo 


X-A- TABLE Etc, 

1 X T-* — io tfa -^^ CO ^ t^»a^ttsr-flc«sot*o 
1 1- *5 T ^5 j^ *— -^j- S «04wi3iou-^noo<0Q 
PI- V 02 *— c^ k:= CO ^ ^^iCO'sna'-tnooQ^ 

■ ?I ■5* ^- i S *— tC TO J^ -f ■«■ w e-1 CI M ^ — i: 

* — t* h— f»; ~>^ *-; to r— cjaCC3Q^'94r^r^-f-inv>*Sa 

1 1- ■:! ^ ir: "** r^ Oj -— ^- O a S_ cd i— ^_ t-_ ia_ t_ rt 

^ , ^ ^ ,., - . _ _. _. _ I- ^ t; 13 T rt d««i 

— rlt^e^io'K -- N 1^ r^ cs' id" ?f o" 6C tT'ii -* crcratTo rf 


> --• 1^ iS e» c~- ^ O rt -- B I'; ?i a c; I- -r — IX W 

I — f^Vac— ac t^^cii^^ 

»«oo o o o D4oaoDooeDOQoaa 
Ig ft w a gi g* c^c^ -^ r^^ ^ r-* r-^ .^ «-i ^ 

>66 6 0060060666666660666 

. 3 


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18 vi-j-.N -wnn THE I.-. •3ir.-.i-sii microscope 

1 imtTt—iv*-. ■iii'l Willi hiji j«owere a rapidly increasin|f, 
• „nJijir,ii,..n oi tl)i- il«pili nf ttir ifan«<(limeniiii>nBl inuige. 
iniiiM'i'nr s«t.-iiiiD <if an obitx*! i^ utaenified 100 times in breKltfe 
LlistATicv t»-'.vn-n ilie [ilantni of pATte Ijm^ one behind the othi 
iii^i^liiiiivii l'.'.'.".>0 tiiiit^ at the ruiTespondini; parte on the azii 
tlio •>l>'>v: i^ in nir. TTiOO tiiuei^ wbt-ii it i» in water, and 6600 
« hi-n i; '.> ::. lVt;iMil» Iml^iani. 

T\:-p^ • \.>-<«iii- tlistiirtion in thf case >>f high Bnipliticatioiuu 
l...»iiir. -i-l'' s,! c<inipl«lc II hindrance to ci>rreft upjireciatia 
f.>.'.i'. ii--r:.' ■■". "''"■5" iiiii'roMMpical imase as «t first iippeare. 
:.i.i<-iv:».:' . ■: * ■'■■ ;>"!" Is not " iu,.ttrriif wnsatum only ; it 
1,1. -It.. :■ ■ ■' .'■■■r ■we': ■■.'■•.— mid, tWrviore, the peouliarity of 
,,i.n,H.' i>'«-'"i- inwt the Htt.i'litiLMtioii, would not pi 


:iv -Aiiiiiirv nt" the ■■l-;-ft «> long ;is hidient pd 
. :<• .-1- ■ .[ I '/i.-'-./i'iH. «*.■..;..>/ ini;ij,t? Weil" fiiund. I 
.... ..•>i;t. a,-. »uol). iiiu~: U' ^iniultaiii-ouslv viciU 

. ■ -il'tm-'-iiibU' ilejitli i-jn ('oiivey no (.imceptioB 
. . .11 ii.-ii.iii'> iii.nsi'stfnl hy the "tijeot. 

, . 1 .i>|.'ii'j.)iii'tiiiiialiiTinililii-;itioiiMt the dojith-dilni 
. .,.'.11 "t iiiiiiLiiI iiisiruiiicnt^ the visual s}noe of 
..... '.i.i>'.iiidniiiri-inilepihn<ihe.iiiiplitii?]itioiiincnMd 

..-..t >i,t >i|i|iro\i II mil's III ii line luiri/ontiil seetiimof tij 

... L. -j..!!.'!'. HlLicli lit one iidjugii.^r.; -'t tin- tVious is pUtol] 
.,..c I)' lit Iwi) iHLi'ts. the limii! '::' which <is reg.tnls th 
.1 'iiiiiioi in !i very dillerfnl nii"..:,Tr. 

■,.i,u<"hitv'ii !•/ tlf '■•I' t-i:/rr-v.v* a certain depti 
..!,.> 'villi; successively depicted with jierfect sliarpnMI 
. !i< iiMiiii, whilst the eye. ndi'^-fi.: itsi'lf l>y cimsciooi 
, ■..- itiiiunuiitliiiion, obtains virtuil iaaces of greater* 
; ii-i..ii. This depth nf iici>T^:L'fiaiiim, which pl^ 
v.iicr'-fOipical as in or»ii:.iTV viMun, is who 



Hwm figorc* are modified li>- Ui« ni«diuia in wliidi llie object b 
placad and by th« gr««i«r or Ims Hliortnesei not] \et\gih of vi§)oii. 

Secondlji the peiveption uf (lupth is aiuiUt<.<d by the itiseiuu- 
liolitj of tti« «yo to Bni«ll defecta in ilio union of the m}'B in tli« opiio 
iHUkKv, nn<l (horafore to&inatl circleaof confusion iii tbeviMinl iniogCL 
Tmuveno tections of tbe object wbicb Are a liltle abo> e and below 
Uw «xact fooul Adjustment an aeen nitbout prejudiiriiU efTectn. Tke 
totnt rffnet »a oklalnril it lltr to-fi/lrti jiriiririHiiiH or rif/i/h tif fimi* 
"t an ot>j«ctivr. This may be dotermined itnnipricjitly by rlrbniog 
tltn mIIowuMb iDftgnituilc of the circles of c»i)fiiMon in the micro* 
wopiaal iBuig« by thv viKUnI anglo under which thcv appf*r to t h« oyfi. 
It is found tbiit cino minute of arc denniCK the limit of Kbniply 
<lefin«il viKi<H), two to tlinw minutiw forfnirly dixtiiirt viHnn, mid tivn 
to six minntcs tJio limits of riiiion onlyjusit lolrrnhlr. Thin U-injc 
detarmined, thu focal ilrrptb dvponds niily on th<- rcfi-nrtivr indi^x of 
the luedium in whicli thn object is ptnccd, the ampliliuition, and the 
Aii«)a of ajierture, niul it a dirvctly nroportionMi to th« ^«■fractivl^ 
iitdJex of the obji-ct mnbuni, and iitvi-rsely projiortianiil to tlie 
* numericul apvrlarc ' of Um: obJ4«tir<', n* nl»i to tti« Hrat [lower of 
the ain|>lilic»tiuit. Thcae lUHtnoc tb» vi.-^il)!!; iin^'lc nf ullowuble 
indiHtinctneaH to bn fixed at y, tliu nptrrture itn^lr iif the im&K^' 
forming |H?»<.-ib to tm 60' in air ; the itejitli of focus of an object in 
*>ir will llieo be — 

0073 mm. for 10 tia:ie«nrD)t)i(iR*Iian 

U-OU „ 31) „ 

CHWJa M 1"" 

O-OW* .. SOU 

o-tnon „ 1000 

O'AOOai „ 3MK) „ 

Hy limiting or enlaffcing titc allowable mnpiitnde of indislinctnnw 
in the inMg« we corrrsiiniidingly modify these fii(iii"es. n» we should 
do vritb media of different refractiv* indices and inci-eiisnl iiprrture- 

It ix ptnin, tbi^, that th« actual Jrpth o/trUion muat iilnmyit bo 
the i-xact sum of thn aeeommotialian. d^/it/i and /orni rlr/it/i. Tbo 
fornNtr exprejiu>« Llui object apace through which the iiyn t>y tlif^ play 
titttccoinmcHlation canpi-nntiutoand necuro a xbiirp imnjit! ; the latti-r 
uyrta the amount by wltich thia object- IlplUM^ la extended in ita 
Unita— reckoning both from abovv and below'-becniise without 
pcftfnct aharpn*!** of imago there is stitl a aufficient dintinoUieiat ol 

A* tlio amplifioatioit inciensrex tbe over amplification of the 
depth -dimension pmuntH in«roa«nalyunfnvourjililei»?liili()n liplween 
the depth and width of the object-KfHicH ocvetuiible to uecotiimodation. 
Wln-n low [toocni are employed we )i>v» n^lutively <;re»t depth of 
vUon, because we have lai'^e aecommo<i«tiun-<leptli ; but lui we piLw 
to nedEoin powers, the uccomuiodutiou- depth diinini^lies in rapid 
ratio, bewniiiiiK e(|ual to only a small depth of focuH ; while when 
the uagiiifyiuK po'i'er is greatly inoreiut^d the accommodation depth 
is a factor of no moment, ttnd we liaveH-ision largely, indei-<i alinrr'.f. 
wholly, depeudeut ou deptli of focui. 


^^sIO!^ with TnE coirmrsD mctroscope 

Till) folliiwing talile sUov.'s the totnl itrpth of vwioii from twi 
3,000 tiniPK :— 



lull [V[l(]| 

FolhI Drptli 

Dfpth at riiNni. 































It haa boen pointed out Uy Ablx- tliat lliis over-ninplilicfttion of 
depth-ditiietiMdu, though it liuiibi the direct nfiprecintion at solid 
forms, yel. is of feTwit viiluu in extending the indirect recognitiim of 
R[Mce relation*. When with incresseof luajniifvinj! power the (ipjitli 
of theim(igel>pcnme>iin<ti'Pttnd more flattened, the iniagi>of diffemtt 
[ilanf s stand out from eiich other more pei-fertly in the wimc ratio, 
and in the same degree are clearer and more distinct. With an 
Increase of amplification the micro(ico])e acquires incrensingly the 
jipoperty of an optical miemfomr, whi<.-h prewnt« to tlie ob«r»Tr'a 
eye sections of a tinenem and sharpness which would be trapouible 
to a nieohJinienl seption. It ennliles the observer by a ncrirw of 
adjustments inr consecutive planes, to roHs/ni" the solid forma of 
the smJiUcst natiirnt objects with the same eertninty iw he is 
accUAtomol to »f with the nitlced eye the object* with which it ix 
concemi'd. 'I'hiH is a. Ini^ ndvnntAge in the general ncii'ntilic usn 
of the inxtrumcnl ; a Rrcjitcr gnin, in fiict, than ci'uUl ha exp«tctiil 
from ihn niiiilii'iitinli of Ktcrrosciliio iihBcrvntinn. 

Stereoicopic Binocular Vtiion.—Thissiibject has been eJnborately 
conniilfrc't niiil pitrtiAlly expounded and demnnstrnteil by Prti(*Ktor 
Abbc! : his exposition itiflers in ^ome important particulars fraiu 
that of tilt! original author nf this l>onk, but in its present incomplete 
fiinn it appcoTN to the editor to Ijp the wiser way t<i allow I>r, Cur- 
penler'H treiitinrnt of the subject to litand, and to pliice l)elow it aa 
complete! a diKust of Professor Ablie'a tlieoiy and explanation of tbe 
same subject us the <Uta Ijeforc un will admit. 

The adiuii'iible invention of the ftrrron^ti/f by Professor Wheat- 
alone hoK led to a general uppriH'iutioii of tho value uf tlie cimjoint 
■u»« «/ birih 'y.' in cuDve/ing to the mind n notion of the mlid/orma 
I.A objects, 6ucli AS the use of either eye siuKb* ''"*^ ^^^ f;enerat« with 
the like ceilainty or effect iveii ess ; and iiftt^r several attempts, 
which wci-c attended with viuious degree uf aucctsa, tie principle of 


the vterMMoope has now be«n npplied to tli« microttcnpe. with nn 
•drontagewhidi lliuae only can trul j mUinntc who Oiki-- tiw Author^ 
linva been for sonio tim« acc-UHtoiDml Utworkwiili thi- xU-nioicopii; 
binocnl«r' upon ubjeds thnt iii* pomlinriy minptwl to its powon. 
Ax thf rrault of thin a|»)}|i«iti»n canimt In- riuVilly umlt^ntooil with- 
oatsomc knowledge of one of the fundiimi-nlal prinoiplea of lnnocntl*r 
TMon, a brifif lu-coutit of this will be her* intro<Iuc«(l. All \-iMoii 
(Ip|imk1s in thp tint inHiJkuiM! od the formation <»f & piotum of tltc 
oligect upon tho rvtiua of iht- (■ye. just a« the <.-aiiier« obscnra forms 
K piHum upon thei fj^mincl f-ltaa pUcnl in the focui of its leiu. But 
llw two itnngrH ihnt are foniie'l by the two eyes reapeciively of any 
■olid olijoet tiMt ta pboed *t iw greitt <l)i<(ant« in fmnt of tliein are 
far from baiiw ulentjcsl, the jwrapective projection of thu object 
«-arj'ing with th« point of view from wliith it is seen. Of thin thu 
mwlrr mav ouilv conrince himself by lioMin;; up n tliio book in 
SiM-h A poattion that it« back shall be Mt a moderate diatance in front 
trf the tumn, aiul by lonkinK at tJte book, tint with one eye and then 
with thi- other ; for lie will fiixl that the two views he thua obtainx- 
are oMa-jitUly diHermt, m that if Ite we<« to represent tJ>e book as 
be actunlij aeea it with each eye, the two pirtnreK woolil by ntt 
inmns oormpond. Yet on looking at the object with the two eyes 
ciitijoiiitly, tlMtrp isnneonfiMon liotween tli* iniag«:s, nor does the 
mind tlwdl on cither of theni singly ; bnt from the l>lrn'Iin^ of the 
two a conception is |p»ined of a solid pmjn-lin^' body, facli ai; could 
only la- otherwise aoiguired by the «ense of loiicb. Now if, instrail 
of lookini; at the colid object itself, we look with the nt/lil anil if/l 
eyes respesiixely at I'iriurr* of tlie object, i-orrespoHrfin^r to those 
whicli would be formed by it on the relinie of the two cvei if it weny 
placed at a nimlerate distAnee in front of them, an'l thi^r \~iMint 
pictares are brought int4i coincidence, the winie conception of a solid 
projectinj; fonn is generated in the niind, a* if the object itself wen- 
thure. The stereoscope -whetJier in the forms originally devimvl hy 
h<of(CRor Wheatstone or in the popular modiKcation long mhm- 
Qtiently introduced by Sir 1>. Brewster— simply srnc* to bring to 
the two oyea. either by redexlon from mirrors or Ijy n-fmction 
through prisms or lenses, the two ilissimilar picturr* which would 
aeoarately rrrprrst^nt the solid object aa set-n by tho two eyi-s reaMt*- 
lirely. tl>H»n Ijping thrown on the two retina- in the jireciae jMaitinns 
tliey would have oceupied if formi-d them ilin-ct from the aolid 
uUect, of which the mental imag*- (if the picture* have been correctly 
lu:«n) is tlie pnH-i>u> i^mnterpiirt. TIiu.h in fi^. 69 Ui« upper pair of 
piiCTturea (A, B) when combined in the atervriKcope Hu<,'geat the idea of 
n prtijiyiin'j trunciited pyramid, with the !<mall Kijuare in the centre 
Biid the four aidcK sloping irqually away from it : whilst the combt- 
nation of the lower pair, C, I) (which are Identical with the upper, 
lint are transferre<l lo oppoaito aidM), uo leaa vividly brings to tbe 
mltid the visual conception of a. mvttinff pymraid, still with the 

■ II lis* bemme uowi—jy to tUitioKaiali llu biaoftiUr ini(ni>K<>[iii wlikh ghvs 
Irac Hrr»nf«ffU *4wl> Itjr (Iw omnliiiii^cm nf liru diHimilu piolatvt trnin a 
Uaotalar wkitli amply euabloa ua to luok <nUi holli I'y" >>i inuf** which an- 
(■■aUallr UmiUmI ip. innj. 


nsioN vmn THE coMroiTTD sncnascoPB 

MiiiiU Kijuurfi in tbo contiv, but tb« four sidtst Klojimg equally < 
\var(U it. 

Tliua »•« %t» tlutt by «tiii[>ly ^rominij Ui« pictures in the stproo- 
8co|>e, at) AS to bring bofori' eutcb cyo tha jiicture lak«ii for the otiier, 
a ' coiivemioii of rulif^f ' igi pruclut^il in Mm rtwuUmg solid iiHAgo, 
the projecting pnrto b^uix uuicIk to recwlc and the receding pftrU 
brought iuU) reliet In like inanm-r, whttn w-wml objects nrr oMn- 
bineu in tJie K»in<> uroased piciui-e«, their iijijMin-nl relative ilistAni;<w 
»■« revenied. lh<-- i-eiuuter buiii^ brought nearer and the nenmr 
tarried backvardH ; so tJiat (fur cxaiiijilt^) a sWreoacopic photognph 

Fio. se, 
repreiBenting a tiuui sluDdiug in front of a mnss »f ice sliall, by the 
eroedug of th« pioturea, make the lij;ure appear as if imbedded in the 
ii^e. A like ooiiveraion of I'elief may also be made in the ciiae of 
oi-taal Holid objecta by the une of the /•ii^udo»wp«, &n instrument 
devised by Professor Wh^ilntone, vhith has ibe efiei-t of re^'cisiu^ 
the perspective pi-ujections of objects seen through it by the ivio 
«yes mpecti\'ety : »u that the interior of a basin or jelly-niould i« 
luade to appear na a ]>rojoetin;; eolitl. whilst the exterior i» made to 
nppwr hollow. Hence it is now cuslomaiy to *peiik of "frrmteopir 
vinoD as that in which tlie conception of the truenatunkl relief of un 
object is called up in the mind by the normal combination of the: 
two pvrspectivo projections formed of it by the ri^ltt and left ©yes 
n>«pcctively ; wliilst by /iMudoteo/iie vision we mean that • conYW- 
won of relief' which is produced by the combination of two rrffff^l 
pcwja-ctivc pmjecliona, whether these be obtjiincd dii-cctly from th<i 
object (aa by the ps"udosco|>e) or from "crossed ' pictures (lui i:i tho 
stereoscope). It in by no ineans every solid object, however, or every 
pair of Btereo*ri>pip pietun-K which can Iwcomo the anhject of thin 
^■onversion. Tin- degnre of facility with which the ' convcrtotl * form 
van be apprebi-ndol by tli« mind np|>i?arK to have htreat influence on 
the rmdini'ss with whii'h the change ia produce<l. And wliiln there 
nre some objects the interior of a pituiter mask of a face, for ex- 
ample — which cun alway* l»e ' converted ' (or tiiriietl iimide out) at 


oiM*. there are (rtliers which reflUt such coureisitm widi more or leas 
o# per«ial*nfe.' 

Now it is easily shown thenreticully that tlie piL-ture of mir 
projecting object Been tlirouf;h the iui('ro«oo]ie with ouly tile riykf 
jutiid halif of an objective having an even niodeiute fui-jle of aperture, 
must differ Mnitibly from the piciui-e nf the same oliject received 
ihruugh the /'fi h.-«r.<I of lh» «ntne objective : and, further, that the 
difierpRoe between such pictures must itici'«MH> with t)ie an^^ular 
aperture of the objective. This (iift'enmee may be practically niiule 
appAmtt by ndAi>ting n ' stop ' to th« ohjectivp in »uch a mnnner ns 
tiic<iv(rr eit'hiT the right or uic left hiilf of it« aperture, and then by 
<ttr«^nl!y tRu-ing the outline of thr nbJMrt n» seen through wwh hall. 
Ilut it IK miirf Ml tisfitol drily broufiht into viirw by Inking; two plinto- 
giu|>)iii: pirturri nf the ohjtvt, onr tlimufrh rrtdi Intirnl linlf of tlie 
ol^iectivc ; for thpw pietnrc* when pniju^ly piirwl in thi- steri^- 
•CO|)a K>vc ft magniticd iinn(:i- in rrlir/, liriii]inng out on a. Iiirjic Kcnii- 
th« solid fomi of llin ohji-ct. fmin which tlu'V wi-rc tnki'ii. What iK 
nwtlcti, tlM-nrfttn-. to jiivf th<? true Kt^-wospopic power t»i thi- inicni- 
soofM* in It inmns of m> bix^ting the cone of rayn tninwiiittRd by tlit' 
obJ4>ctiv(> that of itH two lat^rnil halves imi- nhnll lie tniimniittivl to 
tht! right and the oilier to the Wt eye. If. ho«*fvrr, iIm iiiM^ thuft 
formeu by the ruilil lialf of the oliircttve of a coninuund niicroRcopi? 
were seen by the ri'jf-l eye, «tnl thut fonniil by t^e Irfi halt were 
seen V>y tJ»6 Itfi eye, tho resultant etiHf«j)ti(m ur,uld be not tUrro- 
nnrpir but fttfttii'itei-jiit, the proji-ctinx parts being mode to appear 
receding, and n>>> ivr«i. The reason tA thin i.t, (hut iix the niieroitco[M- 
itKclf revermw the picture, the m_v» proeri-iling thntuxh the riyhl uiul 
the UJi haiul halves of the objective niUKt Ih- made to croui to tlie 
If/i and the ri^l eye* re«pt«tively, in order to correspond with iJie 
itirrrt v'wvr of the object from tile two siilc* ; fi>r if tlila secudld 
rvretval doea not take place, the etlVt-t of the first revemal of the 
iinaf^ prodttced l)y the inicroKcu]>i- exartly eoriv»p<uid« witli that 
prniluced by (he ' craasiiig ' o( the pictui^s in the stereoiu»>pe, or hy 
that reversal of the two perspective projeotioiiN formed direct Iroin 
the object, which ih effected by the psi-udoiicdjie. It waa from n 
wiint of due appreciation of tins prineipli- (the tnilh of which can 
now be practicnlly deruoiintrated) ihnt the i^ariier atteiiipta at piTi- 
dneingft stcninncopic binoctilnr mii-'rovco|)e tended ruthcv toprocluco 
a • jiseudos'-opic eoiiverftion ' of the objects viewed by it llian ti> 
represent them in ttiiK true relief. 

/n tontrniiiiilittrtii'n lit tliin *jydiiiiiilii-n of binocular vision Dr. 
Abbs, a« wo hai-e seen, has demoTistmted that oblitjne \isJon in tbi- 
■Bicroewpe is wholly unlike ordinnrv' vision ; then? is, in ^et, rti> 
peT«pective. Thit pcmpertive shortening of lin^s and Kurfaciit In 
'■tiliquo projection is entirely lost in the microsropr, and, lu n cun- 
arnuenoo,' it ii (.■onti^ndiil that the Kpetinl di>isiniilarity which is the 
niMon rPAre tii onlinnry stereoscopic effects dues not exist, but that 
Ml euentiiilly iliffeniit rooilc of diuiniilarity is found 1>eLween the 
tvo pietor'-x. Tlie outliiif or ctitilntir of a niicrosciopic object is 

< Fur a toUa disciuuon of thit tubjert mf tlir Author'* Jfinfol Phgaotogi/t 


unnlterecl, whcllier viewed hy an utiftl or mi nlili(|Uf ]>en<.-il ; tlioii; is 
no farvsliort^iiiiiiij, ihere to simply Ut«mt dUpliU'viiirnt of the iinits;ee 
of conweulive latere. But AMwoOMUtDilKtbnt, whilst th^^innntm' in 
wbicii (iiMiniUAr piotum on> foniMxl in thr biuoculnr tnicroacope is 
<Uflbreut from that by which their nra broUKlit nbout in ordiniu-y 
ateraoscopio Tision. ypt the nctivities (if tlir bntin ntul tnind hjr which 
lli«y are so blcndot) m to fn^e nsp t(> s<-nsntioiui of noHclity, (Icptli, 
arid pers])©ctivc an- pmcticnily tdMitioO. 

The fact thnt Intrnil ihiLjilac«tii«Rte of thn iniAgn ttrc *et>a in thw 
microscope doprixlK on n pi-culiiir jimpcrty nf uiiiTOKcitpic ninplilitui- 
tJoDi whicli in ill Ktroiij^ imitnist to the lueUiod i4 onlitiary viKion. 
It dopondii rnlin-ly i>i> the fnct, eiiuticinbil ii)h)vc>, that the *iiiphfi- 
OktiOO of thi^ dnpth 15 inrgcly (■xaKK<'mtr<il. Ili?nL-<- noliil viiiioii in 
til* binooulAr miiTusco]!!:' is contint^d t(> hiTfff uiiil ciuirae objiwtit. tJir 
dinwuionii of whiL-li are litrgi> uiuitJpW uf tlio wave- It'll fi:tli. It 
Uieroforft follows tJwt when uiodem« or larK^ iii-ertui-oi hiivi- to Ix* 
employed — thnt xb to say, whenever deUiieAtiour«t)uim( tliL- employ- 
mxmt of ubli(|u« rnys tli« (-lenients of the object nrv no loiij,'t!r 
flopictmliut itolid ubjei-tsaeeu by the- llal«^deve or tliruugh th« t«lci«oia- 
would be dopioted ; uev«rt)iV'Ieea the brain iLmn;^'H tlivm wi thnt 
th« cliArnotvmtiM of lulid viuoti iir« still present«(). 

PmfeMor Abbe deniotistniUa ' that iti an npl&natic nHt^tii ]]oncil)« 
of (Ullereiii obliquities yield iihntioil iinaKea of everj- pliinr obj^rt, 
or tif a HinK's l^yer of a solid object. This iit true howei-er large Um^ 
nperture may be. 

This oHrrieit with it, as we Imvo said, a total aWnce of ptTxpw 
tiv(! nnd lui rsxeittial jteouietncnl difTereuiie betwtwii viswiv with tim 
binocular mieronfopf and liaion with the unaided eyr. 

An object, not quit* llat, asa cuned dialotu, wlitrn obsen-cl wiUi 
All obji-ctive of wide aperture will present points of gretit indixtinrt- 
nau. Thia hu been by snme liupposed to arise frotu the assumption 
that there ¥ras a dianmilarity between the tnwfi«s fonacl by the 
«xial Hiul oblique pendls ; but this is not so. It b wholly cxplic- 
»ble by tlie 6wt that the depth of the omect U too ifreat for tJic 
jimall depth of vision attendant' upon a large aperture. 

It will be seen, then, that ho long am tlie depth of the obj«ct in 
within the limits of the deptli of viiiioii. corresponding to Iheapeilun- 
an<I aniplilitvtiiun in use, we obtain a distinct jMralUI /irtr/rvtiun of 
all the successive layers in one common pJaoe pecpendiouUr tJi tlw 
AXIS of the micro8c.>]>e a ground ptnn, M it were, of tlie obj<K-t. 
ManiSMtly. then, since depth of vision dccreas«e with increa&ing 
Apertun, good delinestion with thi^^ must be canfined to ihinitrr 
ttbjtvU than can be successfully employed with objedivea of narraw 

Stereoscopic vision with the mioroacopc, therefore, is <)tie solely 
to difference of projection exhibited by the difletvnt pandlactic dii* 
plarementd of the images of iiticcctaivo layers on the i-ommon i;rouiiil 
plftne and to tlie jfr--'fition of' rlnfilh, not to the delineation of thn 
plane layers themoelvrM, For, if there witr dissimilnr imaf(«s per- 

■ /ewni. M«iii ii. rol. W. pp. 91-31. 

ABBE mt HTBRGOficonc iisinif 95 

oepdblv kt <Iifft>reiit ptanea, tli« out-of-focus Lajren musl •|>]M«r euo- 
(tned mihI ■■<> rinii/n o/tUfilh would be potsiUe. 

Now HterMwcope vuion requires, «8 bIkiwd tijr Dr. CMrpent«t', \imt, 
tbe deti>i«atin); {wncila shall be so diikl«d that one portion of the 
admitwd i-oue of light is L-onductM) to ono eye and Hnother portion to 
the otlier eye, If this diviaou of tli« image iseflectad inasjrnimeitrical 
way, thecrvEssectionof, ^.j^.a circle must beradarad totwoscaBictrclea 
npraentingoneoC these two nrrangemeDta Men inOnodP, lig. 70L 

% I 

Vm. to. 

Dr. Abbe'a tlMfory is llmt the only coiididoti iiMfvvary frr oWAa- 
«n>/(t« allcct in any binocuW KyKM-in i.i that thii c iviiiinrdM or 
their niuivn1i!tiU (Jiould Ihi di-pirt<il nccuriling to diagmm O, 6g. 70, 
and iiiT i-^ud'trojiie irffHct ncoinling todiagmrn I* in t1irsiiin<- figure ; 
mkI ho drmonBtiatcM that all otiMtr circun)«tAno(w, m«h, o.g. hh [lie 
croaaJiur of th« imAges, are wlwUy iiumutcrial, 

Orthoacopie vitiiou in idwivH ubtaiiied trhtn th« right half of tlie 
right pupit and tli« left hiilf of the left pujiil onlv are employed ; 
p«cu<i(Bcopic rUion in tlie oppoftiM (.■<>nditioD& * It in quite indlf- 
i«r<!nt wWllier the effect i» obtained with crciHiiig er non-cnwaing 
rajK, whether tJie imnKe )>« erect, or invertnl, nr semi-imerted. and 
whntu%'er luuy be eom|)oueiiU «( tlie opti(.'al iirnui^ment.' 

Tbe olMM-vaiit n<)ulrr will ])erceive it ix at this point thitt 

jbvre i* a radicaJ div«r;|(enoe from the int<-rpnitntion eiveti by Dr. 

rflbipcntcr, wIh\ tt% wo hnvi- ««pn nlmve, inxixtiHl lliat ortliuBcapie 

viaion is not to be <i)>tiiiiiii] in ii biiiuculnr with niin -erecting eye- pieces 

tinlc«i tl>e axeet of the two lialvca of the admitted oooe from «iM 


Of course we niuitt keep clearly before us tlie fact thtt in micro- 
MopkC Ttnun it in n<it tlie object but itd virtual tnin}(i' only llmt we 
MV. This a|<]>arenlly »otid inuuje is placed in the hinovulitr micro- 
■ewe ander cireuinsUiMies ibuilar to those of connnon ofijrrti- in 
m&Mry vlsioiu Clearly, then, it is tlie pentpective projeetioiui of 
tUa mti;/f wliieh tvqulre to W ompiinxl to llie pii>ii><-tioiie of solid 
ot^ts ill ordin«« vision, iu inspect to which tbe critiTiji of oriho- 
wofne nod pseudosoopic vision liave been dellneil. Hut it can bo 
gHunetricidly deuion&lrHtcil that ri<;ht-eye porspecLivf nf the ap- 
parently bob d imn'jf is AlwayHobtninoil friirii the ri^'ht-litind portion of 
the entergent pencils, left-eye pei^icctive frum tlie left-luiiid portion ; 
and il in ijiiiU imnitiltfial, o* rtganUlhi* rrMitt, uAieh imrlian of the 
tmrrtfrut rays it mimitUd lit/ (Ac ru/Al or fAt U/i jiarl nj ihf <A>}rctiee. 
The innnucr in which the delineating /"uo/« are tmniiniitted 
through the system nuty be nuch ilk to r^iuira crosung over of the 
Iky* from the right-hnnd luilf of the objective to the left eyepiece-, 
attd vw* vrrwa. But it b not cMOntial to Inniicutiir elTect. In the 
Woihun and Nacfaet binocnlnr (pp. 9>*, 99) cnasing nwv is requiral 
bteaiue the inremon of tlte penciln in not ehiinj|:«l by two reflexiona. 
It the delineating pcnciU have been reflected an nvn ntmi&irrof timea 

UNOCt'LlH JIICntist'OPE!* 


in th>- wiuH' piniif, it will be neoevMry for tl)i< nys {n ctom ; liut if 
tliry havi' liMti rHIvcttfU an mid nuuiWr of tiiiie^ ii U not only un- 
rnomuiT', boi in (IpstJTiuiive of orthci^cotrtc eHwt. jirovUlHl unliiuii-y 
«yff-[>ipcr» (mni' ertx-'tiiid) are MiiptoywL Ht'iicc iii theStwiiliffiiKoii hin ■ 
oruUr it in iwit onlv liol iwjiiiri-il, l>ut wouW^fin' p«?UiUiNfu[iic(?fli!Ct. 

Friacipal Forma of Biuoculai' HionMcopes. — Tlit> lint )>iiM>cul«r 
of a pTKclical chitmcter was Un? arrikiiKeiiietiC I'f Pr»fi-(i»)r J. I^. 
Riil(k'l), of Nt^w Orleans. It wm <levisiHl in ICkM ati<l coiistructccl in 
IK'il'. mimI a i]e8cri[itioii of itn luittiiv .iihI it£ f^iie«is wa» giveti l>y 
him in tli« aecoml >oiiiiiieof tlie tint Mri<« o( the ' Quarterly Joanuil 
of )Iivraeo]»cttI Soienoe ' in the year l^Til.' 

A rep«tiit.lH»tio» of hisori^iiiinliiMtruiiieiit ia))r««riite<Uii li^. Tl, 
And th« amngMOenC of llie prluus by which the hinoouhtreSect waa 
obtaiuMl i^ «hown in fig. i-. 

It will lie Mwii that the peni-il of mys «in«rpng fmro the ImcIc 
lens of tl>e conihination ' in ilivideti intci two, each Imlf pitasing re- 
«peetiv«ly into tlw right niid left piiains ; the path of thA raya U 
iii(it(Tat«<l at <i, A, e, </, tJin <^j«ct being at o. 

To •tfctnv <s>in«td4!»ca of tlie images in tlM lield of rww for 
varying widtlw^ botwrm tli« oym I'rofmsor Itiddell ikvisod (!) a 
ineana of regul»tinf; the inciinatiim of Iho |iriiUR« hy mounting theiii 
in hiaged fmniCH, so tlml, w}iil« thrir lower rdgi?s, ii<uir n, 6g. TL'. 
remain always in fuiislliO i-ontiurt, th4> inclination of th? intc'i-nal 
ndectio;; sorfncr^ cnn lie vni-iwl by the iiction "f thn niilird head in 
frant of the pritni )h>x ; ( i!) th<i 

lnw«r eiids of tlw hit>ui-iiUr l-r-r* Vi— ra' 

inhes are cotntectrd by tmvpl- 
HuR eoeketa, nmving on oua 
and the sane axis, <m whidi 
are cut oorreiqxinding right- 
ami b-ft'bnnihHl scrrwK, Mithitt 
the widtli of tho tul>c« luiiy 
oorreapond with that »( tho 
]>riHiB8 ; and ('i) ihn np|M-r 
emls of tl)(> ta1>Cii ni« r>»n- 
nect^ bv nu'kn, onn iii:tink; 
ubovn ann t)i<- other Iwlow tin- 
iuiin<- [niii'iii, Kothiit ri)j;ht' ami 
Wl-hiuuh-d inoriMuenls are 
coatmnnicatvd by taming the 

Iliiii iiut^uRient eouhl otdy 
Iw uhim] in A vcrtif-jd pudtiiM), 
att abiiwii in tlw^ ti;{Ui« (71); 
In nttvinto tliia <.'ouiii<lorHhIe 

drawlxw-k Ridd<?ll in<.iint«l two right-angled F"""* >" '"'"»' '""I'n 
whiph limld U- [.lipiNe.1 over the cypiwes. Tliia iirniugemeut in- 
vert* ihi' iinag.- in A"fA planes, and it is seen through tite instru- 
tunnt an in nature. 

Thi* Nyiiteni of bino«u3ar excited much interwt in EiigUuid im- 


Pill. T*,— Amiii)ii'n>i'»l 1^ I>rlun* in Na«1iel'« 
<Ua«DHicii"c biniiculu' TnirrcHcupa. 


98 VISION vmn the rosrporxD ancRnscopE 

mediately after its )>ulitiuntii;>n, and Mr. Vciilmm in r>mtloii ftn<^ 
MM. NucheU of Pmir, booh sugg(Mtc(l nnU il«vi«?d a vjirirty vf 
Imiw-uUr svstpniK. 

VulMt's Binocular. ^O no of th«-AC {not now, we have i-wunn to 
l>e)tpv«, iKlv.Hnii'cl or piiii'Inywl liy its invrntor) vmn thnt devUftf! lij- 
MM. Nacliel, constmctril mi tlii" muthod slmwn iu lig. TS. The 
coDp of rnvfc insuiup from the Imrk lens nf tlio olijei'tivp ineetB th«' 
flat HUrfaw of a jirism (/i) (lUciil nliovc it, wlmse secliim is an e«|ui- 
Ut^Tfll tiinii^Kl'". ""<' '-* cliviilfil ljy n-Mfxioii wittiin Uiis jirisui into 
(n-o latc:inl linlvcs, w)ii<1i cross iwcli i>Oi('r in its interior. Tlit ray* 
<i '/ tlint fi>rin tin- right Un\( t>i tho irtmi; ini|>iiii;ing very obli(|uely on 
ihu f.icf of the jirisin.xulTcr total ri-Mexioti, eiuergiiifilhrotijili 
itfc Ivft jiicli' (MTpi-iiilitularly to it* aurfaci-, and tlipn-fori- uiiderKOin); 
no i-ofnii:tic)ii : whilit tJjt? mya "' /''. fonnitiK tht- lefi Isnlf of tlii^coue, 
arc rprti'ctni in liki- niutinrr towards the liglil. Kadi of these 
IN-ncils in ivu-iveil liy a. Iut<-ra1 prism, which ii^ain chmi^-PK ili direc- 
tion, !>» OH til n-tidi-r it )iui-iillel U> itti ori;;iiiHl ooui-s^, uiid thus IhA 
two It«lv(*<iA and if' t of tli»-orif[inul jieuctl nine completely separated 
from eiM^h other, the former heiii(; received into tlie left-hand body 
•if the iniL'ro<icope ((!■;. 7^), and the latter jiito its right-hitnd IkhIv. 
These two holies are piirnllel : Kiid. by iiieuim of an atljui^tiiig ncrew 
«l tlieir Uise, which alleri) the distance heliveeii the eentiftl and the 
latei'*! prising, ihey can be separated from or approximated towards 
each other. »• thnt the ditTereiice Itetn-eeu their axes can l>e brought 
into exact coincidcnci> with the distance between the axm of the 
eyes of tlie individual oliserver. This iiiHtrumetit gives true 'stereo- 
scopic ' prujeotioLi to the conjoint imu;^ formed hy the mental fusion 
nf the two distinct pictures, nnd with low powers of nioderftt«- 
aii^^idar apertuiv its pei^ormance is lii;,dily sntiKfnctoi'v. There are, 
however, certain ih'Hwbacks to it-n fjeiioral utility. First, every ray 
of eacli penril ^ufTers fui, reflexiim^, and Jina to |iass through Ji'tr 
surfacM: this necessarily involves a con wdeitible losi's of lic;ht, with a 
further liability %•• the itiipnirment of tho image by tho Hmallest 
u-ant of exnctiies.* in th^ form of pither of the prii^ms. Secondly, the 
luodiAnicnl ftminceiiipnta ref|tii.iito for inrying the dintancc of tlifr 
liodies inviilvr- nn iwlditinnnl lialiility to derau^i^mcnt in thi- ndjust- 
mcnt of thi- prisms. Thirdly, thi* instrnmcnt can only )>*■ used for its 
own iqicsrial pui'pow; so thnt the observer must aUo hi- pmvided 
with un onliimry singled indicd micrnsmpi' for the exnniinalinn oE 
obje<:ts uii.iuitnl to the piwi-rs of his hinnctilar. Kouil.ldy, the pnrnl- 
li'linm of the bodie.i ininlves pariLllelisni of tiicnxes of tlii" "li«er»*er'» 
eyes, the niiiititetuincr of whii-h for any length of time is fntiguing. 

Wenhkm's StereoRoopic Binocular. .\11 iliese nbjecUonit nre- 
overmme in tin- mhiiirfible tirr«n^Hncnt dc\ ised by the ingmiuity of 
Mr. Wenhani, in whosi? hinouular the cone of rays prooiwding up- 
wanl* from tlit- objective is divided by the inlei-jio-'^ition of n, prii.iit 
of th(' pecidiiif form shown in ti);. 74, so placed in the lube which 
carries the objetlive (figs. '\ 76, a), us only to interrupt one hnlf, 
"f, of the cone, the other hiilf. if /». goin;; on continuously to the eye- 
piece of the principal or light hiiiid body, K, in tlieiixis of which lh(» 
objective is phiceil, The iulcrrupteil liiJf of the cone (ligs. 71, i-'i, a). 


on Its entrance tiito the [irioni, ih scan.'«t|y aiilijvi-tnt Waiiy ri>fraetivn, 

ainre tU axiiil ray Is jierptMidioukr to l\w avrhux- it int!«ts; Imt 

wiUiiii lliB [irisui it ia bulijettnl to Iwti frltrj:iiin* nl h nn<l e, which 

•«n<t ti forili li^iri ol>lii)ti^l>' m th« lini- 

</tnnanU ilie eye-fiiec*' o! the seoowluiy 

or Ivft'lMixl IkkIv di^'. <■% L): aiid slnt-f 

«t itA i?iii^r^iic« its Hxiol ray is ai:;niii 

perp« III lieu 111 r tu ilit- nurfni-i> of tlie f{'*"' 

tl BuHVi-H no more refntcliuii on psMsiiif; 

out (if the pmni tlmii on eiil^rin^ it. lU' 

this urntiigeiiieiit l)i4- iuiago r«««ivnl hy 

the ru/ht pye is foniiwl hy tlie my* which 

huvo poued lliruu;;h the Uj} hidV of lhi> 

iibjj-ctivo, Hiti) have vwat> on without miy 

jiit4-nnintion wluitev«r: whiUt. tlip ininj.i' 

TtKr-\w^ hy llw> Irt'i ey<' is foniMil by thi- 

ray* whit-h have |ns8e>l Uiroii^h tho ri'jht 

\im\t ai thr objcctivr, anil liavir liwti xuh- 

jectcfi to Iwo rvHexions within thij pi'iKin, 

piuaiiijt thmuKh only tu-n stirfati-K of giniu. 

variitlMin of ilislnnoe hptwcn tlur iixrji of Uic^ eyes in dilli'ivnt in- 

'hvtdaMls IK mndo bv (Irawin;; out orpti»liin^' in the ey^-pitivs. which 

|L H 

The (i(ljufitini<iit for the* 


PM. W, Fli>, T«. 

WeiikaBi'4 (Unotooptc kuuMnil«r Blm>>tii>u. 

kre mornl RonwiitaneouHly 1>y ine»iis of ii iii!lli-<l-hnHil. im Hhown hi 
fig. 76. Now, itllhouKh it inivy lie objedi-d Ui Mr. Wi-iilmin's nii-thiid 
(1) thnt, a^ the nys which jxlas throuf.-h tin- pmin niid i>n' ohUiiuely 
redectvd into the aecontbiry ho<ly traverec » )uiig>'r (iigtuuci- tliiiu 






thflK« whicJi piws on unintiMTiijileiUy into the prinoipal IxhIv, thv 
picture formtN:! \>y thorn will 1«! siomfiwli;it larKT thmi tliat which 
is fi>nup(l \ty the. othrr nHt; ivnil (2) tliiit thi" piclnn- ffirnifil by tlio 
rays which hiivo liwn siiliji'rti'il to tlifi actiuii of the ]jri*in must Ih' 
inferior in iliKtiniTtmaui t4i that formed t>y the utiiiilriTUfiti?)! hntf nf 
tJir coi>« of my* ; tlu-so olijoctioii* are lounil to h^iv*" no iiradio*! 
weight. For it in w«:ll itnown to those who liuve tfxix^i-iiueiittid 
Upoii the ]tliMiomenii of Klereau;«)>ie viaion (1) tli'it n slight <lifl*!r- 
«uoe in the «iKi of the two jiictuTca is no bar to their pi-rf^ot com- 
liination ; iiml (2) tJjat if one of the pictures he ff^ad. the full eftect 
of r«li<!f is friven to tJie imu^e, even though the "tlier ]>ioture }» 
faint miul imptrfeul, proviiled tliat the outlines of tli^ latter an? 
suflic-ientJy dislinct to n-jiresent ils perspecti^■e projeotioii. Heiioe 
if, iiiKkud of the two eiiuivlly hii'/-'i'n.-d ]iictures wliii-li tire obt^innlile 
1>y ftlM. Ntti'li«t'a onKi'iil i.'onstruction. we had iu ^li'. Wenhnnrx 
one good and one ind\fffi->:nt picture, the latter woidd be (led<ledly 
prefwrable. But, in pdint of fact, the deterioration of the urenMl 
jnctui* in Mr. Wenhani's armnKentent is 
h.'ss toiixidcrablr thnn th:^t of huih pictum 
ill theoriginnl armiigrnirntof SIM. Nncbct; 
so tlint the opiicnl ppi-fnrninnoc of tlie Won- 
hntn liinoculni- is in overy wnv KUporior. It 
\\\\s, in nihiition, tho^r t'nrlhcr ndvnntngcx 
nvrr tlif prci-nlin;; ; Fii^-t, ihc iin-Htirrconi. 
fort in using it (espifiiiily for koiiip li'n^h 
lit* time lot.'ether), which rrsnlts from the 
.■oiivorfieiicn "f the nxe* of tlie f y™ iit their 
usmi] miglc for luodpivitety nr iir ol>j«t8 ; 
-pcondly, ihut this iiinnculnr nnniignmcnt 
iloeo not nece^itjitc n s]H>i-iHl inMiiitncDt, 
hut niny Imj applied tunny miei-os-'oiw which 
iit ciptkbic of carrying; the wei^'ht of thn 
M-conilnry bmly, the priNiii U-in^ sc> fixed in 
II movable fi-arne tlmt it niiiy in n inonii,-nt 
lie takrn out of tlic l.uhi- iir rephici'd then?' 
in, ^o that when it has hi-cn reniovtsl tli« 
pnncijml body acta in every resjtccl as nil 
oniinnry micrniL'ope, the rntirn cou« of my« 
paskin^ uninterrupliKlly inti) it ^ nvid thinlly. thai the niniplicity of 
itit ■■oii.itriirlion ri-mh-i-s its (k-raiix^-nifnt iilinint iunioB»ible,' 

Btephesian's Binocular. A new form of stpn-nHcupiu binoouhir 
lift.i been introiluci^d by A[r. Stephen son. ■' whii'ii liiis ce.''l4iin dis- 
tinctive fcatiirra, and <it the time Ml'. Stephennun ib-vised it he waa 
entirely unnwan- ihut any |>art of the nielhoil be employet! hud Ijeeu 
omhI bv anothi-r. He bad, however, itule)ieiidenily cunceived Uifl- 
deir* ilevici! (or diiiiliiig llie bcatij iia a ]iui-l of lii?i very ingenioua 
iniklruincnt. This he (hicuvcred and nckiiowleilKed iibout rliree 

' Till- Aoltii-ir cAiinnl iJlnw ll>i>< KpjinrtuiiUjr In |nU» vitliuut di-tiwinji hi* *■'■■■'■ 
fi iViv lilHTnlilv Hith ultlvti Mr. WuiiImuii ttvtiy pfnontcd In ttiv imbtm lliU Im- 
partiuil iniinitfnn, hy vtliipli. tlwn) nui !■■ no doulit, ha iilmlil li*T« UiXt^y )n«> 
AtVil li Iu* IiinI f^luten to niAin Ihp dxc1u»it« riflit to li. 

' iivHthly ilttroiC'ipUal Jonriial, Vul. ii, \lS10l, p. 01, »n<l tol. tli. lISn),p. lOTi 

Ki", "7. — Bidilrir* biiiornlnr 
ixluiiu. flk apfjled by Mr. 


sTRi-HEssosy mxocri.\i{ 


After Ui« full (Icacriptioti Mid cotnpktioti of his luaoculuiJ 
' ooiw of raya piiisiti^ upwards froin Uie ol)J4H;t-glit*s nief^ u )u)ir 
iif )iriBiiiH (A A, lifi- ") 6x^ in lUe lube of tli« microscupe imnMi- 
(iJNti'ly uW'e lli« isMWriur oombiiuktion of tbeohjecLiv«i, so as tocstcli 
tlif- li^lit-ntira oit their emerRence from it ; thesn it divides into two 
ivec aiul belwves >s tlescribed in t)i« R!<l<li-ll ;<rt-MnK, utiicli, iti fiivt, 

7 are. As tli« cooe of rays is equally ilivided by thf: two |>rifcins, 
its i-ao lialvmarvsiuiilarly acted oii, the two |(ii.-turei> nre equally 
[iUttuiiwit^. Hiul of tlie same siM ; while the close it|>]iroxin>attoii ^ 
I ptisiNS lo the Ijftck l«iiaof the objective enables evrn high powen* 
I be used with very littlp lou of light or of deflnitidn, provided 
Jiat the angles mid surfncos of the prisins am 
^worked with exaL-tnns ; and bs tho two tiodies 
unn be made to ooarerge at a ^luAlIfr angle thau 
. in the Wenliam srraiigomnnt, th(> oliservcT looks 
Dtigh th«m with more comfurt. jtut Mr. f<te- 
41 's ingr>i)ioDK armngemciit is linhle to the 
*l drnwtiaRk of not I»ring coiiv^rCibli- (tiki? Mi'. 
Vviihain's) into nn ordinary monociilnr by tho 
witlidmwAl of A prinni, so thnt tlu^ u*c of thin fn-iii 
of it will lie prolutbly rmtrictod to tlKWc whoilisir.- 
to work with a liinnrulnr when lauploying high fto. 7». 

puweri^ III imlrr to ntoi<l :iJight vrrcirsi nriking 
frotu iIh' impinging of Ihn cnntml my of iUr cntir, at itn iiner^geiico 
frciiii thf. objective, againKt thrdoubli- migciif thr priMn-coiiihiiuttioii, 
Ml', i^ttiphimson has dovispd a Rpcci«i form of subntJigi* ■■oiidensw 
{also madp by Mr. Browning), which causr* the ill^iKiirjinijiK' iiiys to 
tat frtjm th« o1>joct in two 
»n»to pencil>s whidi will 

rik« thd tur/eirft of tjifl tVH< 

as. 'niiH coii^;(t« >if two 

vylindi'ica) h'UMts A and 

l£, fig. 1^, nhoH^ focal lengths 

ax i-'A \n 1, having their 

Eenrvrd facpn <>p|>n»;ii to each 

'other, as idinwii in Miction at 

C ; th« larj^irr and less comirx 

being pliicin) « ilh itn |>laii<t sitlo 

(lownwuriU, BO a* to n'ccire 

li^ht from t)i<-mirmr, or (which 

IK pivfeniUe) din'ct from a 

^Luu|>, Uitderthiiteombinatiou ^"' '"■ 

a movable ntop, » ith two drcnkr oppninKS, us shown in fiff. 
T9. Tlie lamp bi^iliK placed in front of the infllrument, the two 
apertures ttdmil KimiUr [wnctb of liKht from it, so that each eyo 
eives a cuuiplelely etjual iUuiiiiiiation, iLiul no cunfusion can occur 
Di the impitigiiig of the rays on the lnwer edgos of the prismii, 
M'ith this arrftii);cment tlie Pmlura markings are sliown am tigtired 
by the lat« Richard Deck, while the curvntares of tho scnlo comv 
cat witli tho dietinctoess peculiar to binocular vision. 
* UohMs Uitfv»tvpi<«l JQinial, vol. «. \<. 11. 

r r 



Flu, nl"— M.I1.II. I. 


But nnt* nf tl"> ^■'^''^ luhiti) !«;,'«»• attpnilniit on Mr. St«pli«ii- 
UMi's cnTirti-uctiiiii is its capibility of Iwiiig oonil)iti«d with an 
tret-liiiij iiimii(,'<'iiieiit, whii'li i-eiKlera it npplif/iltle to purposea fw 
which tin- WViilmiii Linot-iilar cnimot l>e conveniently userj. By 
the iiitfT<^»ilJ<iii of Ik pluw silvered niirrov. or (still b<?tter) of » 
reflecting prism (fig, tfO), nhoie the tuljc coiitBinint,' the hinootilnr 

prisms, rnch hnlf of thp cono of rav!" in 
iio (li-tlcctcil that iW imnjti- ik rcvpi-sed 
i-^rfirnthf, tlin rnVK I'liti'liiig th*- prism 
ihi-fiiii.'h thn surfiici- (," H, hcnig rrtipi.'t«l 
by 111" viirfiHi- A I!, wi •& to puss ool 
ii;;iiiii hy thf siirfnc'- A (' in ihft <Hrec- 
tioii of the Jotli'tl hii(-». Tinw thn ri^'ht 
tivi'ly iiitii tlic ri((ht nnd th<- Irft lMxIif«, 
which iir* iiidiiipil nt n convMiient 
iin^lf, ui uliriwn ill fi;t- "1 J •" 'hnt — 
thi- slii^i' iH-ing hi •lizuntiil (he inittru- 
niPiil bet'oiiies II must uHi>fu1 com]K)unit 
(iisstTftiiiK iiiiL-rtiHti'pt-. und as thus ar- 
raiif^ed by -Swift, with w<-!l adjudltvt «iilJi 
for the hiiiuls, hati but (iiw ^tjuiils for thu 
imr|>i)srs iif i[iimit« liiKsections uinl Ui^Himl* ninuiiiiug ii|«rali<>ns ; 
inilci'd, till' v;ilii« of the orPCliuf! binocular coiiaisU in its applic- 
ubility to llii" pii-NiiiE; ouf of vi-ry ininuW ohjeci", such as Duit'niut, 

I'l'lijcystiiui. or Foraiinnifrrit., 
und li the proM'tution of 
minute dlKsectioiitt, <wpi-c)iilly 
ulieu theae IiLive lo lK^ cnrriecl nn 
in tluid. N<> 'ine who Iijis only 
liius worked iii'nioi'ul'rrly t'nn 
iipprei-iiit« the guidance (Imval)lA 
from liiuof'hir vision when once 
(III- habit of working willi it has 
l".'eii formed, 
Tolles' Binocular Ey»-piece.-~ 

.\ii iu;,'i"nioiiM eye-piece Ims t»eii 
•.■oiisti'iii'ltitl hy ^r,Tollei4(1liinl4>ii, 
U.S.A.), which, fitted into the 
liody of a nmniR-ulftr microscope, 
iim verts it into nii erecting Kterpo- 
spiipicbinorulnr, Thi* mnvereion 
is i-flecleil by the intri-position 
of ft iiy!it«>ni lA pviKiiiK similnr tn that nriKinidly ilcviiied hy MSr, 
NiK'het, but m/iik- ..n a Inr^'er Bcule, lietwci-ii un 'orfctor' {re- 
wiiiblini: ibiil iiMil in thn rye-piece of n day-ti-lcscope) and n jwiir 
of onliiiiiry Hiiyyln'iiiun i'yr-piec(!s the '•rnlml or dividing primii 
l»eiiig plaoi-d III "r nfJir the? pinne of the secondary iniagi- fimnifl l»y 
the si-m.'t'ir. whilp tin' two eye-pieces are pliici-fl iminmliately nliovp 
the Iwii hilrrni pnMiii, unci the i^>iiihiiintion thus making llint 
diviniun ill the |H>nci]K fnrming l\vi m-t.'oiidnry image which in tho 

FlO. M, — RMptipnwin'* crwling 




Cachet IniiocuW it iimk«*iniluipi!iwilKiauwginf(from l3i«olijoctiTfl. 
As all tlie imngi'-foniiiiij; iiivs hnvr )•■ [mim t)irctii]Kl> ''>■* twoAurfnow 
fif fiMir l<-tui-« nnil Iwn priMnf, Ih-auIi's hitstJiiniiitf iw<> intf-nuil re- 
(lrxi[<ii> in llK-latli'r. ilix suqimin^ Unit Profi'Xuir H. L.I^milli, vtiile 
a/liiititiiig n tticH of liifiil: xhniild (i>i'l ;iI>Ir tit kjh-hIc of t)i« ili'tiiiition 
■of ttii« i nit mill'' lit lut not infi'i-ior to tliut i>f riliu-r tlic Wnnliiiui or 
th<i Niu'lirl liiniM^uliir. It i.-i uliviounly a. ^'ri-iit uilviiiitu;ti< tliiit tJiis 
ryC'piccc MUi )ir UMtl witli any riii(<riitu;(iTit! niiil W)l}| o)iji^clivi:H of 
iii|(li jiowcr ; I'tit »« itst i-ffiM-'ti»i>i»'»> iiiu>( iIpih-iiiI iiiwii extnuinltnmy 
■MKimuj u( workiiiansltip it»oii!it must nt'oeauirilv )ie i^'rvat.' 

Abb«*s 8tflr«oioopic Eye-pieoe.- Pig. 82 ivpn?acnte this in- 
'«traiDfiit in hriili.iii. Till' lnjiiy A A' mutaiiis thtrr jinsnis oC 
crowu gluiBt, (I, /', uiiJ />'. Tliti tni) L'y■!■]lir<^<« B, B' »»■ lot into ttit> 
top pt*t(v tli« formei- twinji fixi-cl, wliiUl llie Iftttcr has fi lut«ral 
didii4[ ntoveineiit ; the luttom ]Amln cnrriiM th<! tuU; C for inserting; 
Um eye-[Hece into Um nitcrosooiMi tubi! lik« iin ortliimry uya-piece. 

The two priniuH n nnd i nre uiiitwl m> iu t^i (onn a thick plat« 
*ilb [«rallel aides, th*ir ooiitiiiuitj', howover, VieiiiK broken by nn 

' )^ Aittrirum Janntal o/ .VciVihv, i«1. luriiL (IMti), p. 11), uid toL x»!i. 
4tM6). in. tH: uid JVourAly JfirnMC Journal, vol. rj. IISTlt, v *(- 





rxcmlingl}- tliiii htmtutn nf air Ikk tlinii 0-01 min. — indin«<l Ur 
thp Axlt ut nil miule (rf SK'.V, Tlie cunc! of mvs fnun llie objective ^m 
in (liviiiml in)/) t«'i> pftrt«, one t>piiig tnitiHmittnl and tlw utlierl^ 
irflfrctnl. Tlie li-RDHtnittocI rny* pn^ tlimughn, 6 without deviation^ 
iiiul tana an iitiaxe o( tUe object in llw- nxiiil eyc-nie» j8. The niy8> 
rpHi-cltnl lit tilt- iiiiRle nbown hi tin? figiii* i>«s8 tiin>u<:h the necond 
Kurfkctr of tJi* i-miii i (up-'ii whieh lln.-y nre init<lent nl right iniglM).. 
ttnil rin(!r(ctiij; »t nii invli nation ••( 13" with the horizotitAt mi's ^ 
lot«Ity reUectivl iiitu the pye-l*i<!fe j8' nt im lumle of I'O" by lh» ^ 
liypoUienuHi' oui-face of the viglit nn^lnl ei|uilnter«l prism !•', ihfl 
ivxh of which iJ^ iniiknt nti ungU- ^-f 13^ with the axis of tlto 

A'ljuattiietit for [liH'eimt tlitlnni-ex l>i»tW(vn the eye* iit effi-clfld.: 
by the aci«w l'. wliiih imivi-a tlie eyepiwe j8', tfiRether with th. 
pmm i', ill a iJiiuiiU?) dini'iinn. Tin- tulx-^ of the eye-pieo« can 
also be druwii out if Ki'''it<*r srjuki'atiuii in j-eijuiitsl. 

The ey^-pieeea liavi- ihi- usual twn leiiaeo, but ave of special ron- 
structiou in unler ti. etiualUe the ]eiii;th of the direct axis and tli« 
doubly reflected axis, and in itpile of tliia inequality obtain sharply 
deBn«d intag«« of equal anipHllcation with the »auic focu!^. 

Stereoscopic vi«ii.>ii is ubtaiti«<! by halving the ciint« of rava 
abiive the eye'piec«<*. This is eft'ecteil by 8^lllpillg off half of the 
FT>aI imn^ of the obJ«cti\e opening firmeil above the eye-piect'K at 
tlie so'Cnlled 'eyc*poini' ff or 0', which wprchenls the O'lmnum 
ciiw»- section of all the peuciU emerginf! fi-oin the eye^piece. A cap 
with ft semicircular diaphra;^" '* litted t» llif fve -piwe {sh-iwM in 
llie ligure over ^'), the stniiKht fdgi- of which is fxaclly in the 
optic axis of tlic eye-|)icoc, and can lie raineil or lowereil by ncrewing 
so i\n to obtain a unifiirni hi»ectioii of the cones of mys fi-om CTcry 
pohit of the Held. 

The height of the diaphragm U regulated once for all fur the 
Munc length of the micitMcoiie-lube by finding the pohilion for which 
the aperture- i II lagt- (whicJi on xvithdiiiwing the eye fi-imi the eye> 
piem ut viiible us a bright circle above it) show>> no pnmllox agaiuat 
the Ktmight edge of the diaphragm, i.e. >7i> that on moving the eye 
laterally the image alway* appeal's to adhei-e to the edge. 

Ill ndditi'iii III the abiive cajw with diaphragms the instiiiment 
in Kupplie<l with xi-dinaty capt- with cii-cuUr aperturm. as in fi. 
They tjipcr slightly and Miiiply nli<lc into th» eye-piece, so that 
tliey can W ri-mlily changed. The Kiieeial feature of the insiraiueDt 
ia Ut, tJial it is rii|nble of Ix-ing u»ed with the highc*t powers ; and 
2ndly, that it i» nolnewssai-yt'i cover up half of ™ch of the eye-piece 
tubiw, thus hning half thp totiil nm<>unt of light. It is sutKcient if 
one only (the hilernl une) ib half >ih>tcur<Ml, leaving the other free. 
A» the normal division i>f light betw«'n the two tnlies is two-thirtls 
(in tlie axial) and one- third (in the Intcral), the total loss of li^t is 
mlucrd t<i one-sixth. 

The field ""f »ii-w in the a\inl cycpipi-e in this arrangement in 
any eime newsvarity nppf/tre linj;h|pr than tlmt of thr lateral onfi 
seen witli thf same eye, and in n-gai-cl ti> this Dr. Abl)e re- 
lUarlcH that tlu- diiTeniici- la-twmi the brig-htiuim of the two fields 
in biL'ooular obufrvntion ' ik not only no ditfrct, but, on the contrary^ 

a dccitlnl iulvnntii|[«.' Fur cxpritmco has long provi^d thnt, to 
■JiUm n giinl sXvntmcopic rffrct, it in <>n]y iieceuury tlwt •iD" ini«gi: 
•hiiulil he lus jM-rfcct nnd clmi' lui pcHSible. whilat Uie ot)u>r mny, 
withrmt ■ppreciiihli> (liiui<)vn[it»)i:«, hr of wnsiblv Irss pei-fectioii. 

It miglit thMyfoi* b«> niiticiiintiHl t)iat thU would apply (iis tn 
flirt it (IiNw) in tlic KHRiR wjiy tii (liiriM^ncc <>f luniinovitv. Mowsivoi-, 
lU) ail'lilionnl fnct tanaX )ir tiikm into nc^niuni — tlinl lh« two eves, 
e!>]H>;iiilly ti( inicnMciipUtA, iilway* utiitw unri|iml srnsiliilitv to lii;ht 
«s th<- miult iif (.iniilniil unciiiial tiu*. TIm; Ipm used eve, wIiinm? 
Ncntr-iipwi of vixinti in always fi'su thnii tliiit of tlie one iimi'e fre- 

aurntly rx«rct>«l, aliowi! n (rmiti-r xT'iiMMUty ti> lifcht, «nil the 
ifi^raicv in Ml ciinAiilcmblR thnt thi- !■■» laminoiix itiw^ t4 tlio 
latrnil cye-piHCF. wlwji vit^wi-il with the !<•« fwvHiiml (gmivnilly the 
(It-ft) t-yi', MviuK t'lwi bri^'htpr tlian thn oUirr wIhti vw^-pil with the 
esi-rciwil «ye. Th<- uii<'i{u»l diTUion of tltn lijcht ■* tliin-pfutie ft 
rcJciiue vlmiii-iii. iia it nrrviH to I'lpiuli^c t hi.-> pliyHK>lof;i<al (lifl«rance. 
n>f olni^rVM' lull otily to tnlcp ciirc thnt thi- Jiiui niti^l ryp ix n{iiilir(l to 
lhelnlvra)ey(>-|>ieoe : UDcl 3rdly. thf in^nioux mtm tinmen t whi^iy, 
by xitiiply turning tlu) uip» witli the iliitphniKui*, oiitioM.i)pt<: itr 
eu<i'B".TJi"ti' i?flWt can be proclufpil !ii»tHntntm>unly «t j)Iiii.'<un--, It 
is mure parlicularly iivnilnblu f.n- tub.-t i)f shurt Iviiglh, for wliioh 
ill* Wpiiluiia pristi) is iiuipplicublf. 

TIr- stei*oB«i(iic binoculur i« put to it* mu^l ailvuiitatfi-'ouH ow* 

when Bpplieil ejtiiwr t« ofnujuf (ibjecta of wliiwi« Holi'l fomis wi' iir» 

lirouit of Kaiiiiii^ an exact apprtciation or ia liimqutrrnt oliJMta 

rhich Imv(- »tich a tliiclcncM lui to niakr tlitt urounilr itistinrliun 

tweeai thdr nmrrr niw) thrir mon' ri'imiti- pUmit n mutter of itn- 

ortance. All xtiin^scopie vinion witli th« niionixoopp. *» far as it in- 

anythint: mors tlinn ninn.' secinjf with two m'c*. drpetiils, ns alreiuly 

utrm. i-xdnsivdy ufion tii<- umiiuiil ii)idiniitir>ii [>f tli(^ ]ipi)cili( wliicli 

hlonii the tvo inins^-s to thr pluni- of tlu^ prrimrtition, or the iiKi.-i of 

The fnicrosrope. By uniform hnlvinjt of lln- pi-ctfil* whether hy 

jiriani^ alwve the ohjfctixe or tiy (HophriijcmK ovur the eye-piefi-H- - 

the dilFereow iu tin- directions of tht- illumitiHtion in rej-anl to th» 

|;|W«piunti<in reaches upnixixiinntidy the hoM of the iiiikI<^ "i Hperture 

^of the <>bj«ctive, pn>vi<ltM] that it« whcilii uperlurr ii iilletl with ra.y». 

By th*i anc-xi(ted luilvi»|f we ha^e bt-eii i-DiisideriTi;r. the dii-ect iuuvi,'?' 

I_i« producnl by n pmcil the axis of which is (leqiendiculiir to the* 

' «l the prf]Hii>tii>n, and the dt^Hn-'tM] ima^i- by one whose axi« 

: inFlin*<d about a fourth of the angle of a))erture. 

Witii low intueni, which nllow of a relutively ooiiMidorable 
^epth-penpedive, the >lisht differenL-e of inoliniiiioii, which rentaiiw 
llie latt«r CAW, [s quite sufltcieiit to produce » vei-y marked diC- 
frr«nce in the porapeclive of the suoodfiUve layera in tho imagM. 
But with high jowers the difference in the two imatccs doi-s nut 
ipace— eveii wlien iMith eye-pieces are half-covered with the 
I of tlie nnf;l« of aperture, «o long a» ordinnry pwitrul illuini- 
nittion is used. For in this co«e th« incident [xmcil dona not fill 
th« whole of the opening of the objective, hut only n reUtively 
[Ruall centml |ian, which, as a rule, does not embracs more than 10* 
iingle, and in moat coses c&nnot embrace moro without the 
oWsnuM of the tnicroicopic image being nflectcd nnd the focal 


ilepth aUo bung uiiiieceaaarily (lecretis«l. But n« tlioiie parts of tlio 
proptimtinn wliich Miiecinlly mIIow of iwlid oono^tion are alwHya 
fnrmntl l>v ilirrL-t triiuiiiiiitlod my« in o1)8ervflti»n wilh tranauitUid 
Ujflit, it /olIowH timt uiuler theec cimiitii^Uiioes tti« di^ranoe of ths 
two iningM in fouiideil, not on ilio wlioto (ipertur^^iigle of the o)i- 
jiKHivi", liut ou the iiiucli (.mnllcr anjild of tlin incident and tlirccUy 
traiiMiitttti] ])pncils, whicli only allow of relativ«'!y Hiimll riitrcrpncri* 
• rniidiiuitioii of theiiimgp-foniiing myn to thn iiropruntinn. It ix 

nvidciit, howcvw, that wh^n Alii<>rtiv«ii 
of short fopUK and ponvsiioiirlinxly Inrjp? 
iHii:;!'' nrr^ ii»"l. ii cwiisiiitTnUy grnitiir 
iHUlimitintion of the tw<i iiiiii^'w with tn- 
xjii^t to (itiriillax ciin \n: jircKlticcd if, in 
[iliti'i' of niir nxinl jlluniiiiiitiiix ]icnoi1, Iwii 
["■noils iiiv UM»1 opjiiujtj'ly inflinf«il tit tht? 
itxJK in KUfli II way that iriioh of iltn 
iiimfics i» prixluwKl by i-iie i)f tho pencils, 
ThU kiitrl iif doubli- illuniinntiaii, tbou^i 
ii (viiinot bi- itbtuiiiwl by tbu ainipit* 
.. niiiTur, CJin bi- l>n.^ily [iro<luc«(l by uiuug 

witJi thL« fimdwiaei- u itjaplum;^ with two 
■openin^fs (6^, s:i), ptiiued in the diiiphniKoi nUiite under the coii- 
doiiwr. We then have it in our pi)wi<r to uie. ut pleasure, pencils 
tit imriwer or wider tLpenure mid of greiitt-r or lets incli nation 
towurds the axis liy making llie openiugs ii( diflerent width and 
different distance ajiai-i. 

With dia]>lim;,'m» of tlii.* form ( which cmi I'lisily l« mude out of 
«ttril-liimril) tlip liir^jer aperture angles of liigli-jioww objective* may 
be iiKul" i(-i-iif to iiiteiuufy tlit? stereoBCipic Hiect without einployiu;; 
Willi |ii iM ill, which are pi-ejudiciul Iwth jis dimininhiux the vleaniess 
of tJie iiiiHf^ mid the fiical de)>tb. 

Of course with this luBthoil of illuminaticm both eye-piecea iiiUBt 

be half covered in <irder (luit one iiiiu<;(! uiny receive li)-bt oidy front 

one of the two illuiniuuting cones, niid the other oidy 

J I from tlieolhor. The division of light in both theapertunv 

\^ ^Q images will then he as nhown in li;;. ^4 ; and it is evident 

r T that in iliis caw the liriKhtness of ihe inia;^ for both even 

Fi<' HI together io exactly the luinie ua uould be given by one 

<>f the two con«n alone without any covering. 

Tlie mirthnd of illuminAtion here referred t<i —which was erigin- 

jdly nwonimendrd by 51 r. St«phciiiiM.iu for his binoculiir niicro«cop»— 

has, in fact, pnjv«l itself to lie by far the best when it is n question 

of using higher powf rs than about IWO times. It neccssmity roquireti 

very well corrected and propfrly a<tiu8t*d objectives if the sharpncMi 

of the image is not to suffer ; but if these cr>nditionB arc KntisHed it 

yields most striking stiTcuscopic oftecte, even with ■■bjectives <if 

'2 mm. and less fural Ieng;tli, provided the prppnnition under ohser- 

vatiiin presimts within a snuill lieptli a HUtlicicntly chnracteiistic 


No&-St«reo)copic BiDocalaTa.^Tlii' grrat comfort which is ex 
perivncL-d by ihe niiLTij>i'(ip]>t fi'oni the mnjoint uhq of Irath eyes tuia 




Fio. se. 

> th« inv4tnUon nf moiv tliAn on« nrmng^n^it by whieli thia 
Ilfort can b«r xceunil uhni tJioKi- high pnwoni ni-e r«<|uire<t which 
nnot be «npl«y«i with Uk- onlinary rti^rtnoscopic bincx-utur. This 
I (tceomplinhnl fiy 3[cs-irs. Pnwi-ll iinti I^rnliLiid by utkin;; &(lvuiitH;;n 
thn f4ict nln-ndy ml «■««•( I lo, Dint whni a jwncil of niVH falls 
'>li(iucly upon tht- liUrfiici* nf n n'fmcting mcflitlix i\ part of it il 
F-tlpi-^ral without rntt-nng (lint iiiiilium nt nil. [n tho 
bW«' UMiftlly ovcupiinl by tlnf Wcnhjidi prLinn, thpy 
inti^qiow nn inclini*d pUb- of gluH with pnmllrl niAvn, 
Ihrough which onA portiun trf tho niv« proivinliiig iip- 
wnHn frmii the whok itjwrtui* of the olijii'tiv*' ]ia«u>!; 
into tlu? i/riiici/irt/ boiiy with v<>ry litUi- I'li.iTijjo in i(« . 
, courwc, wlitUt unothi^r portion is rt'flefl«?il fixmi its -itir- 
tnto n rtH.'iJiiixu!«r pri&iii *» pW'p'l an tn direct it 
oltliquply upwiiriis into llm nfrnntlart/ iKidy (lijj. S.'i). 
fAlihoujth thoD' is a iW'tdixl i(ilf>M<(>iic<! 111 brijililni-«i be- 
Iwn-n the two iinnK^Hi tliat formal by the i*flectwl invH 
" ffing thi- fui)t«r, yet there is inarveflfiusly liltip loan of 
'dfttinilioii in<>iUier, e\en when tb«''^Ol]i»f an iiiih objec- 
tive is ose.!. The rlim- and pmm an* Hx*<d in a ahort 
tube, whivh tan be readily subfttilut«tl in any ordinary 
binoi-uhir iniLTuoctii)* for theoneoontainins tlie Wenhain 
pmm. Othrr ari-aiix*'"eiiU were lonfi since devited 
_ Dv Mr, Weiihani.' and 8ubse(|ueii(ly by Dr, .Suhnider, for aaeuriti^ 
'liinocular viiion «ith the higlieKt powers. We have iwwl the tatter 
«f these with pi-rfeet Mttiafacliun, but all that ik r^<tuir«il Is at th<- 
dispdoal of the ii.tu'l(!rit in the nniin^'iiif nt nf Powell an<l Lfudaiid. 
To tliose who liave uard th<-se furni.i of binocuUr habitually it 
1 bepu a frequent Hource of surprise and jwrplpxity that, although 
arvtiL-ally audi a fomi an that of Pi>vr<-ll and IjeaUiid's is non- 
«t«reo>icopi<', vrt olyeota studied with hixb jiowern luivt- nfiptareii as 
, if in relief, and tJie elfect upon the uund of slereoscopii; vinioii has 
en distinctly manifeal. Tlie Editor was conscious 
tliis for many years in the UHe of the Powell and 
Lealand fonn, with eren tlie .'^th of nn inch power 
of the achroiiiatie construction ; at the time he int«r- 
prated it as a conceptual effect *, but it always arose 
when the pupils fell upon the outer halves of the 
Itamwlen circles. The explanation, Dr. A. 0. 
Mercer considers,' is due to Abbe. Sinco (Kg. )<G) 
wlteu the eTe-pie<ws are at such adistMnreApnrtthitt 
tlie Rnnificfen circles corrropond exactly with thn 
pnpilsof the eyes, cctitre to centre, the objci* apprurs 
flat Hut if thn rycpiTOrai hf racked down, *o ms 
to bn neatvr togetlifr, thi- cr-ntri'S of ihn pupils fall 
U[>oii the •■uUr halvPK of tlii- Kammlen circli-!', anil we 
loivi- thv ronilitinnK pf orthnscopic etliwt ; while if they I« racked up 
MB» Mto I* nioi-n »-p»rnt>^l. thn centre* of the pupjla fait on tho ii»i»er 
halves, anil wo have |iM>u(liiM'opic ell'ect. 

< TtammeHon* o/lh* Ukrutf. Hoe. ViS. ml. xir. (IHOS), p. lOS. 
' Jimm. B. M. S. wir. ii. t«I. ii. p. IT I. 

f f 






Tbe OptioKl loveitigationa of Oaais. -I!i>foi-c ionviiif; this itwtion 
of our »ubji,i;t, iu whii-h u<- Imvi' t^iiileavnureil, with a» muuli cliiir- 

tt<!SS OS we coutd cnniinaiut, Ut- 
eii;(l>k- thi; ^ncmt MMidcr to ouiii- 
jiri'tiend witli intoltigibility ilif 
jirinfifitt* u/ ihrnrrlUii! ami i)t- 
jJifil o/iliet Its tliPV ivlnti- Ui tlM» 
Iuuutiaci>i)P, wr lji<lieve wt' shiil) 
nerve thi- hi;;hc-i- intvnwCA of 
iiiii!rint.-upy, unci llif wiinU or <)«- 
Mi-Bt of tile miiri^ luh «iiL'wl iiikro- 
(A'opitiil experts, if we i>ii<leuvour 
Ui piT^seut ill a form eitlipr<levoid 
of tt^i-luiiiiility or with iuevitaljlt- 
tii'litiUiilitien exjilaiiied » (/itn'ral 
iiiitliiirf iiiid iheii an itfi/jfienliwi 
vf llif j\tmi'ii» tliiifitric tnvMti'j'i- 
lii'tm u/ Vatim. an t-tiiilieiit Ocr- 
niiiii tnatiieiUHtimii,w|ioaiiion^l 
niKiiv other brilliant Uboiii'^ in 
n|ipliecl mil theniii tics fyjioundfl 
(/(« /(lie* !•/ llif r^/rorliiJii o/Hyht 
ill flir cntt nf a vo-iu-iil si/MUm y" 
x/ihrrii-al ttir/'tces, lineituj rnrdia 

', o)' variuii* rr/rnrlive indiffali/iiig 

\ liflii-ffii thrill. 

• Although the assumption 6 

U[H>ii which the foiinulti: of Usum 
n-nl are ciat coiiicitleiit with tho' 
coiiiiitioiis presfnted by tlie l«nv 
com bii Hit ions which nit- employed 
ill ilie coii)<ti'uci.ioii of modern 
objectiveh of greftt upcrturr", tli«- 
ivsults, nevertheless, fumiiOi nii 
nduiii'jdfle prefcntntion of tti<- 
path of the TiiyK nnd the positioiiK 
of cardinal [loinls, i^vfu in thti 
microscope us wo know anil ti*(^ iL 
We remember that th*' nittTm- 
scnpe IK l»rf!;t;ly uae<l in EnjiUnd 
and Aiiip-ncii by men who eiin 
only employ it in their morr or 
]ffH btii'l' ri-i'essions ftimi prufi-s- 
Kionnl and ■.'nmnir-rcial pursuits, 
hut. w ho ofliNi i-mnloy it with en- 
thusiiisiii mill intcilitifnt purpose. 
^ O Much scientific work maj' be dono- 

by such mi'ii, mid it will pr»mi>lft 
the accompliohuieiit of this, in our Jtidgnient. if the fre<|u<!ntly ox- 
pruBned de«ire lie met which will i-nable such .itudfiits to utiders(aii(f 

I TliU Sicatv in inntlly cxujiKmtBd tor tlic uko at cluni«& 



in a general but thoroughly intelligent nianner the principles in- 
volreil in the employment of systems of lenses. 

Many such either have scanty knowledge of algebra, or in the 
■cODtinuotis pressure of other claims have lost much that they once 
possessed. We believe that in these cases the folIowiDg exposition 
■of the dioptric system of Gaus.s, with a following example worked 
-out in full and with every step made clear, will be of real and 
prvctica] \'alue. ' Without some intelligible understanding of the 
ultimato principles of the microscope no results of the highest order 
can, at least with moderate and higli-power lenses of the best 
modem construction, be anticipated. On this ground we commend 
the study to the earnest reader. 

Let BN, SN' (fig. 87) be the spherical surfaces of a lena of 
density greater than air, and let P R S ;> be the course of a ray of 
light passing through it ; C, C, the centi-es of the spherical sur&ces. 

Let PR, R S be produce*! to meet the perpendiculars through 
■C and C' in A and A', 

Let C R = r, C y = r',' /i = index of refraction out of air into 
the medium. XN'=(/, the thickness of the lens. N R = A, 
N' 8 5= b'. These may be considered as straight lines. 

Let the equatioii to P R be y — ft = wi (jt — O N) . , (1) 

■ „ „ ES „ y-b =»,'(.«- ON) . . (3) 

-or, ij~b' = m'(x-Oii') . . (3) 

S;, „ !,-b' = m-(x-Ois') . . (4) 

From (2) and (-1) 

i' -h = m (O X' - O X) = Hi' X' N = w' d . . (.5) 
Now sin CRA = ;i.siiiCUB; 

or, '^ -^ . sill (; A K = ,x . "^ J* . sin C B R. 

Xow C .-X. iind C B arc the values of 1/ in equations (1) and ("2) 
when a- = < ) C ; 

.-. C A = /i + «( {OC — OX) = & + mr ; 
and similarly C B ^ ft + m' r ; 

.: (ft + m r) sin A R = ^ (6 + m' r) sin C B R. 

Xow CAR, CBR do n<it in general dift'er much from each 
other, so that fur a lirst iiiiproxiniiitinn we uiiiy consider them Ui l»e 

,■. ft + ;ii r ^ /i (6 + m' !■), i.e. ^ iii' ^ in —-T- , /,. 

Let - - ^ " ; tjieii fi III' =! lit — ft K . . , (6) 

Similarly, sin C !S B' = /i . sin C S A' ; ■ 

f V c" \ ' 

or. ,.,.^ -sin C'B'rt = 7--: .sinCA'S; 

^ If either of Llip cnrviilarott inj turned in the opimslta direction the sign oJ the 
<orreiipouding r rnanl be chang^. 



and, aa before, 

C'B' = 6' + rn'r, C'A' = 6' + m' r" from eqnatioDs (4) «nd 

,', a§ before we may take 

h- + m"r' =,>.{b' +m'T'), or /. m' « m" - '^~ ' 6'. 


., „ -. ^ y-t-lfi' 

r _ ^ u', then /jl »»' =a W — ft' u' . 


From r,) and (6) J' = 6 + ?!--*« d=h(l- ^-"^ + •!^^j 
„ thi8Rml(7) ,«" = ;in»' + 6u' fl -—W "'-''"- i 

andfn.m(G) =m-6«+ft«' (l - '^") +-''-"' 


- = A, 1 - = y, 1 + - =/,«'-«- - as i 

then . //^ijo + «m) , , ,, , ^ 

Ncivr let X, Y l>e the coordinates of P, the point from which tfct 
ray of light proceeds ; 

then by (I) fi = Y - ».(X - ON) ; 

substituting in (f ) 

?/ = </Y + m (h-g .X-Oi s); 
„i" = i Y + «, (/ - A X - N) ; 




Also, if Y ^ 0, y ^ ; or if a ray proceed f mm E, it will aft^r 
refraction pass through E'. Also ui ^ . — ^^ = m",thatiM, 

the ray will be equuUy inclined to the axis before and after refrac- 

E and E' are called the ' principal points.' 


OE = ON-^ 7'=0N+ 1^ 

« m' — 1( —flu It' 

= 0K + 

fi(it' — 1() — dn u' ' 

OE'=OX' + ^-, ^=ov'.i- 

K u' — « — ttlt u' 

d 11 

= 0X' + 


fi.(n' — v) — du u' 

Secondly : If m" = 0, or the ray l)e parallel to the axis after 
refraction, we haie from (8) 

b^ — «i, and the equation to the incident ray becomes 

V + m = m(j- — ON), or ?/ = hi Ta- - ON - j ; 

1 + 


.: when 1/ = 0, 3 = O N + = N + ^ 
=! F, suppose. 

It' — n ~ d 11 u' 

If m := 0, or the m\ l)e pai'allel to the axis before refraction, we 
have from (8) 

b' ^ g b ^ ■ III", and the equation to the refracted ray becomes 

y -■' m" = m" (j- - O N'), or y = Jii" ^a' - O N' + A ; 

, d u 

.: when2/ = 0, .r = 0N'--'' = 0N' ~ .._. /^ 

K )(' — J( — dull' 

= O F', suppose. ^ 

F and F' are callnl the ' fnoal points.' 

OF = ON + - A' + <?."' ) 

fi (n — n) — '/ if It ■ 

OF = ON'- .'"'f'", ,\ 
fi (« — j<) — d u II ) 



Tlifi f.Hsi] rlUmuLV -/'=0F-0E = OE'-0F' 

j<(«' — n) — dnii k 

Hiiuilnrly. it may Ih> rIiowii thnt if there be two lensra, anA &iil 
Hoript iiLiiiljers i-i-for i«i tlie lirst anA wcond l*n» respect ively, wliito 
E, ¥.', V. V i-pfiT lo ihp cntir*- nyisti-in, niid if 

i = 0E,-OE,', 

'i = - '/ = l'\ ("i' - "i) - rfi Ml "A 

OE = OE, + 


oe' = oe; 

i'.»l + ^l"'. + <^»'l«1 

O F = I ) K, + 

f.d'i +■''•1) [ 

We are oow pre|vii"eH t-v wv«ci- iki fxiiniph «/ tlir Cuhm tn/Airm 
bytrncinft n ray tJn-^iiitli two ornuu-r li>nsi-Ki>ii nn iixiii, sIiuwuik Imw 
Jlliy c.i'iiiiiaiili' iiSJiy bi- ftruiul lhrou;jli two or iimiv |i>ii»pg ou tlmi [ixjs.' 

The QftoBi syVtem of traciii{f a ray through two or more lens«s 
en an axis illuttrated by meanB of a worked^ouc example. 

'['"•" li-iiM-'s, I Uiui ','. 1']^. f'^. oi" 'in n\i* ry nrp yi^i'ii. Sk. I xn 
a double vonvex of crown j[ inch thick, thn refractive index n boiii;i 

^ Kwii*nilf«Tiiiff iHir ulijtfTl. 411(1 tliv auuiiiiid vottLtlilioni orf myoi* tor vhMti Wr 
rriU, ne di> nM ueniuitf t" iirrliUM? tliii wiili Ui? lallnwlnit luitBi, |o nnind thr 
rMll*TUl Uiv lullHO k(ttu>lu'i1 la ivrtiilll lllrtllli-liuilliiwt I'llinvairint, 

• mMiiH iiifiiiilv. A |iliiii<> Kiirfiif'i- ■■1 u li'ii* in rnliaiilcrcil a ■■|<liiiiical (iirliwe nf 
*u liiftolle nilint. Aii;r niualon diiidisl b; « - U; uiy iiiiinlm diviri«^ l^-U •• v ; 
wii iiiisilirr lunltiplinl bv U h 0. » pliin. or minua. iir iiiiilllplicil bj luiy uiunlipr S* 
•>Jl m. 

ThiiUiiotiUf in llu< nilH lur Die IranltiKnil ut al)(<>bnuiul meni: 

In the mitltiyiirnHnn i>r i^iririi'n t^t like rtffm th^ wtuil f niwAy* vtMt ; Iml U 
tluf iii)iu>> Htv AlHimilnr it !■ uliiuyi viniiii. 

Ill wIitiliKK, aAt\ nil llir: li'tiiK luei-tlini Ihnt hnvr u plui tiKn : Ilia" all Uu> lamu 
irllli ■ iuUiu>> ■1k" 1 nubliuct tlw Iviu txuu thu gKHlur niiil aSx tb« ali^ uf ibi- 

In miblnuilimi vhuiice tiie tien <tf Ui» ti^rru to tip kublrncUd unA llicm oild in 
aoMrduirr xitli tbv jinivuMK riili:. Eiun|>lL- 

- U 


An niini|>l» '■■•■iini in lb* uiiii'ImI •■i|iinl>iiiu fil uid (lU, ]i. III. nf - . — « , 
I1UI Uuui tlm • •• I'liaMf*'''' "'I'* ■ — l>y llu> uuKntiti' i-iicn iii IroKl nf Ilia (nctinii! 
In tiii>,r. lit. liinri'itr, llivi* brinK A -t in front ot llio Imcliun, llic nmlt xnuiiu 

K\.\)II>].E AITEK OAVSH 1 13 

3, U)i! nuliuH nf tli« Huifaoe A U } uv<1 Oukt of B I iitch. Hn. i l«ns 
IS n |>lBn<v-c(>iu!«v« at dint ,V iix-'h tliick, the rvfroctiie iii(l«.\ ^ iM-injic 
^. tlie r:u1ius of the surfaoH C i» K. "*«* ^^"^ Mirface D is pliini>. Th.- 
disbuwe lietween tlie lenxfa, thtit is. froin U to C m«aMited on th>^ 
Axia, is J iiM-'li. Tli« problem is to titid tJi« conjogste tocuaof mny 
;firi'ii |-oIiit V. 

Ill unler tn avi-oiiiplish this twu points hnve tiret tobe found nitli 
regnnl toench tens, 'flieite puiiitn are ontlrtl pri11cip.1l )ji)iiita (mw 
P P", y y ill tin. (tiS). When iJie nwlii of curvBturv r mid t^. -/, 
the thioktiehs, and it, fi,, the ivfructivc indices of the respective 
lensM,' ftre known, the distance of thene points from th« vertites. i.e. 
the piMnlfl wherv tho hxim rnts thcnir&tciaiof th« lRUS,cnti Im found. 
Thuithv applying Profpwur Fuller'sformaliRto lens 1 the ihataiice of 
P from tht^vMtes Aran )>e deti^nuincd — wc p. Il6(i)— wmilnrly P 
from It— p. 116 (ii). In tlio same wkj' the pointed Q' fmni C ftnd I> 
in lens 'i cftn Im) nieosurad ofT— (v) (vi), p. 1 IT. 

It must l)f! pni-ticularly notiwd that in ntmsuring nlT any dta- 
(Anon if thi> ntimlifir in + it mu*t lie iii«siip"1 from left to ri^t, 
and if — from right to loft. Thus in (i) p. 1 16 iHTcnu.te thu sign of 
-I.IK is ■«• P licit -lAt* of nn inch t<i tin- ri^ht of A. And in (ii) 
htiT^usi- -21 i» — P' lii» 'ii of an inch to (iw left irf B. Tin- ■anic 
mil* n|>j>lirK tJ) the rndii ; thus the nidiuBof A, U'ing mivmuiril from 
tin- vrrtox to the crntrr or fnim left to rijjlil, is + ; hut tJic iiulitiii 
<»f B, bning iiinuiuri'd from tlic vortfx to it* ceiitrw or fnnii ri([ht tJi 
left, IK - . Similarly with tJic I'oncnvn surface, C being mexaured 
from right to left is — , 

In both llie ""xaiuplps hffore n* tin- pointi P P, Q Q' fiiU insiilc- 
their renpt^tivi' [enwK, l>ut it d'lr-K not follow that lliry will ilo so in 
evpry iiutanoe. In »>me form* of menisci, for l^x■lnIlle, they will (nil 
ont&idv the lens nlto^ietlirr. 

With regard Ui the focus of the len.i it fnllowx the came rule ; 
thus,/ in Irtui I is niearared tii the left from P, and /' ti> t)u- right 
fr^ni P' ; KiniiUrly in Ii-ns %/'' is niumured to till- ri;|tbt fn>m (J, 
nndy"' to the left from Cj'. 

Having determined tlie focal length of each lent, tin- diKt>iic4- 
tiKtwern tlie right hand prin4^i|.ial [xniit of the first lens P' and the 
lirfi luuid pritif ipal )ioint of the wc-diuI 1<-iih Q must next be found. It 
iniuiifestly is the dinlnnei' of B from P' + tJie dintanue B C Letween 
the lenacs, Q beiitj; ut the [Miint C. Tliereforr 

VrTien thefif thiTt> d«tn lia%-e lioen obtni nod— that i«, the fiK-fll 
len;[th of rach lens, and the distance between them — wi* are in a 
lukvitKm tonnply tiie formulie(ix)ftnd (x), p. lit*, to find thi> principal 
points E and E' of the ci>m hi nation. 

I la tlw «<rk«il I'UI aiaiiiiiW iiu iluaiiipliai liaa b«ni nisil* hvtirtvn llin r, r* uf 
nna Imi* aiul Ihr ', r* <,l llx- ntlirr !''iii, u,< wtU u ul ^ lu^ d, bvCiMUe irhm Um 
|iC^ulp«1 ivniiu ui-l ("ral 1i.''ii,-(li ..i» il.-l. rmllii"! fi-r cin" ]■<»>■ Ihiat wqmuiunB sm 
■M4 MKaiii iiMitnL fi ilir Mill' li'ii"» ouli ilill<'n-»i v*1iiK luaifnwd Is Hino nuy ba 
eqnutly irrll iiwd (<a Uie luil Ii-iik Tuo uuuiy ilijleruiii toimu sN apt Xo oo&IIUo 
tua iiluiUnt. bIuIi- (hnw *h-> tttr tjiiiiUar viLli matbiniiiilival Mtpmvion* irill aildcr- 
rntami tiiv utntfgrmrtil. 



In aeleoling tlio rnluc of the fccus to Im put iiitn the cquntions. 
fur l)iilh letues, th« U«t mmt \m> Uikcii. tlint i«, in Wix 1 (iv), or 
+ •^7, iitid in lens - (viii), or — l'!<7-"i, 

It will be iioticeil thnt thp value of K Iwina n<-;jntivf, it will 1^» 
iiiea.3uri>(l -.'tU inch to Uir loft from P. SiimlnHv, E' i* inmnun-d 
■8^*2 inch to thp left fif>in Q . 

^ aIho is l-^J* to thr left froiii E, aiid ^' 1-28 to the right 
(rom K . 

Tliese four poiiiU, E, E' and 0, ^', tire callwl ttift cnixliniil 
points of thn (?iiinl>iiiat)on. 

Hprp it \h- fihuervv} thitt iti thin work it hus \>een necv^nary 
for wM)t of N|)ii<'^ to imtriL-t tlii.- priihlem (u ilry Iciist^a, thiit i*, U> 
tlioaa ciLM« whci-R till! luy t!ni4TK<^s from the (.■oinbiiiiitiou into nir, the- , 
Kiimr medium in wbidi it wits iriivolHii;; on iiii mergence. It i* oil | 
thftt iK-coutit Ihut the vulue* of ■}> and * ai-e the amui.'. 

Having now oblaiiied Uie four i-tuxliuti] i>oiuts, we may nt Wicu- 
[irooci'd to lind ihft oonjujfftte of j-. 

Let r et|iial the dintATice of ihe point a.- from tlie fo<.'al pliine ^, 
iDid 1/ the dialiince of hn conjugal from eft'. Then by foimuhi (xiiiy 

J- ,/ = ,!>', nnd as j = l iiieh, .v = i^^— = 1-G384. 

This numoriciiUy (Icterinini>s the position of the conju^te pUiie. 
If the rays incident on the cnmliiiiELtioii are pantUel. them^^x , 

and y ^ ™ . ^ 0, which in«iins that y U coincident witli ^', 

The following is the jjiruiihic im-thod nf tindinu the coi\)U|i;at« of 
V. From V, %, Hr^, dniw n line piii'idlel to (lie iivix to tnoel E', Utd 
fi'oni Ihe point where it incetx E' draw n hn« through N, tlw pcniit 
« hero <(i' cuiK tin* axis, to AV, 

From V dritw another hnc through M, the point where ^ cat»i 
tlie nnix, to niei'l E, and frcun iJie [xiint where it merits B drftw » 
line parallel ic> the axin, cutting the other line in W. W will be the- 
eonjugale i>t V, which was rei|uireil. 

If it is re([uireil tii lind the conjugjile of a ray ]ia*!iing through 
thret! leniiei oil nil iixis two nf the leliKes lUUsl he coud^ined nntt 
their four canlinul points found, 

The principal points aud the food length of the third lens muM 
then W calculated, and then conjhineil in their turn by fonnuin- (ix), 
(x), (xi), und (xii), p. 1 IW, with the cardinal jxiints of the douhie i«ni- 
liination. d is taken as the distance of the lirstt principnl point of 
the combination, nearest the tliini len«, lo the !^^eeund princijuit point 
of the lens, nearest the combination. A frexh set <>{ iiuvlinal poinlii. 
in determined in thin manner for the three lenses, 

So also with four lenseA : the CArdinal |>oint« of each [uiir lietng- 
found, they are vouibiiied liy llio iwme fonniilte. and new caniinni 
points for the whole conihinntioii of four lenses are obtained. Simi- 
hkrly, the cardiintl points of tive, mx, or any number of lenses con. 
1m' found und the conjugal" of nliy point locJiliseiL 

Finally, no one need he diRcourngeil hy the appearance of Ihi- 
leugth of the calculation ; the example is given in full, so tint any- 




ime actjiminted only with 
vnJgHr fractions and dect- 
nuils can work it, or any 
other similar problem, out. 

In lens No. 1, for in- 
stance, the numerators of 
the fractions are all very 
nmple, and the denomina- 
tors of the four equations 
are all alike ; bo, too, in 
the equations for No. 2 and 
in those for both lenses. 
Farther, f is the same as 
f, f" as /", and 4.^ ss ^ 
llaice the problem is much 
shorter that it looks. 

If the conjugate of a 
point on the axit is only 
required, and if the prin- 
cipal points and foci of 
each lens haVe been de- 
termined, it will not be 
necessary to enter iuto the 
further calculation tu find 
E, £', and <^, ^', tlie cardi- 
nal points of the combina. 

The method of proce- 
dure is as follows : If a^ is 
the given point, its dis- 
tance from /, the focus of 
lens No. 1, must first be 
measured. Call this dis- 
tance X. Then the distance 
of o its conjugate from the 
Other focus, /", suiq>osing 
lens No. 2 to be removed, 
can be found by formula 

/s = -897, as = 1-65; 


This is the distance from 
/ to o. 

As the distance from 
« to/ is positive, the dis- 
tance between /' and o is 
also positive ; so o is to the 




Before procnciltng it will 1]» u well to oxomfue oUit^r i>osai1>1e 
tiuses which iiiiglit (wcur. 

Hupposn thnt ;p wiik aI iIik piiint /, then x would vquul 0, and I 
o = 3& ; that i«, a would lie kl mi iiiliiiitt! ditiUkiiw from /'. If. uii I 
the iither hund, thi* point x waa to the rij(bt of y! j: wnuld be uc^:*- 
tive, and u would h»? atsri iiegiitivtr, ln'cause _/'* ia nlwiivi> poaJtive ; ! 
o would thi'ji he niuiBured off to the IpH of /', and the conjufjKt* I 
would Ih.' viriuiU. Tliis meiins ihiit there wdl be no real imag^i 
because Ihe niys wdl l« ilivcrgcnt .m the/' side o( the leiia, t 
they bud coini- from boixk- f.fcus on ih*- /' side of tbe lens. But to ' 
return, TliP poiiit <> hiivinj; been found t»» be the ewiijugate of .«•, 
due to the aule inlluence of No, I len& we have next to meiwiure the 
distance between o awl/", nnd. by applying the aiiue formula, tiiid 
the distance of its coi\jugH,te froiii/'"', owiug to the exclusive eAact 
of No. 2 lens now replaced. This distance Of may be found thus ; 

F<. = F /•'+/'" = -847 + -543= 1-49; 
F/"=FB + BC+Q/"=-2l + 25+ l-8-5=2-335; 
F/" _ Fo =o/-'= J-aS.'i - l-49 = -»4.\ 

Calling this distance O. iJien, by fomtulft y O =/" *. we shall find 

the diHtfince of y fnim/"'. whifh we shall call y. y = *'--- ^ - -' 

U ■iT4-> 

^ 4'li), which is jMisitivi' ; tlicn-forn y Hi-* 4*16 inchiis fromy" to th«' 

right b a ml. y is tliwefore Lh<! conjngntit of j% dui^ to thr influenw 

of Ijoth lenstH 1 un<t 1'. Similarly, th«^ conju^ttr of any point ■>■) tlin 

axis may be found through any number of lemw*. 

LetM .1*0. I : Dittt. -Rmliua A = ' = r ; radius B = — 1 ^ i^ ; 

foci, //' ; thicknew = ^ = rf ; ^ = ^ ; 

P = principul point m«ii- 

sured from A ; P' = jmncipol point niensured from B 
3 .1 


- 1 


!(' = ' 

- I 


- I 

— 1 


^(«'_«)=:^-;-;;) = -j:./„.'=ix 

fi lu' — n) — rfiiu' = — ' + 

P = A + 


B + 

/i(m' — W)— (f HH 

= A + 


, = A + 

. = A + 




= B- 

, = B + 


= B - 1 





•' ■ ^(«'_«)-rfiiw' .n?~ 19 


= P - -947 (iii) 

i-f ^ pi '' ^ p' — ^ ^ p' 4- i? 

•' ^(M'-u)-rfw« ~^^ 19 


= F + -947 (iv) 


I^ma No. 2 ; DeUa. — Badias C = — = r ; radios D = t» = r' ; 


I u 

toci, /", /'" ; thicknflse =— -=rf;fj=; Q = principal point 

measured from C ; Q' = principal point measured from D. 

^-1 ^-1 
«__ —^ -^-,u-— _ 0, 

p(„'_„)_d„„' = ^_0 = |-* = -8r>3; 
(5 (S 

Q = C+ --'("l^_^ = C + l=C + . . (v) 
fi {«' — w) — dww' b4 


Q'= D + _^-^f-^^ = D +ilL__L5 = D - .^- 
fi (« — «) — ditw 64 16 

= D - -OGl'S (vi) 

/"=Q+ ,— ^--^ , = Q +^ = Q + ^«* = Q + 1-875 (vii) 

p(M — w) — dwu 64 8 



/» (m — m) — a II u d4 S 

= Q'— 1-875 .... (viii) 

Bo(Aien«e«.— Diatanceapart = BC = ^ = -25 ; P Q = -21 +"25 

= -46 = B ; /= focus of No. 1 lens = -947 ; /' = focus of No. 2 
lens = - 1-875. 



E = P4- 

if _ 

= P + 

■46 X -947 

/ +/' -i ' ' -947 - 1 -875 — -46 
= P-3U 

-^» = P + 


E' = Q* - . 


/+/' - c 

- = Q' - 

-46 X - 1-875 
■947-1-875 - -46 

^ -1-388 ^ 

* = E - I-'" = E - '^*1-^ ~ ^-^^""^ 
^ /+/'_£ -947 - 1-876 - -46 

=^-:S8==-^-^« ■ 

A'= E' + _■'>- = E' + ■^*/ ^ - *'«'5 
/■+/■- -947- 1-875- -46 

= E' + ""-!^4= E' + 1-28 . 

.^ = ^- ^ = *" = 1 

- 1-388 
> ] ■6.184 





■ric progrwwiin of tlm nioiiiTn ntii'mwiipn from its cnrliest 

tA iu most {H^rfi'ct fvriii m ii>it: iinlv full of intToKt. Imt is 

ina lull nf tbn most valiiablf iiistrurtion t'> llio prwticnl micro- 

'•(-■opikt. In n^nl tn llii^ dt-tuili' of tlii.i, our kiiowlMlgv hu bcMi 

j^>ail^ curiclu^ (luriiiK ri^X'nt ji-arx. Thr imtiijanritiii kiiowjcdjfo 

unci xn»l ill UiiB nuttUT jiomuuhi-iI b^v Mr. Ji>lin Iltnyiill, jun., ndcI 

tlK- uuiciun ftiul vnlunble collediiMt of utiocOHtripM mnilfl I>y frank 

C'rix)), Fi»{., Ll.It., running '^ Uit>y do llii'ou^'h all tli<- hiKtnry of 

tiw itutriimi-iil, from iKi mrli^t ejiiployment to ita Intot funns, 

i'Imvo funitsliHl u.i with u. Vixai-Xv*^ of the detuiU of ita lii»lurjr iiut 

jBi'HHi'd tiir iiur iiiuui'diiLte {nvdi^L'eBBors. 

Wt! iiiik}' ubtniii iiiufli iiuuf;ht into tlie nature of wlwl h india- 

jM>ii!uitileuii<ldi!sirBlil« ill tbu iuici-0HCiif<e, both uii it!i mt^vlmiiiciil aiid 

^opLical sideo, by j> tbou^-htful perutal of tbei« details. It nil! ilo 

liorc to <-iiitbt<i Iho student to infer vrliat a K<>0(1 mioroK. of)i> ^Jiouid 

\if. tbiiii iIm! inotil trxhuuiitivi? ai'couiit of tbe viiriotiiw of insli-ument 

nt thii tinif pniduoi^d by tli« !tevi>iul makers (alway* well pn-M-ntixl 

iu tliPir n-'ju-ctixe ontalo^oii) caii po&sibly do. Availing ouroelvwi 

of till? mutiTia) placed at our disponl by tli« Ketierodity of theno 

;:»nlleiii()n, we nball tberefore tr^ice the mnin points in tbe urifpii 

and pTi)irreu of the niicrast.'ope ii& we now know it. 

Mr. M^iValt' ;;ives what wu niuat vtinstder iinansurerable realms 

' lookiii;; upou the nitciviscope, ' as we know and employ it,' lui a 

ntrivtly iii4>dern invention. Its occurrence at tlie perimt when the 

, spirit uf modern scientiBc retetuvb was asserting itself, and when 

Jie nweHsity for all such ai<U to physical infjuiry and e.\periuiontAl 

'raBMtvli w«i-e of the biKfaest value, is a* striking m it is ftiU of 


It ntay lie held aa fairly estAhlishcd that nmi;;nifyiRg lenses were 
twt known to the ancients, tlx^ Kiinpl«tt optical in>>tniDient« as wo 
aiKUrBtam) them having no pUc^ in thi-ir dvilisation. 

A largn nuinbor uf |iaMn)j;i-K tiikcn from anciuit authors, and 
having an apparent or suptxiM-d n'fiTi'jicc to thi^ cm ploy men t of 
ma'jnifying iiuArumoiitK, bnvi^ Im-i^ei •jnUc-ctiil ami i-nrefuUy critivisMl, 
with till' molt that all such poioai^cis ca» Ih; explaineil witJiout in- 
lolvinij this usumption. 

We Ivu-o froni Pliny the (■Idcr and uthem, that orystiil ft'"'** 
£]led with wat^r wc«v employiol for cuuli-ri-iation by focu.-uun;; tlie 

* Ciwlor Lrelant uh Ihe Mi^veopr, IsM. p. 1. 




sun's myH iM n l>urniii){-gltt»x, ami tbut tli*-^e won- usofl touroduofr- 
i-p>iti(in ; liut thi-n- is no Imcc of siifi^'cslioii tluit llietu! reinuiting 
gloW ociulil n<^t 4UI nintinifying iiistniiiieiils. 

Knnccn ('Quost. Xiit.' i. 6, ^ -■>) KUles, howfver. thai 'letter*- 
tliniigh Kiniill 1111(1 indistinct iin- M*r-n tintiirgMl and tuon clislinct 
tliidiijKh 11 s'"''" "^ jjlai-i till«l wiili wau-i". He «1im> Btat«H lh«t 
'fruit Dppt^ii's larjier wliPii spi-n iniiiiprHpd in H vase of gland/ But 
lie only loneluilm fi-"iu thin tlnit nil olijecU txeo through wal*i- 
n]>pi'iir liirjjrr than thi^y mt^ 

In like niimner it oould b© shown tliat Archimedoa, Ptolemy, 
nnd othpia hail im knowledge of the princ-iplwi oo which refrnclini* 
took pUte at cuned surfucefi. 

Nor is thei-e any auHeiit mentJoo of xpectairles or other airls to> 
vision. (Optical plieuomena were treatt^I of ; Anstotle and the UrrMc 
l>h]rsici»ti Alexaiuler dealt with luyopy and presliyopy : Plutnrch 
lreate<l oE iiiyopy, and Plitiy on the night. But no klluKioii in niad^ 
to even the mnst simple optical aids ; iior is there any reference to 
iitiy Hui-h instruiiieiita hy any (Jreek or Roman physician or author, 
lu the fifth century of the (.liristian era IhetJi-eek phvsicitin Aciius 
says that inyopy is incUT'ahle ; and similarly in thr tliiiVpnlh 
century another (Irc^-lc physitrian. Actunrius, *ftVM that it it an In- 
fit'niity of sight fi>r which <irt win do nothing. But wicr- ihr- riul of 
thi- thirtj-eiitli mitiinr, which in after the invention of spcctacWi 
tlioy arp fre(|iirnlly referred to in niodicnt treatises nn<l otlin- wurkx. 
If w(^ turn to tlie works of ancient artistt we find amount their 
eiit gem* »onie works which n-vea! extreme miniitpni*™ of dctwil and 
delicacy of eivrcution, and some have contended that the*" coulit 
only have l)ef"n enecnitpd i\v mt^ns of lenses. But it is the cipininil 
of «xperts that there i» no engraved work in oar naiionitt eolieetion 
in thf gem (lepftrtmfnt tlint i"ouId not have lieen engrave<l liy n 
qualilied modem engraver hy means of unaided vision ; and in 
refei*ne« to MtniK very minute writing which it was stJitcd hy Pliny 
that Cicero .-uiw.SnlinuK and Plutarch, as well a* Pliny, nlltide to thext^ 
marvels of wnrkmnniihip for the purjiose of proving that Komo nu-n 
are naturally endowed with powers of vision quite rxceptionnl in 
their excellence, no attt^mpt being mufle to explain thi-ir niinutO' 
detiiils a* the ri'BuH of using magnifying lensM. 

These and many other instances in which ivtiTnncft to leHROft 
niuai have lieen made hail they enisti-d or liren known are coii- 
uluuve ; for it is iiiL-onceiviible that even siinpli" dioptric liMiaes, to- 
say nothing of BpectacleH, mieroscojieK and tcl.--i Mp,--.. muld have 
been knowu to theanoienu wiUioiit refen-nc<- In tli. in having h*en 
made hv many writers, and eeiiecially by such men lut Oaleii uul 

The earliest known reference l» the invention of s])ortnclea la 
found in a mnuuscripi dating from Fluif-nce in Il*9!I, in whloh the 
writer says, 'I tlnd myself so preyed by age that t oaii neither 
rend nor write ivithout lliuse glasnea tliey call H]>eetacteB, lately in- 
vented, to the great advantage of poor old men when their sight 
grows weak.' ' Giordano da Kiralto in 1305 savH that the iuveution. 

> Hnllb'x Opttrt, CunbrulKf. ITHH, 9 t«Li. ii. pp. 13, IS. 


of i^)Cctaet«« dates Ixu-k 'twmty ymT«,' which wnuUI Im nitout 1285. 
It ia now known that tlioy wtm: invmitnl by SalviiiQ d'AniiitCo <le)ilt 
Arroati, » Florentine, who died in 1317. Hi- kopt tho iMH:i«t for 
protit, but it WMa diiK<nve««d and pabli^lit-d linforc hi* <hiith. Jtiit 
thero is a abifculiir evi<l«nc« tlutt ■ Irns usr>it for the put-jMup of 
■uaftnilU'iitioti wriui in pxiHtenw- as tmrly ni hiHu-pon I.'il.tniid l.'iiO 
for nt thiit (ini* Itaplincl [aintMl ii portrait of T*o[n« Irfo X. w hich 
ia in tli* P&liuan Pitli. Flort-m*. In (bi.i pii-ture tJi* Pojic is driiwti 
boldinf; a luiiiil inii;;iiiti<-r, evidently intendi'tl to i-xuniimr cArefully 
tlie pages of n book o{>«ai befotv him. But tio inKtrutncntK ocmu- 
pwntbto to tbft oMMleru telewnpe and n)i<T<Hc(ijif atuMt nirliiT tfaan 
tin beginniuf; of the oevoiteont]! century und thu cla<i)i]{ yean of 
the sixt*eiiUi ceiilury respeclivAly. 

ft is, of rourse, known thai th^rc is in (In- Brilinh MvtiMini a 
miMrkable piert* of ri>ck cryW*!. which i.t ovnl in iilin[ie awd ground 
to » pUno-ooiivex form, which wan found liy Mr. Ijaynnl dunnj; iJn* 
(■xcavMiolM of Snr^n'A Pjtlaii> nt 
Niniroud, uid whidi Sir Ihtvid 
ttn'VHt4T tiplici-wl was a Irnn ilv- 
Kigm<d f"ir the iiurpvio of lnii<;nify- 
iiig. If thin could bp K(Abli«lted 
it wnnlil of coune Im' of i^nnt 
iatetwt, for it hu iKwn found 
pamiblp to fik tli« dat*- iif its pn>. 
dartion with great prohitbilily ax 
not Later thnn 72I-T0r> o.r. 

A dmwing of (his 'lens' in 
two MKfMcUi is shown in Hgs. 89 
and W); bat Mr. Mayall givm 
atrong and clcnr rvawna for con- 
dodiog that itii Icmticulnr rha- 
jmder im « dioptrin iniitrunti-nt 
hail (ii-rbiinly mil l>n>n iiiadc out. 
TliiTf an* clinidy strin' in it. which 
wiHild prov.' fiifiil foi' optiial piirpiwf-v but wimld lie cv«n ft0U|;bt 
for if it bod Ix-iai inti-mlnl imndfcomtivp boss: wbilp th<! finding of 
iba '<»nvex' Norfacn is nol Mnoi>t}i, Iml ])r(idu<vd by a liirKtr num'jer 
(if im^lar facrU, mnkin;e tlir curviitun- <)uitr unfit for oplical 
pnrfioMW. In truth, it may lie fairly tjU(i-n iik (»<t.-Ll>tishnl that there 
IS no Avidencn <if any kinil b> justify un in Iwlii-viiif; that letiaes 
for ojiticol iMirpiiwa wcrr known or Uiiecl l)efore tl»e inveution of 

From tiie Kinipl« KiHM-tiicli-h-nK, thi- tmnsitii>n to lenHes uf hhort«r 
and alwrtcrr fovna, iind ultimiit4-ly to the combinatiou of lenws into 
a f0ni]H)Und forw, wuuld Im- in nuch ati a^^ "« thai in which tin* 
inveniinn of siN-ctitrh*!! aroae only a inaltt-r <if titiw. Hut it is 
ikiUitHrt, iuijioftsibic to 6x tbe exact dait- of the pitxlgctinn of tlin tint 
nii(TruM:o[ie, as diitiiiKuii'b^d frapi a nit-re maf^iifyiti^ lenn. 

Ttient in net'ertbelMs a contienl on the {uirt of th'^c beat nlile 
io jml-e that it must liave Iteeii lielwfi-u l.^^O an<l 1<>09 ; while it is 
firobabte (but by nu meajia oertAiu) thiit llitns and Zachuiiax Jiimueu^ 

Pia.lID.— All XHyiiaii -biu' (T). 


speccaclo inaken, "( yVM\i-h\jrs, Holbuid, were tli*- inv«n(ors. But 
it would a|i))«ai' thaX llie tukrlmt ijiicroscope wilh cunstrucicd for 
«bservinf; otjiecU by ruliected li«lit only. 

At tbt- LiHin Colteutiun of ficieiititif tnHtiiiiiivntfl in London in 
187C an old uiioruni'ape, wliioh liiu) beeii fuuiul tit Middle l>iirg, wm 
.aiiown, whicli, Proff^ssor Hitrlinf; coiisitlered, uiij^ht poM)t>)y hnvti 
Ijeen uiiwle by tli<^ Janh*.eiis. It is drawn in tig. 91, nnd v<>niii«tK of 
a L-onibination of a L-onvex ubjei-t-leiis nnd n (Minvrx nyo- 
li^ns, which form tias uot |<ul>li8bed as an Act.unl (i>n- 
Htructioii until 10411 by yiiiitnna, which, as Mr. AInyall 
(xiintfi out, does iurt liarmoniKO with thr n**uiiijitiiHi 
that this instrument was comtnintcd by imo ot tim 

h is strictly a compound microscope, and Uw dis- 
tdiici" V>plwpeii the b'Tise* can In) n^RnlatMl by two 
(Iniw.tobe*. There ai-c tliirn dinphraeini, nnil tbis cyw- 
Icim lic« ill a wiH>d eell, and in iidd thi^rr- by a win! ritix 
hpniTig in. The nltjrct-lenii, ti, jk luniti- iu tbri «vtu«l 
iristrumcnl, l>iit wan originnlly li\»l in a xiintlar way 

tt cunnnt Iw (111 pnjsy tftnk- — it It Iw ftven a poa- 
-libli- .inc In iMiiiitely d'-tfriiiine u|nin the nrttutl indi- 
viduiil <ir individimU by whom tbt) <'()mpound mirro- 
Hf'(j|w was first invrntiil. Kefcnlly some valuable! 
evidence hns bec-ii iidiliK^cil i-Jaiming ita facile invention 
for (!ali](Hi. In a ineiuoir publislied in INHFt' Pro- 
foiunr G. t>ovi, who has inadp the qiieKlion a subject of lar|:|nnnd 
continunua ri'seareli, crrtninly addiirm evidence of u kind niit «aKily 

HuygliMiii and, following him, niiuiy otlieni assign tlie invention 
of ihc! enmpDUnd micniKrope to ('ornejinx hrebliel. n l>at>-hniAn, in 
the year 1621 ; but i( has berri sujiifc-strtl that he dfirive<l hi* know- 
ledge fnrni ZiLchurias Jiuiuen or liis father, Hans >lnnssr>n, Kpeetncln 
makerit. ill Hulland, about ihe yeur l.VJU: while Fontana, a Nwi- 
polibin, i-liiimed the diicwvery for himsi-lf in liJlf*. It is said that 
the Jan6ri.ens ]>reiient«d the Hrst niiL'riisco]»- to Chitrlos All«'rt, Arch- 
iluke of AuNtria ; and Sir D. IJifwuU-r states, in his 'Tivnii.wnn thu 
Bdicn:i.''(^oj)e.' tbui one of iheir miiToscopes which they prexenlnl to 
Prince Maurice was in 1GI7 in the |)ossession of Cornelius DreblieJ, 
then Matheitiatii'ijin to the Court of Jainea I., where 'he made 
mieroscapes nnd parsed (lietu iilFa^ his own invi-niion.' 

Nevertheless we ni« (old by Viviutii. an [laliiin luntheiuaticiaD, 
>n his ' Life of t>altle<i,' that 'this ffrtiat man was led to the diaco very 
of the iniernscope from thai ot the t«leBcope,' and that * in 1612 he 
sent one to 8igisinund. Kiti|; of Poland,' 

We now receive evidem-e IhruuK'i tb^ reiiearchea ot Qovi that 
tJio inv-ontion was solely due to (ialileo in the year UilO. 1*1 1 iff mil 
Uovi undenitands by 'simple microscope ' nn iiistrutnenl * coTulstinK 
*if a single Iotis or mirror,' an< 1 by ' compound mici'oat'opp ' one ' coii- 

* All! It. -troll. Sfl. fi'j- fiat. Siipali, ml. il, i».rii>< U. ' U nucroMO|Hn onniiKWia 
in>«ntatoda(liJiW.',rmin>. R,.V..S. Pi. [V. lami, p. STI. 

i-'jii 111 



sining of Mvoml Iciiw* or ft suitable oonibiDalion of lenses niul 

III n [uuipKlrt pabliiiheil in 1HX\, tn<oX\nn of the itiveiiUoii of 
ih« binocuUr LcIc-kcojm-, iiii\i ]M>inted out tluit Obortti; a spectuvle 
maker, in 162'i, usvd tho Dutch t(>W.-ope im n microK»p«t, nncl ntutod 
thai wich it 'ii tnitn Ap])n>r«(l aa Ui'go nn n pe* ; so thnt one mn 
ihRtinjki'i^l) itK hend, iti^ fprt> nnd itii hair n thing which Roemed in- 
LTi'dilijt' to uuuiy tnitil tWv wit^iosspd itwitJi fulmtrntion.' 

To thiH i)Uotntioii he Aiiiliil : — 

'TliU tr.iiixfonnnlinn of thi' tnlntrojw into u miorosiJiipo (or. im 
OfiticinnN in <>iir oviii tlay vmiUl siy, intii a. Bnickr \rn*) wnu iint nil 
invi-ntion »f tho PriTiiehujiiiciiin. l!iilil('iihiiilnoi-i)iii|ih^hcil it in tho 
yar ICIO.iiuf) had minoutircxl it bitlir kiinii-d liyunc of hi» )>i)|>ilH, 
John Wi (fid i'r)>o I'll, a SLVldiiiiuti, in ii work whii^h the liit(<-r hnd 
jitat ti(ddii>hi-d UK'i'ini't lh« mud ' Pen-^niua/ionf ' of Ilot'lfy. llvtv 
an* til" I'ltwt nonii of Wiidilt^rliorn (|>. T) : 

' Eifii imiK' nihnindiilia hiiiiiK jviftjiicilti ptirftwlioneii expliiniirv 
ItA <M>rmli<ir: wnnUB ipM- iudex «'%l intt^-erriiuus l-itul <>hi(>ctum pro- 
nriaiu. Quid iiuod eniinu^ uiilli* jiuKSUa el ultm l'uui jit'ijue viitcn; 
tatlMarws ohii-ctuni, ndiiihit/> [ici'ti(>icillo. stiitini <-erto uogiHWcaa, «we 
bunc SocruU-m Soi>hrv)tiii-i litiunj vonii-utcm, sed t«iiipuB non docehit 
•■t i|(ii>lidiaiu(- noUiirum rt.'i'uni deteclion*^ quaiii ci;rei,'ie |)enpioi]luiii 
»uo fungatur iiiunere, uam in lioc totft omnts iiulf-uineuci «itA etl 

* AurliuiTntD, paaeis ante diehus anthorem ipmim Excel lentissimo 
D. Creiuonino purpurato philoGopho varla lutrninicin acitu ili^iiiisKiniu. 
<4 inter c-.elen* quomuilu ille luiiiiuioruin aniinuntium or^tiu niuiuti, 
«t wtwua ex ]iFr»picillo nd vn;;ueiu distinijujit ; in jiitrlicul'in uuU-in 
de quudem inaeotu quo'l utruni(|u<i li^het iwuluin nicnibniiiH crostiiuB- 
cuht vt^iitum, <|UH^ tatueii st>pte foraininibus k<\ instjir Inrvie ferreie 
militia catnphracti ttfrebcatu, vinni ni'ir'ljet speciebu;^ viMbitiiini. Kn 
tibi faoakj's Wodderb»rn to Horkyj nouuin Hrgiiuiriitiiiu, iiuod per- 
apirillum |H-r (.<uiii-<-iitraL)Diieiti rMlioruiu niuHiplit'ot obieciii : am] 
«udi priua quid tibi ditturuft sum; in cioleri^ miiinnlihus eiuHlem 
UAgnitadiiiiit, \f\ minoria, quorum ptinm iiliqua spteiididiores haWnt 
oculua, fceuiini tJUtlum Apparent cum auiMgupcn-iliisAhinjue purtibua 

To tliin Oovi wills : — 

' I httv« vi8he<l to nMnUf ttii;^ paasasQ of Wodd«rborn tpxtvully, 
«o that the honour of lukvin); hern th« brat to obtflin from tbi^ l>utch 
ti>If>«<y>pQ « eiimfuutvl inicrriwopp should rcmnin witli (.hdilw), which 
ih*- IntiT callnl r»fiiutlino, and th«t the nlory nf hnvinft Rihiood tlin 
KrfJrriiiH lolcscojip to It inicrosci>|>c (in U)2I) nhouhl r<'i>t with 
L>rabh«il. The apologintA of tJifl TusuHn phih»o)ihcr, by oltril'UtinK to 
him th<i invonlion of tho micro»co)H- without ipi'i-ifyint; with wlmt 
uiicnwropi! thry wrrr^ (Imling, dcfniudinl Drt-hliel of a lui^t whicli 
n^Jty ht'longH lo him ; but the defenders of Dreblwl would ncL un- 
justly in cIrprivingOnliWof a dixcovtiry whiuh incimt«cl»bly uushiB.' 

I turn now to Wnddi-rlMirnn oct'ount, publisJicd in 1610 (the 
4ate of thi- di-dionlion Ut Hiinry Wottoii. KuKli^h Aiubn^aailor at 
Yonice, is October 16, I6I0), whivh n«ds Uiuh :— 

124 T1IR insTOnV AXTJ EroUTinK or TnE MlCROSCdrE 

' I will tint n»w utt^impt to pxplnin nil th« porfcwtinns of lliisi 
wonderful on'fiii'f' l our HPiiac iiloni- in n wifr juiign of the tliinjfftl 
whicji coiirt-rn it. Itut wh«t tnniv- ciin 1 any nf it tlinn thnt by 
pointing a. glaxn to an objivi luon- llinn n thousnm) ]mcrn off, whiirti . 
docs not tivpn Kct-m olivr-, yoit iiiiiumlinti'tv rt-'c<)(|;niB»' it to h*i 
SocroUtx, ton of SojiliroiiicuH, who U iipproAclune ? But timft and ' 
the duly cliarovcriM of aew thiiijaK will U-arh us bow tulmintbly theJ 
glass dn<!X its work, for in llnit ntimo lit^it all the bnauty of thnllj 
inatnin>ctit. I 

' I hfJtnl a fi!w diiys back the nutlior hinist-lf ((inlileo) narrate lo 
tlio Mn!it Exciilli'tit 8i;{nor Ci'i>in»nius vnriiHiN things moat de!dmi>le 
to bp known. «n<i unionxit i>tli''r» in wlwt inutiner h« perfectly diii« J 
tinfcuisli*'!! wiih h'u t*'lti*cojn! the or^pina of motion mid of tl)B nenaeAl 
in till" Miiidirr nnimul.i ; iind os[iennlly in » c-«rtttin inaetl w-Iiirh hiis 
oaoh i^vf fovfti'd by it mtln>i- thiL-k mt!tiilii-»no, wliioli, bowpv^^r, jx-r- 
fonit*?d wiib m-iiMi lioli-s. likf ilte visor of u wmridr. allows it ai^ht. 
Here hiLiit ihon a ii«w jirouf tliiit the ^luas concent nitiii}; its ruyit 
«nliir;^H the object ; but niin<t what I am about lu tell thee, vie. tii 
the other niiinmla of the Mime stie and even iiinnller, some of whiHi 
liave iieierttielet* briffliter eyes, tlieHe nppear only double with their 
eyebrows and the other adjaoeiit parts.' 

After reading; this document Uovi jud^f^ee that it ii> iinposeible to 
refuse Galileo the credit of the inveiilion of a cum/Hmurl tHicni*y-ji' 
ill 1610. and the Application of it to examine some very niinule 
animals ; and if he himself neither then nor for many ywrs after 
made any mention of it publicly, this cannot take away from him or 
diminUh the merit of tlio invention. 

It is not to 1)0 believed, however, that Galileo after these lint- 
experiments quite foi''jot the niicronrope. for in jirepannf* the 
' Sd ;*;;'"*"'* ' hotwopn the end of )G19 and the middle of Octolier, 
1632, he spoke thus to Ijotario Snrsi ?!ef{enMinti (ana^nm of Orali<>' 
Unissi iifi^lDncnM') : — 

' I inight toll Srvrsi sonicthinj; new if anything new could lie tiild 
him. Li't him tJiki; any Kiilntance whatever, lie it Moue, or wood, 
or mortal, and holding; it in tho mm cxnmine it attentively, and he 
will MT nil tlir colourn dintrihutM in the most minute particles, and 
if lie will maki- use of n tcK-Kcope arranged so tliat one can seo very 
near objt-ct*. In; will sns far more ilistinctly whit I iiay.' 

It will not thi-i\'fore Iw siirpriKlng if. in liVi4 (according to 
anme b'ttni from Home, writtni by (lintlntno Alenndro to the 
fituiuui M. de Pi'in-Hc), two micro^ropi-K of Kuffltr, or rather I>rebbel, 
bavin],' ln-ru sent to ihr (Ordinal of S. HiiMinnn, who at first did not 
know how to use tht-m, tht-y wt-rP *liowii to Calilm, who wn» thon in 
Koine, and he, as xoon as he- kiw them, explntncd tlipii- vrr<, as 
Aleundixi wril(» to Peii-eao on Mnv 'H, addhi;:, 'Onlileo told mc 
tJtat lie bud inteiiled an urvrAin/'- which ma^iilies things n* much 
aa AO.OOO tiui«». so that otie sees « lly as lar^- as a hen.' 

This assBrtion of Galileo, that lit; had invpnteil a telesfopo whtidi 
ma^iiiti^Hl W.OW) times, so that a tly apjifars as big as a hen 
must, wiihoot doubt, be referrerl to the year IfilO, and from thi- 
raeunre j;iveii of tlie amptiHcuiion by the stolidity or voltime t1i<^ 


liitvar anipliticittinii <m it is iisiuiUy exproianl dow) would havs, 
bwo rcgual to nnnHhing lewt thnn tiie cubic root irf ■10,000, that is, 
tttxiat 36, niwl tlint in pretty falHy the relative nize. of « Iljr nnd 

AkAiidro'* Iv4t4;r of Mn; :;4 {Hit) does tiot stat« at ulmt litn« 
Oalileo mw th« UrliiMXHie And explained tlie iib« of it, but anotJi«r 
Ivtter of Fkbnr'E to Crai, nnMuig&t iIm- autu^rn)>li lelt«n in tli« 
puMcniun of D. K, Itonoiunpngiii, says (Maj 11): 'I was ye»ttaxb>f 
ftvwning at the hou*p of our r^ignor Ualileo. wlto lives near tbc 
MndiJwia ; be jiuvt^ Ibn Ciiiliiial di Zoller a ma-^iiiticent vje;;''^"" 
lor tbii Unkf of Huvaria. 1 khw h tly vliicli .Sifiior (JkliW him> 
nlf showed nw-. I vat. ahtotinilt^l. niid told Ui^or Unlileo that liu 
w«K aiM>t)M--r cn-atur, in UiAt li« i<howx tbin^ that until tiow we 
clW not k»ow bad bwn' So Unit cvpn on M«y 10, 1621, 
timlilco bad not odIv Hen the trU^Kcow «i' Drrlibcj, and expUinod 
the tun of it, but hail made on* liiuiKvlf and snot it to tii« Uuke of 

Wr liu-k di>cuiii<-[ita to 3Jm>w luiw this uiicrMeopc of (ialilen was 
nuub-, that is, mhi-thcr it had tno r<Hivtirf[i^ot loiiiWe like those of 
bnd>U.'l. A k-tti-r uf PitircHr of Mnivb It. 1624, cays tJiat 'th« 
Hfrct of tli« nhva in to tdiou* the- object Dpnidit down . . . and so 
that thr nstl tiatunil motion of tbi- aiiiinakulb whit^b, for oxnniple, 
jcont frocu mat U> went, M'ruis to g<t ooiitniriwiMt, tliat tx, frou w«8t 
to eaat,' or whfttJter it woji mil nitbi^r coin^ioiicd of a <-onv<'x and m 
cnricavp lent, like tliat aiadc i-urlicr by htm, and UKfd in 1610^ and 
tltcn idniost torjiOtUtu for fourti'i-ti yi-nrii. 

It i«. howi-vrr, very prolmlili' that tbi« last was the one in 
iptotion, for P<-ir<Tsc. anxwiTinj; Ab-audro on July 1, 16:^1, wrot« ; — 
'But th<? acchini' mentioiiml by 8ik'nor tiuHli-o, which makiM flJM 
likir h<-nx; ix of biii own iitv«iitti>h, of which he miule aim a copjf 
for Ardidiike AIIhtI of piouM nii:ni<>rv, which umsI to br pliiced on 
th« ^Txiuml, wh<-n- a tly would he Mren t[i« aixu of a linn, and the 
iiiairunii-nl wax of no ^riater luright tluui nn ordinary dining-room 
taUe,' Which de:u-ri]>tii>n an^'Knn far bvttirr to a Dutch t4'le- 
■MWM UHctl «» a niicro9Co|K', in ihi- uine way cxuctly am Galileo 
had UM^ it, rallwr lluji ti> a iiiii-mtH-'ope with two convrx 

One cannot lind any fiir(h<rr {Kirticulars concvruin); Galih^a'K 
iHvAiu/ixi (no lie luul ciiriHti'n<--d tlit-iu in tlie year l'j'24), either in 
Bartholomew Ini|ieriali's k-tt*T o( JJept^uiber 5, 1624, in which ho 
tliaiiku rJnlilco for liai'iiiK Ki'''" him one in rrrry tenjf fmrfiKt, or in 
that of Gulil«o to Cesi of Sejrtembpr 23, 1G24, accompanyliiK the 
cift of an wvAi'i/iiiu, or in Fedrrit'o Ceu's answer of October 20, or 
in a l«tt«r of GartlkolouiM) Balbi i^ tlalileo of October ■iX^, 1624, 
which ajfeukh of tlie li>nKi"n «ith which Balbi \t. awaiting ' the little 
iioeitiatf^ of tlte new invention,' or in that of Galileo to C«Rar Mandlt 
of December 17 in the iKUue yenr. in which (Wliten ntyK to the 
learned Bolo^tte ' tkat lie would have sent bin> nn urehialitut Xn 
see close tlie snmllest ttungs hut thr )n»truiuei>t niaki-r, who n 
nuiking tli« tnbe, has not yet. tiniKbcd it.' This, howi-ver, in bow 
Galileo qwaks of it in his letter to Fcuhirico Ceaj, written from 


Flomire <>n 8opt<tui1)ftr 33, I6'J4. tunr« thiin three mrnillis kfUT Ills 
(tc}Hirtuii' fn>ii) Itnmft : 

' I si-iirl jiiur BxoeUcRcy mi aeeliiitlinn, by vrluoh to see close ihtt 
KiniMfM. tbiii;;!, which I hope may Rive you no aiiinll ]>lMiaiire atifl 
<>nteriaiiitiicnt. m it <ioes me, I hiive been tonj; in fletidin;; it. because 
I coul'l not jit'rt'ect it befnre, Imvin;* exijerieiiced some diHk-iilly in 
tiii'litif* thi- wiLV of cuttiii;; the ^liiHse^ perfectly. Tlie object inuiii 
be |)liiL-(-<l on the moiuhle eirde mliiuh is at the hose, aiid inovetl to 
Hee it nil. f<>r Ihiil which one sees at one look in butn small part. 
And liecwu&e the distance between (he lens niiil the object must be 
tnost exitct. In Icxiklng at objects which bnve relief one must 1>e able 
to nmvi- the glnss nearer or further, arcordinB as one is looking at 
thi«or thatpn.i'l ; therefore the littli'tubeisnin'lenioviihlp on it«stai>d 
or guide, n« we uiny wish to caII it. It must nlso he used in very 
bright, dear wettber, or even in the sun itself. rcinemlieringthnttJie 
object must be soffieiently illuminated. I have rontt-m plated very 
mnny nniinnls witli infinite admimtion, nmongst which the lloa ia 
most liorrihle, the grmt and thi^ moth the most lienotifiil : and it wns 
with gre-itt satisfaction thai I have frvn how dieit and otlii^r little 
animals manage to walk sticking to the glois and even f<i't ujivrtinlK. 
Hut your Ksoellrncy will l»n*i- the opportunity of olwrviiijC thousand* 
and thouaimls of other ch'tailx of Hie most curious kind, of which I 
'*S you to gix'c m('a<roiinI. In fart, oup may oont»-mph>teenill™*!y 
the greatness of Nntun-, ond how »ulitilrly Bhe works, nnd with what 
unRfieakabW ihlijjcncc — P.S. The litlk- tulie is in two pieces, nnd 
you may lengthen it or shorten it iit plcnsun-.' 

It would he very strnngi", knowing (ialileo's character, that in 
1G'J4. niiit after the atl/icks mailc on him for having jierhaps n litttr- 
tiio much alloweil the Ihilch ti-lcKcope tohi-considpred his invention, 
he should have l)cen induced to imitate Dreblielfl gla-^s with ibe two 
tiouii-x h-iisra. aiidbaie wiahid tJininkr Iheinjiiissnshisowninvfnticin, 
wliilst he had always used, ami continued lousi- to the end of his days, 
telescopes with n convex and « concave lena without showing tliat 
he bad renil or in the least appreciateil the |>roposuJ imufe by Kepler, 
ever since 1611, to use two con vex gUsses in order to have teleseojuM 
Yfith a, lar;{e lield and more powerful ami convenient. 

In any cuse it is impossible to form a decided opinion tin such a 
Matter, the data fniling ; hut the very fact ttutt fruni I S'2i onwards 
(lalileo thought no more of the urrhvifimi (prolsibly liecause lie found 
it less powerful and less useful than the orchinfe of Drebbel), oa tie 
had not occunjed himself with it or had scarcely remeudtered it from 
the yenr I6I0 to 1624. seems sufficient to show that the ori-hinfimi, 
like the microscope of ICIO, wasji small Dutch telescope with two 
lenses, one convex and one concave, and not a reduced Keplerian 
tsIcKope like that invented by Prebhel in lC2t. 

The name of microscope, like that of telescope, firiginat^ed with 
the Academy of the Lincei, and it waoOiovanni Faberwho inventivl 
it, as shown hy a letter of bis to Oesi, written April ]ti, lii'J-'), and 
which is amongst the Lineei letters in the possession of I). H. Bon- 
compngni. Here is the passage in Falier's letter : — 

' I only wish to say this more to your Kxcelleiic}', that i*, that 


Ton will cUdcb onl5 nt wlmt T hiive writUtn cuucwniinf; Uir nrw in- 
vcntioiw iif Sigiior (•altl«o : if I have not jjut in fvtiy tiling, or if 
niiythini; uufjbt to be Ml uiiHuid, do uk licat you Uiiiik. As I nUo 
iiMintiun Ilia ii«u' <k\-A('i/' to look At Ktiwll thtii^ muI cull it mk-m- 
■m{M, let your Exix-ll>')Ky i>e« if you unuld like U> mftl tliat, as iJh- 
Lvi^'uni ;{ikve to the tirit the iiAiiit- of lelt^copp, tio tlipy have wislird 
til ^vv a (.-ou vent VII t iinine to tliLt ulni, iiiiO rightly su, Ih'ciiuai' tlii-v 
ure Uw lirst in itoiiie who hAd one. As <tix>n mi Si;;iic>r Rikit>'H. 
epijiratn U finished, it may be print«tl iho next (Uy ; in the nirnti- 
whiltT 1 will got nn with lh« rest. I humbly reveiviice yuur Excnl- 
leivcy.^Froin Kikim-, A]>ril 13, lA'ifi. Your KxcelleiuVs niiKt 
Iminblesrrv.iiit. ('luvAXM Fadkk (Lyiiceo).' 

The Atilx- Kr:rxi. in n work of tii« oii the intention of the micro- 
itc'i{>i-, thituijlit tlint lie in if; hi CMticlUflo flvHii the possa^ of 
WudderlKvrn, i-f^^KKlitcrd nlioVf, that (tnlileo (lid not invent the coiii- 
IHiUttd n>i<;ro««>pc, hut giiv« n rolivenicnt form to t-li* fiiiiple niicro- 
SL-ope, nnil in Uiikwuv mk good iMinvmtvul it, fur the lAtiii word used 
by WwidrrU.i-n, j-fntjiirUini,,, • Eignilii^d i»t thnt time, it is cle&r,' Itexzi 
s»yB. ' no otlwir opticjil in9.truinriit than spcct«cUw or the ti^lewope, 
iiover tim nii*^rocu.t.pi-, of whioh tlions is no mention whatever in any 
bonk puliliKhnl At thnt tinir, nor in any mnnuKcript known till th«n.' 

Iliit Uraxi WH.S not iniiulful thiit im Octolicr M>, ItilO, the ditle 
of WraldTlMni's rrmiy, tJie niinMt of miornnopn liuil nnt yot be^cii 
tawntM), nor that of ti'lMcnpi^ whieli, ncconting !•■ Knhpr, whk thf 
idra iit C<si, acconliii^' to utlifrx of (iiovanni IVmuiano, of 
OepfaaloniH, at tho end, iicrhnpx, of UHO. but inorti prolmbly nt the 
time of <jnli)«i'« journey to Uomc inm March S9 to June 4, Iflll. 
If, ihcTpfore, the wonl inii-roscoiM had not yrt bei-n invi-ntpd, mid 
if tlw lekKr<>j>i-, or the orx/iiair, ax it wiu tlii-n called, wiiK bv all 
tuui>ed p'rufiiciltHui, iiiii- cwnnot ten why Witikhrrliorii'K ii^riiiirilhiif 
oannnt hnve ba-on n muui'-^binle (telracojin) >mn,Il<-i' tliiiii tile uHUiil 
OIMS. MJ that it ooiilil enitily Im^ uwd to look nt nnir otiji-cl-i, but yet 
u rnnnoi-rhialf nitli two leiiM-s, one eonwx luid one conciive, like tin- 
oUmth, oiirl, thei^'fure. u renl cnuifuinn'l MifniM-<r/i*, ultliuugli nut 
mi'ntioned liy lliai imnie eitliev by \Viiddprl>irii or other*. And. 
brailles thai, how i-ould it be that \Vi)tl<lerbo[-u beginning U* trmt 
• admirabiliK huiun peif>|»icilli,' tli&t is, of the Iflttwiif in the limt 
lii»e. ahuuld theiibuve culled/wrj/iiViV/NijianiiiglelenHin the eleventh 
linn of the suuie pajfe I Keiud's mistake is easily expliuned, remeiu- 
U-riiig lliAt lie had not under his eyes Woclderborn's essay, but only 
kiH:w H brief exlmei leiHirteil by Venturi. 

It thus Appears as in the higbe»t degree probable tliat (.Inlileo, 
in IftlO, was the inventor of the compound microHco[>e ; it v/a*, 
iiul]ii«<)uenlly invcnteil, or introduced, and zealously adopted in 
Hwlland : and when Dutch invention penet.rntrd into Italy in Ki'il 
(ixlileo iiilempted n roclunnition of his invention (which wiln undoubt- 
exily diblini-t from that of l>rebhel); but ■» these wei-e not warmly 
seconded and reepomledto abroad he iilloweilth(rwti<il<- thing to puss. 
Neverllielew the fuels Uori gives iirv as interesting iis they are 

In r*{;»rd to the lUscoverj' of the aimplo lenit Govi pointi out 

r28 THE msToKV ash rvoijtws hf the ainnwroPK 


th«t nft«r the year 10l>0, niincts liaviitK iv>peiied to hope and in- 
t«U«cU to Ktudy. tliOTO lirgiiii to (Uwn some lislit uf science, no that 
iit 127t> a FmnciM'nn monk, Kogl^r Itocon, of llcli«stpr, iu hia' Opw 
MnjuK,' ilnlicntod niid prrKnntot) bv bim to ClemMit IV., could show 
nuui/ marvellous thingH, mid nniongst tl)«iK tli^ cllicwy of ciyitnl 
lenecsi, in onlpr to show thtiigx largi-r. ond in thi^ viw \w says niak«< 
of tlii'iu 'nil inKtruiiicnt tiinfui to old iiii^i niid tliiwi^ wlkOKe sij;ht m 
woakcncd, u-ho in kucIi a way will he ablr to mv the lifters buf- 
ticlMitly rnlnrgtHl, liowt-viM-Mimll thryarr,' AkI<>ii;;il<. nodocnmcnu 
nnt<'rii>r to liim Am di.^covrrcd, Hi^r 1taci>n may lin conHidprml thr 
lirrt invnntijr of ciinvrra«nt Iciim-s, nnd tliprpliirr of tlio nw/ilr micra- 
iini/,f, ll■lu'l^vcl- small tlii^ ciiliirj^i-iui-til by liU li'iinrt may hB<r« bean. 

Ae, liowcvcr, tJiat man of mrr ^iiiuK. tlii' inilintnr of cxperi- 
nu-ntnl nhyjiirjt. Iiail lirmi^ilit on liimxi-lf the hiiti-cd of his onnlempa' 
mric*, tlii-y ki-jil him (or luimy yi-ars in |>ri*im, thi-n shut him up in 
A coiivi-nt of hi.-> oilier in tin- rnd of liiH \otig life of iinnrly eighty 
ymrs. His writing;!! Iiu<l to In jiiddrn, at Inutt Ihoxv tnntingon 
iintui'ul Nci«oce, to itavp Iht^m from ittst ruction, unci .10 the inrciitioti 
of Imisi^^ or tJlti knou'lol{:«^ of tlivir um.' to puliirftt^ imnjp-it niMl to 
idlet iat4> the iiHirniitieH of niffht, reiniiined unknown ■>■- forRiottnn in 
the ]»i^:«r« of the fniuoUH 't^tpun ?tliijus,' which only cnnio to light in 
173:1 by the i-iu* of Sainucl Jeltb, a l»«nieil Englidi dootor. 

A Fioreiitiiie, by ti'iine Salvino de^h Ainwli, «l the Mid of the 
thirteenth oentury H 13W0) (in fiaron'ii lifetime), had tll<^rpfo^R thu 
ijlury of inventing; Hpei-Utcles. nnd it was q luouk of Pimi, Ah^undiT 
Sjiina. will, suddi-nly •■hiiritiddy divulaifd 
tlip M>ci*t of their CO ii«t motion and ii«e- 

Prrha]w Salviim iIpjjIi .\mijiti and Hpina 
ivally (liHL'oVrrpii riiort? tlinii Kopcir Itacnn 
had disciivereil ; that is, they found out thi- 
u^v iif uuuvemiia letutu tor long-ni^'hfxl 
people, and of diverging h-ns^s for short 
M;;ht. whikt the lCiif;)i«h monk had only 
Hjiokpn of the lenttt^s for long sight, anit 
[>erhiips tlii-y iiddiil U) this lirst invention 
the i-iipiibility of varying the focal lengths of 
the lennen according to necil, and tJie othci- of 
Hxini; thfui on to the 1 isor of a cap to fciwp 
them fimi in front of tJie eyej^ or to foKtmi 
ibi'ni inw two circW made nf metal, or of 
lionp joineil by a &nmll i-liwtio bridge over the 
Roue. Elowever it may be. the diHcutery of 
sppclaclm. or, as it may be cuIIhI, of tln- 
ftin/ilr mirntffii/ir. Way lie iH[uidly <liviiU-d 
between Hoger Itat-on utid SaUino <Wli 
Annnti. IraTiii;^ ei^pecially lo tJie latter tlie 
tn%-cntion of tpectacles, 

1'hn purliest known illustration of a 
Kimple niicTCMcope i» ^iven by Deacartt^ in 
Lhiis ■ I>ioptri(|ue ■ in 16.17: fig. 9:» ropnxluccA it. It is pmc-lioally 
ideiitiml with one devioed bv Liobcrkuhn a ceutur^' after and idK>wn 

Ftln. ra.— t>f>euUi>'iiiniiilii 


'Fta. M.— CiuBpuir>i luiemwoi* 

are two small micTMCOpes which 
it is a8irra«d liave bwn Iinnded 
dovii from giennntivn to gcne- 
retion ftince the diHolutJon of 
the AcctulMnia dpi Oimimto in 
IfifiT, with t^i« trnilition of 
linviiig bfipn cimsli-ucUMi by 
Gnlilisa. Thi-y nn- slmwii in 
fig, 9S, but from the saperiority of cMistniction of thi-s.i iiutni- 
iiK-nts it U very iniproh«l)I»i that they hnlong to thn dnys of Galileo, 
who di«d ill Uii'2 ; aud then w » spccinlly iiitcrvstiDK c-oiupound 



niicTOsei^ie, by Giuseppe Ouupani, irfaicli was published tirst 
which is presented in tig. Si ; ita close similiuity to ' Clnlileo i 
scopes' is plain]}' apparent, iiiakiag it still more improbable 
th«!se could b« given u ilat« prior to 1642. 

In a jonmal of the travels of M. de Monoonys, puMish 
1065, thnv is a d^st-riptioD of hb microscope which in of 
interest. He Etateji that the dii^lance from the object to the] 
lens w one ioch and » half ; (lie focm of the first lens is odb i' 
the distance from the first lens to the second is fifteen 
inches ; the focus of the secoiifl lens, one inch nnd a half ; 
diftAnce froni the second to the third, one inch and 
Hght lines ; the focus of the third lens, one indi and 
eight lines ; nnd the distance froni the eye to the third 
lens, eight liiH's. 

This would f»nn the data of a practical com- 
pound nitcrcisco])e with a Held lens ; and as Mou- 
conya had this iiistriunent maile in 1600 by tlip 
'son-in-law of Viselius.' it b-^romf* itnJmhIr in n 
vry Itiffh firi/rtf tluil !<• hitti tnu»l lif altribultd 
Itif «ir/Mirt Jerier of a microtcope ivilh a Jielil- 

In 1665 Hofjke published his ' MitTo- 
grapliia,' giWng an ai^count and a ti^ro of 
his compound niieroscope. He adopted 
thw field-leils employed by Monconya and 
gives details as to the mode and obj(>cl. 


of its eipnU>}-iue»t, which am «t mm ititereatitit; and inMnio- 

tire ; for tbi*v iiltow qnit^: cWrly Uiut it wax nut employed i>y him 

tu correct the apbi-ritnl ubfrnltiou of tin* 

*yfc-lfii!i, but ni«rcly to iiu-reAM- itie siw of 

tite li^ld of riew. He lell^i us that lie used 

it 'ou]y wlieii be bad oouuion to aee tuuoli 

of »n object at oiico. . . • But wheiiever 

I had ocoaion to «xnmiaQ Uie eaudl parts 

o( a body mora accurately I took oot th« 

middle ji^last (field-lcna) and only made 

use of otie eye-glass with the objecl-;>liia8>' 

Fi;;. 'J'l IB a reproduction of the ori- 
^imI drawing, and the general design ap- 
pears to be daiowd by Hooke. There is 
a bsll-arid-sociiet movement to the )>o(ly, 
of which he wi-itea : 'On the end of this 
arm ( L>, which Blido8 on the piUar C C) was 
a small ball IttUnt into a kind of socket V, 
made in the side of the brass ring O, 
tlifiougb whieh the small end of (be tube 
was sciwwcfl, by means of which contri- 
vance 1 ooald place an^l lix the tuho in 
wlMt«m^vrr poslare J desir«I (which for 
many n)iKervniion» was exceedingly noom- 
mry). aii'l a(IJiiste<l it most exactly to any 

ItDeMllnrJIy be remarked that, useful 
M the W I -and' socket joint is for many 
(torposes in microscopy, it is not advan- 
Ugaoiuly employed in tins instrumont. 

Uoolui devised the powerful ilhiminnt- 
in^ arrangoDinnt seen in t)ic figure, niul 
eTnpIi>yfil It sta^e for objects Insod on » 

fraclir-nl know&dgoof what was rr(|iiirrd. 
lo described t im^ful method of r-Klimat- 
inj; magnifying power, and was an in- 
dustrious, wkt■^ aod thoroughly practical 
olnervi^. IJut hn workr<t without a 
mirror, and th» Kcrv^w-fiKiussIng i-irrange- 
ment iwcn in tbc drawing must have bneii 
as troublmoine lu it wns faulty. But as 
a mtcnisvuptst, Hooke ^''^irifd a Eurojiean 
fame, and k'^vo n puwcrfut stimuluii to 
wtcrasi-opy in Rn;[li)id. 

In lO^S a tkacriptiun waa publiahed 
iu the '(lioruale dei Letteruli' uf a com- 
poond tuicrosoMM by Enstuchio Divini, 
wlii«h Piibri liad |»reviously commended. 
It was sttitfd to he aliout 16^ inches 
hi;;h, and adjunlable to four dilTerent Fio. M.— D!*1dI'* Fcmpound 
h-nj;UiH by draw-tubcu, giving a range ol ■icwwoiw. 


luafinilicatioi) froni 41 W 143 dinmcters. Instead of the usual W- 
COQvex eye-l«iiB, two planO'Coiivex Iens«« v/tre applied with tbeff 
convex mirfiLCts id eoutoct, t>y which he clnim(>d to obtain a. much 
flatter field. Mr. ^Inyall found in tlit Musno Coppmienno At Rome 
» tuicroscoi>e aiiswiiTiiii;; so closely to this drKcription that hi> doea 
not hetutute to refer its origin to Divini. He iniulo the sketch of 

■ it given in fig. 96. 
But the optical con- 
fttniction had been 
tntupprpd with nnd 
could not 1)p esti- 

putilishi^. in IfiTl, it 
tr*utisf contnining « 
<li-!(i^i for n inttTO- 
■copi-, of which Ijg. 
D7 IS an iltuKtration. 
Th« KTolli wpro of 
«bony, linnly nt- 
tiich<!d to Ihi! luwo 
and tci the nollar 
eiidivlinK iiw fixed 
ewitra) jwrtionof fh« 
body-tubi'. An fi\- 
titrior kI id iiig tabe 
carried thr ryo-picco 
«lii>\<> on th«T fixed 
tulxr, nnil k Kimilnr 
hUiUhx tulx! cftrriod 
the objrct-lMis helow. 
tliPBi* Hliding tubes 
nervinR to founs the 
iuiH;;^! and rcguliitct 
{wilhincertain limits) 
tlie nuKnitiaition. 
He also suggested a 
screw Brniiigcment t* 
lie applied beiientli 
the stage for focux- 
ung. He devised, or 
reooiumended, seve- 
ral i-onibinatioiui of 
lenses for the opticnl 
part of the micro- 



Flo. ffT.-^hAvtnn d'Orltwi*' oouipuiiiiil mieroMOTC 

Boope, uul refers to combiuationtt of thrvn or four beparato lenses, 
by whioli objects could be aeen erect, which he considered ' uiucli to 
lie prefenwL' 

He al^'i invented a binocular fonn of microscope ftnd pablislied 
it hi his wcn-Ic, 'Lii Vision P»rf:iite.' in 1677. It coitsiated of two 
ooini:ound microscopn joined togeth<?r in ooe setting, so as to be: 

to both «ij-e8 ttt oncn; « segiocnt of wch object-lens 
... to be uf oii«.meli focus) was groiiiKl awav to ftllow the 

«onverjcM»t axist BtartinK from tho two eye* to meet tit nbout 16 
incbea di»t»iioc at th« coroinou fonjs. MediiuiLsiii was iirovidotl for 
ragolatiug tJie width of the axc« to TOrreawnd with Uie obwri-flr'a 

Fiff. 98, showing thn optirnl f»iuit ruction, is c-nincd frota the 
oHginiil itiiiKniD (' Ia Vision Fttrfaiti',' tab. i. tis- -, ]>■ ^0). 

A drawing of thiit htDocuIiir, us knowh to 2ahii, «-n« f^iven in 
the firvt Million of his ' Oeuhia Artilieiulis ' in I6HA (Fiiiidutii«Q UI. 
p. 233), and ii roproduoed in %. 99. 

lu IG72 Sir I.i««c Xewtou aaainiuni<mt«d to- tlie Rojal Society 


ti not« niiit dingniin for n rvt)cc4ing luicniMcnpc; "Km Iinvr. liow«rer, 
nocvMmcp thai it vrms r-wr cohkU-iirUkI. Dut in IflTSJio^nn-ciihocrk 
hcgnn to a^ml to tli« Knynl Booiety liis inicrfMcopiciil iliiicovcTies. 
NotJiiTig wtut known of tlic cpnutruution i>f bin inntniRiftntii, except 
that' ttipy wirre suduIo microvt'upt's, «v»ti down U> no Into n period as 
1701*. W<' know, liow<!V<'v, thiit his tuicro!K!o|Mw with nircnnnicttlly ' 
rougli, nnd ihul o])ljcutly thi'y cooaiated of !uia[>]i* l>i-(.<unv(rx lonsM, 
with workwl surfucpa muuntiil l«-twii.-ii two jilulns of thin Dtetal 
with minutt- apeHuns through which the objectA wivi.' directly Men. 
At his deulli Lei'uwi'iJicwk tjinjueathed a i-nlnnL^ of twtsntv-rix 
of his iiiifiTjieopea t<i the Royul Society ; uithitpjitty, thi-y hftvo 
niyxtrriously diiui]>piiur)!<;l. But Air. Mayall was eimhhil to ligum 

oni? lod^i-d in th<^ tiiu^i'imi 
of Um Utrecht Uiiiv.Tsity, 
wliifh in ftiveii in ligs. lOO 
iii'l Inl ill full ni/i-," It is 
-1-11 nil in ltd sides. The letis 
is SOPH ill the up[H.-r third of 
ihc pinte. It iiiiA a j-im-h 
focus. Tlie ohjwt is bdd 
in front of the lens, on the 
jmint of a short rod, with 
scirw nrran|i|onieiita (or ad- 
justing tho object under th^ 

M.iny uodificaiiona of 
thi« and the prveeiUnj; in- 
Ktniiiii'iitn nn- found vrit]i 
soiiin mrly Kii^'iisli fomis, 
hut no inipoi^ttiit i-onslruf- 
tiv« or optii'ftt modification 
innnr-dintrly prcsente itself. 
Itut n'lnu; iiig<tniou§ arranfic- 
Toi'ntx nro found in tii<^ 
wniph- niicroNcopps de vised 
hy MuiiKi^lirnhroek in tlie 
nurly yi-iirs of the ci^ht^eqith 

F\<>. (no. F,o. 101, 

firiiull fijfiirpd n inienwcope in his ' MicroKi-npliift Nova ' in 1G8;. 
in whith opiicnl mudifiuitions arise, Divini luid, hk was stnted, 
combinvil two plnno-eourex )«niw«, with their coiivi-.v »urfjiicji fa«in'', 
to fonn ail rye-piece: thin i(tm was camwl furtJier in i6(lfl by a 
London ojilicinii, who uw«l two pain* of thaw lenses ; Oriiull did this 
atao, bat ill addition lie u»d two similar (but sinallur) Ifna-s in tbe 
«imo niaiiner as an objectivo. The fomi of the inicirispon.- itself 
was copied from that of Clk'nibin dXDri&iia (fig. 97), but wu 
iDodiCied by the appItcAtJon of an external scivw. 

In 1G9I iionnnni nioditird prrceding arrnii;;eiueiits by dei-jjiing 
a moatis of clipping the obj(>(Tt liHweea two plates pressed awuy from 
thi; object-lens by a spiral spring, the focussiutf being then eftVcteil 
by * 'screw bwrrvl.' 


TliL-i Ky.->t<!ui tif foruwing w*» employed in a more [imctic»l iarat 
by UurlBoekcr id hiUI ami wiu n<topc«d by Wilboii in 1702. tl 
bedUne a rery p»])uUr form for the loierascope in the «ighteeiitli 

W« are incl«bu^ to Hi>nttiini nlsa for originating a horiEonlal 
form of inicrO(H;op<% whicli is intrrfMing and vrhich, in a drawing of 
the ilutrUDMnt, is shnwu tu possess a siib-!itAg<! mmfuunii funttnurr 
JilUd wilA /iteuuiurf arrttr%ga»mU Jar iHu%iiitvit%tvj tran»parrrtt 

Iti UMlaoeker'it microscope ' tli« l«Mi*-(-nrri(<r Alt, fig. 103 (on 
which the otttl P, conlaiiiing ihu Iphn, is Hcrnwoil), ictowk into llut 
body OC, QD itl 0(j ; thi- thin hmss ptut^s F. and V fit within tlic 
body, the porliuun i.'Ul out allowing thriii to slide on tlic short pillars 
OC and y D. and thospind £]>ring pressing them townnls CD ; thp 
object- slides, or an luiiiiiaktdf cagi^ ti H (hingr.l at 'i !j lo allow Uie 

lid (i to fit into H, Anclcising the cihjrtHA ht^wrv^n ntrips of talc)^ 
alitli! Iwrtwci'n t!i»' plati-i E and F vihvu in jiCKiition, anil iJio ''screw 
barTcl "IK tiU inUi llie ncmw-Mwkt-t (.' l> and ifgulatcji ih^j foctix- 
sing; a cnndi-nsing li-ii*, X, fit* im a si-c<>n<l "scn-'W-bjirrol," L M, 
which i-H B|i]ili<'d in th<t Kcrcw aMckitt of I K. Tbix arrungcniRnt of 
the c<iii(li-iiu!r in tH-ttiT than tht^ plan iidojitfd I>y Wil.ion, aa it nlluvs 
the illutuinatiou ti> Iw fiiciumct on tin? ohjf^ct indi-peiuli-iitly uj the 
focal adjuntnurnt of tlic objei^t to the iiiogiiifying lonn ; w)i«rea!i in 
Wilnon's niicroacope, the c<«id<-n«w Imiiig niouutwl in I K. wriUiout 
fkcilily of adjiut4ne«t, remained at a fixed diatuiiL-e from lii« object, 
ud hence tlie control of the illuinlnacion was very limited.' 

In Uarrw'M "Lexicon Techuicum ' <1704, 2 vols, fol.), andor tho 
word microBi-ope, MardiaU's t-ompound mici-oacope (Jig. 103) i* dn- 
Rcri1>«d and ligured. Several iiii|Hirtant iimovations in niicroKCiipii'al 
construct ion were bere etnlmdied. (1) A 0ae-adju»tmimt aon-w K 

I o HN Mar s h .vl l s 

New Invrnird 


For Viewing ilit 
ClRCUl-KTlON or ihc BlOO D 

Cain Syvtal* to U4^ Scvt. 

P». 10*.— H«it«l'« micniKni* (111*). 


iit GOlineot«d with the sliding socket E, supporting the nrrn D, to 
which the body-tube is screwed : the focussing could thus be con- 
trolled in a far more effective manner thftn byany sjrstoKi previously 
Applied to a lui^ge wicnHcope. The prerious syst^nis involved tho 
direct movement of tlie hody-tiilic fither hy rotating in » screw- 
soirket (as in lloolcp's) or by .'4iding in n cj-lindricnl socket (as u^ 
Divini's «ii<I Clifruhin's) ; in n few iiistnnccts the object wns movod) 
in relation ti> the objci^lnns, but nil these plans worn more or less 
defective, cspcidftlty with micmscoppsof liirgeclimen*ions. Mmrsbali'K 
liyKtem was a distinut niri.'hniiiciil inij>rovonicnt, for thi? object cnulil 
now he viewed ■.llli-iiig thu actual process of focuuing, ns thi'^ iiniign 
would remiiin slrnrlily in ihv ti<-l<l. {'2) A fnrk, N N, i* hrvrr^ iipplii-U 
with a tliunih-ncrcw cliinip, O, »n iho pillar itmlf. (3) Hnoke'^ IidII- 
An d-KorJcet joint, which wax ujiplivd U> thi; arm I, is hi-rn shiftiil to 
tJit! lower end of tin- pilliir, where it wnulil giv« the lucivi-niimta <if 
iiicliniitiun to tlm wlioli- niicroncopi; iiiatr-itd of to tin- lNjdy-tobi> ">n!y, 
iM in Hoiikoji ; the Imll L could Iw liyhlly c!iini)itHl by ihf si-n^w 
eollur M, in whith !ik)ts wi-w cut to jjin- Hprin;,'. (4) A i>[in(tvn:>iu{i; 
lens on joinU-d iirni* Jipi>eiii» : this prob/iliiy wna the tirnt applieutiun 
of NUcU adjustmttntn lo thi* condenMtr. From the singular piwitioa <if 
t)i« candle beneaUi the condenser, we may infer, without doubi, that 
the mirror was still uolcnown as a microiioopical accessory in 

In fad, in no microscope up to tliia time has there tieen any 
ti'ace of, or reference to, a mirror ; hut ui 1716 Hertel einjiloyeil it 
and introduceil sonte other considerable niodillcatious. The ^ut-ral 
appeaniuce of the instrument as nrisinully ligUfMl by Hertel is ^-iven 
in flg. 104. Not only tiave we the mirror Lilnw ijie httim-. bm nisi* 
above the stage a con'-ave uxftal mirror reflecting light throuKh a 
condenser on tlie object, while tlie fttaj^ in movable on » pillar 
support. The body tube is hinged and is inclined by a screw -sector 

Tn 17:18 Dr. N. Lieberkiihn de\i8ed, what had been employed 
in principle by Descartes n century before,' the inslrunient that lias 

, ever «noe l>een known by his 

^J^ nnnie, and which is still of con- 

' '• sidernble value to the micro- 

Btvpist. Fig. 105 is « reproduc- 
tion from the aailicst drawing 
known of Lieberkuhn's micro- 
scope. A A is n concmvc mirror 
of silver ; from its fonn the light 
is reflcrt(Hl from it to a focus on 
the objfict C". The mirror is 
Icnit, or objiHit-glim, Ik insrrlnl 
anil adjuNti-d. the eye boin^ ]>lace(l bifhind in the dirutitioa I> at any 
point lh« iingle lens oi' a oomhiniition might r(!i|uii-e. 

'A Poiktft Reflecting Micnwpoiic ' wa« ligurcd \>y Benjamin 
Mnrlin in his ' Micrographia Novn ' iu 17lL', hiiving the intnrcsling 
feature of a micrometer eye-piHCi- dtrpeniling on a (rcrew witli a 


Pu». lOSr-LWbcrkilli!!*! niii.i....i<]|w. 
pierced in the centn- at B ami the 


iLin niinilM>r of thre»cls to the iiich, and by wliJuh accurate me*' 
iramviits could he taken. It wns csIImI a r^Jtrfltiv/ microscopD 
becaitiM- it hod « mirror fltt«d into its cylinilrioal base ; but a whs. 
in rmlity, k compcKind rcfmctiiig form, anil apfieurs to liAve a koo^ 
cbtim t» bare bc-n the on'^nal fmm wIi«iim- tb« iinHlern '(Imm * 
itiicrottfoiK-s werp t«l:pn. 

Itut .Miirtiii origitiiit^yl a large number of iniproveoients iMth in 
tli« npticnl nrmnKiimnnU and the m^cliAiiistn of the inti'roscyipre, and 
w*» an oxn-llriit lunkcr. 
Hr MfipliiHl m«k-aiMl- 
pinion fuciixsing ndjnst- 
nx^nlK, to the conipouml 
inicroHoi>iM- htfailuMl in- 
ctiuinu inoviMni'iits to 
tbc pillar i.'iinT'in;; t}l(^ 
. Rtnjtv and inirivr, and 
Ke fumUItiHl tbi* bIh}^ 
Kith rv«t«iif;ular movv 

It iriu to tliL) maker 
that th« Inle Profewor 
Qui-kett was indebte'l 
for an early inicruwojiL-, 
<A wluoh he evidently Ui 
the ta^l thought highly, 
and which he aubse- 
iiui-iitly fpi\« to the 
Royal ilicKiseopicJil »hi- 
cieiy. Adrawinfi'of tlib 
tnstroiticnt is given in 
flu;. lOfi, aitd should be 
deaoribeil in Quekett's 
Own words. He enys ; 
*It stands nl>ont two 
feet in height, and io 
supgiorted on a tripnd 
b^e^ A ; the central part 
or sleiii. [I, i« of tri- 
anj^Ur tigni-e, hnvin^ n 
nn-V at the back, n|)on 
which the tx^ftf., O, and 
fraluo, I>,Kiipporting th« 
mirror. K, am ca[>ubll^ 
of heinK niovitl ti]> or 
down. Th« ooniiHiund 

Flu. lOA. — M&tllu'* lorw uAiwrvAl tolnmoope u lucil 

Ixidy, F, ia thme inckcs in diameter ; it ix <-oii>{)nM!il <if two lube*, 
Ihe inner of wbicJi (.■oiitaina the eyi>-pioce, and can Im- raiwd <ir de- 
j>mx«l by r)wk «'id pinion, so as to imn-asi- or iliruitiiiih the niag- 
iiifvitiK i>oiier At the l>a*e of tlw trianj(u!iir Imr is a onidle joint, 
O, bv which llie iiistminenl can Im im-lint-d liy turnin^j the Bcrew- 
keaii, H [canuected witii an cnillos m-ivw aclin}; upon a vrom- 


wheel]. Thp ami, I, sapporting the compoiuid body, is swpplii'd »U 
It rack and pinimi, K, liv which il cao he mov^l Imckwanls iiiiJ (o 
wanLs, and a )oint in placed lielow it, upon which the licnly can) 
turned into a horiziintfil iMjsitioii ; anuther Imr tirrying a stage f" 
mirror can lie attached by the screw, L X, so ns to convert it inb 
hoiiziintai microscope. The stage, O, is provided with al! tlie oa 
npparatUN for clamping objects, niid a eoiKleiiser can bn appliei^loi 
under surface ; the stnge itself may be removed, the ariu, P, < 
porting; it, turned round ou the pivot C, and another stage 
exfjuiijitA workmanship placed in its st«ad, the under surforal 
which is shown at Q. 

'This sta-,"} is strictly a micrometer one, ha%-ing rectaog 
movements and a fine adjustment, the movements being 
plishb4l by fine-threaded screws, the milled heads of whicti 
graduated. Tlie mirror, E, is a double one, and can be raised, i 
depresseil by rack and pinion ; it is also capable of removal, uidj 
apparatus for holding large opaque objects, such as minerals, i 
substituted for it. The acceasiiry instruments are very nu 
and amongst tlie more remarkable may be nieutiotied a tuh«, M,(^ 
taiiting a speculum, which can take the place of the tul>e, l{, anj-l 
form a reflecting microscope. The apparatus for lioldin(janirii«lc 
or other live objects, which is representetl at 8, as well as a plate I 
glass six inches in diameter, with four concave wells ground in : 
can be applied to the stfigR, so that each well may be brought J 
BUecession under the magnifying power. The lenses Iwlonginj 
this microscope are twenty-four in nuiiilier ; they vary in 
length from four inche-s to one-tenth of an inch ; ten ef them 
supplied with Lieberkiihna. A small arm, capable of canyingsin 
lenses, can be applied at T, and when turned over the stage tJiel 
strument becomes a single microscope ; there are four lenses suit ' 
for this purpose, their focal length varying from -,'(,tL to ^'f^tii of i 
inch. The performance of all the lenses is excellent, and no 
appear to have bf>en spared in their construction. Tlvrt 


improvements <iii all previous oiDstmctiotiH. He appliwi 'tt 
glasses ai A, n thml nenr B, and a fourtb in thectmionl part I 
B and C,' by vhicb he iDcreaseil ' the fie)d of view aii<l of 
draw -tubes were at A luid B, by which tltesc lenses ccald be sep 
more or less. He oUo arrBnged the (ibject-lenses, or 'liutta 
and i, to be eombined ; seven 'biittdns' -were provided, 't 
silver specula [* Liebarlciihns '] highly polished, each having a i 
fier adapted to the focus iit its ooneavity. tme of whioli is repr 
at p,' and the ' buttons ' wjuld also be used with ' any (uie > 
iipeeula ' by means of the adapter, rf. 

The body-tube, ABC, with its arm, P (in which it scrpwi-d i 
aiid stem attaehment with the fine oiljustment were elearlv mo 
from* design which Caff origiuated. The large i\tiry head, I, new 
II pinion mid nu.'b for raising or depressing the bi>dy-att.'ichin(ail| 
tlie stem. The sta^ iind mirror were adjustable on the sti>iii. 
lar;ge ratchet- wheel controlled by the pinion -handle, S, gave 
re(juire<l inclination Ui the stem. 

Nos. 1 and 1' were ivory and glass 'sliders' for objects, lol 
applied in the spring-stage No. 3 fitting at T ; the ' hollow iit K [ 
3] IE to receive the glass tube No. 10.' No. 4 wiisa dinplintgni Ht 
in the lower end of No. 3, ' to exclude some part, of the light wh 
is reflecteid from the mirror Q.' The forceps. No. 5, could be (J 
'in one of the *mall holes near the extremities of the stjige, ^r in 
socket, K, al the end of the chain of balls No. 6.' No, 8 w:i- m imi 
composed of n series of hn] I- and -socket joints, similar to tli-j -v.w 
employed by Jlu.-^clienbroek, by Joblot, and by Lyonet, and wuifl 
tended to be applied at W, when the stage removed. No. 7i 
a box of ivory in which discs of talc and brass rings were pacb. 
No. 8, a hand-niHgnilier ; No. 9, a sliding arm lens-carrier fittiiw4 
Z, when the insti-umeut was refguired to be used as a siuiple mitf 
scope ; No. 1 1 , a nxl of wire with spiral at the end for pickiug I 
soft objects from bottles il-c. ; and No. I'J, an ivory disc, block I 
one side and whit^: on tlie other, titling at T, to earr}-opn(|ue(ibjei 

jnsEKS Mi('iins(roi'B 



*)m1 oonttnictiMii of the instrument, there ts no micruscope tliat calls 
for farther comaHnratinn Mve one— by an Ki;gli&h maker iiamed 
Jnnn^it WW <«llr<l JonwV 'MaU Approved Compound Microscope 
Mi<) AppHmtOK,' iinil nitbough, in principle, It doea not diD'cr from 
A'lnmc'i' inrtrunw-nt, %. lUT, it jei pr^Kpnt^it ditl«n»icoM iif tlt^nil. 
Its date wnc 179^, 
mm! issMO in lig. 10^ 
which is takrn fmni 
thn oriKiiut] liyurr in 
jVduiis's ' EsMtt/s on 
ibn Micn»ic«»pe.' 

ii^ Iripotl, and tbi* 
sb-ni inclii>es u)Kin • 
eoinpiuui-ioiiit on Ibe 
top of Ibi* pilliir. 
Sir. MuyiUI juaXly n-- 
luuks Utitl thin WAS 
tiw brsl avHteii) <lv- 
lifted up Ui thin <lnu\ 
The nrni cnrr^'iiig the 
bo(l<r-tu))e «ii he ri>- 
lAtMl Mi the top of 
the limb E, and ia 
ajito provided with a 
rac!:AD(lpiuionl>. An 
«xira currier, W. is 
provided for »i)eci«l 

SaqMoea pivoting at 
,so tluit objects will 
reoMUD in the optic 
Axis tliouf-h the stHj^ 
lie moved in mre. 
Tltere are nleo clips 
pmrtdM) for tho 
staget There is a con- 
denser at V, which 
slides on tho stem Uy 
the socket «. The 
mirror nlso alidnt im 
tbn sti^i. Thrro is 
jtrovkJcd a nAatitijt 
nvlttpiii diM, P, of 
object- IcniiKK, and a 
brasN c»ll cotiliiini 
* high pow*r, of ^„ i>r Vr. ii'^h focus, which nn the minoval of the 
lcn*-di*c cnii be scritwwl into the nose-piece. 

Then.' wen- *1im doaigned some intareiitinfc fbrma of refleotitig 
micniBcop<!, t^i th« detAils of which we can utVord no space, their In* 
flulTIl<.1^ having Um-ii of no value in tlie development of the microscope 
A« we know it. There was a reflecting uiioroeoope auf(|fe8t«d by Sir 

Josat's near tmmovtm euHror.vo 

Fin. lUN 


Isaac Newton in 1672, and one w&s devised on the prmapb^ 
Gr^^rian Telescopeby Barker in 1736 ; another of the i 
form was made in 1738 by Smith, which was, periiapa, 
perfect of the Catoptric forms. 

An outline of its constraction and the path at the li|jbt 
given in fig. 100. It was for eicamining transparent objects I 
similar to the Cassegraiuian telescope, but with an extra ] 
piece tube to permit the focussing by movement of the 
The object was placed at MN ; the image was taken vp 
concave, reflected on the convex, and again reflected to thet 
He advised the use of a condensing lens for t^e illumination, ' 
vent 'the mixture of foreign rays with those of the objeot,'( 
the instrument gave confused images of distant objects when il^ 
used as a microscope. 

Even without a condenser there are good images attainaUe^ 


r- ::::..■. ::-.-^^ 



tin i)tiv«l<i]>iueut irf thix now i»xtn>n»-]y ^-alnnble «ncl iMuiuti- 

runtcnt. An exbnuKtivo afimunl of thn <inrli«it rfiscovery 

Olid |ini»rn-AWVf u{>|ilii;:itiuii lu uur nwn diiy iif iirhmnMliiin. »•■ fnr 

L»« it cnti Iw Kivi-ii ji) this treiitUc. will In- Coun<l in tin- cli»|it*T u» 

p«liji-ctiviv. Wf mil h^-rc ouly nttfttipt. (iM' llii- tiikuof minpUtUtness, 

n vrtj- lirwid nittliiic i>f Uw fucU, 

3Urtin iippiiirs ui linio i^oiiKtriicUwl un ocliromntic objoctiwi in 

175ft, but no n-Mills of prnclii-ul value wit^> obtiiinnl. 5!«rtin lini-ina 

fiinii«l llic juilgiii<-iil that liin ai-tiiviiiiiitii.- iiiicroMTc>|«> w«» mil niuol 

0.1 r<-ttw(in]L[ uiic-Kiwoiw willi which h" fiiiiiiiin-il it. But it <;it- 

Ftuiiilv fjivfs him ii ]>tiicD<>f int#i«yt in ihe hisiorj' nf the Ai'hri>niiiti«ni 

ul object-itWtm f««r thv iniproseojH-. 





Plo. 110. — EUlk'a kqiintle juiirniivTr I 


In 17flS Enter )ip^ti to dincnts tlie tltcorv of itclirotimtio 
. iktiil iu I'll, in hiH ' llinptrion,' he enti-i^l upi>ii llw 
Kt more ooti&tdentblo loni.'tli. A |>iiiiil of his, named Xiuhnlaii 
FiUK, |iu1kli«hi?(t ill St, Petersburg, in 1 771, o viilunipwititlptl 'l>eI:iilMl 
iDHtruL-lidti for oarryiriK lenw* nf diU'iTi-nt kindk in n Renter Je;.'iiee 
irf [■■rfectioti, wilb > < Inscription "fa raici-oMMipit which (niiy jwu* for 
tbt? ut08( pci'fi'ct of its kitxl, tikrn frviiii the diiijilric thTiiy of 
anitrd Kulpr. aim! tiuuin cninprphrtiKililr In worktiu'U by Nii-|i<i)tu 
FOM.' This WBS liAiiiJiitcil into ( liy Khi)(<-1 in ITiK, biit. nn 
' nuilt of thefw dij>CU««>nii>i nf lh<- lln-or^' <if ii(rhi>>mrilixni mn bo 
disoovoTOi) mrii^-r th»n ITDI, when Fmii\<>iK lli-i-ldsiiyiler iiuulc nn 
achroninticobjin-tive whii'h wm pn^si-nti-il l>y Hiirtiii^'tji the iiiu«i'uin 
of the l*nire«iij- i>f I'tirrht ; but it wiis fur from snlinfiictory. It 


146 THE inwOlRV ASD EVni,rTl(iS nV THE MK'RnsCOPE 

vBA compoBod of two biconrrx crown-^'likid leiiies, unci a IrimiicHvi- 
flilit leriH ptncccl lii>tu-r«ii tlirm. 

C. Chevtilior tWl« us ' thiit brtwpeii 1800 iiiiil 1810 M. Clmrl««,of 
the ' Iiistitiit,' PjiriK, mnilc «nmll lurliniliiiitic li*ii-i(!i ; but lhi\v w*r<* 
to» iiii]M>rfn'l t(i Iwf «f ival H'l-vic'. In li'll Friiuiiliofcr miuli' 
ncliromntic doulilcrtsnitli lui grout suc-i-ess : mid Jii Ip'':;^— 1 aiiucbro- 
QiAttc mifroscnpo was mtult! Iiv ilic M*->ini. Clifvnli^r, witli Enor 
di>ub!pt lensus nniuisi-d jn-L-iirdmg lo n pliui dpvist^l by Sellique. 
Thoir'.MiciiisciijMMrKulcr'folluwwI.aiul in Iffl!" AimVinjn&triietedu 

hi>HxoiiiJil iiiici-oscopo 
on nchiY'iiinlif princi- 
ples which wits A|><>ksii 
wdl of. IJut u'liile 
up to a very recent 
Hat« it wa§ coniinon 
to aw>prt tJiat the lirst 
lo AUgg^st the pUn of 
o^imliiniiii; (wo. tlireo, 
or four pinno-coiivex 
slniilnr foci, one klwve 
ilip iitlicr, to increase 
tlic power nnd aper- 
tiii-'^ w'a8 Selliqae in 
l''L'^, it IK now known 
tliat tliis liad liei-iian- 
lioipated bv MnruiU 
(cli. V. p. 303). Sel 
li<]Up's plan w»s cur- 
ricil int« execution by 
till' llfssrs. C)ieMilier. 
Tlie instnimeiit em- 
bodying this plan w 
shown in flj*. III. 

h) I) report to the 
A«i<li-nii<i Roysle d«* 
Sci^-ncpR, tlie weli- 
kn'iwn ninthemaliciftn 

Kill- III.— rtiJli.|ui''« iicliiumiitu- itiiii^iiirfjM. ilHSa-ll. 

Prfsncl xnvN, iTonciTuinx this iiiii!rOKc<i|H', thnt in coinjiaring the »b- 
jectiviw with thrm'of oiiL-of Ailiimx'.i1)f-Nin»n-iirhroin>klic inKtrumentH 
•^tltiit up til II ninKiiilii^tiun of two humln-d tinies Sotlique's wan 
decidedly nuptirior ; but beyotid lluil iiiftanifionlion tlierp was no 
supi'ricirity in the achraiuutic ftinii. niicl lie jirefei-i-ivl Adnms's form 
fur j)roloiijf()d oliservatinna htcHUso it gave a Inrjtcr field than 

Tim niccliunUin of thin tiiieriiHcoiic! wax Kimilnr to the English 
inoilet of Junes, nhown at fi^. I OS. Tin- focuMnng wiui hy rack and 
pinion acting on tlie nlage, the pinion ivnmining si itionai-y niid not 
trarollinfi on Uiu nick. Two tlmw-tuht's, A innl It, wora applied 

■ iJM MierMcaftt, Pui*, IWIfi, p. Ml. 



■within tlw Ixxlv-tube, C. the upper oue bavins; n h)c<inaMV« Iciw, 8, 
ftt UtfliiWRrt'tuI.RPTvitiK ■<£ K'l aniplili«r, whicli wuh pruluiblj* tliu fimb 
applicAtioii "i a ' Itiirlnw lens ' lo ii Diicn.eM>[>e. 

Illuniitintton for cijincttic olijecls wax acconipliAliiMl by a lenticular 
prinii, F, which wius ifiin)uilli:<i, bimI coiinecbH) with a riiif! etiihracinj; 

l|»i- IxKty-tuW. 

W<- h'-nrn fiuiii KnMnvl that tho migo of mt^iiificutiou was from 
40 to 1200 diumctrre. 
Tho oh}<-cl-xl**s*^ wftv. 
i-ompoAcul ttitlu-r 'tf two 
(louhli't HyxUtmi for 
't(>w-|M>w*tr work or of 
four douhliit nvxtcRis nil 
Mirowrd lo^Ttln-r for 
hi|j:ti-|>ower woric, and 
(W(» (HTuhirsi were pro- 
viilralof (lilTiMWntiKiwi'r. 

It JK interest iii;{ to 

IAmK oni^ i>f th<' urlivHt 
:DMwn EnKli^i Tiimlt-U 
ut iIm* achrotaalic- mi- 
croitco[Ni Itesidt' that of 
Se)li<iu«. it «ua tiiaile 
liy Tully the oplic-iau, ■>( 
T^>tiit"ii, •nhii at Dr. 

< IiKIIIJi'k illSCjklll'll lull 

lin-ii wofliiii;: at lli« 
nchmtnati-iiiiK itf the IDi- 
t-i<iH:ri|K-. S.'lh<|iH''iis a 
Rinnifiwl mm li Ilea t ion of 

Olll- of thn heat friHIM AH 

niiMlt? hv Ail«lu^ Jom-s, 
or IWIoimI. Tully, aid- 
ed \ty •lo!iepli J. Litid-r, 
iliNOOVvred thai ^t^nt :ii.-- 
niiut.-y of n'orkmuiiship 
atid complete Bteailin<-SB 
in tlie Maiiit were need- 
ful for n«hroiiiatic ini- 
■?rrM«4>pe«, and to this 
ttiii l»e adoptH stmli, 
«ach a« wera used in 
telescopM, connecting 
the bwly-ttiW with tbi> 
haw, Th" instrunn-nt i" 
•hown in tig. 1 1 'i. H<- also provi(I<-d cncchaiiica] moi'enientd to tli« 
-uiui>. hut ■»> lini- ailjustnii'nt wiu applied. There wa« a suh'Sto^ 
pruviiird with a tT>t,iti]tg disir »f jfraduati-d dinphragins. This micro- 
irnjM' wnn iiuulr in all prnhidiility about tli<' ynar IS26. 

Ax coni]N>md with Miiglr tensnt of iM|Uid power. fn>iri which b<> 
nndi light wa« inrrilabty ntoppMl out hjr the siaaU diaphragm that 

Fiu. 113. — Tulljr'* BclimmBlIc mieroHwpr. 


It witH iKwilfiil to une iu 11T1W M tKCurc n fnir iniaa^, the objecliw* 
us^l witti thU instruiiw-nt fpwi- n voat inci-MU« of li];lit 1>y ptiniiil' 
tin;; tlic p|[i]>lo.Vltwiil ot' the full ujioi-tuiv. 

All pxtii-mt'ly iiitere-iiiiiif: inNtniiiit-iit by C, ClievaUor, inndt- very 
pTohMy iiix long iifU'i' ISH, ituil Iniiriiij; much ream iblai ice to that 
of SellK|U(-. is hIiuwii in li|,'. ll.l, Tl is [h-ovhIwI wUli u rewjlviiiff 
(li^i of i]ia(iliiu^m^ n|i|ilif(l Mo\. tin- ilarfe clutmber under tlie sUine. 

and this is a, yhm wliicli <ibt.iiiiecF 
a pci-iiiiineril jiUce in tbe inicm- 
ML'u^ien of tbe future. 

Tlie re])ort of Fresno! Cflu- 
oerniuj^ Sellique's ochronitttJc 
iniciYwcnpe deterniitied ProfesMtr 
Aniici, who for nine year> hiul 
iih>tiic)on(>(l his experiments on 
nchroiiifttic object •glaLfises, to re- 
vomtnenco them in IKSfl, Mid iiii 
!(*;;" he e\bibiteid in Haris iind 
in T^ondoii n hnrizontnl inicio- 
-co]n-. The ithI niHTlty shown 
in it WOK the npplicntion nf 11 
right -/iiiclerl prJMn iinrnedintoly 
nbdvc tile objective to deflect 
the i-nyK thnxiKb tbe borirainliJ 
lH)dy-tul>f, The nhjoet-tthuwi^ 
Here ei>ni]:o8ei) of tlimc lenses 
h"|KTpi»e<i, each hnvinjt « focus 
of ihi'ce lines niid a KTefttly in- 
ci'eft»''d nperture. It li«d iJmi 
exirii eye-pieces by menns of 
wfhieh the ampliliv'ation ««ilil 
hi' incn-AKefl. 

Meant iinc tbe Kubjrct (tf 
iirhninintism was nn^ginK tho 
iiltention <if thn luiBt dintin- 
guixhcd Kngliib niaibi'iitiiti- 
ciiinK. Hir John Henchd, ^ir 
Oeorge (then Profwwir) Airy, 
Pnifcssor Hiii'low. Sir. Curl- 
iliiiKlon, :iiid several otheni, worked with snnie viRour nt the subjeci. 
Also, (or some yemn, Joseph J. ListiM- bud been cjirnestly working 
i-spennientjdly nnd niiitheumticaily "ii iIm- .»n.nii' subject, and he 
ililu-ovei'ed certiiiti jirupeiliL-K iti Jiti adni<miilic ronibinntion, which 
■«'eiv of iinjKirtiiiice. iiltijoufjli thi-y htid not lii'cn In-fore observed.' 
In IS'JO 21 piiper fi'Min I.i^ti'i- uas receivMl and publinbed by (he 
Royal Society. 'find piittin-* the giriuoijiien it Inirl down into pi-active. 
Li&ter WAN enithled ti> ohtuin n t-om hi nation of lenses ciipable of 
traaMnitliiii; 11 pencil of "iD- with n lur-if ci irTe<:tf<i liehl. Tins pa|>er 
Mid ita i-e*.oU>> excrli'd .'i very jiowerful inlluenue on the iininedluti- 
improveiueiit of English a< hrotimtic object-fthuUHw, and Cormnl » 
I fiih ObjFci;vM, Ch, V. p. m*. ' Trmn*. Jtoy. 8ei^ l<a IHSB- 

I m. — C. Cli«i«If*r'i Mhromivlic 



pMraanebt basU of A'.lvnuc»iueiit for the inioraKtuiHS not onl^ in 
its Optical, but alsa iudirectly in its ntechauiaJ voust ruction wit) 


Foi- convewiejioe, at 
ttiiB [Mtiil we may ad- 
vance a little ill ordiT to 
caniplete our brii-f out- 
line of tlie tnecliuiiical 
HpplicntiuD uf Hclinuna- 
tisin to object -kIushOb. 
Mr. A. Hobs lieo«iiie 
pmcticftlly Acquainted 
with tlie pi'iiiciptea of 
achivimatisti) ns applied 
to conibiimtioiiB uf leii-.e<i 
in working with Pn>- 
fMSor B&rlov on thi& 
Rubjcol, Mnd liHviiig ap- 
plied Liiilcr'8 principles 
with grrat success, he 
<li«coverod, ns wo have 
already pointed out in 
Ob. I.,' that by covering 
the object under rxami- 
nation by a thin film of 
(tIrss or tftlc the correc- 
tions were disturbed if 
they bad been adaptetl to 
an uncovered object; and 
wo bnve seen that it was 
in IS37 that Ross de- 
vised ix ninijile moans of 
convftiiig this. He was 
an iiulpfutigftble worker 
ill the interests of the 
atlviiiiconieiit of the me- 
chniiicnl uf well as the 
optif-id side "f tb" mi- 
crosixipo. Fig. 1 1 4 pre- 
sents nik wrly form of 
one of Uoss'* rarliest ini- 
cxAinple, which i* n form 
isKiied undrr Prilchnrd's 
iiiinir'. I'ho Ktage is 
iLcliiated in diagonal di- 
reelioiiK on either side of 
the stem, nnd it-; tfcm-iml 
fond coincidos witb one which Mr. Mayall assigns to Andrew 
Pritclinrd, wtuoli fi;:. 1 12 illuiitnitAs. It has the same kind of stagf 

Via. lit. — IVItcliaril't inictT<wi>iip tritli 'Cottluiontal' 



ow to rweive oitlior 
i-«nn support f>r 

amt nnit ■;metiil count met ion, init it will )>>■ seen that it has 

,al«o n cnrtiMiK Hpiral fin« luljustnii'iit, «hi«b i.i plainly an nncovi-n^l 

* Cdiitinentnl ' foriii. eithi-r a(io)>t«)l in Etij;;liinil from ti. Oli(-rhiiuiu.T, 

»«■ it may li*ve even [in'i-wii'd It, It is iiit«restinK U> juAo, h<iw<!rpr, 

tiMt th« Kal>-!it*f,'varrati^iii«iita in liotlt thwH^ iitalAiiCi!!! ni« thosuDO 

: Umim einploynl liv ^^^>llaat'3^ in cbiiti«utioti vrith hit> cvleliratvd 

LiloubltflK, nrt luvount uf m btch mts ^Hven in the Pliil<iHi>j>hioiil Trans- 

'itctitMis i>C thul ilat«.' 

Tlw B<>H.H fumi t-uiiiiut h« inclinMl, iior can tlie Pritchard ; and 

' the fin* adjusttitPiit in itie foitiu-r is cffectwl l>y means of a long 

acrev [vtsiiti;; up t)ie |>illni' nnd Acting on a triatigulnr dieatli, witbiti 

,whid) the Bteui Is applie*!, to 

uove with mfk *iid pinion, 

toji of the stem t>ein;; 



• letts or ti)p limb of ihe 

leocnpouiKl ImmIj'. Tiio soi'i'u* 

lb actuated by* lar^e, grtiiiU' 

iatod, ifiilled h«ad abov-e thv 


The stage Unt SMppiirts, 

evtdentlr ■<■ ctiuUe di^«'cli'>n 

to be e(rect«<] without Doxuit' 

bjr the weight or prvsHurf of 

ftbe ban<lB. KectauKUiar tno- 

cbanicid uiovejueiits are cm- 

pbiyeil actioK diagonally on 

either aile o( the ^lem ''V 

nther fine screws soibat tlie 

luntloaa mn alow ; wliiuh i« 

n (lesidemtuiu not al«.-ny» 

0011(1 even in aitir >m?«i iii- 

iiments to-day. 

It was A. K'lss, Ii'iw.'ri-r, 

Iwlio succmiifidly sppli'-d thtr 

] long -lever »y stein ol line nd- 

r jti»1n>ent acting on th<' n'.w-- 

piece, and to which wo «h>)l 

a^tn refer.' since it id lh» 

bBMwt perfwt inmti- "f ncowii- 

]4iahing tine niliMKtnH-nl tloit 

wenowknuwnf.nndin itsiiioN* 

perfe4-t form is UKrd in 'bi- 

microAropciif the bi|i;beAt cladti 

(ihnt ))y Mivxn. Po«'«dl and 

f ix«Inn(l) tnadn in ihv prvst-nt 


But A. Kosii al an phiIv period worked out n ' Jncl;«»n ' fiimi o( 
tnicm«ci)|H', with the limb bupporling tlic hody-tulie. Ko npplwil 

I Trnoi. n<y- ."iiv. IfSU. ' p. 101 

« ^ 

Pill, til).— A Ruw micMMOptu. 


IJiistinrni ill tliU Ui act ujian tiie nme-piccc only, wliicli, us 
-SuljKei)ui!iitly Hve, is n v<-ry iiifcrioi- iiicthiKl. This iiistni- 
ment ia riiovrit in fix- 116. Kims triitl vaHoub modilicAlionsof tliu 
tine i*(lju:itui«tit uncf iiimUl. but tVim iiUiut 11541 Iio workcitl t»\\y nt 
llielnoff-Ivvfi-iiii-thiiil aa an[>]ieil t» l)i(! iMisivpJoco tfiroiigh tin* ' crtMa 
lyrta' uiid biini^iit it U>»liij|;li state nf pi'rfri^tioii. Btit tti^ ftill|Hi8si- 
liilities <ii till* m«llii>il, iis I'liiicimiMl il* Ki'iiutivcm'su, vrpm. nover 
utilia«U by Ribs, aii'l wure subafnuiniily juv-vwxl inUt iicrviw by Pun-t'll. 

I» 1841 PoudI miiile a mien »«(>]»' with on Bxtrcmcly ddicnto 
fliM! Ailjustiut'iit iipj>liiil tf> tbe KtA^. The iiii'cIiiiiiiMii nml rh<^ 
worktUiiti&lii|i u'l'i* ext'L-Utnl (vre ;,'!«■ uilraiviiij; of (hr instriimi'iil nt 
lij;. ll'X t»)t Uiv priiR'iuIe wna by no nii>anii tin ailviinct^ in tW 
Oirevtion of the wants ut tne modern inicroni^DpiBt. An iMljuntinnnt 
wliich ;^veil)e objovt a inovablv |>We Iwlwwn Uie comUnx-r .inil 
tke ohjivtive WAS subversive of All »««« in uutmpuUilion, ami wimld 
huve mitde soiue i-f tlie liner rtuiulw of iiiu«Ieni uiiii-()Bto[iy alinont 
iuipussible. Our illumination n««cla, aft we aluill iiiib^-<iui>nliy »>«, 
cetktriD)* aii't foL-uKsiii!* with tti« accunii-'y of the leiis ii^-lf. but 1>y 
litis «^l^»nf^'■lM'lll iiii|>ix>vMl focu^in;; of tho itti.-i;je ivuuld involve 
inr^itabli' dcrani^mi-iil of tlie illuiuinutioii. 

Sinitti and Itock »!«■> idaiIo a Bne instrument vliicli embodiei] 
tli« 'Javkson' limb and line iidiu.^tn)i>iit. Ii [a illustrated in tig. 
118, boiug thf tirst model ina<le by tlii« liriu in this forui, ntid it 
liax many ff-.»tiirp* of int^'tfsl from the point of view of our preient 
rK|uirr»icnts. Itnt afwrr «'p have onci" secured stetidinetts, the 
cTtifial poinl-i in a microscope nre ttio ijunlity of the fine adjastment. 
And ih'- d^lioHcy. lirmuexs, aiid ewso wiUi wblcli wo can centre, f ovum, 
and otiierwiae inifdify the sub'Sla^e illumination. To the former 
cn-lainly thi« model doe^ itot contribute. 

XVe are now prepared to examine and endeavour to judi;n im- 
partially from a practical jioinl of view the tuents of the princijial 
Knigliiih, (?i>ntJnentJil, and Ameiiciin models which nre offered to 
tlir nicrrMeopicAl public, it is ioiposiiible, no lev^ than it in unde- 
iimUn, to attempt tfi describe all the Tiiicrosci:>pe« of every maker, 
urovnn Ihflpi-inc-ipat forms mudf by ihc incnnsin^ nuiltitudeofopti- 
ciiuis. ^W liiivi- sought no opticians' aid ; wo have wirofully rxa- 
tained »11 the forms that Uy any Just claim to presenting an inxtru- 
m«»t which mepts the full requiirioents nf modern micniscopy ; and. 
nltbiiugh wn h.ivo reason to know that the juHgini-ntA wo nxprcxs aiii 
kluml by the loaiiingrxpert«) of this country, wcr tnkn |)iosolonM|ion- 
uhility for thr^i- judgnicnis. Hnving nought fur twrnty yfuni the 
beat tlint ciiutd Im jirmiurcd in niiciMSc«iicM and objectives, our judg- 
ment i» given with dnlilvrntinnnnd wholly in the inten-*t«of si-it-nce. 

Di examining the principal nioitcrn miiTcaeiiprai wi- shuU point 
owl whatever is of .ihmlute important or ivlativi- vulnn ; and the 
aWnre or pn-aencr of this in any form pi-rivi*iimnl)y m-h^cted is nil 
tliat the rinidi'r will nerd to enable him to iM't'omi? cunvinciid of our 
eBtimat« of tin- valui- of such an iiislrument, wliethiT the fonn be 
illustrKtciI in them jHigcx or fount! i» tlie catitlogur^ of the makers. 

With thix object lu'foru uK we shidl faeilitato it.i attainment by 
alonoe coniiideriiig what are the tttrnlutU of a gooil microscope. 

Fid. 1H. — (Iniilli itnd Il«ck*ii miooKOpn. 


appear t'l ho nbviuuK. Itut ve are bound to admit Uiat it is, in 
wlt^t tometiawM eiaini In Itc htAiicIs of tlifl tint class, (liiOTi,>anl«d; 
uid when thtf )i<-i){)>t of iho oetitre of grutity in tli« Eu;{IiKli nnd 
AmiMHcitn slanda cf tli« Hrst eiate. is raoudered, lliia ia u (atni 
mil take. 

It IK iH>iiited out in tluT section on mieromPtrj-' And dmwing 
dial thi- nptic axis ot Oic mici-oscoco should be ton iticben from Ui« 
table ; Unrrefort- u lirst >.-tiias micrwcopi- wliowi optic axh when 
placed h'Tixoiitally i* eitlwr more or Ipsw thnii this is found wiintiiifi; 
ID !t materia] point. Hut tn possess thix characteristic it must liav« 
a bi^b centre of gravity. 

Xon' it in pcHiibb' t» Krctira xtendine-ss liy ( I ) weight or (3) 
d«iii]{n. Tlie Continental mr^lluHl biu invariablv bciii weight. Tbn 
piUor at tliL' in.HtrutiH-tit i.-> tixc^l in » cumbrous mMiil foot of horse- 
iliot- (oni>, which bittra >o high a mlii> In tbi? wboU- rv'mninder of 
the iiutrutiKnt that it ia ukuhUv sti-ady. Tbi» secur™ tho wid 
cert&iidy, but by oarne aiul unwidity mf-niis. It proiuiwit littJo 
for l)i« iiitlrMmeiit an a wbol«. 

What is waut«d in the uiaxininH) of xteadiuenii witJi tho minimuni 
of weight. 

Palpiiblj-, th« itiechaniml conipematioii for the diflicutly of aii 
eleval«l retitre of gisvity is ui ert*ttdf(/ tu-if. Tlie Inidinx fault 
•4 ipatiy stands cliitniiiij; the llrst mntc is Uieir narmwed baaL-«. A 
hnul base, resting on three poiiita ooly, and tlieie ptu;q^l witit 
rork, is the ideal fi>ra perfect iriftlruiut^it. 

11. Next ill order to lite i-tmirl of tlie microBoope comes what i» 
known a» tlie body of the instrument the tube or tube* for receiv- 
ing the objective ;il one eiiil and the eye pieces at Uie oUier. Tho 
tube of the luunocular i* always provided wilb aii inner tulie c.-iUed 
the •Inif-tuh'. In a tirsi-cbss instrument this lattfT should nlwayx 
be proviiled with n mck 'nnd -pinion motion, and should have a scale 
iif from two to three inches, divideil into tenths or uiillimetres. Tliis 
ettables the operator the moreaccui-ately to adjust apochromaticobjee- 
tives so sensiLive, for iheir be?! action, toHccuriiteadJu%tmeiit of tube- 
lengUi. In (act, il is always imporiant to remember that obioi.*tivea 
are corrected for a special tulje-leiiglli : that is to say. for the foiina- 
tton of ilie image at a certain definite di»tnnc«>. 

Tkf-rr rirr, Anirviwr, lirv X'i'iwf' (jf fub«-i«ngUi, (I) an o/itieaf and 
{2) a ni'f'Aitiiiiv)/. 

n- "fduvi/ inlie-tmiijlh is tn^'asured fr«in th« posterior principal 
forus f>f the objectivo to ihe (interior jirincipnl focusof the eyo-pieco. 

7%« nir^/idHii^l tnhf.-/rnr/i/. stiould l>i> nieoAurod from the toji of 
tj»e tube into which the eye pi«cc lits, and iijion which the bcarinK* 
of the eye-piece rr*t to tho end of tho iioiw-picce into wliivh Uie 
ot>jrOtive in KClTWed. 

l.'nfi>rtunat<'ly ditTcrt-nt niakers estiraato tube-len^'tli differently, 
atMl take dilfcrrnl poiiitK frrmi vliich to make llieir meaaurenienU. 
LrH)kiii;t nl the matti'r bitjudly, tbcn- are two r-itiiiiiLtes for tube- 
tentrlb in practical u«n : tbi-sp arc the Entlinh imtl the Contiiiental. 

What wax formerly knon*n ns the- Kii};listi Btanilard tube bad an 
■ ClwiilR IV, 


opticul IniijcLh for liijjli and moderate jHiwrer olijei-tives oi Ou ineh^n ; 
witli low jiiiwem, liowever, it was lois. The iiiucbumc'^l tube-tnu^tb 
u-iis K j inclii-s. 

Profi-sL-mr Alilit', ill L-uiiiitnictiii!; bin aiwcliroinatio objectives for 
tbe Eti;;lisb l>i«ly, hua tukt^ii ihf uietluuiicftl tulve-leiifflli at 9-8 
inches ^ :J50 luiri. : uiid the opliiial tube-length nl 10'1> iiiL-bcK 
^'J70 mm. Thishus cnused hii iiici-iiLse in the length of the Enghsh 
atancLard tube, since aW jjood miLToscopes are made to work with 
these «bjei:ti\'e§ ; jiiid ihr tiihUlii'ii •</'! rin-k iin'l /iiuion tnltiv "'/rttw.- 
tuht ' becomes of f;i-eat |iractiml vnlue. 

The tiil>e-leiigili of the Couiiu'itlul ntrcfinnieal lube is C'3 incbee 
^ 180 mm,, and the 'ij'tiriil lulir-hwjif' is i-Of* inches ^ l(St) mm. 

The question has been asked, ■ Which is the lielter of ihesn two 
dirteiini,' luho-lengllis f ' So fur «k the iti>iii!;ci in the iiiMniment is 
concerned, tlierv i« not niudi tlilliiivrncc. It is of little iniportiiiico 
whether ihe initial niAgtiifyint; poner of nii ohjcoiive hv inentwrd 
by a slightly lower eyc'|iicce usm at a Iniigrr di«tiini?e or n Mightly 
<lMp«r (higher) (^ye-piecent a sliorfflr distwice. But it is of pmcticnl 
importance to ntrtn thnx ii »iii-ill '/[fi'i-rrw' ••/ luh'-lrii'jtli i'i-->'lurrii n 

ijrfater i-^'ft iin ii'/jr<*liiiritf trith n shiirt l-mltf (hmi u ilh n l-m^ iinr. 
Th» |>rinei|ial dilTi'ivincc, howrvi-r, Iwrtwi^n the lung nnd llie Nhurt 
1>i>dy a> alfordiiig a. datum for their rfsprctive vnliivs n tiiAt whvn 
a short body i» by n jierson liaring nnrinal nr-fi»nin'ff'iliijn nf 
inylit, tlie »iajcc of the miirrosco|>c eiinnot Ihi kotii uidrss thr head i* 
iVDidved fruni tlie i-y-picoo, uhnrrajs witli the long body Uik «?y* 
nwwl not Iw tnkcn from th<! eyo-jiiwi; nl ftll, iw thi' stngi- niii \» jh.-cii 
with the iiniispd i-y. 

III. Armngements for focuuing stand next in order of tmport- 
nn«-. Kvi-ry iniirrosi'opc of the first ela«i is proviilnd with two 
armngrment* for ffMrnssin;;, one « •■•••iriM u'd""/!"'"', ai'tiiig mfiidly, 
Jiud thfi othur uj!ii'-'"ljiwh/t^iii,whi<-h should act witligriMt delicacy 
and precision. A jjnod 'cnar^in ailjnstinent ' or primary movable 
part of the iiiKlrnment is of gnvit im()ortancc. The first ri'iiuisitc is 
dial the body or ni'ivnblp jmrt Kliould move i-nsily, snmothly, but 
without 'aliakt?' in tUr gn«)vc or -itot or whatever clw it slides. 
We have found in prjiftira- Ihut a Iitir shiiiwd like n tninraled »risni 
sliding in a suitable groove act^s bfkl nnd longest. But a liar planed 
true nnd placed in a groovH ploughed lo suit it is not i-nimgli. The 
inevitable friutiun tietcnnines ivi-iir. and lliis brings "itb it a futnl 
'shake.' All such gr<iovrs, whioh are usually V'sbapivl, n/'uM^r/ hn 
eul nnd Miiriini/ un iim- li'lr, so tliiit by " unht^'ning u|> ' the Va by 
moans of screw* the luir or limb is again tinnly gripped. Further, 
tlie bar should not ■ tieair ' for it.^ whole length along tlip groove, but 
only on points at cither end mid lu the middle. Powell introdttced 
these prime flGMntiols to H good 'cuane adjustment' half a ceiilury 
Ago ; yet witat thaiHBn<U of iintruuients in which iheHe principles 
have not been applied have lieeii. by sheer friction wear, soon 
i^Aiiged into useless brass since then ! But iiistrumeiitt mode by 
tlii» timi are as gixnl nfter thirty years' use as they were when now. 

Frequvntly Iwd workmanship is concealed by thf free employineDt 
t'f what is known ns 'optician's gn-iwc' and on ovcr-tiglitening of t]i« 

l*««;r(WlX(i AUKANdKMK.Vrs 

pinion, ilriving iU tvcth into tlw ruck, witkli, of coune. B|iet9ilily 
t-n<l« in ttisiutcr. 

If we d«rin> to pnictinti!}' lost this ]aut of a niU-rost-iipe, we 
iiiiKi rrmovi' t)it> pittioii, tiik« nut the bar, cli-an off the * opticiMti's 
t"r''««- ' with |>clrx>[<-nii> from lioth lair uiitl gixiuve. oil with walob- 
iiiukrr '« oil. him) n-pli»cf thf I'ln in tin; {jrooip, and before n>ttxitig 
th'' pini'nj «■•■ if it jJidrs smodilily Jinil withiiut Iiitriiil hhiike. 

Wlmt hns liwn snifl iibout ib<' ' spriiiginR ' of lln' luir in this Kpeciul 
itivtniM^r' ji|>ptirjt r<|UMlly tti nil moving {iiLrU, in Gtii);<- iind Kub'Stnpe 
ini>vcin<-ntH, aiul whm-icr <H>ii>innt friction in inuuiTt-H : ihiuaIIj* 
applicnblc, ino, is iitK lubrirnnt wf Mi|:ti.'i'''t. An iiistninii'nt Mt 
Diminl ill its iintiic -sn'ou-' f>>r tni-lvo inontliH hircomrs lo iraninbili' 
in niiJHt itf its pnrts bv th<! bnrrlc-niiij< of it.< ' noriiml ' lubriciint tlint 
iniitiaii lit'<v»nen it peril t'> its future if peruHt^'d in in tlint coiiditioti. 

If a 'ooarM' adjuttmi-nt ' bi- what it iJiuuld W, iill lowrr pi>w<T« 
nhonhl Iw excIoMvply niiil pcrfi-cily focu>uil by it, itnil with thr 
hif^i«st ]Ktu'erK obicvts nJiinild Im fouiitl mud fociuncd u]> tu Ijie puiiit 
irf I'Wr vi.iibihty. 

KnckuMil-ptuion work for mirrascopmi Imh tdwn Kfcratly itiiproveil 
by iJte aditptinii of tlut tiuggrKlion of .Mr.,)i>ltn Miiynll, juii- to Cin- 

tto. 110. .'-iiji.)>ii ' iliftccnu'l rni'k' 
mink lot Muto aJj«ilini-i>l. 

I'J'i. — L*M run|>Ii>i tunnt. 

pli>y n 'stepped' diii^onol ruck-work, which gnsitly incmwM the 
diiKHithnev tk tlio motion. Fi^. I IU shows the «rmng4tnM!nt a» lirst 
applird with thive nukt, thv trcth of ciu-h pm-t iHtin^ sH out of lini- 
l» (l»e ext«itt of otie-lliird their pitrh, and thi^ xpintl |>ini«iu Iwin^ 
littitl lA currevporid with Iho n»ikv. Thn eHtet in Muiilur t<i wlwit 
wonid be oUninml by piti^hin;^ the t«^h vi u xianU- mok thrfu times 
u IIiH>ly. but »( the Kinic tinir rrtmning iim Mivnjtth duv to the 
cMreer rack. 

Ch) itccoutit of (liHiculty in thn « orkiuniubip the rauks were suh- 
hei|tiently ftsluml to t«iNiui in ti^. 1:^0, wliidi stilt i-etoinii conaideniUe 
iulvniilu;,'e o\>'r thr itrdinjirv fiii-iii. 

There l» u, dofMt in )'itli>-r niian)acii[ie or tuioroscopist if tht^ 
' tftie tu^ustniptit ' IK n-M>rt4-d In iH-t'ure the object is focunvwl into 
ele«r view, rvoii with thf hi^li<->t powers. 

Illfl Tin* AdJasttMnt — Tlii» jwirt of the modCTn inicrMCopi' 
twnaeasf' an intfortkiM.'O not iii^ily exHRS^ntted, and dHififncy or 
Uid principle in tbo cwiikti-tiction of Uiia makes not only inferior, 



Init for t-rilicnl purposes aliaoiutely uwless, wbat are othCTwiso 
iiuimmenU of exceUent workinaiisiiip and real value. 

There ai« two kinds of tine .■idjustiiieiit tiiiunlly employed : — 

i. Thoae wtiidi »imply mnv^ (h^ noaf-jArj^ whicli receives tb6 

ii. Tliohe whtcli wove Ihf H-hJr hurl;/, or the wliole bodf including 
the coOrrHe ftdjuKtmeiit. 

Ex-ery L-onslructioii of the soeond i?1<l»x tiM pmvrd ini]iniPti<!ftlilo, 
mid even perntdoUM. It inevitnhly lirr-nkn domn jiirt n* tln' piirpliiiucr, 
by practice, begins to rdfilisc tW viihif of pi-rfrct lurtion. Willi a 
largo experience of gtiinU of pviry cIas«, wi- tin- ol)Hgc<l to sny that 
gpiiernlly with one oi- two ycKn of %v>rk it Iusck whAtcver inluo it 
at li rst poRseasiiil . 

To this broBcl Ktiiti'ini^t tlit^'nrc possi lily two [•xpcptimu^ invcn. 
lions still mill jmlicr, vi«. Swift's Mtip h'\'i-riiti(l (.'iiinplicli''><liff«ri'iitud 
screw, tti which wi« shull siiliM-ipiinitly rtrlVr. 

It is, howcivrr, uiii>n this iiiodi-l, with nil it's nuiirsil and glaring 
im perfection i, that tSiL- iiinjority of ContinPnlnl uiicroBCojin arc built. 

A sci'RW of uii e\trenip|y Hiip lliriiul, mid tlii«ref»re of Pxtrt-nii^ly 
shallow iiicJMOn — a iiiicromi'ter screw in fiict - him !•• Im\r l/i' ttfitro/' 
li/Hni/ itii'l linr-Krin'j llir rHlint uvight vf tht '""/'/, witli Iti coars* ad- 
Justjuent, Ic-iises.and ao forth ; while tliem''f"'y'''V of Uieadjustmcnt 
Aliould be t*> give a delii-nt«, n,lm<i«>t impei-ooptible, motion to tlic 
(iijeH-'jlat* tilorte. It needs no K''eHt expericniM.' to forenee the inevi- 
table result ; the screw losea its power to net. and soiuetliin;; incom- 
parably woriie than a loU-ralilc eojirse iidjustment is left In its phvcf. 

Yet it is the Continental ni<iil>-1 that ba» liecome tlie diirlinj; of 
Enijli&h hiboniturie!>. and lluit ntill receives the agipreciitlion of ]>ro- 
feaaoTS and their Htudentr^ True they aiwwer iu the ni«in thi« 
purposes aouubt— the «xii{Liiiie« of a limited i-'otirM of practical in- 
struction. Bui how many of those who receive it are the medical 
men of the future, am) t<i whom n luicrOBcope^not of neceaidty a 
costly one--of the ri^fht construction would lie of in creasinji; value 
through a lifetime } 

Almost any instruments however inferior, could he employed 
succtwsfully with ft J-inch objeclive of ' low single ' (to give it whul 
liM 1>con called ' tlin needful penetration ' for liistological subjects 1) 
to obtoin nn image cori-espondini,' to a ti^ire in a text-book of, say, 
B Mklpighian corpuscle, or a section of kidney, brain, or spinal cord. 
Thff quality of a line ndiostuieiit is never tcst<'d hy these nieaiie, 
for, in jioint of fact, a ilelicate lin*t adjiflnient it not even necesMuiy. 
\Vc writo in tho interests of microscopical research. It certaiuly 
may l>o takm for gmiited tlint the end snURht is not simply to use 
the micro*cop« to verify tlic iltuKtrationx of a tcxt-lxiok, n treatiae, 
or a onuniit of Ircturrs ; without doubt it is a nibiiidiary purpose ; 
but the largnr aim in to iii&pirc in the young «tudont ronfidonce, 
(enthusiasm, and anticipation in the mr^thotUand promise of histology 
ami all that it touches. Bui for this there must Im jinlrnliaUty 
(without cOMtliness) in the mechanical >ind optical clianiclttr of the 
nlcroacopes commended and appro\-ed- 

A lov'priced student's microscope of good workmuiship ftod 


dtwt rimign omM Mwily bi> ilnvised if tJin dnnnni) for it iiniw. 
ted, tluriii'; tlic jmsI {l-w jri-nrv n oTtniii clitss »f stiiilrtits' iiiion*- 
I COpM luivi" UtcH iuiprovwl fjrrntly ; this has lii'^n ii itiiiciniiiUint o( 
the S(H«ii«e ol b*cteriol<);;y, which hin! cotiijiclli'il tht? «-■«• of Uiu «ul>- 
Ktage oon<leitaer. Wt luivi! H»i<l riKiu^h r>( (hi: vuluc ">f tlii» iimtru- 
mwit in B succewliiig chll[>t4^r. hut until iw-fiii yrum liiBloIo^-ists 
did aol u*e it Wtauu- it yuan, imt usml in (iurmiiiiy «r willi Ci-rmuu 
instrument 1 It« [irfMi-iit wf, nevorthehtw, hiLn \\nA t)if t-ireot 
of iBipmving th(><lefinil}on»hlH)ne(l bj' thpohji-L'livei usodhvvviudeutd 
gimaawlly. Boom wIm> iwrveive tliU, tnideovour to attribute it to th« 
iD>ptt»'<tmenl (drprted 
in modern nhj«rtivi<n, 
bdt thi« u not the 
ciwe ; t)w ohjfictivf« 
in nunj' riuww are not 
fvnt nvw, an<t until 
Umi iatmlnetjoii of th« 
J«ntt g)n><> ' th« oi-di- 
nary ictiidmt*' ohjis." 
tiv«i w«!r« not iiuilly 
•ojpKxI a«tbi> Hni.'li*n 
object ivcH of thirty- 
five vi-»rK ii-{(v Itut it 
rould tstailj- hp shown 
olH--of thMieiuirly 
ieictivr^ uunI iu; it 
■Iwayii wu* witli ii '-"»■ 
drmef. wduIiI mipjihk* 
in ihi' shnrpntoM of its 
di^tiuit inn tlicmiijnrity 
of itioiiu nou ntlppliril 
to * studentii ' with 
Cbnttii«iii4il model N. 

But it inuu not 
\m tu|ppo«<ed Hint tt is 
4fN<y the Continental 
nudel ihnt is de- 
fotnied l>y tl>e ndofi- 
tioii of this nidit-nl 
«m>r ill tlte ' line itd- 
j<ntiiwnt ' with <■ hiuh 
w(i itr« doaling. Kveti 
daring thu l««t t^n 
Voint it hit!* been «(>- 

jiIibI to Hinip of th« most iin[><iuiit: anil pxp«>nBive insiriiinenU iiiitde 
tn KiiKlitnd and Ani4>n>-a on what la known ns the 'Jackson ' ninilt^L. 
Tliiji luixli'J luw nne sii|trftin' viitue, in th* [HMseMion of a soliil limb. 
Thiii may Wifcr mwiy distinct forms, but it is «uHici«n(ly i-ejin-sentwl 
in fig. 121, wh<-t«it will ho mhiii lliat tho 'limb," which is Bwuiig be- 
twetm tbo pilbra, and which carries the body-tube* and tlio line 

■ Kt,tc Ubapbir t. 

I'll), 131. — Sois-Jaskfonnidal. 

i6o THE msToav and kviilitius ok the micbosc-opk 

n(ljutiltne»t, ts iu One solid pit'Vf. If nothing were siutrilicccl this 
woul<l In* a boon. Foruierly, tliin motlcl wiu aupplip.1 witli n fine 
ixljuitm^iit wliidi only iiiovikI the iirMi>>|m'i:i>, 1ml an ii priiidpje 
whii'h wo sli.ll! sou wive wrtinH, imd fniiu it» iinj)erfectiim» it wns 
nWiitloiied. find tlir si^ltd Jiu.'k.-«iti Hrtii was rut, uiid Ihc wholo hotly 
mid its ix>:u'§e ndjiistiiiont Wii.i jiivotc-d on tlie U'V^r of thu Hnp nil- 
jUHtiiienl. Thus its normal virtuf (ii w/tW Hiiib) wiis .-^criliccd, mid 
A ' tine a<ljiistmi?nt,' doome^l to fiijliin-, wah ^t'eii to it. 

A cunipl^'x i-ollw, A wpilftc mid n dilT'^it^utiitl sinew liav« in turn 

beeii siiite eiiijduyud to ro- 
deeiu thi«^ iiiBtrutiieni from 
tlie failui-e thnt liiul <nrr- 
tnken it. Partially, or iM>iii- 
pli-tely, eAoh hiui failed. The- 
(liflerpiitiAl screw certAintv 
couice tlienr«ticnlly iieareid 
to success witl) this fomi of 
iiislmiucnt. Itut at tb« out- 
set this is tlip wweonly when' 
it wholly iibnndonii the liftiii>; 
(ittd iowpringofthe l>ody-tube 
Jtc. by thn net inn of a 'lino 
ndjiistmcnt,' nnd its mol-iuu is 
only liroiight into opcmtiou 
upon tJir niuivBlont of n iiose- 
piccp. Tho fiiriii specially 
odoplod for iJiis in8irum«ot 
yrta dovised by iJr. Hu;^t 
Schrocd*r, nnd i» n, n?niurk- 
nbli- armnpfniFnt. mortlty of 
ln'iiig iindcvKtnnd. It is ifhis- 
tmtrd in iTg*, \T2 nnd IS.'l. 

Th« nnnfi-inriT', A, is at- 
liichrd to n iuIh- which is 
littKl to islidf aceumtHy in 
udjustidih- bearing in the 
liody-tiilK', B. Thi- nose- 
]<ieou tulm bwt a short pro- 
jecting' nnn, C, by ni<?niiR of 
which it is pi'e-ssiHl upwiirdx 
by* strong cpintl spring inoun't'd in n eyUndricnl box, I..outsid(! tii« 
lowi-r i-iid of the limly-tulM". Tlu- arm, C. ia nio\ «t ii-,'ainsl tin- niiring 
l)V thf diHrn-ntiid-KcrOM* roechani-iin (with Diilltil lieiid. 15), whioh is 
Ifindudtnl onabracki.*t, E. nti.-ichi^d to lliP upper piirt of the btxly -tulip. 
Thf iliffcrrulial-scrrew iiii-cliuniBiu conaisls of a Bl*el rod. F (coti- 
iiottotl with D), which hns two »civw threads at the lower end. one 
working in a thrt-ad cut in the end of tht- inner tube, tl. and tim 
other in the Idock, H, whidi ii siddered within the shealti, .). 
Wlirn thir milled head is turned to Hip lefl, the blot^k— and with it 
(ho sheath moves donnwanK while the ro«l lUelf, carryin;- tlic 
block and t>lit«th, moves upwards. Aj* the screws are cut reepec- 

Fiu. Ii9. 

Pill. I'£l. 

HvhiWHl«T'H flnp wtjuitniviit for thcr Jm/Iimii 




lively to forty-five nnd tifty-twro tlirmds to th« inch, the resultoiit 
niolioii is <H|iiivii]piit totli« dtlTtMviH^ liclwivii tfiv nuMioii of tiie two 
tcfvwa- tliJit in. to dto tiiiitioii of a. Ktvir of n««rly ^3'> threails to 
ihe ioch. 

Tlin end of t)i« alimtli ix ti|»pc<l u-jtli n amnll spheris K, of 
polisfard stw), urliile tin- iirojrvtin;! m-tu iif Itic tiiiM-piorP tube, 
Nipuntt wtiicll tilt! twl «'<:ii-KM, has u oormjxiiKlJng imiiiwvp IitcI of 
Itolinbed i^(>t«. 

TIw ;n'«at fuuh in ttn.i dcaiKit in tii Its tnoii' of Ado|iti()n (if vrlint 
olln-Twlse the lii;;lily viiliiiilil* pi'iiii.-i|>lf >•{ lti« ' ditf^viitJA) »cn-w.' 
JLs n lioe luljustiiif n( it Mii'luubli^iUy cdti&ints of lu» tiiiiiiy ]>arlK or 
aepnmte [ueoea. It ia u Itind uf |)eiii-il-i.-ii^R wliif^li i.-xtciiiU and cmi' 
inu'W ljy lueAiis of n tlilTi-rviituil screvi- iicthiK on the iiosie-mecn. 
Tlwn it« iiioite of altiicliiuetil to the lioily iiiiikes vihrulinn iiii-^i- 
tkble ; lli« iiiic-roHiMer ^crewr is in iin oxtretitely huil ()u&itiuii, k'^'ii? 
tlie utn»o«t leverage for vibratory action iit usin;; it, uiu) ut th« 
atinc liatf, on account of its hoiglit Ironi llie tnlile, tiiuat he uurlcpil 
by tite ariii entirety without rest. Cotn]>le'\ity of jiarU and infelicity 
of armntrrnivnt innlie tJuH <l«vic« not oidjr very cosllyt but by uu 
UMuiriK drikiml'lv. 

\\V mny now diiv«t our attention to thDji/rmff iiflh' tin- iUx-in-wt 
iiit«t whti-h wi' hnvn Mtjiiiratrd th« vnrioua kindfi of litir ndiiifttiiK'nl, \\/. 
lliat in Vihii'h the tnr»r-/<i'p''itjttjf itentxtrtJlfl by tlK* luljiistnivDl screw. 

An iwrly form »f Uii* wak riiiployral by Amln-» Iio'>., It wm 
n|>[>ltCid ti> A uii('r<>»k'n[M.- Iiii\iii)t n hnr iiiovrinrnt, tlut jjr-iti'inl form "f 
trhiHi — i-o fnr lis tlic lor and thi< a[i|>Hi-atioii of thi> Hti<! mljusliiieiit 
aiT conc*TiiMl — xsKPvn in tig. I.'ll. It oon>^i«tcd<if « Irf-fu/thrnrninil 
tinier inirrt"! u-il/itn thr htir, nitil ni-tuattil by 11 iiiicronieter stii-w 
with 11 niilhtl hi-nd at olio rnd, thf fuU-nim licingat thf other, an<l 
tlie n<iw--ptrc«' lj(>tw<>'n theui. Tliirt M?rvi-<1 iiilinirahly in the liays of 
loir-nngl" I cibji-ctivcs ; hut thtn"n wero two faults U-lmiyiiig to it; 
oiM- WA» lluil tli« ttilie of tho ntjsc-piiMc Imd not n »iiliii-iciit IpnjL'th 
III iMiiriiii: nnd was liiihln to n lateral sbiikr ; the other ytan tlint the 
ixljimtuii-iit M;rcw, l)ciiig iirnr tlii: midiltc iif iIm: bnr. involv4^I irrnior. 

Thr n{>|>licatton of this giritK^iph' in it» vciy hiuhoit and Rioat 
|>rafiH.*tly prnrtital form wiik ndopti^d hy Powfli. i\'\\ iristmiuuiit 
■Im> had H Iwr iDOVcou-tit ; Init the Imr WtD^ of n-lAtivcly xrcnt 
IriiKth. h« no[Joyi?il n li-rrr 1;/' Ihrjirtl iifilrr, the mi<-ri>iiii<1i.''r'>icn-n' 
IWLiitl "■^ •in*' end, the tiosc-piru'at tlii' other, and the fulcrum iN'twecii 
tWni. Thn ratiii of th« unim of the Ivver wan - : 1: coiiaM|ueully, 
lui the screw ha^ tlfty tlirt-iids to tlie inch, n coiopleti.- ivvohition of 
tint kcrew cauiMA a uiovemnnt of tli*; olyjectivt! <•[ in(,Ui of an iitch. 
The jHMition of the acraw in imitit^ili.itrly beliind tlie j>ivuL on whi<'h 
til* bar turnn, and (fain precludes the |xitisibility of tlie im|jni'tiiiiun 
of vihrnlioo to the IkkIv : and, a< the nose-piece tube is \ery lon^. 
and oiily liears uu threw |H>intS at either end, lliis udjuntnieiit is the 
Htiradieat, the tiniootJient, and the inoat i-i-liahle foi' all objectives of 
nny iif the Bevenil dr-vices ulucli have oouie before u>iUuriiii,' the last 
iwxutyyeara. In fact this fine a(|iuwtment has held an titnivnlled 
^itloa for tba piiat forty yeara (fig. 129). 

The (Ine wljuetinejit that was employed as its rival on Uw Mtrtier 



forais of ttit^ .litclcKOU nioilel wiut knowik at the iJiatt-siilf lumr, uml 
it wiut tametiiDfii «mpUiy«d id lli» couiDiouer bar luot-ement micro- 
i(t.'0|ie«. lu poaition ami i]ianiel#r will lie iie«i on the ri'-lii-tioiHl 
.•(l<Ii> of lliH lioily of the Beclc-J»i-ti8oii inoilel, tij". 1 li^. In llie light 
lit uhnt w now ite«(l, we are )ioun<l !•) tAy to the iiilciiding par- 
citaaei' of a mki'oscopei. 'Avoi<l it;' it is liad alike in desisn anil 
ciinBtrui.-lioii. The. screw is bo pUced that tivumr is {nei'itat>l« in 
till; ImkI)' wh^ii it U toucheil, wliilo the nose-piecv tulie ix go short tliiit 
iiUit(liu«a8 of inaveiiifoii. iloes rot lieliing to it. It is only that it 
wiiBcoauuriviit with the liplief iit ■lownngk-s,' and voiiset)uent 'pwrn-- 
tr&tioi),' ill objoL-tives (with which no miiciU work could hi^ Hoiiv), 
ilmt it in pos*il>le to nccotiiit for tbi" toloivition ti'V -o long in miiii- 
LxTs of Kiiglisli microscopes of this wliolly incrticii-nt niljoktincjit. 

SiiHjl'* ivrfii-nl till' In'^r in one 
of tho new forms of linn wljuxt- 
iiienl worthy ot^ »ii'«fal trial; it 
hiM in it eli-mpnts of ifrcnt mmi. 
It «viii, howTtviT, riiily III! n,p]>ltml to 
the JnukKon model, and promitvs f* 
ri'di-i'in tlittt inxtniioetit frnni wluit 
must iithi>i-wiHi hiivi* prnved itK 
extinction us n lin-it'lnxs micm- 

Tilt Jintt /arm cf thin aiijuti- 
iiifn> wns Houiiil in prittoipte und 
in|;etii(>us in roiiKlruiHion, It tK 
diliieiill to ktimv why tin* inventor 

ami pntentee liiia iiimndoneil it 
ftir iinothf-r, the vniue nf which 
UB H nuKlilii-atton has yet to In 

Tlie i-Jirly form nmployi'd liy 
Rwil't u^oirlei] wluit hnd l«ini n 
Khecr nftreKtiity of kII nuccv^ul 
tliie n<liiulmtiit« of this type, vi*. 
tbeaccui'iu'v ami pi^eftion of the 
HttiiiK of the nose pii>crtulie. This 
was done, «s shown in *i(i, 134, Hy 
attaching a vertienl piixni -shaptil 
liar. A, to tlie nuse-picwe, untl slitl- 
Jl ing this ill* in a Iwx at 
the Iwek of the billy. A horizontal 
^^sSMi fl_ \ niioronieler SL-rew with » null<Kl 
he«d, V, nets on n vertical Wnt 
leter, 1), on wliieb a stud, K, tixe<t 
to the prism iMr lienra. 

There is aUoamidjustineiit for 

Fio.lM.—Hirin'ijnkntiint'uljn-Li I. tightening up tliepi-iifin liar in the 

V-grooveit, U B. Side slinke nnd 
■ lii»a of time ' are imjuiSMble with this form of fuliustiiieiit ; white the 
jjuwer to ' tijfliteu u]> ' liy means of the ctipsLan-hended M-rewit 



MljHi^tnoil Mm* to till! Icll kuiil. 

Pill. IM. 

t1«'XU>rily vt nnt at piVMriit n vmiiiiioii •^iSt, mid to lmvt> tlm ri-^lit 
Will (ret? i» tiiifH>rtiiiit. Tliis wH.a jiuialed out In- Mr. Nelsuu iilirii 
thtH Hue >t(lju^Ul>0Ill wa.-i tii-st lilt rod uced, and lii^ liiid u HtudciitK 
niii:ras(-o|*t> fHniiirm:t4H] with llie nuci-oniet^r iiiiUed Inaid on tin- 
left side, iia in tiK. H'k It U i)iiiiiifi~>t, liowrvi-r, that h wmld 
KTHkily iiii]>rove thin iidju!itm«iil U the HL-ivw-uinioii wi^re t-iimml 
lij-lit tliruu;jh aiMl a niilloil liMid plLii-'«d oil botn the I'iglil and Llw- 
Wt -lideb of the body. 



On wliut ]ii-uiei|)le the tiiakers Imve rctuniiHl to the former plan 
of plnciii;; the lieiul on tli« rif;lii hnnd of [lie operator is not cnsily 
•w]>lniii«i! : but tliU is die r^se, ackI it involvos an awkwnrdnosf -ind 
inftcsniico in tlie mauipulHtiou of these instruments greatly to bo 


Tli« later form of tbiN tine lul.iuKtninit consUts in reverting in 
lilt- plan — wliieh we hav* seen kucJi stroiiff rt:B>Lin to condjann — of 
tltrowiug upon tlie tilio-aiijuiitTiicnt Kcn-w ihe uiuvcment iif thi* ^-ntiit- 
Idxtv. A Jai.-fcson liiiil) titUil with this inovcinnit. hnK two sliiles— 
oiii? for conr^p itiljui'tinf^ni, which uiiivrK thi- cnt.iro lioily, includinff 
the tlni>-iuljii>t incut lioi ; in front nf thi* is n, nicont) box or ex- 
viiv/itioi) for the lino-iuljuxtmi'nt movement of lliu body only. Thn 
mechniiLMi) ii shown in li;f. 1'_'l> ; it in |>rc(;iKcly KiniiUr to that in 
tig. Il!4, with (hi- rxcpptiiin lliiit the .ttud on which thr Utvpr Imtni 
is li\pd lo till' Ixidy-nliiii' in;rt<-ii(I of lo the ni<M'']iio(-i'-iilidc. 

Ely this \rry wmpli- iin-clinuiiiii tliP tint- ml incut ij. upplinl on 
the front of thr foiirsi? ftltjll^trn^•^^t, (iml nets on thi- whok- Wly-tul>r 
iind not ninn-ly on tin- mini-- p ire i-. 

'I'luTi- ntiiiuini one otlit-r fiirni of tlu>i iuljuBtnit^nt to bo wjd- 
Kidrr^l : if i* " _/tirnt vj' •iiff'reuiiol •eifu; itei-ivil t,i/ ihr Hte. J. 
C'lmphrlt, of Pelliir, Shi-tlund. Ita obJ«t is to nuppkni the dirvel- 
action scri'w, when the form of the mii-rutivope may u|)|H-iir to miiki- 
tluit It uecesMty. This lias bi*ii tJie fjise with the Contiiu-ntiil 
niiutpl. It was applied by it« intentor to a miero^cojio nuuh- 
1iy himself, and was bi-ou)ttil befum the (jnolctttt 
Club by .Mr. E. M. Ni-hou. 

It is vory simple, and i« made by cutting 
two tlin^ds iu the micrometer Btn-w, Fig. 1:;7 
will UluslrHte the oxitcl metliod. I> is the milled 
bead of the diivct-ac-tini; acrew. Tho upper part, S. 
of the ecruw luis (say ) twenty thi-eads i» the inch, 
and the lower |uirl, T, twenty-live thn>adK to the 
inch. Uisthefivd smket foniiiUR |"irl of the limb 
uf the microscojie. and II is the liuvultin;; M^ckpt 
connected with the support of the boily-tul>e. 
The reviiliition nt D cnunes ihe screw thn-n*) K 
to move up find down in B :it the nt« of twenty 
turns to the inch, whiUi thesci<ew tliraadTeauMK 
iho tmveiling socket 11 to move in tlie rcversA 
diroction til the rnto nf twenty-five turns to tjin 
inch. The coiiibiiicl edect. tlieir'foi'e, of turning D 
twenty revolutions i* to raise or lowerT and with 
it the body tuW Jth of nn inch, or i^nth of an inch foresch revolu- 
tion. Tlie spindftpring below H krcju the Ijeariiij!^ in close coiitoeL 
Of course imy deKir<4l Bpi-ed can bt nttJiincd by ]iroix-r ^-ombuia- 
tion of the threuiN : thus 32 and HO would give dnth of an inch 
t'oreai'h revolution, and 31 and 30 would give g^nth of an iiiuli. 

Thi^ scn-w has provided fur the Continental model wluit Swift's 
vrrticnl lever has done for the Jackson mtKlct ; Mr. Uaker, of 
llolborn, hus adiipteil it and with vcrj' »ntisfftct»iry results ; for it 
ha> parted through that must crucial of tests for a tine at^ustment. 

I'n.. 1J7. — (.'iimiijoH'ii 

cli)I*n'i>tiiil •i7f»w 
ftns ndjustailcut. 



ih einiiloyment in jilioto-niicroKnt'hy, wilti rxcRlleut iVHultK ; ihuI 
we lioj-.- that it iiuiy hecoinf die (•Mn>niJ lino adiuiliiirnt for lliiti 
jioriD of iuii:ro!U.->i|)e in |iIiioe of tht! olil form of ilin-ct acting' tvivw. 

Fnitii lli« fofv^oinK vft- Ivnrn tluit llii-ro nn' threo ty|M-x pf luicra- 
Mope riiodeU for uliiih a &uiUblo tiiiv a<]jtisttiit.-ut biu l>eeti found. 

i. tlio Imr inoTeiiiMit inodel, lor wliivli Powell':! HrM onl«r of 
IwfT is I1m> perfect metfaut). 

ii. T)i« Jackson ■i>o<Ip1, fur wliicli .Swift's vertJcnl bhle l«vrr Jx 
tlw best fotia kiion'iL 

iii. Tilt' ContiiM^titnl inottel, fur wliicli OitiiptH'U's itifl'eFi^nlinl 
tervw is thp most unonili .■in<t ilclicstp device ytt ftii-^'vsl^l. 

IV. The atageof tho nucroicope wil! next cill for c<)uiii<I<-m- 
tjun. Whnt is kiK>wii as a mrfhniii^t'l tUi'ir iiiuDt bff a pniT of vvcry 
tinl'clnsn microscope ; but by liiis wc nwan oiip of perfect work- 
mnnitiip nnd cunotrDutioi), othorwjce it u ftii imppdiiucuc mid i»>t a 

Til till!! rnd »-»■ irould any nt tJie "iitwt then' iiiusi Ix." tJioroosliiy 
Wfll'iniiili' iiiovrinriits fffflrtt f'l/ ffi (iiirf pinion itnil ormr. Thn 
mnptoyiiicnt of Icvrrs, cniiiK, *nil lliiil t'lnss nf stn^p-giar \* in priicti(f, 
ftir critit-nl p4ir]niH-^, a nu-rc nnyhiinirnl raockfiT,'. lii-txr trust «> 
wm! mIuojIi- i\\i\fiii'j- fa to ntovo the objt'i't tluiu b<« b>-f."uiicil by '■ny 
■ucU prm-ticJilly lormciiliiig delusionM. ITiry iirp »iiiij>ly impossible 
U tici.iin)|iiinini<-ntt ><i .\ tii-Kt -class nticroxcutm. 

Till' priiii'ipli- upon w'hivb nluno 11 jHirfrct RiOi-lintii<wl stiip- cnn 
lie c(m.iCruRtiii, mi im to work sntootlily without * liroi tA tiuu-, <tnil 
vwlon; coitstAnt u»e witlwtit duliiro, mii«t be the cin{>lovniciit of 
prioa tiinpr^t plntm slidinp^ in sprung V-dtaiwI gixtonii, nmi lirarinj; 
only oil four points. 

W« iiiiiv test tbo mm-baiiiml quality of Uip tn<)v«iii»Mii: of 11 ntJigP, 
«• in th" auH; of tlm ronrw niljustincnt, by rtraovinK tli>- |urt!(, vVvux- 
log tbnni, wul rcpWiiij; Uiftn, wlieti they should iii-i>rk iiiiooilily mid 
whlumt Kliiikr. Where thn slidiug pnrt« arv tiK'''^'^'*! iiitix'n.tily 
Httiiig and nifrt^Iv plougboiii giviores by priMwiiiK tb«- pinion into tin- 
\9 dnimbic muM of ftinnoth workinK nnil iniikinl rri.poiiAivo> 

Sl^diiw plnt<« to milled henilH will not pnweiit iiirlf. 

Mit bntidi's th<' perfect action itf tlie Nliding p»rts. n iic^rfci.'t 
iiiArhiinicail aXngt- should hnve rqiinl *i-rfii 11/ imitivt* fetit-'ifli/ niiil 
LifUifi'i'iil'f. A eoiiituiin fniiit is ihnt the ntteed of tli*- fiick-work 
ginoj; vertitsil motion is grtmtly in pxci-mi of Ihnt of tl»' acrt'W Kivin;; 
bt«r»l or horlraiititl motion. If, for exiunple, 11 pinion Iioh ri^ht 
Itftvwt, nnd the mek it norks linn twentyfour teeth to the ineh. tlien 
three tunia i.f the milled hitwi {nnd pinion) would caiiw ono inch of 
wovcmcnl to tht? xtugir. In order, tln-rrforc, ti> ^t ibe Kitiie mte of 
■OnBMnt in the Lit<-nil motion thn sxrt-ye should be so pitched iim 
ttm'ta move th« ata^ tlin>ugh an inch with tliree revolutions of the 
RiiUed beiul. 

It is moet deaimble that t/t^ piititiH» thouM he jix«fy not movable 
with the n»o\-euieiitH of th« Ktn^., and tlie mUlft /ir»<f- cnrrying the 
fwpeetivp parts itfi-nld be m nmr tn rtteh othrr n« /irioiil'l-'. Tin- liest 
fiMin is that of Tureeiro, where one (a norew) is hollow, and the iither 
{a pinion) ptuisefl throng it ; tliis permits iMth to bo turned at th« 

l66 Till; II[ST()ItV AND KVOH-nuN lU' TFIK MU'IlOSCnrK 

MunK tinio with one hniid, ^iiviiiff * diajjiiita) luutioii, u w«ll «s tl)i> 
xf|iamtv rfctau^tuUr luwa. -iiiil ffin's jjreul fjuniity ftir inxtitiitly jiro- 
(liiciiif; «ny iimtiDii ivi]uiix-il witlmut (■eimiviiiH lh<- hniiil (niiii its 
|iiniti<iii ; a luml desirable iittribute of a sta^e wbcii llip rapid aiow- 
iiipitts i>f ■! Un iu;{ uiid miiiute oi-^iiiiimii are beiii!- foUowtN]. 

U aiill furtlii-r eidmiices »ucli a stsfje if » pinion is Ltirmd right 
t)ir»u;.'h thf ^tii^e with a luillHl h^d iil each emL 

A wir /iTin \if m'v}nu%'it:ni ntiiijr irtin tUvimtl b<j Jlr. Tuf/r*. and 
hiia liei^ri aili>|>t«d iu tliis country. We regret to aaj' that, fur work- 
iiiK |itirpci<iet>. it it a most uiidenira)>lei fun» : Scxure U ineviud)!'' niiil 
Hli-udiiic-u ia iiuposub!^ It» cliHracIci' will \ye uiiitentoiHl frotu 

nx. uv. 

Il hnn ilire? principal erron, viz. '. 

1. Plates «i) thin thnt tlipy lack ligidity. 

2. '\'\w Upppr ulatA w only sti]>|K)rtc<l nit one side. 
.1. Thi- Tiirrell milled hcailti m-c [lUicpd vertically on the top of tlio- 

Mage, a [to&ition iu 
whicli iheir vaJu« U 
reduced to its luwcvt 
in actual work. 

V\'o liare poiutMl 
out in Chapter IV. 
tliMt tli« iitage plal«& 
iif n uieeliaiiit-al stag« 
sImuUI he suitably 
•ir-i'ltuiinl III A»M- 
•li'ftith* oj' iiH inch «» 
•' 'Jiiiilri;' and tlw 
priiioijilfi on wliicli 
ihey kIiciuH bo con- 
Biructed and vm- 
(•I'jjed is give;i und^r 
tliiit head in detail. 

(.In the ti/ijiri' utii'j' 
[ilii/i- there should be 
a htlijt far the alij' to 
rest upon and a «/»/> 
at thf IrflJiitmi sill' 
Itri/iintl wAtV/i it iVTW- 
Hi't I"- jimJiril. This 
should bo removable, 
lint cftpable of being regdnved with absolute precision a« to poeitioiu 

Tlir >ii»i-liif in the tla;/f iihuuJd ii/i/yyn Im I'lr-jr, at least two 
inches ill diniiieter. There ought idways to be space onnugli al>ove 
the ordiniu'v sli|> when it is in jKisition to iieruiil of the easy inser- 
tiiin of the iuilex finger, for by itB proper usi", fociinsing with the 
highest pi'Wccs may he greittly faeilitate«l. The object is to rniw or 
lower the tlijK as the ohjerlii-e approaches the nhjccl, so as to dis- 
cover how npnrly it mny bo to contact with the front Ions of a high 
piiwirr in appmnrliing focii«. The fontl distance shnnld always be 
frU and not snuglit with the eye. 




. I3». 

-I'liD Tullvn uiuuliiuiiml >Ut;i> iv1<-|'l«l 


Let it be su)>|i>MF<l ilnttweane asin;;* dry objpcl-ffliiuxn-itltM full 
.ip'rinre, and ooiuei)ueutly »liini wurldiiK di.itniw-i'. w'ith (Ii*- n'ght 
liiuu) tli''.c<wrse ml,justtiwnt is uorkeil ; uith tlm pIIww of the left 
anti (>u th« til>le, tUe s^mud lioK^r nt iltt- li-ft liiuul rMtinif oi) an 
Iiiiiii>>vi>Mt> )>ATt ot ttif Ktni;^ wliith siMili™ tlip wbolv (iftiirf. the 
■tulirx liii;^r shuulil i«Bt li-;htl,v oil the mI^ci- of the iili]i, nml thp 
tliiiiiih In- nil iilacvt] as lo f'rnBO tlie obji-ctit'e iis il iulviinit's lownnb 
thn still. The (ouch III! (lit* thumb itidtciiu-H w hL-tbvr tin- objvctivn 
ia Mti liicli oil' or (titly ii ijunrt^r of nti ttirb awu\' from tlic cover of 
the slip. The UH>i'vm«iiI of tlie cottrso »(IJu«tiiieiil inii>' Ik- ru)ii(l up 
tti |Ui or j,th oi ua inch, btil after this lhei« tiiitet )ie it citutiuui liiit 
vMMy ailrani'e. 'X\i« )x>ily iiiii}^ be raekeil down until )>y p.-ittla 
iiHivemeiit the Uip is fniiiH) to louuti the fiY>iit i>f the ubjei.'tii'v ; 
llieik promwl cautiously by dc-licatcly lifting tlie 8ll|) from tituie to 
ttnir, tlte ability t>i do m> i)roving pcrf<x-t i^tfpty until tti« fovus of 
lh« object is ulitiined. In tliis way focnAxing 1>c<coine^ cahv and 
ntpid, a nialier nf touch, aiid not uf die^'oiitinuonn prifcedin'e t<i 
'dim-over ulwi-e tlii- t'mnt of tlio Iciw is'— a scAivh rfi|uiriiig a band 
•tkas Mtiil oft«u. with ith cumhrousnna, (MnsidrTRbli- lou of tiiur. 
Tlie aljoiT simple plan with brii-i pnuititi- will i-nahk the operator U> 
fm-uH an "i\i>-<rl in tJiv lii-td with ii g',,-tn{th <ibjoctivv in t>;n or 

If a /irt/'rrt tnffkiii'ieif Wn^c oannot \>r ohtiinndi ta):r no mi<ldh< 
fOursi*, havp 'i ffrm, wllwufr iJiiin imr with a iin-Mtthli/ tiiiJimi triti/e. 
'[Im CtOgT' Rhonltl bo lilt;i>', nixl tlii' li'*lj[« ilionld i^lidtr uith jmrfrci 
KIM* aiKt without mtehinu whr;n gently piisluvli'miiionecornrr. For 
this piirjiow tln> sidf'-gui'W shotdd Imi long, and only the ends of tlw) 
Imr should beaw on tin- stngi?- Thtt a|Htrtun; should !"■ a« in tlw 
iiH-chatiicnl fllngf, and f'^r thi- winii- rpasoii. 

Mr. NeUoii >;u|i^fvi(d ti Htngv- r.f Inrgi- ««>. which iihoahl hnvc)( 
IJ or Ij inch «|>nrtnn- IbuW in it. mid th«*n hav«> tin- iiitiTirnini( 
linirii Id-twiH-Ti il and tli.< (ronl t»):i.ti iiwav. h) that the "latsi- 
tvMiurai-s II hi'i-w-^hw fomi. Tliia ia fJiomughiy elhcicnt. nnd tin* 
|)rinci|ilc i* »"'n in (jg. Xi': 

It \s n UMtta-r of gn-at Intcn-nt (<• Kii^jlish intcrusonpiKtii lo nntn 

that ihi-ir l.M-iiuan ivillalioi-att-un in tjfniiiiny and th<- Intdiitji; 

llrrman innki'ii^ Iwvc not only tui-ii-nih'ivd to ilii-Kub-stag4-coiid<:iiNL>i', 

nwiil cvarn in it> achroauitic form, but that at l«>nglh th"V \u\\v iiImi 

idoiititl tin- iii)>chnni<itl xtiigi! ; the fonii ndi>pt>>d by Zfn* if liguii-il 

~ tiH- lu-'comiuinyinf: il I tiM ration (tig. I:!Ka). 

It niu>t, liow'i-wr. Iw< iiot«l that th« unual ConllnPiitAl tnodel 
LiuloptH a »i«i(" tliiif with a f^-itifli 'ifietiurf and Iwo./fr"/ r/irhu/ 
tilMi with >«' t/i'liiiy M-f ; ttiui is. wjiiitiiig alniont ripi-ylhing 
rojvirrd U> <hi giaid niiKlt'rn work. 

f>TMi t'f iIh' nioHt pt-oi'iicitl rulM for iho youns micni^copiiit in 
.Uiiit ndatiofi is. 'Have your mounted slidv in a fixed position, hut 
IlirVfT c'l/' it if it can j<it««ibly In* avoid^l.' 

li) addition t" )>t>rfecl rectangular niov«nieniH a lir«t-i'1iw< 

raifrosro]*- nlHiiiht luive matxntri'- rtilitrif niufiun /o ihr tUifff. TInM 

is nsnaliy f^lfn'tnl by rucrk and ]Hnioti. but it is at lini^e dcsirabV 

, to aiove it with i;reat«r rapidity thun ihiH adndt^ of. In very w«1l 


iniulc m^tr-amenLs iLc fuiuiin tm^tg^a the rack v> lightly tl 
rvfitd mutiiiD dulv cosily hw )pt«ii to It. Iii othera the pi 
be rliiffHg a p wl Aiii) rajfUt luuieumit ejected. 

Tbe eaiUv <>{ rutntiou uf the ktsgn riiould be rjoacly uppi 
to ctiiiKideim with the op^c axm^ •(■ th«t in miut4(m tiitt 
■bould DPver )« out of the livid Thpn a fciirly hiKh piyMrr 
Eh>lx>ral« rvctan^tUar epiiiring s^r ha» been ut^cil hy loiui? 

tbr rotntioii of tlw fitagtt. Dincted ArcH «« tmpiBiiiKi ind to tho 
itinltihulo liMik 'MJcntific': but in pntrtke tli«v are atiperduiitis 
in tin- ni<«t riiriiplptn ini^truiurtit Iwyoiicl tlicm> iiiiltL-ntMl. 

Tlwre JB B fiiniptf fnrni i>f ntjir-attujr it»w eiii)tloye(l bv many, 
mkI wv Ihiiik with ndmntag*, whfn tlio ount of n i-<3mp)<-t« 
jnrf Imiiionl RUgV mii>.t !"• forirnnr. This consiMs of a flip ti> rt-ivivc 
tl|f obji-cl, DMtla o( giit« lit Imuw, »t> HrrniiKdl that tlie iM^arin)^ 
iJioll 1h' tfljuu itnd iIm! fi-icrliiin mlucMl to n ntinimuiii. 

Such It nHfftr-iiii'ifT fjin )in nuiHr to work with n'liinrknblpstiioutb- 
onw, mid Mticr (..nil- pi-rwnis hav not suthrii'iit ili-licnpy of touch to 
luovv »• Kiiintl Kitil thill All Dliji-ct nil n It x I -inch »H(lr upon the xlJt^e 
wiili sduutirif^i 1111(1 priTisiiiii, it in in fnvnur of iIm- su)ici--vlrt;j;i> tlmt 
it itt UrtpT, innvfs iJisilv, itiiil cwii lit- t'liniish"! with coovoni^iii 
p»intA of hold'fii'il for fhi* luii)il.%, bikI i-oiiscijurottj' i« imiri' itiiioag«>< 
uhle. AsninM it« (faiployiiii'iit l» ihi* fiK't : thiit tlit xliilp i»i 
vlipiml into n riKi'l poniTiofi ; awl -'lid)}', tlmt thi* ii)M>rtur^ in bm 
Biniill til admit »f thi- i>aiployiUL'iil of thu tiiij^i-r in iiiovii]^ thi- ):|iih< 
lu aMiiat in mptd fucuakiiij^. Hut th«a« aiw defects which luiuhl 
wrtAhily hf ovt'rcoine. 

V, 'i'lie sub4tag;e i» tuMnriy second in im{Hirtikiic4i in ii tine* 
«1m)) iuicn>ic<-(ie in th« BtAge itself. It v iiii«n<t«d to rewive iiikI 
i«iil>l« ut to uee ill the most elKcient mattnrr tho opticnl und 'Aiicr 
R]<finnt(ti« employed in illuiiiiiuita* the at>ji-<-ta lUttnttly with ihf 
v»riuti4 powerM finiifl iift-<lful. trpoii thiti niucli of the lini'^t 
irritic^J work with tli<- iniKh-ni microsL-opi^ Je|)cndii. 

To im-oinptith tliiH a jjootl i<ah'>itfit.i> oiUHt liAW rectaiijiulnr 
tn'ivptiK-TitiS, aixl a nick-'iiiil pillion fi>t*uiw>iii^ adjuHtuw-nl. 

Tilt- v«rti<ttt and laii<i»t ui'ivfiiienls iii-tti iii>t br an plaborati' iis 
Uiawp of ihe Kia^<i\ aintv only a ninall itiovuuient in each diivciion N 
nijuireil. The ohjwt U tn neaure u ceiitriii;; motion, a motion that 
will nuUce the •iptictd axia of iJie HlJ)>'^lH~e eoinhinariona i-otilitiuou-> 
with tJ»« opliciil a\ia of th»! ohjeoiive. It tiiuxt therefore be » 
«t«iuly n>i>tioii ; the Ku>>*sia»i> iiiiiTt move de<.-t(dvely, smd mtiHt 
rigidly remain in (he position in which it ia left. 

A Im'I sub-fttAge iiti>\'es in Jerka, tuid is liable to spring from l]ie 
twtitioii ini«iide<l to be (liul. 

It is not »ee<lful iJutt the motion should be in right lines ; 
Htniion in arr» whose UtHifttU intiv*^ at riijht itni/frg m-e ijaile na 
■■Ifidttnt. A »tt«dy, even, reliable tnoljon tlmt will ennble a c^Hltr 
Vi tif/tiunti is all chat is re<|uired. 

The fiMiiMing Ailjustment inu»t bo uniooth, stMdy, and tirm. 
ai'tiiitc riiifiijy nnd remaining rigid. The rcrent employniont of 
vcbmaMtii; condrnKora of wido npertun-s hiix ted siieh critical 
workew us Sir. K. M. NVNon to suKp^t a jiw nrlju»lit*rnl lotk« 
nfi-tf'ii/f. There nrp times when it is ii gn-at luxury nitd n fitcil« 
jHiiii to liHicat'' and dr«im)-l(t n'9.ult«; but it miiy Ih- <juit<! Kiniple. 
« direct-action seti-w irf line thrwul, orn cone which the nrvoluiion 
ol a (screw pi»hi-x lw>ri«iiitAlly fm-wnnl Uj«<ti the liollom of a iilidihK 
bar to which tlw mjb'ilajte is ti:<ed, or an inclined plane uclin;; in a 
Jot in tlie Haiiin «ny. In fact, any siniiilf lievice for focinwing the 
oondetiaer more sloviy tiinn Uio rack-wwrk will do, pusliing tlie con- 

170 tiiK iTisTORY Arcn E\*oi,t-nos fir -nrK siicnnsropE 

tli-n!ii!r up to. or ciiUHing it to rvioeHci from. Ilic unilcr surfncii of tho 
■tkit!. liut iiu ini?»tiH sbould l>e mnploycnl fur chis «tul which will 
tmpi-rn tliv »l>M>lut«yi'i'tjitHvBi of till' Nul)-^tnu:«, i>r eUc inoro will h» 
lout thitu can be piined. Tho iirntiitj|i>nient in Powell aimi L»tlan<l'» 
HuWluK^ is shovn in Hg. l.tO, )>. Wri. 

It is Almost n niniliT o£ mm|iiilKion tn ivfcr hni« to a recent 
amn^nieiii known ne n «*fiit</iij>/ irii'i-*rif,i/', whidi in, lu) iw ntuiio- 
imjilio-, a siib-Mn^ t.n nmnf^A nn t«> Iw mpnliln of bcJuu iiiovvd 

hllrnilltf ii'il iij Ihr- iixis il* OH "*''' 'I'/in'A A"* (A*- iJijrtI un lit' rlniji 
fiif it* trlltl'r. 

The jsi'lit purposi* of tliU IK to Mvuri^ MijHf Ufumituitiott, which 
practically, at tlii« tiinp the Mvingin;! Kuh-xInK** was (h-vistnl. uuiuit 
obtiiinin); a morr oblique iH'iicil tliuii llii^ cuiulfiiais (bi'ii pniviih-d 
uould cotaninTui, iind kiiicc tJiix nU'i mi^iuit sctidiii;^: inlt> the nbjiTt 
a email portion of t\ i-imfi of lifilit )>' iiir niintnih, iiiiiny tiuiitly 
amiinnl thAt llii* tilimc wnJt tnUt-n in he 'ubliipK- iHuiniiintioii.' 
Itilt whnt<rvor ^rnil.i oblii|U(- liiilit through iin «b)<-ct into ibe 
objective in un oblique illtimiiiiitor. Two fomleiisi'i-s itniy htiw 
nUTiipricnl njw-rtiii'rji uf l-t mul i-.'i rcsp-cliMjly ; a sto|> birhiiid tiio 
bni'k b'lis in ivich hua ii iiorrow sfclor cut out, irprewnliiij; tin- onii- 
(litionx ttf tho iu-cnll«l 'obliijuc illiiiniiuitors' : bv tin- foninT wo 
jp!t an oil atiKle of 134" 10', by iht? l»tt■^^ n xiiiiiliiraiiuleof IGl" 2.T. 
tliCM! ■cctom uf ilw cmv ik li({ht of 67' 3' and HO' iV renptw- 
tiraly are in ertry ttciiiie 'obli<|W illuminators,' and the i>tic utore 
ublt(|U(> tbati the other. 

Whftllic^r or not it ik needful or lieHt Ui n.-u* xudi a sentor ih 
Kcnrci-ly an open i[ijR<lion ; it in iii.tnifpst tli'it liy takin;t tite stop 
wilti iw sfctiir awnv from ciinh (■ondi-tisBriitKl si-tulinK in 'li<' tfmfJrh 
rtiiw uf li'jhf (ormeil by iIh- CTin<it-n«i.'r. we are Htill IL-ting oA'tfMA 
iHHiniti'il'irr, tint ihf Mi'/'iili/ in in •■'/ <i:iwi«ri*. 

Tljtre can be no doolil that n 1,-irKe ajiiilure in a coudensm' 
pn>vide« the mJcroscujiist with fur K'«ttKr wcullh i>f resourve than an 
obUi|Uo illuminator in one aximuih can ewr ijive him. A eondensur 
with fin oil Jingle of IGI ' 2A' h much nioiv valuable tlisui even tho 
inni-uiif;le obtained by a mere aecti'iu of a luminous cone. Tlie 
power to utilise llif eutiiv one in n ■•a.m o( the hi;;hest order. 

[t will be iniinifent to all Uial ve niiiit cimccntivitioti nit well as 
obli(|uity. In the ctitadiopirie illuminator of Mr. St«phensun. the 
one defect, oh in diwussin;; it we point out. is. in our jud^iueiit, «i>iil 
iifi^iiHi-rHlriiiiiiit. due to the lenylh of fucus uf it« cuncvive i*flecU'r. 

Onliiwiy concentration rlependi upon the jMiV't ii/tiir rniu/nit^r. 
If it ia ivifuired lo concentralc the li;;ht fruin the ed^e of the danio 
of a pnmtiin ianip upon an Aiiii'/ii/ifrurti iifllncidu, the condenSM* 
must l>c at letLMt a {th inch or jlth inch in power, which will ^iv« an 
iinu<;e of t)ie tlanie nearly the «ame bice as llie objecrt. The ammttxf 
n/ liijht which i« ii.mceiiti'stcd upon lliit object will of course depend 
Upon tho apcrtun' of the ondeniier. An oldiijue cone of gtvoX in- 
teniiity is liore uhiit is needed : the illuminaliiii; coue shouhl l>o 
equal and conjiignti- to tlxit which exists between the ohject and the 

Now it is cert n in that this cnmlitiou caiuiot be met hy an 'oblique 

'oitUQi K ■ n,i.nnKATi<i.v 



itiumiiuitor ' of tJu- kind coiiinionly utuI«nUHKl l>y tliitt luiiuo ; to got 
iuuumind roiiinet, wliiirh is of courM s liitr i/un ui-u, wr niuat 
nii[il(n- n )it>niUpliorical biittoit — or one jfrniieT tlian ii liMni»plii-rr— 
*ilni^««f in iiiiin^rsion oonUci vich lli« utiiler aurfnL-e of tlic Klulr. 

liU Riay W illiimiiiitled hy a benta from a <[nr comMiialioti, niftilr 

itiliijiii.' Uy ihv eub-stngn liciiig Kvrunj; out of tbe axiii- (~!ntiitMl tliiit 
Aiig^lit is iitMined which can Im got with a i-omlciiM-r of gn-ut 

juTturr, vrr nuiiiifi^tlr obtain oiily a porlioii, and an attciiuutt-ii 
will xtiinll portion, of tW li^dit givm in every, ur at « ill auy, itziiuutti 
liy th*" omtcnucr. 

Tliooivtioftlly pcrfwt illuRiinnlion of nn objective, foroxumplv, 
It Jtli iif N.A. H or l'"i. Would Ik" oblaincil by uuini^ a prerisely 

iniiliir ooii-ctive an n coiideiiwv, wilh itai bnck l^ns stopjied down by 
B djottwl sUip, till! tint ln-iiii; of tlif *i/(t of tin- iKTiphei"*! sector 
rt^|uiri-<l to hi' il In titillated. I'bi- L-onrrof itliiminnti'm would pm-iiwly 
;«i|Tia) tlkitt takm a\> by tti« wlgc-ctivo, and wuuld be of niaxiiDiiin 
111 nil si ty. 

Nuw iIk'KT cuixUtionK nri: more iitttrly approncbeit iiy u bi^i-rtiisii 
aelin iota lie condniKer <>f jircat apvrttirv niid of hotiiogviicous uooKti-ut:> 
tiiui than )>y •iny otliitr iiiennK. 

The t^'lv of obliijnr ilium iiml ion ia not Itvrv iti (lutwtion ; what 
wv brlirvi) cl<nrly sJiowti ix llmt, liowcttT tnii<-b luav lu>V)! Ix-i-ii donn 
by obti<(Uo illuiuitiitdi^ ilciH-nileiit cm swin;(in2 Kuostiij^e-n, and tlin 
luir, till' HUtii; UiitijtK rail bn Ar/trr r/»uT icil/i luimi-rviuii ••utii/rnifrit 
itf't/mil iijirrlnrrt nttd itrrf'-rf Cin'f^rtu.u*. 

Thr Kwin^tij; xiib-sta)|>e, uitli \hvKi- coowiUTitioiis— «» wHl an all 
otlttrr ■ iibliqun ilttniiinali>i-s ' of iU onler — in l^hpr n dr-fect in thft 
mirnuKn|a; unfniitftil upeciiitiintiiiii in fact— wliii^h doe* not ptxi- 
mote tb<> pTDjir*'-" of eitliri- tin- iniilnim«nt or tlic worker. 

Anil thin will n|'ply to th'xu- ('()in|ilpx foniw of iiiiLTii»«i(if' kiiovn 
IU ■ raili^I.' 'cimwiitric' and tlinu' pnivideit witli slagrs llmt ifv.ilvo 
•ir ' turn over ' in an axlx at right aii^le.-< to thu optii^d nxia of the 

Tn a([<lition to thr featurm imuiuiTnti'd hitherto, •< ruuij^Hr ftih- 

'oijr rl^ntihl Ill/Ill Iff f-rtifiilni trilh n riifh-'iuri-j-itiiiin futarif iwilii'H ; 

bis is ojily really Hrei!«i iti onlcr to U**' the jxJiirijV'/ii'. For tJl» 

|»urp<He«uf its auocrvi^ftd etiiployment thiK ia iiiii>ortaiil, but other- 

«i»e ila use a N-cry limitv<k 

Vl. The minor is aixo an itiditipenwlile part of a i:oinplete 
miorusoope. In a liiHt-clau ataud it s)ioid<l Ix.- jJaw aud nmenrt 
and from 2^ to It incben in diiuunter. It oiay be iiiouiitt-il on either 
a litn^le or a double erauk ami. In any niii!i'oxeo[)r, if ibere be 
only one tnirror, it should \ve ftmrtrt. Itiia niiriwr, from its eurve, 
liAK a focUD, a iMJiiit in wliifb tlw reflected ray» nil meet ; and the 
luirror slmuld not be li.ved, but ho mouuted That it tuny be focaased 
on ijie object. 

The plaiM mirror is sunietimos fotuid to nive Mn'eml reftexiona of 
a lamp tUnie nl one time. It ia due to some iniexpl.uncii molecuUr 
roniliti'in of the glasa, and is uiideiirable for the put'{H.<>i' ; but it tnav 
Ite iilivijited or altogether overttime by i-otalins '''"^ inirTOf id ita cell 
until a certain ia>itit ia i«nche)l whei'e all the imagjci wiU be rapor- 


iiii|Hiseil. All iiiirtvirs should Iw so niountetl ns to a<) 


Tin- prf^^nt Editor is greatly in favour oi tlif ••iii/i/'iifiitrnt i 
,•■ rtiitigiiliir prUiii cut with care and [irecisiiiii. 'We get 
laraiis tot-il reflf\ion ami nn double rertexions ; and he 
tliiit tiiier iiiiiigt'S i-;in l>e obtained by its me-.iiis than with 
iiiirnir. It m.iy be in"uiite<l in the fJucf nf the plane mir 
is ti) siiy. llie coiie;ive mirror mav bens usuhI in its cell- 
iith'T Cfll, whicli would have veeeived thp plane mirror, 
i;iibir pri^m may Iw mnuiited and lie onpalil'- irf rotation as i 
mirirtr would have lieciu 

It sliould, linwi'ver, bi" noted that this applies only when 
liillit is ri'<]uired tn !«.■ ri'Hwted at an pxni-t light angle. It is oil 
v'li'ati-Mt Sfi-viee when tlie micii>SL'n|ic is of neeessity u^e<l in ii ripfl( 
ii|iri;;ht position. 

If it bi- u-if"! for singles otliiT thnn riu'lit ansles, there willll 
ii'lfiiftiori ii.s well as rfllt'xion ; ami us the necessary decoinpositilH 
• ■f the light into a 5[«'itnini »itl.-iceoni|iany tlin ref fnet ion, care mvfl 
lie c'Neivisod to .SIT tliiit the rays eiiiei^iiig from the piism araai 
n;;hr iiiij;h>s to tli.-st- incident to it, and that the areas of ilie sqna^ 
t'ai'i's of ilic piis^in ai*e sufficiently large to have inserilied withil 
ihi'Mi a oireh' i-i|iial t" the Imck lens of any condenser u.-^d. 

Some employ what jiiis Iwen known as a ' ••'hit'' rlomt (7/uuiiitalMj 
ihat is, a disc of plii.ster nf Pari.s. or iipal glass with a polaM| 
suHiicc. liut a disc of Mnely gTOiind glass droj'iiefl into thediaphngll 
holder of the coiideiiscr will give a pret'i.sely .siniilnr result 

Mr. A. Michael has, however. ]ioint*d out the curious fiictthatiJi 
../.•ili Kr--,il miri'iir hiHonies an inexpensive and excellent substitntt 

i"Y A /I'll'lilniiii/ j.risiil. 

TTpical Kodern Uicroscopes. — Wearn- now inn position tocuv 

fully inspect tliu chatucteristies of the chief forms of microaccT* 
which tlie nil idem ninnnfactiirers of England, the C'ontiiieut, sM 
Amenca oti'er to the luicroscopist. 



Via. 18t. — PowuU Kod LealAnd'B No. 1 utuid. 


174 THE HISTORY Axn EVOLtTios OF Tin? snciioscnpE 

tnujui-ilv of tlie mii-t-uicopt« o( forty yeant ugo are &)«olutely unelvsx 
for tbi* ol)ji.^utiv»iS of to-<Uy, but tlie focussiiij; nnd &tage luuvetiieiitii 
ot Powell uiid LeiiluTid'e microscope still )iol<l tlie Hrsi plac«. 

Fit;. ]-'J rfjireseiits tlie iiistruiiiptit in itn nii>iiucu!tir form, 'flin 
foot of the iilaiiil i§ a iripo<l in one caslinj; : it lins nn p\t<?n<lcd hattf 
«f 7 X !) itidieu, foi'iaing iiX oiicn tlio &tJ-Hili?si mid the- li^ht^i^t foot 
■of any existing microscope. TIip fci^t nrp plugged with rnrk, bimI 
when the Ixicly is in a liorixoittnl position the optic nxis is (rs it 
dlODld be) 10 inohcH from the inlilc 

The cotir&n ft<ljiiiim*"nt i» ctlivi'-d by n 'lar, conHistiiij|( of a ti]«i«- 
MVe gunmotnl (i-uiipntcd iiriKin in form, wliich l«uir« niily OIJ a 
narrow pnrt iit tho iinj^lc.v. It «^xt''nd« iulli<'icntiy to fitciu n 4- 
hich olijcviivr. The nnn whit^h ciirrir-i. tbi' iHidy is ijf uiiuMial hii^li 
fi>r thp tyjio it rpjin^nit* ; Imt this giv<'s n, liii^ nidtu.i (rum thf 
optic crntift iif till' iiistnimi-nt. unci iniik(v< tli<> coai|>li'ti' ri>t4itii>n ni 
th«' Ktagr nixy. 4<i-<-nt i-tliirLi liu\'i- Ix-cii inruli' to accoiiipliKh Uiis in 
other inrtniiitiiitv The cildi-r Kii« fi>nii fnpiii tbi- sli<>i'tni-is «f thr 
iirm only alliiwinj uf >i two.ihints roiaiioH, «ivd in the JatkMin ituHlcl 
timny ilHri-n-iii di-vices havi? Invu tried, the hilrnt lii'iii;; llu- pliu-inj; 
o( thr f.Un[e pillions in » Mrrticiil jioMtiDn almvi; tlie ataifc (tig. lljlj, 
J). Ifi6), u-hii'h i» an iin<|uc«tioiinble terror. 

Tilt! rotiiiimi *if thi- stajir in the Powi-ll and Luibiud mndt-l u by 
tiKmnti uf n inillod hmd mutt <.>>mt'iMtietitly jil.iced, mid tli*^ ilividi^l 
*irc]>? i.t on a plate of silver.' Ii will aino npidly r»lati> by huid. 

T/ii! imii !■ i>n n piMit, which ullowii it to be tuiiied away (roiii 
tlu) KUgn altfijjtrthf-r, Aiid, bh wi- liuve »lreuily iuilicaleti, the ten)]tli 
of the nnn ti-iit itnetf t*> the vi-it? of ;i loi);;er lever for the line ndjuxt- 
nicnt (p. I*;i>. The inilltHl bi-Jid is plai-ed liehiiiit the Btroii^ pivot, 
iif tin- arm. where vibnition la impiiftable, iiud it is in an eway and 
imtnral pusilion for tile accea» of either li.ilid. 

T/ie luili/ niiiy be. with f,TPal eiise, rutirvli/ •vm-ivdj'ttiin th' nnn ; 
this makes the lue of the binocuhir or motioculur body or uf n, sliort 
or ton;; IkhIv a iiirttterof choii'e, v'hile \i (jives access for dtviiiin^; anil 
iithei' pui-po^>« to the iioae'piei'e IuIk-, an woll a^ fur (he insertion, 
and fucussiiif- fit the lens used with mi a|H^rto^llot4^,* or nn anal^tiiii}; 
pristn. So nlwi it is of serricw in jow'power pbolo-microgrnphy. 

We Imve alivady referred to the stajre <if thi» i instrument ; 
hut it may lie briefly slated thnt it Ih Inrj,'*-, bn>^ (^>m)>!eto rotHtiiiii. 
it hiis one ineh of ii;'i-lii:i;;iiUr motion, Win;; ;.'r.\dHftted lo the , ifttb 
inch for ii linder. Tliero i« llf muk' npfni in the vertical and the 
lal4>i-al nio^ ciiientiS, and the pinimis do not altii- their positions. th» 
aperture of the stngp is ani)>ly Ur^. 

Tile W^c of thn stngis ha* n >tnp plnccd on it» left-band 
Hide ; this is held by n itcrnw, but is renioi-ubln at pleosura. -^^^m 
Twc massive lirncknts under the rtnKe remove all ptvwibility of,^^| 
jtexitrf. ^^^ 

T/i« fuh-ntai/r hiiK rectangular movements by screw in Hth«rdirvc- 
tion, as well as a mtary inoitimenl by pinimi. The oonne luljust* 
nient is by rack-work, and a jijir ii'ljiittmrut is aildeil when de*ire<l. 

) Tlii> i> n»w ii'iuli' n( ^'latmum it dnirml, onA Uiu> tunidi » nbrmtcJ. 
» CI»|'Wr V. |.. BO', 

¥ts. 1.10 iliustraies thii Mu^, aXiawUt}^ iiM umkr side in oixl«i' t» 
«iwbU^ lh« tiiie ai^juHtiiieiii to In- mtcii. 

The verticnl ni>d u{>]ier borizxinbtt niilkif lintds »«• rantriiix 
Si^rewsaetin}! i>t riKhl aiylpBlu i-»cli utiier, whilo the diagonal screw to 
tbt-' Ml in tito milled li«»J, whii'li 
Lsiises tl>e Htage lu rotate, aii<l it 
will Ih* w>en iImI nc thv end of 
tbt'8t-i«H- iiiriitti try A i^a milli-il 
l>«t'l oidlrolliii^ II 8crt>w s|>iii(tlu 
i#n»iuatin)j iti a bIi?«I c-ono. It. 
On iNil.-iiiii^ A, ittiimsaiii) with 
n rwr *l«w iiioiton forws dji (or 
rplrtt*K4, mt tiii^ CAM! iiiny lie) it 

tiiii, C, itiMTtnl in till- \tnfif [lialo, 
^, c>f tlit'xuli'^ajfr. 'Iliih motioi) 
o( C carrir* wiili it tin- con- 
4(>iii«r. At rialit iiukIj-s ti>, and 
fiintiii))!; jKirt of K nt the lufli 
aji ill 111- r Dliilin^ jiUti' wfirkt 
a;faiil^l It Hjirin;; n( tltu u]>]i>-r 

itiiil Iwtwtirii timriiifjs F M mHi _ 

.i-to, wl.ivl. ar^ tixMJ .iiw. tiw ^^ iaor~P^\\ ...d I.«I.«.J-, .„i.- .1 , 
aHOBl ntckwl '^hdc. V. <>f tlif siil>- nil)) £m. ndiii-uuMH. 

aLagn; tbo iniit^r ^liiliii^' |>Ul4' in 

tlin f!UQuti«l aildiiMm U> tJ»« usiidl mcki'd slide in tin- n[>i>licnti<>n t^f 
thi* now tint' luljuKtititiiit u> tli(* i>ub-Ktiig<>. Thi- niii^ nf nioliuii ik 
alMiut j( inch till! ilitlrrent.'o in mdina bctwvcn the Kniallir aii<t 
lar^>rr i>IhU ijf thr iU>»l cono. 

T'Ac mirror irt |>liui« iiiid ci>iii-ii%*(', with dotililojointrd arm. 
TIm' Kniiiti ami u-i>i'kn>:tii>lu)i nf this itii.lrtinii't]tn,n- iif thi; highrHt 
<inl«r. Tli^ »«iii unit ilti- utis.-en ii-cifive i-rjtiiilh* M'rupiilimii ciiri". 
Th"' i-i*-*-!!' EiliUir lias )i4d Miixtf tht-sc mitiimt'iix's in cinrtiint, nnd 
■ ii i i»nl (.■(■ntiiiuDUH, UH)^ f»r «ver tvri-iity yi'jii-s. and tin- 

ii uiirk can he dime with it today. It it, nowhciT 

d4Tf<'ciin-. «i"l thf iiKirumeiit has only inn.i> h«-n •tijtlit'-ncd tin ' in 
Muiin jMiiA. Kvfn in su<:)i&lii;ill ilelails an l\\e .■■{irin^irit; of iti'-Khdiiig 
rUji — ttit^ vi^iy best uti|> that can )iu UHtd — Ih'' )>iviita ni tiu: miri'or. 
and Uh- carefully ^/TUMrii-ondilixiDiof nil (.■ylihdn-> iiittinded to n-crivi- 
a|i|iarHtu.H, alt atv notit- with care and miiMrieulinuMii-aa. 

An itiKtruincnt iif thi« kind may )je made lo a|i|K-ar |>erfpGt ■■■ 
ihe «yr<, litit at llie wi(ii« lime may liiL-k »>ine mrttl unjiortaiit eteuK^ntn 
ak a Biiiblu'd iii«lruiitciil. But tliMi is an in§trunionl of llie bi;;hrtti- 
(ird«r as Hucli, and at tlip snnie timo n very litie specimen uf highly 
UttialitNl bmfM work. 

A i>o(« must lie niatb' liefurc Intviitg this tnic-rascope npoti tll(^ 
am of tlte lulx-s in the liody and tli» Mih-itajg;*. 

Vo«ell and Lealand wpro the only rartkcrewliiwe gauge of tubinR 
bad a yti—iH liiln \ the oixe of tJip tlda; wan HUcli that il u'fiuld (ak<- 
iu n binocular liody a JliiyElii-ninn ^'-inch f^y-pTT, htt%-jiig thi- 
bt^e%t ri«li)-glass poacible. The aixe of ibis li»ld-glii»s dqiends on 
two factors. 


1. The distniice between the centres of the eyes, 

2. Tlie raeclmnical tube-length. 

In order that the binocular may suit persons with 'a 



the stjutdard tube. Wlieii the f^e-pieo« vera ' homn ' in thwr 
pUcM ID tlt» tubes they Just touched eaoh other, the inner aJtlm of 
the biitOL'ulitr tubes beiii^ cut away ; no under the abor« conditions 
a larKiT tii:<l<l than ia thus obtained is almply imposHible. The Ki»^ 
of the flvld-RlasK detennina the site of the eye-picoe, aud tlutl wnji 
Diadif to fix lli« diameter of the bodj'-tube. 

Very wiseh' tliese niakera tiiad« the tul>e o( the Hub-staffe tlie 
Mai»e size, m> as to have oue gau^ of tubing throughout. Tliis 
allowv a Kelluer or other eye-piece to bo used aa a condenser, thu^ 
rtduciii^- the nuuiber of a<lapters. 

Lately thin tiriu Itave altered their sub-etage tube to a gauge of 
\\ iuch,sB recouunended by the Itoyal Microscopical Society. Hits 
ioTolves an adapter where the eub-slA^ appnmtus was adapted to 
the old fT^uge, or wlieu an eye-piec« is used as a condenser; for I^ 
ioch is an iinpossible size for a binocular body-tube. 

7'A'fl Rom modeJ, in iU coinpletest form a* left by Andrew Itoss 
{though not now made to 
ao large an extent a» 
formerly), deserves st udy. 

It was a bar move- 
ment, with a foiit formed 
<4 a triangular plaM to 
'lich were bolted two 
_ lei upr^lit plates to 
enrry tho tmnniona of 
the Dii«n»eope. Tltia 
1ia«e was defective in not 
-lieiiig saflinently ex- 
tended to carry so large 
an instrum«nt, with its 
cmtrn of gntTity so high. 
This in»truineDt is illus 
tratcd in fig. ISl. Hie 
■eoarso • B'ljustnient bar 
was rectangular, which 
is theomticAlIy inferior 
to a prism, but it is well 
xpTDng and works with 
l|[TPat snioothnMa, Tlie 
Knc AilJiLttinent in a lever 
of the «i-cond onli-r, with 
the millifl hi-ad in th<* 
tiiiildle cif tin- Inr, whii'li 
iari>lv(f!i tremor, and the 
tube of ihi! naM:-]>i(.-<<r is 
short, making slinKO pos- 

The Btaffi- moTcments 
Are of iinet)unl speed, the Utoml movement being sioww than the 
're^ti^al. There » no finder, nn<l the rotation of the stag«i is hot 
partial. The Mib-Rtago rnd mirror uv good. It was a commanding 

Fill. in. — Ro»-jB:iktoii inc'lcJ. 


iiistriinipnt id it» diij-, itiid vna of excellent workmaiiHliip diid tinuh ; 
but it wiis not ei{uu.l t4)Ui«atr»Utof oritieid work witli imDiemiou <>bj«<.'- 
tjvwi of Ktcat niicTture. Kevertheleas tlio (IcfdctK of this atand couM 
Imvo hwii ruiwUly coiT«;ted. Willi a luoro exbrnd<M) base, a beitw 
&TTangr>mcnt (>f the line adjust iiieiit, a nieahanical stage c<)n.GtTUCt«tl 
on bfttiT nrind|>l»A, and ibe n>tatioti niada coiu]>lf<lc and concen* 
trie— whicli it was not — tbis would have been, t^virii for our piVB«iit 
rvquirrtnontit, a» ndiiiirjl>le iiiatniiueut. 

I'hiK inipui-latit tirm v>ei« otlierwise advised, howovn' ; uid, Ui< 
Ktcnd of correctiu(; ttie frron of the iustrunufnt wIioko history they 
had ni'ul'-. the^ dt«ivued an entirely nrw muf/fi in which a Jackaoit 
limb was substituted for the bar moveiueut. Pig. 1 3:; liltiMrntee this 
form of thi' iii.tti'uiiieiit, from which it will be itPtni dial the foot also 
wiw <'liiinKe<l for the -^'ot'se by being cast in one piiwo ; tho liase was 
not siitficiently i-xleiided, and tht hinder part of tin; f<iot reae too 
IiirtfC, 8o that it sowetirnes rocked on,/'u"c poiiils, beomso tho hinder 
pint was too wi<le -a flat surface, in fncl. A truo tri^xvl will eland 
lirui on an unt- vki table, but this form will not. It is a f()rm fre- 

Juently uaed by variouv makers now, and la known aa the 'l>cnt claw.' 
t is a hiid de^iJ{n, and (nay be, m it lias been, easily tlirown over 

Tlie introduction of tlic Jiick^rin limb lirout;ht its ineriltthla 
troubles— iiouibly, witli the lin« Ai;|jii»tliieiit — to whJL-Ji ne have fully 
referred under that Waii. But in tho Uoss^Jacksou mo<le] tint line- 
adjustment screw vas placud behind tbo body (as the fi-fure sliows), 
which was an impi-oveinent ; still the liody and the coarse .-tdjustnteii'' 
were both i-Arried hy (lie line-ndjn«tment levci' and ncrcw. 

This form lould not— as it did not — long prevail Its oxiste^iMt 
was ephemenil, anil in its plnrc was put a modi^cnlion of tho form 
deviaeid by Zontniayer. known sulisequcntly »s the Hosi-Zi-ntuiaver 
model. This was tlie Kofs -Jackson instrument with a 'swin^n;; 
;iUb-stago.' This inhtrument is illustrated in d;;. 13.1. It will he 
seen that the foot is a ti'ue tripod, consisting of a trian^'ular Ikuo 
with two pillurK Hmti^ from a cross-piece, which CAme<l tliv trun- 

We ha^'e alreftdy aMcMcod the value of a swinfjing sub-suijn<> ami 
found that in mir judgment it is at best rixlundnnt.' Xu tcicnweojAi 
is conipteK- without n good condenM.-r, all. and much lu^re (him all 
that can Ix- done by n twint.dng .lub-stagc can be done with a slottnd 
■dop at the liaek of thi- conil^nscr. Tlii* clnboraK' ap[>cnda;-e is there- 
forn without juvtithvition. Yot in the iuipnticiice for ]ar;:« illuuii- 
tintitig aperture*, irltirh Htrr not al Ihat tim'. itmriiied Ity coitdtturra, 
ibis pbaK^ of illumination a'as carried to a still greater and mon* 
i!la1)orat« ilevchipuient in tltL- production of a coiKTHtric mtcroivoftt. 
This was a K<)s\-Wcnti»m. known as the railiaf microscope. 

In tbn rtirly day* of this instrum(-:it, when no achnnnntio 
condi'iisr-r exci'iiHug 170° in air wu> to hu olitained, in some vcn 
ilitheult iescitr>--)i<-s nei'iling nil the >,'Tent advantages that conut 
fioni grvat ajx^rlurt', thi; present Editor wak able, with mnoll 
Lalxtur, to gtt resulto with this instrument not . otberwiee attain- 

> t'. tuu (f (■^y. 


Tbo optic axU of this iustruinpnt in capMt of being rotAt«d in 
three jilanes nt rijilit angles to one another with thti object as a 
centre, nml in mlditioii to tliis il lina aawin^iti;; subata;^. Tliis 

will be seen and un<l«ratood 
from the illustrations gi>-eii iu 
tige. 134-137. 

Concerning llio line adjust- 
laent of this ns n JAckson model 
prim an ly ve hare already 

Jiiiif/i^r Uadimj /oriii uf tht 
firnt fl'if in the Nn. 1 of Messrs. 
K. anil J. Bock. Tlie early an- 
cpKtor of it was kIiowii on pagt 
I'lt, but it h.ts undergone iin- 
pni-tnnt chnnges as it ii itow 

Srcspntal (tig, 13f*). It is a 
arkson mmlel, the foot being a 
good tripod, nnd tJto trunnions 
on pillars (na lig. 13S illus- 
trat««}. It hns a short lever 
lini^ailjuKlniont, Jj, acting on a 
moviibU^ nwnpicci), nnd plaoed 
in front of tlin body bo tliat the 
hotiy and coarw adjusttneiit are carrio! by the lim-iuljuEtmont screw. 
Tbo stngn hcu a tutiLtiun, but nut oonipWtv. Thr Ktiige aperture is 
not 30 IaT;g« m it should be; on u pivut attached to the limb th? 



entire BtAg« otn b« rot«t«d, aoM to be set «t any inclination,' thua 
coinplving with th« temporary need ao elaborately tiiet by tiie 
'rwliikl ' fomt ; and the angle ran be rworded on the dirtded plate. R. 
But the RtAgo may be inserted t/o that an olijcet may be placed on 
its nndnr pnrt. The sub-stnge proper ha« no redangular centring 
BiovcnimtA, but these are supplied on a scparatn adapter. Tliere 
an ii'i Hp»rinlities in tlie pharncter of the swinging apparatus, savo 
that tho wholn of this part of the stand nut be raised or Iower»l on a 
ilovetailfd titling in the optical axis of the instrument by the lever, 
Z, in onkr to mise or deprcAS the centre of the arc of thn swing to 
keep it concmtric with the object, an<) by that mcnnx to compensate 
t»ir various tbiikncwes of slip. The sub-stnj^ apparatus ia focussoil 
by the niillr<l hcnd W. The cntim liar is att4ichcd to an arc, J. 
working in the circular fitting, Y, niid rotntcd by niciinsof iheniillf^ 
hc&dX— the amount of rotation being recorded liy agnuluat^tlcirclm 
--and the ■nb-tbwo can be svan^ abovft the slngi- on i-ithi-r siilci 
(or dark-ffround illumination. The mirror lita in a tail-piece, V, 
cr it can bo tittetl on to tli<t twinging sub-stag«, U, Tliere is oil 

<labo<rat« armngcnietit for the rotation of the entire instrument on 
its Coot, which in graduated at B, It is not easy to see tlie practical 
pnrpoac of this, as the object is not in the axis of rutatiun ; but it ik 
prftridi-d in onlpr that ■ the microscope may be turned round with- 
iMit bcin^ liftt^d from the table, and the amount of such rotation 

The workmannbip of ihose maken is of tlie biKhest ord^r, and 
many of lh*ir piecea of apparatus hnve ^reat structural merit, This 
Ibtir leading niioroscope ia an impoain;; piece of Hue work ; but tho 
model we have seen has defecta a^^iiist whioh the tineat work* 
iBanship and ingenuity of device strive without satisfactory result. 

Swift and ^u formerly uia<ietwo instruments of the tint elata, 
eae havinK a bar movement simitar to tliat of Andrew Koss, the 
otiier a Jackaon similar to Beck's. The principal difference was that 
the font was of the ' bent claw ' form. \^'e hiive alreaily seen that 
by their invention of the vertical lever fine adjustment (ligii. 121 and 
134) Swift and i^on have made possible a useful future for the Jock- 
sun liinb ; and their model of this form, with the exception of an in- 
cumplete stage rotation, has the stage and sub-stage movements ; tlio 



Utter, however, dei)en<lG tor its action upon a loose rlof; opposed on one 
side by a uprtng vrhicli is wiunteracted ftt will by aciews, so aU«rtng^ 
tlin ponitioii of the centre of tbe sub-atagn. It ia not the best fonu 
for >o inipirlaiit a part of the timtrumeiit. 

All (he movable parts of Swift's instruments are apniu;;; on PoweU 
and Lfttknd'ij method, and the movements are snioolh and sound. 

Tliere have been many stands devised by Ami^rirnn niiiietanf 
durinjj the pait twelve or fourteen years, but they have been baaoi 
upon one or other of the great English models. And the modillcations, 
wliether for good or evil, have been adopted into tbe recent modifi- 
cations of the older English types, and linve Ifoen incidentally 
described. It shoulii !« re-meniliert?'! tbnt Zentntaycr, of Philadelphia, 
tlevised Ihemoiiel from which the Riss-Zcntmnyer was finally formed. 
Its principal feature was tfl obtain oblique illuniinntion in one niimulh 
by the swinging stngc. The line Ailjostment of this iiiRtniineiit was 
most defective. Tollea, again, who wholly desurvrs tlic very liij;l» 
reputation ho attained, rande an instrument in which h" mounf'd 
tMStUD on a disc, as is now the aise with the IW'k nuKlel (fig. l'(N|. 
NeMT the edge of this disc the sub-stiLge is iniidi- to trnvrl in a groni <• 
carrying the conden.'u-r, or dry combiubtion, in an arc miintl tho 
object as a centre. This was only another elabomtiOD of tiic sanio 
swinging aub -stage. 

In later constructionK of this form, Tolles first ukkI tho meehanioal 
stage actuati^il by two pinions vertical to the surfucn of the itago, 
and subsequently adapted by Ross (tig. Il'8). Tlie tine ndjustmenO 
ill this instrument had th« fatal defects chamctpristic of its form. 

Bulloch, anotlif'r Ami-riiran maker of note, mado some mciditica- 
tions in the Zentmaycr mu<lel, but tlicy were in the interests of the 
swinging sub-slax^, and, althou^'h no doubt ingenious, must pass 
with this transient form of Uie iiiicroscojie. 

An illustration of the leading form of Zentniayer's niicrosco[tea 
is keen in fij{. 130. 

It will be noted that, as in the case of the Ross form of it (tiff. 
133)itachieftharaoteristic— no longer, if ev*r, a merit — is il» Kwint;- 
ing aub-stage. But this has ibe chiiiu of being the Hn»t modern in- 
strument to respond to ihe cry (or swinging sub-stage, and certaiidy 
nft better response has subseiiuently been made. 

Ill the ftt«ge on the complete instrument ia the ingenious ar- 
fMUEeniA&t of n glasH super*i«tAge, which hu been so fi-eely adopted 
in England on a certnJTi cliisa of instmnient, and, in the abaence of 
A complete mechanical stage, is the only substitute to be tolerated. 

Dut another stago was made with tJiis inatrument, shown in flg. 
140, with, however, some modifications in detail. Tbisisi'otdifitiact 
fioni English forms of stage of long standing, 

A modilication of thia stand was deviled by Bulloch, seen in 
lig. 141. It presents no special pointy save the employmsnt fA tL 
Gillett condenser with the diajihrngrn drum tAovn Ih* /eiw** I 

A lat«r d«vi,-lopment of this form of instrument is given by th« 
mme maker some yuars later, and shown in lig. 142 : but the chief 
difI'or«nce consists in the odontion of a stage in which the milled 
heads BUind ujmi the rtji^e, which is the reverse of an advance. 



An Instranient uivle I>y Bausch «iu) Lomh, an<i known as their 
at mwromofif, is illustratfd in Ag. l-l^ It is on the same 
gratral plui, but the mirror and sul>»tngo barn cnn be moved lodo- 
pendeotly of eacli other, ur simultaiK.'oiM^ly whra tho vem on tli» 
mirror is placed in n Kctss iu tJip suli-stng<! bar. 

A speciality claimed for Uiia form ik u ' frictionlniw finn ai^ust- 
ment ' ; but it is one of the many which hav<- the int<itrmbl« burden 
of lifting the ciiUra body of the imttruinirnt to ncdum tho delicate 
focnindjusiment of (beobject'}[bi^ ; luid, althoujib highly ingenious, 
is tbn^fore, to our iniiii), wholly imp»rf<H:l for th« end in view. 

Mr. George Wak-, uf America, devised u pUn of some nwTit for 
certain classes of aiieroscopes. The 'limb' which fames ihe body 
and tho stage, instead of being swung by pivots — us ordinarily— on 
th« two Utertl mpporte (so tliat the balance of tlie uivroscope itt 
groatJy altered when it is much inclinwl), lias a circular groove out 


V- — . — 2l 


FlO. no.— Zentmayor"! sUg*. 

on ritlxT sidf, into which fit* a circular ridgv) cost on the inner riiAn 
of each Kupportf U sliown in fig. 141. Thu twosupporta, each linvinff 
its own fore-Coot, arc cast Kcpamtolv (in iron), so »a to meet to fnnn 
iIm; hinder foot, whrm they are \\v\a together by a strong pin ; while 
by turning the millrd head im the ri^ht supjKirt the two are drawn 
together by a screw, which tiius regufates the pressure made by tlie 
two ridges that work into the two grooves on the limb. When this 
preacure is modenite, nothing can be mure satisfactury than either 
tlie Btnoothness of the inchiiing movement or the Imlancing of the 
instrument in all jiositiuns ; while, by a slight tightening of the 
msrvw, it can be lirmly li.\ed either horizontally, Terticnlly, or nt any 
inclination. The 'coarse' oiljustment is made by a smooth -workinff 
rack ; but tlie fine adjustment is made to carry tho wholo weight of 

Fid. 149. — Daiuck uc J Loiub'* prolutloiuil ttoii^ 

Fio. 1*1,— Mr. Gfor^e W»l*'« iminiii 

'« tatm Dt limb. 

tl»e inechanicnl iiiovonicntti of the stUKe, und recUuifpilar niovDiuinits 
lar lli« Hul^-sUigv. ThU is « lin« inntrutueiit. oiid wuuld be wlmimbk 
ill every wny with a RH)re perfect fine adjustiiieiit. Pauy^l an/t 
Lftilatvta tefonti-elaM iiuanimeiil ia, iii nil eHi>e:iti»l ]iu.rlii:olar8, built 
ttpcm the firiDcijile of tbi-ir Xu. 1 atoiid, tlious'i leas «liibur4t«-. 

Ia iD£truiiKiit4 of die iiecoii4t tliua llieiv is bj- iio nietuui xo gratt 


n varintf. t1ier« lielnp: not ao pcrvat a deinimd fur those ad tlierfr w for 
ttiOiH! of tlii> third olau. 

It will be ufti-ful to discusB the leUtiie merit of the thinl in 
oontnut with tlie lii-^t c]a^ft ; but before doin^ u) it will be well to 
exuniiiie some of thtt best i»&truiii6ut-« |>reeeiit«tt to the purchaser im 
ItfloDfriii;,' to thi*. claw. 

Third-clasB microscopes. — Powell and Lealand nmke one liaviiig 
a quntity of work uol sE<coiid even to tlieir large stand. It greatly 
resembles their second-clAss instrument. It is illustrated in tig. l-t.''. 
The tube length is the same, but the stage and the foot are smaller 
tlinii in llie seoond-cJai^ instrument. There is no rotAry movement 
to the sub-stage, and its centring is dou« by the crossiiia of sector* 
and not lines at right< nngles : hut tliis is in no -vay a defect. All 
the movements und adjustincnta are otlierwistt iis in No. 1. 

As a rule. thiitl-cUss niicmsioiJes niv «itlicmt mechauical )itikge« ; 
in this rrsprt't Powell nml Lealand's is an inipnrlant exception. 
Ijecnuso it lins a stage prciviiW with the most perfcpt mrchiLnism thnt 
Clin he enijilovitl. 

IWk's third -class mid'oscopc iit stiown in fig. 146. It haxagood 
flat tripod font with a singk pillur. Tlio Jiu'kwm m<kd<-l i* used, but 
II pccoHnr tine adjust men t is eini>li>yed. the l(!*cr lifinji plac«-c) liclow 
the stngc the »cmw lieiiig jilaotxl iinniedintely behind the pillar whit-li 
KupportK thu limb, and where it \i. easy of tuccuti. TIid body ix not 
atte<'t«rd by vibriitiim whrii it is touched. 

The lever is of tlii? K»-ond order, and It supports th<! liody limb 
and coarse ndjuMmenl. In IniTt, save Iq ii« lini^ ailjuNtnient, thin 
form approximated jinmcwhat to the Contiiientjvl moitrl. The line- 
lU^Ufrtflient lover i« rathcT sdiort, but it will he found to Im- stendiei- 
nnd slower than tht- direi't-aitinKsi-rew, 

Tlie slngi' in plain, H'itliout mt^L-hatiicnl iuov<-m(Tntii ; but it liiu a 
mo\-al]|i? ^laxx !>ta^ over ihr' principal stngu ; tri thin tlio slip is 
(flipped and the whole Hu;>i'rstJij;L- of glass is moicd with much vaxe 
over a fair area, Tlic HjiL-rlure in the glaon »tu;p? is not liirji' 
viiongh ; it sjiould he cut right thruugli to the front, which would 
nmcli incnwie ita uaefuloess. 

Tliia iniitrument >ilso haa a sub-stage vritli mctc and centring 

Swift and Son's tliird-elasa micrmcope iu its inoU auitablK fonn 
dates from about the time of the vertical lever liiwi adjuHtmrnt 
patented by that firm. It was lirst made from the designs of Mr. 
E. 51. Nelwn. and it jiivseuted three <listiuctive features :— 

(1) Tti« milled head of the Hue adjustment was plut<ed on the 
loft-hand side of the liiub, 

(2) The slAjre was of a hor^-shoe form, the aperture being 
entir«'ly cut out to the front of the stage ; and 

(A)'Tlie body-tulte, uhich w^s of standard sixe. vie. 8] inohes, 
WAS inado in tn'u pieces which not only secured ]Kii-tability, but also 
jiemiitted the> use of both long and slioi't lube«. 

TbiB insti'uruenl is illustmted in fig. 125. It wm alao poateaaed 
of a cheaply made nnd fnirly good ceutriiiR sub-stage, to carry 
Powell and Lealand's dry achromatic combiuatiou fitted witJi a luni- 


out rotai^nnn to ca njr Stops. The mb-Moffe was niAdn bv HiUnting 
Ijwift'ii retitriiig iiose-piev^, aik) providiu); it with a rack-aiKl-pinion 
forsuin^ atmu^iueut. aa illuUratecl in tig. 147. Tlicrc win uIko n 
gnduBted ^tog^-plaK- oiid sJidiiiK bar, n [^m (Vvised li^- ^Ir. Lewi* 


WriKht fiT 11 liiidpr. TIi* evo-piet-ea vrere provided with rilisa, tike 
I'nwcll nml Li-nUmd *, outnidf- tlie tuW to govern th© deptll wliitli 
p*c1) itliould elido iiilci the drnw-tutie, bv which ntmni; tlw oiaphrajpu 
is in the Miue place ulutevtfr the deptb of the eye-pi«C« napkiyw, 

ooon sTLiiEsrs micriwcoi-k 

anal it was iMnUracted (o do nHticiil irork w-icti the liij-liest 

Anitiher/am\ itflMin irmlrtim^nt hnit nTix-ntl}- l)ocn inUxMluceH by 
titft Unn of Vhns.. Daker. of Holbom. London. U nnw in a sugges' 
tioti 1i_v Mr. Neboii. iliat this fomi 
•iMiuld Im a(U]>leit Id lli« C.-unp)ietl 
tUffrtfulinl tertrtr jiw a'/Jiiatiitrtil, 
tanking :i gooil i(U:ilily tliiritoUsH 
mU-riAroiw. It. siiciikl bt- ii<>i«(l thut 
thi< ilifl«mitiitl MTew [wriuiis of nlow 
M-ljiii) lN<i%' otilAJiie)! >>y ineMiia of 
tmtrn* tkrriuU \ it ih tlHTefor* very 
strong. Fii lli« onlinnr)- C'o4ittn«iit.iI 
fiinii of 'tiivct-iM'tini; line-iuIjUBtineiit 
•cn^w, if the motion iB*/>nr, tho tJirend 
riiujrt l»p linn, fl«noe in forms whwc 
Uiat tinr iuliii>ilm«nt is mndp to lift llw 
Utfly, Ihn <lilli;n>iiti»l sorfW is of |;rwit 
vikluc Fig. I 'IS illustratAs tliit lulmimttln iiiKtrutibviit. tl lias n true 
[n{.>0() foot of tlio I'ou'oll ntHl li^iiiliiDcl type, » tiliiin liorM'-siioe •ila^'i* 
uritlianliitinjitiiir, n ■'oiiflfii^T fo'UKsiii',; by n ^tinriil in-tii.n. giiiilu rui'k- 
iui'l-|iiuioii •yxirso itdiiifeUiipnt to tlit^ liody, witli ilrnu 'tube armiii^'oil 
to work with npochroiiMitic objoctivt^ iiiikIo far L-ith«r liHtj; or nburt 


PlO. HM.— N*lti3** lunu 

"niis inxtrumoiit liaa the po^ulinrity tli«t tlw* Kt*gn i* !J iiicli 
Wlow tbo truiini»i>«. which has the HT«cC of lowprintt tin- contra of 
gTkVlty wli«n th« itwUumeut is in » vartie«l or ituiirly vrrlicaii ptmi- 



tiou. WliPd liorizftBtal tlic optic axis is SJ iiidies from 
Tlip ends of the feet are plusgcd witli cort. 

It pi'ovpij on testing tlmt the CaiopljcU differential , 
eijUal to tho mo^t critical wfu'lc, and could bj' used in pho 
grapliy. As a. result wieml adilitioiis were made, such as ] 
pillion fucuKsiug iiitil rectiiDgular uio\ciiieiiti< to tlie Buli-sti 
rack'Work arrftiigeinent to the flruw-tuUi?. Subsetjuenth 
and heavier iii>(Truiiient was niadt-. ha^iIlga j^-incli moreoEJ 
height. In tliiii model the milled head of the dillerent 
placed l»el(jw tlic aim, instead of nliove it, which is nn in 
for photo- niicroyniphic purposes, aiid no speciftl detrimeutinun 
work; and, if re<|uired, a diilerential-si:rew fine iidjustnieiit i 
fitted to the Bul'-stjige. A rotary staijp is also sometiiLies 
this instrument, bat thwe whioh we have Keen have not gi^ 
aperture snttir-ient dimcnsione for modem focussing. 

This instrument in its complete fonu as devised by 
bhowii in tijf. I4t'. The stage has changed it« form ; but ifl 
aperture Im' kept large enough this may be fully iount<'rbal;tui:B 
the rotation given to it, and with the Carnpliell screw filtml b 
the mirror for tlie Gne adjustment of the condensei" is a vi-j . .iliH'. 
tive and uhpfn! luicnsscojie, and may be eaftly reannineiniiil 
amateur and the student. 

Tlierc is not sufficient rack-woi'k to focus tlie 70 iiiui. uh\f 
iii'ifis. but the nose-piece unscrcMr; and the objective is held 
the (uIn- by an adapter. 

TiiiB larger foiui of the instruinout is placed amongst niier 
iif the third clohs l>ecau!ie it is unjirovided witli a mechanical 
but ft suppJeioentftiy and remo\iiljle niwhanical .stJij^e, devii 
Mr. John Mnyall and ninde by Baker, and Swift, and .ilso by i 
of Jena, can eji^ily be added, as it is in the figure (14!*). Tliisl 
t'aiily well, :iiid i*. ii Useful appendiif;t> for more delicate stase »'0 

Fourth-class KUcroscopeE. — These should liai-e a rack-aml-pn 
coarse adiustmci 

movL* fuisily anil tirmly with pres6Ure from ODe wle only. 

TIm) stci^ u hst«ii«'l to th« U|i]irr kuIc at twi> iMnckctA wliicli are 
ratt in one piei'c vriUi th^ limb ; on tfan uiiilor side of thecQ bnickeiA 
ilwre b uDOtlinr |>lttl(' vrhii'li holcU tit" xub'Kta^ tuhr. 

Thin mtilruiii«nt i!i!iuj>)>lir<l nitli lurgp plnnr-aiiit con cjivf- mirror* ; 
luui (^iinniOeriii): tlml it tuiiatitutt^n n sixtJl cUn of microtcope Iiak 
*rry inuth in its favour iiMiMvimiliiryiiiilniiiicnt (or ttic wor!:-t.»t>|t, 
Like nil t)i«M- tiuikr-iV iiiatruuKrntiv tlip fi-ct nn- ]ilu;fi:c^) with oork 


■tflil we kitowr of sonw of chiton luioroMiopcK tliiit Iiav« been in um 
Jorf'irty years. i»n<l are still tho trimtitl 'jounn-j-mon" inatrunKMiU 
<■( ii»<>unter« nnit i>tlM?r woikvr* of variou* i.nli*ii> in many dppnit. 
■MtitH of mi(-ros<»{>y, 

Thi! Mi^irs. iiei'-k nlso niAkv n inicixKomxi of tliis kiuil cnllol » 
'hitetoU)f;icul ilisMctiny inionwraipp.' It !» illu-ttnitpi! in tis- 1'>- : tl"* 
body is rBBiovublp, an^l ' Iohjk ' or itinipli! oiu bi- usetl««dL 


Thr tvro ninitcs (if vnc are stien in tli<! figun!. In this cuue, however, 
the bmly dooN not incline; but in !>iiii]>licitj nod price fur Mi iu- 
\lriimr'iit ri this cln^ it dt^servis coniiiieiidaticin. 

Portable Hioroscopes. — Microst-oiit^s that may 1m' reailily Calcen 
fiYjiii I'lnff til jiUiio, iiiiii which iirt! ytt jimvidiil with tlie arran^' 
DwntA n-ipiirtil fur UNing the [irincipiil nnfiamtuH, lire of iiuporiAnee 
in scmir iiiviwtjgntionsjiind an)<l«jmltl« liy ihi: niiijurity of tho«» wh'i 
hftvi? 11 liviii^t inti-n-st in iiiiiTio!«.s)pic wurk. 

Till- i'iirli((st, nnil nIiH lh« bf«l fona "f thin kind i-f luicriBCopo 
wiix uiwlr hy Powell and IahiIimuI. A.i o[>«i)iiI for u"^ it IsillUAtrntcd 
in tijt. I "fit ; but tin* trip<id fiKit 
folds into wlint lii-ciuiii's ])mctically 
n Btuiflt luir, nnil is hnnt by inetins 
lit a Joint la occupy the IniHl spKcei 
Tlie body niiH-rovirB, «nd the whole 
lies in a very nnull »|mici*, K<ving 
at th(! Miuin time tittin;;s in the 
cKbin«t fur It-niu-t, cunilcnKf^rii, and 
all needful uppaitLtiis. The coarse 
and line ;iil iu^l ini-iits to ilje Txidy 
nro IIS in the No. 1 8t<ui(l. m itre ihe 
stage itio\'enien<a : and r he Hiib-Kiage 

Fki. ISS. — B«ck't hUtologiciJ ■ail diMurtlng inici»»copr- 

ha« iiicb -and 'pinion movements and rectnti^lar Kcctor cvntringt 
while nil the appnriitum providinl with the bti<je«t initrummt cnn bo 
tiinployixl with it. We luive utted this instrtnnent fur dHicatn and 
«rilicAl work for liftmi years and there U no ffdiingoft' in iu quality ; 
nnd whm pnck>-d with the additional apparatint rrquir^l thn vilri i% 
la X 7 X -f inrhi-s. 

Swift nnd Ain Mibsequeiitly niiwle au iiistmnKiit on Mniilnr lim-^ 
The tiipcwl nnd ^l^lJfe arc packed practically .is was I'owcU and Iji'rt- 
land'K, bat till* Ktn^- in this c«se is plain. It earrirs n very con- 
venient nchniniatit.- condenser, to which we caJl attention in itx projirr 

. its fine adjiKtineiit in so utiKtratly Ui«t jl cutDot be used 
rs or fur critifnl work. 
Bent *s«t Dp' » MM) in %. 154, and in tta packed 

i.'il in 6g. I-"'. Tbp owe in wilicli This inrtru- 

Iku;!; were llifl n*\t in nnli-r of time tn manuf^rt^iri- 
it of this kind. This is illiutrnftrl in lig. XTtG. It dilfm 


from the two preoedJcg forma in being a Jackson model. 
oculiir 1><>dy and tbe coarse adjustment have to be lifted aod 1 
by tlie fine adjustment whenever it is used. The stage is 
rotnt'fs, and the sub-stnj^ has no centring gear. The 
packK into a box 10!t X Tjt X 3^ inches. There is a 
Gpociidly made for this instrument. 

Mr. Rou.sselet lias designed an admirable little instrument oCl 
able form but of the sixth ctasi;. It is binocular. The toipocl ] 
the stage is plain, with a sliding ledge. The condenser fo 

itii etini/''nn<i ltii*rii lew l*eeii tIetUr<l b_v euiployiit:; llie Ittn- 
o( Mr. Sti-pUMitofi. This iiistruuieiit U'iltuntmted in tij;. IK'. 
It itt inB<h> l>_v i^v'ih niul Son. The st&ffi is :t lurge. Ili't inbK witli 
ii]wial n^K for tli« nrnt^ Tlw otjjertive and biiioculnr ptirt of tlte 
liculy n-)nain \-rrticnl « focus verticatlr l>}- ■ rark-nnd-piiiioii 
(sMra- inijnRtni*-iit, tbi.-Tv Wing no fin* niliuMm<'n(. Tlie bo(li«B 
kIhivo 0\f )>iii' (imiiiR nrc t'UitAbly liirlitii^l, niimirK boing pliuwd 
iiumIc- thrai to «-fli?ft tlin inmg^. This riHlfxion »lw> caiixps tho 
pTPCtioii of tb<: imng*'. wbii'h i« VAltiabli- to iLp inajoritr mgn^'d in 
iiuu-ct <1L-Mi>ctiiin or Cbc ititmrctioti of rcry Holicnt^ iind ni)nut« orgnn- 

We h*vp now to conniibT fA' tmut jirimilii-r ^f/iwA" nd'ipt«l for 
utnplK niii'nmrtipi'ii. Tlmt in tW f'iriii of n bull's (-yi; Manil in th» 

Flo. IIA, 

liHist complex form pnmibU>. TbiH instnnnfnt boMKiin iiitrinr.tslinln- 
jttiu-p hdw^n the itaixl-iniipiitii'r iin<l the o>in}ib'Ii' microM-o|H-^ 
lirinj;, in fact, n<»thinfc Dion' tbnii n lens stippi>rt<rd iiixucli ii nuuinir 
iiK to lio canablr <>f Ix'iii^' rrtultly tixrd in n vnriHy of }H]dtiAiis 
Kuitabln fur distcctins; niid for other nuuiipvilntions. It cousiits of u 
ctn-ubtr bnuAfiNit, wlirrrin ixKrrrwrilnxhort tubular ptlUr (li^- 19^). 
ohich i» ' *pruiig ' nt it« npprr rnd, so ni to ^(raaii a H»cond tube,. 
hImi ' Kprunji,' by \hf drawing out of u-bii^h t)i<* pillar luny be eloa- 
sativl l»y Blxiut iliMK- inrbrs. Till* varriiit nt it« upper end a Jointed 
■ujtki't, throutrb wliicba «|unrc tnr about 3.'. incIi»IonK slides mther 
MitBy ; itwl <tnt- rnd of tliis bnr carrii-» iinotUer joint, t« which is 
•ttacbMl a rin^ for boMiu); ibi- l<*rH--s, By k-nstbening or shortening 
tXiiv pillar, by viiniiiK tin- ntijflp wbii-li the aipiare Iwr ninkcs with il» 
euuituit, and by -liiiliiijf that bar tbrougU the auclcet, almost any pv«i- 

Ill Wing n (rosb i>ortion of the lonttrrs to be exajuined into riew ; 
iind, gimi-mlly nicnliin;.', it. will Hi' fonml wniveiiient u> p|j»i-e it on 
llir i>p|iosit<) M<!i" of thfi ohirct, mtluT tliiiii on Uw sanie kiiIh witli 
t!ir iiliwrrw. At n is uliowii th<- })ositi"ii in whiuh it m«y Ik? most 
cmvrnifiitlr sot for tin? rtihw^irti'm uf i>!ijfL-(% i-ontaitied in a plnt« or 
tnmj^h. the wiiiR of which, bring lii;:b>-ittinii the lens, would pmviMit 
tin- UK! of Biiv nis^nuliiT mounlwi on " liririiroutJil ami. The powfrrs 
UKunlly )iup|>lii->l with thi« iiirtniiu<-nt bi« oih- nf nn inch frn-ut, iind 
K inoikI of ritlicr A hnlf vr n ijuiin«r of mi inch, lly uiutcnvwing 


till- {>iUiir, thf^ whole is ininilf lo jmi-k into a &iiiall, Uiit mac, tho extremc- 
jjorliibility «!' vrlitfli is n givut rt-cuiumeiidntiuu. There is, however, 
II fiiriii of mounting (or this iiiHlrumeut, wliich was devisetl by 
tJuL'ltetl, which hiis superior iidvantaFl^fi. It) the form JllubtniteJ 
we iirt- oljligpit t') mech:iiiicTilly iLriiiiifce the lioiiwmt.ility of the leu*, 
which, of L-oui-sc, is iinport*iit. In Quekett's forui the loiip or lend. 

i» ^1) hunf( iti a Hhk that 
it h»« a peudulous motion, 
fliid with fvery chaii^^e in 
the po&ilioii OF aii)'l« of 
the Imr, the lens, by tjie 
action t'i grai'ity, becomes 
perfectly horiiontnl, TIiiH 
is by far the best form of 
moiinling. Although the 
uses i>f ihtH Mttle instru- 
mi-iit are grenlly liniiteil 
by its want of KtBgc. mir- 
ror, iVc yet, for tliP cla** 
of ptirpncrs to which it i'" 
suited, it has advanta^ei^ 
owe ]"'i'haps ciery othrr 
form thai bas been <!<■- 
vised. \Vh*:r<', nii the 
otluT hand, portatiilitv 
luay 1h" alt'igfthcr !<ncn- 
ticc^, and thp inHtrument 
IK to la- aihiptcd to the 
mnking of Inrgc disscc- 
tions under a low ma^i- 
fyin;; powrr, siinn- such 
forni a» is representnl lIi 
lijj. I .iO— construetiMl by 
Mtssrs. Baker on tbi- basin 
ofthntdevise"! byPriif<'KM)r 
Huxley frir the ust- of hi* 
Pniuticnl Class at South 
KriiHinRton will \xi 
foinid decidedly prefwr- 
able. The fraiin-worlc of 
the instrument is solidly 
constructed in iiiidio;^ny, 
all ita aurfaceJi \<eittg 
blackened, aud i* so ar- 
ranged an tu •■ive two up- 
rights for the support of the nUtge and two obli<jue resiB for the 
hflUiU. Close to the sutnmit of each of the^e upri^hti^ is a j^roove 
into which the staj^-plate slides : and this nmy be either a square 
of nioderatoty tliick glats or a plate of eboiiile havin;* a central 
IMirfonitiou into which a disc of tlie same nmtenal may be iStted, 
ui RK lo lio Hush with it« siii-foce, one of those Wing readily 

Fi.. iw, 

gubatituletl for ihc otber, ns nuy beet suit tl)4> use to be aauiv 
t>t it. Tli^ lens U cnrrieil on nn ami uorkitii: (xi <^ racked stam, 
whk'li is rnUed or louerfd hy n inilietl-timil pinion atUclied to n 
{■ilUr nt llie fur[li«r ri(,')it-li«ii(I cnnitr of ilie %tage. TIte length 
<4 tli« rack i» sufficient to Move tlie ann lo V>e ii())uU«d to any 
fueal ■liAlntice Ix'twecii tn*o inclii's nod » (jiMi'icr of nn inch. But as 
tbe b<rii;lit of tlie [lilbr i>^ not ■^iifficicnt to nllow ttie use of n lens of 
thre* iiiclres focuB twliicb is very uscfti] for Inrg" diMPctions), llip ami 
t«rn'tiig: Ute lenM-s is inndo niUi ii itouliln Ih-iiH, which, wh«i its 
[Wtttion in revprspil, an in thc<loitrd<iu(liii>- (which in rcwlily dotictby 
auM.rewiiiK ibo milled bn»d tlmt iittnchc* it to tbi- top of th« nwkm 
iWbi), gives th« additionul inch r<-(|uir<il. A* in tlif (Jiifkclt inicro- 
t^'pe, a coiiipoiind Ixxly niny >»■ riujly litttsi, if <l<-virr'(l, toa M-pamt<* 
irm ciiiwil'ln of Iicinit jiivotcd oit thi- siini"' stem. The mirror frnmi- 
is lixed to the wixidi-n hiwn of (h» in»tnmi4'nt. oiid place* for the 

Fiu^ 1&9^ — 'iMbtMnUffy ti-^^vVtUn nucr^yn-jyty^ 

U-xtft^ iin- made in groows Wncnth the hand-sapporte. Tlie »d- 
t«iitB^-i 'if ihtH t^cneml design Itave now lic«ti luiiiitfactorily deinoii- 
Atnttixl hy iIk- larii^e xna tlut hnK Ix-en made of it : but the details 
^ its construction (i^ncli an the h«ig)it an<l ^lope to lie giten to the 
linnd-nvts) may l>c rn.«ily nddjitcd ^> iiidividiinl requirements. 

A rery ]wrtAbl« nimplc niinrowojie ttnml has lieen designed by 
Mr Ward to take a Sti-inhril x 10 nplnnntic Wp. It omsists of 
a circular labln on tbrMr lemi ; it has a mimir, one «ido of which i» 
'iminj.'Ml (or whit« cloud ilTumtnntion ; it aliio hiu a bidl'icoyn for 
'HNinur. Tho focuttiog i* p<-rform<fd by a iili<ling nrni, aiul th« wliole 
Mcktt into the reniarknhly Hiiutll arm of n ca^e wboon outside di men - 
«MNks ttr« 3^ X 3^ X 1 inches 

But the very bort form of tliwiMTtiiig micruacopo,^f(fM;>« or nBiplr 
trtuft which we bcJiavf t» 1>e at pririwiit conKtructed is mule by Zciim. 
Wr illustrate thio form, fig. ICU. It bus n large firm xtaj^' 4 inclii« 
M}uat« and 4^ inclicxfrom Ute table, to which wooden aroire-sts can 

DiwEt-nxf? MicnoscopRs 


U- ntfiirlW. or I:Ot, in lany he rk'^in'tl. Tlii' iita;,'e Ijmh a htTjuri 
ii|M-iiit)^>, 3 X 3j| incli^s into wlik'li ran be [itai'itl eitlier ii Uul hiiuo 
|iMUr. lira gliiw suWittitf, or !i iiieMl pliile uith u Imlf-iucli hole in 
it. trii<l<>rit>?4iili t\w etigp ni'<> blnck ami while sci'eens which cnn 
NNdily 1*0 tumpd iivi<(«'. Tlie nnu vrhk-h in fuCusixHl Iiy ud excd- 
I^Tt §pirnl rjck-work luIjuKtmool caiTies villier a Xeiaa dissertiiifi 
111 ii»c"[i(', wliivli witli iin'i without ils i-ouiiiv^ eye-lens j'ielrfii wis. 
•littV-iPnl (uivrerM, varrin^f from lA to lOU <UAmetcrn, or Uie ni-iii will 
MVP ilie Ki-ry tine ZciKii'Nteiiiheil Ivit/v. 

The iiixtriitiipnt U provided with » lar^e plants niiil c«iicjk\-A 
niiiTor on n jointtil ami. The utility of this >iiti|>lv rnici'i)i*coi>e i» 
very urtiit, aitil u'i> tto not hei>iitato to pro»ounc« It Uie Iwst thiug 
t»f its vlnv* ««• hAVf ovi"i' s''On. 

A vuhHtxiilial atwl daliorale form of liinn'ftittff miirt'tctifir, r/rrwM/ 


Fm. 101.— UkIi'* 1 wmUr ibuHtiaji BKiiMMqw. 

Jiff l«l' Str. H. It'ft;, Is riprr-i-ntwl in ti^-, Ifil, From the aiii-Ies 
€4 n wjoiirp iiinhogniiy tjn*o iti^rp rise four htroiig limss pillars, wliidi 
mpport at » hriglit of four inclirs n hrniw plat«iS'^ iiiclicii vijiuiv, hii^ iiig 
« ceutral ajipiture of iitic inch ncross ; upon this \v*,\>- a hra^ 
phit4*, of which the clinin*tcr iii iHjaiil lo thi- siite of ilic iirceeiiiti"', 
■umI which is nttnohitl to it by a ri-votvina Httiiig that surroumli^ thf 
cMitTttl aperture-, and can bi> tigliti-ticd by n larjjfv milled he«d 
Ixmrath ; whilst mImvc thin is a third plarc, which xlides c-a^ily over 
thif M^-ood, \n-i\\\t Iwhl ilown tij^mi it by isprings which allow ainovo- 
■iMtnt of IJi inch in aity lUr^tctioii. The (oji pintn has an iipi-i'tuiv 
of H inch for the rcci-jitton of \arioux glauns hiuI troutth^ suitable 
for c>o»taiiiing obircttt fur disMctioii ; nud into it ccut ho littc<t 
a si>riii;2' holder, miitabh! to nvcivc nml secure; u ^hiM hHiIu of tli« 
nnfitinry xiir. Ity turning the Ini^' circular plate tint ohjcul Ululer 
ubttvrii^ion may W ea^^ily uiadc to folate, inithoul diiiturlnnj; ila 


rrtiitinti t» till' (iptiuiil jiortioDH iif th« inatrunitiit : urhiUl « travers- 
ing iniivuineiit may )ie Kiv«i) to it in tuxy i)in.-ctii>n hy acting tijimi 
the Kiiinllf-r platv. Tlie l»flOiiin<l Wvk {lillnr timtMus & ti-iaiigulnr 
littr with rack-and- pinion lunvement fcir focal udjuilnieiit, whiH) 
cnrrics the hurixuiitiil arm for the suppi-ri of ili^ leitWB ; this una 
call In- t«i-iiecl nvay lowawls the left iide, but it in proviilwl wiUi 
u ntdi) which cheeks it in llio upposite direction, v.lien the l«ns U 
exactly <iver tiie centre nf tlie tit«t,'e-a|)ertuFe. Beneath this a(M--rtu»- 
i.1 a ciiiicave mirror, wliicli, when not in uge. lies in a recess in thi' 
uabo^ny liase, ao as to lea^e the upaca l>eneaili ihe <»taj|;e entirely 
tree to receive a hox (.-mitaining ApiiarntLis ; whilst frani the nffht- 
lianilhack eoi-ner there cnn he miscii u stem oiiTylnga aide cundent- 
][){{ lens, with a hal I 'iind -socket niovemeot. In addition t" the nin^^Ie 
lensts And special com Iti nations oi'dlnai'ily uwd fur the pur|XMes of 
dis!<eotiim. n. binociiljir iirianui-mi'nt was devisi'il liy Mr, It. Beck, on 
the principle applied l>y 31 >f. Narhef, aiiotit the snnie <lato, in their 
stereo -psendoBcopic micro-oopo. Adopting Mr, Wi-nliam's method 
of iiUowing half the cuneof rnyeto procrwl to "iiccye without inter- 
ruption, lie caused the otlier half to In? inieret-ptcil hy a |wir of 
prisms, and to lie by tbem inuisniittwl to rli'- utbcv rye. Hut we 
tind its utility to Iw pinctioally limited by thi^ niirr»>\nicKs of itn 
fieUI of view, by itjf drficiency of light nnd of mn^nifying power, and 
hy the inconvenience of thi- iminii<-r in which the eyes have to !« 
nppli>:'d to it. 

The Continental Model. Our i>nc pur;><ue in this treiitisc ix to 
«liiteavi)ur t^i promote what we belicvii to Itr thi- higlx-'sl interest* of 
the niicTosciipe ns a mechanical and opticjil instrument, as well an 
to further its iipplicAtion to the orcr- widening iin-a of physical 
iuveeti^tion to which, in re«enrch, it mny be tliivcted. To this end 
throughout the volume, nnd esi»-<'ialty mi the snbjert of the value 
and efficiency of appnratuii nnd inrtrutneiit*. me h«v« not bf«it«tod 
to st*to ilctinit<^ly our judgment, nnd, whem nee<te<l. the Ihwis on 
■which it rests. Incidentally w^ hnve expressifl perhaps mm-e than 
once our 'Hfip/imi-ul, and with mirselvf s ihnt uf mimy of the leading 
Knglinhnnd Americnn niicr'>sci)pisl*, n/' tlf j't-rm 'j' mirri.trtiiir kmurn 
n* fA«- C"ntinr>ilal nutilA ; imil un cherish sti-onjj hi>|>cs in the jn- 
U-rent* of tile ncience ">f niiciiiscopy that so entpfprisin*; and eminent 
a lirm nx that "f Zitl-u, of Jena, will bring out a nxHtel that will 
eom]>ort mor<> complrtrly with the needs of mndiTn niii-roieoMcnl 
iTsi«ri-h thnn even the best of the moik'ls that they imw produce, 
ft i« to thin hoiisi", under the cultivated nuidanei- <\i Pr. Abln-. (hnt 
we aru Indcblfd for the rtpk-udid jjerfeclion to which the iiptieid .fitle 
of the microscope has been recently bi-iiuj>ht : and when we know 
tliAt the ' Continental model ' liaa in the haud-i ■>f the lirni of ZrIkk 
pBiwd fnan an instriinn^nt without inclinnti»n of the bmly into nii 
instrument that doeu so incline, and from an instrument without 
sub-Htnge «ir «)n<lenfter into one pi-oviiled with the latter of thew.' abso- 
lutely iniU«|>en"ahle appenda-jes, and finally from an instrument w ith 
m ]>erfec*tly plain Binge with * clips ' into what in now a atjige with loe, 
dmnical movements — we can hut hope that these conceKiiona to what 
luu bolon^-Hl to the best Ensli"!! models for over forty yean may lead 


to Ml entire i-MVinstraction of the staiid — a wIioIIj new model— tn- 
tAoded lontwtnll tlie rwjuin'nvenU of modem high clam work in iill 
drnttii It'll Is, «imI with n ttiic sdju^jueitc of the most r«(ined cLvui. 
^ e otnnol lioubt, if thiK woro m, that the aania jctmitu which liiu 
flo nobly <>levated the optirnl riMiuireiiienU of the inHlrumeDt would 
nrt with <«]nnl iHinN^s on itc roiihtnictiun and uiMhiiiiiiiRt. 

W* linve wi-iti*n througlMHit ihii* boot too fntiiklj- of the eminHit 
iwrvi<-es of MesNn. Kcia to th<t fiirthomncQ of th« iiiti^mU nnd pro- 
graHsion of thn microscopi; lu n snifintific instrument to be niiiiui)dei-< 
■4onH in niAkingn|)lninc3itiniKt4;nf thi?* quality of the i»o(/<>/ on whidi 
Uiair eljtbonite and in Konw kvoim* luviutiful stauda ure built. It 
will (w Men tliat w« evrrywhrtre justify our jndgmenta by [iliiin and 
(««i)y wMnprtJwndivl lyuiwinn, i»n(l the vi-iy eminence of the iiinkora 
n'nd«rri» it innimticnt thnt pmrticiil nticrcKCOpiHts should, without « 
■bwlr of bill*, nitsi^s tlin viilue uf i\ stAnd which i>> certainly not 
l-uiJt tin liui'n that contributi! to » higher and still mora olbcii-nt 

.\t t]i>- Mmc time wfi do not Uind ourMlrirM to tho fact that ut 
Englinh riinrki't for tli« 'Hartniwk ' niodrl has hnd very much to 
do with th<- p«r[H-tuation of the frron which thnt form r^nt«iiis. 

Th« rtaaon of tlii.i it ii not diflimilt to trace. T/i' iii'lu--iien 
mflhorl advanced hut iilowly, in pnwtiee, upi>n tho proftrssioniLl 
•ctii'itic'x and eren the prolesdonal traininf; »f nn-diriil men. Tin- 
country which was (he home of Baoon and Xi^wton, mid IHarv'oyand 
Hunter thiM>r(-tically acciept«d, but was not ()uick to npplyj Uit> 
iiietltods of inihiction to the irorfc of ibt modicnt schools. Th«oi-y and 
mpiricism hfl<l a powerful ]>li»ce in both the ti-aching and pni<:tiefi 
of mrdicine in Kngland untd iIm n>rliiTy<'ar$of the T>ivsontoentui'V. 
Medicimr wfiJ< ntmttulely unilllWt<^d by ll-ii^on until thn latter half of 
the H'veDli'^nth ci-ntury. It was not until Uic early yennt of this tien- 
lory tliat the nxidem hcIimoI of medicine ln-gim it* t«n<-lioont oarmr. 
'iitt at that lime fA* niirrfj*:n/ji"— one of tlio inoit powerful in>itm- 
ilx whidi can be tItoujiUt of in the npplitntion of cxpiTiniental 
and dfdui'tive tuethodx to the science of inMicino — was lookt^^l upon 
vid tri-a^'<l by tlie fcteulty ns .i pliilodophical toy, a nierv plaything 
iho rich ilil'lt'inli. Rut in sjMle of ttiia the inicrmcopo wa« 
Eight nmdnatly to a IiikIi state of ivrfrotion, and by the end of 
the tint (hint of the century waji rcmai-kably advanced asapmcttcat 
ttihtrunirnl, oil itit eaaenttala liein^ more or leiia completely det-elopM. 
Meanwhile, iHi the Ojntiiieiii, the mici-oscope was reKiircbsl by iho 
Faculty IX* a •H.-i.-iililic inslruiiienl of great and incivaaint; valuo, 
iH-ing used to good purpoee in making important dtaoovnries in 
iinatoiuy, liistolosy. anil biology generally. 

Tliia wiu gradually m/toni in /Am munlty, and there iirom 
tlowly a (leair^ t>i employ the same instrument in Englaiid. ISut, 
although English iiietrutnentu of the most practical and relatively 
perfect kind, representing the large cxporienoc uf ninny careful 
anaateun, were easily accessible to our mrdical men in their own 
(oontiT- — liecaose it was on tJi« Continvnt that tlio inrostigationa 
TcferrM to luul lie«n made—it was nothing leas than the CofidVnbr/ 
meroteopt that waa sought aft«r and ol>tained. Because earl/ 



obaervntioiie of a hiittola^icnl cluuiictvr (und th^rcforo of a nature to 
lie bejroiid thrBphoi'ci'f tin- Injr Ainnt^^ur) hiiil liM-n Kuccoiftfullyiiiado 

nitli n cpiinin form nf niici'osvupa 
oil UiL' OctnUiient, it vm |>racli- 
i;iilly oi^mI thut tliis mast h» 
till? iiiiut iiuituli]i; iimtruinpnt for 
-lui-li II |iur[ioitc : but tliU wtM an 
iiifri*iioe niiulv without know- 
hiitir- o( or nrfprt-nc*' to the wcU- 
kniiwii EiiKlinli iiiodrU. 

liel US cun^fully cxntiiinp thi» 
itutrunient. The typiciil fonik 
was tliiit Tiiiulc Iiv Hnrtiwck. 
Seen in its beiit [iniuiiivi-xtatr, 
we have it in one uf Zeiss' in- 
ntiumetitB reprmeiited in ti^. 16l'. 
It ia » iioii-iticliiiin;; iiistruuii'tit 
with u abort tube on a nnrmw 
honie-shoe foot, in which Ktewth- 
neiss i*> obtAiiied byalieer wmjiIiU 
It has A Bliditif; lulie as u coiiru- 
iiclju&tmenl, mid it ilirt-ct-nctiuK 
wrew for the line aditiMiiii'nl. 
The stage is suihIJ. and ihc iiiicr- 
ttirein it isrelnCi^ely still ^nlnile^, 
uf no son ice in rejioliing thi; foc-us 
of an object by t'lucji with n 
high power. It is pmviiltHl with 
spring clips, and a diajibnijltui im- 
mediately below the stAgt-, and n 
concave mirror. 

>'iii'>?ly used, because up ti> a coui' 
liiiiiiti\i.'ly Iftledftle (ls74) it wiu 
i-e),'ni-de<l l>y many on llie Con- 
tinent as a mere elegant jJay- 
thing ; its true value wa& not 

On this model all tlie inicro- 
Bcopps of the firm of Zeiss, xf 
J(>nn, are constiticted, a>i they 
nvi} tiseil ntnioiit exctasively on 
till- C'Ontinent, and ai-e regftnlMl 
in innny of the iinivi-rHties nn<l 
iiirdiriil schools. Iwth here and 
ill Anirrica, as posfiessiiii; all the 
<|Ualitics required for the Ix-st 
InologicRl researdu 

If we examine the finest of 
ihcv instruments made up to 

Fk). li& 

lt^>.'i, wg are iiiipi'esi>ed, as we 

alw&ya are, with tiie bnkuty aiul ore of th« workiuiiiiHhip and gnisli 
fit t)iis finii ; liat there is the &aiue lieavy ht>n>fr■!>JlD<^ toot, KtMuly 
«tK>iit;'i '^hilc thn in6tniinC'nt in noninetiHiuf/, only neeiUewly heavy, 
requiring rnmiiion in^^piitiily alone to Ret equal ateadin<«( with 
one-{ourth thn irright. Diit uiice this iiiBtinuient luia btwii adupti^l 
td the EiitflisJi form liybeiiii; tundv to incline to any aii^fle up to Otv 
htnizoMal, the foot l>ut inseciiiely bitlaiiced the iiistniiii«iit ; it iw 
notdidiraltitAicis not iincoinmon to topple it over. Tntleeil, in t)ifir 
pbot4»-inicnigrNj>lii« ontlit the Memi-s. Zeistn practiotlly aee thin, foi* 
they supply atuiUief foot to ic/nVA (A-* mK-r(>t>f*-/i' in ^itinfial. 

It niiiKt not )n^ forsntteti thai this want of balance is with thn 
short, not thi> hnit; liody. 

The diaiuMin' of tJw tube is small, beinj^r 8li;;)itly oter seveti-eifjhtJis 
of an incb. 'Dint this ts seen to be a di^ai I vantage would uppeai- 
certiun. bpciiusn th<? i'A'4offr<r/Jiir uiicnff^'/'i' nuiiiri of Zvis» has a 
ttiiyr U-ty-tiilu' ; and in their recent "Apiinidis' to their latest 
ratuIogiK' thrymlniit that for certain purp>*es othrr stands made by 
llieni. 'owini^' to the limit«l diameter "f their tul)es. cut off the 
tieW ; ' a rignificant flint for tlwwe who would narrow theEiiglishimrly 
At tlii« pi««tnt rfntn out of righle'-n models ten arc mode with 
inclining bodii^i, and thmn Jiavf sliding conrso adjustmrnt. Itlit in 
the twi-ive ni'xiels for IfS'j ti-u incline, while "niv two are rigid, 
ami eight luive rack-work ngninst f"iir liaiing vliding tulws for 
coAne nd)n<itnifint. This in a nuinifct oonforniity of the prim itivo 
leoclpt to thn KnglJKh typa 

The f/ifv( (!'■/(«-/ wrw. only slightly m«tifie<l. obtains uiriTprBally 
ill tliese modeU, We have already plainly *aid that this is not suf- 
licientiy delicate in its action for criticol work with an npochromatic 
objective "f 1 -4 or 1 ."> numerical aperture, espi-cially as a micrometer 
M-rew with a nm-osntnly dcliaitc thread is Itound to carry tlie i-om- 
liine«l weight -if thf body, limh, eoiirse ndjurttneiit, and the opp^ising 
spring ; that it will wwir louse untler the stn-jw of ounetant work is 
irievita'ile, and thus its Utility muKt l>e wholly gnne. 

Tlie IM49 mnilet has a n<'w form of linf ndJuslnir'Tit, thenltoration 

bnng that the mirrometer Kcrew actx <>n a hardeni^l !it<i*l point ; tliia 

HMV cnuce it to work smoother, but ns ii<> weight i* taken off, thi-m 

buiflictilty in discnviTing any reason for its admitting of mor» pro- 

Elongnt nsp without injurious wear. 

Thr nfnyr of thin iiistrumi-nt, in common with all linilt on the sam« 
natel, has thni- fiindaniental i-rrors of iltriigii :— 

i, n«' ttax*- is *o narrow that tin- eilgi-a of the 3 X 1 »lip<t are, in 
! Contiuentnl stands, allows) to project over the ed(fr-s, Messrs. 
iM have protitably dctiartftl from this fuolt, \>\ h'i^iiiR to their 
hu-gi-r stands a stage in bim more like the English type. 

ii. Tlie atones have an ajierture so tiiiiall a» (o limit their useful- 
nam in focusing wltli high powers. 

iii. Tnst^^ad of a sliding ledge they provide, what still mor«i 

efficiently militates against easy and rapid focussing. \'\t. apring 

clip*. It is unfortunate that no stage oti this model a<lmit« of the 

nee of the finger U> aid in reselling the focus. Thin genttn tilting up 

.of tiie otijeot, aa we approach the focal point, would uve buudreda 



of cover-glasBes and ol>jecUve fronU ; atul we liave rauon to knov 
that not a few are broken with tlii»i foi-ui of sta^H ; but wv hav« , 
iii!ver M<oii [tut foruaril, and do not know, a siii-;le reiUMm in juxtili- I 
culioti of a HtiiitU aperture iii the stage. 

Another im|Mi'tant point ih the nbiience of rotation in tbeonlitiary 
Contiii«utal fitaiiiL 'iVne rotation in n stnctly Englinli fraturct, liut 
iu value U great, and it is nn indispensable adjunct Ut practical 

Messrs. /i^iss havo introducrd a substitute for a rotat^- Stage; 
they have doao this hv makin;; the budjr uid iimh »i>li(l wM liie »taff«, 
»a that the whole rot«t«a together. 

Pcin-tiiMilly ther» i» only cnw point in favour of such h movement, 
and that is, thnt tho object rtwnainn fxnctly in the same positlou taj 
regard to thn lipid. But against this anangcmrnt th«re is — 

I. Tho liability of tlirowinf; Uic optic axi* aliovn the stage out of' 
ovntrc with llmt ImIovt tlui stage, and tliis tiiaugh thv' workmanHbi{» 
he, as it is, of thi- lii^hcKt onl«r. 

^. Tbo rotation of a oiicroacopn object for onlinary oxnminAtioii 
i« really unimjiortanl, uh thero can bo no top or hutUim to it. Kven 
for oblttiuc i I hi 111 illation it is not roijuiTml, as it U alwnyii cAsi»r ti> 
rotate tli« illuuiiiiatinK pencil. The only iiiManora in wbiu-h rotation 
of tlie object la iiuiiortunt are : (a) irAc-n lit eitftial i* jKi/arifi, and 
then it Is a dislinul disacl<ra»taK« not lu be able to rotatn the object 
independently of the body which csniee the uutlywer. In ihort, tli« 
sta^e rouitinf; iTidepeiidently of the body would be jnnfi-rnhli- hccausi', 
if it is ri-i|uirv(l to rotate the uhjccl on a dark polariKrd ticld. tlie 
polarising and aiuilyaing priaiuacin be net nl tii* jint/irr amjlr*, Aai 
tbi^ t)ii' object rutatdd witliout disturbing' tiie ncliitivo positions of 
the pri.tiiis. 

Hut this cannot he done with the arranxeineiit of t1ie2ci>i« model, 
which rotates )>ody and stage. 

(/)) Fur /Ji'it'i'iiiirr^ni/iJii^ jiur/Kitft. — In this case in Ihv. Zeiss 
Ktatid the head of the line-adjuiituient screw ia gaun-d to tiip focussing 
I'od : so, manifestly, rotation of the body becomes it[i|H).Ksi1ile. 

Dms, by adopting rotation in the fomi obotieii, the luglicKt end* 
for wliifh the microscope stage tlioMliI rfCuU'r catiiiot be itccumplisliix). 

Tht fuh'glityt is often quite wanting in tbe coiniuun Coutincntal 
forms. This was true of tlio flartnack stands, with rure cxccfition* ; 
tlie Nacbet iastniments were proi'ide<l with an elementary form. 

As we have seen, until quite recent times, the ei>n'len*tr %om 
rtf/arrffl on thf ConliitfHl as « »«»r^^iiomi, j/" imf it fuolUh a/'iiluinfr, 
but that prejudice has bti-n killod by the ll<|lit thrown on the whole 
question by (l)the chromatic (ItSTll), and now ('2) the aehrotnatio 
condetisor of Alibr. Hut even a r.,mi>ouHii condenser was in use in 
England in the year 1G91, and tJie licst work in England unoe tlie 
invention of a^ihronintism Iiiia never been done without onft. 

In tbe mounting of the Abbe eondcns«r every posuble ingenuity 
has l)een displnyrd to inakii it do its work without a sub-fltaf^ ; but 
a [lermanent centring and focussing snb-stage into which tliia optical 
errftnp^nirnt could, ninonytt olinr, lit, might be made for biilf tlm 
labour, ingenuity, and co«L But rather than this, we have tlie con- 



dmser iiuut« to alKle on the toil-ptvor, &ikI to Iw jammed with & 

/( iku Aer^ore n*ithrr eentrtni/ nor/anuviHt/ ytar, but, striking 
u it inaj a|>prar, » duijAragm, whicli cniinot l<e uM-d with, nnd is 
110 part of, the coiideiiser, M rnjipfie'l tcith mfthttninil n-nlriHff Ami 
rack-work focussing moveineiiU ! Th»t in to »y, /Ar tlrlimlr r^fnirr 
irfaa o/ttieai combiiuilion mny tnk« cart of iUttj: but ii (liiuilimgm 
aperture most be centi-ed \>j ineo}iaiiisin jiikI focuMeil bv tacV ! 

We know titat the idea titvolved in a nek-vrork flikphmgin i« 
tbo gmduntion in tlie angle of the «oiie of illuuiinatioti from thu 
pkne mirror by racking a certnin MieU diaphi«gm up ur doivn. 
Bot this oai) be better done by an iris diaphragm or, more ]>erfectly 
nill, by a wheel of diaphmgius. 

Now, in reality, notliiiig i« so important as the centring » ml 
focttcsing of the condf-nwr, afbrr wo am one« provided with perfciH 
ohftetivM ; so tluit wliiltt the iric diaphmgrn or a rotating whe^l of 
diaphnigmA would mert all the ca«o of the racking and crntriiig of 
thna^ then u nothing in thi^ br«t ttandx of whnt is dnubtleiis t)io 
Urgest and mont fnlight^niil house for the manufnctuix^ <>f micro- 
KopM in the world, tu itupply the iiidiiipenMblv lufth of tii<! inixjent 

We olnervr with pleasun- advanct-n in every din-ction to wliich 
wt ^11 ntt*n]tion to dnfpcts. Thv more recent instruiiiirttts an 
laarvelN of in^-rtnuity ; we pniM>nt, in tig. 163, the latf!«t and lincst 
form tif Zi-ixx's Im-sI iiiifr™M:ci|>is 

Therp is no fault in thr workmnnjihip, it u tho bp-st possible. 
TJif lUtir/fi vi*tt/ in/iiNlly : there i* nnthin;; tii comniand conitni'nrlii- 
tion inany |>artof the model ; and, KeejiiKthnt the McxArs. Zr'i.s.s hitre 
now progrefcod m far a* to furnish (heir lir>t -(^liis.s etatid with thii 
Engluih meebnnical movement, and rvfii stagi' ii>tiition, we can iuit 
balwve tli-«t tlieadvnDlageKOf tbi-w- iinj>n)trmentA nill iiinki- [>tnin 
thvg^nti'rmtvniiljksi- thiit would accrue from an i-nliivly nevr model. 

The Furchue of a tf icroioope. A ilcsire to jiosiieNs a s^no<^ but 
not co«tly micniM^ipir is rxtivmrly common, but an ii rule the init'nd- 
ing purchaM'r bus little kiM>wleil)|;<! of the inatrument and docs not 
pnifewi to know what nn> theindlipenuiblepurtJiof Kuth ainippariitus, 
aor whjit pnrta may, in the intemdK nf economy and hia ajieoial ob- 
ject, U- diintiuMl witli, iKLting him atUI ponessed of a sound and 
wi^lt-iiindr inxtrumi-nt. We may lirieHy ntnaider this matl*!-. 

Tlte lint qui'Htion to be asked when a microscope is to be pur- 
clittaed ht ' What is the orrler of importance of the various parts of 
a mioroscu|>e I' In answering this (|uery it will be to liomo extent 
true tliat nubjectivity of judgment will appear. But we believe that 
the followin;; (able of the relntlvc order of imjiortnnce of tlin pnrtx 
of a microoMpe will commend itself to all worki-rxof large ami broad 
experience :— 

1. A coarse adjustment by rack and puiium, 

2. A sub-stage. 

3. A finoa^ustment. 

4. Mechani^movenienUtosub-stJ>gt',i.«.f6cnningandcentring. 
&. M«chaiucal BtAgo. 


*. RAclc-wt»rk Uxlrnw-tubc 

7. Fin>l«r tu stage. 

8. Pl&iti rotniy Btage. 

9. (ImdavthMiand ndi-wnrlc to raUtry nUtgo. 

10. Piti« iu]Ju)itin«nt ti> aubntu;;e. 

11. Rotiirj Hub-stAfte. 

1 2. Centring to rotary stage, 

Tlii* tahle jjiv-ed in order the rel'itive xiiltji-sor tlti- Jifvijrni jmrts : 
MiVA, a nm-rosL-ope vritfi a r«ck-iiiKipim<m coiimi' mljustnwnt nnd a 
sa)>'ata;,'e Ls to be jiwferwl before a tiu'croiioojxt with a mck-and- 
piiiiuit coarse adju^tdtetit, h _/i>«c lufJHutiiifni, but nn ruh-iUitijr: Or, 
a niuToacope witJi a euariie aijjustineiit bv rack auil pinion, a Kub- 
stagey fuid a fin« a()JuBtineiit ii« tu tie preferi^ befuiw imo with thn 
■UM oonrve (Mi|jlUt&ient and a niecliaoicnl &ta^ uiovomeni, Imt 110 
cab-sta^ or fin« ac|ju»tnk«nt ; an<l so 011. The last item is of Inint 
importance^ and the iinpartaiico of all the othura is in tbe onler of 
their numeration. 

Anoiber matter of si»i)o si^iilicance to tite tiro is the relative 
Taloe from tho point of view of tiino i;onstiiiie<l and, tliei'efoi'e, of 
primn cost, in producing tho MvemI kinds of inii-roni-opcit. The 
No. ! standi of half-n-doxon nioki^ra may lie n<-nr the snnie cost, hut 
may ni-verthrlcKs havo involved tho con«iiinpt)on of very dil]erent 
tpuuititica of tho highest clius of s)>ill(Hl Intiour in their production. 

Manifnttty tho firet tiling to Im look^^il nc in a mi^roiicope making 
any prrtcnfiions toi^uality U tlnrMoi-d'/'.- uf iltf in'rimnitiJiip ; and 
this should carry witlt it the <iaostion, hour niucli >niu-liine, and hovr 
much baud work and litting. tjiciv is in ii. Aivsgi'uduated on silver, 
for examjile, are very atttnctive, and wilJi iiuiny are most inipres^vo; 
but tliey are simply marhlno work and 'juitt; inexpensive. 

Ill tlip two givat typtw of moflels tho hnr movement and tlio 
.lai-fcuin liiidi, the bar »>i)vcnient invnlvc* more Umn double the 
itclual hand tilting ; while n rinl^ adju»tmrnt with a movnhlff nowv 

Ci(s-« tak(» twice the fitting of ono in whi. h the w ImU- body is moved 
y ihr tiiie-adjuMnicnt iKTrw. In tlio tamr; "any n iuf--lntnirfit tl'ii/r 
whichisnuidcof maclun^'plniicilptatn^ slUlingina machiiir plnnghnl 
groon.', in much Icaa costly in timt' and i|iiu1ity of labriur tltnn a hiind- 
uiadp aprang Ktag& 80 a i>7i/'-i>tir;/r ]iavin;t a moviihli- ring prmivl 
)>y two Ki-ivwt agninKt a cpring has virry far Ii^h work, and work of 
a lowpr L'liiaic, than ow with a tnin nvtjinjiol'ir i-entring movennmt. 

It will folkiw, tltcn, that a Jnckiton-liiulH-d microscope with mt 
niorablc noa^-nk'ce, with a marhincimwle niecliutiical aXii-^o, and a 
nun'aUc rinj< for sabulai^', will not have involved inoiv, perhaiM, 
than a third of Mm nkilled work which mam be expended on a well- 
■Muln iiuttruinent of the Bime aixo with a Imr movement. Dut, if we 
com|Mre the niige. of prima an ]>reaented by English and Amerionn 
makim, we rarely tind an <>(]uivaleiil difference in cost. 

T1m-ii the tiro will be warned by this not to puri-haiie api-etentiouH 
instrument with u bur movetueiic and mechanical stage for, Nty, 'il. 

Bat t/'rt tnu'-prinvl intilruui-tnl t* I" fr* /luivlninfl, if, aw is almort 
certain, it bi- a Jucksoit model, Bee that it hns n rack-work i4>arw 
adjnaUuent, «fichew Uie a/mrt-Ur^r note-pirer, and liavv a diilbnntittl 


■crew fine Adjustment, n large plain tUga, and ta 
centring sub stage. Such an instrnment alionld ba ofatiAl 
5t. 10a. 

Although not frequently used, it would be dwng our «| 
perfectly not to refer to a ipMul filEB of aiiorOMMipa ImI 
ehemioal purposM. This is an inverted microKepe oamm 
structed by MM. Nachet on the plan deviaed by Dr. J. Ic 
Smith, of Louisiana, U.S.A., for the purpoae of viewing otged 
their under side when heat or reagents are being i^^med to 
has lately been improved by its constructor with » specUl 1 

T, whicli nuAM or lowm Uio ^tiige. Tlw pUtfortii on which tlu> 
upticoJ KpmmtaK n^«tl) ran ))p inui eil in redaristiUr lUrectioiis \>y tJvn 
two Riill<-d hi'iulH, <i, T, «ik1 i« furnished with iwo «rittclu*l«<l scnifs 
\*j nmnx nf which it niay be bniught witli ox>ii.-1ii«-«s into Any ]>OKi- 
tion prfivifrtuly recordu^ no thnt any point of the oVij«.t may >)© 
idnmrdiiitvly nr-found— an nrningimietit of cpeciftl viilue in ciiUivntKHi 
«xpc-rini«ntM. On the iiXtige is « vircnliLr glASs cpII, C, for holding 
ibr duid to \te exaiiiiiiMl ; in the bottom of tJiis is an a|)«<rtDrr wbich 
in i-lm4Hl )>y a piece of thin oorvr-glttHs well coniontiod round itx 
ittgps. thus nllowint; tlie of hi^h initgnifying powern hnviiiK a 
rpry ■Jiort fionu; vliile ita top is ^Tonnd flnl, »■ thnt a coi'itoI thin 
pluo giMut may l>e cImcI/ ti(t<;d to it liy the inlon-fiition of n. little 


111/* IkbttMiar; inknHMiM^ umiI tor clientlcftl work. 

^c«uw or Klyi-*riiie, \\m w1io!i> Ix-ing Mocurpd in it" pliu-i' liy three 
Mmall nprif-tila. The cell is furniuind nUo with two aitiitll jiUns 
Una, R, R, ■■itli which iiidinrublK-r tulK-s ai-e conninTteil. By this 
Mil — which iii»y be mwle to mtm- a> » tiioiat, n wann, iinil n, gtin- 
chauilier — experiments on tli« rarefurtinn nnd cun)pri.-uiio» nf air, 
and on tlie niMorptinn of pucn, <«n be luiidf w ilh Krcat fitcility. 
For 'culti^ntioii' experiments wimlW cell* an? pmvidfd, which ore 
altAchtxl to brus pl*te« ao iirnuigcd ob to liavc k lixed position on 
tliic Htage. 

Tbe Bauaok anri Lomb Opticid Company have now eonAintl th« 
fi&tive *eith Ui^ vniinary rrrliati /unit of microtcifif, the principlm in- 
«xdve<] 1>eing (they bolMn-e) entirely new. Tliis fomi of instrument 
ia particularly otUptcd tax chemical invest igatioiis, for the reason 

Tia. Ion. — Tlia Minr iunlniniont clmngcj inio an otHlniLry [(inn. 

tli*y iiclhMt! to the Burfni.'e of the blid*, imt\ are thnnrforo in •>«» 
jilaiit.-. It iH iiU't un «xi'eUeiit dUdei-tiii;; uiiorvwcopt-, iwit inpnrtiuJIy 
crvirtlnK, ofTera iiu liiiHtnince to inaiiipuUtioii with nnv piiwer, and 
uiAki-H it eouvMiieiit lo oliBPi-ve the objert clircwtly. TIhto urn two 
foriiiB, tlie ' Lnborrttoiy' and iho 't'nivereity.' This Lalx>rnt»>py 
niioroscDjM, wlieii use<lasun invent instruinnnt, in Kh»wi) in 6^. 165. 



Till! iiiim>r'bAr Hwiiijipi on nii iixU in lli<^ uliini' of tho sutge to noy 
pdiiit aliovv (If below it. The iiiiiix>r nml Hub-iitagv aro ndjiutalile 
va tilt- iuim>r-lmr. 

Tilt! Kub-alAx^ vnmm a rvmilvinn^ (linpUru^n, nnd ix dxrd on a 
pii^t, HO Uiftt it urill nwlnt; tn ami oal of tln^ optic iiKis, Allowinf; 
tii« iKilHiiser U> he aitai-iic<l nii<l r«mly fur inKtniit ukc. On Uio slide 
ia the anu, to the luwer fcicle i>f wliit^h is fiutojivd tliir priKni box. 

Ou tli« upper borixuiitul aurfaiv of iliis in IIm ii(HH-]>in.t-, with an 
«xtn wtapter fur tii;{b powera, ami in ilu! obliijUL* nufiicc in a «crcir- 
MckH for llie l«o«ly-mw. 

T'> tr»n&foriii the tiLMrunieiit into an <inliiuirv uiicroiic'dpc (fi^. 166), 
the tube ix uuscnweil, tb« milleil hrail al the front <if tin; itnii 
loueoe'l, which reJeamw tho pri^in box, and the nriu is nwuii]; i>it its 

Flo. ICT. — The ' UniienHjr ' iDiFroKiipc m a chemietl init niawnl . 

Axis frt'tn lirtwrcn the piUars into an uprifcht iWHition. T1i<? tulx^ is 
now nttnc)i«d u> the opjicMciiA niile of the noae- pieces aud alter th« 
ifflp-'ClipK nrv revetwinl it U nsidy for work. 

The 'Utiivi?r»ity' uiicmacope (ti^ 167 aud 168) is in its gencrol 
coiutnicti<in niitiiliir to tlie precMlinftt except llial the (»inglc) pillar 
unci tliv iinn are not japutinetl, but are of ItraMi, and t)mt thi; 
tnrtniniMit nwiii^ on an axin whii'b is the Baiiic as that of ihtf 
mirror-liar. The Mage ooiisista of a ^lass plate maunt«(l in a braas 


The pri^in um^I for inversion is that suggested by Mr, J, Law- 
rence Smith in li^ill, baving,' four fiices, with angles of 57°, 150*, 
48\ an.t tO'\ (Ik- raya l>ein- twice totally r«fl«Cte(l. 

Tank microacopea (slao cnlle<l oqtuiriutn niicroiic<ipe«) hare, for 

low tliR front l<!n« of 
thn iibjt^ire^ «t a 
propter ilintAncP for 
tliu fiicuK, tnay (lira 
Iw plunged iiit^ tht-- 
a<]Uuriuni. Indr^nl, thf 
tube of U)« iiistruiiii'iit 
may W so prot<vt<il 
lis t<) vfiirk fur wmii* 
tieptli, aiul Iwvtf »ouiu 
n>nK» ill the w«t«r of 
a ifoiHl'&ized ta»k. 

A beautiful ttutru- 
uieiit of this olaa& haa 
l>e(Mi <teviB«d by Mr. 
J, W, Stephen san fur 
the «sa]iiiiiatii)n of 
livitLg objects iti an 
Aquarian). A brnaa 
bar U laid across the 
aquarium, as showo in 
the w,>otkul {fig. IC'J). 
Toiidiust it toaquari« 
of different widths the 
support on the left ia 
Tiinde to slide alutig 
the bar, and it van 
1)0 damped at any 
given by tho 
upper millnl hcnil. Tho 
II 111 led head at tho 
side, by proning on a 
lonto plale, fastens tho 
hnr sf^uroly to tlio 

IVtVi-tHrn the enila 
(if tho \mt slides an 
ami carrying a sprung 
siickrt, and the arm 
can 1x1 clnnip<!d at any 
. givttn point of tlie imr. 
Through the soclcot in passed a glas? cylinder, oenientwl to a liraaa 
■-■ollnr at tho ujipftr end. and closetl at the lower by a pie«! of oover- 
glass. Into thin cylinder is screwed the body-tube of Hip inicrtHcope 
witli eye-pitwe and objective, which are ihu* protected fmni the 
vat«r of the aquarium. The microscope is fucubscd by rack and 

I';,.- 111-. 
Tba ' Unirartit; ' mioreuupn flxcd (OT UprigUl um. 



piniiin (iiiilK-d hmd jost bolow ilio «}-e-{iieiw), niw] in addition tiut 
U>jM)tiv« istcrt-wwl to * d raw -tube, m tlutliUpoiiiti»n inthec}-Iiiiil<T 
!B^y be ap]>Toximn(Rly n-gulnUvl, 

Tbtf arm uf the uickitt U hinged to allow of tlii-> nncrtHnipe l>eiii({ 
inclineil in a iilaiie piunlld tii tho siclcis u( the iiquiiriuin. Tlie lower 
millMl Iteail cUihih tin- linigr i»t any desired inclinittinn. 

The Hiitket iiUu rotati-s on Uic nrm, so that the uiu-rawMipc (i«ti be 
iacliDed in » pLuie panilUil li> tlif fivnt of the ai[UAriuin. Tliun any 
point of the u^tuntuii tun lir rrncliii^l. 

A very convenient fc)riii fr)r siiiiio clasjie« o( botanic*] work with 
\'«r)- low puwens itiid alxo for niiiini-juiii work of a geiitind kind, w«a 

r « 



^ jtf 



/ ^'" 


Fiu. tea. 

dAvitrd W Rom nnd Co. It mny 1i« cither monoi^uUr or binncitlur, 
and hnit n mck Riovi>inent on a. hiiriKontnl luir, ^'iving it coiit<id<-roble 
tntigir ; unti n riu'k focnl inovcmciit on thi- Ujtright bar and on 
the kcKly, with nn nitililioiud ncivw niovvuintit in a i)ircctji>ii to aiu] 
from tlw iibsrrviT, »> tJiat aU llm princi|ud nxiveiut'nla tti« arr«iii-;i!d 
in its construction. iLt gfnrnil I'tiunwlpr will b« undemttH"! by the 
iUoBlrnlion pvfu in lig. ITU. It is extretudy useful in the gcnoml 
Aiuly of BDuli tanks. 

Mr. C. Collinn's iMinnriuiu mifroncope (ii({, 171) differs from all 
other forms in that il u ajiplicHl tu tho oide ol th« a<|tMi-iinD it«olf. 
Thin ia«oc<>iti[>li!ih«l by tunkint; me of n simple piiL-umnticfippfinitus, 
The hcud of the ' Nuckvr ' ia vbovrn cu tli« left of the di-awing. v, ith 


»n iniluirubbor riiigsuriviuiiding a central pUbm. Tlic- ring iKftppUn) 
to Uie f^lnaa tturfrice ot tlie aqtiariuni, and the air is i>xhaa«t«<l hy 
■crewinK round the hwiil of tlio piston seen on tbp ri^Iit. Two tumx I 
are sufBfienl to fiist«n the xuekcr wcurely. The ithI to which tlw 
HUpport o( tlie bodj-tubo is ntt»ch«d pjinseH through the Kurkcr-onn, 
iinil t-aii be elauipei) nt any height <lc>iii-e<l. 

Pi'ofessoT E. •Schult7«hn«<lc9;ignedari(l UeeAT8.Kli>nn«niicI Tktiiltrr ' 
hftve made the micrOMmpe, tig. 17l', for the olisen'atjuD of Mnall iu]iin- 
tio organisms in on oqunriuni spceinlly oonstruL'teil for th« puqioM-. 
There are tlii-ee pnrts : (1) the mland, the greater )i:irl ••! whii^h is ', 
nickel- plat ocl ; (•) the nc|unrium ; (•*<) the ilium timtiii^' iiiiri'or. 

i''iu. no, 

The stand vunsi«ls essentially of n ntifrosr^ope-tuW, wliiuh is 
supported in a liorixoDtAl position upon a trijxHl in sudi a way that 
it on be moved in three ditTcrmt tlirei-tions by rack und piuiim. 
The colaraii of the tripod cnrric* a nifk mul pinion, by which the 
tube ts moved verticftllv. Ontln- lulie whichoarrieathemck iaasliil- 
iii|{ pieoeuith n seconi^ ritrlc for the horizontal movement from right 
to left : upon this slide the microscope is 6x»l in a horiiotital posi' 
tion. an<l can be moved bnckwnrcN and f()r«-ariU in a. tube jirorided 
with nu-k and pinion. There nn- thi-rcf'trf llirec moiement* — verti- 
cal, horixoDtal-Iat«rHl, and hoiUontal-vigittal - bi> llml the orgnuiatu 


ofasxrvrnt cmn bn follonetl hy tli« tuW ua it moves upon (he glaiw 
wnll t>f t]>o a<|iLariaiii. 

Th<' iu|nnnuii> (nnsistc of n eUiid witli n frHni« whkli otrriM tltiv 
•qiMiuiii pro^r, 10 cm. in lircndtli and lieiglit and 10 mm. t» 
ihickneu ; thu inny be n-plncctl l>y otlipr«. The frainei is mail« of 
Imiat lAf.jui'rt^l I'lnck. Iho nijaariun) itK«lf ronuMti^ of » hoi-sMhoe- 
ibapttl [>i<V4' of gU««, holJi rion of which nm i-losoil l>y plnles uf 
ttirtrT-gliuu, li'itving t he iipppr «id open. It is t liux p<«sil>lc> to olMene 
an orgiuiinni upon trithir of tlic twti thin sid^-s with nn ohjiyrtivo giving 
nliiwnroiiipliticfLliniiof L'UOtoilOO. ToscrrcnolTthf sHpcrfluous liglit 
niwi tht' nuiiicrons nllirxions in Uw? ni|iioriiiiii, thi' frnmr <»rriM n 
(Uftphntgiti aimngrmont which cad lie iippltiil on «ither xtde >t 
plrtururv. Diis ooiwiRts of n Kliding plaio nhivh uiovra the tiro- 
wiriaontiil guidea ; it is diviiLcd into thn-e pnrtk, nnd Uile on oblong 

Vw. m—CoMauf* *qiuriaBi iiiWic«c<>|w, a|>|>&rd Co Uiv tquorliina bj ft 'tatket.' 

(ipening in ont of Ihi' 'tiviuona. In this opening ft thin i^ato Hiidiii, 
»nd rati Ix^ clniupr)! «t utj' point. In thiti pinto again is a cirvuliir 
u|>crtnrp, nhich rim lie closed to n gT«nt«r or less pxt«nt by variouo 
dinphntgnm kvpt in pimitioii hy * smuU spring. 

If on animnl or ollirr small organism is on lUv upper left-lnuid 
ronu-r of lti«- stdu tumeil tuvrnitlii the m)i,-rosui>po, tli« sliding plute 
ifl first diovmI m> that the vt?riii-»l longitudinal opening lion in the 
Ifft-hutd thinl, Ihd nuiall plai« is then m^ so that its opi>iiiii^' lipK irt 
Uw up]WT thinl. Tf. oil the other hand, tlie itninial is nn tUv ri^lit- 
bntid Hide, th« larger stidilii; plate is iiinvnl •.» that ihn longitudinal 

Xning lic-s on Iht- right, and if the animni i* towardx tin? Itottont, 
luuall alid« with its opening is movr«l downunni*. Tim two 
Hlidtnu plaits luv now ui directed thnt light may U- thrown by Um 
tnirmr ihinuiih the nquaritini nml upon Ihr imimnl «n tho front vide. 
The iipcrturc van Iw further reduced by diaphntgins. 


Tht' mirror ib coiieave, 10 cm. in diameter, anil fUed 
Rt-iml with a. ball -Hiid- socket joiut bo tliat it cHn be ndjustcd ' 

An an tidjunct, and odrairoble aid to the student of the ' 
pond, ae well as a. simple and eoay meaoa by which specific : 
microscopic life luaybe be found and readily t.i.keii, we call aC 
to the t^uk microscope of Mr. C. Kousselet. It is illustrated] 
173 and si'arcelj' needs further description. 

One of Zeiss'a Hteinheil ' loups ' or aplanatio l^nisee, to ' 
have referred, ia carried on a jointed arm, which is clamped 
tank,' the tank being nowhere deeper than the ran^ of 
the leus employed. The arm moves on a plane paralli^l to i 

It so froiuriilly luippeiui Uot a minuUt ubjoct is lost tiiiiiply liy 
muoviiig tlir pocket l^na for an insdtiit to bike tip tho pi|><-tUi ; 
In th« nhorn nppanktua 
tlw (viiB TcmMiis in lilt- 
MiiiioM in wbii'li it has 
hetn ^aaed, 

MicroMopM liAve tiwn 
•mnge'l in mmiy w "v ^ 
la 6u-ilitnU- cIam demon- 
ItrfttioD ill mii:rosco[>i<.- 
work, liut wn liuvB oft'ii 
IMMi«> tlwt in inori- simiilf. 
dbi-ietit. ami iiii'X|K-iiBii i- 

tWl ttuit gUKgt-'St'Nl li.v 

Dr. Uc-nk. Tlio instru 

meiit iR infuii^ liy iitljw-liiun 

iuout4^ tutwiMi A wooden 

Mppoit tn H horiniiitiil 

hoaral, wliicli nUo carries 

> stnnll Iiimj] aluelied to 

it in the r«iuir«l iraailioii 

(bg. Iti). Tlu) object 

iMving boiit Kxed in iu pUce, nnd thi- conrw ndjaxtnient mucle by 

(Kding tbi> Ixxly in Uie out«r tul^-, tliejin pnru tnav llicii bo iraiiiov- 

«Uy sccurwl, nothing being li-ft niovwblr: oxcrpt the eyc-tubi*, by 

dMing wUii'li ill r>r out tJie tine ndjuslmpni may be effected. ITiut 

I'lu. Ku, ik.iunriii'ii ri iv|UArruiit iiiirrtiM'o|v. 

Fn. l?i. 

tJie vrholp nppMntus may )>?> pomod fnmi hand to liuitd wiUi the 
grvste»t facility, »nd without any prol»»l>ility of diaarrangi-im-nt, 
and every olwcrxer nay mdily ' fo<-ux " (or himself, without any risk 
id [touring l)i« object. 




TiiiR chn])t«r on np[Mknitiu> iiccrasory to t}i« micixwcupn iniglit Im 
pjisily iiiiults to (iif\i|>y the wlioti? of tlie jpatf we propose to (liivatfi t<> 
tlicwitirpreiiiniiiitorof tho l)ook ; th*> inj^nuiiy of succecKJvi! luicro- 
Kcopisuamltlie variety of comlitioiiepivsL'iiled by aucoe«(iiv« Improve- 
inriits ill Hie mioi-oscope itself Imve i;i%cii ori;;iii to wch • viirioty 
of Appliances and acCTSsorj- nppni-ntUH tlinl it would Xtt- iotiU: in n 
practical Itnndbook to attempt to liguni find (l«6crilie. We [>ropo«', 
UHiri^ore, only to dcscrilje, ani! to rxpUin the tuode of miccesnully 
vrnpluyiiig. the es^scotial utd the bcM nccetaories now in use, tk'jflect- 
iiUC. <iv iiiily inoidcntnlly referring to. tha«o whicli are either sup- 
plixnled, orwhirh prrsfnt DioditimtidiiK ctthernot iuiportaut in them- 
sclveii, or accouiit<-d for l>y tin- fiitt of thdr production by tliflfcrenl 

I. Uicrometeri and Uethods of measuring minute Objecta. — It 
in of the utiuoit JTiiportjinif (■) Iji- iiMi- uitli iit'ciimi-y, and as muck 
tnnijilicity as pnsiiilili', to iiirnsiiic tlir olijects or [mrts uf oUjccU tliat 
are visible to lis through the niirrowojip. 

The simplfiit mode of d<iiii;j this is to project the inaB;niIi«cl 
iniOK^ of tho nbi'ict by any of tlir iiiHhixU descrilted under ' Camem 
Lucidu and I)rn»injj ' (p, L''t,'(). If wi« oftrrfuliy trace an outliiiit 
of the intagp, nml tln'ii, without diaturhiiig any of the an-angeuu'nta, 
reiiiove thp "hjfvit t'rcini thi- rtniCf, nnil rfjilnco it witli n, 'sta^c mi- 
ciwiiet^r,' which ii MNipty a stiji of tliin gliuui ruled to any dmiii-d 
scale, such as Iriithx, liuiidnHh.lis, tliouBandtb;^ of an inch and up- 
wards, Tnuv now the pi-oji-elt-il imnjfi- uf thiii upon the aiiiie paper, 
and themmnKarcnt onct^ l>efore lis f'lrniakingnroniparisoD between 
tlip ohjrct and a kiviini umlr, both hnioK titagnilii'd to the same ex- 
tent. TIk- amount of nia^iiilication in no way alTccIs the prahlem. 
Thufi, if the ih-awn piiture of a certain object exactly tills tlie in- 
tiirral 1«lwe(-n tlut drawin;; rejireseiitin); tint 'til inch, the object 
nuMwurc]; the '01 inch, ami wlii-ihi-r we are ftnplnyjng a nia^iifyii>)( 
piiweriif a hundn^d or a tliousand dianu-tiTs is not a factor tlutt 
vntcrs int'i our dettTiinnntiuii of the site ut the rjlijitct In fwA, all 
ilrawingx of mii^runcojiic objecta are reiidemt m\ifli more practically 
valunhle by having; the ina;;nitled scale plaoml benc-ulli thi'Ui, 60 that 
lunasurnmentii limy at any time lie made. 

In favour of the above method of micro- ni rami irmpnt, it will be 
notinl (l)that noextra apparatus is re([uired, (2) tlmt it la extramely 
Himple, and (3) ibat it i« accurate. 

The most ethcient piece of appaiutua for micro- incasurenieiit is 




-irithoat doubt tho wntKw uicromktkr Grs-PiEcs ; it wiw iDVCntod 
hy R&mjMlcn for t«)««po])n°, tind if well i-oiifttru<.-t«(l In u inaat vnlu- 
nbJe adjuRct U> tlw niM'i*oii>.*opi.t. It 18 iiiimIu by Htretubin([ acraKC tlio 
field of nil ojcv-pipiie tvfo extremely liiu' jiat-all^l vrirai, ouh or lioib 
iif wbji'li PBH lie sppATntvi] hy ilic at-tioii oi n iiijcroiiietor Morww, tin; 
■cirrmuftirrni^ nf ilin brius hmul of which is divided intoiL<.-oi)ViMii«nt 
nuiu)»-r of [Mirts vliivh «u<\'Ptsiv<>ly pHss by an index a& ibv iiiUImI 
h«iul is turnfd : it is seen in tig. l'.*!, B. A portion uf tbe Md of 
vivw on »iip sifln in cat oH*Bt n)^ht angles to the tilnments by a scale 
fonncd of a tliin p1nt« nf bravi linviiig not<-hi>i nl itti eiige, wbose 
diBtan<w I'DiTtuipoiulK to thnt of tlio tlin-ads of tli« screw, every (iftli 
notc'b b<Hng iniulc dnrprr than ibp rr-^t tn iimko the work »f enume- 
ration Mwitft. In tlw originni Ramsdi'ii <iyf-inr''c one tiliuiietit w«,s 
»tatioiiar>-, iIm: obj«vt hi-lng brought into Mi<:h a pntdtion thnt one 
nf iu ettjjtst iippeaml to tomb thi- lixoct wire, the otiier wiro being 
Moved bjr th« niicrotix^tt^r s^rew until it npfMMrs to lie in oontnct 

vilfa thv othni- <Hlg<! of ttio objiM't ; tbi- number of entire diviuoiiu on 
tlie scnlo thi-n ]dio«-K liow ninny conifilttti' turns nf tlii> Hrrew liml 
liwn ina«l(! in tbo trjiMratidn of the wirrK. wbih' tl](> number of index ■ 
jKiintii on tbe isigi- of the inillfd lii'iid aJiows thr vului- of llie f luttjoii 
of a turn tliat may liavt^ Wvu made in lulditinn. IjKuatly a si-rew 
with 100 UiriNuU to tJu! indi i> cniployi'd, which gin'K to oaob ilivi- 
lion in the lu-al*- in lh« «yit-[>iiH*H ilia vnlu*! of -|,'in^ of an indi, 
whiUt tbe «<l)ee of the milkd lieatl ia usually divided into IOO jiarls. 
Botli wires or liUineitU have taw* Iwm made to mot o, a «. rew 
and (liviiied h<t*tl betiiy fixed to the wire iJiat RaiiiiMUm inadf 
Btationary. Tliere is no advaiita^ie in tliis fitan, nnd it invo|vi-K 
nHedlf^tx complexity in caloulatixii. Tlie be* method, thci-e .an be 
110 dtiubi, is the one einpliiyed by Mr. Nelson, wliicli in t<> buv<> one 
ibreai) tlxed, but ual in the rcN/iv of tJie eye-pievv. hut live notches 
iu tlie Mrale from tbe centre <>ii the «de furthest fn>i» the utcw- 
liead. Thin not only [lemiits of a niuch larger objivt being lipamiiH). 
but al&o ket^w tL« average of mmturenient.'; in tin; middle »f the 
Q ' Md.' This is not only convenient but iiii}i<>rtant, bc(nu«o tbe 
I iiBBgtulictUon U not uuiforui tJiroughout tbv licld. Jf tho poww 




emploTed is high, in order to «fiect the man of tlie gnat 
tion, one wire (the fixed eentrtU one) will be in the nr' 
field, the other at the m&rgin, and the cwmpuiaon will 
on account of the unequal magnification at the C|y»-|iii 
ont the field, whereas if the wire be j>laoed fire notchea on 
both measurements are brought more within iba centre tit 

Messrs. Zeiss now make a Ramaden mierometm' 
provided with a glass plate with croaaod lines, which 
the eye-piece are cnrried across the image (otmed by Uw 

Fio. ITD. 
by meiiiir^ of the mensuritiK sci-cw, so tliat thr ndjostitient J 

SELatSiJ tiCBt 



Th« flal iiirirr ))os liM a screw nttacliM] U) it uliich vngo-^-a vhh 
m h^tul on tbe i-xtcriitr of tht> oul«r box. ThJii ^iven atMiul one uic|l 
■if «-rew iiiiiwiiH!til to lh« innvr box, vliid) <auMB titt webn to 
tiuvcrw tli<; licUl i>( th« mii-roeoopo. It iiiiihi be remnmbrred that 
lliui in no wiiy ndVscts tlir inovciiieiil i>f tlin inovHbIt) wrIi from the 
6xn\, whkb mil a](Mii- Iw nccomplished hy lui'iiiuj^ tliv gmdaated 
ntii-rou»-tcr )i(«(l iia in llui old fonn. 

Th*! ' cyc-pimM' luliii'tcr ' portion «l th« iiintninicut i», lu it« naiDA 
itaplics, iiionJ}- nii Bdajitei- to tnkii the opticiil part of ]i»t(iijv« c<im- 
[Ktuatipg v^Q'pircet <>i various powvr*. 

Fill. ITI.~' K*kcp'> sew iarm ot tav* nucrnniflc 

Iinni«.ili*l«ly billow tlic «*<■!> Ls nii iri* dJApIirntpi). TbiK jM^mntci 
• <linphnigin In Iw umiI Kuitnblo to i\tri pownr of tin* «)'<■- pici-u 
rmpUtyrd. A guitlint; linp at right angli'* to the webs liaA rH>eii 
dribiol. Caru luuit Ih> (Ilkl^n to obMT\'v tliul vilieA tlie i»ovabl« 
w«b coinddes pr«ci>i-lv willi tlir Gx<hI vrub, tbi- indicator on lb« 
gTMltuitMl \tiMtl ntands al xeni. If tlikt is fcit tlu! ca.->«, the tiii;;er 
HfTPw niii><. Ixi )ikv<mI, wtiicb will li1M^ntt(• the Ki^aduated h«a<l, and 
ihi-n it rati lf•^ pliu.-«d ill ila prot>er pcuilSuii and li^otd. T)ii« j.i of 
nnivrr^l a|>]ilii-Alion to all svivu- in)croiuetei>. 

Kour |niiiila are (gained by tbis aiT»iij|[emwil : — 

(1) Ttie oiuipeiiuittiij; t-y^-piM-« yi«ldn fsi' )it^tt«i' (K-tiiiiti>iii wlieit 
timwuiinK witlmpocbmiuatio objective* tbiiii eilliei- vbti Huj-^honitui 
nr R«uKd«'n funiis. 

('J) DiRi5i«nt-powH^ e}'e-pi«cpa i-an be employed. 

(3) By ine«iia of the screw which moves tbe mirronieti-r wi-h* 
ncrOM the lield it in possible to perfonii iiien«areuieniswiib tb'.' wi^ba 
ii|iiidistaiit from the centre of tbe field, and thus rlimiiiRtn errt>ra 
■lav to diBtortiun. 

(4) Tlie precealiiig advantage is secured wlthuut au-ntioing iha 
Jienetit of a fixed ^jero web. 

To M*» (A' fmr mirnitiiriff ir'iV/tftKWMit slii>ul(l nut ImiuMifrlpd, 
*a tbe mrtom hn« Iwon. like «n ordinary rye-pim^- into the toW of 
tl>e mii-rosoope, hnt t* fJmul/t hnrr. n jii-in »Uin'l quiu infle/irtut'iil/i/, 
fireventing actual ermtiu-t with tlir^ body-tiilje. 

Plate II. gives tlw; modi* of it«cinph>ympnt, the ill UHt ration Iteing 



made from » photogmph hy Mr. N^lwin. Tli* niirrotiictcr cyp-piewn 
it will he Rwn, in litt^d into a Bland wlinlly indrprndtnt ot tW 
mlcTOecop^. This connists of a sti-ong upright, litwul into n uiaHiiv* 
tripod or circTjInr foot. Tiio fiiot in rithci- tunc Mily rest* on three 
points; tlip ni>rigl!t in citpnhlc of t4^1l>scopi(- OKt^n»ion l>_r n clninping 
bub» ; a short tube whivh tukes the nye-piccu u. 6xttl to tliis upright 
1>y-B coinpaxs joint. 

Tr> uw it tho ohjcrt to Iw mwisurwl il p'*^'^' '" powtion, imd 
the inicriuc^pi' iiiclinnl iti the UEunI way. Th« oitUnary oyc-piem 
is removed, uiid tlur .tcpnrntr ittnnd with tin' niirro) lifter in it.< pUor 
is put in fnint of tW nmrroiwopc'. tlir rxtcnsion tulu- liciiij; iiiistil ur 
lowritid until thi* titbi> at the tnp of it, cai'ryin^ tin* miciniiiftiT, ix- 
nimlc continuous with the tulip of the niicmi.n-ojie. as wen in iJif 
ciruwing. It in. well to li'avt* fitmi Jth to j\,^ th.* of an inth of spacw 
LfVt.neiMi tlie iKxly-tubp aTid th<' mii-ronietiT iuIh-. Ii wil! I* n<in- 
needful to (-mploy con*i-tion» to romjx-risato for tlii> incn-aa*)! length 
of tuW. If the olfjeetivf' Iw provided with a ' i-orreition i-ollui- 'thv 
.idjuitment must l>e wiyHrei-t^l; but if it is iiol »o pnn iited the lulic 
o( the niiciu^i.'ope muol lie shortened exactly aa luuih as the tulie 
carri'iiiB the uiicrumetcr will have l«nf^h«iied it. 

BythiH arrun^ment it will be found that ntaiiiputation can be 
otrpctcd withimtUip vibration of the microscopical ima^ which is ine- 
vitably thei^raultof t1ierevotvin;;uf the micrometer screw -heiwl when 
the micrometer eye'picceisplnoed.asit usually hasheen. in the body- 
tulx" of ilie microHcnjw. The t-onse<|uence i* that much more minute 
spoi'cs can be mcASured. and with niucli ^.Teater accuracy, ih: 
NeUoti has repeatedly spanned the m'nn''! •'^ ""^ ^'"^^ '•y means 
of a s1fi<^ micrometer in the focun of the ol>iective: this was replaced 
by a mounted specimen of A'iiiifii/'f"ir" /'••^»'-i'(/'i,andhe Imscounled 
ninety 'hi.v lines in the j n'ocit'' "f "'> ■i^'-'h 'ty timkinj; the movable wirtf 
pass succMsivety over thorn until the lixed wire was reached, Ujr 
siinilnr means the Editor hn* measured single objects less than lh« 
raAnntbof an inch. 

It will Iiavo la-en premised by the careful reader that the stAg» 
micromcttr must l>c uxod in every set of nieni-iircments : at least we 
wouhl strirtly cmphnsiw tliis as the only ni-curatri and scicntitic 
metlirid. It lias linen iMlvisnd that a record of <v«npnriK>ns with the 
viiriouti lenKC-« in the posscwion of the micro5>'0pifil should be made 
once for all. \Vr dividcdly deprecate this incilio'l, uiilcwi it he in 
such Uttirilv viiluelcM work, ns is tminctimi'K dono, whcrr IrnKcx ni« 
uncorrei-t<Hl and ucriirai'v of tulu-- length forgotti-n oi- i^ioml. Thi- 
correction of an olijcctivf and th« tuln'-lcn^th oujjht to van' witli 
even: object, and therefore a, com]mriMin of the irtagi- micn>nu-t<-r 
and the screw -micrometer shuuld be made with l.'^'cryIet nf measure- 
men tn. 

Morei>ver, the majority of sta^e mieroroetew cxhiint ttry con> 
Mdemble discrepftncjes in the sevctul intej-vals hetwiini the lines ; 
it is well ill the interests of accurttcy to talco ihesi-rcw value of each 
under a liiKh power, lind the value of tli« ai-enifte, oiul then note lliA 
particular npace or spaces that may lie in iigreeiniait with the avera^ 
and always use it. An illustration will make this clear. 




proTides a sta^e micrometer of 1 nim. diviijed itit« '1 and 
followinj; are tlie actual lalueK obt.iined fur encli of tlie '05 

'^- 8-40 



■ ^3:^ _ - 


8*38 mean yalue. 
latenee it will be seen that the last division, 8-38, agrees 
aoeut, and ia the beet for all fntora um.' 
\g thus obtained a screw- micrometer value for a certain 
item), the Bcrew- micrometer value for any other object 
•wn,'the size of the object may be found by simple propoi- 
iB, viz. if 8-38 is the screw- micrometer value for -05 mm. 
UiAt for a certain object, the size of the object is 
(i) 8-38 : 6-45:: -05 : a;inm.; 

6-45 X -05 n„oR 

answer is required in fractions of an English inch, all that 
•emember is that 1 inch =: 25-4 mm. ; then 

(u) 8-38 :6-.(5::^-l^:^ inch; 

fi-45 X -00197 -0127 AniR.R- k 

x^ — — , -i- = =001515 inch. 

8-38 S-38 

s stage-microinet«r is ruled in fractions of English inches, 

xwe the serew-micrometer value for tts'ho*'' '"<''' = 4-257, 

for the object = 6-46 as before. 

<iii) 4-257 : 6-45 :: -001 ix inch ; 

. = e-^«><^f>l= -001515 inch. 
4 '257 

I nvmbar gino tor tcicw lalae the whole nomber etandH for a complete 
jt Bunbar irf nrolatioDi of the ■ciew-heul, and the decimal, the poiiiou 
iM MBd ofl bajmd thii. 



If i1i0 ntiswor in required in metrical moasumneut, then tu I 
(iv) 4-25; : fi-4r. :: (-001 X 2.V1) : xtma. ; 

4-257 4S57 


In this ooiiiieclion it will lie ns well to k>^'^ two ^-xninplas of 
Ksl* oompariioii wliicli iiro i>oiiietiiiies required. TIiuh you Mvn n 
certain intt^rviil <>ii n. metrii'iil stago inicroniKter wliidi y<>u know u> 
boActniraU-^ aud yuu wish tu compare an Eiiglisli nUtui- mii-nminUr 
witli thin SL-aie ill order Ui land out vrliich jmrtifuliir interval nf , n'jp, 
ini'li oxr*WB witli ii, Sup[>ui6 05 mui. = S-3H si-rewf valm- nx aWvp, 
thnn ;ill tluit i» iwceasa.ry is to find the point to wliivh the scniw 
iiiiuroineler niuat bo set in order tluit it inay acounit«l}' span tlie 
j^o inoh. Take 1 inch = 25-4 unn. as before ; then -OO) inch=02">l. 

(v) ■Oriiiiin. : '0254 mm. ;: 8-38 ; x screw value ; 


= 1'2")7 screw value. 

Conversely, if a nietricnl scale ia to becompored witli an accura' 
Eiiftlisti one where '001 inch = 4-2^7 »crew %-alue, then the screw 
value for '0-*> nini. may lie found thus : '001 inch ^ '02A4 mm. 
(vi) ■02-'i4 miu, : -05 mia. :; 4-257 : j- screw value ; 


-Oft X 4-2.^ ^ ^.jj^ ^.^^^ j^^ .Qj^ 

■025 1 


A eh«a}> gitbttlHtlm far llie ncrtivr-micrtimrtrr h«* Ijeen deviseil by 
Mr. U. Jaclison. It <'on«iBt8 in having a transparent arbitrar}' acahi 
inserted intrt nn ordinnry Hnyglienian cy«-pir»^ iti the focuH of thi.- 

oye-leiie, so that it 
^ will be in the saniv 

plane as the niafpii- 
tied image of the 
obji^ct to be mea- 
ning. It U seen in 
fig,. 17''. The method 
of uning it is preoiftcly 
.siniilur ti> that of the 
Mcrpw mifroniet*r: the 
voluft iif , ^..fl inch or 
-,'g mm., M the c-ase 
uiay Im!, i* found in 
tenns of the arbitrary 
scale, Thn vjiltio oif 
the objtwt in t«rmsof 
the aame kcoJc is atoo 
found, nnd compari- 
son ni.-idi- accord- 
ingly. .\ll that ntwl 
lie IS to substitute the term* of the arbitrary scale for sor<«w 
values in the preceding examples, an<l they will meet tlic caw. 

The arbitrary scale slioulu bo vnpttble'of uio^'ewent by a acrvv, 

Pici. ITS.-— Joukamrt ^ytt-pioM; nUFraOMCfir. 


otherwise the appliaii<w is liaixUy oa a«ur»t* na tin- tint mriliod of 
mil-run letry by dimple drawing described aliove. 

<>f all Ui« iiietliwU of microiuetry tbe mi«t iu.-ciiTnto in llmt 
p«lf«mied by phoUhniirrofcraphy. A negative of tlie A^jfrl to hi! 
Tirm—* is tahfR, nnd then, without nny nltenttion in talw- or 
cAinera-lcnctli, thp inagniliH inmgr of the stA^e iiiiirroint^^Lr ia pro- 
JKted on tli« giXMind gint* : thix if spnnned by meitiis of u pair uf 
airing divirfrts. The tif^tivc tilm is tlieii iwrnttlied by thftie 
ifiriilnrs. 'ITiPii yon nrc in a j>osition tr> iiinko the liiost >ocurut« 
isramirr'nMint thr iiiicmB(x>jH> in capnhlo of yielding. 

It in rxrmlin^ly iiu|iortant, whrni pcrfonniiig uiii-romelric 
ni>-aAiirciii«-ntK, to rr-rnrmlicr thiit tin- pri-ciMr rdg<« of nil objects in 
ih- iiii<To»c<Mw iir<! ticvfrnrrii, r.'oiiM>niH-iitly it is impouiiblo toasoer- 
l«in froin wbat point to whiit jioiiit tlin iiicfl.iiir(imrnt in to be made. 

Tliik, whil» lutr«lly nlli'tting \arjti^ and uounn i>bj(t:t*, becomes 
cupn'uicly important uith Hinnll cibjivts. 

Inati^d of » n-«] t^jje to mi object you j^l diflrnction bandn. 
Tlim^ liHiKlti altfr with focutt, and also to • ;n'<!utrr extent with thi- 
iui;(lt* of the illuHiinittiiijf cun« as well as with tJie aperturu of tho 
•ilijrctive. HeiK« it «n!iuea that the aciuiul^' inifromttry of delicate 
•>>ii(<cta j|>re»nt« one of the most dillicult matters enuounterwl in 
practifal niicrofscopy, At the present time opinions differ y'^'i'iy 
as to the treatment of jiartii-ulHi- cases. 

Tlie following plan of Sir. Nelson's is the out«-ome of a long series 
irf experiments : — 

1. The fot-ui and adjustment to be dionm may Im ti-rmifd Ibiit of 
fill- 'blni-k dot'(MH:- Elimination of errors of iiiitrrpn-talion. p. 356) ; 
ill (>th<-r words, if tlie object wen.* » Hlt^tulvr titiinK-nt it would be 
r«|>reaputetl while with bliick ed;^s. Tliese bloi'k edKes are due to 
diffiuctiotL If the lilitmetit is % ery slender and ihf illuminating con© 
small, there may be seen a while diiritkotiun Ml^^e outside the black 
ii«ie, and perhaps another fniTit bW'k ont? outside that a^iii. 

2. Redutv, as far as prwM'ble, the e.vteiit of these diflVaotion bands 
hy (n) using an objective with as large an aperture aa possible ; (k) 
bif using ns large an illuminating oone ns possible. 

3. Me»«(ire from thv inner <xlge of the inner diffraction Itand Ut 
thf inner edge of the inner diffraction Imnd on the oppi*ite side. 

4. Bnt if the dinmMcr of a holp be rc^nuired. thon thi> measure- 
ment must bn made from the oHl«Trdg(f of theonti^r black ditlinotivn 
liand to thi' nnti-r odjti* of otitrr dilTnictinn band on thn oppiKitp: indv. 
It must not Ih' for^ntti!n, howitver, that thcsi! rules only apply for a 
particuliir fririu nnd a pdrliculur luljuHtmrnt. 

II. The Camera Luoida and its Uset. -There am a InrKc number 
of "mtrivanc« drviwi for tli<- purjiosi- of imablin^ th<( iihsi-rver 
to "y; th«! imago of an obj*i!t projivi.-d on 11 surfari- upon whii-h ho 
may tra>-<- its outlines, but tliey rtsolvo ttirmselvc-K jniK'ticjilly into 
twii kimls, vix, 

1. Tliose which jirojwt the mii-roscapical image on to tJie surface 
provide*! for tin- dniwing. 

3. Those whii-h project tlin pimdl an<l paper into the field of tlie 





We kIihII descrilie vrlial vre i-onsider tlie ni<wt pniotiokl forms <J 

In point of iuiti<|uilv WolUiftoH* ntm'rn liiclrlix tUiuui tJie ]>otit 
of lioiiour : but to use it tho iiiioix>MM>p« must l>e placed in a faort>^ 
zontul pMitiou. Its, general form i« stiomn in %■ 179. Th« raja 

on leaving tlie eye'pieoe, nbove wliicb it 
i» lixod by a collai-, «nt«r a prisiii, atul'l 
iift«r two iiitvmal reSexiouB poas ia\y 
wiirflK to tliP eye of tlie observer. It 
ciwy to M* « jirojeotion nf tlie mii-ro-l 
Ki-opic imotuo with thiM imtiuiiipiit, 1>ut it' 
is when ttn ilcsirv «t the same lim* to 
spi- till- mjici' nnd the lin(.'er>i liokliug the 
pi'iii-il tliiit the iliHieulty l>egins. ThoJ 
cyi' liux tu bt! hcJil jiiKtK'iiii jioxiiinn thnt ^ 
Plu, 17^, ihv ulgu of thn pri.Mu hiM-i-ti. tho pupil, 

so tliat uiip-hiilf iif thi- pupil rri-i/ives tlio 
inicnMCOpii' imaxtt nml the iithi*r linlf tin- imiijttw nt tlif paper anil 
tha hoiiil ent|>l(iyi-il in (trnwiiix. If this hiHH.'ti»n is not <s(Uftl, too 
ntlch of on« tmngi* i.« M«n at tlie i-x|H-tiSi< of the uthir. This was in 
aotne )«nso suppiur'tl to \n: i;i>iii|fen.iiit4-il by tin* u>i- iif ti'iius, iis siM-n 
in thi- liautt' : but llirt iliflii-tilly of ki-irpiii;; the (-yr prfHH«rly in onr 

Cisition hiis ceiuaed thU itiHtruuii^iit to fall iuti> dii^UM-, Mtvri'Ml nunemS' 
ang now dc^Urd frt'e from tlii» defpot. It Iiiia ii<!\'i-rltn-li'£< one 
spn.-iiil jKiint in it« fiiv<iur — it does not i;m-rt tlie itnn^, (utuunK llie- 
rifcUt t" be turtml to the left, mid vU-f rwrwE. Tliin Lt lui lulvitntoici!' 
the vnluc of wliioli we nIiiiII HubM!r|ueutly see. 

A Kiinjde i-nniem vm-t niiide by .SiH'uiuifriii); by means of a kiiimII 
inirmr or dreiiliir n-fleclor, which in pluceil in the [jatli of the 
eniergenl (K-ncil (it im anjLcle of ^•<- to the uptii' axis, iliu» n-llectinj; 
rn.y, fnun ihc iiiuige upwartlH. TUp in»lrunieiil in seen in ft;;. lSt> 
iind tdiden on to the evi-iiiw-p. The i-elieclor must 
be Mculler thiui the pupil of ihe eye. bociiune it is 
tbruuKh the peripheml portion of the pupil that tin- 
nijtt, nut Mopjied nut by the niiiror, come from the 
paper »nd pencil. Hence, as in the >vise of Wol- 
liL^ton n caineiik, the pupil of the eye iiiuei be kept 
{H>i'fcetly centivd to the biiiuII reflector. A>> tlieiv 
is but one reflexion, the imafiie in invvrtml but not 
transposed. To Fiee the outline of the iumgens it is in the micro- 
Acopp, the dmwin^ must lie made u]>on tmeing paper, nnd invertedi 
looking nt it n^ a tr'iinaparency from the vruiig side. 

There is considerable variety in the experience of (liferent 
mieroecopitts AS to the fuciJity with which the-se two instrniuent* 
ewi lie used. The differeiico in nil prohnhility dq>cnd« on the 
greater normnl diameter of the pupils of the eye* of some ob*erv«r& 
in eonijinrijion with that of iithers. 

Dr. Lii'it'l lU-iilf Hpvi»r.(l one nf th/- nmjiJrul entn^m*, which 
hn* tite wlvitntagr of Iwing thoroughly elbcient. It oomiitK of n 
piece of tinti'd K^ofs, plaeisT at an an^ie of i'y' to tin- optic axis, 
in the patli of tin- rinergent pencil. The tirnt surface of the glftn 
(vftects the nwgiuHcd image upwnnU to the eye, the |iaper nnd 

Pin. liU).— fl>iu|<lD 


ptitvll lK>iri(,' M«ii tIii-ou;:h liif sUik. Iti iu simplest form it U km'ii 
ill lig. 181. The gl^JH is liiilMj to n^niter tlip liprotid n-tlexioii frwiii 
lilt* iiit«rri»l «urfitc« of (lie nUsa iiuipi-nitivc, Tlie tvHi-xioii of tint 
inxa^ is ideutioil with thul of Sunnuicriii^E'v. 

Fig. 1838howa»tittinf; MloptMl liv R:tii vh nml Lutilh fiirthBt«Mi/rn/ 
fill/ emitftra, I|. is nuule 
ui vulcanite, unil the luilf 

rine to which (he fntme •^^\ I 

itoliliiig the iwutr*] tint 
jius i« 6x4^. Ht« on the 
«»p ot thi-i *iji-i>iwp, nn<l 
wttli >ni(Ki.-iwit grip. 

litfii/tr tckirk /fi-i-Jiri iftf 

tmaijr (^ (hf /-iiier an'l 

frttril iulit (A'' H-ii-rtrMt'/i' 

Imkr is tin't tlint ilcvisiil 

liT Aiiiiei, :i(liijttitl to %^^^^^^^^^^^^^' '^^PTl 

llip liorixritilnl ^^^^^^^^^^^^^| 

l>j Ch.'V«tier. The f\f 

Imiki llirmigh the niicro- 

trofw M th<* ikl>jt«t (as in 

Fuv. int.— UmIt'i- tuiiwni. 

Flo. IW, 



-Buuaeli aiul Liuiib'B litliiig t»r BnUo'M 
■■•rotnl lint pamvra luciHii. 


tordinarj' view nf it), iniitnul •>( l(v>kiii|f at its projection a])Oti Hie 

jnpiT. the iiiwHeirf tlit- inici»»ii»iiit WiiiK|iroj«ct«a upon th9 field— An 

vrmnt^emeiil wliicli is in tiinuv 
K)wct»tnun!iulvniituj{e"U-< Ttiis 
ElTovteil b)' cQtuliiiiiiiK n 1*1- 

fomt^il xilver-on-J^bas uiirror 
^witli a rtltecliiig pimii : mid its 
an will lie tiiMieratt.KMl by llie 
riocoiiiiKtiiyinf; diB^niin (lij;. 18ni. 

Ttie r»y all proceeding from the 

oliject, «ft«r piueT^ging froni tli^- 

Aye- piece of the iDicm«ixipe, 

pawn tliroiigh the oenirni pcv- 

fomtton in the oUlii^afi mirror M. 

wfaidi if ptnoti in ^nt of it, 

and HO <llr*«Lly nnwimlK to the 

ny^. On th« otlier liniii), thp my 

«*, pnwmliiig upviinlx fmin thp 

tnwing point, cntcn llifprinni P, 

isiv0Mt4^ from it.t ini-limnl Mir- , 

liwetn tliv iiidineil surface of tlu' 

mimir ^I, nnit i.i l>y it rt-flccti-il in the «yc at h', in iiucli pnmlleliHni to 

llM-my 6piiic*vdinKfr»tn tlirol'lci't thnt thi- two 1>l4'nd into one iinuKi*. 


Tli« Editor IiiuubihI witli <;nvit fiioiliiyiind HUCcefctaouii#niilt!tt»vl 
iiy 1>T, Hugo Scliiiider ntiil prwlui-eil by ^Ii?»i-b. Rritn. It in tigurt-d 
ai 184, Alia consists of n c«>in1>iiiutii>ii of n ri'^ht-nngM |)riHm {6g. 
]8'i} A it CnncI u, I'lioinboidal prisui D £ F C, ko arraii^l tlmt w)iMt 
iulju«t«(l very iicnrly in oontnct (i.e. sepiimt«0 byimlya liiin Etj'aiuiii 
<it nil') the fncnt ItC nnd 1> K nre pArallel, mid i-ciiiHe<|ueiitJjr betweei) 
D E ami B K' Clii'V net togotlier iis & iliick iMirallel plute of f(\M>n 
through which thr ilinwinj: pnper nnd iieiitil n\n be B«<en. Ilw 
rhi>raboidiLl t>n«m i<^ »> oonirtninml thiit '^lifii the fuce U F i« iipplivd 
ftt light nugVii t« Llif oplio Axit. of the mifro^i-ope, the uxial my H 
pnMP« without rrfnu'tiqn to 1 on the int'i^ninl fm-e E F ; whence it in 
iidnlly n-Bi'vtiil to J in the fiicii 1)1!. At J n p«i-t of the ray i« 
nrioftril to tin- l^y■■ by ••iiliuarif ivHpxioii in the direction of J K, 
mid II port iniiiMiiittt'd to J' on the foci- A C of the right-angled 
prism. <■>( tilt" hitli'r » pniiJon in idso rnflninrvd to K liv oitlinnry 
r«fli*xio(i iit J'. Tlift hypiiUi«?iiuia' fnw A 1' is i-iit. at sucli an niigli 
tlint tlic rftflt-xiou from J' ooiiicid™ with thnt from J at the fvn 
]}oint K, tliiw uliliftiiig tlie Hivotidary mlI<-\)on to »trengthrii thAJ 


CMiwn luriilx- 

Fiti. las.— Diftgriun «i)iliL(uliig ScliiWa'i euiuTft Ineidk. 

Juiniiioaity of the iina;^. The nngle G is nrrangml n that the 
extreme inar;jitial niy H' from the field of the B nyi^pimn- atrikm 
Upoti IKJ lit Ik point junt l>eyond the nii;,'le of tot*l n^Hfxioii. the 
tlifihtctiOD-linndKiit the liuiiting /inglf licing faintly ilisocmiUe at 
IjrAs etl^ of the field. This angle givck thti gremfnt amoiini of liiihl 
by oi-i/mory reflexion, short of tolnl i-pflpxion. 

I II UKP, the iiiicroseope >ihoiild hr ini-line<l at an nngh' of *ft', mwl 
the image foiru^^ed ihi^iugh the ryp-pipi:i- as u>^uul ; thn ouinem it 
then platn) in position on the oyc-pie<-e, luid puiih«l down uiitd tli« 
iiiiagi- iif iJte object i* fully and well Morn. The di-iiving pitpor innsi 
be tixi?tl upon i\ TaIiIc on a Invel witli the rtany inimediuti<ly undi-r 
th« i-antcra. The ohnervrr will then we thn mii-roHi'opical iina;p- pri*" 
jrCtiuI 01) the paper, and the lingers eiirrving the pencil point «iU be 
dearly in view, the u-hotr pupil of the eye lieing aiailuble for l»th 
intagc*, the diaphragni on thn instrument being considerably larger 
than tile pupil. Tlie eye nmy Im removal as oft«n as required, and 
if nil in alloweil d reiuAin willmut altPiatiou, the drawing nmy lie 
left and reconniieni-eil, withimt th* slightest nhifting "f the iuiag*. 

If a verlicnl pouiion of the niicrOGciipt! be needful, tJits nuty be 



flone liy incUiiinpf tlu- tiibii' aii<1 <lntn iii^ jiujicr U> an aiij,'I(i of 4T** 
flitltrr in front or ut tlti? Mile of (Iia mii-niscitiic. For nccumtp 
(Imwiu;^, In all aziiiiutb!>, theilniwing jinper nhoulct "f oiwnw- i-innci<1« 
with the plane i>f tlte o|iticiil iiiuiK**- 

ThiH caiiien may In- ii^ed uitb a lianil-tnajcnilier, nr with Kiin|>k 
Imm used fur ilisnei-iiuii and otli«r purpowH. 

FrofieMor Al>>>e has also ilevb«d «n Insirutneiit whicli we 
have uitetl with i*-iiiiplele stx^cess. T)ie aci-i>iii)ianyiiiK tlrawinjc 
its- lt*6) will a( iMiLf nhon- llie NiiuiiHoily of iis action. The 
:iugc of the |:i.'>|>t-r Ami ]>eticil iiiiiiiiig, say, in a veriictvl tlirectiini 
i&^ llg. 186) it iv{|t:H.-te') liv a lar^e iiiirrur in a huri2»rital dit'eclion, 
W, to ft cube <rf glaKS which has a EJtv^reil din^onal plane with 
a HitMll circular hole in it in the viBtml p>>inT »f the e_ve-|iiece. 
The tiiicrowopic iiimse i» i^en directly through this aperture in tin- 
tilvering of the jiriHin, while the siUeteil plane of the pri'-ni ti-aus- 
lails the image iif the pnppr and the opemtor's lingers and pencil. 
Rjr tl»e concentndt J' thusnl>tained of t.lie bundle <>f rny: reaching thi- 
eve from Imth the mu-rnscope and the pnper, tJto image nrKl the 

l^u. IW. 

pencil wttJi vhidi It ia to b« (Iniwn are Hcen cuineidentaUy witlmut 
any utraininK of the eyes. 

Thit iiwtniiiieut reiniiiy-* the jiaper lu be |>)iioeiI in a ptiinr 
pMmlld to tliat of the ohjet-t ; tliun, if the mtci'Dftvopt) in vertical the 
p»p«r inu-it li« lw)ri)»mtiil, and nVo ivrwi, and it prewnla the inwiii.'P 
pradaely an it is M«en in the microscope. For the purpose of dmwing 
miiiplj, and where the oinerver luin had no expeiience in the utie of 
» canM-m luolda, we ^lould l>e inclined to recoil iiij end this one aa the 
■iiKtrutniMit prcBeiiliiig to the tiro the greatest fncility. But then- 
i» a n«e to be made of thecaniera lucidnlo whid) thit one does not so 
midily lend itaelf, which is none th«l«ss of gtvnt importance; that is 
thi>d«!t«nniniiigof the magtiifying power of ohjf-ctiveK. h is iiianife->t 
tliat Uie dintaiti-e between the paper anil the rye of theobset'ier 
<»tini>t be 40 readily detemiinixt in thi* rair as in thoNO fomiA of the 
instrument wlierw ihn imagi' of the paper aiid iit:ncil h wen direct. 

With "lie or other of the fon-ftoinjt cnntrivanws evrryime may 
Intni to draw an outline of thi* mii-n>-t('iipir image ; and it i* ex- 
iRtoely deairahle for the (akr nf nccunicy that e»erj- repre^'ntalioit 
«l an object should be btMcd on xui'h a delincntion. Some pcr»mis 
will uae one instrument more nwlily, wine another, the fact being 


(lint tlll^^e isu wire of 'kiiuck'inthe use of Mcli, which U cvxniiioiily 
uLHIuired liy timvtioe aloiie, no Uiat » pei-aon ftL-cu.ttnnied lo the uku 
ot Mijr oue of them does not at tirst work well with nnut )i«r. Alt1iou);h 
Konie persona »t once acquire the power ol' fttMin^ 'he iiiiu«;e and ilw 
tracing point with equal diatini-t tiestt. the oiuie is niont fny|ueiilJy 
otberwiae; «ii(l hence uo one should allow himself U> be lialllt^l by 
the tulure of his tint ntteuipt. It will sometimes hnppen. especiidly 
wlittn the Wollfttton priKiii ix ecnployed. that the want of ])ou-ei- to 
eee the pencil is due to the faulty pOHition of the eye, too Ui';,'e n 
purl of it being over tlie pn«n itself. ^\'hen once a good position 
has 1>een obtHiiied, th« oye ahoulil W hi-Id there as stwidily as po«- 
sible, until the tmcin;i; nhnll hnvc U^rn coni)>lvte(]. It is eswntial to 
keep in vitiw tliut the pro]ii)rtioii between tJtc siw of tho tntcing and 
thftt of thir object in uHfipi.nil by thr (ti«tnnca of tlw «y« from th« 
paper ; nnil hcni-i- that if the microMuipe bo pUcod upon » xapporl of 
(lilToreiit hright, or tin- eyii-iiit-Li- he elovntod or dopivnod hyn tihvht 
inclinatJon given to the biHly, the scale will bn altorod. TIiik i< is, 
of cotii«e, iMTuliurly itnpiirtiint to licar in mind whnn n wrii-n of 
tracingo in bi-in^ nuidn <>[ any xl^t of ohjircts which it is intMiUod to 
ileliiient4! on iv uniform scal«. 

A rnluabht luljuiu-t to a cnniem lucid* is n tniinll jmniflin lamp, 
-■ieirn tJt the Iclt of PJatft III,, which illualmt^-s the corn-ct mi-thoilof 
uninit the oimeni luii'lu ; tliis Lamp i.i iiinipit! and is capable of iK^iiiy 
raisul or lowM'cil, Ittli-il with a )iaptrr shade, for a, grcttt deal of the 
Miccc^K atteTiilant <m the u»e of the caiiipru ile|]enils '«i tlie ri'lali%'e 
illumination i4 thi* inieroaenpic iinn^e on the one side and of tlie jiuper 
•ndlinj^ersftndpctit^ildf thei-xecut<>nl on th<-utlii-r. It ig not a mailer 
tobf^di-terniimil \)\ rules ; ]*rKonal iftjuution, siiruetiniesidiOByncmsy, 
<Mrmiiiii'* liow thi^ li^ht Khali be iv^^Uted. Many linishea nii^-ro' 
drauBht.-.iiii-ii vise ii fi-ehUf light in the iin«.;{e, and a, strong light <m 
till- liiind and paper, and Olhem eijually bup(.v»(u1 inanipulute in 
the | inverse way. But upon the adjuiitjnent of the res[H-c- 
tive sources of li^ht to the personal comfort of the drMUjjhtoinan 
will depend his success. 

Cnre must be exercised in this work in thecaae of f^ritic/il iui'tij't. 
Tliese most not be sacriAced either by racking; the condenser into or 
out of focus, or by reducing itn tingle liv «, diaphragm. If the in- 
tensity of the light has to be reduced, it must lie done by the inter. 
portion of gloss screens, and this is Ijcautifully provided in Abbe's 
camera. The illustration of how the roiious apparatus for the use of 
the cnmeratucida should Imj di<(po"xl, given in Plate III., may be pro- 
fitfdily studied. Ltoth mirror and tmll's-evn are turned aside, and the 
hand nitil pencil are illumtnatod by the iilinde4 lamp. 

'Hie lamp illuminating the linage in sern, with such a screen of 
coloured glass as may l>e found needful, and the lamp illuminating 
the paper and pencil, and rtirefiilly sJioiloil almvi", i* afno wr-n at the 
eye-piece end of tJie lH-«ly-tul«'. <)ft*-n, if the iniajre i« too bright, 
we lind that bringing the lamp down to illuminuti- the paper niorn 
intensely HUtfires. If not, n«e si-n^ens ; thn ilhniiinntiiig eonr muxt 
not bn wmpered with. 

III. Th« determinfttion of magaifying power is an imjiortnnt 
and indepirndent branch of this subieci. Fur thin ourpoM, and for 


tlie reason given above, Scale's neutral tint camera ' is eminently 
suitable — indeed, js the best. We can eaailyand accurately measure 
the path ol the ray from the paper to the eye. What is necessary is 
to project the image of a stage micrometer on to an accurate scale 
placed ten inches from the eye-lens of the eye piece. There must be 
complete accuTacy in this matter. 

We can best show how absolute magnifying power is thus deter 
mined by an example. 

Suppose that the magnified image of two yi^'^nt^'*^ ^^ ^^ "^<^^ 
divisions of the stage micrometer spans -n)ths of an inch on a rule 
placed as required ; then 

(i) '002 inch : "8 inch :: 1 inch : x power ; 

X ^ ^ 400 diameters ; 


fiw it is obvious that under these conditions one inch bears the same 
proportion to the magnifying power that TuVnths of an inch hears to 
y'gths of an inch. 

Suppose, now, as it sometimes happens, that the operator is pro- 
vided with a metrical stage micrometer, but is without a metrical 
scale to compare it with, there being nothing but an ordinary foot- 
rule at hand. 

Let it be assumed that the magnified image of two y,\n 'i^'*'- when 
piDJected covers -^^ inch ; then, as there are 2.^'4 mm. in one inch 

(ii) -02 mm, : (-8 inch x 25-4) ::\ : x power ; 

X = ~j\-^— ■ ^ 1016 diameters. 

If the reverse. is the case, liz, tliat you have an English stage 
micrometer and a metrical scale, then, if the magnilied image of 
two jj'ng inch spans IS mm., 

(ui) -002 inch :4^::l : ^ i 

^='J9^'^J< ] = 354-3 diameters. 

The above results indicate the combined magnifying power of the 
oi>iective and eye-piece taken at a distance of ten inches. The arbi- 
trary distance of ten inches is selected as being the accommodation 
distance for normal vision. 

The magnifying power, however, is very different in the case of a 
myopic observer. Let us investigate the case of one whose accom- 
modation distance is five inches. 

Here he will be obliged, in order to see the object distinctly, to 
form the virtual image from the eye-piece at a distance of five inches. 
To do this he must cause the objective cot^ugate focus to approach 
the eye-lens ; consequently he must shorten his anterior objective 
focus. In other words, he must focus his objective nearer the 
object. Tliis will have the effect of causing tlie posterior conjugate 
focus to recede from the objective towards the eye-lens, and the fact 
of bringing the inverted objective image nearer the eye-lens brings 
also the virtual image of the eye-lens nearer. 

' Fatte ass. 


Sliorli-niiiff the fuciu of tlii? objective hoA ihe eifecl of iiicnosliii; 
ita |K)wer : l>ul us UiLs ult^rAtion U propoi'lioiiatcly verv little, tin.- 
increase in ]xiwer is verj' Hiiimll ; but tli« shoilenin^ of tlie ey<>-pitM3(> 
virtuftlfruiu ti^iitoliveiDohesliiuitheofioclof ncarlj halving* it«pow«r. 
CoiiBequeiilly the conil)inrd rrttult n( the eyo-piece And oltjeotivK in 
tlie caae of hnlving the cyp-piece virtunl b to nonrly halve tlie power 
of tbe iniorowopp. The iiicrwise of the objective power ia practii-ully 
«o Bniall thnt it mny In* neglecteil.' In j>riwtico it is fuuiul l>y UH 
that if tho iiiiBge i* piiji-etfil va « ground gliuw screen ten iiiche» 
fruiii the eye-piece, tbp iinnge is nCfirly the snine siM" whetJier 
focU8«.-<l by oriliimry or niynpie sight. Tliw i» in hnniu.'ny with 
Abbe's (lemonMriLtiim (j>p. '_'~>, '2>i, Rit. 28) thnt ImiIIi ininginK im- >eeii 
under the Ninie vtHUiil nii^ic. Itiil, on the other hiind, if it myopic 
xight eonipiire« the iiniige with n .hcaIo, the nik^iilicjiticin will be 
lew thnn with urdinnry virion. 

To find the precise iniliiil power of any leiia, or U> find thn exitct 
multiplying jiowiT of any eye^piecev is not so easy, A Intioriou* 
CJiIculntinn, inviilviiig the Knowlerl;^ of the diBtaiieeH, lliickiiAsii, unit 
refmctin! indices of the lenseH, is required. Bui « very ^lpp^^lXl^lnU• 
det>T^nnntion, ButRcit'iitly iiccumle fur nil praoli«»l purpo>ie?i, may Iji^ 
fnwily iniide, fspecially if one liao a photo- tiiicrojfraphic enuiera **__ 
hum). The princmle i^ as follows, mt 

Select u iena of medium power— a ^-indi is very suitable. Now^B 
wiUi the itiicrwoope in a horimntal position, and wiih a powerful 
illuuiinatioii, project the image of the stii-^ micrometer on to a Boreeii 
distant five feet, measured from the back lens of the objectit-e. If no 
plioto-niicrogmphic cainem is at hand, it will )».• neceuiury to ptM-f(>riii 
the cxp<-riiiit!iit in n darkened room sliading the ilium iuu ting source. 
Divi<le llie inOKiiifyi'in ("ower thus obtained by (i ; the i[Uotirnt will 
give the inititU power of the lens at ten inches to » very nearii[>proxi- 

The rensoTi why the result is not perfectly accurate ia tliut tJie 
ten inuhea must be iiieii$ureil from the pusterior principal focua of 
the lens, and that is a point which is not given. But in t'le case of 
a power such as a ^, it is, iii pi-iu-titi', found to lie very near the luck 
lens of the objective. So by inking a long<listance, such as five feet, 
the error intriiduccd by n snmll dixpl'ieemcnt of the posterior prin- 
cipal focus does not materinlly nmcnint to much. 

There is n further crr-ir introduce*! by the approximation of tJie 
objective to the stage niierometpr in ordrr td focus the conjugate at 
such a distMice, but this is smiill. \Vc fun see, therefore, thnt llii4> 
error tond» to slighlly ini'reiisr the initial magnifying pnwer. 

The initial power of thr> j iH-in^' fouml, ami iU <'onibined niagiii- 
fying power, with a givi-n eyepircr. bring known, th" combined 
power divided by the initial ]»ower givnt the multiplying power t]f 
the eye-pieci*. t'uro must lie of courtx- tnken to notice the tube- 
length * when the combined power is nienaured. The initial power 
of any other lena m/iy be found by dividing the comliiiK'd power of 

> Eiigtuh IfrrJiuiiiV, ml. tlii, Nn. IIH'.. Arlicie on ii»>u*iirciucDt4o( masaUjlac 
potti-c iif imcniwatic oWclivn Ly K. M. N'rUin, 

• Ihid. Tol. iitviii. Ncs UH|, ' OiitictI TuWlciigtli,' by Fnuk Cmp. 



that loiu witli the »yv-pircc wIkim uiultipljriag power haii b>?vii 
ilfWrmirK-il. Iiy llm multiplyuig powei- of (hat o]re-piec«.' 

VoK^ptMei. The Uinu aotK-fuee primnrily m«Mts thnt pnrt of 
n micrvficope into vrliich tint objtvtivr icrrwK, but tlip lenu in ulsu 
aiipliod to vnriouK piHVs ot ii]>piii-«t u.s wliicli ciin be litle<l lielwe«ii t)i« 
Doee-pioooof tlH;micruHi-opi»iu)clth»obi?<'t!v^. Tbi.'nan?, foriiititniici% 
TDtnting, cKlnttr, oentriuj^ cbunnns, nnd Aiuily>itig n<i#«.p)et^«s. 

No«e-|ii«o<», nlUiou^li lh<ni^it to be i>a, am not n in<K)ern idea ; 
our prednccwtoni ut « ciuitiirjr «go omploynt jtiiiiiliir infMits. 3lr. 
CVi<^ has rvct^iitly uc[|mred a niicmn:npi% wlikli pnowssm a double 
Arm, nt ttio i-iid of which is 11 ii'll for 
ppCT-ivitig dilVi'iviit leiutta. Tliis oi'Il lits 
m*cr t)i« piid of the iiosc-piM:c, aitd ko 
Icnop* the Ht'i'end <>)>j«ctiv«s which inny 
[» inserted in ponition. It dates, in all 
[■miKtbitity, from tJi« eiul of the aeven- 
Iniith or the early [»rtof the «i)(lite«ulh 

But in the early days of t}i« mk-ro- 
•row r»lntinh' di^-s of obJM-ti yen. as iJio w n 
in ttii. I S" (or, p(:rlia]i«, older still, a lou;^ 
■1(11 1^ till led iiliilt'of objectives, an oil H8 tig, 
\>^ Mitowaf, wfiv frMjuently employed. 

It tti con till uiilly deHiribltt tu be a1>I« 
fi >iibititut« oue objective for anotJier witli ns liitle ^^peuilitarn of 

Fill, m.— !*•■' '■■■".■ ' 


fd nh- 

Fw, im.— SliiUne pUl* at oUlvelin^ 

tiiiiT- and ti-oublfl tut pomibli-, 10 as to be able lo exittninn und^rr a 

lii;ili''r tiia;;iiifying power tli« de- 

ijiiU of an objoct of whiili a 
,,|{citerat view Itas brvn olrtained by 
|in«tiui of ■ lower ; <>r to une the 

lower for the purpow o( jimliiin 

4 iniikute object (such 11* a jiaiti- 
LOtlar diatom in the niiibrt of u 
lidd^ul) which we wish to sub- 
mit to higliiT ainplitimtifin, Tliia 

was convcnicntlv fffcvtoil by the 

ni»ir'pir»-e of Mr. C. Bi^oolte, 

which, ix-ing scniwwl into the 
tiject ttwi of the iHxIy of ihe 

_ DicrtMCOpr.cnmc* two objectives, 

fith'^r of whirh may be brouh'ht 

into po«ili>i4i by turning tlie ai'iu on a pivot. 

6g. Iti9. 

■ KngtiJi IfMAnwu, Tol. iIyI. No. IITS, ' Ueatunmvnt «I fuHvt.' b/ C. M. Tfijton. 


Pio. IW). — BrooWii 

HMO. pl«» <U> niiulll 

by (iwlft. 

Thix u kIiowu m 



Tlt« most gonpi-ntiv useful of all niN(e-pM>c«K now in uxo tue tlu 
rotatinjt forms wliith Mi.ibic on<? to cany two, tlin-c. or fourobj<'ctivM 
on tlip microBcopdi At oiie timr-, nnd 1iy iiii-i-<- I'otiition cocli is mtca-it- 
sirely broucht ceiitrol tn thi^ optio axU, h-mi in fig*. I'JO, 191, 192,-] 
M 8Uppli«a by M(«.frK. Kcot. It >■ ftlniOHt unMncitamrf now ta4 
point out th« iliMiclvniitA^c of tlioan olcirr ami Ktnvight formi wliiclnl 
involvwl till" dnngt^r of kmifkintt nut thi- froiit leii» of tin- nbji-i-tivi-a / 
by bringing it into (^oiiliict with Bonnf piirt, .if thi- stiij,'* wliilt? tlisJ 
otlipi- olijfotive wna being focuHsecl. Tliis iilij^ctioii hiis boen i-iUirrly] 
rnnxiviil by tbi> iiitriiiluPtiuii of thr Im-iU form by JIphbpi. Powi-11 aiid~ 
Lcaliinil, luul olhi-in, sbiiwii in li«H. 1 WO- 192. Tliert- fUii be no doubt j 
that for oriiinnry dry Icua work soni*? auch iJevict.' Is itiipenitivo. 
8onnf, Lovrover, who do » >ery large utuoutit of iiiii:ri»copioal work' 
prefer to uwe two microaoopea ; tlip one » tliird or fourlh class luicra- 
Mcope, with (itily a coaraendjiiiieiiientatid a 1 -inch ob)e<.-tiveai]d mirror^ 
tb»ot)i(^i-luiviii;{»ciuirtie and Bne adjustment tind ft i-inth objectJv«" 
with tt ■iiiinb forDi of condenser and plaiip mirror, all fiiieaitcl higlME 
power wurk Ijeiug left for n special microscope. 

Th« one drawlHick to the use of n rotating noh^-piece is the oxt 

I'l-. m. 

Vu>. 1 02, 

Wright it thrriws upiin tlir' tiiip ndjustmcitt. Ah thU subject is fully 
tn-:it(!<l of under the heading of ' Micriigcojic ' no more will Im- tiiai<l at 
prv^wnt thiin ilmt a double noHc-pieee is to be preferred to a, triple., 
tuid a quadruple need not bo entertained fur a delieitte imiruiiieiii 
uiileia It is nniuired to liml out in liuw sliort' ntimealiiieadjustiueiit 
Riuy be ruined : for let it Im^ noted tlint a S-ineh, 1-inch, ^iiicti. and' 
^•iiich objective of Engliidi make weigh together S^ ox. without auy 

For the proper une of a rotjiting nose-piecr' the length o( tlie- 
objective muuntti should Ite so armnged that when the objective is 
changed little focal adjtuliuent will Im- nccewuirj-. 

Aw excellent calotte tiOH-piecf for four objtictivea i» Rude by 
Zeitt ; tht* is so arranged llittt only tin- optical portion of the objd.- 
tive is screwed into the no^e-piece. This plan much lightens it, «o- 
that the nose-piece and the four len^fx wvixh 3} ox., or only 1 0£. 
more than nn E^nglish ^-inch with ii nerew eullar, and } ox. inofv than 
an Kn;;lish J-inch of wide angle. 

A centring nott-piece has been made with the riew of placing 



nay ot>j«l^t■v« <<nntnLl b> tim axis of rotation of Uic «tage, It is, of 
oMrtt, mudi chi!n)H^r to vciitre ftti okjectivv bjr mauuof * DOW-pieca 
to th« axin of rotAlioH of tlin vtuge tluiu to cvntrv th* rotary tUtgfi 
tollM-obji'clivi-. Ttii*, Hk<' iill other ad»pt«nt, is nn Mlititiona] weight ; 
tmt hen' tliiMv ix vi-n- liitl« to be gained by it, fitr if the rotniy stAge 
u well niAilc- nny Klijrctivc will li^sufficaaatWt'Viitmtlfor&Upnicticnl 
pnrpoHnk Mr. NeWn, as we have s«ea (^. 147), [foiiitircl uut, aI 
a time whpii tliv nub'stagA was codtly, tliut bucIi n tUKU'-pipoc lunied 
ai»id« (lowti, with ii turn-out rotatiti); riuf; for Mopi iic littisi Iwlow, 
oiMle a very elBcif iit rfcttuigiilar centring; »ub-staRe at a hhinII cntt. 
Sub-stagM ar« now quit« vominou and cheap, aiid G«iilnii{; iioto- 
[lircra are seldom usmI for any pui^ose. 

Ni^xt to tli« rotating probably the thawjimj nom-fnei-e is thn 
most imjiorlant. We do not know from whom, aud when, the idea 
«l nn omins^inMit by wliich an objective could Ih< rapidly altucbfil 
or d<rtiu.-hi-d oritfiiukiM ; but certain it is tliat the idea is admirable, 
sad one which u scarcely yet as fully appreciated as it altould be. 
U will be quite iuipoMible to go tlirou,i>h n tithe of the appliaiicei» 
vUck hare been invented for this purpose ; it will be suflicteut to 
Ity down some princijiles and tuention a few in which tlioee priii* 
tiplm are fultiUed. 

TT»e fir»t i>rinciple is that tlie objective or nose-piece, adapter, 
« whatever else is used slioidd ' face up.' Tltis means Uiat a tlange 
turned true in the lathe should ' face up ' to the tint Aide of the iiose< 
piece, which has also been turned 
true. Tliis 'facing np' sliould be 
uwde tight by a sctvw, inclined plane, 
cr wwlKe, iic. Unless this is done 
you have no guarantee tliat the axin 
fif iht objective ia jmrallel to that of 
tlu- )>ody. Tlierefare all those appli- 
ances which merely grip the (il.iec 
life, or an adapter wrewed on i > i lj< 
jective, «re simply of no \.tlui^ 
. >i>dlr, the appliauoe, whatever it 
should lie light. 

Kachet's changing nose-pieoe 
which fullilt none of these conditions 
cannot l>e called good. The nose- 
|uece is large and heavy, even for the 
mnall objective it is intendol to tako, 
the screws of which are i'(iiniy in dia- 
meter againi^t tJte {i of tliat of the 
Society. The objecli^vs arc held by 
a spring clip on a small llangc. Of 
course, srTew-i.-oUftr adjiittinpnt with 
such A device would t>o simply im- 
posuble. Zeiss's sliding-objcctive 
cbanger is iuobI elaborate and ei&cient, although, as we tliink, much 
hnavicr than it need W. It consists of ft grooved slide which screws 
on to the DOse*piece. On each objective ia screwed au adupter to slide 


Ptit. iva.~ ZcIh's (tldinc-DlijoPtlvD 
cliBiigKr. « illi alijvrtirti iii potilion. 



Fid. lot.— Th« ohjpotli* OuUclinil 
(nilu lliv bolj'-iilitla. 

into tHn gix>ov«<1 nosc-)ii«c«i. Th«sc mlnjitM^ which airt u-Mlgti-sluiped 
and ' fjM"'- lip,' Imvp two novel fpa.tnr'"*, tlii- tirKt liriiif.' ttiat tlicy iir« 

(■jich fittitl with nvtiuiaulnr coiitriiig 
A(lin»tni<-ntK, which piTiiiit the (>lij*ic- 
tivrK til lir pi-ntrwl to one onother ; 
iini! tin- sfcond i», thnt tht-y huve 
ftilnpti'iH to <<(]iiatiM' the Icti^i t4 the 
olijf'ftiviii, sriw-hm i» ehniigecif olyei:- 
tivt'N ix iniidi' httlr change vt fuoiil 
ndjuKtiiient ii rfiiiiin-d. Ki^^ 193, 
lii'l »}n»w tlif nattii^" of this iirmni^e- 
mvnt. In Xi^Uon'* etmn^pni; none' 
piece It small rin^' with tliree Htuds in 
NCrcwed (in to the ohjective ; a uosb- 
piece i» scrpwi-d <ni the mivTviMM>p«> 
linving tJirec Jil'it* iin<l three inclined 
pl&neit. Thcrefoi-e. hy plnciiig the 
studu into the clot* Jiiid ftivins the 
■ ilijettivn 0. qmirler of a turn, the studs 
run up the inclined phuiesthuiit^iLUBin;; the flfiii«»-Rtiiface up ti^tly. 
Mr. XeUon Ima points out a far better umi simpler meth-nl 
whirh dinpeunes with all extra apparatus. 

Three portions of the thread in the nnse-ptece of the microscope 
Ttsi:If are cut away.nndnlso three port inns on the ivcrew of tbeobi««Civ«, 
Tho«e portions where the thread is left nn the objective pam throrugb 
those Kpii«!N in the none-piece where it \m% Ix-en ent away. TlM 
8crow mgOKfit just as if the wholfi jioitw were there and the objec- 
tive facM up in the usual tnAnuer. Thii plan in no way iiijurM 
eithrr the nucMecope or the «1ijectiveii f<ir use in the ordinary way ; 
thn* inirut "bjectives will OL-rew into the none-njece, and cut objec- 
tives will screw into an uucul nose-pii-ce. Tin* plan is similar to 
thnt employed in closing the breuoh of gun*, and it w'us seeing ono 
of tlienj in lt*!*2 which su^tji^led to Mr. Nelsmn to adapt the saino 
principle to the inici^)Si-ope. Subsequently it has lieen foand that 
in ISfiy Sir. James ^'oean had jiroposed niucii the aanift ]ilaii, only 
cutting away two portions instead of three ; it is curious that «noh 
ait excellent idea was allowed to drop. 

An •tiwlytiug noie-pieoe is that which carries a Xicors analysing 
priaiu for polariscopt^ piirpose«. Tn some the prixiii is li.ied in tho 
iioee-piece, whercais it ought to W capable ijf rotation. La«t)y wo 
haven rrmb'iiuj noso-piec« for the purjiose iif testing olflwtiro-ii, 
yiv. Nelson, in a paper read Iwfore the Quckelt Microscoficitl Cluli, 
February If"?."!, statwl that he had observed that certain o^octJvM 
performed better when the object was ]>laced in a definite nriinuth. 
With ft view to eliminate any possible alteraliun wliich might ai-iM> 
from the revolution of the object with regard to the liglit. h« had 
designed a revolving nose-piece which enableil the objective it*elf to 
lie rovolved true to the optic a.vis when any imperfection in it« 
performance in a particular azimuth could 1* imme<lial<>ly noieil. 
ThiK plan had, however, Iwen previously in use by l*rofe«or Abli* 
for a liniilnr purpose, but not, as we beltere uade public 

ai*ooEsnos» as to hndeks 245 

^^^^ . finder is « verj- intportaiit and viiltuilili' iiilditiou to 
n mioifli^^ By ite means the powliou of any [wi-ticuUvr oliject or 
|«rt ot an obJMt in a mount cmi be not«d, so tlwt tt uiay Ijl- found 
nfoaa on mij- subeeqneot occMion. In working on a microsoopp 
irithout A find«r it frequently happens tliat in the pruaecutioti 
of special reMinreli, or in llie examination of unknown objects, 
lonvetliing is seen nhidi it would be of the utuioet vnlue t'l recur lu 
■^in : Iwt tlie aniount of time lost in transferring tlie object to a 
■IJiDd witJi a tinder is an great that most experienced niicroscopista 
do all their setirch and general work on tbeir beot iustrunients witli 

Ttie UM-fulnMS of tJi« finder has caused a large number to be de- 
\iEe(I, but. n* in aU cases, «"» only connder those which we believe 
embody thv )>rtt pmctiojd prindploa. 

The finit, Mid by fur th(^ hr-xk, in the graduation of the stage 
plates of tt me«hnnicnl ntugr byilividingan inch into 100 parts both 
on the vertical nnd horisontnl pIntrK. The vertical stage-plate will 
tfcsn iiulicnti' thv latitude, and ibi- lioriznntnl pinto tlw longitude, of 
the object, tlw sli]> Ix-tn^ always pnwtcd clow home against » pre- 
par<^l stop. For ns-my yi-nrii M'-si-rs. PowcU nml Lnaland havr supplied 
their No. I Ntiinil with thiikinilof lindfir ; nnd its pominnrnt piuiition, 
and eoMt in use, not only givn greater facility in special rrsearcltes, 
but in reality atlAt-li a new miIuc to cvirr^' sliilc in the cabinet. Huch 
a worker at critical inutgea att Mr. Kclnon hiui wc4>k« of doeo work 
'brnrd ' o« thi» UMa of bin KJidcit. A »till bcttttr plan it to ' tog ' 
ill books in which thtf nlid<!ii uv numbered. The result is that tlic 
labour of d»y» and weeks can I* in a moment recalled for dcmon- 
ttnttinn ; aiul m) nccuratc is this method timt an objiict no small as 
n Ibvl'riniH firrmo or a upecitied minute diatom iti a thickly 
icnttrml mounting mar be at onoe, aiiiI a» often as wn please, 
n^1ac<:«l in tiie field with ■''vcn high pnm^n. 

Thcite finders of c<ourM) ai« only suitable for the microscope on 
whi^ tlie * log ' was taken. It in lx?neticiul, and rviti needful nt 
timea, to interchange sp<N;imens or refer an object to an cxpt-rt >it a 
distance. In ttiat cnae a minute ilot muy lie pUic^ed on the cover, or 
a single selected diatom or othtr obji<cl may br lixed upon nnd ita 
latitude and l<>tigiTudeasTeadonthemicn»oO[)e«f the sender marke^l 
on the slide. If tlie receiver then pUoea thi.i on his microscope ajid 
centres it, the dUfrT^noea in latitude snd longitude may Ih> noteil, and 
will give the i-otislunta for the oorreetidti, which luust be uddetl to 
or MibtnMiCed from the figures given by the sendf r. 

Mr. Nelson liaa made some very practicnl suggestions touching 
tlie iiiipro\-em«nt of finders. He suggests, what we hcnrtlly accord 
with — 

I, That the stage -stop shall be always on the left hand of the 

S. That the zero of the horizontal graduation shaU l>e on tlie led 
hand of the scale. 

3. That the iieco oj tlie vertical graduation shall be on tJio top of 
the scale. 

4. That when the finder is placed to 0, 0, a spot marked on tho 

246 ^TsEcaesoRY Xi 

liottom edf^ of n 3 x 1 inch brass t«mplAte Iwa iticb«s from thn *ti: 
slialt be in the optic axis of the instrument. In other woritx, Uu 
latitude and longitude of the centre of a 3 x ) inch glats slip Khalt bfl 
60, 50. 

5. Tliat the divisions shall be in jJigtliof an inch, and thcscalc 
oue iueh toiig. 

If these vciy simple siiggpslions were ndopted gpnerallv, an olije 
found on one microncope could lie easily found oti aiiT othi-r. Thil 
like the * sociefy's screw ' for ol>joct-gliuse« an'l n aniverHhl Kub-itt 
dtting, dGSor\-«H, in cho intereiits of it)t«mationnl iiiic4X>M)op}', 
rtinsi deration of optitiann, 

111 pnictioil 'liigpiig' thn um of a hand lens wilt miiibk Mm ok 
server to rwnd by Mttimation vory nccumtety : hnlf a divisaim can * 
wry npiirnximnti'ly judged <if, and this is as t-ldor an will \tc n<\t ' 
vfith tin- higlir-jit powftTR. Wf hiii'i- found, foi- vi-ry tli-licatc worb 
tllat wr could Idl' with nd\'nntagp Intupen the division)! thus: cav' 
* long. 41 ' ; but if slightly nvi^r but not nn i-aliniuti-d half, '11 + ' ; if 
half, ' 1 U ' ; if nionr Uinn this but If^ than 4'-', it in \i>s}tf^ ' — ^S^H 
For logging [luq'tiM-x the Itins »(• re«>iiiun'nd in one of Arias' 'loup^^P 
magnify in;; mx iliann-lfrit. Tb«iy an' ndwiiniblt- iiialrumenls, and 
an? funiislifd with a lmndl«, whieli way In- ustnl i.r not at the will of 
Uie workiT. 

The othiT tlnd«r wt- dmire to coimidcr is called after iu i»ven' 
and is known hb ■ Midtwood'a finder.' ' 

It consiHCa •>{ a micro- photo|,'ra]>h, one giguare inch in siKe, divid 
into '2S0O little mjunres, so thai i^acb i>< ^gtl) inch si|uare. Each 
sr|iia re contains two nunilx-rs, oiii' indicatiiit; thtt liititudt.- and one the 
loii^tudc. To 1u^ any object thn ulide containin;i the object inu&t be 
reniov«^] and the slip holding the micro- photo^ipli tubetitoted for it, 
tJicn tht! tixiin! in the gijuare whidi nioat nearly njcrees with the 
ceiitrsof tlie lield IS noted. Of coune, both the object and tie 
Alallwiiod lliider must Im ciirefully made Vt abut against the stop. 

Tltere are two dr&wbacks ta this lender. 

1. The divisions ai-e not tine onou(,'li, so Uiat it is only suitable 
for low powers. 

i. The removal of theslide, and its substitution by the Mnltwood 
finder, renders it extremely unhandy u-hen UHin<;an iniRiersion objec- 
tive, all ttie more so if the condenser liappenD to be immersed as wall. 

If the Alaltwood finders are loode alike, they are then, of courm, 

I>r. J. I'anttcsek describes a finder,' which appeam to have sonic 
aih-antago not poseei^^ed by tiMt of ^Maltwood, which he considers iu 
vonipsrinon ' timi' wasting ' and ' minute.' 

Two lines arc drawn on the stn^ at right anglea, intt^rsrcting in 
tlic optic axis : these are marked 0, Liiii-s a millimetn* apnrt nn^ 
drnwn pnrallcl to those on the upper half and tho left hiilf of tin* 
lindor, tbiis fpvinfc horizontal Hniut in tho right upper <|ua<lrant, 
vertical lines in tho left lower cjuadrant, and siiuan-s in the left 
tipper one. Ktu-h tm of tlie lines is marked as shown in fig. 19S. 

' Trmn*. 0/ Ihi- .Vfffii. Soe. now tarin. lo). vi. IBIiK, ii. M>. 

> ZtiUth'.f Witt, mir. vnl. v. IrUW, jip. W-*t: i. S. U. S. IMS. f. lU. 

II of 




nii«n tbe object ia in tlie li«l>l note ia tukfn iif llu: two linm »ii 
Tiliidi tW left &nd upper miI«s »( tin- aliile twi ; tliu», 42; 11. 

DiftpiinijWi. — Tb«r« art* three kinds of dinpliroj^ut in ate. 
First, the POtninaneM fonn is Uiot of a roLntiiigf iliix! of u»vi>ml apur- 

Kl... IBlV— IJv,— .k'l. fm.l»f. 

tnmt gnuluated us to ftixe. Seotwdlj*, a Keri<>(i of n^piiniti! smnll (liM;ii 

••( mHnI, with a titiglo ooQtral aperture, which lits iii » suit.-ibia 

(arm-r. Tbtnlly, tlievo is what ia known uh the ' Iris ' Uiaphni^iu, 

whicli, in ita now form, made with aixtveii iihuttiin, luia been brought 

to Kteat perfection by Mr. 

Baker, and ia also lienut-ifully 

iiwdr hy Zeiss, nx uhnwn In 

fig. 19^. In whatever fonii 

ihe diaplmgni tnny bo which 

is iiv UM) u-ith tbp mirror, it 

is iDiportAiit tliat it tihouki not 

he plBcml t<io ntuir th« objact, 

att rbnn it doc* not lit: in tb« 

path of Ihd cotii!, but at its 

kpex, and will not cut (Jic oomr 
'itnle9« it br rso(Hr<hn;()y miinll. 

A vrry HUiall diaphnigui iip!r- 

tur« i* objeoti unable, an it i^ 
J« to introduce dilTniu- 
I tional rffects. Therefore it is 

1>Ht<'r to u>e a lur^er auei- 

tu^^ further away from the stago than a pin'hole iwitr thr ttiLg4% 

When a diaphragm is u^d in connection with il cnii<h-n«T, it 

abould tie placed just behind thit bnek l«ns, and never nliovi; thn 

front lens. Cakrtte diaphragms placed ctose under the stage, and 

Fill, Ituy^^ZiitMt Jni iliii|>lir»i[in. 



latrlv Ixtth li^re and on the Cone 

whidi hii.\^ litmi much ir 
nrt n niuituki*.' 

A vpry gt><x\ way of uutlitiK clown a twie Iroin a mirror is to have 
thi- (lln]>lini;;iii litted »o that it ciui Iw iiiaile to ndtnnec or recede 
from thi* oljjpL't- The udiuDtAK^ tlms j^uuied i« that one .ipenure i* 
mode lu<ti>Uiedt)t)-i>f Mveml. Itdao{>cnmtaof ciirefuladjustiiirnt. 

Tliv iri^ <lin|>hriiK">8 lisvt- now l>een hrouifht to such |)erf*>e1ion, 
iLud Ktv 9u cuuiptirAtiiely )ii«x)>etifiive, tlial tlt«y Imve superveiled fov 
^pmeral work »tid Dnlinary purpjaea all othera ; )>uC vrhat^^or ilin- 
pJirtifpra ia u«e<l it should it<ori «ntil{/. Iri« dinphm^is work unte- 
tiint-KSoatiflly that the microscGijie niay be moved l>efori>t)i(-Hiiiphnign>. 
So, too, v'ith the diaphragm wheels, some require a |>air t4 pliers 
before the^ cun he rotated. Tliia ia nwtly accounted for wlivn ure 
examine the way in which they are tixed. The iisiwl method in to 
screw the wheel to the under aide of the ine<tel stage. Nnw, if tlH-m 
are neither washors nor a shoulder to tJie wri'w, it is morr tlian 
prolmlile that wh^n the (linphrngm i* rotated it will m-i-ew hii and 
Janili. TliP jiUivhiiRrr nmy ctisily <il«i*rvc a matter nf this kind. 

Coodenteri for Sub-ctkge Iliuminstion.'— Thix cnndraipr i* nn 
atwnhilely iniiiispoiiKjibif pari of n cftmplcte niici-o«co[M'. It* value 
(nnnot l>f oivr-rntr-d, fi.r the nl>ili(y 01 th»' Ix'tt iMwe* in do thi-ir 
Ix-xt work, even in thr ino^t ^ki1ful h.-ituln. ix d<-t<-niiiiii-d by it. 
Pericction in the <'orreL'tioni' of nlijert-f^liuunit it iiuliiijH'ji.-uiblv ; but 
t1i».ie who KUppooc anil aflinii that lliis is uU that vr diihI tliitt Xh*t 
obit'cli>'r in tin- ijiiiMiiwnpe- cannot undiTstnnd thi- nuture of modern 
triticttl work. Thi- importnnce of it i-ould not haii- Ixt-n n-alised in 
the w'n»p in which wo know it in the mrlier dates of thr hi.itory of 
till- inrtruiiiMit ; but at a<i early n period as IG9I we pointed out 
(p. l.'{-'i> thnt a drnwinjzof Monanni's horisiontnl mieroHK)i)e showed 
till' prrnrnci' nf a compound condenvcr. It tx, in fact, of aoui^- 
intt'H'st to noti" how our modem condfnsrrs (frndually nrose. 

Tlir miCT'CMOdpe that iiliion;<st tlie older fonn* (ll)94) appean- 
moat ellicieiit and Kuilcd for tlie examination of objects by trans- 
milled liitlti wan that of Hartiwcker, p. lH.'i, fig. lOli. It will l>e re- 
membered that it not only waa furnished with a eondenser, but with 
n focuniiiK armngi^mrnt to ^f- unmI with it, which wax not in any 
way affected by a change of focus in the obj^-ct. Thi« l» « feature 
vi-hidi, altboufcb not iJien important, i* of the utmost imjiortance now. 

In the fon*ctioii nf (lispention in the h-nntr* ■•mployeil in the 
dioptric form of luicroacojM" ui much diflicultv wnn exiH-rienced that 
seveml efforts were niJHif to iinxluie catoirtrtc forms of ihe inHtru- 
ment ; the most succeHnful of tliese wn« tliat of V>T. Sroitb. of (,'am- 
bridife, in 1 ?38 : but ihis and all ullnr formn of refiectins microiteopo 
had but a brief exiatence atid piiMHl for ever nw ay. To tlie iniprave- 
iBent of simple lensea much of the mrlici- proj;rem of microacopio 
inveHtiifatioii is attribuUble ; and tlmt known ai * WolUston'a. 
doublet,' deviueil in }i*'2'J, wils n <Iecidcd improvement in all reopect«. 
It consisted of two piano -con vex Iftiiim ; bui tliii wut again improved 

> Qnrlciftt. Unrrv, Joarn. ToL iv. ]>. 191 etiat. 

' The irord 'ouideoHt' tlimii)[1ioul Ihi* wurk in aiiiilinl Ui ojiUml tpplUnMB for 
Ih* (uliHilagv; «)»( ia knnwn ■■ Uio ' bull'4'F]rD' ■• n^t called ■ ' Minilenatr.' 


,wrbo nlUrwl thf ti'iii. ilUtAiK-iw niul plHc«d m diupbregu 
IpDM-a. WhMi thi* iihji^ wn« il]uininiitc<d with « coo- 
dmsfr U»i» formal whut w<u« (Iw t)C«t dioptric microscope <if pre- 
kchrotnktjo tiniFS. 

<iood nwnll*, uithin cortttiii timiU, may be obtjtinwl hy means of 
tbv bo»t PriU-hnnl douhlittii. Witli » ^^th inch tbo surfoce of a 
strong P()dum "--iilit nutv hn lu-m »■ 11 nurfnco sjrmmwtricnlljr sc(ir«d 
<« rngnivwl, but the Editor hw* iw-vcr himsnlf Ijpon ubb I" rcvonl tbe 
' f xclKntMtion ' mnrk« ; and »» tliii; is thn i-xpcrii-iii-i- of other *liici*nt 
nqwrtK, it vamy be tnltfn thm no ii-Nolution li th™r w** nt-ri>iiipli'.1i«d 
{Mprfr-aiehfOnaticdkyK; tlicsf Ioim-s in fftct, ovcrlnpprd tiicdisi'iiverj' 
<if iii'Iininintuiiu. 

Bat tbo practical mallH of thn umi of nrhniinntic ]inis«8 vion 
Inl thti mo«t f'Xpericnced mrn in its tbi^Ty ntid pnu-tt<H> tii permnt 
tbit if it wi-ri' good for the Irnw^ wliii'h formed tin- imager', it viw 
nUu gnnd for tlie OnrndlMlMT. TIuik Sir l)tivifl Brewster ill IM."!! imI- 
TDCntrd MI Ai'ltrmiintio condenter in tliese remiirkiilili- words, vii. ' I 
haiv no hrsilntion in Mj^ing tbnt the [ip)HtrntuH for illuminiition 
rr^uirr* ta br an itr/fct lui thf ni/pnriilxt/'ir n'«>V>»,anil>>n thi«iU'tount 
I vrould reciMUUiend Ilint the H/iii,iiiuttinf/ Irnn *hontrl hi- fterfeetlt/ 
frttfitftn fhrotnntlr atui fuhrricnl aljerralian, nnA thiit tiio gnwt«*t 
an be tnkttn to cxelmk nil extmiieout li;{)it Imtti from tbe fibjnct 
and En«m the *j-e of tlie obncrrer.' This iti n judxnw-nt which every 
tuJvancc in the ooiLttructinn of tin- optical port of the niicni<tcoi>e, m» 
B*ed by th« nwHt ncconpltsbed nnd cxpenenood expcrU, luut fully 

Wf hMvf no knowlMl^, from ad iniipection of th« piece of 
apfMishi* itself, of the conatruetion <>f the compound sah-stoge con- 
(Icncrr (if Bonnnni (p. ISA); it do«H not ap[>Mr to haw attracted 
OKich ntb-ntion. nnd of course it whs quite impossible to secure a 
triticnl imagcv by it* in«ms. It wii^ tocussed on the object merely 
(0 obtain a* bright nn illtnuinatiou aa poeaible in order thnt the oa- 
j*t1 might l)e neen at nil. 

In the o'tiiifiiHer used by Nmilh in his catoptric micmecope 
(p. Nl) «e huve tlie earliest <1"3K) known condenser, by means of 
whti^'h n di»tiiu'ti<in iM-tweenA ' critital ' im>ige— tliat is, nn imnKein 
■riiich a ahnTji, cliMtr. bright deliuiti-jvi is given throughout, freeman 
sU 'rott*uu<'M'rtf outline or detail — nnd an 'uncritical ' or imperfect 
image could b* mad«. It was not. apparently, nt the time it was 
Sret used ^onKiibml to ho no important ax wo now know it tn bo ; 
and it is prohnblv tliat tl» mode of focu»>ing the light apiin the 
object by Ita m«<aiu wa* to direct tho instrument to the sky wnth 
«W hand and to use th« liicimvox condenser with the other. In 
1837 Sir O. Brewster writes of it with appreciation, ssying that 
*it performs wonderfully wnll, though both the apoculn have their 
polish cxmsiderably injured. It shows tli« linea on s(>mo of Uin tmtt 
otgecta with %'ery eonaiderable Khnrpiii-sa,' 

Ko adt'ance was made on this condenser for ncnrly a cuntunr. 
In l(tS9 Wollaston recommends the fdcussing of the ttnagK of IM 
iHnjArttfftH by meant f4 a jilano-convcx lena of J of an inch focus 
upon the object, and (Joring in 1 $3-i sayn concerning it : ' Tliere is no 






moiliticattltHi of tUyl>i;!it illuiniantioii au]M.Tior to thivt invented hy 
Dr. Wt^laaton.' Bui Bir J). Itivwstvr ulijett^il Ui UiU. ouiiteniiing 
th&t tlie «NiinH^/^r&( itself nliuiilil be focuik^d upoii the object. Ho 
preferred « Hersoheleinn doulilt- 1 pluced in the (^c ucis of Uui 
miomeoope. But wliU»t iheri* ja » very cle*r dilferen<.-e betwnui 
theseauUioritiea we L-un now nee that boiji wore right. 

Ooriiig, who WHS e.\m n l^uler in tlie mii-roscopy of liis daj, us«l 
difi\ised liayliijht. find iia the letis lit? employed was a pIati<:»-couviuc 
of ] of Ml itioh focus, the luethod of focu^nv' the diaphniffia uhh aa 
good as any othei*. Iiei-ausi.- iho iliaphvA^in wa» placed nt a distaiwu 
from t}io lens of at least five times its focus, so chat the ditferenofl 
b^tweei) diA])hi'agiii foiiie, and ' «rhit« clou'l ' focu-, or the focus&iiiK 
of the imn{;e of a white cloud upon the ohjpi't, utu not verr gretit. 
Unt UrawstT was writing of n cnndlp-llarae wh<>n ho insiste<l on the 
lirin^ing of the ciindenser to a. focus on tlie uliji^rt, and in this hn 
wiw, lioyond all], right. 

In ]^W Anilivw ItOM ^ve some rales for tbo illumination of 
ohjprts in thn ' I'rnny Oyclopndia.' These wi-i-e — 

1. TliAt the iiluminntingconB Khould <m|uu1 the npertura of tho 
ol'jective, and no nioi'o. 

2. With dnyliglit, n nhitii cloud being in foras, tlie objoct was 
to lie placfl niwrly at tlic npi-x of tlie Tin- <A>)f>:t was seen 
b«tt«r HoinetiineJi above, anil soineticiK™ beiow tlm H|iex of the cono. 

3. With laniplight a IrtiU'n-eyo is to Iw \itnl tii {Kirnllelis'?! th<i 
r»y^to tlint they nmybeinrailHr to thosn coming from a wliitoehiud. 

Of the old fomis of cnndtmsnr, that di^vixitl by Hi: Gitlott was, 

there can bo no <toubt, tho 
Iiest. It was achromatic, and 
had nn aperture of CO", Kig. 
I'JT illuiirateK it. It nns 
titt««l with n rotating ring of 
d iaph ragm splarcdcloso behind 
the lens cumbinntion, Thia 
wiL* fornn'd, as tlio lagnn 
shows, by a conicftl ring witJi 
ii[HTtun-s nnd stop*, and on 
account uf the targn nnm- 
ber of n|ic-rturc» nnd stnjiK 
it would ntlmit, which, pru> 
vidcd tht-y nm cantfully ' cen. 
trtnl,' Mrn of K'reAt valui' in 
practiml work, as well as from 
the fttd thut they are so placi^l 
as not to i»t«nere with the 
stage, mak«s tliia arranftcnipnt 
of diaphrasnis and stops au tfxoellent on^ ftod it is not cleur why it 
has fall«n nito disuse. 

It had bfH'ii llie custuni to recommend tlie useof (hia iiutjniment 
rackMl 'rU}^*r ivilliin or H'MinU itn jucii*. Carpenter employed it 
without and Quekntt wiUiin, and one or oth«r oi theee uetliotts vran 
f;eiieraL But in tlie use of good achromatic oondenaen with high- 

Vw, lOT. — CliII«tl'B criiiiluiinnT, from ' He-f{t 



Fill. )&!>.— r»«Tll tnH Ualaad'* 


. it «ooii b«CAine inaiiifeHt to practioAl workors thai it la 
, as Sir Oitind Urewsler i.>omt«(l out, lA« »»ure^ «/" tig^l m 
Jveiu»ett trjf lA^ et/ndfufr on the object tluttit raitlly cricicBl iiuagr wiu 
to tje otrtAiiwid. Anil Mr. Nelson I'ndilydenioiuiraced this inct oven 
vith tlie cnn'l«intrT (iil]«tt linit (l«vi^. 

The ni?xt oondenser of any inonicot is a most \-nhinb1o ono, itntl 
emiatJtutMOiMof timgrmc ni<Hl<>rii improvemeiiUof th'- mi«r<woo[)e. 
It WHB an achromatic <-oiHl>^ne«r "f 1 7 0' deviswl nml nianufnctiii-ofl 
by MeMTS. I^'ivrcll nnd [ji^iilnni!. Wo linve< iintl ttiiK iiiKtniniPiit for 
tw^nlV'tivfl yenrs on fvi-rv ViinVty -.f sulij^rt, and Wf" ilo not hoiuito 
to olliriii lliiit (01- tfi'iH-rai aikI oniinnry ci-ilicnl unric it is Ktill un- 
aur|MM*d. Vifi. I'J^ itluxtniti:^ thi< apparatus. Tlii' op'.ical combina- 
ticmUa 1th of »» inch ixiw'it, anilitis 
tlMrr«fora luori- Kuitalilf for olijiftivcg 
from • Ith of nti indi iind u|iwarr)N ; 
hot by lyoiovitiK the front Inns it niny 
be tWMl «-itti nl>J4><-tiv<!s n» low aK ono 

Hnriii|> givi'n to tit\» tfuulvnuii- »> 
hi-^h a [iliu'i^ 11 1 noil ^t I' vim tbixnof dui- 
iniinrdint)! tinu'w, it' inav be witll to 
upecify what thi- (vciuirRinciits art? 
whtcti a condf ntM>r nn{>loyMl bi crilii-al 
work with hiKh jw^ers thfttlil ine»U 
It in nc«(lful tliatw#t>)H>ut(l bea)il«(I) 
to obtain nt will tJie lar^Jit 'lolid ' cnn« of light ilevnid of uplirrit-al 
nlwrmtion. ' Uirwutly spliirrical nbcmition ninknt itM-lf apparrnc 
thfi comlmtxT £u1h ; tJiat ix, wh«n, on oc^coiint of uiuIiT-corivctinii, 
ttin centml ntys ore bmufifat to a longer foc-u« tlwiii tho nmri.'iiial 
rays, or wb^n, beoaiuie of over 'correct ion, tlic; niar^inal r>iy» h»\'i' it 
longer focwa tlurn tbe ci^ntrul. 

But (:;) it is aIho Bti Hbs(>]uti! nuentiid that if n condeiMcr is to 
I* of ptsctioti servieo i I must hart! a wiirkini; ilistnnce MUtficieiitly 
lorifit to eiuible it to lie focuoHil thnm;{b uniiiuiry allpA. It would 
tn- an odvuntni^ if all objecta ninuntt^il for critirtl liiKli-prtiii>r work 
wrv inotinted on slips of a fixed ^'iluk^, siiy 'O'i iuub. whicb would 
W ' in«)iuni,' 'U'l inch l>eiii^' ai-eoMtit«rl • thin,' and '07 inch * thick.' 

It in plain, howevei', iliat tocombine a lar;^ aperture with a (;reat 
wurking <U>t1anc« the skill of tJte optician is fully taxed, for this can 
only be oti'oniplishMl (•■) by keepirig the dianieter uf the leiit^s Just 
Urffe tnouKb to transmit rays of the re'iuired aiif>l« and no mure ', 
(i) by working the convex Imusi to tlieir edge ; («) by luokin;; the 
fiat letuee as tliin ns possible. 

Mow it tsduetotlie eminent Firm whose condenser we have been 
coiuideriufi; with such apprecintinn. to say that the condenser 
rtfen«d to (•/) transmil^i the largest ' solid ' cone free from sphencal 
j^rraCion ; (e) that it has tlie greatest working distance : {/) 
Ikftt iU chromatic aberrations are perfectly balanceil. In tho po«- 

I Thik U oat of 1^« inADj oxpreuont which «rfl incrjU-blc to tiie pn»lic«] qh d 
•|if«i»tuii; il is iWRiJy r«siYta>i«i>t, anil RMnnii n/ull Ccrae ol tidlil, * coii« villi uon* 
>i[ It* r%y MoflHd out. 




Hoafiion of tli^Hc tlinn^ I'luoiitinl ijunlitiea it Iiok Ktuoil u»riviill«d fo 
upwiLnts of tliirty ywirs. 

The i-emovnl ot tlin front leiiH of iliis <M>nd«iiHC'r, which mny 
nsftiUly uiiscrrwott, reduccvi it in jiowt'i' aixl angle, nnd th^rafor 
tnftlcea it suitdblc for objwtives of lower (tower. Tliin, however, 
rather ad odnptjition itivolvittg cuni promise thnii iu\ id<«tl oondms«r^ 
for low powMx. When the lii;(hest cla«» of work has U> b« dons it 
ia jit^(ftil to Ativ ei'fleiuirra imil*d 'w Ihf. }ioKirr <if the okjrfiirf lud/. 
A low 'power ooiideiiser of nucli merit la tandie by Swift nnd 8on •, 
it beeiiis, in its n>lati<in to low powers, where Uil- c(m<li>i»rr of PowpH 
aud LeAlnnd ImvfH ofl*. It cotiM«tsof two doublrtB with « *iii)(h- front, 
iind is much I<»wit in botli power and Aperture lluin timt tif tlic Initio 
nuikeis ; Imt by sHdinR off the front cnp itit<i whioh the front lens i« 
burnishrd Iwth ]i«wer nnd nperture nmy i» further reducrd. It L* 
ochroinntic, nnd is a prncticnl nnd iiscfid iiiHtruiiietit (^a]1ablc of Md»p- 

tfttion to any mien Bt-ope. Fi;;. 
199 isKfteneral illunlnttionof 
this nppliaiii'^. 

A coudeiiBer hariti|; con- 
si<l(Tnible value, wid Kjiecially 
ndnpted to leiuai of low jtowi^r, 
and up (I) tlioae of ^ inch 
focus, has ,iusil>eenconatruet«d 
(ind ])tai-ed )ii our hnnds by 
niesNrs. I'owell and LealaniL 
1 1 WAN made in Teaponse to th& 
Mim«&t HUfQcmtion of neveral 
leading niioniauopiMd, »nd in 
lunuy respects fully an»wen 
it* purpow. It isaclirMDatic. 
has a nunienoal niwrturr of 
■83, with an aplnnatic aprrture- 
of -a, and (or dark-ground illumination 'a poaseissed of th« lii)th«it 
qualities. Its power i» a ij incli, and will prove a miut uarful 
adjunct to the photo- miiTogmplier. unce it will enable him to ^et > 
Ift^ge iUf^^ of the source of lijfht on the object; hut it« abenutions 
are not so perfectly iNtlunceil as we could de«iiv. 

It is poMeswid of a new feature so far a« the condenser of tbeoo 
makers is i-onceriied, having pprrannentlv placed beneath the 
c^ticitl arrangement »n irit diii/iknifim, and in addition tlie con- 
denser mount is Huppticd with a bcHcm of diaphragms and stops which 
are plaowl in a turn-out-nnn earner ; thJB provides the worker «ith 
facility at w-ell as accuracy <if method, since hoth of these can bo' 
used under the same ndjustment. The aperture of the cone trans- 
mitted by the condcniHer with t^ach diapliragni is rngraved upon that 
dujihrMgm, nnd with tlie stops for dark gmuiid : the aperture of the oh- 
jectivo witli whicJi the stop will yield a dark ground is aUo ent^aved 
on it. Thia cmbodids the recommrndntionn we havn mado lielow. 

We give an illuntration which is self-es plana tor}' of this appa- 
ntuR, fig. :!00. 

BefoTR the introduction of tlie homogen«oiu Mystem, and the 
production of auch gratt aperturr-x by Powell and Lealand a«. 

Flu. I iw,— SHI It'll ivmaoniior. 


iVth of nn inch fovait, tlis cono 
uvliroDiatic t.'Uiiilenser was as 

« 1-5 in a |th, a '^'^tli, and a 

tmnainitted by FoneJI'a dty 

UrK« an l-miIiI lie utilised. Itut 

witli aperlur.M §uc]i as theHe. and 

tli« mi1iaequ«Dt introductioti of 

tiw apacliroiiinti« systeon of 

leosea, mui-li larger vonoa were 

rvifiiired. Toin««t this necessity 

l^]weUall<l thour^iit 

sUh';^ntioii of English expert8, 

iiiaile tint a ctiromatk condenser 

on tlio homogeneous system; but 

lliis was subsequently sucrcednl 

by an iMlironMlic iiiscninwat of 

KKMit value «n t}w> snmo systmn. 

Tliis rOdibirialtmt ronviitK nf a 

do)iU>x front vriiti two doublet backs ; it ia nmrly of the lanio piiw<^r 

AS tlieir dry arlironiatii' i-oiulenttr, but is of itiuch ^rnttiar ajwrtun:. 

It bas linen lirr>i][,'lit uUl mon* roceutly to a very higli fXnU; of |itT> 

Via. «M,— Powdl and LMknd'i nra Imr- 
famr eaadmmw. 

fill. 'Jrll. 


Poiii'lJ Atiil tvi^^l^id'h liifb'[«p>iftrr lU-JiTiiiiiAtir rondcnM". 

(action, liavjog ati aperture of 1-40. It will work tJiruu^li a niouiit- 
injK slip of -07, anil f»r aperture and working distance is, liku ita 
dry predecMSor, (|uit« unapproaclietl. 

We pment a g^iternl view of this iiiHtruineiit in Sgt. 201 and 
S02, but it will bo found thai oUier ntopH than iIunm Ulu.-ai-at«d will 
be Inquired, «otno of thesis bciii^ of litlk' ui' no value ; while tlm 
«top8 tnade witli riu)i;s may be made iiiucli l<->a expensively of tiio 
nnie forin, but without the outer riii);H, hivin;; meivly lliii^ ]x>iiitM 
or arms to rast upon tb« edff» of the aocket which rowiven thcui. 



In iu origin&l form it IimI tlio gtvnt disadvantage tlutt tb»| 
1(KUS»ing hoc) to bo v.-hollj' diitiimingcd to chango a diaplirngin. bub | 
it occui-i-ed to th» prc*Mit Kditor th/it this could b« obi-i«t«l \>y 
sliding tube carrying uim ring wbich ivcei\'cd tlio diaphragmi. ' 
Originatly tlii« vf»s tixiil m R, (!«. '_'0l', thr Korlcct fortliodinpbr^nxl 
being turned oiit from Hint poviticm as in K, Wr. -HH. By ptacinip > 
this 01) n Klidin^ tu)>t? vre can hIiiIc the (liaplirngm rarri<>r down, i 
quite within n-iti-h (<f rye und Imml, lut iv( A H, lig. '201, witliout iii 
the least disturliing llii? optical comlnnnttrui ; utuI whcnadinphragiu 
lias b«Mi rcmovwl ami another rcjiliu.'ed in li, fiR. 201. is tumod in. 
and t1i« whole in ulidden up into po^iition by tncani of A, a« shown 
in tig. '-'0-J. 

As theaf Eilii^ta ar« na&nng tlirough the pren^ Mcwsn;. Powell 
and L*-al)uid hn\'v placed in our bandit an eniin-ly ut-v oonitcniwr, 
strictly n]n)oliroiuutic, eiuployui;: a fiuorit* k-na in iIk* cmnbi nation, 
nnd pi-('sfn{in;t£e!itui«a in the lii;'hesl de;;ree dmindiK Wi-tind iU 
N.A. to be 0'9S, its focal leiiRlh long enouKh for a lliiclc slip, its 
aplanatic ajwrtuie ■9. It will l»e of grent value in critical work. ' 

It is easc-ntuU for ideal Uluuiinatjon vritb trauunitted liKht ( 1 ) that 
the illuiuinatiug axial cone should lie approximately eounl tu tJie api-r- 
turn [>f the objective used ; {") that the object should ue ]>lace(l at th» 
apex of this cone. 

If an objective breaks down with this ideal ill uiuf nation, wlilob 
I» ,very probable, we must be content to sacrifice ibe ideal ; or, ns 
is aUo exceedingly prolmble, the object under exauiiuatiun lacks 
oontmut, the ideal method must l)e modified. But if we havo a 
■iiitnblc objivt, and a perfect objective, it in the dlrong conviction 
of sonir Icndius cKperld Ibat, as we increase the cone in aj^rture, we 
iniM^-ase the perfect rendering of the im«ge, until the point is n-ache<i 
whi-ri! the cone from the condenser is equal to the aperture uf the 
obj(^'tivc. aiiil, wluitever l>e the object used, it is advisidile not to 
exceed this. With the movt perfect ohjectivea of the present <lay, 
we tiiid ill practice thai the best rcsulls are obtained when a cone of 
ligtit in u»ed, which, on the rrnioval of the eye-piece, is found to 
occupy three-quarters of the ni-cji of the back lontc of the objective. 

No oondenser is sufHcicntly free from spherical aberration to 
transmit a cone eijual t^i its oicn npcrturc. Condensers are all more 
or less under-con-ected, nnd consequently focus their centrtd rays at 
a greater distance tlinn tbeir marginal rays. If wornck up thecon- 
dejieer so that the marginal rsiys arv focussed on tho object, the focus 
of the rays which pass through tlin centre will lie beyond tlie object. 

It is well known to those practised in mici-oscopy that in the 
case of a narrow cone, from a well -stopped -down condenser — that is, 
a condenser used with diaphmginx uf retatiM-ly small diamet<T tlin 
illumination is at its greatest intensity whi-n the object i^ at tlin 
apex of the ilhtminnting ctme. anil if the condenser is racked either 
upordon-ii the intensity of the illuminatiim is mpidly diminislic<l. 
But in the case of a cnndenaer with grcnt aperture, if it lie mck(«d 
Up the mnrginnl rnys will have their full inti-nsity, while thos^ wliioli 
passed through the central portion of the oondeniier will have n 
dimini sited intensity. 

The exl«nt to which this will take place will bo wholly dependent 


M the amount of umler- correction ]>reaeiit in tJio 0Dnili*nMT. In 
«MM^ c«D<li!n«-r3 the under-correction b .10 w>ri»ux, that fi oUtnin 11 
wid«-, or prcn » moderate cone we bo enfeeble Uw «nnlml cone hh 1 
(o trdncre ii almoot to DKre aoiiulftr illuininntioii, which is not k 
dfldntljli^ <jnality. 

It will I* seen then thiii ihe ajierture of the <xmp of tight tmns. 
lEiilti'd hy .1 I'ontlenset plays a very ini]>»rt»iit jiarl in KiviiiR tritival 
qvalitj- to nu ilnaf^e with clifl'ereiit olitedivrs. We aliouM thi-rr-forc, 
totwc » cowleiiser accurately, be iible todetetuimc the njierturi' of 
ihi- cone we are usin;-. 

We tuay measure the total aperture of a oundenser just as we 
ilo that of an objottive, viz. by mcianH of Abbe's npc rtumeter.' But 
Utr- ^fri-lti-^nji^rture cannot be meMsured in that wiiy ; Uutt is to Kty. 
tlw n[>ertur« of the Inr^^t a]>laiuitic cone, or cone free froni spberictt! 
alirrnttioii the condenser in o«pable of gir in «, cannot Wsodiscot'ert-d. 

To do this place the condenser in the sub'Stnge and an obiectiie 
on the nose-piece, focus bot/i upon an object. Let the edge of the 
bunfi-llame be used, and so nmuige the focus of l>oth optit-^al coni- 
Unatio4u that the edge of the denr image of the lamp'tlnnie falli 
cFntmlly upon the object. Now move the object just out of the 
BeM. remove the eye-piece and exuniino the iMck of the objective, 
a&d if the aperture of l]ie npinnntic illuminating cone wgrftilfr tlmn 
(hat of the objective ir will show the linck lens to lie full of light 
(ftll. 203). Thei*f(.i-e, if the nperlurc of the objective is •.I, we know 
tlmt tbe aplanalic illuminating cone cannot b? lesi than 5, If now 


Ftci. M*. 

KiQ- sod. 


ve ctoae tbe diaplin^ so tlutt tbe iinago of it just iippfnr^ nt the 
back of the objective, we are able to determine the H[H>rtun- of the 
illuuiinatinp cant- with tliut given opening in the diaplimgin ; thu» 
in fij;- 204 it in a trifle less tlian H N.A. 

In a similar maimer the apertures of the other diaphmgm open- 
tnjfn can l>e di-tt-i'mined. 

Now let the diuplirugui l»e opern^ to the full aiieri'Jre, iind nn 
objwtive with a wider aperture, say 'S.'i, be usecL It will prThiifn 
l« found that Wfore we are able to fill the back of the objective 
with light by racking up the condenser two black apol* will 1«? 
formed on either side the middle of the disc. When we reach 
the disc of liglil llu»t ia largest (fig. 20t>), any further rucking up 
cauim tlw apiirarance shown in fig. 20fl. 7'iif loit jiviiU (■'j'orr tlif 
•iji/wirmic 0/ (A« 6f«cjt s/mlt indieat't thf Inrytitt a/jitimfic ajif'rture 
of iht condrHurr, and u tli« limit of the eoivlnwr for crili/yji ^ry,rk.* 

There are tB*uy other condensers of more or less merit and tise- 

■ ChapUr V. 

> ■TUc Bwk o( til* ObjKtlta and the Condenicr.' E. Kt. S'cUon, Eng. Mtth. 
ml. ilrtil. No, ItM. 

fulneaa, 1>ut wo imi*t <«iilinp our coD«ideraCion to tlione l)i»t will 
])ri>vi> of grcAtc^t valur. for tli^ir several parpoaes. 

A foiidrHtMr trt'wn tig llir ' \\'--h»trr' waa lii'iil iiiiula in \>Mh, 
uiiil ia still » yrxy um'ful one fw- lou- powers. It i* tlie miiiih u 
Utat mode by Kmift,' Init witliout tlie middle comliiniition. Its 
Migle is In*. kikI itw rnnui> is not so exteoiiite : Init its diief 
«oinnmtdatJOD in pi«a>Min;i( tl)rse< (lunlitie^ id tbitt, Iwviii;; uiie ootit- 
hination lew thiiii Swift V, it in of necessity lover iu price, And on 
tliut account will Ik< wi'Ii-i>di<- tn soiuo workers. 

Iti its pri'Jicnt funn it rpifiwsi iu primnry ooiistruction. It is 
»nw uiftde with it il'>ul)lc fivmt niul a single bade, inateud of n sinj^lr 
frtnii Aiid It d<iul>l() \mc\t. 

An •tc/irviititii- •■«ndrniier which Iim boen very liirKi-Iy «wp«l tn 
Kii»lHtid anil Anit-rit-A, nml wlii<rb luui seourad a gmit ileal of ooiii- 
iiieiidiitioii. is tliJit of Prnfi.-K.sor Abhft. The optical pri'luctixns of Abtw 
lire toil well known iind Uhi vnlunbI«,BM n rule, to make It nevdful !•> 
ibeullier <haui>rrfi'ctJyfriink tNiiiceniing Ki) im porta nl « piece of itpnn- 
mtus ofi tJtis ; nnd tli<-n- oin In; no doubt thiit tlie wide popnlitnty 
*>t tliis instrument i* due., not m> uiiich to intrin^^ic merit, aa to the 
fact ibnC it bits Ix-en (tni|il<>y(v) by thiMc diielly wlio, prev-iounly 
ignorant of the ^-nlur of nut; i.ondenwr, hiive aX onoe perveir«(l tho 
«iihanct«l valun of the nvtulU yi<'ldi'd by its irnvms 

To ihotr- wliii huve niiiilf tbr scientiti<! iixn of the luicnMcopc «. 
eareful study in En^lniiil it hiu Ih^bii n pcrKiiitont -.oiirce of m-n-t 
tlial it \\af, Ix-rn no Icnts nnd pbirtinaciouxly tAught that t>he ' L'orrisjt' 
histoloxii'd niicroncopc must lii' .if tho Harlnaok tvpe, ami that it 
^lould be used with nariiiw-niiglrd dry IfiiSfs, jietbapa « Itli-incli 
fiK-u>i, luid no illuininjitinn but that nfTonled by a small ooneiiw 
mirror, the focal mint of whii-h in cxti-cmdy doubtful or unhnoun, 
nnd iu pmclice wholly dinn-jinrilftl. No iloiibt a student iiulMntnl 
«ii ibese lines would Iw lUitonUlii'd iiid<i.-d whi-ii he exehatiKnl siirJi 
n practice' for the illuonnulion and iinprovi-d inia^ alforded by mii 
Abbp oonil''nKiT. 

I.' sunlly Kuch exchange of illuuiinntinK luelliod presages an <•«- 
change of instrunicnt. for thi- Krii'iiiitionlly imperfect and wholly 
un»ti»^tory ' tool ' that la in the majority of case<i put into ibe 
liamU of thr medical studrnt will not lend itiself even to an Abln- 

Thft fact iN that a lnr;;e pi»rt of tlic admiration tliat has been ex- 
pressed for thi* coiuletu«r baa rcaulted, not from a comparison of its 
rasnltc N-irTr (lt»*r o/ nfhrr high-tliin aehrauuttir roiitf'nf-rrt, but of 
itnitgM obtained without any sub-stiig'- optical arrinirenienttt at all, 
plocwl in contrast with the results obtained by usio^' tliis condetiM^r 
MUDst the inme(ibji*ctivewhi-ti uu-d without itH aM, Uiit that er^n 
tncM itiiaifcH are etitii'ely inferior to th<' imaKTK cbtained by tin' 
higher onh-r of achi-ouiatic eonilenwttw wc only require the practical 
testimony of Pnifetuor Ablie to prove; for fir hat xini:'' yw/tc-r'/ nn 
itehramatii^ otHdfnttr i^ muth uieril, tu which wo give consideration 

In its niost perfect fonu llii* rhrmnntir condrnKiT of Abbe's oon^ 
aista of three ungle Iwnxeit, the front Wing beinisphericsd, and tlie 



two towrer lenses fonu a HerMdielcimi doalJet. Tliix comhiiuttion is 
iJiowtt ill % 307, and Ui« gouenl foi-in of tho insU-uinent lu nppliecl 
to Zeiim's uwu microecopes is fthown iu %. 20S. 

Tlie jiower of tkb coiideiuer is low, and its ap«rtur« in wry Urgn 
(I'3C); li«uce lieywid Ili« (act that it is not nelu-oiiiulimHl it h&it 
cDonuous aplierii-nl nlicmtion. Tlin 
itia(uic« betweieu Ui« (od of tbe ueiilnl 
portion «ud of a tutrrow annular tonv 
wboae internal diainet«r is ;Jtli-iiii'li is 
VfltU-iiK-li. Ita aplaniitic aperttii' 1 
therefore only 5. Kow, uhilst it 1 

of 110 iticoii8idera)>l« cbaract«i- lo 
Kv« ail ucliramatised condenser, »nd 
out* u'liicii, with grvaXier or leas eiilliu- Fio. nor.— Oii" 
■iaaui, all workers admit, yet the point 'J •*!,!« -, u, , 

<f vitftt iinportance is tbnt it sliould bt* nplanatic ; tlio b«st coiiuloiutfr 
i> nlwuyi tliiit whivb vill transmit tlie liirgMt nplanntic conn At 
tilt) clone of diis section we fuinish a table of tliv rctntivti qualiticit 
of the ootwieiisera of tJic Wjt construction novr nccrssibUi to tliu 
niicToscopiBt, and a refercitc-e to tliia will sbow that Fowoll and 

lAill«nd'ii ity Mhtouutio (fig. l!>tt) with th« lap removtd is in thi» 
rcKpoct as enJHent o* ihh fonn of Abbe's. 

Thin I'uridt^riMT cuii be vxtiA either drjr or lioino{!«n«ously ; but 
o( oi>ur!U! with objevtivfs of grntter aperture tlwin 1 '0 the Imse of 
Llic iilidf ithould alwuys )h^ iti oil-coiit«ct vrith tlw comloneer. 

It (;ivi-!i t\ii- priti('i|vtl iiiuthtioAtioiis from diroct to obli<|U<> i!1U' 
tnintttioii with tmiisniitlfd Ii(;lit by changing nnd iDOviDg ft set of 
tliaptiraK'i^ pUoed in » movable fitting and the diaphmgiii mty be 
inuved «cceiitmally to thf optical axis of thocondensfrr by mnring the 
willed head. It givee durk-ground illuminntion with objoctivm of 
'•* N.A. : for such illuminntion, in furt, it in, pvrhariN tho Ixtst illu- 
roiiuitor cxhini, and shou> ■•Ijiccl'^ on n <lnrlc ground with Kpnrkling 
bntliaiicy, ■•iiid may l-c useil with polsrixrd light. 

A ohroninlic con<lenser, somi^whnt similar in roii^tniction to this, 
and of lo*' price, is hiwIp liy Sl^surs. PowrJl and T^mland, but it is 
of inach hif^her powi-r. so that the distonoo between tlie foci for 
the ceuirul and pcriplifml nys is not so great, and on this nroount 

it yielda n aoinuwliat lancr 
Hptiuiatic cutic. This in- 
ttruiuent with its din- 

[ is mo^^ I'onifnirnt in 
form, and can be handW 
and udjttdtdd with smii'-r 
facility, tliuii that uf Al>l>c. 
Thii tize of thi-ir ivupoctivn 
back Icior-A ix niffiiitioant in 
this rOK^rd, that nf Powi-ll's 
)ieiu;t /o incli. and that of 
Alilw's being If^r, inch. 

Tile diapliru^'nu Itist. 
210, A ) havp n ci'ntral n}>r|-- 
turt-. f"r the pui^io!* of 
ociilriliff. and tJw 11IOVI-- 
ment is made by ini«ii* of 
a slot at the to]> in whicli 
the arm A fits, and anotlwr 
ann,lt,iHpkce(la( t)ie)o<*cr 
4'tk1 so as Ui give r(«d/ coniinand uf llio rotation. This pLan atlous ot 
the uw of one or two ol>li(|ut- jKtncilx incident 90" apart in axtniuth. 
TLi'coiuleii«^r til tisniountwl is only intended OS on oblique illuiuiiialor. 
ft fonus one of the i*st iij tlio vay diiinp condenwrs whtu it i» 
mounted in a ptniu tu))^ mount with a Icdgo to hold the diaphragma. 
I) in the optical purl of tin- coinlrnMT plncrd immeilintoly nhoxe Hie 
diaphragms and in oiliuimi-ntion contact with the lta^e of ih? blide. 
The circular diaphnLgm U tixtil into the inner tu)>e attitcheil to tlie 
^ul■-sIage tube, C, just below tJw [lOMtion of the ami A; the otlier 
diapliragm is screwed to it by a scn-w in che eccentric hole, shown 
in **cli. It will be seen tJiat whm the diaphragius are pliMWd 
IOg»th«r in this manner t)ie uiowmdnt of the ai-m will produce the 
eh&ngta in the light an nl>ore tnentionod. 

, -JO?— IVwell .'11./ [.■ .;.>„l'i 

Flu. AID. 



As w« intiuiat«() above. Profeiuior A)>b«! hux dow pnxluc^ aii 
nrXn/unttii' rotttUmfr., oateiiKibly for um in higfa-powttr phoMgntphifl 
*i>rk, liut iubkctof much mure geneiul utility. ItwmsisUof nshigle 
Inmt witli two doutile bockx, &ii<t it imijectic a aliup and perfectly 
•chrDOWtio inuj^ of t\\v source of lif^lit in tho plane of tbc o)»«ct. 
ttn {towtfr tH low, Iteiiig J -inch focu-'.UDditliuii a latala))rrtur«of l-O. 
ICA^ri-iit nuijrTiority ov«r tbe chnmiatic funii is tlmt it tmnunita ft 
iciocli Inrh"cr aplanatio cone tluiii tlml ; for wti^rona th« fonnor gnvo 

Fitk. i;lL>*Ab>r^^* AchroTtiAtifT nnuU(Tiii«>i- 

onlyan nplanatie vm» of ■•>, this inatninir^t yields a similar con^ of 
"flS. Liki" it« prrilfsTSsor it is lurj^ :iiifl lienvy ; and. with ^tpsi dofpr- 
mfpniidTV*poct tiioiirt'ontiiipntal iie-igIil>oun=,wp would siiggr>t that 
tb» iK a too gnnf^ral clinractmEtiR: tho hnck Ions in this cimp in 
lanra than an inch in dinmHor, wliilt )mroIy \ of an inch is utilised 
wlum it in timitinnittinji; itx lnrgr«t couo. Th« iiisti-uincnt is rrprc- 
Mtitod in lig. 211. hut ™ vrry pxeellent nioditicfttion ill 6(tinj|[ it to 
Engliuli iiiicrcMcoprK ha* Imhmi made V>y Mr, ('harlcs Ikikrr, tho 
nptiHan, which is shov,-n in tig, 31?. whirr- it will be «wn tliat 
tile tittin;s for dinphi'nginn i« 
eunvenieiitly placet, and an 
frix dJAphrn^iii can )h^ nKcd with 
gmt cn*n I)q1ow this. This 
*tum-oat' arm carHi's a disc 
nj metiil to reoniro the dia- 
pliiKgmss stops ^^- Ovrr this 
II fitted n ring into which scn-w 
adapter*, which willDttowothr-r 
condcnsiTH to bft uanl on tlir 
One mecluiniHni. 

Tlie nurtol dim- Hhoald have 
acenti-al aperture a« laiiUA aa Pio. iii^BJwr-. flitin« W AI.W. whro- 
the lareest tiack Imi of any of nulin oonilBnnr nisd in Enjclidi micro- 

llMcoinbinattonJi toljeuw^lnitli »op». 

thi- mount. It nliuuld l>o thick enough to recL-iv« two atopi or dia* 
phmgrnn at a timv. This power to alt«r a diaphrvgiu or atop M ■» 

■ a 




to Kccurt! liny i^quirMl arrauKMueiil of aperturM nnd itana witlinat 
in the Inutt ^imurbing any of tlii! adjuHtnienU of the coim^iuner itt tk 
pnxticml gnin of a very vatuuble kind. 

DiitpliruffiUH should l>e uiiii'ltHd with the numerical nptirture bh«j 
yield, and slops lilitmki be tiiArki<d with the iiuniprioid iL{)«rtur« of 
tli(! (.-Diie they cut out. Eiiijiirioil uuinbora mn mialendiitt and 
vidui'h^sK. ThiH specinl inai-ktiiK iieetl Dot involve two miU of dJK- 
plink^n'^'' "'th two coiidciiaer oiuibinutions, one (or hiiili nnd th« 
oUwr (or Ion [mwcnt; the iliflerent uutuericul apertumi for each 
iiiiiy 1)6 niHiked on either ude of the diaphre|[t» or «top. Memory 
cuiiiiul fail if ve i>i»ke th« Iuickt side ct the dl&i^raf^ iiidiiiitv the 
uporturen for tlie lnwcr-powpT cundenner, aod fief 'frM, 

We may note thiit for dark -ground work stops should be plac«l 
cluso to tlio back lens nf thu I'lmdenspr, uud in the caaeof ii dia- 

!ihrah''u — vhicli is ir«» iinitoftniit —a.\\ inch of diHtmicc should not 
10 «xcee<Ieil. 

The iris diapliiiiBTii i» for ;gir>nprail purposes luore coovenieait than 
the usunl cimilur plulft, Init it hnx (he drawback of being incaiiable 
of setting (o any f\iu!t kiu. A tU'licjile point in an image, caught 
with a iiTtaiii-isin-il duiplimgni, is not rcgniiie)! with eas« uid cer- 
tainly with th« iris,' and mny involve^ much putjence nnd labour; 
bat n wcll-iiiiiHi^ Innj'' plate of gmfliinted diA]>hrHgin8 will wlioUy 
rvniow thts ilifhcully. Momovir, for testing object- glasses it is 
supremely impoi-tant that a iin-tnl ilin]>hr«gjii be ttseil, so th*t thi> 
conditions nf illumination niaybr rrndilynnd ncirunttely reproduced. 
Messrs, Heck provide n, condenser with a nxivahte top, carn-ing 
fi'oiit lenses of (lilferpnt power central with ihe backs. Ita chamcter 
will be i«iulily t«cn in the illustration given iu fig. 21%. Thi» com 

hinn a. high-, low-, or medium -iiuiucrsioii 
or diy ocndeiiser In one ]>i()ce of ftpps- 
mtns. The first lens when l)TOB|(1it over 
the back combination baa a low ang^e, 
nnd is intended for use without fluid tin- 
lilKlologicAl subjects. The next, is a iull- 
npertore lenj^ with which, by revi.hinx 
the tliaphragiu, thu angle can Iw vaiiei) 
frciin I SO' downward)!. Tlie thinl lens, 
with full aperture of diaphj'ai^ui, has nn 
nngle (if 1 10° in glass ^ 1 '23 K.A., and 
im truncated, cutiingoat the central rnys. 
The fourth lens has also an ajwrturv ttt 
r^'j, nnd is similar to No. 3, but the 
pnil^vry IK paint«<l uv<^i- so an to allow pencils only at right angles 
to pnes. IngfniouR as (his arrangement is, it is likely to interfere 
with th" (Nin-ivlioiis; and as the aperture is not exceptional ly gieat, 
it cnlU hcri' for no KjK.-<;inl notice. 

It niny hi: nf M'n'ici.' Ut tho«c who are unable or tndispooect to 
Jipeiul consiilerable huiii* upon c-undensera to stale ihut an excellent 

< It Hill b* uri;rJ tliiii ii;Hiiiiir<-i f>ii 1v fiiu'tly rapn-lnced with Iho itu la 

The Mimtt* U (1) Uiot Willi >ul>--*.iiglwl c.-»iliiiiwr> • lirjr uliitht ilifl (w i>in in 111* 
■>|>«1ui« niskm a <nty HmA diSsmiM ia tlw sagtc ; a mnilu dlfficranoa wsald be 



achrofDAtic condenser can be nuil« hy pliu-itif; a Zetmt 'apluiiAllscha 
I^peii ' on {j'tntihetl's forniuU id Uie iMili-HUfic-' Th««e iirv iiiad«> in 
ivo (litfptvitt piiwora, vut. 1 uii'li uhtl H inch, An<l we can fully 
iMtify to tlipir being tlie iimhI useful huna-letmes foi* ordiriArT work 
iliat L1UI b«> eniploj«l. Ureal credit is due to Dr. Zeiss for liriii;,niif( 
qni SDch excellent nchronintie leiiaedatso lowapni:?, nndiuinieeliiif; 
a vant long nnH generally felt-. An aclimmatjc Inup t>f this kind 
if nlnxict an indi><[>enfiablc kccoingMiiiinent of a niicrnwiipic outfit, 
iiiwi if « tube to receive il l>e arniti'.'e^) in the »uV>-«tnge these lenses 
niake renllf excellent ccmdeniterH for low pi.iwore. It need not have 
« centring inih-xtnge, but only a central fitting. It 18 ii'>t of course 
^oalifieil to s-up[>lnnt the wmdenser of larger and nmre perfect in- 
■trunwnts but it ix enpnble of raising studenbi' and otJier Miraple 
inirrt»e<ipLit to a much higher level. 

Without a c<>n<lciiKnr the mieroscopc is either (by constnietion) 
iii4 a iri^-ntitic inntrumcnt, or it io an iuHtninieiit unKi-ieiitifieAlly 
iimhI. It IjecoRint a tavrv ' Diagiiifying glnj«.' It in tin- adaptjitiun 
fur and umi of a condeiiscr- though an Kiniple as ii lii'iiiitplii-rical 
lens fitted into a singi' plate — that mivr* it to a iuicnm.'ii|ip. 

W« have alreiuly n'ferre<l to th*! nature of thp iiiiTcbanical 
nrrangMDcnts nunlful for th«' ei>ndi-n»er in a Ki-nci-iil way (Chapter 
III. p. 169); we nijiy luld here that tbi- Kimplnt fortu of Kub-»l»ga 
U-ing a lube fixed centrally in the: t>]>tic axis of the micniicope, lJi« 
Minpl«xt fomi of 0)n(ii'ti.tcT-miiunt will Iw a tulir .ilidinK intii this. 
It niUAt not »cr<-w, il muiit push, and thi'n- nIiouIiI lie a little 1>el«w 
the liaek lens a ijioiililer to hold the diaphmgiiiK, stops, glaveii, i:c. 
Ceutnng smr i« nttl necessary with students' and I'titm-ntJirv niicro- 
Kopen. TlU! Nti^ht diKpIneenient^ duo to iiiTyiiig i-eiitrcs of dilTercnt 
ofawctivm will with such niiemM-opi-^ prove of no luiiment if the 
nVstagv is once for all CAi-efully li\i-<.l centrally in the axi.-i. 

Whil we rn]uin! to do is to <-initrf the iningi* of thi- lump llaine, 
OK jimu) with a low-power li-n.s throuKh th" condenser, so that it 
in in the middle of the field. This ean lie ihine liy mnvioK 'he 
np or the mirnir, and until this is xatisfacli>ry the bp>l. i-esults 
at Iw obtained. To obviate tlir iiici>nvenient'e of having to n- 
tlte combination in order t<> alter a diaphr:>giii ' "r Mop, in 
this wimple ni<^mnt an internal slidiug lulie may lie used, Miniething 
like that descrilieil in the latest form of the apiiehroitiatie condenMr 
of Powell and l^alaiid. It will t>e a further advantage to have a 
^MMntte' eell to fit inU) the bottom of the sliding tube lo recei^-o 
_ ulouml glaaaea ; a it{Hml shit-fticusaing arTungeinent may l>e added 
' with mlvantage to thia Icuid of mount, acting like a pttcket pencil. 
Fur HtuileutA' atid eleinentjLry mierutioopes — uow so often and ao 
Uiwiaely without vondeDseni — this Ls a uiool inexpenMX'n and moat 
eonveuient arrangevDeut. 

B>|qm<iab1i* in tti# ecu** ot i plioUrgn|iliii^ Ivun. i'Xt II ii in uiiaU ifc]i«rlumi vDoh 
w M* hJiIbdi Bicd in pbalo|:ri>pLii! lan'm wlicre llii: difflcnUir bHw* >n iho pu* <•' 
Ik* Bian>MD|in, (8; It it iu lliv tmmll BiicrtnrM Uml tlia Irii bil' to n->pond to 
lb* noMmcDt of Uie l*Trr, 

t /ount. tloy. iliera. .Sop. wrin ii. loL iii. on Zuiui' l<mp. E, M- SiiI«.ih. 

' In til* l*«il>rif»J luiEaax" or oibki' oI ininiMOcpii>t« ■ ili«|i!iriwio ith-hii" r liolii 
III ii|iMt«m Ihnt a ■ lurica diaplini^i' mMUia thai Iho npcninc in tlin ili^plmgn 
fltti, diw, 01 in* ■• Ur|^. A ' irtop ' is all epaqtM 4i*c alaiipiuij ont DPnttnl rajr*. 


An rpilome u( iln prtiiL'itiiil ]Miints iiiiiy be of service. 

1. A Mib-BUKe tulie ftxi-d L-entrj>Hy in tbe body irf the niier 

3. A Hptml slotted tuW tu pu^ into 1. 

3. A tube oartyiiiK tbe ctptiotl coubination of tbe condcnwr^ 
sliding into 2, witb & piti uiuving in thi^ spiral slot. 

4. A loiij; tube carryiiiK Uie diapliMijTiii and slots slidint; into .H, ^m 

5. A cell tarryiufi colourwl slaiwe* sliding into tlie boltwn of 4. Wt 
CofvtttM^rn rrtfiiirr tpTuil nwunliiii/joi' tw* wi/ii (hf fitJiiri/ieti/ii: 

Tlion lit least two ' turn-out ' rotatins rin^js Rr« requii'ed lo liold 
solenitea. i^wift niakcti»u in^euiuu^ murium in /iirva mount for 
employing, aiiioii<;&l otiier Uitn^s, llit- i-ondens^r with the gwlunifopn, 
to whu'Ii wo call ftttenUou in ik-suriltinf; tbe polari-icope. But wo 
know of no plan equal to that found in tlie best siiind of PovreH and 
I^nduiid, Tlio 8ub-6tago lias a double fine, one pinced concentricully 
within the other, The inner one revolvrii by n inilleil bend mid 
riH/inve^ the usuaI «ub-]^tAge apparatus. The outer one receiver a 
mount of three seleiiitr« which revolve, and ftn> pinred on *tuni-out' 
nruis. On tlio upprr jmrt of this mount of seloniiri) is « screw, ulucli 
recei\'es tbe optical corubination of their dry ncbromatic condenser. 
'When this is scivwod in its place we bat-c a con<lcn«?r of the first 
order, with a mount of throo platen of selenitcs tnking the place of a 
mount of diaphrngniK Sic. Now from lbi> u>nl-r psrt of the Gub- 
Btage into tho inner and rovolving ring is Attcd tlic polai-iiier, and 
this IcAvcs little to )>e iluired in practice. 

We would odrisp the inicnucopint to avoi<l condi-ndT mounts 
which carry their own centring movements apart frnm the sub- 
stage. It is with regret that we liTid that this pUn has been adopted 
in Abbe's new achi'ouuitii.' coiulenser. It is manifestly better to fit 
tlie rectanv'ular nioveiuenta to tlie sub-stage, and then thej becouie 
a\~nila1jle for all the appniutus employed with the Kub-sta^. A plan 
which require!) that each piece of sub-stage apparatus which needs 
centi-ing should lie provided with separate fittings for tliis purpose 
can have nothing to recommend it. 

Wp give on the adjoining page a list presenting the most import- 
ant fcJituros of the most importnni condensers, which wn boliere 
will >)r! of service to the student and worker. 

Till- Hplimntic aperture nvon in the tdblc mrims the X.A. of the 
grf^at^st. solid conn a conacDsnr is capable of tivinnniitting, the 
source of light Iming the edge of the tlatne pinceii in the axin. 

The cone tmnsmittcd by any condeiiKcr is iiABUnicd, for pnu'timl 
purposi^s, to lift a Kolid one, so long a* the' image srien at thr Inch of 
the objeot-gliuH when the eye-piece is n'niov>>d ((h<- condcnuer and 
flame being centred to tbi; optic nxJx of thct objectii'e, and the sourcii 
of light focusMwl by tliu condenser on the object) prMnnt« an un- 
broken disc of light. 

The moment, however, tlie disc breaks, that ik, black spots appear 
in it, or its [x-riphery brenk* 1111'n.y firiiii ita centre, thi-n, as wc have 
sltown above, sjiherical nliermtion conir-s into play, and thi- limit of 
apertm* for which that pondenser is aplaiiatic has iK-en excct-deil. 

The limit given in tlie to-ble is for the edgi; of tlie llainn an a 
Kource of light. When, however, a single [toint of light in the axis 
ia the source, tlie condenser will lie much more sensitive, and a low^r 


iw for tlie apbin&tic BpOTtiin' th™n tlint pv.-u in tin: taliU' will tw 
lined, itut lU A single |>oint of li|;lit t« mI'Ioiii, if evur, practically 
1 in micnmxipy. it wn» diNam-rf iH-ttnr to pl*«! in tlio tulili? u 
'pnu-tioil rattier than * tlkonnrticnl uul i>iv>Uil>l,v truer hmuIc. 

It Iiiu txvn steteil tbnt titc )}«it tliirk ^'mutulii am oLtiiiieil w)i«ii 
t ttop IN used whicl) is of just a Hufliclmtt *izie to givn a xuitaUle tlurk 
Bcl<) simI nomonr. 

\\1i*n savh n stop h»» bten choirni, nml exc«llent nuiults are ob- 
tained with, say. Imlsma -mounted olijtjcts, and llien in tliu pUce of thin 
liriii;; nniinalcnW in wnter iMt exnniiintl, it will pn>lial>ly lje found 
lliiit a dnrfc tirl'l cnn ii<> luu^-r I>r i>lit--iin(-ct. 

for aninialctilM in water uul ' pond lid- *' s^iientlly n stop burj^ 
tiiau tliAt vmplogred (or ordinur}' objccUt will lie n«crssaTy. 

X^. SJL. 

I. PowcU and Ixobuul'* dnr ochronntio <1SS'} 

3. ., ,, top lens i*iiwivci) . 
IL .. .. bottom tons only . 

4. Swift'a MllrMaitljo 

A. „ u top Ico* Tcmot'ed 

«. Abb«'*e)iioiiuitlc (3 Icnws) (1S»> 
7. „ ^ top I<ms remuTfd 

flL Ponlt iiii't LmUiKlV obnmnlio (Abbe's tot- 


•. PoiraUaiid ljmJao<r«adln«liraBMtlc<3»^l> . 

10, „ H uMd dry , 

11. « „ lop Iniumnairfd . 

If. AbWtMlitoiiinii-- (ISSS) , . . , 

IS. „ _ top lona removed 

IL Powdl ami IxnUnd'i lav-|ios«« adiramatle 


IS. rtn'vll and lA'slncd's Apocbnimiitiit (IS^I) . 
10. Zei«'« 'aplacatisdic J.npui' \at^ llcld 





Other lUnmbiaton. — Tn the crane of the history of the inicra- 
■S^e, n ]ttr);-: iiuiiilinrof HpiM:uilpioCOTo(»l>I»riitu>ilmvclH>:'iidovtsecl 
lor th<- purpoM) of ocooniplishing soine rcnl or supposed end iii illiimi- 
n&tion. ilany of thcno navo prorcd wholly impmctirnhli- nnd hnd ii 
nirrtt nphenwirftl existcnm ; innny niore iinv<>r accompli" hi-d tho ••ud 
for which thry were suppoMKi to Ixr conntninti'd ; nncl ii still Inrgcr 
nunib'T liavp tn-<-n supfiTsedi'd by ht^li-oliis* condcnHftr*. 

Till? fcT«at inajnrity of tJioja- illuininatoro wnm d«vis(Hl for the 
prodnctiuii nf olilii|un light. Fii th« Mrnsn in which it was t-niployod 
a few yi^am a^ it is n^ndirrvd nocdlcMt by condenMTs of gr<-Jit aper- 
tnre. All thnottlii)uity alprcMatnendedcun ImolitftinnI with Pnwi^I 
ami LrsluHt'si oil-achrtimatic coMtonscrr. But tint to igiioro lliiian 
who loay kUU ttcstm it in its older fomi, «*<• may mv timt it is fotmit 
in itH vtiry best form in StqihoiiHon'K oatadlopmo lUuminator. By 
it* mMUiM a gnnt^r ohiiqruity can )>n obtniii'il thnii ran be uttliBOd 
even by our inoil'^m objrictiveK. Tho f^r'niirnl form of this imtrunwnb 
ta Men in ti^. -lli butthn priiioipln of it« construction and operation 
will bo UMilorttood by th<? dingratn given in tig. 'JIA. The illumiiubor 


c'OUsiaU of n M-giucmt, C, 1>, K (cut from tli« edg^ inwanb), of 
plano-convex Ichk of crown gla^s of I'itich ntdiusof curvatiiK', tlian 
Una li&ving n dininctcT of 12 iiicli, Aiid tkerefove m thickneSKot 
0'2 inch, tlic upper siirfiwv iH^in^ binck. Tlie Memeait has a lenfftb 
of 0'56 inch and n depth of ori inch, and is therefore alnio<>t a, 
square- Tho cun'cd surface is silvered as in the cittoptric tens 
<Ie?<cnbed in 1879. It is cemented to a rectangular piece of flint 
•;lius. A, I), {-'. K, thn mfractivc index of which is 1'6'^2. The 
thickness of the flint hdixg 1}-\H inch makes the total thickness, 
whet) tlio two are ccRK-ntcd tos«ther, rather more than hnlf an incli, 
The olijcct of the flint ^Taxh is t«-ofold. In the tlrst pLac«, it 
dispotn of thrm-fourtJw of the Bpherical nbornition of a concaye 
mirror of the>e dinit-nsionH, oiid mahlcK ono to use light whidi is 
practicAlly p(irull<-1 ; and, in tJii' nccond place, it linx the very obvious 
one of sc-urina gri-!it*-r npcrturc, whicli is the primnrj* consideratioiu 

\ tUtui illM'I'i f "■' 


Tio. US. 

Flo. ai<.— StrplwiiKOD's eulgidioptric 


Aslig. 31 >'i shows, rays which enter the flint glnss horiaont«I]y «« 
refltK^tcd at the silvered surface of the crown-glass sogm en t, and 
iiperturca nro thus ubtaine<l mngiiig from O'T? to 1'644, N.A. in 
llint and I'^ili N.A, in crown. 

To ohlnin the smaller apenure", the plane surface of the under 
part of the flint is utilised by cementing to it a segment, K (mther 
more than half), of a plano-convex lens of radius O'lT) inch, ■ii'ith 
a focus in crown 0'04 inch above the upper surface of the llint, 
and, therefore, at the upper aur&ce uf a slide having a thickness of 
0-04 inch. In order that the rays may be received from tho 
iiiiri-iir Itoneolh (and nut horixontally) a small right-angle reflecting 
prisni, G, is placed with one of its sides fipposite to and parallol wttli 
the receiving "i'lo of the flint-glass. 
. Tho aperture of tlie illuminator is only limited by the r«fractjva 


indrx of Iho flint-KlaKt UKnl. [f it In di-^intl to luivc a InrgPraper- 
tnn- than I 644 N.A., it rnn W (il>tAiii<^I by UKing a flint-gluwi of 
hivliT refmotivn indi^x tluii l'£-'>2, TIh* nptrtura is Always within 
linlf per <H-nt. <4 tbi- tvfnicti\-n inilf^x »f tlu- gtas* (tin a filid« of ihe 
tiimc mnUTial 0*03 iitch thick), m> Uint if ^litioi wrm us^tl of the 
n^mrtirc Indrx l'T2l tliii api^rtiin- of iho tlluminntfir would be 
l'Tl!j N. A., Mjvl ati on tu »ny r'xtt'iit with iii<-rru.iiti); di'iivitipu. But 
whrairo uoe glwtti tKyoiid iibuut 1*65 wp Ki-t iril'i difficiiltii's, iwreiy 
dma glAiH is *lw*yii mon' or li-xx ecilounil. uhicli, by iiurnchiiig tlio 
mors rcfmn^bte rays of thn ipi'utruiii, hns » lriHl<'ni;y tu dimtni«h 
th» ttttetln Kpettvrv. It in, mmifivpr, ili^icult i« wurk, ami in* 
rdm thii om of some fluid »t i^uut ns rrfivti-livtr uh itsi-lf. With 
flint of I'Sn we hftve monobroniidc of naiihilinliiic to coiiiiirct the lens 
iind tho slid«, atwl tbi" rIiuji is pmctitiilly coloudi-sv iinrl iii*y to 
"•nrfc : aUivi^ a11, we luivt? iin ajir rluii- wtiidi cm-tiIii liy S jicr ci-nt. 
>> ImlHAJD Atistsotf 180*. To coniMict tin- ordiuiiry cniwnx'"*'' "i'li 
ihr iltumirmtor, tbelv(iino;^ti(«usu]Hliiiiii (oil >>i-'fihfi-ui%r)uM-il with 
tiitr obJMtivo can idwayB l>« «mplov«id, hut with flint k''>-->-'' stiilt-Jia 
wint mtmctWa flnld, such ns ud of ciiuin or uiotiobmiiiidv of 
lut^ihthiUinA, is Meettuy. 

The oil of oiaaia if pure will aofEm t-v ah npertun' of, any, 1-63 
S.A., and. in tlie opinion of moat ])«>pt<'. it liiu n liiu uiipliMuant 
smell; but fur the full aperture of 1'644 this ia iiuulticivut, nnd 
loonobrotuide, or ttuine equally refraoU^f iiicdiutu, mu%t lie employed. 
Such Bolutioiis of biiiiulide d uiereur)', idlliouKh ino-imiiti-i, an- <>1)- 
j^ionahlefurthiii purpose, fmin their ehoiiiiciil iictioi), ■"■d for other 
nsMim. The U^ht (it>t»iiiiil by thLi iiii^ciiiou-s instniment ix fren 
frnm colour, since it in ohluiiied by n-llcKioM, but in arlunl um* w<- 
fitul Un wMiknea* to lie ui tfie fi>i:t ttiat il.t ItKlit InckK inlmiiiti/. 

Of bll (lie older ohlitjue il)umiimti>rs IIVmA-tiu'ii ivjfrx Ulnminator 
vu probably the boil in it^ d»y, aUbou;;'h not finy to iituiiiiKe. mid 
ibt remit* can now, <if tiiui-se, be »urpiis*<l by tin- U-at coiidtriwr. 
It is ootnpOMd of Aglasa cy1iu<ler(fif;. UIG. f) Imlf an inch long, niid 
faor- tenths of an iui-h in (liaiufler ; om- nidi- of winch, 5iaitinK {r<nn 
the bottom eilf(o, is worked f^^ ji fmlished imt-iH im iinijlti.f Gl" with 
tiie bASe. TIm top of the <'yliuiler ih |<i>!inliMl dut. whiUt its lower 
turboe is convex, betng poliafaed to a rediu-. of ^'^ of an inch : clone 
beoMttJi thislA&t is set a piano- t-on vex leni^ of l]-iiioh focus : und 
the oombtiiation is set ecccntricHtly in a liilinK. i '■ ft>lii]>led to be 
neeived into the Hub-stage. The parallel my*, /j'j, rellecied up 
into it from tlte miiTur. are made to conrenie, by the convex surfaces 
at the base of tlie cylinder, at such an alible tliat if their course 
were continued throu|:;h fflass they would meet nt the point A, almve 
the I^Ihss slide c ; but by impinffini:; on tbe inclined polished surface, 
tbeyare re6eoted to the Hat Be|*niental top. jroni which again they 
would tie reflected obliquely downwards so iis to ni<>et in the point 
t, bat for its being brought into ' immersion-cmtJirt ' with the under 
•ide of tlie slide. PnsKing upwnrds thmuph the slide, thpy meet in 
• point, ff, A little atxtvo itx up[>er AHrfov>\ in the optic axis of the 
in)erDaco|>e, to which point tlie ohjeci nniM be brought ; and by 

gving rotation eithsr to the object or !•• the illuminator it may 
' illuminated from erery .iTimuth. Il'or convenience of centrinn^ a 



J .1 J 

bUck lialf.rtlliider. .-. U «r> tixwl I.y tJw wO* ol U>e cyltmlBf 
xhntif Hilotuponitaiippor nurfcu-e W brouglit into thr. (*iitriMil tJw 
Iklil o( view of n low-powiT i.liiectiiiN iw focn*, y, will lie in tJw 
optic Hxis. Some skill mid pnicuce »re n^oired to luw tLia a)ipttnitiiB 
u> lulvjiiiUftft but ■ it will ninply repay tlw iroulJ* of uuuiwi^ 
iiii; its (liiriL'iiIiir-.. It i' '"-it suited lo thin, fl*t obJLvtH ; willi tJiotie 

timt nn* thick iui<l iri«;^lHr 
• lintortion i.4 uiiav<>i<Ub(e. 
AlthouKhspM?iaUy desired 
nt n 'tilnck-Kixmuil' illu- 
niiniitor, it iiiny nbu bo 
imido uw^ful in the raaolu- 
tion of dirtlcnlt lest -objects 
)iy IrnntmitU't Itgbl, iho 
illtiiniiiat^n' bein<> lowered 
t::iii) > co]our«<l xpei'trani 
H]>|H--iir!< ill tlie lit'kt, tli« 
rayu of wbicli bi'int; out 
ibeir iii3irkiu;;>« witb r»- 
ui'irkHbt#(lintiiictnf«s. For 
ii->e with eitiiiT of tbesn 
iii-riiti;;enietitM for 'bUd>- 
^■rouuii' illniuiiAlion, it 1* 
)>«ll»r thiit tbv obJMts 
rtliould Iw muuntrd 'dry,' 
•'■ipi*ciaUy when tW-y are 
ut be viewi-.! utxler 'im- 
riif-rniiiii ' objectivra, bal- 
-iir.i iiiuunt^l objccls being 
'hu- •*«! Iwtter with dry- 
iVout iilyectjvea. 

Tliii waa followed by 
'•line' »nil 'liutton' ilia- 
luiuiktora. Mr. VdiiImri 
devised the Minple iUumi* 
iiittor Hhown in lig. 217. This oonsi»tA of a semicirc^Ur ilim "t 
Hhsu (Moiniiwhnt rc*oinbliiiE the hilf of » button), of hnlf nn tneh 
ill ilinmpt4Y. tliOKidcK of which ait: ti'dt'-iied. while the i-irmilar ttlft 
U rounded «nd wHI p-Mixhcii ton tranxrerAe radius i<f ^thof iin indi. 
Tliiaoiticenimt^-Hthcli^ht thrown ufM-n any purtof tt«drcanif<-rm)C<- 
\ f^ on to nn o^i'i't iiioiiiited on a slide of thi^ usimI 

lhickno« with wIio^a under aide it it brou^iL 
into imm^rKJon-conlAcc by the intiTvnntion of 
4-itluT wntflf. glycerine, or « mon» n-fmclive oil. 

* "ilw'liio^iViIior"" ^ ■^* '' ''•^"'^ ^ so Htteil to the microTCopo «s to 
illuminntQ the object from nny aEimntli, it shonkld 
Iiavi^ itK flat sidrji gnupml in a clip, which innyeillxT bri mnantvd 
<in the niib-Kto^ or iittiichc<l to the under side of tlio HtAji^, tn rither 
!.■*«■ )iii\'in]{ itN diiimctnc section brouylit up to tho undvr nirfncr of 
tin- iibjrctnlide. tty gi\-iiig rotation to the object, th<^ illuminntor 
rciiiiuning tixi-d, thf illutuinating beam may be made to croa th« 

fill. ain. Wcnlmni'ii rcHoi UlomiiiBtor, 

rABA! llJ.rMlJfATOKS 267 

fonu«r ill anydirvclkiii that U litted to bring out tXa markings; itien 
Mlowecl ainall lieiiiixpherical lenses of vaiioug foci RttAchvd hy oil or 
ntiier fluid to the tMse of th« slide, niid cAUSi-d to rec«iv« ubIi(|Uo ntjrs 
fturo thtf HiiiTur, t>r» lou- - power objfsrtive, nrmnired by 'swinging' 
uik-Bbi;{e to (■irovariouHan^ks upon this letrn. Tho ditHculty vith nil 
ilMse wiM (I I thnt (I lens nt dirtVmnt focto^ wax needed for <Hfl'eroiit 
powen ; which, however, wa« reduced 10 its tniiiimum by the nsn of, 
«r. (hrev leti>e>' of ^liiToiTinl foci, gn»ttiv than hMnispbrnw, nnd no 
iDounted oit the mck iltaL takes the sub-otsg* as to contra! or alter 
the position nf ilic fnrfli point by mckin;.' tip or down ; »nd (i) it is 
not ulwnys po)i-ihje 10 i.*p< KUflicirnl inl^isilv of light, bythdr mMnn. 
It wu the arrivnl of Abl-c's rhroniniic fondeniser thnt put an i-nd, 
for »l] practic/d porjwiij-s. to siicli deviciTt, When it wiin i^mployccl 
«ilh oil ^rent obliquity with fur greater iiitenKJcy wax tlicimiiimlialtt 
trsuU, and ihi', sh we Imvc itocii, wns Mirpnurd )>y Powell'ic TIn« 
|iiu-iii.illnr ijunlily trt obli<|Ue itluminnlion wnK ttill furllier ndv.-inoi-fl 
ijy PuHrll'n •truiinilnCjiiriii nflhrir Intrat eviul'mi'r, whick cotvtlilulfs 
iiw of the nio«( ublitiui- illuminutorK extnnt : iitid llist too with 
kbotidaiK'e of tnii'itK'^ liuku It is, o( courn', vi-rv chrumittic. But 
■ti cnii obtiiiii all ilt^mbh! obliquity with all iiintlfal iiitentuty with 
th» Pawi-U und LmlandnchraRiaticoil-fond('nsi-r with 11 idol, mtd that 
•jiiite without <-olour. Hi-tk.i- it muNt \w \iioVcA upon us th« IwHt 
<ihli([ur illtnniiuitor nt ]in-iu<iit raitde, Wi> hnvn written enotixh to 
niidcr the ',:r»uiidii of our xtr(itij(ly vxprcKnvl judgrnvnt cl«ar from » 
^ir1ic«l [loinl 'if vii^-, iJiitt, (or tlic puqHi-w of oblniniii;; vTitical 
iniitg>--<i, aiid Kittling IIir Wst iwsullii from i>bJH.-t';{lai(irx, ' Kwing:ii;{ ' 
''Ub-«tageH and concMitrie iiiiRroft.-ojir-M iin- nMHilcwt, and mny )>h 
III inch ievAUB ttbibotmtionK. < 

T'> i^vtt cunipli'tciii--'Ui tn thiK part nf our subject it is needful tii 
rvfer to thi^ KHiT'i.i^Nn and i\\v. i-aiiakoloid, although lh«y biive 
jinatirnlly h»t tlicir iiiii>ortAni-c to th<! :iii('nM>.'opi>:t. 

A Spot teiil i« a •.-ondenM-r with n pcrnmutint nxiid Ktop fixed in 
it to rut <)tt' tin- O'nind myn fnr tho pur|ioii« nf ohliuninK u dark 
^und n|Kin uhicli the illuniinatrd objcirl Hex. (Ibviimdy it ia not 
4 oonmi'tndablr acci-SMny ; it ik u cond<'n'>''r, but buviux mjiiyl stop 
it con uidy Im- u«nl for this luixle of illumiiiutiuu : and, in tbiu form, 
it esn oiitr he umnI critintlly with cjl>it!<]tivcs of 11 certain delinile 
ApcTturn, iMcnusi', in onlir l<> obijun a good d»rk -ground iUumintLtiou 
tlie ><top ought t» Im- Huit4>d to lli«a|ierture of theobjeotivn employed. 
In other wuni^, a kpot tvns oidy docs i» a limited and imperfect 
iiiy for certain object' Kla&^«a what 11 good toiidpiiser with [n-opcr 
Ktopxi wilt ihi for ull witti which tli« spot lt?u8 cou be employed. The 
^t IK tluut (he best rocent condt-Dsen render needless ntucU cosily 
iml.-*tng<' ii|i[<Hntlat^ and, therefore, Uie younger inicTOcopists juny 
Bell afloni a tine condeiii^r »t onoe. 

The paraboloid, or parabolic illamiiiator, as dorised hj- ^Ir. 
UViihiiiii. and sub^ijueiilty impi-oved by }lr. Sliadlxill, ingenious 
and >>einu(iful inatruiuent aa it is, cornea under the same cAiegory. 
It cQiunKit of a paraboloid of glass tlini reflects to its foi-u.i tht- i-ayK 
which fall upon its internal surface. A diagrammatic section of 

' aiaiilrr 111. |>. IMI. 


this instrument, showing tli** course of th« m^ through it, is p;!ven 
iu tig. 318, ibe shatW portion representing tlie paraboloid.' The 
panuld rays r ¥ r'' (llg. Slli>, enteiing its lower 8Urfiice perpendicu- 
larly, pass on until they meet it« pantboltc fturfare, on which they fall 
at fiuch an augle as to be totally reJl«ct«d by it, ami are all direi-ted 
towards ii« focus, F. I'hetopof the paraboloid being ground out inin 
n Bpheriml crurve of which F ik the centre, the rays in eniprging from 
it undergo no refraction, since e»i-h falls peri>endicularly upon the 
pnrt of the surface Ihmtigh which it pfistcs. A stop placed at !$ 
prBV«nt« any of tlio rays rrfiected upwards by the mirror frani 
pamini; to the ohjrot, which, hring placed at F, in ilhiminAt«cI \tf 
tllP mys roflwtod inli. it fixwii all sidt* of the pnoiljoloid. Tho(ft 
niyK wJiich piu» tlirough it diverge again iit various angW ; and if 

Flo, Vl-^ — l'iituLn>liD 

Flo. !UW. 

tlie least of thMOi G FlI, be greater than the angle of aperture of 
the object.glasi, none of them c«n enter it. The Ktoji S im aitoclieil 
to a stem of wire, which pnmi-s vertically through ilip mruboloid 
and temiinaU'S in a knob bencDttb, as shown iu ti;;. 21^ : and by 
inenns of this it mny hn pushed upwards so as tu cut o(T the les^ 
divergent rays in thdr piu<i«^ towards the object. It is cUitued 
that this instrument ho* grcftt capabilities of giving- dark-ground 
illuminattoTi with IcnKm of *wido apertures': but that has applicKtioti 
to tlw lenses conli'mjiomn.' with its intriMluction, and not U> wide 
apertures ns iipplinl to tho Icnsea of to-day. In cooiparison with 
what can Ik- dtinn with condensers it suffers grently after we paw 
tJie ^-inch objctctivn, altliough it does give exc«lleut reeutta with 

> A puafaolio iUnmiiwlor *«•■ Br>l ttfTi-wl I>t Mr. Wmlmni, whcs, homiwr, 
•mpbTBd • >il««T ■{wciilain Inr tlir imrrniM. Al-iiit llii< xiuiu iiiou Ur, mulbolk 
danHd ui lUinulu coniUtinci' i>l £lw lur tlir Mtina |iiiri>>i>H' lurw I'niui. J/k*«c> 
Soe. mr. i. vol. iit. IHSl, pp. M, IMj. Tho two prlncl(il«a ua comtiiMd la tba gte« 



xttf low nownrs such m l^-inrJi, 3-iiuih, aiut S-iiich objnetivrji 
vlwn empfoytKl to iltaininaUi ]nrge objecM audi »a wholn inHi-uu, 
Umow this itit>lruiii«nt givM mom diffbsioD of lif;lit over tUf. wliotf 
(tf A htrffn object than s coiid«ii*rr (loeH. 

Tliera ia ttnotlii-r pAraboloiil whiVh, for i-ompl«t«iima, w« must 
hriellj ovn&idcr. In oHcr lo lulnpl tliP paraboloid for (lai-k-j;ruuiui 
mamiiuuion uiidor objoctivm of wid« angle of njierturif, M%: 
V^iibaiii ' loDg Biuc<> conniruclnl a _fial-toiiii"t pamboloid, titt«i) 10 
RJeot only Tit-yi of su<rh cxtmnr oblitguity thai Xhej would not 
fMis out of Ihe llftt Kurfiuv nf thn pntttholoiti into the under iiurf«ct> 
<i{ Um slido oiilcSH n till!) of t'illi'-r witter or of Aonie liquid of Iiiglior 
nfnctive ind^x (shcIi ns iiirpi-nitm- or oil of dov«) tot* interposed 
Wtween ibein. WIh-h llius i-nnlilMl lo filter thii slide these rays pass 
on outil thcT ihl-M. tint covi-r, from which (in the cmc of dry-front 
ol^tviiven) tfley art- rcfl>Ttc<l duwnwnrilii upon the surf4kco of tha 
O^Fot, giving it a l>right illumiiuititin un a [x-rfi-ctly ilnrlc field. Tho 
if^inl viUuu of lliia iRKtruiii*-nt, huvrevcr, not bring then under- 
ftciod, it wan not oomtruoUsl for lali? : tJio miui- |>riii<:i[il<' liitving 
IjKi) mont n«Piilly taken U|> by Dr. t^Iinuiids — an immrrsion-pnm- 
UiloUl W4La Njm^ially devUnl by him for unn with iminerxion- 
objrotJvMi of witti- a|>erlure. But it ultvrly fuitii for iIiik jxirpou'. 
The ohjrol Iws to lie mouuteil on tlw alip i/ry, mtiA then-fore the 
nloft of iImi tiiimenuouotijcclive at uuvi^ is ttu'rilioed, iK-cniixn thit 
>ir-ii|Ai;n tuit.wi^oii the obj«;t and Ihe fOv<?r- -which is cHsentiut t<> 
tkic illiiniiiiAlor -dcHtntyH hoiiii>»eneity awl reduces thn oI>ii>cti\n 
]iracti(wlly (A an air hun. It v-as hi-licvod thnt Amfihi/Ji-Hm 

C"nW''n waa r<'fi'>lv«l by ila meniui with nn oil -object! v<-, but tliiu 
nut l>een, nod v-<* beli«V)> vnatiut lio, eslablijtlind. Ity tj-onct ami 
oui?ful »miii(niUli'>ri we Iiavf iibtaineil faint n-solution of .-1. j-rOn'-ida 
«ttb It '/'y IciiK, but an iinmrisinn letui u.->«(l dry i» mi i-oiiiplctdy 
QUI (.f I .)ir.> lirtti tliat it will n.-«olve nolhiiix- 

PolATising Appaiatu. — In unli-r lu rx'imi»<! traiiHparuiit ot>jt-<'t» 
br poUriwtL li)tht, it ia aeeeuaxy to einpluy houiv misinsof /ii/'in.'i'if/ 
tio ntyn Wore thi-y pnaa tliruuKb the o}i|i.-l-1, and to :t]>i>ly to tliinn, 
in aniiie j"»rt of their tWirse between ttie obji^t-t ami the t-ye, an 
luuifff'iinj luediutii, These two re<|uireuieiil<i may In; providitl for 
in dilFoi*iit mod«s. Tho ]Mj<tri''f nuiy be either a l>uridl« of plate* 
*S Uiin ifhtia, uM<d in place of tho mii'ror, and jiolaHaiti^; the vxya bv 
nlk-xiojt ; or it may ben 'single iiimge'or 'Nicol' prism of luelamt 
({Mtr, whii'h in DO coniitruct<Yl ns to traitsmit only one uf the two 
nya into wlii*:)! a beam of onliiinry lij;,'ht is made to divsricate by 
paaaiiiK tlirough this fiubstancf. (Jf these two metiioda the ' Niuol ' 
])ri»in u tbfl one gencrnlly prpfnrrMi, tli« objection to the i«lle<;tiu|f 
pjlariser beiD); that it citniiot lie iiioile to rotAte. Tliin [lolariHiii^ 
Minn in uaublly fixed in a IuIh.* (lig. ^20, A), furnished with a 
Ufgo milled Ixind at X\tn bottom, by which it is made to ixttate 
ia a cotinr whii^h suti-w* jntn the )iuli-5ta^ tittinc. t'»r the 
niuiliftfr » srooiw) 'Nicol' prixin in usiudly muploynd ; and this, 
llxnl in » short tubir, may bii tittcd eiiiu'r into a eolUr interposed 
bMwveii tbe lower «:nd of ihn boily and thl^ objective^ or into a cap 

' !l'raii(. ilitrote, a-.t:. X.S. ni. ir. IMO. 



A, Fittitucnf |<oUn*infC B. Filling ol 


plaoetl over tite py«-pioce <hg. 220, D), in tlip ■IoimI ai tlic ordin»r_r 
«}-»-piec« cap. The fonn«T iimngoiiiMit, which in uptviAtty ndapMd 
. Y, for«wvrithtl«*1)iiiocBl«r 

thjw nf not liiniting tht- 
tifld, but it Ktopn n eood 
ilctil of liglit ; niiiln in 
111'' inttcr till' iinitgo i» 
lirielitiT. hut A gond At*} 
of ihp ninrgin ni tli4i fidd 
is I'ut otT; nnd it » 
itl«;i%^impnrla.iiMhAt tH 
polnmiiig prism Ehoiili 
bi' turse, so OS tinl to iici 

wiiNpr. thus rutting on tli*- 
lig^it when it is ukihI ; for th<' polnriKing iip])iimtiis miiy hn wtirktfi 
ill coinliinnlion cither with tlic acliroiuntic^ comlciiJier. Ijy whiiiti nipaLn.-i 
it may !«.■ oniployml with high-jiowpr olijoctivitii oriiHU 'jiiirk-gri>uiirl' 
illuniiniitiir, wliieh slmwa nrniiy i>l)j<wta .luch as (lie horny i"ofyp«iri«t 
ot isophytes -gurgeously jimji-rtisd in <'oli>urs ujion n (lark ti«l<il, 

Kor brilifring <>ut i-irrtnin i-ffpcla trf colour by the Use of poliiri-ar<l 
li)(lit ii is, ;i.i iilrciuly stntt^, iloadnibli- to interiJOne u p)iit<- of iiflfMilr 

lii'twpen tli» polariwr and llie obji-iH : 
mid it ia ndvaiitH^Mus timt thiu should 
W vtoAf- U' rf'volvi'. A v(-ry convimicnt 
in ml I- of HlVclinj: thin iH to mount ill'- 
si'ti-nilfi plati- inn rcvolviiij; <'ulhir, wliit'b 
tits into tJi<^ uj>pi-r rnd oi the; tubi- tbnt 
n-cnivcs till- ]>ciUri^iiiK priKin. In onlrr 
to obtttin tln! (jn-utMl variety of colora- 
tion with ditTt-rtrni objircts, lilms of 
m-lfnitr of ditlVit-rit tliicki)i-«.ie8 should 
lip muiiloycil : and this laiiy bo a«cain- 
pliabed by HubatitutiiiK oue fur •notlicr 
ill the revoh iiiK loliur. A atill gnmt«r 
^tirieiy ui»y be obtAin«<l by monntin^ 
tliiif tiliiis, which wparately Kivo thn?e ilitlereiit ctilour*. i» rotUn 
Tx-vnlvinK in a fmnit? rviteiu tiling that in which hand-iiiaK"i<'<'ni arw 
usually mounted, this frHiiic heinj,' fitted iato the Hub-stJ(K« in iwich 
n nininier tliat t-ither a shigle eelenite, or any eonibi nation of two 
n-hiiitw, or all thtve tog^'ther, may be brought into tlie optic axis 
nliovt! tho i>oUrisinH ))risiii (!!■*. 2"JI ). As many as thii-teen (iilftpeni 
tints may thus bi* obtuiiieil. When the coneTructioii of the tiiicru- 
si'npi- (toes not rva^iily aitnitt of the connection of the selenite plate 
wiili th« polHrining prism, it is convenient to make use of » plate ot 
liraits (rig. '2'2'2) ujuiewhat lar|;er than the glass 8U<le.i in whieb 
obji-i-ls are onlinarily mounted, vdtli a ledge near one edge fur 
the slide to rest against and a large ciniibir ajierture into whii-li 
n ;;lnsii 1^ fitted, having a film of selenite cemented to it ; tJii^ 
■aelenite stage' or object-carrier being laid ujion the stage of the 


1 ... .11. 




nicTOMOpv, titc alid^ «oiitainiti^ tlto object la plnocd u)>oii it, niul, 

bjr AH Enf!«iu«us inodificAtMHi cotilrivrnt 1^ Dr. LeMon, the ring into 

wkklt U>e &ielriii(« [lUte is 

flU*l btuti;^ made ntovnMp, 


lor anuther, whilst rotntioii 

imv b>^ cif pii to lhf> rin^ hy 

■nmiut (t( i» taiig*tit-mrew 

(llnl into the linus ]>lnlr. 

thf varirty o( tints fcivrit 

In- a wlniiit- him uticlnr 

|rtUriH^I li^lil u so K^^^''?' iiii^'nwwtl hjr tbn iiilnrpoation of a, 

Malinjj tiliii of luiot, tliat two spirniteit rrrl unit hlnr — withamk'U 

b1nt. Mv tiHiiiil to ^v« th« 

ntinruerietiit <x>toureo1>- 

lunnlitc from Miy nuinWr 

dt »«t«nile fihni, Mtlier 

iqMnitt-ly or iii conibiiiA- 

lam witli facli other. 

Til'* DoinjiM-t ■|>pa- 
nttts ntade by iSwift la a 
gnipml snb-stiigvi iltuiui* 
mior is useful und <x)ni- 
UlrlKl^blr, MmI is <n{ln)i}<> 
of ntlajitntint) In most 
Etitfli^i Viiicrcs^iiJrt. It 
it bbown ill tiji. ^>:!:i. Tli>> 
■jttvial adv*iiln^rof IJiU 
«unil«ti»^r lit« ill its liav- 
ii^ thtf piiUn-iiny {irisiti. 
th)^ B^Ieiiitc- Milt mir»- 
fibns, lit*- bU<-k'groui>() 
obliqu<--!igbt Mof-s, 
ih4- iu<iil<"iutor, all 
hrougltt cUmi- und'T tin- 
bKlc h>lia nf tbr iirhro- 
BMlic : whilst il i^xnl'irii^ 
JtcrIC iill lh« tiXMt iiR- 
tWit npplinnviii which 
«ab-stj^(p ii( » fcoml 
motlvmto iiii«ro>««iir> can 

TV «!«(■ of wiANivAivi- 

•li^ifiM* in mirruumfie 

tnark. nuwviUlly bluo light, 

kltfaougu of li-at moment 

than in prr-ai-liroinivtir day*. Tin* usual mpllioci of ohuitiintc 

(wlourcd light in to pu" iiunli}|)il through i-oloui-rd glnw, or ihroufib 

a cohtundaotution, Kuoh lu IIh- iiiunionta'.tul|ili(it« uf copjicr ; but llu» 

iaaiiMWt iiuperfMt and unutt^fiidory rnvthod, and dote not give 

1^.. 33n.— Kkift'. illniiiiniiilnic lud jialnriiiHiE 

Htoifoolironiatic liglit. Tlit^ Messrs. Zeiss hnvn const 
apparatus (afWtr HHrtnn.ofc) for obtaining moiiodiroma 
a triif forDi, which is i]lu^tl•!lte^i in fig. 1'24. 

By iripiiDs of two prisms, P', P', of strong dispersii 
spectrum of i-oiisicle ruble length is projected upon tie 
lieneutlj, sothnt with liighpou'ers the entire field ih illu 
iieiir iippronel] tit umnochroiaatic light. Tlie light enters 1 
lueDt thifiugli the slit S;>, which ie adjustnble ill width ' 
S', and piisses through the prisms und the lens Jii O, for 
trum ai. S/i^-, where I he object on the stiige is supposed to 

By moving the -ilil by the screw S' the. .spivtruiu 
pnss over t.lie obiect, the different colour's following in I 
The instrument may be useil for low powers with ordiuuji 



live {>owrr, to u-hi<'l) tlic li^lit ih niliiiilt^il by a linrrow xlit, it 
Miltitu<lr> of (Iluk lines iu»k« thi'ir nj>))«iarNii<«. The «xi!tt«fiii-«> of 
these was originnlly iiolicifil by WiiUitsboii -, but sm Fr>tinii<ifrr timl 
Hi1;g0ctc<(l litem to n tt«in>u;(l) inviiti^tioii atid tuA|)|ie{l t)i<!ii) out, 
they ive known iw i'raunhojrr line*. The graftter the ({is|>er!tioii 
givm by the niuItij>ltcaiiiM» of prisum in the s|>ectroacoi)e, tliH iui>n> 
M tJwm linvH »n> »eeu ; uid thi'y Ihsit conNidcmlite niaK»iH<^tinn. 
Tbey Twtalt fiMin tlw tnl«mi|>tiuti (ir iili9ior)>tiuii of certain rays in 
thn BoUr »tm«aplMir«, according Id tin* law, first stated by An;^truiu, 
iImU ' niv« wtiicb u subitAnm abeiorlM nm precisely tliose wlikli it 
niiitK wfii-n nii>(](! »«lf-luiuinouA.' KirchhofT thowea that while the 
DicnnihiMi-nt v»]>uu« of sodium, putauinni, lithinin, Ac. give a 
ifwtmin witli I'lianH'teriutic 6rv/Af liui^s, tlw- sumr rnpours intercept 
ponionn of tltv Bjiectruni, so as to giv(^ thtrk tin<is Bt the jioints vh«re 
tfa' hrijiftit oiiva ftppoar«(). ahaorbitij^ Uioir own >i|>odaI colour, but 
•flowing niy> of other colours to ]mss thtuugli. Again, when ordinary 
light U nitde to pou through coloured biKlirs (sniid, liquid, or 
gueuus), or is reflected from Ui«ir surfaces, so us to nttii-t thr rye 
witli thi- seiLiation of colour, it« spectrum is commonly found tn cx- 
hiUl ulisorption >'itfl», which (iirt<<r from the Ptnunhofn- lines, not 
unly in their ^rcAtcr breadtti, but in Win^ inori- or Ws nrlmhiHs, or 
ckiu'ly, wi that tliey cannot bo rcwlvetl into di^ttncl lines by magni- 

ficaiidii, while too much di»p«rKion thins ttieni out to iinliHtinctncna. 

Now, it is by iho cluiraclcr of tliiT' IihikIs. and by Uu^ir position in 

Ikeapectrum, that the colnursi of ditt'etE^it substancvs can be moat ac- 
«BrMt«ly aiid icieDtificnlly conipan^l, many colours whose iirijirt-uions 

•n th« eye are so simiLtr that tJiey cannot be diKtinguiKhctl bning 

nwdily discriminated by their spectra. The pur[KUP of the micro- 

tpectraaoope ' iii to apply the 6pec- 

Iroaoopiotest to very minute (junn- 

titina of coloured substances ; un<l 

it fundamentally conBi»ta of an 

nrdimiry Fyc-|)iece (which can \>r^ 

(MkA into any microscope) with 

Dertaifi special modilicatioDS. Aa 

onoiimlly deviscti bv Dr. Sorhy 

«aiT w(>rkc<l out by Jllr. Browning;, 

the micro- spectroacopo i* eon- 

rtructed as foll»WM (ti^. ii^t) ; 

Alx>ve it* eyc-glaw, which is achro- 
matic, and mailc cngiabh! uf focal 

•djuatmelit by the milled head, B, 

iliere is phiwd a lulic. A, fojiUinini,' 

» leriMiof tive prisinii, two of llint ^l:o-'~ 

(flg. ■Zt('>, F !■') interpoaiHl Iwtnetin thn-t 

nf crown (OCC), in such a niiinnei 

that the enieriBent niTB, rr, which 

have been seiianited by dispersion, leave the prUnm much in the some 

direolioin a» Uie unmcTgent n*y ent«red it. Below the eye-glass, 

1 Wii Jo not m^o lh« «h»m(«,l»«loa«npU(i»tioii» •iKwId'nw; ''"^ ■ "^^'" k it 
■ouM b« mow hutMaions with anslogx to taU Dil> luitmnwiit lliu tftttto«>t«n>toj.t. 

Flo. Stt,— UicTO-t|iMtraKO|ic>. 

I'ln, ■i'li: 

ACCEssony appabatiw 


in tho pliuNt (if th« ordiiiiiry Kto)), is a ()iii}>lini;;ni uitli a nnrriiw slit, 
which ftniitit thr lulmiitiion of li^ht <lit;. -'-i'l) : itiis luin Ih? adjusted in 
TerttcfilpcMfitionby tlicniillodli^iul, H, wliiUl thi'UnyidlJi of tli«> i^lit is 
rcgiiliitcil by C. The foresoiuf;, with aii olij«!ti>*i; of xuitAblo powOT, 
would be ftU that is needod ^or the euuninitlion i>f the fipertm of 
objecU pUeed oii the ata^ of tho mioruscojic, wlicthcr <^>«c]u« or 
tivnapiirent, aoUd or li(|ui(I, providefl that tJioy tmnaniit a Kufliciont 
nniount of light. But a& it ia of gr«4>t iuiportanre to tnako pux-t 
ooniparisouH of such artifii-ini spectra, alike witlt tJi« ordinary or 
natural spectruiD, and with each other, prori^iou is luiule for the 
lormnliuti of a second spectrum, by the insertion nf n riKht-anglpd 
priam tliat covers one-half of this slit, aiid reflect* upwanlx the light 
transmitted through nn apertorp weti on the rixht *i<le of the tip-- 

piece. For the pnxluction of the 
ordinary spectrum, it ix only- 
requisite to reflect li^ht into this 
Aperture from the Htnall mimir, 
I, curled at the ude ; whilst for 
iii» produrtionnf thespet-truni of 
any sobstAiK-e through whidi thft 
lif^t reflected frcini thJR tuimir 
mil lie transmit te) I, it ia only 
ni^pssnry to place tlie slide carry- 
ing the wction or cryautline Him, 
or the tuho contAininK the solu- 
tion, in th(! frame, I) X), iidnptvd 
to receive it. In eitlu-r ™«n tbta 
••cond speetruni h wwn by the 
eye of tlio oUserver nlongMrlr ol 
ttiftt orodnced by the object 
viewed tliroogh the body of the 
niicroacope, ao that the two can 
U- exActly coniparerl. 

Thr exnrt position of the ab- 

Borption blinds is as important na 

thot of the Fraunhufer linej ; 

iiiid nomrof the most coiuinicuoux 

^ i>f the Utter aHbrd fixed iKjint^ 

To. aBr^^Bfigbt-llne q*oti».mioroni.l«. "* r.-femice, provided the «>.»■ 

spectrriRcopo be employe<l. Tin- 
nmount of diapcnioD determines whethrr the Frautdiofer lines luid 
absorption bandit are seen neiirer or fIlrthl^r npnrt, tJteir aictuat 
pontwn" in the lield of view var>'iug affording to the diaperiiion. 
while tlicir relative positious arc in constant proportion. The \itaX 
oontrii-anoo for meaauring the Kpectni of absorption bands i» 
Bi'owninfu's bright-line micrometer, Hbuim in fig, 227. At R ia a 
Hmall mirn>r by which light ffom the lamp employed con be re- 
Aficted throuKh E D to the lens C, which, by means of a perforated 
atop, forma a bright pointed image on the sHrface of the upper 
prifin, whence it is i«dected to tlie eye of the observer. Tlie lotntion 
of a wlieel worked by Ihc milled head, !kl, carrin this bright point 



«t*r tlir Rprctmin. nnil tl)i> cxAct amount of molioii tiMy lie rMid nfl' 
10 tninnt'i <"'''■ '*■> *'■'' ^■>'laAt«d circle of tlie wltp^l. To use Uiis 
•ppBrmtus. the KmutilMfrr tines must l>e vi«Te<1 l>r Mn<liii-; linght 
(brlight thraugli llu- upt^ctmccopc, Atxl the ponitions nf tite jii-iiictiml 
ItiHW cmn-falljr tuctwnrvd, tlie rcMitig on th« micromcler-whMil lietn^ 
aMM down. A «i)rcti-uin mA]> may Uira be dmwniin oardliuard, im 
Ricnl* of f<|U[>l {Mrta. iind th<: tinm inwkeH on it, mn Hhovru in tbe 
»per hiilf of tig. iJ^f*. Thu lower half of the: somo ligure sliowra itn 
alNorptioii H[MH.lniiii, with iu bnnds at certain dUtAnc«s from th« 
Pnaahfttr liar.*. TIa' cnnllioard (pttctrum inap, wlwn once drHwu, 
ifcoBld be kept for reffn-nci-.' 

A lifgiiirxT with thr niitTru-tpActrcMCOpo Khoiihl firnt hold it U]> to 
1^ xky on u ch^ur <)ny. wtlliout tlio tnti'n'tmtion of thv micrascope, 
*nd attte thn I'ttVcIs nf 'ijfiiing ami chilling the xlil l>v rotnting the 
•trcw, C (fij;, lll'.'>) ; tin- liiit-:! can only In- well wm when (lio slit ia 
ndnord to n narrow oMniii^. Tli« Kcniw H dtiiiiniiibr!« th« length 
«f tl>p slit, and oauMs* tli« Kpeotnim to Ih- itrt^a as ulirond <irn noirow 
tibUm. Tb« Kcivw R (nr in xntm'. pattiTim two Kiiinll sliding-knoba) 
i^ulat/ra th» i|unntity of li;;ht lulmittiil through llii? Mguarv aperture 
JMn b«tw««ii the {minta of the springit, I> D. Water tinged with 

■ to 19 so ■,(. SO &n 1,0 t» t 

Vtti. T'' .iLiili<if<Hr lintm. ljnw*r 

1l*l< ' Ti:l*ljOn to tilKk. 

piirt wine, tn«t1<ler, and hlood are ^uod Uuids with which to com- 
amoe tliia stwiy of absorption bands.' As each oulour ^'»rieB in 
nfruixihility, the focun niuat be luyu^ted by the surew R, &f-. 22o. 
^^KorAiiig to the fa,n of tlie Hpeclniin that is exainineiL When it is 
^^piiml to BM the (i|>M-trtiiii oi mi e)ice«<lingly iiiiuut« object, or of a 
■tuill portinti otdy ut a lar;ger one, the prisais are to \te removed by 
witliilrawini; the tube cotitainiiifj them ; the slit should then lie 

ried wide, and tho object, or part of it, brought into the oeiitra ol 
Held ; tJie vertical aitd horiiontal slits can then be partly shut 
to aa to eticlose it ; and if the piisiuii are then replaced, and a 
Boit&bla objective employed, the i'oi|uirrid sjiectrum will he seen, 

' Mr. Swift Im> 3ni«nt ui imi>n«i>i1 nitrro-(jM«lKMMi|>«. in vhldi Ih* micro- 
■ivtnc ■ppitriilu* U uiubincd with die oHiiuT^ ■|>«irOKf>pia efo.piHe. and tvpa 

rni no fc* lirT>DiEt>t '^^ ttif it^d tl onf>, Otiior iifinTiiviniiuntii iloviMtd by 
Sarhj aad ■ ww tann dtriwd by Mr, F. It. Wntil K(ie bcm rnrried into 
viMlUton lit Hi. Hllnr. (8m Journ, of Bey- Mtemv. Set. vol. i. 1»Te, p. HU, 

* A mtIm nf apM^MU, In •nuJI tub«)^, lot the •In'l)' <il »kttirjiUaa-tpBetn, U 
kapl an aile b; Mr. Bnnnnnit : uhI tlw diiwtioiui pvui iu liw Mow lo Wart inlA 
Ihe Jtrtcw-qHefrOMvpf iltODid be tMcIallr *UeU<k4 I«l 




unrnffiected hy adjnrnnt al)jf cU. For ordinary oliservatHas ol>j«ctivc« 
vS bom two iuclics to J-incli focu« will be toawA mwt Bnitabtc ; l>u( 
for very Uiiuut« tjuiiotities of material a hi^Ii^r power inu«t be 

^ i>in|>lo]'ed. Even a single 
nxl blood -corpuBi-le may be- 
iiiuile to show the clutrtkcter- 
iBtic absorptiou bands rr- 
presetited (after Frofeesor 
^;tolc(«) in tig. 3^».i 

!i//iii(l» in tfM-h<trr» or ttnnll. 
rriit, ihv ttrifxW'ir n/iffirtim 
micro*!'"}", A^tvrWvA by iJr. 
Sorbj- in tire ■ l*TOC«MliDg« of 
tbo KoyJ Mocioty,' No. »■.'. 
I ^t>7 , p^ •13, is fixttvinoly con- 

Thf. rfitftrtil oculnr iy 

fi^tftet JortH nf /A' filem- 
tptetrtuifopr, Tlii'iKAn opinion 
rxprrswd liv I>r. Sorhy and 
■ itlirr oxpc^rU, and it it mnni- 
fc-Kt in tlin ohamctt'F of that 

inKtnuii«Dtl Fig. S.IO reprcw^nts a Kudional viitw of thr instrument. 

It will Im seen that tlie lowvi* part is an ordinary cyu-piitce with its- 

Piu. sw. 

i'lo. sai). 

Fio. 311. 

two lensss but in place of tlie ordinary diaphragm tlinre is s alit, 
adjtutable in length and bi'oadth, xliowii in (1;;. 231, By btuilyiog 

■ Pm liirllirr infi^rmntinii im "Diir Bpwtru m Htlliod dl Delpctlnc Blood.' iMaa 
impanaul [i^pci b^ Uc. bocl*)' In, itoatM^ ili«r«re. Jatiin. tol. vi. loTI, i>. 9. 


thi* lignra tlM* mi-lhod of Riljustmcnt, witli two acrews, F and 11 
the pmjts:liiig lirvT, wliirh rarruw n reflect ii>); priBrn, can br 

lil^ umtiTttdnl. 'Die uppoi' [Mrt of the instruiiimt swings 
Klnat th« pivot, K, *o thitt hy nponing th« slit the ejrn-pwcv can lio 
uccti for tDcnwinK ni) objrct, the tiit Ixring tho (lUphragm. Tho 
uppt-r portion cnitlaiiis thr prisms, ami nlno a scale in tho tubr. N, 
which IK illuniinntr^ hy th« niirmr, 1>, The iBiAgpof the solo is 
rt^llevttwl from thi- upper ■arfnrv nf thn liuiC prism to the tryn, And 
«bi-n proprrlj' udiustisl i^vvn the wnvr-h-ngth of the tight in anj 
part of th« KjMvtruni. TlMtn- is nlso a mipph'mi'ntnry ftttgf, not 
•faown in tho fisuri', afian which n ipiyitniii oiin bi- pLiccil, and itit 
light thrown up through th«- slit hj rvllcL'tioi) from the ]>risi:i on tiie 
tft'.'«r shown in fij;. 230, i(lonf;iti(l« uf the light from thr object <Mt the 
ttMffi of tiie mi<.-r(inctipi>. tliuH (.-nablintf the Kptictra from the two 
aaaicot to bi> (tirettlv 

The Kethod of Using the Ilicro>speotroscope.'-Th«objecti> to bi* 
[DTOtigStedareiif IW'i H>rl8, li<]uiili>ii(l sdHiI. Colouring nubstAiivna 
asehloroph.vll, tin- t^luui-in^ niutt«r ut Imir, blood. Jic will fi-ei|uently 
come untier miin.-si*i.-1n*vcopii-invwt*gation in tin- form uf a solution. 
In )ti*i»>nil w* no«l •M-itiv«Iy say imythinft com-CTnin;; the piviHiiiitioii 
of the solution. In rt^fervnce lu th(> (.■hloropliytl ■•{ the |)liauero^'aiiiti 
Mpeciolly, the partic-uUr{iai^of thtr plniil from which the [irqiaj-tilioii 
t» to tie HDule, AS, for insi.inoe. tli^ foliii;^ teat'es. ia put for a short 
time ill l>oiling wau-r. th«n <jui<:kly dried by means of bibulous pn|>er, 
on'l tbcn innitersed for a lon)|^r time in absolute iilcoliol, ether, or 
Imvxiile in a dark place, for the purjioaeof exli'iiclin;; the chlorophyll 
eolourinK Hintwr. Tim concentnition of the solution tliua produce<l, 
which inllurncea the inn-nsity "f the nbs.iiptioii uiectrum and the 
number and hni^th of the aWirplion Iiundv depenun n&tarally upon 
thv time durin;; which the iiiat<-nnl i» in the extracting oiedinm, as 
wpU na on tite <|u»ntity of tlic inateriaL 

ComiuMity alio n solution of leagcimcentratimi will give th« aune 
intensity of spectnim if a sulKi-iently thick layer of it be usc<l, The 
nltttion can generally l>e examined in an ordinary teat-tnbe. The 
-twt'tube is filled and carehlly corked and then lai<t on the stego of 
the tnieroecope, or hel'l before the opening of the comparison nriinn, 
ox Uie case may be. For tb" latter purpo>je (bringing liquids before 
tbe opening of Ute romipari-ton prism) a small open troagh of gbuH, 
with two parallel glass platesin vpry useful, b'orcxaclinvortig&tioos, 
liowever, the tmngh-lliuk in profemble. It is a Sn«k whom two 
■idm, bfu-tc and front, are pamllel, fumishrd with n cnrefuUy fitted 
jITv.Ufwl-gla*« stopper. It should be lille<l quite full of tbd solntiou 
niul then laid with it* t>road side i>n i)ie stngc. It is especially iji- 
dis]ieiisnb)n whnn we wish to Mtulr the comltinntinn speotrum of two 
aolations. In that ease two flantH «i* filled each with a different 
•nlntion and both laid upon tlw stage one upon the <ithiT. Fur the 
purpiwe of examining smnll (|iu>ntities of any lii|ui<l, a sutficient 
depth ha-ing ulitnim-d with vi-ry tittln material, v^rticnl glass tubes 
ottotflied to horinnit'il jdates am u«ed, n* proposcwl liy Mr. Sorby and 
shown in tig. '2^'2. The narrow tulMM arv made of various lengths 
from M«lJonK of lNLroin«t<'r tuliinn;, in order to present different 



thickni\iK)'K r>( the ctontAincd fluid, tho brouil tulie Iieiiis biglmr oa 
one hUlf tlinn tin- nt)i«r, niid thiui rnniititulinK >> wiH]K'^-iihn]<c<l viM, 
which, nlicn lilleU and oloiml Uy n thiii ci)Vi-r-Kli"»>. wilt piiiM-nt u 
vaiyiiig thU'kuMS of fluid for study nnd coiniHkriMin. If ilif nlijiH't 
to nn inv(9li;^uted IS noi a solution but a. [>re|iamtiou of ihi' kitid 
vliii^b we cuinmoTily mnploy iti microscujiiu iiuiuiripd. w(^ must lint 
of all liriiiK it into the (ouu^ of thn cil.jective tiyiit«ui. To ilu tJils we 
inast lint remove llie tulie iHuvriiia tin- jirinms. oi*n the siit suiiK^what, 
nnd uur tilt! ttppitratu^ us ii sitii]il« oculnr. If one huH to dii«l witJi u 
nniitl object wliicli wuuld not enlin-ly lill the slit, but so tlwt ray* 
of lixlit niifjlit come in by it nuil iliaturb ibe 6|)ectrum, he sJiouM luni 
the cumiMiriBoii prism so a.H to shut up Boineof the *lit, iiirithout> huu- 
ever, lettuig in the li;;ht upon it, and then bring the object li]> near 
to it, njid from the other side push up the dhorloning apparatus a& 
close lis is necftAiiry. On (ho other haii<l. hhould the olgect cou&iat 
of a number of single ininuie grains, which would caum to be drawn 
acroiH the Hjiei-truin, iu tlie direotiou of its iMigthi porpendicular to 
the Fmunhofer lines, a like number of dark lines, one must adjust 
the Diioi-oscopt^ m thr.t the object will be n littlo out i>f fociu, some- 
what above or bnlow the true focus. In this way we rIioII get a 
uniform spcptruni. Thes[>ectrumcan also bn im proved in KOineothftr 
cases by likdwi.v: throwing ihe object somewhat out of fucns. 

Fi.., 831, 

niaminatioii by Beflexioii.— Objects of nlmoat evm- deacriptioir 
will reijuire at times to )>e esaiuineil and studied by vrbat ia cnlW 
mflccted light ; the lif;ht in this case is tJirown down U[>on tli« 
object by vnrioDK devices, and ia reflected ttpwmnls througli llie 
ntijRctiw. This has been called 'ojuque illuuiination,' wbicJi, how- 
■!rer, is not a rnmprehonsive, nor even sn accurate designation. 
Only a sinAtl proportion of the objects examined in this way are 
't|in(|UR ; tho Kam« diatom, for example, may oft«n with advantage 
\ni exAininod with transiuitted light, being ti-anspsrent, and again 
by miitns of an illumination thrown upon, and Tellect«d up from, its 
5ur(a4:o ; nlna r condenser with a central stop when used for a dark 
Round shown obipcts by reflected tight, but it is mnnifeistly not 
'opMud iltuuiinntion/ The designation of this metbnd of illuniina- 
titm IS con,te(ju<'"itly mom accomniodatini; than iiccurato, 

TlH'n- an; two vrry Kimpio means of obtnining this cDperticisJ 
iUuniinntinn wlini ]nwpowerKnreem])loy«d. The tirat is the 'bull'*- 
eye '(which IN nowherp in this work called a 'condensiT '; this would, 
fts it oftrn liua dune, Utiul toconfiision) ; it is enough todesignAt«ita» 
we have done. It is * pUno-convc-x lens of short focus, twii or thrw 
inches in diuuieti^r, mounted uiwn n separate stand, in aueh a manner 
as to permit 'tf itn )j«-ing ptiicrtl in a grPAt vnriety of positions. Tlic 
mounting »liown in Hg. '.'3.'( is one of the best that can be siloptpd : 



tiie frani« wlikti cnrrin ilic lens U lx>ni« at Uie bottom upon aBwirel 
'hiiU which nllovs it la be tuni^ 111 nay ivxiiuut!i; whilst it may 
' inclined Bt any ungie U> the hori»>n. l>y Uie revolution of tho 
rlxontnl tube to which it is attache'l, ki'ouikI lli« otJivr horirantal 
vhich iinijcctB fnnu the at«ui; hy the HljdiiiK of oiw- of these 
I within tho othfr, nsnin, the hnnxuata] arui niuy be It^nglhonccl 
i thortenvti ; the lens iii«y l.e beLured iu any jioaition (o» its weight 
u »[■! Ut drag it ih>wii when it is lucliavd. uiileas the tubes Iw niiKli> 
io work, the on« into thr other, lomrp stiffly tliau is couveiiiinit} by 
mcnns of n tiglitvntng collnr milled nt iU ei^en ; and fiiudly II19 
konxoninl urm i> attuchrd to 
n tprilig itoolcrt which )Jidr« tip 
aiui clown u]x>n > rnrtimi Stem. 

The |>Inno aiih: of thn 
bttUVttyv shuuld W tumcil 
towards the object. Sonm 
nuL-roaoopist* liki! to haw 
their bnirs-ry*! ntliwhetl to 
Kuiu> piut of the microM30]H- ; 
bat if thL-> it done, care mutt 
U- tnkcn to iittachil ton dxfd 
fort »f thi' titivroEicopi', ami 
not to eitliur the RWH.-Iinjii^'u) 
tta^ or to thi-- boily, us i.t so 
ofl«n done. If it is lixeil to 
tlw incchnnitKl iitntfc, wlii-ii 
din object is movixl the light 
will n^iuire t4i Ih' nuuljuK^vl, 
to Mky nothing of the jirolinlilK 
injury to the xutgi'i by tint 
Wright of tin- bull't-eyr. If it 
a dxM) to thi- IxNly the liyht 
will tn- ilisuhiood when thi,' 
focuiof the objective ia«lt«rcd. 
Heiuv Uh> bidVs-eye ahouhl 
either hn vc u weighted se])aiute 
ttniul, or be iittui.'hed to the 

Till? oj>tJc»l effeotof ouch a 
iioU's-eye dJfTers accorditif; to 

Pill, WIS.— Die biiU'».c]rf. 

ib» ude of it tui'iietl towards the light ttnd tlio condition of th« mys 
which fjill upon it. Til* poailion of fwi*/ Kphpricnl nVicnution i.* when 
ht/tinrfj^ Hide is turned toward* jwrvi/Zi-/ or towniil* tlu- l"iil dircrijiny 
rays, 00 n »>.'<(] u en tly, when uwd by dnylight, its ;i/ni(« uide should b« 
lunied tiiwni-diiUwa&^'MV.aud the same poulion should l>e given to it 
«r)ten it is uaed forprocuring converging mys from n lamp, tliix Ixiing 
pUcetl fotir or five times farther od' on onv side timn tiie olijuct ix on 
Mie other. But it may alw 1>e employed for the purpose of reducing 
tlie diverging raye of tlic lamp to pnmllelism, for use either with the 

' Ri7uiate1*r4ballV<ij«Ulikriiiiiiiuituliuu|>UuuiivpnicDlandInoip«n>iTC; — Avni. 


pantboloiti, or witli the pani)>o1ic npei^uluiu to Iw prcfwntly dnncntied ; 
HD(I the jihnf side in then to be turuedlownnU tWlnin]\ wliichmust 
be pUced at Bucli a distance from the buH's'i'TC thnt tlii^ rays which 
liave posBed tlirough tlie latter shall fonn an inverted imnKC oj 
th« Uuip fliLiu« on the n-all or a distatit screen. For viewing niinnlr 
objeirts undor high powers, a einuUer lens mny Ik' usril to <>ht«in n 
furtlior concent ration of tlw rays already broiifthl into I'onvnrp^ri' 
by the bull's-pyp. An ingenious and fflpotivo inodo "f uvina the 
bull's-^ye, for thfi illuminnlion of very minuti? iibjcetii unih-r hi;;hiT 
power objcctivc.s hnji l)pcn deviw-d by Mr. Jnnu-s Smitli, Tin- miiTii 
scopo bfinK in po»ititin for observation, the liiinp nhoutd Iw? nliw'til 
eithrr in thtr front or jit the tAAo (m most oon\'Anii-»t), no that it-<<;, turned ■•dgcwayK to llie rtAge, vhnvld lie at a ;^nu-what iov*r 
ti^pri, null at n iliMnnon lA ol«iut thtrir inchiKi. The buU's^ye should 
be placed betw<^i tin' ttag". and the Iftntj), witli its plane Hurfkce 
upperinoxl, nnd with itt cnnvcx surfiicv a little abovf th« Kftx^. 
Tlifi light entering its con\ex surface n<-nr the innrKiu turned lowanl* 
th(^ lump fnlln on itH ninnc siirfutTc iit an angle M oblique lui lo be 
alninst totally ivlli-cled t^^wl^^l^ the opposilt? marten of tlie coiivex 
surfui*. by which it in oiindcnsHl cm to the tilijet-t oii the stnge, oii 
which it nbould ca-it n shurji «iid brilliant wed;;e of H;;ht. The ud- 
ju!>tiii(.-nt ia best inudi- hy lirst placin;; a slip of whil* card on the 
HtaK^. and, wlifu this i> wd! illuuiiiinle<l, lubntiluting the object- 
ttlide fur it. inakiuK the linal adjust iitent while the object is l>eing 
viewed under the microscojic. Ni> difficulty i* esperienced in getting 
gCMwl rewull.-' with powprs of from liOO to 300 diatui?lers, but lii^er 
powers riripiin- ciiivful nmnipulation nnd yield but doubtful rAfUlts^ 

The sw-oud simplv method uf Nccuriiiji this illuiiitiiatioii in t«luLVe 
tlio couwive mirror of the luicrwsi^o]* capable nf being used abov* 
the sliige ' ao that the souivc of light muy by its means be focusMvl 
on the object. Neitlier of these plans will answer for other tliau low 
powers, whei'e there is plenty »f nioiu for the light to pass between 
the objective nnd the objeet. The ingeoiouM u^e of thn buH's-ey* 
vuiployed by Mr. Jauies Smith, a* detailed above, incrtMses the pos- 
sibility of magnilieiition, but it uee<ls practice and care. 

An illuminator not mi well known, or M least mi much uwd as 
its merits justiliet), is Puuell and Lealund's small bull'ii-pye nf }-jiirh 
focuS) which slides into an ndupter lixed into the eab'ttngr. and 
Misoeptible of ita raek-niottou up and down. The object ik plncrvl 
on « niper-stage and lie» coiulderably above, but parallel with tin- 
•irdiimry stage. Tlie bull's-eye, capable thuf^ of Ix-ing raised or 
lowered, and of being moved by sliding away from or rh"* to the 
noiintfid object, ha:^ ita planeade placed agnimst tbredge, ami nt right 
angles to tho plane of the slip. Ity this ui«nns illiiniinatinn of gn^it 
obRquity can l>e obtained, and very surprising effocts swurf d even with 
high poweii^ It waH niui-h used by the Editor nnd Dr. Drysdnle in 
Uieir earlier work on the Mtprophy tic organisms nnd in the days Im. 
fore homogenefius lenses, helped us over many diflicullieK of di-tail. It 
wan the tir*t illumination to actually resolve the ilin7i/(i;)/<-iir>t/i'//ti'-(Vit. 
Jt could Iw %Try oaMly obtained with a «tndent'H microscope provided 

■ S-M Journ. Hoy. Slierot. &>f. vol. >il. ItUM), |>. Vt» 



with XelMin'fl upen stage,' for on lliU the bull'H-eye could "be iiloci'd 
kipi»8t the Mlge ot' the slip without aiiif ■i]>ec-ial HpjiAiutiiB or lilting. 

Ai)oth«r nml popuUr ucthotl of 'o|iai|u« illumination' ii by 
UMtis of a speciftliwfl fonii of niirror, i^eTierHlly of }iolibh«(1 ailvi-r, 
«alled A aid-! T*fii^ftr>e, aii<l lixnl ns in the case of the l>ul)'a-eyc, 
aiul for the same reasons, to an tniiu»rAl>le fxirt of the tiiici-UHc.-o|ii'. 

The mannerof employing thisrellector, AH proviiU-d with Powtll 
bh'I Lealnnd's Iwst stxnd, is wvn in Ilnf- III. Thn ami of the 
side reflector is lixed to an immovahlc luirt <ii tlic tlnnd, nnd is thns 
aiiaRecte*] by the riickitij; up^r Aown of tln^ body. The lamp placed 
OH ihe right of th*t obu-rrpr is set at stidi n \u\t^\t that its beams 
(idl full upon the reflector ; thiH, by mrAtis of ii luill-ftnd'socket 
joint, can Iw fiueily mnnipulnt4rd until tlin full iiua^ of the finnie is 
«Hise'l to full upon (ho objnct. For tltD miinr (nirprao a. finimltotie 



^wii/iiiii IK conniionly ciiip!oyi-d niounti-d fitluT tin the obji-dive, 
U ill itnck'.t form. liji. LTtl, or on an ndnjit^T, iik in Crouch 's^ 
»liowii in dg. 'ili't, whi'Tv n i-cilUr Li iii((-r[>OMil Ijctwocn tlif lower 
*nd of tlM^ boily of thi- niicniBcojiL' and the «l)j«:tive w-mi ut A. 
JViu' i« mJ n oimnifiuliMi: fAitn, for it inoreaaet thi- diituiico Wtwi'i-n 
(Jir ubjt<i:tiv» und thr Weiihani biiiocuUr ])ruui ; and ua llie bin- 
nculnr in Kjieciftlly Kuitiil for tlie kind of object usually examined 
with this Npeculutii, llii.t incrMSMl distance acting dt-triinentally on 
llwt Iwhavioor of the biuoeular prisms, *nd cauigng the available 
racking diatance for the focua ui objectivea of very luw power to be 
short<!ne<l by the widtli of such coIIjit, is to be avoided. 

Th» bedt plun without doubt in to Httach the &|>eculum to n tivi^d 
{uirt of the HtAud. MS is done in the Powell and Lealdiid, the Koia, 
niid the Beck Htaixk. 

A mmliji'''iliim t.flJtr fifr/ibolif r^eetor tea* difvitfH hy I>r. Sorby, 
And luw proves) to tie very u§eful in certain inveetigntionH, nuch an 
th« microscopic structure of nielAts. It consists of ft [uirabolic 
i^evtor, in tlie centre of which, in a eeiiii- cylindrical tulie open in 

' aiayiKt 11. |u 1DB. flir ItX. 




&uut, is ]>laced a siiiaU i>lnnc rc^fli'ctor wliioh «>><•» hulf of tho 
oliJMtive. mid throws the lifilit (iirrvtljr ilown U]ion the object nnd 
Luck tlirouh'h th« otbrr hiilf. It i> sliown iti Gg. 236 with tbu 
cylinder iii pWo, nnA in the i]iitl<r(] Un«s witli th<! mium tum«(] out. 
This umuigement itllowN of two kiuda of illumijiiitiiju, olJiqud niid 
direct, beiuu readily u»rd, as tJiti yliuv^ rafleoUir iit ntucheu bi iiu 

iirni so tliut it can be iiwunfc out of tli>- 
«-ay when tiot rvtjuired, an fthou-u in 
th« tigure. 

Dr. 8orbv wag able to got results lit 
the exutiii nation of polUlied M-utioiu at 
steel not otherwise attainable. 

No upa<]ue ilhnuinaUoii, however, 
)ia:i yd Burpafesed the venei-alilt- 
Lieberktihll ; the \x-st cxprrls frtvly admit tliat the finest criticAl 
images U> Up olitnimrd by thU method of illuniinntiou are aecured 
hy tho Liehrrkithn, This moilc of illuminating ojuiquo objects is 
by mcnns of n naall tmncavo xpoculum reHc«tiii^ directly dowu upon 
thorn to n focuii the light n-flcctwl up to it from tlip mirror ; it wiis 
formerly much in une, but in now coQii>amtiv(i1y seldom employed. 
This conc»vo spwinlum, t<Tni«l a ' IJidwrkiibn ' from the celehroted 
microccopiKt who invontnl it, is niudo to fit uimn the end of th« 
objoctivc, having » [n-rforation in IIk cmttre for tho pnKMgc of th(! 
mys from Iht! object to the Inm ; and in order ttint it may receive 

Flo. 884. — S.)rb);'ii muJili.nlj 

'II tit 

Flu. *1V. 

its li^t from a mirror Lenoath [fig. 237, A), the object niusl bo so- 
mounted as only to stop out Uie central [tortitm of tin- rays that are 
reflvcted upwards. The curvftlure of the B|iecutuni U so iMlapt«d t» 
the focus of the objectire llial. wlien the jailer tit duly ndju-sied, the 
r>yN reflected uji to it from the inirrur ahutl Itc made to couvur^ 
stronzly upon the pail of lltt- object that la iu focus ; s separate 
speculum is coniiequeutly required for every objective. 

tlEBERKt'HV— lli>W Tr> MiH^.NT nlUBCt^* FOR IT 285 

// Itaa tutt maHiffKi lirauiiifkt : the tirat uue, lial of requiring a 
arfiantte CifirrtiiAii /irr ttn-A lijrrtii^, is a dilHcully whicli ill Uw 
Mianot tilings CMinot be ovcrcoiue. Tlie radiuKof the I^eberkiUm 
■tut niter with the focas o( tlw olijeclive cuiplt>jr«<l, nnd «Hcb sliiiuld 
havv a crrlnin itniouht of pUy on the objective to allow for aliKbt 
•hrmtioiu of foctu, for if wp voiploy paruld rays it is obvious tbat 
llif Lij-lfi-rkiitin wilt focas iw4irer to thaotgeot than if divergeiit nys 
an-uvd. TliiniainotbyntiAllowanceheiii^inadpCocoiiipensaicil oit 
ihp tiilw which Rli<lM the LieWrknlm on to thcnoeeof i)i« objeviive. 

T/ir rtfottil f/ivi wAiii-jt him rr/nriHCe la id' '/ffiat ifviy in M'A»eA 
oA/nrb hat* to hf inoiiittft/ in <m!er to W suitable for tli« Lii?l)@rknliu. 
Thivootild ba«ULly«v<ii(b-dif profoMionnI and othar inountcn would 
Wttnd to the following idnipii- Hag|;v«tiona : — 

1. Slidn ahoold Wfnrr Ij* covon-d with |iap«T ; it Jk witlicul use, 
aiwl fails na an oronownt ; iwd upaquo glaw Mlips ahould l>c «ntirc>ly 

2. Th« ring of cMnnit aboutd not be niadc of greater width than 
ia ncw«!wary fiir (pcuritjr. 

A. A stop nf paptT or vnrnixh itliould novcr bo placed behind an 

Let every inMi(|ui' mount b« also a trantjuin'^il «ii<-, since it in. 
often inoit nicfiil to rxaminn an opaqne »hjr-ia nfliTunrdu )>y tranx- 
■itted light. The Ntop kIiouIiI ntwnyx )><■ u M-jiamtc on<- ; iJiia iimy 
be a disc Ofi u pin held in tliK Kultxtage, or, what ih ntill siinptrr, ik 
piece of luoderately tliii^k ' cover ' glnv, cut to tbu 3x1 inch iiixe, 
ur ratlirr shorter, Rhould havo n unwll disc of Brunswick bU^k put 
on it rimlrnily on llif ' ttiiii-tnble ' ' nnil thix niny tm placed umlrr 
the xlidc wlwn titr liclinrkiihn is to be used. 1'hrrr nmy be two or 
Ihm) Mucb Klips witb stone of difiermt siiw i in this way vwry 
mount ntay )h- exnniinrd eitti^r with the TJoherkiibn or by din-ctly 
tmnnmitird light, niid of t^ournc by linving a largiTr stop thi' name 
object may !>•■ i^amined by any kind of rclI<H;ti-d light. l^Duiy a 
«mlnabtrr pn-]xumti(in has IxTn Mpoilcd by jJiicing a »tup on it which 
cannot lie n-inovrd. 

i. It wiiulil Iw n most npprccinbiD bvnetit tu IJii- caune of miora- 
xopy, am WL> liavK nlnWy hintinl, if a unifonii gitufci; <rf tbieknen of 
■lip aiul iliaiiirter of eovi-r-Rliua were adoptetl. For the llttvkiiess 
(^ tbf slip .the Atli of nn inch would pr»ve luuat suitnble, and for 
llif diajiiBti-r of the covi-rglawi ^ of an inch wHuJd be iiiotit conve- 
nient, and if tltc thickiwu of ih<- ei>vrr-glaK9( were uniformly fruiti 006 
to '006 the fcatii w'lulil Iw xtill jn'ritter. Certainly no mount ott;jlit to 
be finbibed without iln- ihicknvK" of the oorer-glass being marked in 
tfawwmd point u|<i>» it, nud a nurrow ringof didUu cement should i» 
put round every cover glaaa where there is ereti a probability tlmi a 
luiniof^neous leiu will be admiaatble iu examining the object mountrii. 

Very minute cot er-gUaaea — such an thou i^nths of an inch in 
dkmetei^an^ 10 tie wholly condeniue<d. They do not allow the con- 
ditions requim) by modem niieroacopy, being adverse t"> ihr nnploy- 
_Bieut of oil-iiiimeraidn lenses in anything like tlie most efficiitntwny. 

LieberkiUins caa be uuetl with objectives as high as ^ of an 
I CluiKcr VIL 



inch focui of ■"' N.A. For higher powers than this n perfectly Bat 
Hueeuluiii mny rL-{>lace lli« coniml form, t>eiDg iUuniiiiat«d by a con> 
(fenaor witli n. stop, and racked up well witliin ite focus. Thft 
oblique luiDular ring of light fullii on the flat speculuio, And is 
then reBeited o» the objert. 

II tu mi lint ion f'>r liieberkijhn iiinjr ba 
either from iho {\a.V of the liuiip finnift, 
reflected by the plane mirror, or hy th* 
edge of the flame, th« rnys hoin^ n-nder«d' 
pantllel hy n buHVoyi-, nnd rr>llccti'<l fm 
tho pl«n<' mirror to th<! I-icbcrkJihn. 

Very nftcn thi> kind of illuminatioa 
ix Ko intnniw thnt it n»]uiro« modifying. 
It IK tinr-ful for this nnd mniiy otiii-r pur- 
piiKPiK in itluminnting ohjcirt* to liKvo a 
nmnlt /mmr, nionntrrl likf (til nflittarif 
/luiCn'fi/n ttnnii, into H'^Aifh a filttle of bliM 
tjhiA* may kr innrrtfil nwl ulannl brtutt^u'i 
thf Jiijht iinii till-, mirror. Two inohes l^^ 
four iiicbi'.i is n vi-ry uM'ful kUh for aaob' 
11 iton^n, mill twri> or thnw plates may 
Im' l((-pt of vnriouH •Icpth.i iif colour, so 
Midi th«v tnay he ctiujiyi'd as ctrctiin- 
stAncfu reiiuirc. 

Mi'-nnra. Bi*i:k make a small and uwoful 
lu/fitmirJififi; which cnn bn iil«) iiud 
for this purjiDKc. which in illuKtratrd in 
ii;;. 2:W ; but what is ri'ijuiri-d is not mrroly 
a tinttii i/Iiia; hut a comhindticm nf dif- 
fen-nt tints n-sulling in a. foirnct blue not 
I othrrwisi- klliiiniibt(!. 

Thei* is oni- uthi'r kind of niflectod 
illuiniiiutioM ■•itiploycit, produuxl by thn 
vertical iUuminator, whioh, although it 
has Weil in ns<- for some ydirnt, has i^- 
ceivi-tl an ucoMftimi nf value from thn um- 
plo^'iiient of imQi«nion Leiues. It i:i pro- 
iluceii by a dmice which aeourea Teitical 


Smith, of Gene^To, U.S.A. 
The piinciple of 'his illuminator ia t<) employ llie objectire as 
its nvrn illuminator. Tliis may be done in sevenU ways. 

1. Thdt of Professor Smith is to place a tipeeulum with au aper- 
ture ill it in IIh) body of the niicrosi-ope at an angle of \y to tJie 
optic axis ; opposite this speculum is au aperture in the lube for 
light to enter. Tt will be uri<kTMtoo<l by &fp. 239 and 240, whii.^ r«- 
prment n longitudinAl section of the uoie-piece nt C of the vertical 
iliuniiniit'H' nt r, nnd of part of the objective at '/. ii i&tlie aperture 
for otlniittinK thi^ liyht to the speculum b. The path of the beam is 
<lepict«l, and it will Ik- stien that on being reflected from tlie specu- 
lum it pnfuif I hrough the combinatioii of lenses, lualdug the objectirs 





fenaer,' &nd is brought to a focus on the object, which is the 
if tlte objective. The rays now proceed from the illuminated 
■lee more, tbrongh the objective upvarda, and pass through 
riaal aperture in the speculum to the eye-piece and the eye. 
loIleB, instead of this, places a small right-angled prism just 
lack of the front lens of the objective. But this only gives 
I Qlmnin&tioii in one azimuth, and from experience we are 
Ito express dissatisfaction with its performance. 
Tf !■■! II R. and J. B«clc, in place of a speculum pierced, 
f a disc of covet'-glass. The cover-glass is mounted on a 
k fig, 240, in order that it may be rotated and oblique light 
idtT the mUled bead, /, A, fig. 240. We believe that it would 
better in practice to fix the cover-glass at the angle of 45°. 
will be seen that by tbts plan total reflexion may be obtained 
direction and transmiaaion to the eye in another. 

Fio. 239. 

Fio. 3«l. 

. Powell and Lealnnd's method ia to fix a piece of glass, wurk'nr 
at an angle of 4-''° to the optic axis, with a rotating diaphiagnk 
ant of the aperture a<lmitting the light. 

P* KM th^sr iiatrHments the edge of the lamp flame should bp 
si in front of the reflector, so that the rays may be reflected on 
ke back lens of the objective in a line parallel to the optic axis, 
i£itance from the lamp to the reflector must exactly equal the 
*BK« from the reflector to the diaphragm of the eye-piete in a 
■itne «ye-piece, or the eye-lens of a negative eye-piece, otherwise 

inji will not be focussed on the object. 

lUi illumination is only suitable for objects mounted dry on the 

•,tnd with immersion lensea. No good result was ever obtained 

Athe inunenion lenses were brought ioto use. 
Of ill the light which is caused to pass out of the front lens of 

koliJKtive, through the oil and into the cover-tflass, that which 
haoldiquity less than the critical angle of glass (41°) passes 



throuf;ti Uic cover oi«i oliject nnd i* Inst ; liut uU tlie iijjht which i* 
of ffTKtttirr obliquity tluiH thi- criti«l iin^lp far k'**" i" toUllv n-fln'twl 
fram tin; ini'irr tur/a/'if <tf tlf •^•ivr-i/liita, and comes back tJinw^ch 
rhcoil iind llm objecliv*' to t)ie eye-piece iind the eye ; ihey mm, in 
fnct, nil optically coutiiiU'ius, bo th»l the upper surfiic« of tite eaver- 
ehus has ceawed to exi^t optically, the only rellcKion b«in(c from iU 
toiler surfacL*. It in here, therefore, that the oil-imiDernon HyKU-m 
){iv<-s II iipw v&lue to this illuiiiiimtor by this me&iis, eiiabliii]^ it to; II larger aperture otherwiee unavailing. 

When this illuiniiintion in employed, if the eye-piece be removed 
Jti)d the hack of the objective be examined, it will lie eeen that all 
iJuit jHirtioii of the back of the objective whoso aperture exceeds I'O 
isbriiliantly illuminated. ThinAnnnlusnTpiTKcnts, andiKproduce<lby, 
t]ie excess of aperture beyond the (i<iuiralent ftngle of 1 1:^°, of which 
itia also a mcamtro. The internal dark space tsu the exact diameter 
of that of a dry objcctirr nf thi- Kamo focuJS and i« the niaximun) 
space which it can itself utilinn mi a dry object l>y tranKniittod tight. 
By means of tint tnslruinr-iit carrfulty uccd, thi^ most stubborn 
fruatules of ditliciilt tests nrnt thi> most crucial lined oltjoctn hare 
been rmolvMl ; white it is eminently seT\'ic™bl«i in dctomiining 
whether any d^-mnunted olijcc-t is in optical cnntact with the cover- 
glftM or not. It it be not ko it is invisibli> with the vertical illumi- 
nator. 8o oIko it is instructive to fxnmini' Uie ImekK »f ohicctivM 
of viLriouK api-rturea with this imide of illu mi nation. A dry objective 
will bo wholly without the bright nnnulus, white an imnierKinn of 
I'l N.A. will Iiavp » iiiLrro\s- annutus, and tiint of 1-4 or t -5 a bmad 
nnd still hrondi-i' one. In tliis way. by prartice, a fair approximation 
of the aperture of nn objectivw may Im olitainnl. 

It is not the /ifmnlalr sixe of the annutu>, lint the relation of the 
qize of the annuluK to tlmt of the whcitelim-k, that niunt becKtimatcd. 
Thus a jth of N.A. I''J will Imve ns lircKul an annnluw asa -^^th nf 
1 'i S.A., but tlie diameter (if the back of tlie ^th in, nf course, much 
larger ttmn that of the -g'jtii, and tliiN iuvulvM tJie 
necessity of a relative comparixon. 

In examining; obji^ctH with those hi^linr powan 
which focus extremely close to tin- cdvering gUmt 
the tjli^tttost inadvettence in likely to lea^ to m 
frai-ture of the rUss, and perhaps to (he destructioii 
of a valuable slide. This is a serious nutter with 
Miller's diatom type tilide. or Kohert's l«st lines, 
or with many others that are exjiensive or pcrha|>8 
imposiiible to replace. To remove this snuny.' of 
'ianger, Mr. Sl'iJietuon e'liilfivtft/ ih' lafety tia^t, 
shown in fig. 241. The frame on which tlir slide 
carrying the object rest8 is hinged at itjt upper mrt, 
and kept in its true [Million by idight ifprinipt, 
which fpve nay directly the slide is pressed by the olijective. It i> 
found that springs finn enough to ensure the steadineos required 
for high powers may yet be sufficiently flexible to give way hefore 
rerr thin glH«i) is endangered, and a glance at the stajje tbows if it is 
mode to deviate fmm the nonnnl jiosition in which its upper Mid 
lower edges arc parallel. 

Fio. Ml. 



UmAiI m this piet-e of iinpRrntas ntny iio, it is tm-ertlielesB qnito 
■UUKCmry i( the front o( t (h- stnge of ttio microsoope be either ojien 
«r Iniyi- eiii>UKh to ivrinit "f th*- nm of the finder on the «!;,■« of the 
iKdp, wh*n it is msj- t<. (li.iixvrr hy toiii-h wliether or wlieii the 
(rout ft( the lens is in fK'Iii.'il mularl willi tlio <ov<t rrf the odjeot. 

ApplianoM for the practical Study of living ftod other Object! 
with the Microwope. — SUiijf-j'iirrri>t nnd fiW. — Kor bringin)* iiiii!eir 
ihc objvci-Kl'i^ >» 'iiff^i^nt positions niich >itiinll opnijuc oViJei-ti^ as 
Mil bo conreniently bold in a pnir of fon-cps the Wnz/r-y^^nr/M {Vi%. 
!43) sapptieil with most iiiicmix-opcK nfl'ords n tvoAy incnn». These 
uv mounted hy mnns of a joint tipmi a piti. whii-h Itt« into ft holei 
ttther u) tJie i.-«ri>er of the stitj^ itni'lf or in the ohjcct-pUtform ; the 
ohjnirt U inserted hv proiw- 

■ - - ■ ^s^-^ 

ing the pin th»t projix-ta 
from one of tho bludrK, 
whereby it is separated 
from the other ; itnd the 
bladee close xKnin hy thi-ir 
own elasticity, an n« to 
retain the objrvt when 
the preMV r<> i* wi t hd ra u n . 
By sliding tlin wint itrm 
mkieh bears the forrnps 
Ihraogh its sofkct mnd liy 
noving thnt KnclcH vi^rti- 
felly (ipon its joint, nnii 
the joint horiwntally 
npon the pin, the o1>jii-t 
nay be brought into thf- 
Beld precimtv in the poi>i- 
tion requirKl ; and it tuny 
be tun>ed rot) ml and 
Toond, BO that nil xidex of 
it mny \x rximiiniwl. by 
limply giving a twiiitinK 
movement to thtt nire 
lem. Thi' otlinr ex- 

Pio. Sia.— Btii£U-torrep*. 

Pio. Si3^-t>t*i(e'lbnMy*. 

Flu. 3H. 
Tlicpa-pronxed lorcapk, taew ii<lju-<tn><'nl. 

Pia. MS.— T1i« itag^tiot^ 

iraioity of tho stem often be«r> a anudl bnuu box filled vritJi fork, 
uid parforated with Iiolw in iU side, seen in fig. 2 13 ; this alTordH u 
tenire bold to common pins, to lli« lieadu of wliich ffmall objects can 
he attoched by v.wn, or to which diitiB of curd itc mny \m atuichid, 
vhereon objrcts iirp mourtod for ln-iiiK ^'ievfl•d with the Lielirrkuhn. 
Thin metho"l "f mounting wng formerly much in v<iguf, but llrl^ Ibh-h 
IfM employed of late, since tli" I.ii-berkiilm has unfortunately falliyi 
into oompantive disuse. The forceiis iu Sg. '241 is also often of 
treat practical ralue, and is adjuated for holding by « screw. 
That which is knowni as tlie »M;;r-n'rt, for tJie purpose of holding 
«m]1 hard bodies, nurh as minenUa, apt to be jerked out by the 
angular motion of the blndea of the forceps, or very delicnto nilMtnueeB 
that irit) not bear rough eompresuon, is very uaeful, am! is seen 
in Gg. !24A. The !ltage■^-tce fiU into a. plate, as is the caie with Bedt'e 



disc- holder, 6g. 210, or i( luajr simply di-op iuto a aia^ fii 
lh« ligur«. 

For the «3ianiiiiation of ol>jectA which nmiot be conv«niciiU<r 
held in the Hinge- forceps but which am be tpntponirily or perma- 
nently nttMched to discs, no inenM IB comparable to tlie riu/e-hoiiti^ 
of Mr. K. Ilfvk (lig. 2Hi) in regard to the ^ili^ it ftSords for pi«- 
Hcnlin^ tlicm in oveiy variety of position. The object being atlacli«d 
by sum (having a sdiaH c|iiiintily i>f glyceiiiio mixed with it) or by 
goldsixt; to thoMii-far^ut' a uniitll M»cken«d nratallic disc, thi& is 
titt«l by a short stf-ni projcclintf from it« nnder surface intv a 
vyliiidriciil linldci- ; and the hohlcr carrying tli<- dii^c ovn be ni«de to 
rolatf iirouiid a vertical axis by turning th'* inillcil head on thi- 
ri^'ht. whii'ti nctx on it by inuAiiJt of a khiuU chain that works through 
the horizontal tubular -lU'iu ; whilst it <-aii brt mtuW to inGlin<- in om- 
ftidt* or to tliv other, until its i>Iunc iHi'DnicM vMlical, by turning the 
whole moveintrut on the horizaniul axis of it« cylindrical socket.' 
The Hii]>)iorttng pinto liL-iuf; ivrforati-d by a liirgti aperture, the 
ubject luay be ilium iiialrd by tJie Lit-bcrkuhn if fti*jjrcd. The dism 
lire iuaerled iutj> the holder, or are n*mov»Ml front it, by a pair of 
forceps coiistructc-d for ihi- purinise ; nnd they may bi« safHy put 

anay, by inserting Iheir 
status into a ptati- {icr- 
furatt'du'itbluilea. Sevcml 
KUch plat^ with inlvr- 
veiiiiig guards to pruvont 
lUeni from couiiii); mio too 
diHi^ npinmition, may be 
pAckeil into a Kniatl l-nx. 
To the valu<> of ihii llttlti 
l^eM of apparatus t1i<- Author oui bear the Btrouftivil tcatiniouy from 
bia own experience, having found bin study uf the /ui-n/iKni/Vm 
l^reatly t'ai-iliiatrnl by it. ■ 

Glui Btage-plaie. — Every uicroat-ape sliould be furuidiwl witlia^ 
piece of platt- c\nss, alHitit 3} lu. by 2 tti.. to one moi^gin of which * 
narrow strip of glasi is cenieiitvd, ho lui to form a ledge. Thix is. 
oxtrenidy useful, Ixitit for layiug objects upon (the ledge preventing 
them — togotlit-r with their covers, if used— from slidiiij; down when 
the mii-roscQiw ii innliiied), and for preserving the stage from injury 
by the spilling of jityi-« al«r or other saline or corroiihe liquids. wln^n 
SDch arr in u>«. ^uch a plate not oidy serves for the examination 
of tronspnront, IjuI also of o|>a<|ne objeclA ; for if the condenung 
loDS be w> adjiutcd as (*■ throw a side li}>bl upon an ubject liiid upou 
ity either tho <liAi>lira^u |>lnt«i or u sli]> of black pixper will aiToiit > 
diirk background ; whihtt objects mounted on the sniidl black discs 
suitable to the IJelwrkulm may conveniently mt on h, insteiul of 
being hold in the stajje forceps. 

Qnvring Slidei and Stagei.— A numlTrnf contrivances have been 
dcvisnl of hiiL- V'/iii^ for the purpose of watching the life histories of 
minuti' Aquatic organisuis, and of ' cultivating ' such a* develop and 

' A loiall pair of (omp* lulaptril Ia tikp ui> miiiata idijiHiH msf be fitted ial» 
the cylin^riwl lioldcr in |ilae« ol « diic- 

PtD. MC— Bxik'n iliK-holiler. 

GltOVriKG SI,!I>E3 


Bullipljr thenia«lv<!H in i>nrti«u!ar HuicK Oii« of tlif tiiinplfst and 
Must rffwtive, thnt nt Mr. BotlrriJI, rpptVMnt«d in fig. 347, conKistA 
0* ■ slip of ebonitr, thn^ inc-hts bv one, with n w:itji»l ftpvrturp of 
thrnp-fourtliH of an uicli nt iU niicif r sidi- ; thin nprrture is rwluci"! 
W A projectinK sliouldrr, whcn-on i» oviiipdUxI a diiw of thin gUuKi, 
whw-h thus fonuH the IxjiUoia of « icll hollowi>d in tlip thickniw* of 
(III- rlioDiti- 8hd«. On *«ch wdn of thi* cpntrul coll 11 mukII lat<-r»l (sdl 
in«ninuni«ntii)g with it, and aliout n fourth of nn inch iii di(imi*t<T, in 
•Irillrd out to the Biuiii> de]>th : tliix .HiTM-sfoi* thrrvcrptiouof h supply 


of WAtci' (ir Oliver fltiitl, « liidi is iiupurtcil, an rec|uirail, to the central 
'srowiiiK' <"e"t *hich iatxiupM.-il by pUcing* ihiii yhiaitoverovfi- 
llw olijeola inirottuiW int/j it. with liie int«rpO)>itiAiiof h rin^ of thin 
|«per, or {it a «rw»t«r lliirkness lie rvi|uirMl) of n riug of ctirdl>ourd 
iiT vulcaiiil*^. If llic fluid hi- iiiti'oduci-d uito out- of the hit^-nd ivlls, 
mill be drawn oft' from th« othera - oiltirr bj- the u.*-, fnnn lium to 
timi', of til* Kiunll kIbm bjrriiiKe to he btTcviftpr de»i.-ril»pil, or by 
iliniuU aonrniii^l hs U> proluce a coiitiiiaoua drip tn'" »ue and 
/rum the oilier— • <^ii&lantly renewed nu])])!/ i" furiiiihtHl to tlie 
(iTtiiral <«11, wlu<;h it 
"Blcri on one nid*-, 
aiid le»v«s oil the 
other, bj capillur)- at- 

Dr. ZetfW nwl 
Br. Stmitliix'syroteiivi 
ditle ia ahowi) in liy;. 
i\^ Two seiiiicirctcs 
of aaplwlto varnish 
»rv brtislied on the 
ktide, one beinf! rather hirgsr than tlie other, so that lli^ ends of one 
haU-drcle miglit overlap the oth«r, bul not so clonely as not to penuiC 
ifae entrance and exit of air. When nenrly dr)' n m)nut« qaantity of 
l^rowiii}; fluid was plncecl in thi* eentrr, upon wliich a few spom 
were aown, « cover-glaM W'ing placed over it. which ndhi-ivd to the 
snui-dri«d varnish. The slide was placed undttr a boll-gloSA, kopt 
•Ump by being Itneti with moist blotting- paper. 

hr.Jti'btox'f ;;nminif Ji'ir will lip iiiidrrhlfKHl from the ann<-x«d 
^etch, flff. 2411. The sbadrd parts nn- pieces of linfoil fuxti-ncd wiUi 

Fia. uiB. 




kIivIIhc gtui- tn n gloss filJdc. TIm; Diinutn fungi or itporex tn Iw 
p;ruwii ni-p plnocd on * glow covit Inrgii fii:)agli to covit tbc Uiifoil, 
with ft droplet of tho fluUl n^uinsl. Thin, u(t«r examination to nns 
that no pxtninrout mnttir i» intro<lui'Ttil, i.i filaued over thi? tiiifuil, 
iind the pdgp* fcwt^in^ witJi wiis .lofleiiitl with oil. leB^iu;; irrv Uie 
spKces, X X, for rntruoc'! of "ir. (irowinft aJiilea of tliifttlt4cri|»tio)i 
could be niiiilv vliraply willi thin jcIilx-i itintmd of litifoil. 

Dtt/fimjerantl Ifrfftdah'it Moitl 
Stage/or CutUtHuotui Obmri-vrttioiit, 
It h needful in workitig out the 
life liiatories of miiiuU> fonas to 
be abt« to ke«[i tlie orKatiisms i>i n 
noruiaJ nod undi§turbed condition 
for ootiK^tiiiies wtvks »t n time; 
oiilynHumll drop of fluiil v-oiitAiiiiiig 
the or^iiisui cad he uikW ohserrii- 
tioiv, and this, without projwr pro- 
vision, ia coTibtantljr evnporatang. 
To orcrmit tltis, iind stilt to employ rery high pitwors tn proloDgcd 
study of 11 ifivrii orgnnism, is thouhject erf tliis device. It coimists i-f 
II pluiii ;:Iiiui titJiK'*! lig- ~^^i "• "■ s** titled as to slide on in th<! plarv o( 
tlir- iirvliitary sliding Rtngeof a l'o»eU and [icaland nr Itins Ktaiid. It 
JH lhu< tuiH-qitilileiif the tnei-hAiiii'Al motion^ <'(>iiininn ti> thoM-!>tji^'t>. 
Its fuuridiition, Kg.'J~iO, n, r. ik plato glasx, nimut thetcnth of nil iticli 
thii'li, in onlcr lo giv<> it (innni'i^s. Hnt thi» in ton tJiiok to work 
through with n coiideiimr and high )mwcr«, and theroforu a circular 



X X 

Flu, 14ti.— MwlUoi'i growing lUfie. 

Fill. SSO. — DuUln^r uid Dryidale'a iimliit cniitiiiunus invwiiig *Ugt. 

opetinre, ft. isnut through il. nnd a thin piece of ^tiodghiw, r*. >t,r,/, 
is fixed over the under surfnee of it with Cnnada L-ilsnm : this mar 
lie as thin ns thr cnnilenser may require. At the end of the nrnx 
o, wliidi eKtttnds S()me distance Iteyond the »ta;,'e to the right of Ihf 
I'nader, hut when the nrrnngement is set up on tlie niicroscopo to tlio 
left of the uperator, a brass Koeket with a ring attached is tljcril with 
mftrine glue It is marked in the drnwing y. p, ff. The object o( 
this ring it to hold n gloss vessel, lig. 251, about 1} or 2 inchM 



<Ieep. It simply drops in, and tlie top, m, htann sUglitly Urger than 
tlie riiijb !f, %. 2^ it is prevented from slipping tliroitgh. 

Let tu KQppoae tJt« aUtge to be in itM position on tJi« nitc-rosoopi-, 
uid Uie tosmI, tig. 'Oil, insortotl in this manner inU> ff, fig. 2-'>0. .K. 
piece of good, n«w lin«n is now cut to the slwpe (imwn in lig. 2-'i1, 
the pHrt a Imng long onoogli U> mtch to the end of the gla&tBiii"n, 
uid tJton At b bonl uvrr, hnving the pnrt in tli« 
"eaMl, fig. --''1. vhii-h i* iuMTtRd into ^, lig. 250. 
Iti position i» indiciitn! in lig. 2'iO, by tlie dotted 
linco, A, A, /i, Jcc. Itut bcfom it i* laid npon ttie 
sUigp a circuUr iip<trtun:, J, fig. '2M is rut out, witicli 
mojit bv niui'h larger in diiimrtrr than the covering 
gbus whjeli it is inti?iidi-d lo u»'. Wo lliorofore 
rmploy kiiwII cov^ri. 

T1i«gliui willi thr Itapof liii^n in it isnoir filled 
vitit wnt4!r, ■11(1 the liiiiTii JK wi^tt(») and wrung sn ns 
not t*i drip, nnil tlie wholi- i* very ation, by capillnry 
octioR, L-oitiUnntly iind evenly wet. A drop of th<? _ 

lluid to tn cxaininnt must now be placed itt A, tig. 
I'.'iO, u'.il th« coveriuf; gluss, ■', mu&t be laid on. It will in- uvn ttiiit 
thrrc ii a brotut, cl«iir s]nica between the covering gliua iind thr linen. 
We now mant to form a chamber into wbidi the »)>ji-ct-gluvt owi 
1>tt inaei-t4^d, and wbicli shall enclose a portion of thu cuniULiitly wet 
linen, and be to a very large extent air-tight. 
The consequence will be that tlie evaporation 
«ilhin tlw ch»niber will lie always great«r in f,^^^^ 
•^antity from tli<> linen, uu account of its con- 
l4nual n^newal. tlian it c«n lie from the lilni of 

rndtwil. the moisture in the cbamber is so 
;^reiit utidt^r faiournble oircunutances Uiat it Fin. MS. 

ntJuT increasen tluin allows a diiuinutiou of 
the filu ot fluid. TIil' manner in which we effect thiK is simj>l(t. A 
piece of gloss tubing, nliout 1^ inch in diameter, i" cut to about ]{ of 
an iiidi in leni(tb. At one end of this a pieoe of thin short coou- 
tcbonc is linnly stretched, and a small hole is mode in its centnt. 
I'lg. i52 givet a drawing of it ; « is the piece of gl/iw tubing, t, Ik 
ih» ctretobcd elastic film, wliich is feecurely tied on by miuinK of n 
gtvovfl ill tJie glass at d, and c is the aperture, '^l>^ bottom «1g«, r, 
should be CArefolly ground, Tliis is laid in the position in uhicli it 
is looked at in t)i« drawing, on the linen of the KtAge, the aperture n 
Iwing over the centre of the covering gloss. The objrct-glaw is now 
racked down l/^r<^ug^ the ttnatl hiiU, e, 6g. 252, and odjusttnl to focus. 
Tbv eaoutcbouc should bo thin enough to afTord no iiiimdimiTnt to 
Ute action of the tine ai^ustment, when it will be si«n tliat it clospH 
the object-glass by its elasticity at the aperture; and the ^eiitln 
prteaure forces the under edge of the chamber u['On the linen, so 
that littto or no air is admitted, while if the under edge of the 
chamber l>c cjircfuily ground it will suffer the sto^, linon and all, 
to move nndor it when the milled heads for working tlie mcctuuiicul 
«tage un in actioa. 




A drnviug of tlie aptmnituii in working oi-dcr ingivdn in perpen- 
dicular s«otMW at fig. *^'>:t. T1i« parU ». n in tbin tignrn re|wo«iMit 
tli0 fflau stAge corren[)OndiHg lo n, n, tijt. 'J>tO ; A En lioth lignm 
iftiuins for thp round ni-ertui* in tlie thick k!iu« : />. in fij;. 2!iS, car- 
responds to the ttiin glasi w liicli covei-a thi» nf«nur*, marked i", '/. 

f. J ill tis. 250, but in the- 
forui of Ilti> device now ui>cl 
hv the Editor tli« tliin glnM 
floor is c«lnent<^d to tlip 
Uitti'iK of llie pllltp xUias, ft, «, 
thu.4 uinkintt a cell njuat to the 
thickneaa >>f llie wholn atajie. 
Tlie iinen i» nmrketl in ii<rtt«l 
lines in botli tiKureit: li, \\^. 
2S3, repre»enta llie covering 
gLiss, |-, ill flg. 250;^e. fig. 
253, is the piece of (;!»»* 
tulnng, §hown in lig. 252;//, 
tig. 2S3, is tJte strecctied caou- 
biliouc »een tit '< in lijf. 252, with llie ohject-gJ«« y. penetmtiii); 
and ti^litly filling; up the aperture r in tlie flgvre, thus furmin;; tlit* 
moist chaniber, i'/>. rli, bv enclo'iing parts /i, h, tig. S.l.t, of the liiu-n, 
which from the gloAs vomcI to the left nf ihi- ^tage is by cftpillarilj' 
^dwa^F's renewing it« iiinisture : nnd with '>. lig. 'I'i'A, sunk as a cfll, 
l>y tJie attocbioent of the thin gla*s floor to the under side of the 
Kta;^, as dmeribed .iliove, this annulnr flap of linen o^erhaiijcs, Imt 
does not He upon, the ttoor on whicli the di-op of tluid with iU living 
inlukbitatits is plat-e<l. This ii u great secunty against accJdentnl 

It will Ite seen that the instrument must be horixontal ; but tlien- 
IS iio inconvenience arising from this if it 1)e placed on n. sufllciently 
low support, and it will be fnund in practice thnt it mny lie worked 

Fl". KM. 

Flu. Wl. 

for A long time uiihout any otlier diange in tlie arrangement tlinii 
the screwing up or down of the fine M^justiuent. The difficulties in 
working arc few, and can Iw Ijcst discoiered and overcome in 

Dr. DnUinyrr'n Tlfmi'i-uf'ilir tftesffr' for Coutinuou» ObMrvatitmi 
'It Ififf/i Temi-eratu/vt. ft fmjufntl.v hiipj'Oii^ that either for the [lur- 
pow vi experiment, or tlie study of ^cial organisms, the student 

WAItll rciSTlSIors MOIST ST.^OK 


uvmU a uiiiilur coutiiiaoiu ^taite to llie nliovi.', but one in wliivh 
ivrying temjienitureti umy lie obtninml aiid l:<-i>i. at nity ]K>int (Utif 
4t the » ill of (he o|««tor. TIiib in very iaii-fucUtnly iifrtnii[>li>i!i''.| 
by Ui# followjn;; deviM : Th« staK"? *"» iiiu<k iiK d(-s-cril«Hl iiUivi-, 
1igt it wfta Hioile lM>Uaw aiiU uatec-tiglit. 'Die »liol<! iit4t^'(t i'« si^mi 
in penpecUve in li;;. 2bT\ Ac A, » A nre two i^rooved pieces uf »cili<) 
rnvtal whidi penuit tlie atage to slide ou tti the stage of au oitlli'ary 
■licrascope, and jMrtake of tlio lueclianioal iiioveineuts effected l>y 
tJte iiiilleti beads; B is a veiseil for w^t^r wilti a theniiomcter n 
of ftiiltifieiit delicacy for iiidieatiii); die temperature ; t ia » luer- 
■rial regtilator, eai-ejully mode, but of the UHual p&tterii ; c biiiigs 

Pro. %-n. 

tho pu {torn the wain ; >/ couveys as niucli of tlic ^as ns is allowed 
In <«tM|>e from hetHeeii tbe top iif llie nierc^iiry, mid tlio liotloni of 
cb» goM <lelivery tulM< to the burner t. Tlie reiculation <tt this 
apf*riilu.i HO as to obtJiiii n !>tatic tempt-mturr-, ni is n-ell known, is 
ft nuttter of detail de]ieiiiiin); chiefly on the can-fid om-- of the iiier- 
curi*] w-'fewploK /i and the height and iiitenwty of the bonier •-, 
A temprrature miili* as accuntte as is necU-il <wn be obtained f"r 

jntrpone reiiuired. 

"nie BtaKV A i* placed in position oil the iiutr anient, and two 
-openings in Una hollow stage nt •rrt (A) nm connected with two 
Hiuilar o[ieiiinKii ii> the water ^'ensd, vii, >//> (B). The whole tit 
carefully filleil with vr*ter and raised to the re<|uired lemppratun! 
and rofOilateil. 

The manner in which itacconiplihliestlH-Mid draired isastollowi. 
On the centre ot tho stage (A) will be seen n wnall cylinder of gla«*i 





tlitK U grauni] at the end placed oit the HtA^e, (tnd covered witb a 
Hort of drumhead of india-rubber at the upper eiid. By fxaminiii^ 
V wiUi a leiii it will be seeu that a cell is cnanter-suiik into tlift 
upper pinta of the hollow ata^ at e", and a thin plate of gtuNi is 
ceuiented on to tliis. At -t another disc of glass is ceinentod water- 
tight, ^o that u tilm of wanu water circulates between the upiier anil 
under surfai** of this ^lass aperture. A f;lass cup is pliioed in tlw^ 
jacketed receptacle / (A and C), and this aho h lilled uith Yrati«r, 
A jiiecc of linen is uow laid on the stage (A, •/) with an aperture i-iit in 
itti centre slightly less than the cotinter-sank cell in which thfi glaiw 
diac e" ia llxed, and a 6ap from it isallnwctl to fall over into tlieglaa* 
vwoci/(A and C), Thus by capillarity the water is carried coniitantlr 
over the entire face of the lineD. Hut the glni» cylinder seen in A w 
made of a much larger apcrtaro than thn cell and tlie opening in the 
linen, and c<Mise4]Uently a largo nnnuluK of the linen in enclo«ed witJiin 
tlie cylinder. The drop of flui<l to Iw oxnniinrd it placed on the aroall 
drciibr glass plate and covcrwl with thn thiiintict glaAt, the drum- 
hwd cylinder is placed in poxition, thn mint of a high-power len« 
is gently forced upon the top of lh« india-rubber through a small 
aperturo, thus forcing the lower gmund aurfacc of tho cylinder u(ioii 
ttie tinen, and iiinkintfthe Kpace within the closed cylinder practi- 
<ully nir-tight, but still admitting of cnpillary action in the lin«n. 
Thus the cnclownl air hecomc* mitu rated. 

Itv complete circulation the wati-r in the vtusel ti (A) is but 
klightly below that, uitliin the jacket of the Hta^-e, and thus the 
mpour as veil as the stage are near thexnmc therninl point. 

For the ndinissioii of illumination and for allowing the us« of 
vnrioUH ilJumiMntingnppnmlus, a !arj(i' bt-vellod aperture f (C) ia 
made between the lower and upp"-i- pintii* of tin- Ktagf JKckcrt, wtiicli 
IN found to KUpply all the ncconimodatiou needed. 

There are many other fonnii of hot atuKe having \arions special 

purposeis and xomeof genoraj 
application ; a good account 
ol these will lie found in the 
'Journal Koy. Micro. Soc," 
vol. \ii. Her. ii. pp. 299-516. 
The Liv6-box and Coic- 
pressors.- -What i» now so 
well known even to the tircr 
ns the ' live-box ' wiu origin- 
ally devised by Tully, atnl it 
was afterunrds improvt^ l>V 
Varley, who iti ilie place <if 
a level disc of glasa for (hir 
floor, as well iis the top of 
the 'box,' bevelled a pieoe- 
of thick glaM4 mid burnishid 
it into the top of the tubr,. 
where it foniied the llnorof' 
this 'animalcule ease': thi« preventnl the draining off of the 
water at lh« eiljjo by capillary uttraclion. Itut in that form « 

Fin. 3S9. 




FAannot be umnI ■uccnafully with it, nnti thrrrfom n dark 
gimml cannot be f mjiWitL But lui it tit RoUftinr iind Infusorw 
|ri>i'mJly tlMtooiLitttuti- llio rttixon iFftre for thin pinrt'of iippiinitiix, 
ftiul OK li dark cmund g)v«a resulu of high r*lu4! to tay tiothing of 
dwJr Imtuty' — with (hrse fomiK, it lost uuch of its voluv. 

Th« whol4> pieca of ftftuuntuii is sevti nl A, li^. 2M>. A ifwtioii of 
it is M^t■ll nt B. A moaificntioa was nubspqueott}* iniiite by Kwift, 
•bicb JM xeen »t C, -vhvrv l)i« glass floor-plate is set>ii to be buniisluil 
into the limiw |tliite, anil thtt oov«r-){lian is made to nliile tlov> 11 upoti 
it im n tulw. Bui in lliiit still Rrtrnter difficulty arose, wlitn llw 
(oi-ui of the Iniii wiis n>Kirt«r thiui the length of ttie tube ', tJien, 
indnKl, thl^ motion of the 'box' wua limited to llie differeuce iii 
diuiiiPti-r Ixttweeii the cell in which the gliuwes woi-e hunk nnd the 
olijfctgliuw -in fnrt otOy low powei^ could be employed with it. 

Mr. ItouiuH>let lias overcotiiti thid difficulty l>y a device which in 
Aliown ill lig. 2S7. 

In this tJni ghiM plnte bewlted for the floor is §oiiiewhnt redticc«l 
ill iliainfler, but the outvr ring in enlnr^^l suflicieiitly to nilow uny 
liij;b power to focus to tlte very edge of this glius floor. An object 
lytuj; iinywhrn! ovi^r tiie floor cau be reitched by the condonaer from 

Via. £17. 

Iieliiw, itiid by iMtti high Mid low powprs from nbove, uikI when wi'U 
iimilc it acts well ns a comprcscor. A drop of watoi- so Atuiill lliiit 
n rolifor may br minblc to swim wit of tW tiehi nf view of » \- 
iiiL-h objoclivp mn br readily DTningi'd with it : and 11 litUe pniotice 
'Msbti-'- th<: ofM-nitor to rinploy it for many useful purpo§efl in the 
stDily of ■(■one! lif.\' 

Iko oompressor or compresioriam is a mon- i!|iilioriiti< device, 

I Miinswiiiit 'ifttrr tbi' .'i^imi' kind, but arrangv<t to give tlie operator 
wore Ofuiirnlr! (^tntrol over the iiniount of pi-i-.uur<- Co which tli« 

' •ibjert tM Mibjvctinl. Mr. Hou.uM-li't ban coii^tnictcit imi- of very 

I viiitHent ft>nn. The bnvelleit j^lnxs in llii.s r.lio ik krpt kinall. willi 
mpect to tJie Btjre of llw cover gliiJai, and it act* with perfectly 
pnmlM prt*iure U-'twit-n the two ;^lniM'H, which in drlitnle work is 
of oonRtderatile iin|>ortJiiice. 

The cover- Klast is held on an arm which !icre'n-!idowiiona%'erticiil 

. pout agaiiut a Kprin^ : an the sci-ew is nuited the spring mi.-u'K t.\w 
■.■ortn*- gloss, and by an iii;^nioun spnn;; catch It in ki-pt central with 
Om gUsa-plate Hour. This win nevi^rlhelrM In- rcliMMsl, and tli" 
Cntiiv cover cam be turned aside lo j'ut ■m a frwh object, clean and 
M foKh. It is simple, light, un<l, bt-tng jiurallel, can br ii«^d with the 
higheat jiowers. 

■ An exlreiuely handv frrrrtiblr foinjimmf iHseen in (ig. Srifi : it is 




tile l-iiigs liiive Iwo xliiu covers iiiiient«(l to tlicm ; ihc low«r tAntr 
U •ttiw'htHl to a rwl turning in n pocki*t, whilai the Ui>p«T ouv IHW" 
on n. Riiltt.-<) linid whii'ti irliini)ti( if rMiutriNj ur rdc«a«s it wlit 
rm]iiiriKl for iiiore fimy i-lcuiniii;!;. Vun'itie pr<'K«unMt nt the covn 
gliuar-K nnt >il>biined by tuniiiij; tlip milteil ii«*d in the wiilrr of tl 
pbitr, nnd tlio uicket litti in thn sta;^i in the. luiiiin |Nitltion «a tl: 
*Ft»ft<- forcfT» Ac. 

It in ■onietiuieB rw<iuii*<l, in \-wry delimti- wiM-k on very niiiiii 


Pig. ii»,-~SlT, Rcwlaiid'ri rovmlblc gompT MW . 

OT^^tisiufl. to I'oiiipi'psB tliem vorv -ilowly nnil with grml cm« wliile 
vre are examining them with tii^h jhiuits. Tliv i\ivi?T-gl«s« is at 
necessity thin, ami the prmsure from tlte minat« dimenaions of the 
obJMit niURC br vi^ry I'miudemble in oixWr to liot uprni the orptiibui hi 
aII. Fin- thix pnrpOAE> wi- have foiiitil m noDMiwhat heavy campre8»>r 
tiiodc by Powell nnd Lralimd of ^tvat rnlnc. A strong Imss iilalf 
forms its \utne, mid u \v\vv arm oirrying the cover-gUw >■[ tfap 
ooinpreewir is rnis<.-d and tnwered with a lino screw. The oover-KluK 
may be ceiiieiitMl on lo two or three sepnnit« fnint8 which m-ivw 
oil. and may carry coviT'giaK-i of ditTei-ont thicknesses. The cnver, 
to prevent fle\ui-e, is iitnall, hut will work easily with any of ih" 
modem high-|Ki«er lenses. The whole liody of the ring to which 
the cover-glans in ntliwheal in very aidid and strong. A glosi slip of 

Tin, -jnn,— I'ratruiv UnUce'B pardkl comprenor. 

th« ordinary aisa> sli|« in, and is clamped by a, upring on the huM*, 
and upon this the cover-gla^s is bixtiighi down in wimpn-Ksiiin. 
An object the ftOOOth of an inch in ihit'knras ran \m- i-ciuhly 
held and cinprewwl by it* mpan», but tli" fii>nt» niii>t Iw 
tunieil with (TTCHl cnre, and the i:iivi'r->.dii>*r» uwil fur <'(>menting nn 
must be much largrr thnn the aperltire in the ring, so tJutt a Urge 
lurface may lie ■.-eninnted. 



RtwcntlyPinifeMiir Y. DeIn){flhiu((l«4vi>M-<l a very (ulniirsUe (onu t<t 
^iiuj>rKa(>or for ttie most ddimti? oh«crviitiiinH (lip*. 'J^9, 260) in whioli 
the [tT«miT« in efliwtvl Uy the octinn tit a screw on an inclitie<(l plmiv 
K, anal wiirkjii;- »;,-aiiiiii the <it>n!i([ R. Wh^i tliir screw b, tunie>l 
on one aid*?, i)k- ii[i|i«r piirt ol i)ic citiii]ini(Kor <-fin he rnised uii Ui« 

F)a. MO. — PrafnBCiF I>«l*g«'^ iiantlal <ioinfirp**nr. 

ptvi^tx B B', AS shown iti S|j. 2110. Tlie fmiii? liotditi)* the upnor 
jilnU^ hiis n ptiilMl UKitioti oil l\\e pivot D (mid Che cori-egpntuhii); 
«na on the <>ppoMl« HiHe), and tlie Crnme cnn lie rletitcli«<l by pnvn- 
ing th« pin C and the correttpoiulinf; one on tlie «ppiBit4> mU; 
niuxiTi^ tlM- frame -holder u< ^>nnn <>])eii slightly. The two j-U-jiw 
htnng ithlouK mid Iviii;; oruMwl, it In eusy to nild ft driip %i{ liiguid 
daring ('i>inpi*anii.iii. The tniiiLii'i-BSor win be revenieil, and in tliiifc 
riLHi' nuts nil the three -.iiiaU pillii-a, wliicU iir« high eiiuuKh to allow 
llw tiiillisl lii'M'I 'if 111'' -.i.-[vw Ui olttir the Htaj:^. 

The zoophyte trough in & l»rf:^r livebox ditVereutly coiistniuted. 
Till" f<inii tliat huft proved otic of the bet^t up to our own day nan 
intrrxlucr<d by Mr. Li«ter in It'H, and iii well known. It in drpiuted 
in fig' -^1- l>eii<^ formed of nlipt 
of ghucc, mill) ha«n Iodep honioii- 
U) p\M9 of glnss pi|ual to thn 
inside length of the trough, »• 
thai it may Iw movetl frciTly 
wilhin it. alMi a Klip of (jlua 
that will lie on tlic Imttoin and 
SI! it. wi'h llin cxwptioM of tlif 
thi«kiie«i of tliis Ioom: jilnti'. 
To uae it. the olip Ls put upon 
(he bottoui, thr looii^ pUte is 
placed in front of it iiitli ita 
bottom e<Igf InudiinK >lii? iiiKidL* 
of the front gltux, a niiinU iiury 
wedgv is inM'rti-il betw«?en the 
front ^aM of thf' tn>ii<{h and 
tical plate, which it •ervea lo press backwaitls ; Imt this prwiuro 

Fl". 901. 

tlie up)ier part of the loQse ver- 



IB k«pt in ch«d[ by a small ntrip of bont wKnlcbonr,' which U plooei] 
between tlta vertical pktu And thn l*4tck ginn of thn trough. By 
Doving th» ivory wedge up and down the ftmounl of space left 
Wtwwn the upper pnrt of tfie vcrtiail plntn wid th<! front gimw of 
tlir trough will Iw precisely reguinti-d, noil as Ihfir lowrr margins 
lint nlwnys in clot*^ nppoKition, it is ittittinit tlic one will incline to the 
oUiur wiUi a, constiint iliminutiuii of the tliirtAnt^n lictwMtn thrm from 
aIkivc! d<iwiivr»rrls: An oliji-i-t (Imppcd inU> this Himco will dmcend 
until it ru«ts IxHwi-nit tlitt twn surfncrs of glusc, nTi<l it cun be placed 
iti u pT'iiitioii of grmt ootivvnicDcn for obiwrvution. 

By vi'iy little contrivnnca tlmi' troughs with their coiittints niay 
)>e Ui^pt, when not unitt-r examiimtioii, in luach Inrfcer ai|uariit, 
ubtiiiniii;; tJie Mlvniitn^i* "i iii-mliDii niul cuobi«A&. 

Mr. Botterill di-visi-d n trouali which i» made of two plutcs of 
vulcuiiile or metjil uliiob ncn-v i(i;,i'lher, and '<elwe«n Uiem urn twi> 
plates of sIao, of the pr<i]<er sw.-. of any d(^ir«<l thicJcu««», kfpl 
iipnrt by half a ring of vulcanised iiidiurublior, the whole ttcing 
screwed tiglitly enough together by three milled li««d» to pn-vrnt 
Ieiika-;e. Bat leakajee or Uie fracture of gla88e8 U not uocomnioa^ 
with tliig otherwise convenient fiirni. ^^^ 

j\ji excuUenC, though iiluilloxf, trough was tniule by Mr. C. 0?W 
Dunning, wliich we iUutitrate in fig. 26'2. Thp lowor pUte or troiigli 
firoper is mode of metal, 3 inches long by I k widp and about ,^ 
thick, with an ovnl or obloii'; {lerforation in the centi'o, and the 
iindf^r side is recesseil. as bliown in tig. 262, B. In tliis receu is lixod 
by Dieans of CoiuuU bal&aiii oi* shellac n piece of stout careriii^ gbiKC, 

forming the bottinn of 
A b gK - ■ ^JS I. " ■•~'1W» . -- . -f Sl the cidl. tliu revest bi-ing 

Mitticinnily deep to pre- 
vent the thnt glass 
bcitloin from timing into 
actual nmtaot witii the 
atnge uE the luicroaooiw 
or with the tiible whwi 
it IB not in ude. Two 
pieces are provided near 
the bottom edge of the 
t-ell: thecoT6r(lig.262,C> 
ia foruied of a piece of 
tliiii bniss, rather shorter 
than the trough, but 
al:out the same width ; 
it ha^ an opening formed 
in it to correspond with 
tliat in the trough, and 
under tliis opening i» 
cemented a piece of covrr-glass, Tlie cover [ilalo is nntcb^d at lh» 
two Ifotlom corners, and at the two toi> corniTs are formed a 
couple of projecting ears. In nnler to use this apparatus it must be 
laid Hst upon the table, and tilted <|uile full of water. The object 
■ Waicli'*i>rins ur ntlicr I'lntlic mcUl >Uoatd iiul be ami on umiuit ol osMUtion. 

Pro. MS. 


KXAsnsiNT, urFcaomA 


to be examined U tlien placMl in tite oel), aiul in»y be properly 
MTuigM) Lht-raiit ; the cover is then lowered fr^ntl.T tloviii, tJifl two 
nat«br!iBtth<-t>uttoniedge§bein!,'lir8lpUce(lHf,iiiniitth«piiu ; in lhi« 
wftv the Kn^ieHliioitswnter will lie driven out and ihf wliAli-nppnrntuft 
ni«y t"« wipe*l tlry. TIte iri*pillftry nttractioii, aaiiUtnl t.j- tin- w.ight 
of xhe cover, will be found sotlicient to prevent itny I'-nknK'- ; nnd 
tbi' pins at the Ixiltoiii prevpni the cover from ididin^' down wJtna 
the micronvope is inclined. Tliis Jtoopliyte tr*>ui;h gniiu-jcint two im- 

tortnnt >|UBlit)e8 : first, it dop« not leak ; second, it in not rt«dily 
■roken without groM carelMKHPSS. The shnllow nenn may Ix- ovcr- 
4Tiine by pliicing nn «bonit« plal<^ with the requii-ed ii|iertutv betwirvii 
the two mounted gIftMCS. 

InfuBoriu, minute Atgw, Ac; however, can be well seen by 

piftcing a drop of the w»ter rontntiiing them 00 an ordinary slide, 

■Ml Iftviiis ^ Ihin piece of covering glMM on the top ; and obj«ct« 

(iftoniewhat grefttor lhicknr«« cnn lie oxninincd byplacin>; a loop 

or ring of tine cotton 'tli rend upon an onlinaiy slicle^ to keep the 

cOT'ering glaMi nt n emnll diHtanci'' fnini it ; nud tho object to bo ex- 

Ainined being plnccd on thfl ulide with n <lrop »t w«tcr, the covering 

i;lua in gMit^ pn^srd down till it touch™ tho ring. Still thicker 

objftM may bo riewod in th« various forinn of ■ cells ' lierenfter to 

Ih- de«-ril)e>l, nnd itK, when the (■■•IIk an^ filled with flnid, their glik<« 

covem w-ill luthen- by cjipiltiiry nttnictiiin, pri»'i(le<l the t)U]>crfliioii4 

niiiUturrt thai nurrouncU their edfTM be nnnii\'<<)l by lil(ilting-]>n{)er, 

ibey will remain in pliirii whvn the ratcruscojii! is tnclini^il. Alt 

annular trll, that tnny lie used «'itlier n* n ' liv«'.|«ix ' >>r aa 11 ' grow- 

i«i; slide,' had lat«ly been dpnwd by Mr. Wdwr (U.S.A.). It is a 

■lip of p)iite->glafa, of the luiinl sitf. nnd nniiniiry thickneis, out of 

•liich a cirt-ular 'cell' of jineh dinmrttr is ground, in such a 

iiMnner that it's bottom is mnvrr instnid iif runcave, its shid lowest 

jwrt Iwing in thn centre nnd the dii'jM-^t mund the ninrgin. A 

MiuiU dmp of the lluid to be nxnmincd being pliiceil upon ihecentrnl 

eiHivexity (the hi;fheKt pnrt irf wliich slunild W- iilnuist with the 

fc-pnernl Mii-fiM^e nf the ]'liite), nod the ihin gliLu oner being plnivil 

upirn It, the dnip vpii-iuls ilM'lf out in u ihin tihn, without tinding 

it« wuy into the deep furrow nroinid it ; a,n*\ thtis it hi)l<Ls-»n tho 

eiiveriiig g^iwi. by csipdbiry ntlmi'liou, "bile the furri'iA' w-rves as nil 

air.chiunlier. If tin- ciiv<'r lie I'emented d"«'ti by a r\n}t "f golil wJt" 

or (laiiinuir, tfi that the eva[H>mtion of tJie fluiii in pn'* entt-d, (-iiher 

aninuil or tOKftlable life may tbii* l»e tnnintaiiied for some ibiys. or, 

if ilie two nhoiild be lialanciil (a» in an iU|U<ii'iinii), for tmnie weeks. 

Dipping Tab«B. - In evrry i>)ieration in which small (|uaniitteN 

of liquid, or small objectn coiitnitieil In liijnid. have to lie dmilt with 

I by liie niicnm-opiHt, Iw will tind it « \'eiy great convetiienee to 1m 

I pnrided witb a set of tubes nf the formiii ivprewnled in tig. 3A>1, 

1' init of Koniewhal larger duneniion^i. These were fomieHy de-vigimie(t 

I as 'fislting tulte*,' the pur[uiKe for which they were originally d«- 

I vned b»^-ing been the tiitliiiig out of wnterlleas aijii/itic inxnct- 

'I larvw, tho bugger aniina!i.-ul««, or other living objeetj! (li.vt.intriit«hnbl« 

either bv tho unaide<l eve or bv the assistance of n majrnifving glawi 



B V. 

applioabk, •>{ viiui-M-, Ui Ui« ^Wtioi) trf mtiiuM plants ; nnd tUrv 
Kinv be luriipil I" mnnir ullivr no L«)i>> useful puip(i»ee,caRK< of whicti 
will t* speciliwl li<-n5ift*r. When it 18 datiivd to wjcucp nii iibiii.Tl 
which cnii In' mh-h irithei' with the eye iklniw or with n nin^iiifyinK 
;tliiHs ""*' "* thcw lulitw i« poKsetl down into the liijuitl, iti iippiT 
■iritice hu^'iiir; Ixh-h pn-vjously cli»w«l by th* fon-fingpr, until it* Inwi-r 
orifiM la iimn«li'it«'ly abii\-e the object ; the tiiiger bcinjj; thii-ii n- 
nii'vetl, the liquid Huddcnly tint* inti) Uti^ tuU-, pmli- 
nbljr c&irying the object up with it ; iind if thiii in 
HCeii U> b« the cmw, by pultiiijk' th«> tin^cnr nfiniti oti 
the top ol tJie tiil)c itft contonlH nmuiin in it wIm-ii 
tltP tulte ill lifl«d out, mid may lie dr|H>Kit<Kl on n Klip 
(if KUtan, or on the low«i- diM- of the iu]uiilic Ihix, or, 
if toufopiou» for either r<tc|>lailc, limy Ih> diM!hi>r>iifl 
into a \»T)fe glaKN cell. In ihuH fi.-diiut; in Jitrn f<ir 
Any but uiiuutf objrttis it will lie 
):4-ii<ri:i>Uy foiinil cimvimirnt to t'niploy 
the ojirn-moutbi-<l tulip C ; tli<np with 
wiinlli'i- oritiiT*, A, It, \nwii imiploytsl 
fcir 'lishiii(j" tor iiiiiiiiiiliiiW ic. in 
hmnll lK)ttlr-S irt' tul»% ■•!■ for seWtinj; 
niinutf- objects fnim tlm oell intu which 
the wnti-r tiiki-:i niJ by tlip tuW C bim 
Ix'on ilisrlinrjTf'l. It will Iwfiiund vftvy 
con veil ieiit to hii\r the lo|»< of thc-sc 
lost blown into sniM funndv, u-liich 
shall bo covered with thin ^heei indiii- 
nibber, or topped with indinruliln-r 
nipples, which by conipntwion iin<l ex- 
pniution CAii then be regulAte<l with 
the srmteat nicety. 

In denlin^' with minute «<|iuiie 
objevtB, nnd in a greut variety of otiher 
nuinipulationis n riiuifl gfa** tt/ringf '4 
the |lfttt^^^l i'i"pi"e»ente»i in fi|!. 2fi-), mid 
of aImiui iloulile the dimenKi'jiiK, will Im 
found cxti-nmely convenient. When 
this in lii-inly held lietwcen the fore itnd 
,ui)d<11(r tini[<-r«, nnd the thumb is in- 
nri-tnl inlo the ring nt tile summit of 
the pirton-iiMl. Kuch cuniplete comniaml 
it Kiuiietl oviM* till- pint^iti tluit its motiim may l>o lygulntMl witli 
the ;{iwatR<l riici'ty ; and than minute <piaMtities of flui<l may 1m 
reinovetl or nddnl in the vnrious o[H>ratiiinK which have to W 
j«rfonuMl in the praimnition nnd moiintinK of <ibject« ; nr «iiy 
tuiniit« ubjet't tnuy Iv Ncln-twl (liy the idd <>f the simple micnwcoiie, 
if uecetmry) from iiiuon;^t li numlHT in the »mic> drop, nml trana- 
fem-<l tiiuHejuiritf sUji, A set of sncb syrina<% witli [Hiints drawn 
to different deyi-e^i of tinene»s ami Ix'nt lo diHi'it-nl cnrvaiuren, 
will lie found to I* anions the mnirt useful 'looU' that tin- worlt- 
inf; luicriinciipiBt ciui hiive at hu oamroiind. It will ol.-ui W found 





rtn. ana.— Ditb 

(tillK tulH'a. 

Fill. •JM^—Glan ' 



tluil if II (liptMii;; tube witli • ithja Imlli Imvn nn i nil in rubber lioUniir 
tutll or tvitl altu'lietl to the t(>|i <if it, il will Act, fur lliu nu^urily uf 

fMOepi. — AiK>theriDiitrumenteinIiuliK[)criinlil» t(ithe<micn»c4)iii»t 
u hi (m! c<>iiinM>ii1}- oonfiiid«recl an appeii(tag): Ui thr niici-uwupeisUiv 
t>irr<-iH for taking up miiiate objects; iimnv foniik iif tlmhavi^b««ti 
(li-viMii, ii( vrhich onp tif th* UKist vi>ii\ t^nifntiK rcprrsriiiwl in fin- -'J5. 
iif Mimi-tliin;; l«w than theiictuiilHiEV. A-ithc fuivi'iui, in Tiinrioc re- 
wnn-iw-s, Imh e cimtiminily tu be- pluiixMl into .ii'ii-wator, it in lietlei- 
lluii thry fJM>ul<l Ijo RindQ ol hmm <h- «>( f;i^nunn silver tlinii nf kIpcI. 
Mncf tiM-' Utter ru*t« far more retulily ; amt as lln'v ni-oi)<itint'-n<le(i 
(likf flUwwting (iirceiR) to wke iv firm ynisp i>f tlm iibjoct, but merely 
I" liiiid it. ihcy niny l>e ninde very lij.'li(, mill tJitir s]>riiig jxirtiuii 


Plu. •il». 

KleiidiT. Ah it i!> eMventinl, lu^feier. to iJitir iililily tlwl tlieir 
)i»iiiii. nliKtiUl meet accurntely, it if well thiit mn.- "f ilie blaile.s kHouIiI 
U- funiihluHl with u ^ide-piii pajuitn>; tlin'U};b « li"le i" iIm- other. 

MiiHt niicitiMMpiiils have at »(>aiv time ex]>t'rie)it'eil tlie (Innticr 
litiu i>. iniiniiiAiit t>' their instniiiiMits and niAuntiii;^ when exhibit- 
lUjC delii«te objects with high ]Hiwer in niixnl awiviiiblirjs nrJHiiig- 
ftiiiii tin- inadveileiiev <ir want «t kiiowleclije 'if winie visitor, vrhi> 
may dii ten-ible inlrfhief by iiim gently tisiiiK ilic i-inii-m- iidjustment. 
MesHrh. [{)*wi made an aiTangoneiii by which the -■..utw niljiihtiiieiil 
rimli) lie ' hK.'kt<l' at ajipven point ; t>ulanei|Uatly iirteful and vimplci- 
mriliiKl viax U't\}i Mfto devi»pd by MeiarK P>iui-|l and Livitand, whet 
a--t^\ >i (lee]> riii^. «» b showii in &K. 3G6 : this riti^c lias two [linit and 
Bsi-i*w ;>rt>jei-tinK itiwaixlx. When the sorew 
1* withdrawn llie riii'p> van If slipped over 
llie niillt«d heads oi the onarM- ailjualmeiil. 
mid by screwiiij; the tiiiiaU screw ' home ' tli'- 
riiiif lannot be withdrawn ; but as they an- 
[•nine Upon the milled hea«1s, tiw hitter can- 
n-rt lie t/riiughi iiif > action ; the rnij<B wnijily 
rfM.I\e upon tli« l)ead» witliout briiiginj; 
iheiii int» play. 

Other funnis of tho wine appliance have 
Ix-en niadv by thin finw ; and MewirK Iteck 
liaie made thetie rings with tOicht nMKlitica- 

ti.inK tnon- rwrntly. They are the mtwt F,„.3(i4i.-pm«ilM»l Imi- 
efficimit nHMnn <tt mrtinti-nittinK the danger IumI'- (irtitDCliiie riuu (or 
\acUhui "n public exhibition of delicate m-r*- ^,,..i.n^..i. 
"ibji^et* nndt-r hi|fh ptwem. 

The forrjpiinKeoiistitute, it U bnlievwl, al! the ni.Hl iniportunt 
piec-s of appiiratUN which can lie ooiisiilered in the lijiht of acctawirie* 
to the niirnmcojie. Thine which have Ijeen ci.iitrivMl to nfl'unl 

C' fi.r the pre|iaruti<m and mouiaing of oIijfolR will Iw de- 
in a future chapter (Chapter VI.). 


It U munifE«t tiittt pvrrytliing in th« fonn and (wuatnictioti as well 
as ill the niitun> of the o])ticnl and uiechanit^iJ a<.'otn!u>ric« of ilio 
■iiirnwcoiie, exists for, and to iiiakt' mon- elticit'iit, tJitt npi'cinl work 
of t)ie n/ifrfi'rv, or image- form ill); lens combiiialion, wkfch coiistitutL-a 
tlie basis of th« optio«] proporti(« of thi« instrument. 

The devrlopRioiit of tlie U)i>dern objective, as we !iavc nIrMuly 
iMeeii, has l>crn vriy gmdual ; hut thciv are definite efiochs of very 
nmrked nii<l iinponant iptprnvcmcnt. Our aim tu ibv Ktndv >iiF 
objeclives ts priu-ticitl, not antiqiinrinn, and we may avoid i-lnhoratci 
reuearchis on the subject, of ni}H-afliriin\(itie UnurA, and rr/tfi-tiiv/ 
»prriitii, which \m\r; bccn (iutficii-ntly indicated in the thini chaptrr 
of this vcjiiinii', Wft nmy also paj« ovrr the Mrlier Dttemtita at 
nchcoiuatiHm ; lltf Iriir hinfiiri/ ol t/f iimrlrrn olijftlir* beghu /rom 
the liuir (/"If I'* afkromnlitni linn Itrrujiimlfif tvorfcfti rtiil. 

Tlie lir»t movement of a dcfiiiiti" chaineter towardH thid ohjei't 
was made, it has lieen recently shown,' so early as 1S08 to l.s| | liy 
Dei-naiTlino llarvoli. wlio was Ciii-ator of the Physical Labomtory nf 
the Lyceum of Kmncia. fttr. Mnyall discoreretl a referent* to thiH 
eflbrt to miiko achminntic Ipnuc*, and through the courlt^v <if the 
President o( the Athenn-um of Drenna iliscover"! that Maiitoli vim 
nn amateur optician, and tlint lie had taki-n drep inI«rtHt in th>> 
nppticfttion of achmmati*ui to thr microndipf, nn<l that a mjwr <rf 
his on thn subject hail bi*« pulilislml in tht- ' Oommenlarj for f hp 
voor l^tO^*, ami tliat his hail nxliibitnl his achrorantic objeftiviii «t 
Milan in 1811 anil olitain(<d the award of a silver modal for their 
merits under the authority of the Istitutn Retain dflli* liitrienie of that 
city. Olio of tli'«<' olijpctivui was found to haii^ Ixwn ' re li^'iouiily 
pnawrved,' and wa« K«'nrroua]y prtaentwl in ItifiOto the Itoynl Micro- 
acopical Society "f Londun. With it wiu forwarded the * Procettao 
V«rbalc,' or otficial n*i.*ord of the awards, notifying Mar»>Ii'a exhibits 

Hnd tJie award of a Hilver methit, and the actual diptom/t, dateal 
August 'iO. liSl I, signed by the Italian Miuist^-r of thr Interior, 
jtlaraoli'i! <ibjoctiv« was a cemented combitiution, lianng the plane 

taAn of tlio tlint pn>(H'nt«d to the object ; and if this waa a part of 
tJic intended condrucliou, of which there appears nmnll room for 

doubt, Martoli pr«eed«d Chevalier in this, aa we sh&Il subsoquentJy 

«e«, ^-ery proctioal improvement. 

■ /num. Boy. Uk. See- 1800, p. 430, 



It luut bi.Tn, l»ovf*ri-r, emAtaaary to afcrwlil tin- lir»t |>ntrticnl>l« 
aUvtnpU to ncliromatUe o1>j«wt-j;la88eB to M. SRlligiim. In Itijli Uiia 
opticun iriui th« first Itnown in France to Buni>rinipn«c< two, three, 
or four BchroRuttuHid platio-iyHivex *<loubIetti, thiit in t** my, jkuth 
«f Imsrs. ThMM objectives had (heir convex Kurfium preaeuted to 
the object, which gave tJietii four timf« as niucli apbi>rionl nherraiioti 
u irould hnvn b«eti llie oise hud llieir yjiMitionn }hvu n-vfim^i,' aiid, 
M «R hMvc just seen, 3lAntoti rv^-vreed them. This :M.i.-C9ii)itiited an 
ntnHsivp rnluctiou of tlie Apertures, which, nevertliolewk. Mill tuo 
iraiiifmtly displayed ai> obtrusive abermtion. ^till tJio c«n<«pti<Mi 
of na nciinnimt'tweid combination liad been embodied in nn iuiliitl 
nMniit-n In 1825 M. CTievalier per<M>iv«l the exact niituro of tliti 
miilake imule by M. Kelligiieii, iin<l mn'le tlie lenw-K of It-iA focul 
Wngth and more nHinunntJc^ and inverted them, plnvinK the planu 
ddc iif the flint towardk the object. 

It is »>iiiewhiit important, n« it it inteivsriuf;. tu uolf tliAt llio 
>d(« of Uie »uperpi)siti<-n of 11 raniliiniition of lenscsdid not oriifinntw 
fnna theoretical r»ni^iilcni(i'»i« of th« optical prineiplm iiiviilvi<d. 
It in scAroely concdMiblr tbiit Hhere them was manifeet i^iutmncM 
<af tlte position of ■ pUn<>-<H>nvi-x h-nx for ItiMt MpherioU «l)erniti»ii 
(a principle now thoroutflily tnnltmtodil) thi^rr could have been in- 
tti;;)it en-rtigh eitlicr to <let<M-'t th« jin-iM'iicfi «rf the two nplanatic foci 
or to discn-er « mt'^thiHl of iMlatK in([ them by inductive ruuoning. 
EverythinK in tlii> hi«t<n-y pi'inta to Iiiippy n«eidrjtt aa the prima) 
step ill ftchniniatiseil nliji-clivM^ and this, with very high pmb- 
BlnJiiy, appliM to Ikfanoli'« work ilk to that of Chevalier. 

Thv form of three r-nporinipouHl tdniltar aohniuiatif d<>ublet« ia 
precisely the combination <>f the Ptrrnch 'butUinH,' wliicb liavr Ixvn 
■oM ill tbouiuindx until quite recently, many of them being numntcd 
at Eii^iUhIi iibjectivps. 

At tlie suggestion of I>r. <ioriiig, Mr. Tutly, in lliis coniitrv, 
vitlxiut any knowledgo of what wn« beinK 'bine on the Cunttniiit, 
made an achromatic objective in IrCil. Thiil wan a sinj^le ooinbtna- 
tion, being an achromatic iincrincnU'd triplet. It was, in fiuit, « 
miniatnm telescope nhi<>ct'Klau> and i" iltna- 
trateil in fig. 367. Tuo 1<'iiki-s nmdi- on thia 
principle by Tully, hai ins ,'„ and ^„ foci, wore 
toand in pmrlicr (no thick, and in many 
ways iinperfrrt ; bikI he a-aa iniluci^l t>i make 
UlOther KinjE:le triplet nl ^"n fiicuaand 1 ti° aper- 
ture, and itAperformaiWO wan aaid tube nearly p^ snT.-Tutlj'i uhio- 
cqunl to that of the j^^. nutic iripln. 

Hnbaeuuently a dindilet waKptai'e<t in friiiit 
of a sintiliir triplet of boniewhat diorter focu4, forming n doubla 
minbination objective of 3(** aperture. This yma pronounced to be 
« Kreat advance upon all preceilitig coinbinatjoua, eren thuao which 
hwl been pm<)ueed upon tlie Continent. 

A note of liater'a at tliia timo apoii the objeotivM of Chevalier 
ii of intsrect. He found tltom nueh stopped down, and, in one 

* C1i*p«et L 



iiiHliiiR'e, tie oponni thr slop nii'l iin]>i'<>i'wl tlii' etivet. lister anra : 
'Tlie Fivufli itjiiiciiiii kiK'WH iiotliitif; o( llic \alue of apertura, hut 
he him kUowii u- lliiit fine jiprfoiiiuiuof-- L. not cunfiiiwl to tripli- nh- 
jtwtivfiB ; nml ill sucifwiuUy cmiihiiuiij; twoaoliiiininlit:'^ he liiia ({iven 
nil iiii]Hii-tiiiil hint piN'linhly « itiiotil Ijeiiifi hiiiwelf n«quMiit«l wiib 
it* wiirlh—thrtt 1 h"|w wi)l lt«il to tlio nc(|uihitiun of n penetrating' 
|iower Ki*at«r than cuuM ever lie iwnbetl with mii? »I"no,' 

At this tiiiit^ PnifMsur Ainici, <if Motleiiit, one 
i4 the leniliiif* minds wh^i iutist«<l in giving ita 
fiirni to llie woiieni iiitciwcniio, hud Im-n ljitffl«t 
//^ by the rlilticultics preMntwl liy the pn>li)piu of 
iithroDuitism, Bn<t IukI Uid it asidf in fitvinir of 
(he retlecting iiiicruBCOpe, hut lie now rvtunipd 
to llie ]>mct)rnl rocoiiuircrntiiin of Uic pixiHuctioii 
(if im iichiiiniAtic kiiK. Ax u rr-^lt hii apjuwr^ 
I'lhrtveCMintmctwI iihjcctivwi.f gnsiH-rnpiTrtuiT 
thiin iIkiko of Cho^nhrr. He vixitnl Tjimdim in 
|S27, nii(l hroii)cht with him »]H^-iiiiriiK <>f hi> 
w<ii-k. which pnahifi-cl a mint fiivoumtilc int- 
prcwim. null Kuh,v(|ui-Tit)y hi? luude hii objectivi^ 
i»{ ^'iiifh focus. 

Mwintinic, in tliiscnutitry, Mr. lJ!itfrhi'<iu(;ht 
nlM>iitiin important p|HK-h in iht- pxulutii>ii <<f th(? 
adiniiuiitic nhjcrt-fjInM hy thf ihsfuvety <•( tJi*- 
two apliinutiL- fiK-i of it ooiuhiniUioii. It hiul 
• HX'Opiiil his uiiiiil for sfvenil yeaiii, hut in 
JHiiunrv IK3U a. vr-ry iuijHirtnnt )ui|M-r wax iiiul 
to, and pnl>)iiih»(l by, thi- Rnyiil ScH:iety, written 
by him, in which lu* |Hiinls out how thr nber- 
rntions of om^ douhlrt :uiiy he iK-utntlisnl by 

Ak thr Imsis of n luivniK'opic objective, 
eon»idfn> it ciainpiitly ilpsiiiible lh»t thn fli. 
lenn shnll \iv. plftii(>>fiiiiiiivi-, nnd that it nhiill 
JotDt-d by 11 p-niimiiint cement to the convex 
Id Hi. 

Kor nil achn limit ic nbji^i-t'xI'^MWJciiiiHtructeil 
he made the geiierid infi-n-'ncv thnt it will Iiiup 
on one side oi it twn fm.-i in itj iixin, (or the 
iTtya proceMliiiK frnni whioh the Kphericid Jiljfinitiim wiJl lie truly 
coriTWted iii ii iniidemte Hjiertui-e ; thiit for thr i|»ics between tlieM- 
two pDtiitn iti H|iherii>nl uberiittion will lie (iver'(.'^>rrec(e<l, aikI 
beyond them rilhiT wiiy iini)w-oorn'rle<l. 

TIiuB, let o, h, %. iC.S. lY-jiresent oiith nil object ■•jIa.'*, anil bi- 
rcu<{hly ombiden^l k» ii )>l'ini>-(-»iivex lens, with u curve, a c b, 
lumiinj; thiiiti;,'h it, »t uhioh the >pbericid nnd chromatic errors 
i»re eorrecleil which lov Keiiei-iitiHl at ihe two outer surfawss, mid 
let the gUai> be tliun free tr.)iu »lierniti<iii fur niys, / </, e, g, issuing 
from the nuliKiit point,./| hf buinf;» ]>erpeiidiculiir to tlie ranvrx 

> ' Pviivlnilini: iiicaiit 'rcvlriiitc* ]>ti««r in t)iii*r ilaj*: hvullndei, Uinvlol*. (o 
■ncreaiMi n' »p»rturB. 



Tif^ aiWi. — TJiF two 
■fcpl&iiAtic Foci oi m» 


mutKie% mod «rf to the plitiiv one~<ancl«r thrao einrum&tunrw tho 
ancle uf otDPT^ncts ff r A. inucli excAeclx that of incidence, / it i, 
bong pnilxihly Alniiwt ttii«e times as grant. 

If lli« mdiiint in ntiw niwle to Approach the glnss, so that tlje 
<uaise of the my, /'/e g, sliitll bv more diroi^nl from lh« nxia, an 
the angles of int'ulriK'p iind rm^i'^nct? iN^mic ntoro ncnrly equal 
to each irtJMr, the iphrricnl «borniti<in proriufcil by thp two v,-iH be 
foand tu boar a Ium pmportion to tlm ojijuMing error "f the 
smgle correcting cui^o wh ; for kucIi h i'^.u*, Wxvrviatv, the myh 
will bo over-«>tT(XTtoil. But if / still njiproiithiw thu glnvi, the 
angle •>f in<4<leiioi' continues to iiicnriL'U' with the iii(-nutsitig diver- 
gence of the my, till it, will exerml thiit iif pmrrKi^nco, which hii« in 
the meanwhile bt^tMi diminixhtng, nnd at li-nKtli tliL' nphrrinil error 
fXKiuevtl by tlicm will n^cover ita oriKinnI ]>rop<.irtion to the nppo- 
ate enxw of the cum- uf correfti'tn. Wh»!n_/'Iuut rtviched thi* point 
f" (at which tJu- angle of iueidenondoeanotexceMl tluU of vniergnno)- 
«i luucli aa it hiul at fint c<nnl^ Hhort t)f it) the ruya a^aiii ]mmn 
the glass free from apherinU alirrmtjon. 

K / be rarrieil henvo t4>warthi ttio glaaa, or outwards fnuu ita 
4iriginal place, the angle nf incidence in tlt« fonner cattc, or of 
.^Hiiiigiiiii II in tii« tatter, Ifecmncs HtNproportionatvly effisetire, muI 
djfither way the abermtion ex(-i>eiU tln' corrMition. 

How far Lister'* iliscovei-ips wert? aflt!ft*<l by Aniici'ii w(irk it is 
now 4uiti' iiii|H>cuiible to *ay ; tlii-n! caii lie but little ilouht that niHiie 
inlliKnce LI due to it, hut it is eigually ett^iu: thiit a profound knuw- 
li)Hgc> of the optic* of tliat time woa the onlv fnunilntioii upon which 
the facta in LiMer';^ paper eould liave \K*-n built. He whh n mni) of 
application atid an enthuKiaKt, and it was inevitnble that he shouH 
rxert n powerful intiui'iiee upon the enrty hi.itory of the npticx of the 
Hucrx>Boi>pi'. Tlii.* i* the more eertniu when we rememlier how few 
ware tiie men at tlmt time who knew in anr practical si'iise whiit n 
■icTiwiOope wa.^ ; nnd wo find thiit in 1831, being unable to fiiul any 
ociticion who f-arrt! to experiun'Ut suHioieiitly, Luti^r tiLU;;lit hitnnelf 
the art of lirim- grinding. "Tid he nude an objective whosr front was 
a nenisiTUs pair, with a triple middle ouinbination, and the bai^k a 
plat»r-(-oiivex doultlet. He declared thia to l>e the beat leus of its 
lannediate tune, aii<) it hud a working distance of '1 1. 

One of till! iinniettiuie coutiequencea of the publication of Lister's 
pamrr waa the nkpid pniductitm by profesaional opticians of achmmatic 
i<)>icvttvw.. Tlie data supplied by ListM- proved to Iw of the highest 
talu« in the actual production of these, aiid the progress of improve- 
laetit wa-s in eonsequence, and in comparison with the time imme- 
lUately precediuf,*, remarkably rapid. 

AtHirtw /foM began their mannfaoCure in 11^31. Ho was followed 
by Hugh Powell in 1^34, and iu Ilj39 by Jamra Smith. It is of 
inure than ordinaiy interest to study in detail thn work of thin im- 
metliiitu time, and the following table givingaliat of objretiveji, with 
their foci, apertures, and niode of couslniction, with the i In tea of 
their production, will give a fair idc* of tJie work of Andrew Rosei 
ia tiie maiiufacttire of early lenftis. He wan tlieearlieot of tJtethree 




BiigUah DuUcers, nnd undoubtnllv oarriccl tlio palm both here oiulun 
th« 0(>ntui«Dt for tho excellence uf hu ubjccLtvct. 

1 Loch 14'^ two douUeU, leaz 
1 ,. IK' ain^ triple, 1833. 

t.. Sfl'thrwpnin. H«*<. 
: w :: :: J'^'"- 
, " *|t}triploffotilni»cllwi»cloublebooki|'J^j'"*].UitMr'jiron»ialii, 

1 l i** - .. -1 


•I "i — J 



Example* of tlieee old lensra nre extant and iu perfect pmervA- 

tiim, mid ti>r ci)n'*i-tioii tliejr ftre cain[Miroblo without detriiDUit to 

any onliiinry civiwii «iid Hint glass nchnnn»tic of the same s|]«irlur« 

of th« prespnt dny, 

^— ^^-^ An wuiniple ttf the construction of the ^-inob 

p ^— -^ focus objective <if TfH", coiMUtting of three pttini of 

^"^ !!r^ lenB08 ftrnin^l with their pUne sides to tlie ulfject, 

f^ ^ the position <'f liiirt ttbcrmtion, is shown in 6g. S69. 
j^" "y. The foci of these thn><;piiini lire in the projiortinn of 

f^^ — ^ 1 : 'J : 3. In lt<.t7 ihw iimker had so ioinplet*]y 

fin ses^A t-ln. corrected thi' rrrors of sphi-rioil imd chixMttftlicttber- 
conAiiutiao by iiitioii thiit thir i-ircumstuncii of covering «ti object 
AndMv Com. witli a plntc of the thinnrat gluts was found to dis- 
turb the orrcotioii.s ; tli&t is to say, the oomc- 
tion» wen; so r«latively [lerfect thnt if the combin&tton were adapted 
to (in uncovered object, co^'cring the object with the thiunent gliuit 
introduciKt refractive <li»lurbanc«N that destroyed the high quality 

<rf (he object i^'e,' 

Lister's paper of 1830 
gave the obvious clu« t» 
a metjiod of neutniliaiujr 
thi» ; that ia to any, Ay 
Iriu tlittancf : and VUnc 
Applied this correction by 
monnting the fi-ont lens 
of nn objoctire in a liilie 
which slid over anoilier 
I II I Mt carrying t he t wc> 
other pain. Tlierv wks 
a Knuul guide-pin in an 
L-»haped alot to limit th» 
amount of raorement ; for 
II HC'ii'wvt/ ohjrct* the front 
couibinalitin was drawn 
outand the pin was turned 
into the fiHit L ; and for 


Fio. lT0.->4MtU>n<il *djattia; cbjett-glua. 

covered objects th« conibinatiuna were closed together to their limit. 
Suluequently tliis amtngcmffub waa inodifiMlty the introdactlott 
■ ri(f« Cbkptai I. 



of » screw arrHngptnent, m in 6g. 370. The front pair of iensM f.i 
Scad into * tube (A) which Hlidoo ovpr an interior tube (B) hj wltich 
tlw utbM-tvmpBJni am bold ; nnd it is dravn up or dovn by means 
et ft oollar (C), which works in a. furrow cut in iho inn«r tube, and 

ri « servw-thrMtl cut in thit outer, 10 tltat itn revolution in the 
II to which it i* tixMl by th" one tubo give* n vcrticftl movement 
til tho iither. In om^ pnii nf tlM^ <iut«r tube nii oblong iilit is made, 
■c svcti al D, intii which prajodii > miiiiII tongun Kcrpw«Ad on the 
iRnor tube ; «t tho wdo «f tho fiirmcr two horixontAl lines ara 
mgnrnl, imo uointin); to tho w»rd ' uncurrrnl,' thi; otlicr to tlie 
word ' cavvrc<l ; wbiUt tho Uttvr in cniwvil by n horintntnl mark, 
which IK brought to eoinciilo witli oithiT of thu two lines by tho 
fotatjon of tin- Kcn^w-voltiir, whcroby tlio outer tube in movcil up 
or dtiwii. When tho nurk hiu lun-n miidr to point to the lino 
'oncovnmt,' it indicates tlul the iliHtuucttof tho IcnMuiof theobii'ct- 
glas* is Nncb an to uiake it suitublu for viitwing on objcrct witliout 
any interference fruni thin glast ; when, on tho othirr hand, thn 
mark haa been bruuglil by the revolution of the ticiuw-collnr into 
eouMiidmice with t)i« line 'oovered,' it indicatj>s that the front lens 
ha* b«!«n broDi^ht into such proximity witii thn otlutr two as to 
pnxldce an * under -curreoi ion ' in tlie objective, lilted to neutralise 
the ' over-covreolion ' pixxlucetl by tli« inturpoxition of a glass cover 
of extrvnieBt ihlduun. 

TliiH nietliod of OoUar correotioa aer\-eil tlio purpooes of mioro- 
Mopy for upwards of tliirty y«ari, but when raont critical inrestiga- 
tions were undertaken and (liijective* had inoro aperture given to them 
it was foiinil that the metliod liad two 
peat faulta. 

The firat was tlmt tlic ' covt?n-d ' nnd 
*Bact> v er« d ' marks w<-ru too crude. To 
noady tlii>, thu screw collar was 
grM]nat4^ into fifty dirlsiona ao that 
intervahi between t^e points ' covered ' 
and ' uncovered ' might l>e recorded. 

Tho aecond, a toon eerious defect, 
waa the norement of ifte front lena, while 
the back remained rigi<l with tlie body 
of thn micTXMoope. The detnmetit of 
tlii« nrrnngcment waa that in corrvctJng 
a wide-angled, cloie-workiag objective 
tliere waa a <langer of forcing Uto front 
)h» tbroogd the cofer-glan by means 
of the collar correction. 

Now thn object in the arrangement 
M shown in tig. ^TOa enables the 
front lens to maintoui a fixed position, 
*hile thn cunectional collar acta on the 
posterior combuintions onlr. 

On the Continent it baa been the 
pmctioe to Kmloate the ooireotional collar in terms of the thickness 
of the cover-ghuN in deoimahi of a milltm^trt-. Tbiu if a cover glasa 

Fio. 970*.— Ppp*<y>t collur 



Im 0-18 niui. thick, tlie cotTe>utioDal oullar sliould 1>o set to Uwt 
<tiTiKion niArked Olt^. 

In RiigUiid, on the ooutrary, the diviuona are entirely empiricul, so 
that tlio operator hiu to discover for liiniHelf the proper adjustment. 
It i« not to 1» supposed, however, that the Euglisli method U uii- 
■cu^ntific, for when ao operator beconiM expert lie would never for 
on iualAUt think of Adjusting liy any other indication than tliat 
aSbnled by liia own eye and experience. This ia a veiy important 
point Itecause the interpretation of structure, to a great ex lent, dependa 
on acciirate ai^ustmont of the objective, and it would bo fmly to 
(cuppose that an eminent olisorvor would surrender his judgmeDt to 
the predetenniiintion of thMiy embodied in what must be the 
inipmeotio&a in even the most oonsdentious and thorough vov4c 
wluofa fpve^ a piitcticnl form to such theory. In fact, it is the teat 
of accurate manipulation that, however the collar correction be dis> 
tnrhed, the microKcopist will, in getting a critical image of the same 
object, always, by tho t|Uttlity of the image he obtaiuK, bring the 
correction towithin the merrsit fraction nf the same position, althousk 
the correction collar and its di^isiolUl are never looked at until tJie 
desired image i« obtained. 

The frtct that tlic over -correction caused by the co\'er-gla«s wait 
discovered in England, and that means were at once found for it* 
correction, while no himilnr steps wrro taken on the Continent, ix a 
suHicient evidence of thn advanced pi>sition nf thiw country in pi«clical 
optics at that time. 

T/iU nubject n/nndrr- and nvfr-corrftlion it oiw nf targt intfort- 
nncn, and it may lie well fit this point to enable the tiro to clearly 
understand, by evidence, its nature, although iifint it is haa been 
fully shown in Chnplcr T. Takn a single lens, the field-lens of ■ 
Huygheninn ■■y piitc will wt\c admirably, and hiild it a couple of 
yards from a lainp tlnmr ; the rars patsinfc through the fi^ripktrat 
portion of the li-ns will be found by experiment with a card to l» 
brought to II focus itl a /mini On thr ari* nimr*r lb'- lf»a than lAeM 
IMsiing thrau-jh the eenlrr. This in nndfr-corrtflion. The SUM 
expeririK^nt Khnuld be repeaUHl with the plane sido and the convex 
si(l" of thi' lens ulternutt'ly turned to the flame. In the former 
caxe, when the iina^ of the Huiiii^ is nt its bent fociiK, it will bo mr- 
rounded by ii coma, and cvm tlie portion of the flame which ia iit 
focus will laek brightnwui. But with tho ••nnvr tiHr li»iKirrit Ik" 
tiainf. it will bit found tlmt in thft image on the ooid the coma ia 
greatly reduced, and tlie image of thi^ flame brightened, 'Die rat* 
son for thia ix, aa already stated, that the spherical aberration is 
four tiiiitsi us gi'uit when tlio convex side of tJie lens is towanls the 

The piBctioe of tlieae simple tests will be most iDstructire to 
those iinfamilinr with thi^ optical principlea on which an objective is 
conKtrucled. Tliiry make plain that an ai'v^r-mrrfpl'tl Iriu is eiw 
vhirh briiiffs iU jitri/'lfnil rnijs lo n lortyr /octi* than ila ftnlral. 
But a cover -jiliiM produces oifr- correction, tltereforo tlie means 
employi'd to ueutraliae the error i« by the under- correction of the 
objective. If, howcvt-r, the objective oniployed dioald be vtafttf 


Pio. VTl.— An MTlr 
^xii. OMabiiulico 
tiy A. Rets. 

Tided with Mich ineaiu of correctioD. tlie eye-piece muai be brought 
iwuer the objective, which will effect tlie Mtme ivsult.' 

Still contliiiiig our rooaiderHtJoii to tlie y«ar lt^3T, we tjiid tli&t tv 
farther improvetiient wa8 mtide by lister, who employed n triple 
{rotil combination. TliU consisted of two crown plsno-odnvexos witli 
» fltot pIano-coDc«rfl between tlioni. The rcwult of thift was the 
incrau>e of ihe nperture of un iiith-focus objecti^'e to 23°. 

An illniitnition of the mode of coii§truction oE 
thoe lenses is giveit in &g. 37t, which is dmwii 
from mn early J^-inch objective by Andrew Rum, 
hnvin*; bnyoitet>catcli correction adjustment- In 
|K4L*H i-inchof 44^ a J-iiich of 63%find a -f-inch 
i)f 74° were raade upon the mune lines. 

In it<4l the Royal Microecc^ical Snctftty 
■•nIcrpKj A micnnecope from each of the before- 
>ai»nti-iiml Itadtug opticians. The objective* 
■applied with tbem «re still extant, n'prtnvnt- 
ing with nutntl certainty the very best work <>f 
tb» se%-nml iniLkers ; they are consequently rahi- 
abltt OH irlUblc fip<>cimcn« of the host work of the [x^riml. 

Th<- <>bj<vtir«s sBppliad by Jiunes Smith luve the pMuliarlty of 
l>^in)[ sepnniting lenMi. 

The lowMt pow«r is about l|-inch fucux. When this ia used 
aitme n <liAphisgm is slid over the front to limit thi- aftei'turf, but 
we are unable to xav whiit that liniit wom, sJiii-i- the iliivphrugih hn* 
bMn Imt. By placing anothei- front irhcrc the diiiphriiKni would 
hare boen, the new combinaiion Ijeconics nn jj-inch foetus, whiln 
nt another front may l>e snbstitui^l, making the objcctit-i: a ^-tnch 
incus. Thin latter front consists of two pairs, und it is pi'ovidwl 
with a gmdnntnl scrcw-olhiv ndjastmcnt which .M-juinitcs IIkhu 
jjftii-s, bat the arrangement is of a very pHmitivn tinh-r. 

This ohjvct-glnwi will divide the iHHlurn, nmrkx in a milky field 
with a full ctmvt •nd the field is niucti curvitd. 

Then; is aim a Ncpamting l|-inch and ^'inch which is good, 
whilp tjM -^-inch nml the i-inch may br i>inMdpr<-<i fair. 

The leiucn supplied by Andrew K<hh pin^ 11 )£"<hI 2-inch and a 
fair l-inch, but we have mm a better than tiiis of alxmt the loune 

Hugh Pnwptl KUpplioil a l-inch of giKxl ijuality, and ft |> 1> jt 
f^'indi fairly good. The aporture»of tlti> f, and tlieil^-tneh ara 
ti cnunw very Iciw. 

On th« wlitJe it may be said that the corrections are well 
UUnnoed in tlx? lowrr lenses, An<l the apl■Ka^^s m<iderul« ; but when 
we ooine U> tiie higher powers it is l.h*- <lir)ici<!ticy of aperture iliat 
becotneet ho nppreguivt^ly apparent. In IHH Amici niiule a j-inch 
nl^tive <if H±*and brought it Ut England. It wuh understood 
that extra deiutn flint wax vmplnyed in the c<)n»>t ruction of this 
nbjectii'e ; Imt this is pri-Ulinhl« ; and Mr. Ri«s altered slightly the 
furv« (if Aintci's coiuitrui'tion, und with ordinary flint succeeded 

' CiK^fq^omrrirtioii ii alto knomi m ' pMitiio *b«rratiiiii' ; riv»c-eoiT«ctU>u ui 
' M^liv« abamUoa.' 


in oxt«tuUnf; the aperlnre of a 1-inch obj*•ctivl^ to BB', or -69 
N.A. ; ttnd a Va-""''! objective to ISS", or -93 N.A. O/ this 
)att«r it waa afflnued that it was ' the Inrgiwt nnguliir peiicil that 
cnulcl lie puAaed through a microecoiw object 'gtiiKt.' 

In 1880 object -glasws wore nindo with alripUbaeJi eomliinatioH ; 

thMe vere attributed to List«r ; but it i» nl.-<<> aSirtn«d tliat they 

were llie previous device irf Amid. It may well be adistniwd point, 

for it is qiiit^ ceitain thitt tltix device bruu|<ht the 

di-y schromatic obj«wtiv« [lotrnlially Ui ita hif;he«t 

perfection. Tlip (^oniliinntion is iUusUuted in Ag. 

fZZ^^^^^^ ''>-• find undfT tlic ci>n<litionii of itn construction it 

V- i inAybnwclldoubtfil if iuiytbinKwi]Ieversaqwssth« 

/" ^ re^iilU nbtjiincd by KiiKbaU (ipliiiana in achromatic 
[.^""Z^N objectives constructed with this ii-iple front, doubl« 

pjo.i^a. Atripln- iniddlc, and triple ImicIc oombi nations. It maybe 

1i*ck comhiniL' notion) that Tully'ti objective bad a triple bMk, 

(ion by LinWr l,ut il was not ih* result uf intended constractjoa: 

""" it wnfl n fortuimte cutiibinalioii the real value of 

whicli watt neither undcnituixl nor ap]>rtN.'iated, and as a consequence 

its fxistMice was evani-iiccnt. 

In this iianie yejir Wenhiun produt-ed another moditicatioD of the 
ochnmiauc o1>j««ti\'4) of conjuderable value, but more to the ntanii- 
footun^r than the u.-h-t of the micrcscope. It consisted of n tin^h 
front ; t)ie combination b ueen in fi)t. 373, which it will be aeen is ft 
simpler c (instruct ion, but ibitt did not aBect in the Imst the price 
of \Xm objectives produced. Subsequently, now- 
vxer, the form wad adopted on the Ccmtiaent for 
low-pricivl objrctivcs, which led to a rednetion of 

tilt! cost of English objectives of the same con- 

j^ ~^ Nt ruction. 

r" ^ Mantfwitly, the ninf-le front loosened tJie tisk 

^— ^ of technical errors, but we bare never been able 

^ — ^ yet \ai fiwl » idiiKle fi"ont dry oc^iromatio objective 

Flo. ara— A rinRie- vrhich hMji Hhown any superiority over ft ajnubur 

^i™w"""' ""« I«-'---i"K « triple f.«..l. 

Th*' single front employed with two combinft- 
tions at the buck vas. the forui in which the celebrated uolfr- 
imnvrnon obj^ftivt* of Powell uiid Leidand wei* made. It was by 
one of these that the striw on AmjAiplmira ptlivtida were fil* 
rvwilvcd. Indeed, what is known aa the viater-imirurium «jM<mn ^ 
ohjrctivff, deviAed by Profe^or Auiici, was the next advance apon 
the old form ; but it was an advance the optical principles of wbi^ 
went ccrtAinlv not at the time understtiod. 

In Paris, I'raxinowski and Kartnnck brought these objectives to 
grmt perfection, and were enabled to take the premier place nfiainxt 
all competitors at tlie exhibition of 1»67. The next year, however, 
Powell and Lcaland adopted the systew, and in turn they cbRttnced 
the Pariit opticians and produced some of the Uncut ohjectivcK ever 
made. Their ■ New Formula ' waUtr- immersions wem made after 
the fine dimIcI of Tolles referred to below, and liad a duplex front, 
ft double middle^ and a triple back. In 1877, when tho water- 


31 r 


Fill. 87*.— A iditRDiMd 
rcanbinntiiiii li} Wen- 
bun. IDitU. 

imnwrsiott system toudi«<] iu Id^Met point, «pertun'ii lu grrat nit 
rS3 w«ra reached ; and iii Anedoft, Spencer, ToIIm, and WaJw 
prodocvd toCDe extremely fine leDBW of l&rge aperture. 

Daring the y«kr I8OII Wenliaui experimentMl uitb und sag- 
gKtMl * too •mploymeot of a duptrx front ; tlutt in ho any, a fnnit 
«ooili)antion madt^ up of two uncorrected lenaea in oontjudistinction 
to lu) ftchromntiiotl pair. Au illuBtration of 
Uw plnn Huggnrt«d is given in fig. 274, which 
hnrdly npnotn to US M « pmcticable form, 
«nd which crrtjtinly was never bronglit to 
perfBotion or put into practic<\ 

Bat in the nwnth of Au^r^st 1873 Tolle« 
■etu&lly in«d<i, on wholly independent tines, 
» duplex front formula for a, |-glycimni^ im- 
iBMMon of 1 10° boUam angle, which pn«eed 
into the posM-Miion of the Amy Madical 
Slasmin at WMthington. Thciro can bo littl« 
doubt but tliia objfictivn would hnrr^ prodiici^l 
N mw-h dtt'per imprcfision l)Ul< for tlio fnct that it was in adrnnco of 
its iminndiatK timtt. 

ToUtw, n> wu har« hintiid abovc^ asrd the duplex front in th« 
conotniction of Kinu! of hia ironwnion objectivVH, and was fi>ll<>w«d 
in this by the bmt EnKlL-di tnakeni, and in thn cuae of a celebrated 
^-tnch, purchaiied by Mr. Criap, TulW was able to reach a bnltaui 
angk of 96*. 

At tbo tinw that th« 'WBt«r-inini«niioii l«ni«8 were being coq- 
itnict«d by rival opticians with incrciuiing pRrfoutiou the groat 
theory of I'rofri»or Abbe oonceming niicnwcopic viHion, the import- 
aiKW of diRVnclion spncttn, and tbr relation of aimrture to power * 
vaa entirely unknown. In thp abs^noo of this knowledge wholly 
miHtnkcn vnlui; wiut attacbod to potmr prr w in the objective. 

With a foou!' iLM short as tho ^-inch, it wan not uncommon to 
find npertun-n li's.« than \-'i, white obi<?riiv™ of J,, y^, j\j, and 
ercn liigher powers, were mjule with i-xtrnmoly reduced apertures. 
This was done in tlie intermtjt of tho (.■omnmn bt'lief that ' power — 
devoid of ilii suitable omicum»it api-rturts- ooukl do wliat wiu so 
keenly want«l. 

Tliis imprv-iaioii, however, wan far from universally r*liod on ; 
tlierewereitevenileiLriirat workers, who, wiUioutl>eing able to explaiii, 
as Abbe subiWN|uently did. why it watt «o. still urged the opticians, 
in the mauufaetuiv ii every new power, eapei-ially the higher ones, 
to produce tlie largeat possible amount of aperture ; andthe<ividen(» 
of this it atil) to be found in the objectives tJiey tbon succAcdMl in 
•fataininjt. But there can be no doubt that a recklen df^re for 
wuwnifying power, all other considerations apart, greatly ot)tuiued \ 
and the opticians were able to encourage it, for it is for easier to 
consirucl an objective of high power and low aperture tliitn It is to 
nuke a low power with a large aperture. 

Thus a ^inch of 065 N.A. will be far more expensive (u»d prob- 
ably not as well corrected oe i of 0-7 N.A. The 4-inch objective, 
< ITaiUWilUicra /<>«»>. vol. L p. ITS. 


v\en if ft good unr-, is sure to exhibit sphmciil nlHTtntum, irliil^ 
the ii of low npvrtum will ahiyr miiny ininut« (ibii-ctM v/ith con* 
aiderable clcarnwBS fiapecuilly if n cotnpamtivntjr narnw illuiniiuiUng 
ccMte be usni. 

This (liffirrmice bi-c»ni<» Htill inorf ninNmcuouN iw tli<- iliffirrencr 
ItebwMn npcrtuiv luitl p<»w«r gmwji nfliitivnly fcmaUti*, until we nbtain 
ultimiLtt^ly oil nmiitificiiticm mom thitn uwi'ltsw fnun lU utt«r iiuibilitTT 
■•I) account of (li'licicncy nt upiM'tun-, l» f;ni*p (Intoilx.' 

Up t*i IW7J, lic>w(?vijr, thiTo wiis un entire abiBMicir of kiinwU.'d^, 
pt*cn oil tlin piirt of the IviuJctrK i» inioriKtciipic theory, iu%, mmI 
pi'iicti<H% iL% to tht> rtatl optical priiiciptRi thiit vnalilMl un b> ne* » 
iiiii-riiMMipiu iiniiK"! ■^"d consiequenlly Ui und«rrtitD(l Uie en«eiitial 
I'ciiuirciuents Iaj lie ajiiied «t in the best farm <if micnuioiipe. Bui ui 
ISTT Abbi-'s grejvt diirructioii tliwiry of uiieroMopio viiioii upinmrvd, 
whicli bos led to oIiiliikim <>f iiit'oiii)Hkni1>le value in llio pi-incipteH of 
<.imst ruction at objectivM nnd eye-piece^ und, ■« a eunsequence, Imh 
U> si>me coiisidemble exl«»t yiven u new chanwter to the eaitira ui- 
lilrunieiit. Its proinulK^tion hna indeed ini>uf|urated ah entirely new 
epiicli in tin- coii.'rt ruction untJ use of the mioroaixipe. 

Tilt? Keii<>riU eharuL-tur »n<l the detuila of Abbi-'a theory are Kiven 
in the second chapter of thia treatiiie ; but ita pruclic-ul beuring upun 
the theory fliiil :ipplicatioii of the opt^ciil pitrtul the itutruuent were 
noun uiAiiifml ; for in 1)S7R the /imiiOj/i^iifvus tj/ntmi q/ imiinrrti'/n 
o^m-fitvx' it<as itUrodtietH as a. logical outcome of the dilTruvtion 
theory of uiicroscopio viuon. A formula for a J-iJich objective on 
this sy«t«m was prepared by Abbe, lo whom, we teum from himself, 
it bad been nugKesited l>y Mr. J. W. Stephenwui, nf the Royal 
AUeroecopicul Sopirty,* It luut been nireiidy sliown ' tliat the liomo- 
^eneoutt HyKteui wms •«> calleal becuune it emph>yed the oil of cedar 
wood to unit*! the front tens of llie objective to tlie cover-Khuaof die 
iibject, in the Hiiuie way as vater liad ))eeu employed in the ordinary 
iimni'riiou m/ntem : but< aa thei<e wilh a piuctical identity l>6tweeii the 
■■efi'Hctive and di.-ipei-Hit'e indite:.-, of the oil, and ihoee of the i-rown 
n\nt» of the fi-out lens, the rayn uf H)-hc pHased thi'uu^h what wam 
<fweiitinlly a. homogeneous ftuluUiiice in llioir path across from the 
>>hImi in -mounted object to the front leiiH : and a homa^neotu ujfMttn 
<if objeclivea tuuk the place uf the pi-etioua water iinnieraions. 

This was the &nt great step in advance in optical comttniction 
and ftpplication following the theory of Abbe. 

As often happens in matters of this kind, thei« had l*een an 
appnrmit anticipation of thU nystiim of lenws by Ainici as far back 
iw 1844 ; but it is very nppivi>eiit< that Amici employed the oil of 
nii»erxl without any clear knowledge of the principles involved in 
the honingeneous syHtem ; bcin);; wholly unaware of either tho incTMum 
<if aperture involved or the caiue of it. But this cannot be mid of 
ToUes, of New York. We have pointed out that as early as 1873 
he made n T<4-inch, and sub>Mi]uently in llw namf ycnr a ji-inolt 
objfictive, each with a duplex front to wnrk in nift hal«nm, and with 
a Sf.A. of 1-27. TItese objectives were cxuniinfd by this late Dr. 

» TM* a.»pi« ir. » ibiit. 

^ V.K; liUifJourn.Sot/. itirnir. Social, ii. IKTD, p. UST. * Chapter L 


WiNMlwanl, fif liie Anuv Mtxlicul DspHrtnirnt, N«w Ymlt, nnd with 
thM exumiiuliiii) were itlluw«ii Ui dn>{>L F<ir T(iU«ei wt ua <iri(;in«) 
deviser t4 n |>n«L-ti(.-al honioK^neoun Hjntetu thin wna unfbrtuiiatp ; f<ir 
tbe actual uilnxlactJoD i>f tbe svsteiu in u fonu cupalile »f uiiiiei^nl 
■{i]>lioatiou, und worked out iii all xtn deUiU ut an entirely iiide* 
pendent nuiniier, we mre wbully iudcliteil ui Al)l>e. 

T1h> priuciplo w:ui lurt, n^vei-tiieless. an ivudily luid wuiiiilj' 
adoplecl in EiiKUud on its finit in l reduction 
is mi^rt liAVc been a>itici]iut«tL ThLn itnno 
]virtly, Ikfiwever, fixiui the {nd tluit water iiii- 
tnentttttia hud 1>eeti bruii;;lit to mi bi^h a point 
id CutoeUenon by Heasr^ Powell and Lealntid tliat 
ihm eoriy hoBrageueuuH objectivee were not pos- 
•«Med of more upeilure, and were not aeiiNibly 
Miiwrior to tli« best iuiiuei-Nloiis made in Kn^Utnd. 

Tbe boiuogeneous objei-lives were mode with >^~N 

linplex fronts mm) two doubl« bucks. A geneml 
diaiCTuin cf liieir mod« of construction is ann in *^'"- *T8.— C'onibhi*- 

, ,,-. tinn 'IT 'homOB"- 

ag. ^lO. neont" Inunertioii 

Sii loti^ as crown glass was employed in their lijr Abbe. 
iiiniiufHctare, and the itnt«rior front lens wua ii 
beaiiapherv, it appeared that N.A. 1*^5 to ri>7 wax tJic ajK-rturt! 
limit they nmld be ninde to reach. Mmhts. Pow«1I and Leuland, 
however, bu uutM>i(/ ihf ant«rit>r /ront ktUffnatm- thnn i licmlj/Jirrr, 
iamMsed the nperturo of n ,^-inch objectii^ to \-i% N.A. 

Tllia front, from beiiif' ^rreater thnn n heniisphim, prwenti^l 
difficulty ill momititiK ; thu was at lirat overcome by onmrntin^ ita 
pluw surfoce to a thin piece of ffjiast, which was then SxmI in thn 
nietAl. Eventually, however, this forui of cunblruclion wiui i-Imnued 
hy theee maltei-s in a very in;;eitious manner : so to xneak, they 
tetirely invrrlni the cotiibinatimi, and aooomplished ttie end by 
wml-itH/ th« front of jiinL By this niMns they obtnininl uporturen 
■liich have no« as yet lieen eqtiAlle<l by any ether mnkirrx, machinK 
iaa J, ft I'j, and a 3'j,, n X.A. (if 1-50, out of a thc«n'ti»-«lly [HMsible 
*pe«lur« <rf 152. Prof«?si>r Abbe has since, it in true, made an 
objective with a nnmericnl aperture of 1*63, but thi» r«iiuires the 
objects to be mounted and studied in n. nicdiiiin of currecponding 
tf^ractive index, and consequently, in the jircpiit studi nf our Icoow- 
l#dft« of the subject of niedin, not upiiUc»blf- to tlic nr —.ligation of 
•nlinary organic structure*— certainly nm 'if living tlim^s. 

These objectivM fully occupied thr rm 1 - ojiittuntil 1886, when 
the most importnnt epoch Mnoe the <liw<>vi-iy luid application of 
KhroniMlism was inaugumti^]. 

We have alrandy pointcil out in dntiiil ' thnt it wun the grent< 
ticfect «f the ordinary rri>wn nntl dint achi-immticH tluit Itro eiilourg 
md^ cotJd bt eombiivd nnil that tin- other oloum cau^ied out-nf 
bcva imagM, which apprarinl a.'s fringtvi mund tlm nb jec-t . Tlii.twns 
*haC wan known bh Mr r<tnilnaty tfciii'lnrii tprplriiin. 

In like inannrr, it Ium been Known tlint it wns not [KiK^ble in tlw 
flint and crown achrumntic tn cnmbinr two colours in all the xoncM 

' Chapter L 



of ihc uhjcctivi', so tliiit if Iwii mviMi colimrs am cidibinwl iji llio in- 
tcrmvdmtc xodo, tlicy will nut iio ciimbinwl in th« penphvnd iind 
thft cmtnd iKtrtioiis i>f thi! (ibinctiit). 

Thme {imrnrinK^nn, it hiis Iwcn pointMl Mit,' uriae from vhat i* 
Icniiwn ns the irrationaiily of the upectrum. To «>rrocil this «'c luvo 
stK-n thiit Dm, Ahlie, Schntl, luicl Zf !-■« directed t]i«Lr utuntiim to 
t)iui]evi3iii;( uf vitrMiufl LinnjMuniLi which xliould have tb^irdisjwrsiv« 
puwi^Ri iir"]">rlioiiiil t»i their n-fracti** uidioeB for tlie vuriuui ]>artf( 
of Uie s])fttruiri. Only by th™e meaiiB could llie outHtuubng errors 
iif nchrumutUm tie ci>rrect«l. 

It is tiiei-efore » fact iJuit tlie old flint and crown objective^ 
whether (or the miuruscope, Uw wiescope, or the photograpliie 
«ftmenL, f>r«, Mi'ictly HpeakinKi neither n«hroni«tic nor nplAiiAlic. 

OUiaa whoM properties far more nearly approsiniaied the lheo< 
i'etie«l requirements than any previously attainable having* been 
nianufai.-tut«d by the Jena opticians,* Abbe was able to produce 
objectiveti entirely cleansed of the aMowAiry tp^trunx. Froni calcu- 
lations of a most elaborate and exhaustive kind made by Dr. Abbey 
objectives ara made by Zeiss which not only combine three par1« of 
the spectrum inv^tciwl of two, as formerly, but which ara also aptunalic 
for two colours inst^^ad of for one. This higher stage of achromatism 
Abbo has called apncJirt/nuilism. 

A ecncml plan of the construction of an ajxtclironiatic objective 
as mmlc by Zpbw is shown in lig. 378, which, it will hn understood, is 
dinj^immntic, but MilKciently nlostrotcsthe clnbomt«correi:tionsbr 
which the perfect rwiult* giTcn Ijy these objectives arc occomplidied. 
But, in addition to their form of construction and the^>ecial optical 
glass of which they are compoeed, it is nov known that they owe 
much of their high quality to the use of 6uorit« lenses amnncxt tlie 
combination. Kluorite is a. mineral which has lower refractive and 
dispersive indices than any glam tJiat has yet 
been composed, and therefore, by it« introduction, 
the nptician can rt^lnco the spherical and chro- 
matic altermtiou.s giwatly Itelow that reached by 
nchmniatic <?oin hi nations of the known type. 

It is a sonii'wliat depre.uintt fact that fluorita 
is tery diHiciiIt to priicure in thr clear condition 
needful for tin? optician, hut from what we have 
seen the optician can do in thu manufacture of 
glass, we may hi>iK! tliat an ni|uivn!rnt of this 
mineral in all optiuu.1 qualitiL-n may )>« di.icovemd. 
Tlie medium for mounting and immersion 
coiilAct h:Ls, of i;ourse, to be of a correitpun<)inf( rrfnictive and 
tlitpenire index in all iibjectivea uf great aperture, anil it is insisted 
by Abbe that llie ft''^'''' <^ which the movnt is made, both slip and 
cover, muHt, when the limit of refraction by crown glass is passed 
by the objective, be of flint glasa. Tliishepres«tl«asa«in«7»<1 non 
in the case of the new objective juat made by the house of Zeiss, 
and a specimen of which liaa been generously given by the Finn 
to the Koyal Microscopical Society. Tliia glass has a numerioal 

' CUni'lcr I. • Chapter IL 



Aperture of X"^^; it is too cnrly ki oriticUe it« qua1iti*n, Init in 
* &ulMe(|Uent diapUir on thft pnsu-nt tHaUt tif our kiiourlMt(^ ajt Ui 
the ultiniKtv MmcUire of dUtoms, wu are enabled to prvM-ul t}i« 
results of Mtne of the pbotn-iniom;{nipb!> produced b)' ila u)(!un!>. 
Bnt it 11W7 bn noted that reiy miiuh will dopiMid upun tlie N.A. of 
i)i« illuminnlinfr cone wliich am bo irmployed vitb i( — not tbeuretj- 
cally. but prneticidly. 

On ihr wlwlts aud (or itie purjHBes of pr«clicftl bioloffical iiiven- 
tigHtinn, it in to tbe dr}- a[)ocIir(>nmtics that we are mcol indebted, 
Mill from their use we sb;itl derive the larKe«t benefit. 

As nw subject is really of mon- iiajMirtaiice than a cleftr under- 
(ilanditig of tlie dilTerence nf atMiuo of ehroinatic, acliroraatic, and 
Apochromatifi lenses, we venture to preoenl a diagram malic illustnt- 

/. J8. 3. 


tioa, whi«h, while not strictly nccurn1(\ will rorry witii it no erroTi 
M D po)>ular ilhistmion of this importAnt subject. 

In tig. *.'T7, 1, 3, 3, we have rcprr^jcntatioiis, as truly «« they can 
bn drawn, ct sowt of tqutU Hf/hl ; that is to sny, the peripheral u>ne 
vill tmnnnit an amount of light, ct|iibI to that gfircn either by the 
iatarmediato xnne or tho central rirclc. Let them therefore be 
called *ipiilufrnt mnos. 

If we assign a numerical value for the visual int«nfiity of the 
«boU: apectnim, sny 100, made up of tho following parte, viz. 

It«d IG 

Oranfie jellow 40 

Yvllow green 30 

Bine IB 

then if in any one nf the equitucent zonea the whnln .^Mwtrum ia 
brought toa focus, we shall har<>for that xone 100 aJiitni-flWtivit value. 

But tin: imtire objvet'glaiei is divided, an in thrt diikgram, into 
Ihr** c(]utluo«'nt sonea ; con»equ«ntJy 300 will reprt'-w^nt tin- i-nluo of 
the whole lens, provided the whole M the spectrum ia brouf^ht to the 
tame fiicus. 

Ity referrinx to the diaKranw we aee that in a nnu-iu-hraniatio 
lens (lig. 277, 3) we nhati t-et only 40, beeauae only onu part of tb« 
.Ifcotrum is brought to (he focus tn tttt intermetliatv seme ; and aa 
~ aberrntiun cauaeti tti« light which {MUi»ea tltmugh tlie other 


zones Ui lie broujilil to other foci, Uiey for uU [imctioal pui'pi»«'s might 
be NlOpped ouL 

III wie ncbmuuitk leiut wa fame {(!«. 277, 1) in iJie intennedut 
wme two purtB of the spectruiu fumbiiied, aa 40 + 30 = 70, and cia»| 
in tutub of Ui« oilier lonea is rtlno hromjht lo l/if eanut fof.u$, say 
io iJie outer tone, nod iO in the wnli-e circle. Tlie retinll is tlia 
the whole ai-hi-oiunlic lena ^ves n totui of li^lit on Ute principle Ktat 
above of 30 + '0 + 40 = 140, In the *pochromntic systvoi, how 
ever (fi]^. 277, 2) we find in the intermediate »)tie tltreo purt* of tli4 
Hpectruin united; that i§ toaay, 40 + 30 + 15 = t*d; nndtwo in mcli' 
of the others, say, 40 + 30 = TO. Tliuii no apochromntic ol>ji.'vti\-« 
will tpvo 70 + K"i + 70 = 22n. 

liecAlling the suppotiitiotiii we liav« made for the purpnw <rf thi 
graphic prtMcnUtion of u difficult subject, it will Iw smin Umtiinon 
HchroDiatJc objwtivo would give 40, an achmmatic 140, and 
iiporhroniiLtic '225, out of a possible total of 301). 

This iliiistrntion uiigbt be exceeded in scvrrc accunkcy, 
scai-ocly in simplicity, nnd it sufficiently explnins fmni thin [luinl i 
view nlmip the vast gnin i>f the npocliriHnntic i.y-sl<^n. 

ft is intermting to noln that, while tin* inicniscopc in iU twrlier 
fonii t4H>k il« powerful position by l)i>riviwint.r iicliniiiiati.ini from the 
tclcsfiipi', it hiiK niiw ti-il th<" wiiy to thuaixM-hninmtiMtt !itat«, which 
without dimbt it will In- tho wurk uf thf' npticiun in cunstructinjp^— 
tht! totRSCDpi! uf the ininifdiiitf future tii fi>ll»w. flH 

We Wdulil bi'K the iimdcr to l)ciir iti mind in tlie purcliam ol^^ 
i)bjectivfti tliat, wiiilst the »itreoU(i coinjiouiuis with which Abbe's- 
iHuiutiful objpctivr* arc constructed are now accessible to nil "(iticijuw, 
utul whilxt without thes«> Atjbe's objectives could ni-vrr lutvw Itecn 
constructiMl, yot il itors not l/ff nn/f nuwiiM /oUow thai hecitum UN 
objrelivr- i* M*nE ji-Uh the Ablx^-JSi'lurtt ^liiw it u ihfrrfor* IWD- 
chtwnatie ; iJte teeonAirj/nprftniui unml bTrmiivd, tind ih'. /ipher^ao- 
chrojnatie aberration haianerit, or it. is ' iipiK'hTi>miiti<- ' i>nly by nt(»> 
nonier. It im nnotJier ftwture of thrsr nbjw^livrs, whi(-h it in iniport~ 
nnt t4> not4!, that tliey an- sa cuii»ti-nctnl thnt the u]>[>fr f<H?iil piiints 
of nil the obiectivwi lii- in unr jJaru: Neiw om thf lower fociil piitntA 
of tlic eye-piPCffs nrtf also in omi plnin-, it follnws that, wliatever eye- 
piece or whuu-vcr objeolive is UKed, tho optical tubo-leniiCth will 
remain lli« ruune. 

Prcrfewor AbU' liiui fouiiil ' tlmt in llie wiile.iijM-rLure nbjectii-e 
of liij-h powfr tliM* Is nn iiuLNtamliiig prror wbicti lliere are no- 
mean a of removing in the nbjiMTiive alone, but, *» we have already 
explttiiieil, thia in left l*i l>e InLlancnl by an (ivrr-forrecled eye-piece. 
As thia peculiarity pertuin.H utdy to (lie liigher pi>wei-s, a cumtspond- 
ing eiTor liiul t*> be intentiountly intnxluced into tJie lower powers in 
ortler that the smne over-curreclwl eye-pieces niijcht be aviulablo for 
UMi with them. 

It api)eui-!> worthy of not« in this relatiini tliat one of th« best 
fomiB for the I'ombinution of three lenxeH i« tlmt known as StainhwI's- 
formulii, which connislB of a biconvex lens em-mted in two concavo- 
convex lenses. It will be oboen ed by reference to tli« tlgure iUustrM- 

■ ChBptot II. 


iag the Apochnwualic leoH wiiinirufliou (6g. 276) Uul tliis >» iarofly 
BMid* hm uf. In ixKUe iiiaUliu.'vs the eiiva&iTi;; leiusKi jakhiuu miHicM-nt 
dnuity with r«]|pird U> tii« c^iiinU bi-i?(iiiv«x l(-n.i to alUiKvthcr ovnr- 
piwcr it, the rt-sult lieuiu il M-cowvos triple with .t n<-s»tiv« ftwus. 

It is nnntber diatiiicti\-e ftwturv wf lh« 3 mm. obj<*tivi: timl it 
hftK 1 iriphx Jrwtt ; tlius Ze'tm'a 3 min. (= J-iiich f»t'U!>) )i2li iIh! 
rrrors fniin ihrae anmrrectecl l«naee balwii^e<t \iy two tri|>l<> InicIcs, 
i.e. itifMt IcRMo taken tiit^her. 

Hie foci of tJte set of a)KH-)ironmtie lens«e now mnUe hy Zrian 
mn integral diviiuoiui >if wIihL intty be t«-nned it unit leiui of 2i mm. ; 
21 ho i'hoia«a lui u mwniH »t avoiding tlip iiiconventenctut iniie]nruble 
ffiim tiir UM of the d«'inml wvHteni.' Tliei unit lena ia tliervfore * 
liltln lii^her than 1 im-h in p»w«r, I ti the series of dry lenaw tliere 
AM two jMiwei-)* iif lh«' 8&me itj>erlure. Thu» 24 iiiiii. uml IG mm., 
comrapiiudiiiK ■•> Eiigli^ 1 inch and ^ ini-h, each hiui an uiierturi! 
of -3 ; a 12 mm. and 8 inm. = Englifih \ inch, and j incli, hio'e 
«ttch itn npn-ture uf -S5 : while a C mm. ntid a 4 uuii. = } inch aiul 
} inch, have both an nperture uf 'D5. 

Tliepp lire iiLto wal«r-immeii>ions ; a 2'<>inm. = ^>g ineh, with 
K.A., 1'^'%mmI two oil-unnierHions re8pectiv«<ly 3mni. and 2 mm. 
M I inch «im1 ^ inch, both b«tng made either with I'S ur 
1-4 N.A. 

Apnrt from tlwse, intendMl tn be used for photo)^|ih)(r ]>ur- 
fimtn without an eye-piece, in a 70 mm. = a Siiich, alM> a 3I> mm. 
«r 1 li-inch objective. 

With the excyrptUm of the fi mm., 4 mm., and 2'5nim. obji»ti\-ft« 
whirh hHVpthpBcrew-coUiiradiusliiient, thin seriea have HxidmountH, 
Giim-rtion twiiii; M-cured bv alleiuttou of the tulieleii^h. 

Tbi- {wrfoniiani-r of th««« leiiMBi, aa th«y are nnw mititi', ii i>f tlia 
\rty hiRhi'Ht <)nlcr. Tlisy praaent U* the most experii-nt-ed evn unsur- 
unii^fen. They are dim-cl^d with a delicate jM^rft-t-tion which 
ly thin syntt-ni, oi>u|)l«<t with technical exeoulton <i( the lir^t onler, 
out piCBtbly lie made tii piVNiuoe. Tlie uptica) pulinh, the eeutriiiKi 
the nrttin)(, and the bmnswork cHrijunly liave nerei' )k!('ii surjuuved. 

It i.i a matter also wnrtliy of note t)iat Zeina's a|ii>chr<nuatic 
Krint nf objeclixtfi fire true In tAfir tle*i;/iiutiotui an jniufra. The 
{■inch iH KUi-ti, and not a ,*q-inch deni^tnated }-inch. Thin wan 
«]a«lty true of the wiriy a^hnnnatics. A, Row* produced a J -inch 
under that name. One n.iw liefttra os, ma«le fifty years aao. has nii 
initial jxiwer of 41 : and that ti a {-inch hua an initial jxiwer of 21. 
But modern achromatios of fair apertui« are always j^reatly in 
ncees «< iheir designated povrer ; J ar* nearly J-inch. A J-Jneh 
over 40* has an initial power tA 35, and ia a i*,-inch ; ,^^-inch 
Dt]JeotJv«s are in reality j-iiich ; and {-inch objectives of 90" and 
n|nranU have initial powers of 30 inatend of 40, which thoy should 
uve, BO that tbey are in reality |thB : some in fact^ — l>y ni> nicnnH 
uneuuunou — have an initial power of 00, and ara actually ^th-incb 

1 Allliooch llu loci ot Um ItrniM va vxpi««*ed in inl«ic*Ti>. willi Uid ttngla •»»■ 
lion «J Uia wakr InuBenhm S't mm., tlm* an ItuMDronient dccinid fnotimis in tna 
ioHial n^pB^^iag poim «t all Um Miiw asMfil thv** or 1-S uid S imu. Ukua, 


This is explicabto enougli from the niAkcr'G mint iii vipw ; it is 
far ensier to put poui«r into an objecl-gla§s tJiaH apurturr. It ia 
easi«r to make a ^-inch of 100" thao a \ with 100° ; tho rtisuU is 
that low powers with suitnbly wide apei-turc4 am ■.■cixily. 

In the Zeiss apochroinntic sArics of ol>jci!tivt« tho 24 mm. of '3 
N.A. and la mill, iif S-'i N.A. may be conniilcrt-d lu teniH-it of the 
very high^-t iirdci- ; ihf- n-lntiuu of their npnrtun- tti iboir povnir is 
such that cvrrj-thini; wliich a. kcon und imincul cyi; li capiibh- of 
tnkins cogniKoncc <if u 7VMtlmti when the ahjeedve it t/itlding a 
vuigi\>fii:n(ii>n rqual to tuxlw. limm iu initial patixr ; for tliia purjxieo 
an cbjcctivc iniisl hnvc 0'26 N.A. for cuch hundre<l OianwttiRi of 
wa'fiin^d luiignilinvtion. UniliT thciw condition!! wi object in Hcen 
in the niorit iwi-foct mjinner pnt.iih)e. 

It may tic well fur the nludrnt to prove lJilS| which muy ba 
rpatlily [tone, 

1'iiko ii Huilidilv ulijpct, Kuch u« « w^ll-nri'jiart^d [>rub(HC'is of 
a. blow-fly, nnd cxaniini' it under critical illuiuinutiuii with tlie 
24 iDm. '3 N.A. (= 1 inch) iibji>elivi% and a \i comjieDitalinK vye- 
ptccc. Note with fliiMi iiltt?nti<in rvury [uirliculur of the immtpn 
tltd rcMilutian of the jiuinu of the minute luuj's, the form of th^ 
odg^s of the uut Hucttiriiil tuhi-H, the ttxtent of llie surfaoe takeu iut<> 
till' ' lit^ld,' uud the leUtiun of ull llit> [urtsto the whole. 

Now change the objective for tJie 16 mm. -3 N.A. (= ](, but 
with the Name apertuiv). Nothinji more is to be seen ; the oiost 
dexlei-oua maai]>uUliiJn cmuioL brin|{ out u siutjle froJi detail ; the 
KMilutioii 13 ill uo eeiiae carried farther ; thecut auotorial tubes were 
in fact, in uur judxinmt, Ix'ttvr .-u'ctn with a lownr power, while with 
it nil of vuurse a smaller (-xtent of the object occupies the 'lidd.' 

Itnaniu fact be wjircely doubtMl thnt the picture preaented 
by thu j in a tlistiiiL't i-etrogrtvwion in every jti-iuie comp«re«l with 
ihpit presented by the 1 inch wli<tn boih are e(|ii4dly well imule 
nnd li;i,\'e equal )L]>er(uri-s, viz. ■."(. Hut Ijt'yond ail lllis, idutteper 
maif he done by the I'i uiin. -'A N.A. oan be aocompUebed in aa 
(xjually MitiafiiL'Uity iiiiiiiuer by reoioving tlie 1^ ey«-piece aud ro- 
placing it will) piTuctically no oilun- ult^rratioo by lui ISeye-pieooj 
and HlUI hi^jber ixMulm cjui be obtaiiK^I without the HlighCest detri- 
ineut to the iuitif^e by UoinK an eye-piece of 27. 

Nut leiH iutere^lin^ and L-uuvincinK will it lie to examine ths 
MUiie object with a I'J itiin. '65 N.A. (= ^-luch), and an A Z«8> 
ai-liromatio of 'I'O N.A. (= jrda inch) uuing a 12 eye-piece. How 
who may still retain Home conviction an to the lalue uf ' low-angled 
f;1asse« to secure penetration ' can want no further evidenoe toaii 
auth a simple exporiineiit afforda of its entire fallacy. 

For those who prefer it a true hiHbolof;i(iaI object may be sclochsd. 
We diooee a portion of a frog's bladder treated with nitrate of 
bilver, in which are some convoluted ventieln, enclosed in a masButar 
sheatb which had contracted. 

This object is presented by photo- microfn^ph in figs. 7 and 8 of 
the frontispiece. In fig. T the vessel in the fnig'ti bladder is seen 
by a Zeiss A "J N.A., niagnJIied 140 diameters. The object of tha 
photograph is to expose the fallacy which underlies tb« gcn«nJ]f 


Mxwpt«d slateiueiil tliat low-aiigl«d gUasm are the auiat aui(A))l« 
fur hwcokifcicij purpoaes. The aasumptioii U fuutidMt <ui tlie fact 
thmt the penetTAtiou of a leim varies inversely «» its apt^rture, uott 
it IN Mken for grMited that ' depth of fucu§ ' will bo oblainetj, u<>t 
to be »ecur«(l by i»rf^ apertures, and iJierefure it is taketi tor 
^nuitMl tbut we are eiiablra to see iiiln the structure of tuques. 

tn exniiiiiiiii); the itluBtration (which will with udvantJipe penult 
the live «f a leus) it will be seen that scan^ely an endotheliuiu cull 
oan be clearly seen. A sharp outline is iiuwhefe mauifest, because 
ihe iRuaK^ of o4i« cell is cnnfuwxl wiih the outlines of otheni upon 
which it is superposed. We have soon that there is no perspective 
fniper iti a niicroecopic imngv* ; tlierefore it is l>elter to use hiffh 
■Mrtureii iu objectives, and obtain a clear view of one plane at oue 
tiaie AUtl train the mind to apprrciiLte perspective by means of focal 

It will be admitteil that no clear idea of what an endothelium 
eeU in cat) be obtained from tig. 7. 

But flg. 8 (frontispiece) rrpiwcnts tlie same stnicturc slightly 
less tnaipiified (x I!!**) by nif-nns of an apochinmatic 1 S.A. '€5. 
H«t« tiidy the ii)>per sarfatM; of the tabe is seen : hut the endothe- 
litttu cells can bo clearly tniccd, and a sharp detiiiitioa is given to 
erery oell. The circular clastic tissoe is also ^liAplayed, while the 
whole ima^ has an incinuod ftbarpnciw mid perfection. 

Thus, with the objective (A -30 N.A. = -{rds inch) of lower 
aperture the endothelium ceJIit raw fi" wwh ; but when the imiwo is 
compared with that of the objective of wi<ler aperture ('65 N.A.), 
tke former image is found to be dim and ill-denned. The miiscuhir 
Acath is so ill-defined tJiut it woidd not be noticed at all if it hod 
nit heen clearly revealed by the objecti%'c of wider aperture. But, 
Ml the other hand, the objective of greater aperture not only shows 
llie muMTuUr sheath, but it also shows the elongated nuclei of the 
nnscle eellx ; and attho siuue time bringsout the convoluted vowels 
lying in the muf«uli>r shi-nth as plainly as if it wcit' an object of 
Rfhcient dimensions tn lie upon llie tal>1e appealing to the unaided 

We have (xiinted out in the proper place,' that although ' pi-nc- 
Inting power' vnritM inversely as the nuineric*! aperture, it alxu 
nrim in»fr«ely as tli<T nquari- of th" p<'wer. 

Now, frikin what we knnw of hixUilogical t^wchin^ in tliLKCoinitry, 
we do not lMu--itat« to xny that a hixtolngist would not have attempted 
lo examine tlin al>orc objot-t with oven a ZcLw A olijrctive. Hr 
would have advised tli« um- of ' (Ae J-inch,' of pcrhn[M '6^1 ajwrture ; 
bat by mi doing he would hnve Kccured only one-third of the pctnc- 
Iratingpowi-ri^iuf npcrtumand ono-mvcnth of the jienctratiu^' pow«r 
fuS jKiwer. 

It is ninnifint, then, that pursuing this cuurne in the hintJUogtcal 
Iftboralory ilcfeati tlie I'lid Koii^fht, and which it is so desirable 
to atUtin. 

It is abwiluteJy unwise to i»i> a higher pu%otr tluiD ia needful. 
A J-inch where a ^-inch would answer in^'olvca Ima Ju maoy ways, 

1 Clutpter I. 


and woiilfl never ,1k' rnMirted to yf lUr. njmrlurt of thf. Iftvitt rmfintftd 
tivrf (I* i/fvat n* Otr. /niicn" Htnl ti-yitimnUlij prrmiUfL ' 

A givm «tructiirR fa> 1m: HKvn At nil inu>t biivo a ^iv«u up«!rtun' ; 
U) obtuin this, n» oliji'CtivcN now niiuli- fur titbfiraturjr purpuat^ mil, 
tlley arc otiligLfl t4i USA liui hii/h n juhmt. TKi^ mnilt is tJutl in se«k- 
ing tn B%X)i<) wluit ix ncitiiuntnl tli<- Iiikh iti ' pi-n«tnttiu(; piivrcr ' ill 
jin invent)! mtio In thi* aji^i-tun!, it is Eorgott^n lluit wo am liAing it 
invcmcly lu thn njunrr of th« powwr ! 

.Miii-oivcr, the two ik|H>ehr»mulic iibjnvtiveii we h»ve alrandy 
refn-ml to ns ir^\ li-nsus uii! Mjuiilly uliln tn ahow Uie vulue <>f 
jipiH-hiiiinutiiui, nut mi luucli mi ac'count uf lh« i«in<ivul nf tbi! 
nroniiilnry ^jwcli-uin iia fur the rt«lucLiua of Uie nberratioiiK Ocpelul- 
t-nt on tbe irriitiiinality of Um; Hi>eotru]u In orOinarv ucbt'i>inMics. 

Usp (lie 12 mm. -GA S.A. objective, PUoe h diAtoui in ImIhuh 
in lilt) fcicuc of it un ii <iark k'*""')'! ; ibi* diJituni will aUine with a 
idlvni'y wliitvntws, an<l lh« linage will be wboUy free from ttifi,. 

Nnw takv line nt ilif beat uchroiuatica obtainable of Jiiu^b Erwus 
<if tiO° (almuHt i'<-i'tHtnly il ,^„ tn puvrer) a.nA examiue the »ani<! 
4liiit<idi m the Huitin I'ircuniatuni-ea; it will lie bathed in fui;. If, 
liowever, the uchronialic objective la &ii exceptionally good one, lutd 
wt- i-educe itAapertuiv to 60^, we shall get a fair picture of tlin 
dintuni — on« indeed that wn« con^dered criticul antil that with tli« 
apoeliromatic was w«ii. But in comparison it< is dull and j'ellowifib. 
From which it follows that an exceptionally tine achi'oniatii- /„- 
int-b of 60" or '& N. A. will not snffer comparison of the image it jrields 
witli that of HU ftpoclu-omatic |-inch of -65 N.A. 

Speaking genemlly on tlie whole (juestion, th«n, it wouW be tlie 
utniOHl folly fur hisliilogiMa or opticiauit lo abut tti«ir eyea in tJi« 
umt(Tiiticent cluu'actei- of llie iieriee of dry apovbi-oniatios of TUAm, 
Hinging fn-m 1 inch (■24 mm.) to J-incb (4 mm, -95 N.A.)tUid mora 
recently ^-inch. Thev are the moot perfect and efficient Berif« iif 
nbjectives ever placed in the hnudu of the worker ; and unlesii 
Eriglitth lensc« ou a, truly npochromalic pi'inciple and equal quality 
are produced, it inii«t l>e to the detriment of either the opticians 
or the workers of this country, 

Nor need it be stippnsed that the pnxluction of objective^ 
approximnte to these must l>o conllif ; great steps bave been taken 
lately in the reduction of their cost. A remarkable instaitce of tht» 
is proviiled by the prudoction of two objectives by !i. Leitx, of 
rtermany ; they have lately cnnie into our hands ; they are but 
semi-npocliromiitic. The one is low, having an initial pnwcr of H, 
with an Apprtnn-of W ; the other is practically a j-ineh of 'W^ N.A. 
The low power 1im turpusod every achroniatic of its kind we hn*-e met 
with, nntl the higher power can, without hesitancy, lie spoken nf as on 
«xceediii|,'lyp«>'lgln5.s: nevertheleJU the price of these two objectivra 
i> tnij/lhrr Ifss tlmu the price chat^;ed for tlie lower power, if made 
in Kn^tnnd on nehronuUic principles would eertninlr be! Yat 
R«ichi'rt has k\m\ surpassed thi*, and we feel that we shall bodung 
a grrut iirrvicR to stuflents of Kinall means in calling their attention 
to tlic following rvmarkable and low-priced objectives : Lcitx No. S 

> Clu(>iiitU. 



m) N. A. -2 : LeiU Ko. 3 (Ims th&n iirds) N.A. -26 ; Rei(^<«n No. G 
<ith) N.A. -81 : BeichHTt IH'' (,'slh) N.A. 1-24. The IWcIiert 
Nn, 6 is a Ivbm whose low price ia astoiii^iing, whcv tlie ]i«rf<'ctJoii 
of tW performanoe of alt rliat we have seen is taken into iiccountf 
for it ia the riviU of even tme HpoohnnuAtJa^ This fact in of iixi' 
portance to the medical stiidetit. ciiiil to the nptioiann (^nerallf. Ity 
ApochiMnatiHd objectives of the highest ortkr tJie work <>f )>r«Mnt 
aodfnture tuicroscopy will liodonc — that iji iuoi-itahle. To thoroughly 
ttadtntuiid wliat it« very l>p«t rrKult«, l-heorctionll}' and prnoliciuly, 
niost be liMxiaDee the inipomtivo aim of th>^ optician who wuuld 1)r 
■Abcg M t of tlie direct wnntA of his tinto : and to produce thi? neanst 
to these in nhjcctiveg and eye-piecos nt th<- lowc^ poai^ilile price 
is, ftpni-t from all other ixKUCS, to bo n dinvt 1>onrfAotor of true 

Tic Eje-pieoe. — ThewjT-pii^p, sometirowcallwl the ixrulnr, isoii 
optiod combination, thr purpiwi- of which is w to n-frnct the diver- 
ging penciU of rajnt which form th« no] object -imnt^- tliat tliey may 
i»ll urire nt tbi- pupil of the obwrvvr's oye^ Tlwy )uiv<( also t" furui 
It Tirtdal image of the md imnfiie whieh ia pretentcd to them a« ihd 
olijnH. F<ir thin purpone n <«iiil>iiintii>ii in indinpirnxable, but this 
may Iv VBric<!. Tnt-ns are ordinary aiul apccinl ryt^-pirycs. Those 
in 'irdiiuiry one xepunte into two divisiona ; (1) tirixiiivr< cyc-pieciw 
Kud (2) nefpttive ryi:-pienii. Thne am nuuly diiitiii;^uishcd : with 
■ amliif rji'.ytitvtr wet «ui obcaiii a virtual ti/nifff of an object by 
UNing it u a iim))lu micittfcope, becauan itx focas ia (■xtcrior to itself. 
This is not Msrwith thoi*^iifitirrw<kii»^iy,bncAUi(iMt«f(>cas is within 

The fytf-j»et» ia comrann msh t* motive, and i« genorally known 
as Unyghens', and wMnrtimoM ns fJnmpnni'a. Hiioki; npppjint to have 
hmta llie Gnt (ItlSr)) to havn nppHiHl thu Hi'ld-htn.t t« thn •■yr:-t«ns of 
the mwruaeope, although them ii a high prulmbility that Mich n Ipdh 
wss firat owa by Monconya ; hut how fur hv was indc-litird for this 
to thi; i^impnuna cye-picce attributed tu Huyghcna cannot now bo 
deter niinpd. 

Tltiii inxtrumnnt im coinmonly lued oonaiatx of an rytr-Ifni and a 
ficld-loiis,eiu'h beting plano-convex, havinK their con vex aidc-H towards 
the object, their foci being in the ratio of 3 : 1, un<i tho distance 
between them being iijual to half the buiu of their focal lengths, a 
diaphnigm lioing placed in the tocua of the eye-lens. Tlie gencml 
form of a Huyglicnian eyepiece ia bliown in longitudinal oeotion in 
fig. 27 S. Tbia mak<« * rery convenient form of eye-piei;e of i> and 

10 magnifying power ; but when the power much exceeds this last 
anuuiit, llie eje-lena beoometi of <leep curvature and short focus, so 
tlakt the oye mutt be placed uncomfortabljr near the eye-Ieii& Tlaa, 
however, is ita chief defect, and it may tairly bo considered the best 
anlinanr eye- piece. 

Pflrhapa toe beat form of tlie Huyghenian eye-pioco is that do- 
vised by Btr O. B. Airy : its field-glaKs in a Dieniacna with nutii 

11 : 4 the convexity towards the objcct-glaw. The eyp-lpna is a 
crossed eoavex 6 : 1 the flatter side to tJic eye, the iliatiince Ixt- 
(ween tbem b«og twice the focus of the eye-lens, focus of lield- 

323 onJEcrn'ES, eye-pieces, the apebtosieter 

]fn* being three times thnt of Ibe trjt-ha&, Uic diaphragm being in 


Auotber neifative eye-pincr- h tlmt kiiowu as tho AVV/mw, or 
t/rlhoicopic. ThiscoDBisUnfa bi-euiivex Geld-^laas.and an ttchronmtic. 
doublet meniscus (bi-convox «jid bi-concave) eye-l^iw. A vcrticul 
se<:ti<in of ono so coDstructeil in seen in tig. 279, Them eyc-picow 
ii.-iUHUy magnify ten times, niid tlie advanttt»« tbey *Te suppom to 
givecimhintsina large IJi^Id of view ; liut tJiey arenot goo<l inpmctioe 
for thia very rwson, tUey take in n field of view grcAter thnn thi- 

Fm. ars— ""yB'"""!"" *j*i'i<.<-' 

Fin. 2711. — Kflliioc oju.jiiece. 

(ibjertivecnn stund, and iLsa ml«^ c\-ciithc wMitrnof the fipld will not 
Ixnr compBrifion in shaTpiiPx> with tiio Iiuy<;)ieniHn form. 

Tt is a miggestion of Mr, Nelson's that a crossed convex 6 : 1 
lipld-lens and n ineniNL-ua and concnve'Tonvex doublet eytvlensinigbt 
work well for this ftirni <if cyvpieco. 

Positive Eye-pieces. — In tlie enrly compound micrasoopes tbe- 

rye-iiieoes wr-ro a!l positive ; timt is to say, tliey cnn»ist«dof a single 

bi-convex cyp inns iind ;ici lic-t<l-g]n»i, 'Flic driinitinn with thismuf* 

Imvit iK'cn inop-t imfwrfcct ; the nildition of n 

tt _____ V Ht'ld-li'ivs though it wcif n. bicdnvex, not in 

9'' ^ till! corn-ct rntin of funis m>T the theoretically 

1/ ..^1 IwHt dixianci-, must hnvc been cnn^dered « 

If a itntiii atWance. 

I -.. ,-■■ I In this wiiy mattirrs restrd, huwever, until 

l^,,,^^ ^ " " _ ,,-rj III'-' llnK>r(-ti Willy ])frfuct Hiivk'heninn form was 

V. 9 di_'vis(^l. Nothing hiu yet displiured this com- 

Ftu, -iMi. binutioti or >iuccc«ifully altirri'd its fonnulu- 

Object' gitixxns have bennuBrvl ns cyi^pioccK and 
■II tonux oi loiipf orniiuple mii'roscopic ItuiGCH Imve \>cen employed 
for the same purpose. Solttl eye.]aecoK liave also liccn Mscd tiotli 
in England and Ameri(<a, but with no results Hint sur|iii.-»i!d a well- 
made Buynbeiiijin oouiliinatton ; imt tlie liest form of all of the 
combinations which liave heeii tried by us us piiwtivf iiinf;l<^ ey«- 
pieces are the Steinheil Irifttr ion/is; a section of one of thetw i* 

' II li a curioun li-ft Ihftt in pncllce tlic dkuoJ fnnnulB It/r Ilii> Uui(lii>iiiiui ry- 
piws i* radiu* d A*IdJana iwiei- timt ol rjv.lvui^ uid th> dittwioe bMvcen tbcm 
tujiul M lull tlM •urn ol tbvir foci. 



asen En Hg. 280. But a positive eye-piece wax duvited by RAiniid«ii, 
conaistiiig of two plano-cuavex leiisea ut tuyial foci ; tlie dliitanoc is to 
lie equal to twoltiiitla the foeal 1^11^1)) of one. Tlie diuplintgm will 
of courep W exterior, 

Abbfl'i Compensating Eye-pieoea. — We bave already given n 
gmerd 'ifvrifition of tlie unture and Hotion in oonn«wtion with tho 
apocIirinn.itiiT oly'ectives of thi« foiiii of eye-piece.' In the spctioti 
above on oltjectives we have referred to the fact tliat these eyc-pincni 
are mirr-nirrfftrH ; this may I>6 easily seen liy ol>»erviun tlie colour 
at the ed^ of the diaplirasin, which ia an oran^ yellow. If wn 
(•inipnrc this with thei'olouriii the same position viitli a lluyK'i^niaii 
ej^pieco, this will be blue, Ijeinf; seen tbrough tliesiiuple uncorrectn) 

Thor? are three kinds of conipensating eye-piece as deiifjued by 
Abbe. Tli«EA are 

1. Searcher eye- pieces 

2. Working „ 

3. Projection „ 

1. Tltf amrcAer forms are negatiiea of very low power, intended 
only for the purpose of linding an object ; they consist of a single 
fteld-lADS and a doublet eyedens. 

7%« Hx/rtim/ forms are Ijoth positive «nd negative. Tlio 4 power 
U a negative form leaembling the searchrr in that for tho shoi-t 
tube ; the eye-piece for the Ion;,' tube hns a tripint oyc-lons ; but 
the remainder, vi*. J*, It', li*, and 27, whcniirat intrttduccd, woro all 
nautirrs ; they were nubHeiiuetitly, however, changnl for negatives. 
HftTiDK used both, we are nlnA to lenm that the positiviit are l^ui}; 
again introduced. It may \>e convenient bo havo the S a negative 
like the 4, but with re^rd to tho I'J, It*, 27, it k important that 
they should be positives. 

These positive furniH are ou a totally new plan, Iwing oompond 
of a triple with a single plano-convex over it; the diaphragm i:i, 
of course, exterior to the lens. With thcAo the dcllnitiuu is of the 
fineat quality thixtughout the field ; thry prrwint thi? aitinirabln 
condition that with the deeper powers tlm |trc)ppr jiosilion of t\m eye 
is further from tlie eye-lens, which miikes it an Miay to uaa an eye- 
piece of as great a power as 18 or 'il n* »nr uf t «r 8. 

Tilt JirM of these eye-pieces lina, a* w« hplii-xe, been very wisely 
limited to five or six inches. Thcattetn^itontboi'artof EohHiJi opti- 
cians to give tooiirev«-piecr« fields rcnchinK<'ixbl'?pniiifln-'s Li luiermr. 
A microscopic objective with the luweatapi-rture had the licld K'*''^'y 
in excesa of any other optical tuttrunHint ; and to deal with mcb 
feeentric pt-ncilH oa must be engnfcixl by an eye-piece with a Held 
of pight<M'ii inches is a strain not justified by wliat ia gained. 

Thcpowerv of th" working i-yc-pieeca are also airaoged in a new 
way. l^if multiplying powfTB for tnu long tubs ore 4,8, 12, 18, 27 ; 
it will !«• wi-n at oiiets tlicn-forf, that they bear no definite ratio to 
one another, and if we seek tii Kimplify the focal lengths, we are by 
the employment of the metrical system cunfronted witli decimal 

1 CliupUr t., p. 09. 

Y Z 



froctionsi. Hat witJmut fiiitln-i- <-liiI)>iratii>u it may be weU to say 
timt li ix tin- mn*t jti-m-nilly u.ii-ful (■)-t--piece, iwid if only one com- ^ 
ppnsftting <?yi;-pi<!ce is to Iw w?lpctec1, tliti* cau be no question, frou 
n prni-tical point i>f view, }nit tlii.'« i« tlw ln-st lo employ. The 4 ig] 
toi) law, and lli<! 27 is Uiu lii^ili for jjeiieral purjioses, tiiicl the V (iiid| 
18 nri' «uAici«nlly nrar thi; 12 Ui K'vo the latter the ach-autage 
geniTwl vriirk. Wi- cnnuot. However, refmiii from the expn«&ioo ■ 
the opinion that a w.-rira of 5, 10, i!0, or 0. 12, 24 powers would 
in iiiniiy nenftM morv uaeful, iviid would offer facilities in applicatiou,] 
not secun-d by the sierii-s of Abbe now in use. 

It niny lie well ti> ^tte furtJu^i* empluisis to tlie fact that this Con-J 
struetiun of eyi- piece tA not otity t-^eutiaj to the proper work oCl 
npoelirnniatic objectives, but ttiey ^'reatly enhance the images givvnl 
by ordinary achromatic leiisea ; and it may be Dot«d that the M, H, 
and 18 eye-pieces for tlie sliort tube are identical with 12, l!^, 27 for 
the long tube. ^H 

Ilie priyectiot) eye-piece is mainly intended for photo-micro- ^| 
grapliy, but it is alsu useful for drawing and exhibition purpooM. It 
ia a ueKalive, with a siiifjle lield-lcns .ind a triple projcction-lena. Thp 
project ion -leas ia fitted with a spiral focussing arrangement in tinier 
that tlic diaphragm which limits the lield may be focnsMrd on to the 
screen or paper. The field of this eye^piece is small, but ita detinitiom { 
is cxquisil«ly sharp. 

It may not bo generally known that goofl pliolfi-micrographii > 
be obtained by projection with the ordinary compenntuig working' 
eyo-pioccs, but t1ii» ia a fact worthy of note. 

It will perhaps be of practical utility if ve append a table indi- 
cating the focus of the compensating eye-pieces when used with 
the loTi'j and the a/iort t/w/y. 

I tfas 
ition ^M 

FocM Iff Kye-pitee*/ifr tong Bad]/. 

P»wer . 

1 S 

t • 




Foont in miu. . 


rt75 3J7 




„ lacliM. 

1 •■» 

s-e 1-8 




fbpiii ir/ Sj/e-ptrMt /vr tiiirl Doit/. 

Power . 








Fooua it) miu. 








,. inclim . 








JYiJrcfuiii Eyf-pifert. -'J for short and S for long bodies«flO mm. or 3-M 
inchea ; i ioT short nnd (1 (or loBg bodies = IS inm, or 1 -71 in. 

Special Bye-pieces. — ^Tlie most imiwrtant of the»i>, the mieromtbir 
f}fe-intte, we have already considered, so far aa its appUcwtion to 
micrometry is cono-rnf^l.' Ita optical clianiotJ-r may Ije itroperly 
conudered licrp. If it is a neffative eye-piec« tlie roicrometj'r li plactKl 
ia the fociia of the eye-lens, but if a positive combination it ia placed 

' CbsptwIV. 

now TO TEST 0BIECT1\"ES 315 

in tlio focus of the cyf^-pitMip itwlf. Th<: RamMlr-ii fomi dMcrib«(l 
■bove i» thoToa^lT suiffi for this iiiiqMxr!, hut n ntrgntivn form is 
gmmdly (rtnplo^nl, thv micnuin-tiT U-mjt tilacpil« the cyr-pioc© 
in th« (liniihrngm, i.e. Uip fociis of tin? i-ji'-lt-n-i. 

In ora«r Uiat the micromctiT m«y Ins nuscrptibltt of fooiu for 
Tuioun mglita. it a nwwwiiry ihnt tli« <'ji»-l«nii in the ciue of n 
ni^tivA nyn-piooo, and the whole cy4)-»i«c» in thi? cosn iif u pnnitivv 
OTw, Khoald b(! mount^^ in n Klicliii); tulKt ; niid one with n Kpirnl xlot 
will Ik- profonblfi, Mocts it in«k<!!i the work of focussing both facile 
Mid oocnrntfi. 

If only one microinpt<>r nyc-pi^ci- i.-t UMd it should bo of medium 
jMiwirr, Kttch AK H-inch focus ; but it ix an inexpensive nnii n umrful 
pl&n tohavonn wlditioiinl set of lonsfwto screw on tu the sainn mounts 
ao Ilk to mitke the eyp-pivce, sny, • J-inch fiicuH. 

^eetrotoopic, polarising, goniometer, and binocular eye-piecea 

are Mch trr-.xttil under thi'ir res]«'i.-ti>e .lulijei^t^. 

The index eye-piece is one which hn-tapoinler jjUu-ed lit tli>- (lia- 
pbniKin, »o oonstmrti-d tliiit it can In- turned in or <iut of Iliv li<-ld, 
lUuI is WWII to point to the position of an obji-et. It is of limited 
Btility, for with tow powers it is scareely iieedtHl ; «e ciiu reiidily 
iwlicuto by description, to iinother, tim wsitiou of iin "bieot in a 
field every object of whi^li is in view. With 11 liiRb power, on tlie 
Other tuiud, it i% very tlillicult to use such 11 ' fiii;{er.' 

A hfttvr jJan K-hm Ih' magmiientian. if yftit fa U> bKVe n ttrift 
q/ iiiaiJtra'/m» o/ilt^rrfnt nfrtuns to drop iuU) the eye-piwe and 
uimint>'h tbo field of view. Thia not only makes the object to tie 
pnintjil out tann wisily neee-ssilile to the eye, but — iw we have by 
many ywiw of oliwrrvBtion juiived^it iii<Is in close observation upiin 
miriuti? objcctK by cutting; olT n liir;:e urtui of lijfht without ult«rinf; 
the intensity of whnt i-enuiini^ and so makes close observation more 

As it is din«ct!y asKorinti-d with the eye-piece, we shall Bnd no 
betUT pbiee to note the rurion^ imd Iiiiheito unexplained fact, that 
when rejuilvin^j striic or lini-s with oblitjue lifflit the effeL-l i» much 
stri'.njjthiTied 6y f/"^\'"j it J^icoFs nnnhjiiiuj priain i/ifr the rye- 

Teiting Objeot-glauea. — It will have been noted by the attentiro 
refMlirr that tnuny of the more inipoitJint qualities of objectives 
are determined by the princi]>lfci of their cuimt ruction, and bceomn 
in fiict ijufwtioiis simply lA the quality of the workmnnship involved 
in producing the otitical rtnd mecbaniiiil parts of the ()bjeet-(5lns.i. 

Thi" niiaiitv of the workmanstiip may be Icslpd by tj^i-hnicul 
mtun.i (lescrilK-d l«'low, and by llial bulitii power which comes wiili 
exp<!rience. TIiIh can oidy be ■inparte'l ihrtiugb the paths of lidHiui- 
and experinieiil, by whic!i in ever\- ca>* it is w-ached. But, ffi^nted 
that an object lias been iUmnimiled in an intelligent and. -latisfaetory 
manner, the fimt eoroplete view of the image (which must of course 
be a tJioruuf;hJy familiar one) will enable the expert to come to a 
conclusion as to the quality of a giien objective. The character of 
the iinuKe to the exjieri determines at onoo the character of the 
leiiii. Tliis is tJie more absolute if a series of «!ye-l>iecefl f up to the 


luiut powerful tlint can be obtiiinixl) am nt hand. Nothing t«sta 
the qunlitj- of ah obJM-livc ko uncompi'oniisuigljr jw a deep ojrv-picctb 
For lirilliatitif of image a tiiodcmtc pow«r uf eye-piece is a coam 
best ; but the capacity of thr^ objci^t -gloss is clrorty comtnonsanto 
with itA ability to endure high i-yc-piiHc-K witlioiit luss of dinrftct«r, 
Jiiirf even sharpnoiis in the iniiigr. I.'iiIcsk tlii* ol)jrpliv(! Ixi of high 
(luality, the sliarjiiipss of th" iniitgo gnuluully ilixapptiirsi iik iIk; more 
powerful pynpicHJcs arc uw<l, until lit liwt eithiu"ftll <ir jMirt of the 
image biv-nks tip into the 'nilti-n' (ItTtuiln of a oonnm lilli«Kmph. 

A Icnis fini-lv oorrrctotl (with |jtrjiiMi[)i'rtun-) will Iw^iir thti (trcTpmt 
eyp-pin'ing with no tlt>triiiii'iil. Tlu- ^1 nun. anil thi- 1'2 mm. of Zt-'iM 
will RiilTpr any i-yc-jiii-i'iiig noeeisible to the niicniSL'oiiist without tlm 
atnnllctit Kunvnclri' of tht jOtarptinai of itui iningft. Wo huvo in fiu;l in vain tn 'bn-iik lh<^ iiiiaHi''yip|iled by these ot^eclivtei 'down.' 

This miidtf of ttstiti>i i.^ '>f cimtsv lu a liirge ext«nt eubjevtivi-, or 
at InoKt ix (!o:itroll(»l by iiiouiiiinuiiii-'iible judgments. It i^ nuixl 
iinjiiirUint iht^n^forit Ut hiivv a niiide of judgment thai nhall be u<.-c<b- 
nihle U> UiL' bf^inniT and the intei-esled atnateur. Dr. Abbe has 
piiipiLii^d a nit^tliod which la at leuHt aroeHsible to all. 

In ordinary pnu.-tiye micrcMcope objectirm, if teitted at all by 
their puaHetHorn, are simply subjected to a companion of perform- 
auce with other letisea t<rie<l upon the uinie * teat ubjei-ts.' 

Tho reliiiive excellence of the iina^^ seen thi'ough eadi Jens luay, 
however, depend in a gi'eat part upon fortunate illumination, aiid 
DOC a little u])Oii the expeneiioo Hn<I iiuiiiipulative hliill of the ub- 
fierver: Ix-sides whith uny truBtwurlhy fwtiiuate uf the i>er£»inH«n«e 
c£ the leiin under examination inv.ili es the ci.nsidi-niiiun of s suitable 
test-object, as well an the mu^-iiifyin;- power aiid aperture of the 
objective. It is kiiowin-* what is meant by a, ' critical ima^,' and 
lieing able to discover whether or not a given objective will yield it. 
Clearly all t«8ta of optical iuHtruments, which iitv not capable of 
»iiui«*n'(W fxprcsgiott, must be coinpamtive. JIS<i'jnifi/iny yxitrcr cati 
be meaenrod numencally ; it i>i not comparative. In the saine way 
rrsolving poicer is niitihemationlty measurable ; no is f/tnttratittg 
pourr. Hut tlrfiHiiion an<l bri/fiitncy «/ tutiifff, and eridence of 
enilrinf/, can havo do numerical exprcsHon ; they are consequently 

The structure of the te«t-obJect nhould bo well known, and ths 
value of its 'markings' — if intended to indicate microscopical dinicn- 
Kions — should Iw upcnrHlcty anccrtaincd, cai-e being taken that the 
iiiinulr-ness of dimcnKJnns and gRiiciul ilelicicy and perfection of tho 
tesl-i)l)jtrct shciuhl lir ndapldd to th<! power of the lens. A fairly 
ciirrect estimitte n{ the rclntii'e jierforniance of leiise* of Diodemtn 
magnifying powrr may (li-ubtle>.s Ih- thus niiulf; by a <M>mpet«nt 
ulnierver; Imt it in not juiKEitilc front niiy cnui^uiri.-'on* of thin kintl 
to d'ltenninu what may or ouffht tii im the ultininti- limit of optical 
performam^^, i>r u-hclhrr any jxirticukr liMU uiu]i.*r txaniinntion hiu 
actually miclml Lhis^ limit. 

AisuminK the nianipulatitm of the in.itruaient and the illuminatioii 
of Iheobjwt lobi-u.tjir'i-fi-ctniiiiowiibU'.and further that thi; ti-rt object 
hoA been tielevteil with due appreciation of the rc<|uircinuitts uf perfect 


tcrticnl <lcIinMition, n hir c<>mpnriwn cnn only b« drown bfrtwiwn 
ocjectivr* of tlH- iuini« nmi.'nifyiiifc powxr nnd apcrlinv. Which of 
two or nmm olyoctivi-ji ^ivi^ii tin- hrtt4T iinngn may l>r rpadily 
«no<UK)> BMM'rlniiti'd hy Kurh cnmpuriHon, but thr vnluns thtii; n«c«r- 
UuimI liiilil ittjiKl tnily fur (h<- [HirticuUr <-tikM of nhjiiTlx i-xnminw]. 
Ibe butt pc-rfoniuitwe mtliHiit with n given tnu^inifyiiii; piivri^r niny 

Cibly flKwwtl rxpcctftUon, yet slili he Iwlow whiil might, niic) thrnv 
nutfht tA Ih> ubliiinr'd. 

On tlH> ivthrr bunil, I'xtraviittnnt i^xjirvUtionii tiuy tnduceii ImOil-I 
in p^rforniaiK'e* wliii'li miiniit Im n-iilisiid. Tho um[iloyni«nt ttf tht' 
UBt-<i)t|twtA iDUHt ill tiM? LI m<)n?iivi-r (.■nlculiiU^l to lend to an nnlirely 
flvw-iutlMl vMimatiun of thn lu-'tuid working fHiwer of an oliji-clive — 
aa, for exADiple, when ' resolving ])owit' Lt (i!itiiD»t«l bv iu extreme 
limita i«Uier Ihiio by iu p^oeml Kflicif^jicy, or 'definiiiH powrer ' by 
extent of ani]>liti«ation mtbor tluiit by I'leunieM of outline. So thut 
«n obaerver is teiagHed to Mm tbut he can disc-om tUruu^rh kia {wL 
IvBH wluit no ey« e»a see or len.i Nhow, Tim hii{ip«ii« cliiedy with 
Ui« inexjierietioed l*«',-iiiiier, but not unfTt<r|u«iitly niso witli th« moro 
experienced worker w)iu iittvuciitt^n the use of f>reat niuplitlcntion, in 
whose mind !>epir,ition of det»il nieAus ziiiitiysi<i of Mnii-ture. nnd 
opttoilly void uitflnpaoes prove the non-exiateiioe of Anything which 
lie does not see. 

As mncfa time is often lost by frequent repetition of theso com- 
petitive oxji mi nations (wliioh, after «11, leiod to no better reAidt thnn 
tluit tbe obwrrer finds or fMicieii that one leiu performsinhiohnndii 
more or lees satiafacUirily thnn sotne other len§) it seems worth 
white to consider tlie viLlue of a mode of teatin;; which can lie readily 
applied n'haterer its value uuiy be. A sliort and ensv mothod of 
tWting on objective — not by oniaparisoii with others unlv, but by 
iteelf and on its ovn inerits — Alfurds not only the most direct and 
positive evidence of its qualities to those who are morn concemetl 
in pmvini! theae instrunienU than using them, but also yiidds t» 
tlw gienuine worker the satisfying conviction thnt his labour is not 
fnistmted by bulty construction and performance of his instru- 
inmit. It ix, however, to be borne in mind that th<; luicroscopist, in 
any Kratiny of the quality nf his lenses which hn may Attempt, has 
no other obj«ct in view tJian t« acquire such insight intii the upticnl 
-vinKlittiinK of upvyl performanco as will enablo him tn make tlie liest 
ii»n (if hiN intromeiit, and nciju ire con tide ncc in hiit inti^rpmtatio'n 
of what lK^ M-nt ilh well as manipulative Kkill in exAinining micro- 
aoopicnl olijcL'tH. To the oonxA^ctor and oxpt^ of uptioal science 
are Wt tln^ lu^vere^ tnvr«ligationjt of optical KO'ectn and cnuses. the 
<ltflicultiea of t«chniaaj construction, the inventii>n of new leiiS' 
■oumbin»t4(iii!s and the numeroun methodK n! testing' iheir labours by 
ilelicaU! and exhaairttveproecsKeN which reijuiii' ipt«.'ial aptitude, and 
lie entirely out>idi' the *|ihiTe of the niicnaciipist's Usual work. 

Profetaur Ablie's nn«ln of testing obji-ctivi-s in explained in h«H 
* Beitnigp tar Thmrie dm Hikmskops,' 

The proceaa, in our ju<lgnwnt,rwiu»iT«largftexperienceand much 
ftlcill to Im of prnctic*] service ; but it is bojuxl on the following 
prinoiple :— 



In liny «»tubiiuit.iou of lennea at whioh ah obj<H.-tJvi> U ccmdiiuhhI 
thp gimmctricnl il«Uueiitiuiu of the hua^ of luiy nbjevt will be cnorv 
tv lnMci>iupl«t«)L»davcuralescooitliti^a8tbe|>eiicilsuf Uf{lit ronkiux 
fran the object are more ur Xesa perfet^-tly foctwied on tlie cuuju^lo 
ImokI plane of the objective. On lliin ilt^peiul tiiie iletliiitioii aud 
exact (lUlributiuii of light und sliade. Tlie tiiBCuncy of tbit focuraiux 
fuiictiuii will be lie^t asceruuiied by ftDaljainf; the course of iitulatMl 
peiiciU tlii-ectecl upon dilfei'ent pnri« or uonea td the aperture^ luut 
iibaarrln;; tlie union of the several inM>;eB iJi the focal ]>L>ii«. Fur 
this purpa«e it is necessary Ui brin^ under viev Ibe collective action 
iif «ii-|i parx of the aportutv, ceiitnti or peripheral, while at the same 
time the image which each part singly auil separately forms most bo 
distiiijfuishable and capable of compansun with the other inuifgeA, 

I. The illuminaiion must thi'refore lie sore^'ulAted that each sooe 
of the aprrtun' Hhall be tvprpwntcil by an ima^ fonncd in the apper 
ftical plane of the objivtivr (Lr. clow behind or Blx)\-e its b«ck lena), 
»o that only one narrow tracJc of li^ht b* allowed to paa» for esch 
anne, the ti-arta rapreMintins the ac^vral »>n«« being kc]>t as fiar as 
piwwible apart from each olliiT. 

I'liiiR, sup]i«Kiiig the working siirfao- of the front lens of an 
objective to be \ inch in iliaincti^, the iiiinge of the pencil of li^cht 
li>t in Khould mil ixx'Upy a, Inr^i^r rtjiare than -^ inch. WlMin 
twii pencilK aif! employed, one utf these should fall ho na to vxtend 
from the centum of the field to i^^ inch outside of it, and the other 
aliould full on thi* o]>|>wiite aido of the axi* in the outer peripbi-ry 
nf the field, li-nN'inK t.tnw a space of i^g inch clear between its oww 
inner ntnrgin and the ccntrn of the field, as in fi^. 'JSl, shere 
the obji-clivo imaeea of the pencils occupy each 
n ijuartrtr of the cViameter of the whole field. 

If tltnw [lenciU of light be employed, the 
tint xliould fall KO aa to extend from the centra 
of the fipid to j', inch outside of it ; the accond 
nhoulil occujiy a zone on the oprxnitc side of 
it. between tin- ,', and -^, inch (ncaaunxl 
from the centre): and tlitt third the peripheral xono on tho snmo 
side aa the lirst in Rg- -^-■ 

This RiTAOgetneiit placra the penoilH of light in their moat wniii- 
tivo poaition and expoaw inosl vividly any exislinu defect ia oorrvc- 
tion, aince the course of the raya is aucb that tlie pencils meet in 
the focal plane of tlie ima^ at the widest jmNuble angle. As many 
diatinet tinag«s will be perceived as tJiere inny be xoncs or portions <rf 
the front (ace of the objective put in operation by Mparnte pi^nciU o( 
light. If the objective be i)erfect all tlKWt imagOK idiould blend with 
eme setting of focns into a ninijle clear, culourlesn picture. 8uch a 
tuaion of imogm into one is, however, prevented by fnulta of (he 
image ft imiing prooece, which (so for aa they arise from i^tlwriaal 
nl)rrmtiiin) do not allow this ooiuoidenoe of aevend fmagca tmta 
dilVennt juirts of the field to take plaoeat the lame timc.aiid (so far 
an tliey arise fmm disperaion of colour) produce col'mrwl fringes nn 
the edgis biintering the dark and li;-ht lines of the tOKtobject and 
tlie edj^ iii each wpai'alo ioia^, as aU>i of llie currespiMtding oo- 

Fln. a«S- 




nt images in othnr tmrU of tin; liclfl. It tK tu be iHirne in 
vSoA Hmt ili« terrors whiim ar« apparent witli two nr tlirm lach 
pendia of liftlit iuukc neovnuil)' In; multitilicil whvn Uit wliolo Kre* 
<•( an olijective of fanltjr cvinxtrudion is iti nction. Tliix wnuM 
■fipau* t» ns 10 be the Ktrongp^t nnMin fur utili.iing Uie wh»I« un*, 
becaoM what we an.- waking ia tde drfrfU — llits errom of tin? objec- 
tive — and U) ntalci; tbi'si- ns plain a.i possiblt- is u iriHir r/Mff 7Ii]h. 
Dr. Ab)>e proceeds, bowevw, lo ('oiisidei- — 

'1. Tli« ineaua by wliich such isolntrxl p«uciU c-an Iw nbtiiiiKHj. 

Aa a Mpeoial illuminating njijmratus, tlit- ci'iiilfn.Hcr <if Pnifi'ssiiir 
Abbe ia rvcoinuendt-<i, or i-vpn a beinisphtirii'al Iciia. But wi* are 
(owviiK-eJ tlial the illuininuting iipparutits sbDuM \>v iia iiiiirly iipln- 
Batic an it niii bp. This is wrtaiiily not true of Abbe's chroiimtic 
Modi^nser or a ]ien)isi>hcnca] Ima. Th« reason b obviouK : thn 
tpbericul nberrutioD wlioHy j)re^'(rutA tbe rays pasunt; tliniu^U tin- 
MJM in the dia]>bra^t) from beia;: fooussed on tlie object - tlin 
lilvered plat« of lini>!i--(ir thr mime fimt. In the lowei- fuoal plane 
•4 ti>e tUutiiiiiatiiif; leiia iiiii»t bv litled diaplim^iH {easily made dt 
Itteokaoed cnutlbimrd) piercwl with two or lliree opeiittiKt> of such a 
tin that Ibe imaRes, as formed by the objective, may oocopy a 
fourth or sixtli part of the diameteruf the whole aperture (i.e. of llie 
fe^ld heeii wlit-ti tookijiK down the lul>e of the iri&truiaeot, after re- 
voriiig tti« ocular, ujion the objeclive iiun;.>e), Tlie required aixe 
<f tJiese holea, whit-li <te|iemts. Ilr&l, on the f<n.-)il leiijfth of thft illnml- 
■intiu); l<?iifl, luid, aeeondly, on the aperture of the objective, may be 
llius t'lHiod. A tent i)l>ipct i>eiiii( first blmr]>ly fiKTUnsed, caud diU' 
pbraflins faaxiiiK ho)eB i>f viiriiitis xixi-s (tvru or three of Ihe same hIm 
ia rach ouxl) muni 1>e tried until one size is found, the ima^ of whicli 
in the posterior focal plane of tlie objective shiill be al)Out a fourth 
(o a &iilb |iai-t of Ihe diameter of the field of llie fibjective. Uoleft 
bavin;; the diiiiensions thus pxjiertiu en tally fi lUiid to ^ive the re<)utred 
siie of iina^e uiu^l iJien be jiiei-ced in a cun.1. in such a position a^x 
will produce images situate in the tjetd, as shown by fifp^. 38], 283 ; 
tlie card is then Itied in its place lielow the mtidenBer. Wo are 
strongly, however, inclined to believo, partly from experiment, that 
Wter results would lie obtained by putting sections of annular slits 
at tJte haek xif (Ar nbjtclive. It the oonaenser l>e titled so lui to 
iwolve round the axis of the instruiuent, and also caiTy with it 
the rinf; or tuW to which the card diaphragm i« fixed, the penciU 
'd liplit admitted throuf;b tlte holea will, by simply tuniing the con- 
■k-nser round, sweep the face of the lens in ns many xones as thi^ro 
lUO holes. l^uppr«iii;; the condenser to be mrried on a rotating 
inb-etAge, no additiiuial arran^nieni is required bc^idos tho 
iliofkliragtn -carrier. Thus, for example, if a Collins oondei-.ier tittinff 
is a ratattDK stib-stage be used, all that is required is to siibstilutn 
for thn dia|»iVigni which carries the stops nod npertui-oH as nrrungi>d 
hy tlie fnak«r, » diaphragm pi«rcefl with, say, thn-o openings ■■£ J-inch 
duuDel«r, in which drclcis of caid may bo dropp«l, tho i-ard beiriK 
(urvod with hnl« of diflferent sixes accordinif to the ilirections given 
tixiTo. Wo doubt, however,' if any sub-stagK will revolve with 
nficiant accuracy for so dolicat« a test. 


AiHitlici' pinn ntli)j>t^(l Ijy Fir. Kripp, nnd found very <;i>nv«nieiit 
in pmcliocv is to mount n condni^iin^ Ichk (Vrt^vsM>r Alilie'a id lliifli 
cwic) Upon II short picrc of tiilio, which titK in tli<- rcitatiiifi sub-^tatt■>■ 
On l>[>pOKit4^ .liduA <)£ this tiilx.-, mid nt n. distonct; from llii! lovi-i' leia 
C(|tud to tho focal dinUinci'! iif thn cnm hi nations, lUita aru out out 
througli which n slip of Ktout cnrdlionrd ran Ih; pauctl itcri-vi and 
Iwluw the leu*. In the cnnllioiird, huW cf varii>u.i ftiicns n»d M 
varioux tliisliinc™ fnmi each othi-r, luiiy Im pierced ai-L-ordiUK Iti 
plnwiure. By •.imply poiuin); thi- sdip through the tubt^. th« |)eiicils 
of li^lit udiuitied thfough tin; hiilnt (which form iDiaK"* ^ thc»e 
iioles in the upper fmni pliuic of llie ohjcctive) are made to ti-&ven>o 
tJie JieUl of view, unil by rotating Uie sub iitaue the whole faoo of the 
]*iis is swept, and thus M-arched in any direction required. Bnthere, 
ugain, the Bpliericul ulwrration of uu ujioorr«ct«(l voiidenser would, 
with ail ohjeative of large aperture, cuuse the oblique pencib 
under some ooadittojis to pass under the object ; and a1t«raitioii of 
fi>cu8 will not properly alter this— at least without a disturbance 
of the focus of <he obiecli\'e. 

When on instrument' is not provided with a rotating snb-slngc, 
it is sufficient to mount the condenser on a piece of tubing, which 
may litido in the sotting always provided for the diaphmgin on tho 
undnr Hide of the stage. 

i'ard ilinplii-ngmK for experiment may be placed upon the top of 
iL tliin piece of tnho (open at both ends) made to slide inNide tliat 
which curricK the Cinittenser. and remorabls at will. By rotating 
thiK innor tube the pencils of light will lie made to KWiwp round in 
the tielil, mid thus permit each part of the central or peripheral aonu 
to be brought into play. Against the accurate value of this, »ffUMi, 
the spherical aberration of an uncorrected oondeoaer would strongly 

Abbe'i Test-plate. — This test-plate is intended for tlie oxamina- 
tion of ohjcetivi-j iiith reference to their corrections for spherioal 
and chrumntic abirrration, nnd for estimating the thicknect of the 
cover-glass for which the Kplierical aberration is best corroclod. 

Fio. SHS. 

The t«-st-plnt<' fonsiitta of a series of cuver- glasses, ranging in 
tiiieknesH from 0*011 mm. to 0*2-1 mm., silvered on the under Hur^tce, 
Mnd cemontis] nidi: by side on a slide, the thickness of f«ch being 
Biarke<l on the silver film. Groups of parallel lines am out through 
the lilms, and these are so coarsely ruteil that they are easily rMolvvd 
by the lowest jiowers ; ynt from the extreme thinness of the ailver 
tliey also form u i ery ih-licnto tent for (>lijectit~es of even the higliest 



Fill. mt. 

-and wiilfMaperturo. Tlit- ti-nt-pUtcin its nntui-it] sue is tiMn 
I. 'ia^ oud utMs oE Uie oircIcK rtilnrgixl in ntva in &k. 381. 
To cxniuine ui objeetJve of liirgv n]:H.'rt.i,iri^, ihe discs must be 
focucMHl in fUGrouion, obaerviiig in cncli cniu; 
Ui« quality of tlie tillage In tiw ci-ntm of the 
field, and llie variation pruduceit )>y usin^ nltvr- 
ii«t«tj central ntui verj obli<|UC iltuiuiiiAtiKH. 

Wbeo the objective id perfvclly i-urn-i-ti-il 
tnr Kpliericol aberration fur the (larljcubir l)ii<'l. 
ueu of cover-ffUas under exooiinatiua, tJie out- 
linca of tl»e lines in the ventre ot the Beld will 
be perfectly aliarp by obli<iu« illuDiinatiuti, and 
vitoout atiy nebulous doubling or indict in ctiieiu 
ot the minute irregularities of tlie edges. If, after exnttty adjusting 
ilie objective fur ol)lii|ue ligiil, central illumination is used, no alt«ni- 
lioil of Ibe focus should bo necessary to show tJie outliiieH with equal 

If on objective fulfils theAc coiiditious with any one of iho dlitCK 
it is five from spfaorical abcrmlion when used with cover-^lafscs of 
that tlii<-kness. <^>n the (ithcr hand, if every disc sliiiws ncbuIouK 
doubling, or an indistinct appearance of the edges of tin? lines with 
oblique illumination, or if the objoctire requires a dilTerrnt foool «d- 
jiMtmcnt to get equal sharpness with central as with t.ibli<[uv light, 
then thn spheric*! currectioo of the objective is more or lust im- 

Nebulous doubling with oblique illuiuinntiou indicates ov«r>cur> 
rection of the marginal loue ; indistinctueu of the edgM witliout 
marked nebtdosily itxlicates a&der-correotioa of this zone ; an 
iilteralion of the focus for oblique and central illumination (tliat Ik, 
n ditlereuce of plane l>etween the image in tlie peripheral and central 
piwtions of the objective) DOtnts to an absence of coiicurivnt nctinn 
'4 the separate xonee, which may be due to eitlier an average undcr- 
or over-correction, or to irregularity in the convergence of tb" rnys. 

^»o test of chromatic correction is Iwsed on the chnrnct^'r of the 
eolour-bonds which are visihlo by oblique illuminntiDn. Wirh good 
ourroction tJie cilgcs of the lines in thi> contro of the. Held iiliould 
dww only narrow oolour-bands in tli« complcmi-niary ciJom-s of the 
Mcontkrr »pectniRi, namely* on one side yrllow -green to appli^grtfo, 
aad on the otber, violet to ro«fc The more perfect the corrMition of 
ibe n>hrrica) aberration, the clearer thin colour-band app<!ar3. 

To obtain obliquity of illumination extending to the marginal 
ame lA the objcctivi^ and a rapi<l inti-rchangv fn>m obli(|Ue to 
nintral light. Abbe's illuminating appnmtux in manifeJitly defective 
uii uociiunt of its Kphcricnl aberration. We want at least his 
achnimiitic CDndcJitier. For tbo exomiiuition of iinliniiry tnnnerKion 
oijjectiveB, the am-rtumt of which an!, lu a, rule, greater than lK(y 
in arc (1-00 A.X.). i"d tJiose homogimeous immersion ohjectiveg 
Mliieli conftiilcrably exceed this, it will be nwesiary to bring the 
under aurfucct of the tvAt-plale into contact willi the upper U-ns of 
the illuminator by mtiknn of cedar oil, even if water- immersion ob- 
jectives are uocd. We inuy wld, aa u uuitter of experience, that 




hnviTij; imce veFilrxKl thi^ lixhl unci Uin cnmltinser. we buUI, with 
defereitit' tu Dr. AIiIia, thul Hie litfht should <n\ au ac^count be 
toucbecl. which Ui obtain obliijiiity ]\p advhtea bv mirrur obansra. 
We Mltve that Uiia ihould be secured solely l>y tlie inuveueuC ol 
tlie diuphragm. 

For tlie exAininatini) of ob)ectiv» of smaller aperture (1pm tluui 
40' tu nO") we may ubUiin aU the neoi-iwary itiitjt fur the ««ti[]Mtioii of 
til* Bpheiical and i-hi-uniatii: currecttoiui by iitiiting the coitcare 
miiror so tar kteiTiUy that it» edge Is nearly in the liiif (if the optic 
axis, the incident oune of rays tbeii only lilliii-; ''•ne^lialf of the npcr- 
ture uf the objective, by wluHi nieaiia the s}]>netis of the outlines 
and the chumcter of tlie cu1our-bnii<U can be easily e»titnntffrl. 

It is of fun<lniiieiiljHl iniportniiL-e in employing the trst-platv to 
have bnllinnt jlluniiiiation and to use an «ye<piece of high ]K>wer. 
With ohliitue il I uiiti nation the light must always he thr<>wii pi-rpi-n 
dicuUrly to the direi^ou of the lines. 

When from practice tli« eye liiw lenmt In reco^bw the fim 
dilTiftranccs in the quality of the outlines "f the imngi-, thiH uii'th< 
of tnvwtigntion gives very ti-ustworthy results. IHliirrcncp* in the 
thiclcm-sv of i-over-glasses of O-OI or (J'02 mm. can br rreii|;iiisnl wiih 
ob.i(N:tiviw of 'i or :i mm. focuH. The ijunlity of the imu^e initsiile 
th<f nxiii i* not dependent on HphericJil and chiimiatic correction in 
till? strict iieniin <if the term. 

IiidiKtinctnmw <if tho outlines t'lwnrds tlie honlertt of (ho |j<-ld of 
view ariAM, us n rule, from uneijunl mngnifimtinii i>f ilie tlilferent 
xoiiex of the objective ; colour- bunds in the priiiitierid [lorlion (with 
good colour- cs>rrection in the middle) am nlways cnused by ani-r|uul 
mngni Heat ion of the diflerent coloured imngrs. ImpiTt'n-tiMnsnf tliis 
kind, improperly culled 'ourvatoreof the field,' bit- nhimn t-i n ({iwnt.-r 
or IcM pxtcnt in thobcst objective* when thmrnjiT'itui-c isponsidcndilc, 

•J'l-sting an objective does not mean seeing the ino«t delicate 
points in an object ; it rather means the maTnuT in which an object 
of »oriie Kim- in defined. 

A l*-!*! fur low-powers up to 1 of HO'or N.A. '65 ii an object oii 
A dark ground. Nothing is so seonitive. One of tin' l'i^i/r-i»/itnt 
Itecauae it tokei tight well, is good. Far higher pnwers m ooane 
diatom, a Truvrntium Jimhriniiim, is exceflent ; for unlesa ui 
objective is well correct«d theimngp will l>e fringi'd and aurri»unded 
with scattered light, anil the ubernition producrd hy thf cover-glaaa 
is plainly uianifntt, and by accTiratB correction mix be done away. 

Brrf'r nf frtitrinij is one of the special liefects of objective* 
which the Ahlw method of testing iliics not cover. But if we placo- 
a sensitive object in n cl^^tnin direction, and wlien the beat adjust- 
ments have given the best imngn rotjttv^ tlwt object through an angle 
of 90", only a widloBntred objective will giv* au unaltered image- 
tlirou;;h«ut. If not well-centred it will at certain parts gro* 
fainter or aliarper. The most useful image for this puipose with 
medium powers is a hftir of Polyxenwi ia-jm-wt mounted in bataftm 
(frontispiece, fig. (!). 

For higher powers nothing surpasses a podura scale. In t^is- 
particular it has always l>eeii of great rnJae to opticians. It should 



1m •tnhiiKly uiairkeil, ;iud must be in optical coiiUct witli tbo cover- 
gUu ; tliiti may be tested by mcKns of an oil-iiameraion aud tho 
' vcrti<Hil illuuiiuator ' (p. '.*1*4). , 

Th« objectives of widMt npcrturo am not readily tested bemuse 
tbm is uo con<l«nwr niliici«;ntly npliLtintic to do it cxliaualively. 
Tbn best that can b« done is Ut takn k diAtom, such as a Coociuodia- 
m, iu balsam with strong 'sncondariwx'<P]nt« 1. tigs. 3 and 4), witli 
tbe largeiit apUnatic cono tlmt can bi.- obtained, which at pred«nt 
nan l>e tiest aooompli«hc<l with Powell and XioiiJand's nchniinntiu 
MDdausier nf 14 N.A. It luuKt hv a good objective indeed that 
doe* not ahow signs of bn-akiiig down under tJii* strain ; and there 
is extrenw suwvptihility to oo^■l1^ coiTPction to whirli close attention 
Buat be paid. jVuilluminatingcDni'of N.^V. I '0 is proliahlyjust below 
llie p<Miit of ovcr-Ktntin with tJi« l>t«t lensot at prCM'tit at our disponal. 
Trttiiti/ Ir-ntt*, tbercfon', nvwlvu itArIf into two inirthodH, tie. 

1. For low and nxiliuin jiowei:*, <Ui'k (n^und with a Polrcistina, 
or a diatom ncvording to Uit; ]iow(-r. 

2. Viatrini/ for niisliuni powers (an ordtial not n«odful for very 
low povrort) sliouhl Ikt by meuna of a bair of /'oli/jr'nHt lai/urur, 

3. Centring for higti oowen by means of podura Hcnlo. 

4. IKifinitiuii CotKinoaitioua ast«roraplialus with wide-angk-d cono 
oiitaining Khrir{<. brilliant, and clear view of ' secondaria.' 

The apertometer, aa its uaiue implies, \» an iiuitrunutnt for mon- 
aring thf a/xrturr tif a mifnfttopie vlijeelioe. As correct idi-as of 
sprrtHro liavn imly obtained during the past few yean, it luny Iw 
iaferml tJial ajierlometers ooiistrucled before tlie dftitiitiun of aprr- 
(ore was givt^n and accepted were crude and practically uKclcNt. 

Tlio contraveny on tlie 'ajwrture question,' wliich was in full 
iperntiiin M>nie eiffliteeu yeara aince, is not aua]t4>KethcrNatiiifiictory 
page in the hiatttry of the modem inicroecope, and for many reimoiiB 
It is well to jHUis it unolwervantly by. It will suffice to sUte that 
daring its pioffreas an apei-l'imet«r was devised by R. B. Toll«s, of 
.America, which accui-ately measured the true apeitui-e of an ubj(«- 
lire. About the puvme time Professor Abbe gave his attention to 
tho Kubjccl, and with the result, a* no have seen, tlint he lias pven 
A ill-finite and permanent nionning to nuiiitriotl ajiertun; uiakin^; 
k, as w« liave seen, the cjuiralnnt of the mathematical expit^Hntmi 
a ant! u, n )>eing thi^ rcfiiKCivo io(l«x of tlie medium and u hnlf 
ibe angle of npertnrc.' 

Th« application of thin formula to, and its genoral bearing ujxm, 
the diffraction thcry of micrnBcimic vision has Ixien given iu ila 
propter place ; but an thn aim of this manual It thoroughly practical 
no iJiall be partlonrd for even ii Moall meosnro of rcpetiljou in 
tndoavouring to cxnhiin the um- <'f tliis formula in such a manner 
tlMt only a knowlcftgn of nmplc arithmetic will Ixironiiire*) to enable 
tbe student to work out any of the prolilcms which are likely to 
trise in his pmctical work. 

* & knowU'l^ III the meuiSoa of llm lrigDUC>metiiegJ aipmuinii ' Biiin ' in i 
tttmwy >a ■flnnK acT ot Ihu killnwinit qaoalioni. At lbs vhIdh an til found i» 
"-' it i> nnlr ucowMiy to caution Hitur who an ooiwquiuiilwl with Uio am' of 
matical t*ble»toN4 tlul ll»'} hmu tUo ' iiAlunl lias'iiDd not tlio ' log nii«.' 




W» oAii heht accomplish tliif by illintrntion. 

i. If a ceriAin dry objeciisi- tui)<n.Ti angolar ftpertureof 60', what 
is iu N.A. t (i.e. numoncai fiporturo)* 

All that is needful i« to lind thi^ vnluo of n une ii ; in thb euft 
u ^ tlio rc4rH>cCiv« indox of the modium, which is tiir, is 1 ; aiid », 
which is half of titi". = 30° opposite 30° in n twhlp of natural niiM,' Li 
■5 ; sine m. therefore, = •'>, which multiplied hy I gives -5 as th« K.A, 
of A dry ohjectivo hnving W of niigulnr nprrtiire. 

iL Whnt is the S.A. of ii wrtti-r-imiiic-mon whraro aii»ul«r 
aperture = 44° 1 

n htire = I "33, the refractive index of wator ; nni) H,orIinl( 44', 
is 22'. Sine 1*2° fiimi tables = -.37.5, irhiuh mulliplied by l-33 = -A 
(nearly), which is tlie N.A. r&iuired. 

iii. What is tho X.A. of tm oil-iutniersion objcctiro hitving 
38i' of angular a]ierture 1 

I* the refraj^tive iiulex of oil. which is equal to tliat rtf crown 
glass, is 1 -52 ; u = 1 11^ and sine t( from tables = '329, whiuh multi- 
plictl by 1*32= -5. 

Tlius it U seen that a dry objective of SO', a water- inimemion of 
'M", and an oil-iiniiiersion of Ss^" aU have the same X.A. nf '•'i. 

Il will be well, perhajM, to give the converse of ibiH laetlmd. 

iv. If a dry objective is '5 N.A., what is its angular aperture t 

Here because « sine w = ■■'», sine w = — ; the objeclivo beiiut 


dry n ^ 1, tlierefore sine « ^ '5. Opposite •'> in the table nf 

ntvtund sines is 30° ; hence it = .TO'. But as u U half the angular 

aperture of thpolijeirtive, 2»or G0= = the angular ap<«rture required. 

V, What is thn anguUr aperture of a water- immersion objective 

whose N.A. = -nl 

Here n = 1'33, n sin« u ^ '5 ; suie u ^ - ^ ^ *3Tfi ; 

M 1 '33 

« = 22* (nearly) from tables of sines : .% 2u = 44", the angle rc- 


vi. What is the angular aperture <if an cul-inimersioii objeetire 

of -5 N.A. 1 

Here n = 1'52, t» sine ii = '5, sine »r ^ ' ^ - -. = "SSO ; 

« = 19j° (by tablex of wnes) ; and 2n = 38i, th« anglo rei)aired. 

We may ynt furtlier by a iiitiiple illuHtratinn explain the use of 
«i aiiie i(. 

In the aooonitiunyiii:.; diagmni, 6g. 2^5, let n' rcprosent a vcnset 
of kIuss ; lei the line A be perpendicular Ui the stirfacu of tin? water 
C D ; sujipose now that a pencil of li^ht bupin^iPS on tlie Murfiii;i.i of 
the water at tlie point wliei* the pei-jiendicular meets it, tnakinj^ an 
angle nt 30° with the perpendicular. Tliia pencil in pimetruling tli- 
water will be refracted or bent towards the pnrpenilieular. Tho 
problem is to llnd the angle this pencil of Ii},'ht will luakt? witli tlie 
peipetidicular in the water. 

To do this wo must remember that n sine » on tho lur «de is 

> Till* Appendix A t« tliU Tolvma. 


SMPLB nxrsnuTios of the vse of a* nmr. u 335 

eqtutl to ft' Mae u' on the wntcr judr. Tlius on thi' sir side tt ^ I, 
H ^ .10°, utd bv llip tobies of sinm clno 30* = -.') ; «u»t«<iiieiicly on 
the ftir nulo we liarr n Mine h = -S, 

On tho vr»tvr nilr n' = 1-33, and ««' is to lie foDiul. Dlit m 

= *■' = -376 ; which (m 

n Bine II ^ II sine u, sine u = 


ibn taUes sliow) is the imtun] sine of sn angle of 22* ( n«iarljr) ; con- 
wqaenily «' = 22' ; bo the pendl of Hghi in pwising out of air iiil<) 
wAterbiubeen bent 8° from its original dii-ection. Convertiely a 
pmcil in water making an aufle of 2'2° witli th^ ])er|ien<lioulai- 
would on emerging from the water be bent in air S" furlhvr atmif 
from the p«rpendicu]ar, and so make an angle of 'MP with it. 

Now if we suppose tliat these 
pencUa of light mo/w round ihf 
fiirp^ndieulirr, coim* would fi« df- 
Kribrtl, and we can readily see 
Ikat a sulid cone of 60^ in air i» 
dw exftct ecjui^'alent of a solid 
tone of H" in water. 

If we fuHhor suppose that the 
vater in tho vessel is replstced by 
eedar oU, tlu- pondl to air remain- 
ing tltotanwas bcfor<\ will, when 
it eut4'™ tfrn oil, Iw l)ent more 
ihan it wa» in the water, bfieansc 
the oil tbU A higlier refractive 
inilex thAn wnter ; n in this caso 
iif equal t^i 1 ''>2. 

The vxnct position of thn 
f«ncil can bo Hc^termined ui thn 
■una mMineT as in the previoiu 
cue. On the nir side, a« before, 
n wne i* =■ '5 ; vn tJio oil ride 
n' sine u' ^ n ainn u; sine m' ^ 

""",'"=.■2. = S->II,which(by 

taUn) ia the natural sine of 

{'. It follows that the pencil 

luLt l)een bent in the cedaroil 10^^ 

nlof its ori^nal coiirae, and a 

! of 60* in ail' becomes a cone of 

(° in cedar oil or cniwn glass. 

Finally, it is iustmctive to 

tbe Rsult when an inci<len< 

in air makes an angle of 

'with the perpendiculnr; nsine 

I becomes unitv, and m in watei' 

" in oil 41"' (nearly) ; conse- t™ ans 

a cone of either 9'^" in wat«r, or ^21° in <iil or cmwn glass, 
U At txaet equivalent o/t/te Khole itemi^Afrunl rattxant in air. In 


<<tt)Pi' wonts, an<l t» incy thn mn<l*' in whi<!li this ^-iwit truth htu 
In-on bcfoiT stntoil, llir tncu>n-liciil iiinxinium nplnlun^ fnr a <lrv Ifiut 
JK cquivnlfHt. t" n wntor-iuiiQi-rdon of 97^", <uid lui oil-iinmnminn of 
f^ij" aii);;uUi' npiii-tarp, 

1'hP InKt |>robIriu tliut nci<<l m-cupy un i.i tii dnd l)i<- nngulkr 
apcrtuiv of nn nil-imiiiersi<in wliich snail li« cquiriJcuit to a wat^r- 
iinniersinn of 1^0° angtilitr ni«rture. 

TiL (Jn the water side n = 1-33, « = aO*, sine 90° = I, n siiiit » 
^ 1*33. On th<! oil lyiln »' = 1-52 and »' has to he. found. 

A*«' *in»u' = Msine «, wn« «' = li?^Hl!t = 'jI? =-8-5;w'=61» 

^nearly). by th« tables ; ^»' = 12'2<' (ncArlj), th« «JDgl« required. 

It thus nppenrx (I) that dry fuid immeraioa otyectivm hariii|; 
<lifl^nt aiiK:ulnr ngiorlurpj, if <if Hm mmo *igvit>alfnt rtpirrtHrr, are 
deagsatcd by the wnif ti-nu. Tlnw o1g«cttr«« of 60° in air, iwr 44' 
in wMer. ■«■ 31*1° iti (■it. tiavi- iiti-iitieftllv tlio same a[>ortui'c, and am 
knowi) bv tbu snine clesigiintiuii of 'ft N.A. 

(3) Ttic iH'netrating [mwer of luiy objpctivo is proportional to 

.■..■■ and itailluminatinn iiowHr to (X.A.)* Tliprcfurff, if wrdvul'ln 


tilt! N.A. we hiiWf tlic pi^notrating power, and inoreaae Ute illuiui- 

iiuting power four times. 

Ill compariti^ the pcnetmting and illuminating powers <4 tAjto-i 
tives, however, earn mu»t ho tnkpn t« Avai<l a populai- error, byj 
making thrm bntwwai obji-cti*-** of different foci. 

It cAiinol, for example, bu nid th&t a J-inch objective of '8N.A.^ 
liaa half the ponelrRt.iii)» power of » ^-inch of '4 N.A. Neither caw 
it Ix- »ai<l that it has four timwi the illuminatitig power. What is 
mewnt is that n ^-inoh of 'K N.A. has half tlie penetmtin^' and four 
times the illuminating power of a ^-inch objective of -4 N.A. 

But bocauM pnndrnting atul illuminating powent diminish nn 
ihe square of thft foci, a i-inch nlijuftivd of ti N.A. hiw four timet 
the Uluminaling and nearly four timm the penetrating power of a 
J-inch of (i N.A. 

The old tumiencliiturp, in ush bcfon^ numerical aperture w« so 
hnppily intnicluciHl, did not of I'ouran admit of comparisons of pens- 
tralingnnd illuininuliiift jKiwera by in*p<.«otioii ; which, liowerer, is 
A manifest (ulvnnliigt!, coiitributin;; tu accuracy and precision in 
important directions. 

(It) It may he well, for the sake of com|>l«TbRncM, to rc])«at ' hero 
that thn nwolving powei- uf an objective is directly proportional to 
it>i numeriod apertuie. If we doulile tlie N.A. wi? nlsa double thn 
Tcuolvinc power ; and this not simply with objectives of the saxivf 
fud, na &i the cnse of penetrating and illununatiiig powers. Thu* it 
in not only true tliat a ^-inch objective of -6 N.A. rcaolves twice oa 
many tines to the inch as a ^-inch uf '3 N. A., but b>i also does a 
-inch of 1-4 N.A. resolve twice, and only twice, as many m.i a 
-inch of -7 N.A. 

Within certain limits, then, the advantage lieu with lon^ foci of 

> CluptM I. 



iri(!« >Dgl«, bncAQse we tlius secure tbe ^'^^'^-''t nuolvinf; powor 
witii the irroitest ])eneli-iLtiiif; iiiitl illumiiiutin); [tuw-emi. 

Fmm wlint hsN here lieeii Hhou-n, Uieii, it l>eti>in>!H evUl«tit ttint 
tbft titn|ktiiyiii«fnt iif the miaxMcupe as ftri iiiNirumeiil <if precision in 
lugdy >lntr Ui Alibe'o vork, nntl that the iiiti'Oiliii;Uuii uf nuniprinil 
apMlum, with iUi dtrictly Acuuriite me«nuiKi Iibs been » pmctkuil 
l^n of anti)!!! %'nJue. Uiit lliis has been gi-eAtly eniiehed by liLs 
JiAving introtlucrtl « thoroughly §iinple and useful nii^riomtUr. TtiU 
invulviw thf nune principle as tliaC of Xollea, bub il U carried out in 
s simpler niknnvr. 

Abbe's inntTtiinont ispraeontad in fig. :!86. Ic will be seen llut 
it ooniuMK of K <tat cyliniW «f clnM, about three iuchw in diameter 
And half an im^h, with n larf^ chonl cut off «> tlmt the purlion 
Ipft i« somewhat Rl<lr^^ t}in.ii n >«niicircl« ; the part wliei-e ihe Mgnient 
i« rut ix bcvi-tlf'd fivini idmvo dii»-nwnrdi> to nii nn^lp of 49". and it 
will hi'- frvn tliat ihnrttii: a Mimll dim: with nn aperture in it denoting 
the ventre of tlu- Niiiicii-ck'- Thix iiutlruiiirnt iK uwd ae follows. 

Fta. 9M.— Abbc'iaiwHrnMliir. 

The mieniM^opfi is placed in a vertical poailiun, and llie aporlo- 
nrtrr is placed upim tin- sUue with Its circular part to the finnt 
uid Uie chord to the back. Kffuned Uglit, either from suti ur lamp, 
is Bssnmt^ tit )>e in front and on both sides. Bupposo the leiiH to 
ba mcMnired i.t a dry J-inch : then witli a 1 -inch eye-piece liaviii» a 
hrgo field, the oi'titrti disc with ita aperture on the apertometer is 
bnnight into focuti. Tlie eye-piece and the draw-tube are now 
removed, Icnvinf! the focal anuufiemetit uikdudurlicd, and a letia 
tupplied with the upertouiet^r is screwed iutu tlie end of the di-aw- 
tubi-. Tliix IciiH with the t^ye-piece Li the di-aw-tulie foriii.i n low- 
powor compound roicruBOupe, This is now inserted intii the l)oily- 
tabe, and the back leiiaof the objective whose aperture v/a dcaire 
to mcnjtiirR in brought into focus. In the image of the back lens 
will Ik- scf^n «tret«liei] across, as it were, the imu^ of the circidar 
part of tlir aportoinet«r. It will appear as a bii^Iit band, because 
tlio light which enters tiormally at the surface is reflected by the 
lie\i-lled part vi the chord in a vertical direction, m that in reality 
a fan uf 180° in air is formed. There aro two sliding iicreens seen 


on mthvr aitie ot the li^oii^ *■' tl>^ upeitometer ; tbejr .lUdo on 
^•<^rti<»l L-iteulur portion ot the iiuli-umrut. Th^ uunKva of tiusm 
iwTwns i-aii )io iteeii in the iniftge ot tU» liriKltt lutud. TA'^m acrteia 
ahiiulti now he movttl «u ifutt ihtir edgn jtiMl loueA Ihe periphery 
llie haek lent. Tliey act, as it w«iv, um ii dinphragm to mit tJte fau 
luid reduce it, »> llkftt its ftiigle just mjuoIs the aperture of the olyec* 
live and no more. 

Thb augle i» uuw determined bir the nrc of gluts Tietween the 
acreeiia ; thiu we get ah nngli? in riiittit the exAcl equivalent of th» 
aperture of the olijective. Ah l!ie niiiiicrind ttpertures of these artat 
are engraved on the apertoinetcr they can Iw read off by inspection. 
Kei'erthele«« a difficulty is oxperionccd, from the fact that it ts not) 
easy to delonninc the exact point at which the edge of tJie sore«a 
touches the periphery of thn bftcW leiwi, or, as we pn?fer to designate 
it, t/if liinU of nju-rturr, for cnrinuH ait thin cxpivssiuD may appear 
we have found at linnw that thn Imck lens of an objective ia Urger 
than tho ajirrlitri^ of the objcctiw rrifuir't. In that case the «dgea 
of the witH.Ti rrfusc to tmii'h thit prriphi-ry. 

On the wholv wc hiivu found that u. far Iwttw way <rf irmph^ng 
thi» instrument ix in ti«- if I?* cimnrrtion tiriih a ffrtjiiualrii rtilary 
*lafi', thi- edge of the llainR irf a parailin tamp being thv illumi- 

ThuK ; Set till! luniji in ii diivt'tion at rijiiht annh's to tbe cAortf 
lit the a|«'rtoiui'ti'r, and hu)>[«im-' that the indcrx uf the itagn is at 0*. 
'I'lie i*d}(*' of the Aiiniv wilt Ix' Mrt>it in tli« rentrct of tlio bright band. 
The sliding kl-i-cciu Ix'tng dispcnuril irith, rotiilion of fA« tiage trill 
(viH*e the imagi^ vf tli<? Ilamc to tmvcl triwurdK tli« edge nf Uic 
aperture ; rotation is ■■ontinuiil until the image of the ftaine is halt 
extinguishoil fcy the ed^ of the apmiiirr, the arc is then read, am! 
the same thing vt rejxiated on the othi-r siile, ami tlie mean of tJie 
reailings i« taken. 

If the Ntage mlnt(!ii truly, and if the iiuctTunirnt is pn'iperly fet 
lip, the reading on the one aide ought to he ideutieul with that on 
the other. 

SuppcBM- ttial the sum of the ri-adinpt on Iwth side* = 60', tins 
mean reading i.->c(m»ei|ueiitlv 30°, which is the senii-angle of aperture 
of the len.t in y<i*ii, Kmin this chttum we Imve to determine the N.A. 
of the dry j-ineh lui w\\ aa ita angular aperture in air.' 

(i) As liefore, N.A. ^ » sine w, and n sine w ^ n' sine m' ; 
whieh means that the aperture onthe airtiide Ixei^uul to the aperture 
on the glass side ; i» = I for oir : »' = I'GI?>, ihe refriiL-iive index of 
the api-rtometer ; «' i» the mean an^Ie meaBored, which in this cose 
is .10° ; and n sine » luia t<> In* found. 

Now sine 1)0* = -5 (by the tables): »' sine u' = I'filSxsine 30' 
= |-61.'> X ■S = ■*• = n sine ii=tlie X.A. i*iiuired, 

(ii) Again, toltmUhedityM/rir'r/ii'r'i'nriir 'iu. As befoiv, >i sinei* 

, . - I - "' Bine ((' l-fil.') X -^ .. mo9 

s=n snie m and sine « ^ := . ^'t- ; « ^ 53 

» 1 

nearly (by the tAbh-s) ; 2k = 106^, which is the angle require!. 


^' Bine v' = 1-615 x -866 = 1-4, which is the N.A. 

^ rioe u « "' """ "' = Lg^^ ^ jgg = ■92. 
n 1-52 

by the tablea), 3u = 134°, the angle required. 

lifest tbaA if the refractive index of the apertomet«r 
; the oil of oedar, the mean angle measured is the semi- 
tnre of tbe objective, and its sine maltiplied by that 
ex IB tbe numerical aperture. 

be foand the more H«cnrate and nniversally applicable 
?Asiuing the apertnreB oi objectives, as the extinction 
howB precisely when the limit of aperture ia reached. 
(1 LietUBJid'a stands lend themselves admirably for use 
toiiiet«r. The body being removable the lens can be 
upper put of the nose-piece, and any measurement 
ttely nuide. We would advise every niicroscopist to 
e of this admirable instrument, and to demonstrate for 
[>ertnre capacity of his lenses that he may know with 
r tme resolving powers. 

WiTiiOl'T ntt^niptiiijt t<i occupy space with a (liiicuB»ion of ihe i|ne»- 
"t'lim oi the riji'it "t ' iiiieroscopy ' Ui be t!uruii(li^i-e<l a gcimice, we ni»y 
vfintunt to iiMirin tliut h will )w hiil a reoojjiiilHiD of jHucUcal (ucla 
if we cliiitii lis a deliiiilion of min-unca/i^ tlwt it expresitea aud in lu- 
tended !■> ■.-any with it uU that l>elun^s to the science and art of the 
niicniiicrijM? as n BcientiAi- in aim men I, hating re^^-urd e(|uidly tit its 
ihtHirelif-al ptini-iptes iui<i it« pi-HClic&l wurkinjf. Hence ' pra«ti<-«l 
uiicrxai-opy ' will meni) a disooui-HO on, or dJBCUMioii uf. the melhiidii 
of emjiloyin;; thn iiiicroscipp and all il« 6UUple8t and niui« cmnplcx 
appliuncex in llio niiist perfi.>ct manner, lutsed alike and eqtudly upon 
theoretical knowioilgo and prncticjil experience. 

On tlii« condition a ' inicroscopist * nieaiiB (or at least implies) 
one wtio, underHtnndiiiK ' microscopy,' applies liis Uieuretical uul 
practical kn<iwle<lge, either to the further iinpravemeiit and petf«- 
tion of the instrument, oi- to such branoliei of soentiHc research as 
he luiLV prolltiihly employ lii^ ' mici'Osi.''opy ' in pronecutin;;. He U, 
in fact, a man employing specialised theoreticsJ kuo^-ledge and pntc- 
ticftl skill to a particulai' scientific end. 

But a ' 7tiicro»ro/ii<^al »)riflff' Um a TKihlo raiton (fAr^ because 
il is eslabli&bed, on the one hand, tn pnimote -without consideration 
of nationality or origin — improvement* in the theory and practical 
construction of Ixith tho optical nnil nic«hanicnl part* of the micni- 
licope, and to cndcftvour to widen its applicntion as a Bcieniific in- 
strument to o^ery ilepartniont tif humaik knowloilgc, nrcording. in- 
vostigating, and discussing every n-fincmcnt and cxtnnHion of it« 
Upplienition to nvery ilepartim^nt of science, whether old or new. 

In this Minse no more prncticiil dcHnition of a ' micrnKrx>|)ii»d 
KOciolv ' «<n \>r given than is contiiined in the inviduidtlu ymigni of 
iha 'Journal <'f the Royjil Microscojiicjil Siiciety ' from the end of 
1880 to the pi-PM'nt day; and nolH-tti-r Juslilieation for the existencv 
«if such a K()cioty can he niHHtml tlian i» afforihtl by thu work donr 
dirtvctly or indirectly by it, in inciting to imd promoting thi; theo- 
retical iin<I practical progrvsaion of the instrumrnl ancl its «ver- 
widening applications to the expiinding arms of natund knowlnlgr. 

In tliis chapter we pmpusi- to diacusa the lievt pnictical mothiab 
of using the instrtimrnt and its ajijiliances. the theory concnming 
which has alreiidy been rliscusted, while the mode of ajijilyiiig tliis 



knowlMlge to biological ntul othn- invcstignttoiu U «iil«V(l upon in 
tha italiaeqtietit dui]it«r3 of tlie U-ak. 

To Ive^fiu liis work with buccc« - if bis olij<?ft bo gL-iitiiiio work— 
iIm> MwleDt niusl be proviilM witb wjuk- ru<>in, or portion «i u riKim, 
wbkh Im" can lioW aftcred Ui tiin [)ur|Hi»e, l'iil>-!u »i>tx.'i&l ijivfwtifja- 
liouitare u>ul«rt«keii, it is not il lurf)^- an-ii tbit ia itHjuirecl, but a 
cfHce oomiuaiidiiix, if pMwible, h north hhjh-'cI, uti<l which can ha 
AfMitged to rvodily exclude the daylight and ci>iiiiiiaiid complete 

The first reqair«in«iic «-iU be a tuilaih fabh. 

Tlus shoutil be lbi>roughlv_/fr»t, niid it should be rtcUtngulnr in 
iJutpf, A round table, if uuall e-ijieriiilly, is most unde«ii ruble, ua it 
offers no rapport for the Arms on either siile of the instrument ; und 
with uro1ong«d work tltia is not oiily n serious, but lui absolutely 
fittAl defect. 

In A rectnugulnr tnble the centre may be kept climr for micro- 
HCiipicnl work, whilr' there nre two comers «t the hnck, ono on Uio 
left and tbe other im the right li&nd. The former may Iw u*ol for 
the locked fluo or ii;ln« shnde for protecting the instrument whon 
nnt in use ; and when it is in u«o it in nn way tntcrfcrts with iho 
uaefulnoM of tlw tnble. In the nme wny the i-ight-hand wirnor 
may !>•■ iiu>l for tht^ luihinet of objects wbtoh is being workt-d, or the 
M|>j«r8ilus nnilful fi>r une. 

Tlw miuL iuiiHirtJtiit jnrt »f tbi> tubli — that ik, thi? niiddtc, from 
front to back — .010111(1 Iw kept ipiite cU-nr for the puqiiiM» uf muni- 
putation, »ml a tuHicicnt spncv nhoulil be ke|>l (^luir iin eithi^r side uf 
liie instrument for resting tlic nmis, and no ioo»e jiicces of ap|Ntnitua 
tdwDM ever Ixi depoutod within thiw" spaces. This soon bi-eunies n 
habit in practice, for ejcporicnoc tenchwi— >nmt!tiincs painfully by the 
unwitting destruction M a nmrc or Ifrw ^vduidiht upplianct'. 

'n»e spi»c»« til the right iM-yond thiit left for tbi- arm of tb» 
opemlor may lie uied for the work immcdiiiirly in Imnd ifsi^cially 
for a s>>cond nnd simpler niicriwi'oiM'. An iuNlriinifnt with only a 
oanrsT' itdinstment nnd a l-ini:h or n J-irich objtHilivtr will iuffice. or 
m ipfid (lisMvtinK-stjniil will nnswer (-vi-ry purjiost-. Thuse who do 
much pntctii'.it wiirk will find ^.ui^h u plmi nion* rapid and mora 
efficient than llii* cuiubniui! nirtluHl of u nitnry noHC-jiiet.'e, especially 
where iTritical work linx to be done. 

When work i.i lieinji done in a durkenwl room ther« should bo 
on the pxtrenie ri^ht a smidl tamp with a paper shade. (Special 
slMileH for tliis purnum can be obliiiiied from Bilker, of Il'iUxirn.) 
This bght may W ke]>t low iir \xse<\ for itt^tural illumination when 
rpnuireii -it i» nrver ohtrusivi-, and always at liund. 

A lumiUr >i]>ncie on the left liiind should U- i-esen-ed for a small 
rmitid stJinil litled with a Hat cylindrical ifta^s sluide with a. knob on 
thtsUip. Tbe itnnd should be'suilably arranHed to hohl two eye- 
pieem, thrve i>bj«?rti,ve«, on« condensrr, a buttle of cwUr-oil (fitted 
with a suitable |>oiiiter| dipjiM-), nnd a Iiok conOiining the cindensrir- 
BtofM. Tliia ia a mrat U««4ul arraii|C-ment fur finch a tahli' ; and It 
iieed not have ,i diuuiet«r greatt^r tbnn nine inches. 

Thr »iif/-r til'! It'll oftiieJi ii itMr nhuulil be 4i X 3 feet, and as 




no work, sucb oh iiiouuttng or diiueo^g, umy be au|>powd (o be dona 
ut this tiilile, it is well tu cover the nurfnce vrilli ntoixN.i'u. timt 
being very pleasant and Kuituble to work upon. 

It sliould lie i-enieuibered tbat fur u fuU-siEecl niicrusoope it defith 
of three feet is required fur ooiufortAble work. When the uiicro- 
soopels set up fordrawiuK,' the lump Iwing use^l diivcl, 2 ft. I>in. 
is the intrrowest limit in wliifU this i-nu be At'ooinjilinlieil. 

Another point of niuch importance it the /ifi'jlit i\f Iht Uiblt. 
Ordinary tables, being nbout 2 ft. 4 in. high, are too low even 
for largo micitiscopes. Th^u or Ihrte inches higher tlmii tFiis vritl be 
found to grmtly AicilitAte all the work to ho duiip. It is beet to 
have the table tnnde completely, on thoroughly notid sijiiare legs, to 
the height of 2 ft, 7 in. ; but we may employ the ghuw blockH 
employed unrlerm-ath piano feet as an expe<liont. It i« furtlier im- 

fMirtant to have the table 
([uitc oprn undemeatli, 
and not with neitts of 
(Irnwprs on either oide, 
bpimusf with tlm pnr- 
tiirutnr tidilc it will Ih' 
fiyi|Ui'ntly r-iiiiinil that 
two ptM-Kons may sit ude 
by Hide, which a only 
nOKablr witli a vlrnr >]>Acr! 

Till! ni'oompnnyitig it- 
lustrntioii (lig. iJKTl, with 
thii a])pendoil rrffrences, 
will mak« (iiiito rlcar the 
chnractrr of the table 
which wr mrxiiimirnd, u 

I>. SLuinl "( aiiiHunlui- : 1. Bunk; T. IdUit* iiiicRi. .„^|i _„ .l_ „-,i,. -e ..,:»i., 
-«jK,; «. Ho™o,l inlcro«»|»; V Wr.ii!i« p»l; T'"" "" '*" "**'"* ^' """« 
10. IlDUWfaiitiuiil; II. Iiiiclil-mndUlar. >t. 

The tfthlo nboro de- 
KcrilKid is supposed to lie emploverl wholly fur ^'ncral purpiuuw at 
nluiervatioii or research on wholly or {uirtialty mounted objeetK. 
Itut thi^ miori iK<-»piK( wh<> oimK nt mom than ihi.s will rrquirc an 
nrrnng^nii-ni foT' diwicctirijr, mounting, and nrrantpiig hintologit-id 
iin<l nihcr pi-fjiuiiitions, and in sora« cju^rvs n ^ici-iid (abtn for geiwrnl 
purpoM-x of micriiw-opiral biology. Tlieae art; I'lTliiinly not cjaien- 
liid.-s i-sprcinlly if tin- work done is a mere occa.iioiial (H-i-ujintioei ; 
but when- imythinj; like continuity or jii'inodienl n-gulitriiy uf 
uccujuitioii with »ui-li woi-k is inlendi^d, it will be of f;mnt .lervtcc. 

A ilUfr^inf/ ititd iitiiuntin'j table is indf*<l cif ini-»tiiiiable %nlue lo 
thrmewho a iTuo I complete cirrWaiid clcknlinesoi in tJie awompliKhmenl 
of Mich work. 

We have found in inuctico thiit a tJible Brmly made, wilh a hd^l 
of 2 ft. 6 in., Hemioreukr in fonn, and a little more than mlt 
the oircle in an-a on the outride, with the arc of aiiotJier circle cut 
out from it lo rect-ive tlio neniom nilljng »l work — much after the 
fosliiou of the jeweller's lieneti — wrve-i admirably. A rough BUf;gM> 

I Chuptcr IV. p, SJW. 



Tut. HMT.— Mipro«cofi"l'- tuliln. 
(ii<«l«, linohlo t fo't.) 

i. CAbiiiel for ofajnrt*: 
4. Lftmp with ihulc: 

1. Cue tor micmocHipr: 
a. MlnioKtopu lump ; 



gBTaa in fig. 388, whiofa pnnntB Uw plui of the top 

The whtSa araft beneath Bhoald be nnoccnpied, bnt at 

B nukj be pn^ not exteoding more than fonr mches 

■nrmee ol ttw top of the table : On the aide B a 

dimmn, with everytiung reqnired in the form at 

metmort,/on^a, fipattu, li^r-«IulM, &c in tlie npper 

tiulting fHan, miott ^uart,JUes ot Tarions coarae- 

, iui. in the other ; on the A side a single 

_ MfM, antra of tmioiu thicknesses, bone, Itm, 

r etHs ol ^ (uaorted) sini^ wateh-glasMS, ttaininff 

tifterm (if need), mmo with fine teeth, kone» of vuions 

plata Cor grinding and poliihing glass, ^, platiimm 

hteida, three 'No. 3' sable brushes (water- 

Flo. SIM. — DJBiecting and moouting table. 

this way all that is needed for dissection or mounting will bo 
t reach without moving from the chair ; and if liy an arrange- 
wlueh most moderately ingenious manipulators could accom- 
Th of the articles in the drawers has a fixed place, there will 
idifficalty in finding by touch what is wanted, 
ha taUe top may be of pitch pine stained black, or, still better, 
veiy hard wood finished smoothly, but ' grey.' 
W qwce in the figure immediately in front of the operator 
ha snt out to a convenient size and thickness, a thick plate- 
dab whoso edges on the right and left sides shall be slightly 
li^ ao that it may shde firmly inb) a prepared space cut into 
iwfanrt of the table and occupy this space, the surface lieing 
if lerdwith thesurfaceof the table. This plate of glass should 
■da Uack on its under side, so as to present a uniform black 
ML This ia often of great value in certain kinds of work. 
1^ naefol is a^ro^y whiU unabsorbent surface, and a slab of 
ifmrettain may be easily obhuned of the same size and be made 
•xactlj into the same jJaoe. 
a vsing tiiia t^le for lUsaection the arms have complete rest. 



and 1 in tlie lij;ur« vrouM r«pr«a6nl the po&ition of the diaseetiK 

We Hod UmI of Kydcr ■ 

Fro. SSI.— H>h1i> of lUuuiInttian lot 


2 IK K Huitabl« positton for a small 
f<irjt*uenJ (chii'lly liolaiiitvil) purposes, 
answer tliU purpost- iuliiiirft1>ly. 

3 in asmnll leasot of spirit (dilute) fi>r uw witli thfl section knife 

4 IH u ntaiid of iiiinintiiij/ itirJiii, iu suitable hoCtlm, u* Car 
imltnm jii paraSin, or xylol, glycerine, itc. as wcjl M small bottle 
of fVit^F-nlM for bot&nicAl or zoological hittbology i:c 

.'■ is a nest of ap«'rtarB8 in which to place partly mounted olgocti^ 
to pi-ot«ct them fiv>in dust, whiir the balNniu, damiunr, Ac. may boj 
hardeiiiiig on tlie cover so as to lio in a suitable state fur tinnl ntimnt 
ing. A slide may go over th« alnptng front of this and wholly i 
elude dust 

6 is H stand of cements, vamisbca, Ac. audi an arn needful ; aiul 

7 is a tiirn-tablc. 

For the work of diKKcdioD, whnn tJie Kubjoi-t roijuirw roflcclcdj 
light, one of tlie dtisidcrata is a tuoA*' of illumiiintion at uncn conve-^ 

nient Jin<l intense. )Ir. Fmidc | 
R. Cliesbins F.L.8. &v.. wliu 
work on 'Iki« and Bra-kMp^ 
ing' U a proof of kmiwled}^' 
and praotioe of minut« anatomy, 
adopts an old plan wlijcb wa 
have always found udniirable. 
It is iUuslmted in jig. 389. 
Rays of light frotii a hiin]> are 
|uirallelt«ed by a, buU's-nyc full ujion an Abroliitni's priom and 
focuaned u]Hiu the objwt. Th« pri-ini may Iw mountod on a long 
uiany-jointi>d arm ami is of moat varied u.i«fulne«a, A St<*]>he!iaon's 
binuL-utar is. we believe, employed by lUia gentleman, bat it will 
serve admiiably for any form of dissecting iustruuent. 

For the nmre w^'uoral purpose of the prirate Uboiatory a iJait, 
Jinn tabtf 4 feet <> inclies x It feet ill area, of a suitable hei;;bt f<>r 
tho worker, slioubl be fitted as follows, viz. : if lig, 290 repreHrnt the 
rough plan of the table, 1 and ^ are gas fittiii;;s attached lu the maia 
to supply Moir/ii/ie. Itnnttn'K buriirr, Jtc 

4 is a small tube of melAl attached t*i the water umiii, with a 
lap, and bent in the fomi of an inverted n> with the attached le;; of 
the C\ th<^ longer. Tins affords a. pleasant stream uf waterf'irw&fJi- 
ing <tiMirct<inns .tc. ; mwf j/' iht ujwn end be ni'uU with a tcrrti; arvt 
liavr a miiutbly imui" piiKt of tnbiiig Jiltrii,lo tprrtv an to it, thin huii-r 
Htaj/ ff atlofhf't to an iiuHnrnhhfr Ivhr, at the vthrr trwt o/tehieh tt* 
may j'oMlf n Ji 7ir i/Ium iw/tiic*, which will act im uvu/i bulll'foj'lkfjituit 
hon, and scn*i" with the linext dinwclirig work. 

Ti in a glaKK trough for wacte, with a perfoiated aperture, 6, con- 
nected wilti a wnKtC'pipc, thn:iugh which the waste water jac. flows 
innocuously away. 

3 repn'M-nt« the position iif a Thonin micTt)tome,and A It are two 
well. flamed Hat Klides, which may be drawn out eighteon inches, or 
1 Jount. R.U.S. now wriH, IHHT, [>. IWl. 




t'lii. XHV— titbornLxty tabid for microtocinnl 

|ta>Ji<^ fully iti. Thi^ nra found uL tiine* to b« ()t grrat *or%-ic^ 

■ Iwri- tin- KiMc*! is ni>«u(^wluit «>iiliii«i. 
T>ii» tAltlrnuiy lH>littMl 

110 one sUlo (tlll^ Irft) »t 
1 1nut with B set iif (lniw«-rs 
land khulvi^s fur r«i.-Mvitif; 

TBricHu* apparatus un<l 
t 'materials, with Uff^ei- 
|il(uuiUtk«af !iUiiiB,an(l re- 

aKMilo, har<li>iuuK. ninc«- 


wbUc if n ijiior covers the 

whiiitt, tlM> inner side of 

ikis UMy be readily fitted 

to receive drop-buUles ' 

rant&iiiiii^' »ll tJie KtninK, 

rwf(eiiLH, and Hiniiliir tim- 

(«Hnl» iti constant usi-. 

If ibme lie Uibellinl wit)) 

paper IaIioIs saturated in a 

tolntioii of solid pHrnllin in 

lurjieiitiii'-, ai)d nftor thi- liii-penlin* tiiu crnpomted firmlyfiied on the 

liolile, Ui*y »r* titv pcniiniii-nt, and, indeed, bottwr tnikn anything 

we have tiied wave where Uic tuinte of tJii! conl«itH is enamelled or 

«ngT»ved on the liottlo. 
It has been already 

piiuitetl out that tlieri> 

m i-ouditiotiB of re- 

Mirrh in which tlt« , 

iniciiMtcotx^ bns to t>«'in 

n ciifiabiiitly vertical 

|)>«ltion. ThiH was the 

Mae with tht> researches 

on the saprophytio 

Ofprniama Diade <.-on- 

jrnutly by the pre«wiit 

Mtor and Dr. J, J. 

Orysdnh-.* It must 

tiways be the cate 

where certain forms of 

continuous life stajcw 


Pia. 191.— Tcipod (or aune mi«iMMp* ip aa 
upriglki |Hi*itici». 

are einjdoy^ for prolonged or continuoos observatioiu) on tlie dove* 
lopnient of the minuter forms of life. 

In Hu<-h cAMeN the Inble is quite nnnuitable. aiid special Ktanda 
have to Ix- eniplnyed tJutt from their form (jive ^fiat stability to the 
mtnuseopo, and atrord the body and head of the observer na niuvh 
Dnrnmand iind rmso in tuiing the instrument in this awkwnni position 
M cMi be obtained. 

■ C1i>i>i*r V[L p. itL 

C * UoHlUf V>>n>. Joum. roI«. X. lo xvill.; /nnri*. II.\f.S. vol, lil. p, 1 T0I. V. 

I "rt**!!. p. 117; vol. ri. |i, IBS : »ol. »ii, p. IM; rol. riii. p. ITI. ' 

pRAcnniL 3oa»os«-ftpT 

nil i* txK 4aa« bjr mmbb of • firm\y matlr irif 
ikifed fif tM u tiM top BMMle to ncvivic the fei>t of Uwi 
1^ £91 »•« («t&B«<rf th» eMwtmetHin. Tltc 
sepad ftiv *<fl!Mtiimd finul; bnuml f^g^h^r wit 
A. A H tW b«d lor Un> tripod fret nf PowclUnd ' 
B » • t»fek 4idi sb>l» to th» ln:H «f A. A. nr downl 
yaatian. TUs it amiaiy to mrivR the lampi. 

S^ tim ■cwMBMMit thr bxi^ cnn :ki pUcv hsetf as tol 
■•at fuljr, uhI thm is *n urai^nnvnt M tin f 

FkL tM.— t?«lag the Blcrowope ■■ •> urrlgtit j^idUail. 

A. A, to nroeive ntmHirts on whidi tlir nnns luav rvai 
other niamjiuhtliun than tbut involvui in vorkiu^ the 6t 
meat antt the niUlnl Ik-iuIx cf Un- nUtfCf » rcijuin^. Tbi: i 
onng this amnpMnrat is xvit in li^. -■'-■ lu tlist va 
ttMirbolriseniJil'iyed fur (heiiinkinf;i>famtuvr:i )u<i>lttitm<*'t 
A^-inchobjwtiif. But tlm [Mjsiltiui of tiie basiil IiiiMMi. thf i 
«copo npOD ii, the jmaition of ilie lamp ([mrtly -^rn in ihie ia 
Aicvgrottnd i>> the Ml), iiml thi' rvUUve wist- uitli which tl 
initninirni h nt tite i-ucaiitiind of (Ite oUttrver will be 
III ixxIt to use the luioroscopi.- sucnsbfiiUy. «« 
iUsniiiMtion tbo iniviiisiiy ct whkJi w« c»« fully nij on. 



rtnunfualilifA that inFofiY aJniHfii'irt at tiinnc,nn(i unitTSppoiiil 

_Ji.itiimw!., in il« einulojrmeTit, tiul th in •■ lite exatption rather (Aito 

th* rii/t. Wli*t U needed in a wkH-dimIo tamp witb « flat flame : 

heiffht . 


tliii' wc^ thoukl tw able U> conirnl with uraal r«so ns i 
{liKtanoe from lli« uicroMOOjw. XotliiiiK i> r([uiil practimliy to 
^inch or n l-Jni^k ptinflSn Uinp ; thin (pviin tlit? whit^^nt light nrtili- 
«iaUyw!Ce)adl>l«-!aive llie hiKbcl'iDtvii^titsaf tll<Mtlca>»(llWl■l^^t rtcctric 
lijlht. But thvm in nothing; ut preneut ■c(-i%i!iibl« tu Hi'- .ituilcnt of 
tiiia kind. Tbe etii]>lu5nient of the et/r/t of llie iLainL' of a well miictu 
paniffin lamp used with 
K«k1 • oil ' biui HO present 
rind. Uaa in much yel- 
lower, snd not so «itsy in 

Tu f^t the best form 
ei Biii-rtMCopicMkl liunp ih h 
nutter nf winiv import- 
juiw. Wo call tbfnttfn- 
tif'D of the rviuler to t)i« 
bnit siuiplo form of Ininp 
vhtch will iUciinipliiOt 
rv.TV piirpos.\ This is a 
WkIcI ui-mn^ by Mr. 
^I'rlMin, anil tW ilrnwlnj^ 
«f H-hicb is givi'i) in tijj. 
i9a. Tbir lump banin 
paratHn mimI has an orrlj- 
nary ^-iiKh wick bani«r. 
Thn rowrvoir in TfKtmagu- 
UrMudfl>t,&l X 4 X Ij ; 
it serrt* tJiroo diitinct 
puri>"*^''= IeI, it wiIllH>ld 
»Df!)ciiM]t oil to bum for » 
whole dny ; ^ml, pt^miits 
the Innip to bo iowfrwl 
Bear thf^ tablt? ; 3nl. nidt- 
MtJt tbe heat ctmductcd 
by iIm metAlcbititney nntl 

«Dv«ntH till' oil binling. 
te IniiiP in pliicpd at on« 
ugle ol the reaenroir to fto. sm.— L«mp Oerl-d bj Mr. E. M. N»Uon. 
tOiiUe tlte flame to be 

pbeed very nvar tbe Btafie of the laicToecope, which ts ^loi-eJiugly 
tueful witli BotnekiiKUof itlumiuution, enpecially with retlcotedliglii, 
witli the Iii;;l)or powers, iwd for Powell and Lealund'a aupcr-Ktage 

Tbe hole for liUiiij; the reser\'oir is placed nt the diagonal oomer 
for convenience. Tbe diiinuey is metal witb an onlinary S X 1 
glass slip in front ; the diameter of the tiame-cIiAmbcr iJioiild not 
«xc«ed l\-j»ch, and the (crooves holding the i^iatu slip should project 
J inch from the flame' chain her ; tlia aperture sliould Iw only 1 1 inch 


lamp diiinneys; 2nd, preventa reflexion from inn- ■ uiir,.'^ ol 
chimney, which causes a doulile iumge of flatno ; ''i^l, |itvv«DU 
scattered light in room ; 4th, is not rWMlily hrokon ; slips can be 
«utt\y replaceid.' 

> U it ntf importkot to remove Uir meUl chiniupy utter ua*, ec U UnM nut Ut 
livive it onwlieii iiul In uiw<,*iTiM> lliartapuntlnK porsinn |[»lh*nn>Dii4iIuid cmik* 
undMiMlilo M»iil when Uia liini|> i« t^m lit. 

LAMPS 349 

Bjr rotation of chitnnciy rjther thn ttlgn or flat tids of th» flKnm 
may be uwd. Th« )>iitl't-«jrK is of Hcrachel's f<>mi, vb. « ni*m»rtis 
aud croesed convt-x : it it moutiUNl on nn nrm whidi rotates crn- 
may with tli« lamp flnnic. The nrm is slotuwl m that tbii ball's-pye 
may be foousud to tbe Untno ; it can b^i tixcd by » clamping sm^w. 
Tbe bull's-eye may nIso bo cloratml or d^prostncl ivml lixw! by a. 
tUnipingsriTw, not shown in th«- illuBtmtinn. ThnbuH's-i-ychnviiig; 
once bMn focuiwcd in prnnmirntly olamptNl, nitd it is liroiiKbt into 
or Ijikei) out of pOHition simply by rotation i>f lh« nrni. Tbinv slii>nl<l 
i*- a groove in tbo pillar with a stfinilyiii^ pin on thu Innip to prcvctit 
rotation darins clrviitinn or cteprcsiiini. 

The form of thi' clnniping klti-w ia im|K>rtant ; it slionid he at tJiu 
mper part of the tulu', nnd not at tlm lower, as shown in tliA tignre 
lliis keeps the «cr«w oh-an from oil, wliich alvraya, to u Kreali-ror I«ibi 
nctent, extuIiM over p«niflin ]a-)ips. Th» screw sliuuld be uf ilutt 
fbnn which cloce* a pincliinji ring round the ro<i, and not merely a 
MTTew which sirrews on to tlif rod and bruises it. Tliia lamp, if 
niade, as it sImiuUI itp, vriUi a jnpaniiitl tin reservoir and « rast-iron 
tripod foot (fie. 395), ia (|uil(r ini'xjwnaive. There is oo justiUcstiou 
for a circuUr foot, except that it can 1>e readily and well Bailed in the 
with Imtti-r ap[Mir<mt rtwultAiuid lees labour tlrnn other forma. 

A Mnnll lamp U made by >li'HHrM. R. and J. Beck. We Ulus- 
t«tt> it in lig. '2{'l. 

Tim huip. A, dnuastti of u heavy rttif,') into which a square bruas 
nd, B, tssTrcwMl. Tin* wpiniv ro<l carrip^ a socket, C, with an niin, 
D, to which tl>« lamp is attjiched. 

On each sidr nf the liurncr. And attacliMl to tlic Ann, D, i* an 
uj)ri|i:ht nxl, U, tooni.' of which (lie ditmucy U fixed, iiide|>endent uf 
the rw*r»'oirof tlw lamp, thus fiuihlinK tin- i>liser»-er to revoKe llie 
burner and merroir, and olitaiti either Uie (!<l;,'e ur the flat »ide of 
the Haiw without altfrinx the ]Kwiti>>ii i>f the I'hinuiey. Tlie 
cliimn«Ty, H, i» madi- of thin brii-vi, with two o|i«iiinKft opjuwile to 
<«ch other, into which xli<l(* 3x1 ^'luss »!!]» uf cilJier white, blue, 
or iijml gInsK, t}vf latter wrvin-; 1L^ a Tellei-lin-; hut we do not con- 
iwler the ri'Hexioti \u:ti- aL'i-i>mpliiihed aa other titan an error; it 
Tansc« doubh; mllexion »nd oinfiisee the eondeneed iiuaxe. 

A M-micirflt! Mwiiijjs fniiu tlic two uprights, (J. to wliicb it is 

ittiichnl hy thi- jiins, H, ptiiw-d level with the middle nt tin- ll.-ime ; 

to thix Hiioicirelp i* lixeil a dovetailed bar, I., carryinf; a -ilidin}; littinj;, 

'», which Ixstn a Henwhel ball'a-eye, P. This is complex and iliere- 

ioTf wwtlv. 

Tin* Imll's^ye U fixed at any inclination byn milled heaJ workinj* 
in ■ idott«t) piccu <>f bnu^ K, fixed to the arm, I>. 

For nee with tlw iui«r<»«ipe in an upri^'ht position, when proloriBwl 
inviMigation.i ban* to take pluc^. the Ump becomee even of more 
importance than under ordinary circumstances. The prracnt Editor 
deiiited n lotunwliat elabomte apparatus of this kind, which ho 
alwnys eroplo_n in tliis kind uf olnervation.' But tJie essential part 
of it M only on nrTuugeiueut by which a uiillixl-head moromcnt of 

< Uonl^tg ilicr^ Jtmra. lol. xv. p, IBS. 



Uie entira lump xany tiik« plnrp In i)ia right or the Mt of th« ob> 
serrw, lu w«lt u.h h Kirnitur [Mtwrr to ^l(-%-iit« or ilepress tlie poiiitioa 
of the Aariv. Whfii tho tiiitroiKiijn! U lixtil, »ii<i llie rectAnguUr 
priani for illumination ( in yhnv of the miri-ur) U Axtd »t right an)*l(«, 
tlie oeutriu^ of the Imnii IIjiiup upon thi- olijei-t in more r«ailil}- (lone 
by meoiu of motion in the Inaip. A rcnr simple form of this Unip' 
luubeen nwle for the EdiUir by Hr. CIultIm Baker, of fiolltorii : 
it is wen in fig. 29fi, being uii ordiiiiiry luinp, except thiit tbd millixl 

Iieiul to th« rit;ht iw wc ime^ 
the damo racks up owi down 
the enlirv lamp, ■ndthw luilM 
bend behind, nnd nt right 
angles to thi*, works a nu-k 
and piiiioTk (showit in Iho cn- 
gravinti;] cniTving thn wliolii 
Ump to th« riglil or left €)f 
the raiddlr position. Tiu* lump 
would Iki l>plter, if thr xtudciil 
did not object to t)i<! cost, to 
be mBdvwitfaaiiw-tid r^tervoir. 
or «t lrtt»t t«> luivi! itn Kmuiffi^ 
OMTnt hy mnuM of which tlie 
bull'K-cy<' (with i> cuteh fixii^ 
its focux from the llutoe) were 
M> alhxrd iv to b« carried ut> 
and down and to right and Mt 
with the Inmjk. 

When thi> microMape w 
fixed in its upriglit |KMition, 
and th«^ jii-ixin m arrwix^l ti> 
girr clirrot and not'oliUque te- 
nexitai, the lamp flame, b]K— 
means of a curd, in iit'rubj;e(|^^ 
as iirnrly riglil for the r»-^ 
tiexiun of the ima;!« of Ui« 
tljinw into the L-eiilre of th« 
rii-tcl IIS m»y lie, and then a 
.. ' ~~ little iiioviriiient ill cue or both 

' — niilli'd heads will bring it ac- 

('umt4-ty into the lield. 

We may artnni.-ethomicn'- for Dnliiiai-y transmittiil 
light, tliat is, for light causc<l to posn tlirough the object into tho 
objecl-Rla^i, by placing it upon the table, arranged as *lrMdy 
directed ; the instmiDcnt is then sloped to the reqiiirod porition, 
and a ooiideiwer, siiilablo to the powrr to l)e employed,' is put into 
tJie aub-sWge. The Ump is now put into the ri^ht position, with a 
bull's-eye, on tlie left df the nimcrvr-i'. Tlie condenser is then, iw 
described below (p. '&:>{), to bu 'centred'; when the objiiclivo 

' n<r« ChapMr IV. p. Ma 

Ti... s.':.. 



'liti^be chauK«<l ivs il«%ii«(l. and Uie Ajre-piew olwred Ui suit. But. 
it nhoulil In- oan.'futly iiulrd, lluil if Apovbrunistif |)owerai«re lieitig 
kimI, thi^rv luUBt Iv aocunitt^ luljUHliuent ii( the tube k-ii^h if th9 
Vqt retiulls iirv in lie obtuiiicd ; niiJ with uiiy Meriotih iiici«aM o( 
ibe |iour«r of tlieobjectiveavuDd«useruf higher a|)ei-tui« and shorter 

iJdeii. }iowf ver, as gMxl or better results may hn olitaiiiecl with- 
oBt l1)i< ein|>loyiDeiit of the mirror at all. the light boitigseiil directly 
UinHiKh the L-oiidetiHer from th« Unip Hame. The itiode of arrnnge- 
BM'Ut for thia kind of mAiiipuJAtiwi is pivsented in Vlate V„ whvro 
it will be ob«erv«l that thir microscope in inciinetl more towanU tlut 
horuxnilAl to suit the obsrrvfr ; the Uinip is directly in front of thn 
wb-fit«go, the mirror is turiMid naide, and a framo (fixiil ii|h>ii a hull's- 
eye stAiid) r-arrying ncrevRS of coloumi glass is jilitoil Ix'twi^m 
Uie lamp tl*mo mid thx con- 

Litpiiier (s»b-stAg«). 
By this means the light 
is sent into thn cunden.s'-i' 
■nd u[ion tho ohjovt, and is 
_ lltm tr»trd i» is thn vmw 
(fur ci-ntrinK) wlutn the 
nuT'ir IK u.vil. 

Tlif tint Hlcp in thr 
ihrvotioii itf I'fHcii-ncy in the axf of tlu- micnwcope is U> underNtand 
lif prinei/Jni of' i/Iuiuinaiion, and a knciwlrd];i: of the variouii el1ecl«' 
froducviii by ih^r htiU'it-cyo lies on tin- threshold <>f this. 

Ilnfing given dirbiiU as to thr forms of taiiij/ which uit of iuu«t 
nrficp, we nssumr thnt n pnrnttin lamp with .J-inch wick 'm used. 

If we placv thi" flip- of this llninc (K, lig. 2'M) in the centre and 
nxart fiKtis nf tlio buH's-rye It, A shows the etlcct of doing so. 

If a piecii <>f cnrd wrrc hi>ld in tlic p'lili of the mys proceeding 
tram R thcpictun'ftn shown at 


Till. snn. — Ed^ ol lunp a»uu> ill mitr* aod 
locus of bDire-Drn 




A would not 1mi SM-n— iiutmd of 
it An <'ntiiri{i-d nm! invcrl'sl ining>' 
•i th^ lUmi-- Thr image nt A is 
lined by plncinK thu itye in 
he ny* and oy looking din-ctJy 
at the huirs-ry*!. 

The liifhi U so intMtNV (hat it 
I mort- {lintximt to tnk« the li'-lrl 
of II 'i-inch eye-jitMct and 
' it in the jKitfa of tht> I'ay.-t 
niig the imago of ihtt liull'!!' 
MX> on II mrd. It xhould )h' 
□nticnl with c«rR tltal thi* 
Jiamrtrr of the dini^ A tlt-jieniU Upon iht- dinm<-ler of thv hall's- 
vyt a ; Itut Uw intirntity of ttitf light in A <h-pi-iidN on thcr fucnl 
KtiKtli of B. riui nbortcr l\\e focu», the mon: intrniu' will 1xi the 

We ar« here aasuining throughout tluit the field Ivnx k nt n tixcd 
diatonoe from the buir«-«ye B. 




FiOi JDBu — IIi«inib of |ilMJnii fluno in princip*! tocua 
ot cooMW nilrror. 

JJut if we mow ihrjlamr, ^—Ntill contml — mthin th« focua o( B, 
w* gpt tbn iTBult sh<n*n in I>, &g. 297. Hut liy moving K it-HAmit 
the- focus of H wo gnt the picturti H, wliilo K is tin; picture vhcn K is 
focUMed hut twt fHlred. 

A common error, una mptiatMlty iiMit with, ik tliiit nf plftcini; « 
concave mirror (' (fig. '-'08) ho tlmt thi' finiiii' V. in in itn firini^i/nl 
j'ocHf. Ttio n'Milf. of this in tlint fiarallrl rriifi nrt) ■rnt to !(, 'nicse 
mjni cni hnwght to u fnoui at « diatiince frotii R ntKiut niunl bo twic« 
the raidiuK of th» curvature of B oiul Uion scJattt^ntl ; n tobtlljr 
■diSaimit rCKtilt from wliut in &iine<l at. If tlit; codoivi; mirror, C, t* to 
±t be uf any tue in illami- 

untion, it iiiUKt tie plimtl 
HO lliul E is N>^ Af iu 
principal /oe\i4, but at 
its fvHlrf of' evrmturr. 

The buir»-pyr givis 
ab iUustnilidn of what 
in of wider applicution. 
nStemeduMl of obtuining n, critical image with a coudeuaer by iucaiui 
oftmsnitteil li^lit in !.)ii>wii in Sg.'299. K is theedK«o{ the tUoM-, 
S npiwents the iiub-«tagc condmiser, and F the object. V is thtu 

the focjil conjugiLte of E, 
and F aud E ore in the 
principal axis of S ; tlwt 
is to say, theae are the ra- 
tntions wliich exiat whttii 
u condeiuier is focuiwed on 
mid centnd to ui obji>cU 
Let this be uudentood us 
the law, nnd th«ra can bn 
but little di tSculiy remain - 
iiig in K^tinii; the lirst 
results from a coudeniter. 
Fix- 300 illuKtmlm 
another method of n^Uinf; 
the same result. \Vc may 
illuminate a coivli-n»T 
with light direct from ih« 
flame, as iu fi([. 299, or vtr 
may interpose the mirror, 
OH in &g, 300. M is tho 
plane mirror and,|)tv)pedy 
uiicd, exactly the lotmo 
rrault may he obtitineat an 
in the former case. It ix. 
howc-er, slifthtly morn 
difficult to set up, Inii will, 
on the whole, be nrcfer- 

Nothing can bo of more moment to th<i b»^nuer tlian to undcr- 
suad the practical use of the condenwr. W« must direoi tlie student 

Flu. Wtu— Uode o\ obuinbg crltio*! im^a. 

Pio. MO.— Anotim noUiod of gnttlug crllinl 






■tated oonaenung it in Cluipter IV. Bat the 
■4>«"'*^ be atnlnllj oaaadvnd. Fig. 301 shows a sub- 


Via. 801.— CtmdBiiMr and objeot- 
gU« witli the wms kpertoie. 

r, 8, and an olgaQtivt^ O, 
nd en tbe Mme {Mint. The 
hma an »pertBieeqn«l to that 
ijiJCti Te. iSffw if tbe eje- 
■moved, end we look at the 
ai tbe abjeotiTe, it will be 
foil of li^t, as Kt B. The 
)t but witb tbe aperture of 
tmr cat down hj a stop, is seen in fig. 302. Now only a 
I back of the ol^jective is filled with ^ht, as at T in the 

t does not follow, becanse t 

mB of the objective is fall of 
t 6g. 301, that therefore Ae 
: tio be full of light. The 
bows tbe tright image o/ihe 
/tofne, and it is in that alone 
etdpietu-re can bejbund. If 
aer be racked eidier within 
; tbe focus, the u^ole Jidd 

g Ulwmint^fd, but at the same time a Ear smaller portion 
ctive wfll be ntilised. On removing the e;e-piece and 
the beck lens of the objective, pictures like D, H, fig. 397, 
tt — Z> when within, and H when without the focus. 
odition represented in fig. 301 at K and is the severest 
cen can be applied to the microscopic objective ; that is 
ill the whole objective with light and so test the marguml 
f porfions tit the samf: tijite. 


Fm. 80S. — Tfa« Huna, with tko 
qiertnn of the aoadeiuer oat 

Pio. SOU. — niiuaiiiation for ' dJBiued daylight.' 

obtain the state of illumination known as ' difTused day- 

1 tbe simple mirror when no condenser is used is f re(|uently 
most inaccurate manner. The correct method of doing 

wn in fig. 303. F is the plane of the object, C is the con- 
N*, tbe mirror being placed at the distance of it^ principal 
I the object. But the manner in which it is usually done, 
e of thought or knowledge, or both, is shown in tig. 304, 

A A 



where it is manifest that there is a total disr^ard of the true focal 
point of the mirror and its incidence on the plane of the object. 
From the impracticability of this diagram as a representation of a 
working plan of illumination, we may see at once the importance of 

Fir.. 304.— ErroneouB metlioa ol arruigeinent for ■ diffUBed daylight." 

having the mirror fixed upon a slidinff tube, so that its focal point 
may be adjusted. 

It is also iinpoi-taut here to note that in daylight illumituition 

Fiii. aiir., — l.iKlit from the open eky falls apon the mirror in nil directions. 

a {i/niie iiili-fur f^ves a ciinn of ilhimintifion, as in fig. 305, when there 
is ample sky-room ; but a window acts as a limiting diaphragm. 

In regard to the parallelism of the direct »ofar rayt there is of 
course no question. But the paraUelism of that portion of the solar 


mmrr reflectki* to a itici's from tiif, opes sky 555 

light which go«s to form tbc HrnMinniC in our own higher atinu- 
q>hen> is so ooniplotoly broken up by refraction And rell«xioii 
uwngst the subtil psi-ticti?* of Uiix h>K:h«i- ftUnrnpliont tJmt tlie mys 
vtuch mnstitulo nur tisyligbt faill from every point of the vtnib'le 
li«s««n» (Utough with fcrrotly (luninishn) intonsuy). 1'bitt ia to my, 
»e h.ivo Rt (hAjKMsl It light sourvo cxtondinc ovpr 1.^0'', (c/oVo f/ir aun 
iltfJJ' rxI'iuU )»-rr a rinnti anffl'. itf' hut half n flff/riy. Ht-iiif^ thus 
oanoonihtl t>y nii illimitabltt iinil NRlf-luminoiiN <;xpiiiif>rr of other uii- 
dolatioo*, thv iiucsttiiii '\% n<i tonp-r nf piii-iillpl rays only, but of light 
«insn«tinx fmm im oiit<:r circlo alHivn thn rjirCli upcin <^vrry point of 
^[» oarth's KurfucR ; and n oiirror exfiOMil to nK^b n luminous ntuio- 
sphere nitut liotli nvi'ive «nd raflrct fri>m nil kiiIi-k niiH ii[:>od aII 
uiIm. If, tuiwevdr, it Iw pJKced under tin- sXa^ of n niici-oiicop», 
all vi'riic-al light in int^roeplt-d, mid tlK-n; rtmiaina iiotliiiig but th« 
obUifar im-'idenco <u the sturtiiig-|Kiint uf t)i«> tlinory of illumination 
by ronv<!r;^ikg li^t ; for It acurvely neMln rvpntition llmt ohHi[uity 
of ii>cid«iit.-e gives inevitable rise to ol>li(|uity of reSvxiun ; nud it 
becomes equaJly clesu: tliat in ord«r to »trik« the object the tii/ht 
mnM ftliraytjoll <Atiqiieti/ nn ihi- tntrror. 

Then it follows fi^it uliat liaa been said thnt tlie li^lit fulling 
from the open ^y upon » mirror blls in all <<(ini<eivable dim-tiotis. 
Thus fif;. 305 bIiows the liti«fi 1 to 7, includJn;; an anglo of 30^ If 
notliiDf! intervent< the light of tlmt sky surfuie uiual fall upon Uwt 
nimir, <i h, nnd be reflected on O. Tb« int<-riiie<[iale myii, 1'. 3, 4, 
5, 6, fonn the rinirfn/inf/ UluiniiuUiiuj [H-wil, with of course an in- 
finity of otli«rH tiUint; u[< the spores between. 

Iti otbfr wuriU, every jioint of » mirror is a mdinnt of a wimli; 

lteiiiJS}kl)i?rr. ajld tiiii i» filially true irhrfliT thf mirrar br fJnitr, 
^inniir, i,r (uiirnri, so l"lis mt tjiey nre exjHHiUil t"i a lMiundl(---« »ky. 
Therefore • plane, ronvave, or convi'x minor mill givf a i-oia- uf 
illuuiinatiou i>f wbieli the objei't in its apex, no matter wliat tlie in- 
rlinatioii or distttnce of the mirror. The au^le of the eoue will Ix! 
the wirIh the mirror subtends at tlie objeot— subject of euune to its 
not being cut down by a sto|>. 

As a mutter of fa*.-t, the 
ItouiidteeN sky xi an altfitrai-tjon 
wliicb ia never obtAtned itrpnu.'- 
tice -, therefore it prarlically does 
nuke A diderenM whether iive 
plsiie or concave mirror i^ Ufted, 
and whetltcrthe latter is focmMd 
on the object or not. 

The riotteil lines iu tig. 306 
sliKvr i-aVH falling on ux differetit 
priint6 on a plani.' mirror: the 
i-uiitinnous linos ithow the re- 
dt>xio«s of these mys on tlte ob- 
ject Th" hpevy Hn™ from 
tither extremity of tb'- mirror H- the object show tie maximtim 
ung)o of (Sinn Ibst mirror will give in that {mrticulnr position, 

The induvnce of a limitation (as by moans of a window) should 


Fm. son.— Wltb Uio oimiii nky. lltfbt !■ 
(ocUMnl si hU poinU. 



then>for«i \x ronaidoml. Th« extent to which it is limitini;, so far 
us ite influenvv upon thn ill iiniinii tine cone is concRmi'd, ii xlinwii hy 
nil pxunination of (lur imck ui ttic k-tiN of ihc objwtivc when Ui» 
vye-piocPisiTtnov^d. Pig. 307 stxiwA tlir Iiock of the objn:titc wlirit 
tlie planr mirror ix^t, uitd lig. 301, R, whiii the convnvc tnirnHr 
in usedt ut in tij;. ItO^. T)ic iK-ytuiif r Uiould ntuil y tlmw! rxjirriiniitU 
by repontini^' th<-ni. 

Tfi« corifct niPtlicid of illuminntiiig to niilnin a ilark 'jniHml willi 
the objoct luniinnii> in sliowii in fig. 30S. This inny nlso )><: hmxI 
when, by iiii'iinN of thiil sliovrit by fif.'. 2£'S', ihi-n- i» not pnoogh of 
till* ticli) illuiiiiiuititl even when tli« fliit of thn flumr is uNrd. Of 


1^(1. S07.— Imu^n- Al Uii' Iwk of UlF 
«L Jidi vonluiuilij'llitht txii * filuiD 
niirrcr ■ft' uwi^. 

t«ui^«; it vfill 1m? uiiil«rstoo(I 
tiitit for tlif durk-ftitiuiid re- 
aull u nuitAblt! stop is inserted 
lN-iie*lli ibt? 8ul)-slage coii- 

It Iw4 been shown bv 



Kui. tot. — ntDiiiiiiiitiau fur djtrli itroatul |«j|b 

iDitiiy illuktnitidiiii on iiinnv tulijtk^M thnt oertAin rccutt* in critiunl 
wn>rk van Iw obtAint^l with tin- buH'xoye which tin- not 80 ncueMible 
wilhont its uslv But Mr. T. P. Smith ha^ xtrnW thi* L-Wr r«i;anl- 
iny the atructuiv of (.■ertiiin dintomi. 

fill's lli«rt! cnii be nu doubt, i» due to tli« fact that tlM> parallel 
rays, falling oii the .sub-stafi*^ Hwdennpr, thmtm iu/wuf nnd inomutu 
the unxli! I'f ihp coiif of illuniiiiution. It will \w noiiii^tl ihut when 
llif ItuU's-eye i.i intnKlu«!ii tlif coiidf-nur will need nwking-iip. At 
tile Bume t)ii>e ure pivfer iUuiuiiuitinn at- in fig. :f!>9 urIIUO, rxn-pt in 

vium where iliuininitting ctttvm of 
niii.'cimum iinKlen itre iTKiuired. 
Thus it will \» little ncMird with 
InuiMnitted IJ^hl. exee|>l whnn oil- 
imnHTsion objectix'esof Inrjce nper< 
Hii-e iiiv \iij(^. berjiUhH illuminnlinfj 
coni's up to ■.** N. A. tHit be obtaimil 
wrilh gornl rotideiinern by the 
iiiKthod shown ill lig. iiM). Hut 
whfii theniicnnBi'n|>e ih irf niV'i'Jivily 
u.-M-tl upright the reet(inf,tdar ]iriMn 
or the phino miiTur itiutil bv omkI, 
lig. 300. 

Th« arniii»eitifni iit fig. 308 is Mmintimpx useful for |>lM>tti- 
mii-rogniphy when it ih iitU'i-H-itf impt—^lr to illaminnte tli« wh<4i' 
field. But in oi'diimiy i-jiseH it \n Ix'ttnr to oontrart t]ie fleUl than 
USD a buir»-«ye, lui it iiivuriably iiu|Muni tlie (k-tinit^on. 

Kui. SWy 

-t<«iiie mult 1 

lith rixintm 



lit i«KSrcl Ui iblH liiat li;;ure it will 1m' unJumttiixI ttint(aiiWfi)rv) 
£ n^r«(ieuta the •hJkc of ihe Aajne. fi thr buU'H.i-^-, M tlii- mimir, H 
•he eundenaer uixier [li« »lAg«, mkI K lliv ]>liknc <if ihit olm-ct. 

The Mine n«ult as tlw abuvi? iimy It! dbbiiiii'd bj- tW concntii 
mimir (nn iJiowii iit fig. 309) iiuuwtl of Uie bull'i^'i^i?. But this 
in a v«ry (litfictill nrrangoment, jieldiiiK Uio IwKt nwults only wiUi 
threat ■pplWiitii.xi and carp. 

But tJi« Biipivrw folly nf using n t.-oiK-.ivi- miniir nii'/ n Im/ra-rifr 
msImiwo in lig. -UO, wltrrn C i>^ tlit- c<)Qcave iiiii'i-->r uikI (iu Imfuni) S 
tJiesub>%t«gi<rH»ni)i^iiwr : thistteciiros a reault— un wilt Ih? ^icnn hy ilin 
reUtitMt t>f the light Ui tlic cond«mer(8) — whicli is im I'nr fmni wlmb 
is sought und dnirabh) iw it con wt<ll be. WIuIp uKolbpr l«-M>ri i>f 
prat imiHirtntuW ni«y bo Ipjimt fnmi li^. .111, whiirh illuHtral<^ 

M' •■rn'r ></* H"/ harimj iki- rrti//- nf thr flnmir E '» 'A' /'rinei/iil f'ltenii 
,^' thr huU'it'f^. H. 'n»' mm cdnviT^ii ou [lie tiiiidpiisi'i' S, an ihiit 
it will tm-iiUH! in nil pifiluibilily inipnw^tble to fiiL'Ua it i>n the 
iiby-or. Thin \s n tiit<-nil Wi-iii 'in the Vdluo i)f hiiviii}; the InillV 
eye fax*\ U\ ibr litmp, »> thjit lioih tuny lie iiHive<l to^etlier; hikI 
them »ho»t'l^ lie a tmtch in tin- *l«t i>r Ami which carrieu the 
bnU'Myw Vn AentnAe wh«ii the tliinu* nf the hinp in in its pnn«]>>t] 

Fid. EM^— AlMiuditjr of ntiiic* liuUVfijw Pin. Sll.~A)HunlilXD( uiiiuK > Imll t- 
■nd ■ (xmntc niirror. *}« aitli tlu ednsJ Uie lamp Ouna 

nol ID it* r*tnatpiU leea*. 

The alxivn nre funiLunimtitl )irin(.-i|il»i of illumination, nnd if the 

uUttit U Ui jaicrrtjA a» a nuinijiulittor he niuit (lenionstmte iiml i-e- 

dfnoDntrntt: tbrtii, mmI lincome iniwiterMi theirdetoils and what tht^ 
enUnUnilly I'lit-h. 

Wt- miiy, hiiwcier, with much lulvantaffe give thrm 'i Inrjrtu- and 
tnorv dctniled «]ii]>lit.-ntiini to the |>mctical M>tting up <>f a dnrk- 
ground tUuminatimi, lui iu fiff. 308. 

1^1 ail (>l)Jn*t such aaa trieittUiHtH (diauini) br tiik'^i, aixl ^tip- 
pww that the ulijwtt»e eniploved ia s -(-inch of "28 N.A. Wl- niiiMt 
fint iuljn.%t the lauip and bull's-eye, 08 iii fig. 'J96, and jp-t the iidfie 
al tlie lamp ilaiue pxleii'ted tti a disc as at A. 

Xiiw let a sniail aperture lie put into the eiiiuteiiMKr and n tri- 
ceratium on tli« ataKe mid the H -objective on the ikhi-- piece. 

The mivnMcoipe Iteiii;* put iiit<> pmition, the \um\i xhould be 
placed ou the left'liaml aide of it— a Urop with a fix«i hull's-eye w 
afoiuined — and it t<li4iul(l iu>w l>e arran^^ an tn height, ^> tliat the 
rayn froui the buH'a-vye &ltould fall faii-ly on Umi jilaiie mirror, thiit 



Inltvr Iteiim iiK-)iT:«l ao aa to reflect Uie lienm on the. Imek <>f tli«t 
Nub-)it4i^ condenser, 

Siiw wiih any kind of ISrIiI, foctis, and plscp in tlw o«>ntre<rf tho 
li<'l*l, tlif fi'leenilium, us in tij{. 312 ; iheii rack ihi^ ditidwwer until 
tlif spiull iiperturp in il« (iinphragni comes into focns; centrt- ibi* 
ii. the irittriUittm, mi in «(•■ 313. Kack tho cnmdcnser clonnr up 
until tlie bull's-eye U in focus, oa in tig. rtH. 

Hei« it hdjipejis tlmt tlio buIlWyo is mit in the twitre, and It in 
xiut uitifariiily g/h<i «Uh Hyhl, as in %. 296, A, bul liaa inatrtifl tmt 
Knveentt t^ light. 

This is n case which fi-eqiiently repeat* itwif, hut it Is of ©onrs© 
not inevitable. The buU'n-eyo may bo more "i- less fiUod witl> light, 


Piu. 311. 

Flu, »ltl. 

Fio. an. 

Pia. ais. 

and may ur may nut be moi-e nearly epntr(<d. In \his case we \ta\-« 
next to ceutiv the image of the bull's-eye to the tricentliuni by 
moving the luirrnr, as in fig. 31."i, 

But it will be noticed thai this centring of the image ot ili* 
IjuH's-eye doe* not Tfct\fy ih' <l!_^iiti'jn o/fbt- light. Tliia will b« iit 
inii'e done by moving the lump with attnched bull'^^eye ; tliia motion 
requimto beu kind of rntiiiiimin azimuth lound the wicku an«xitk 
The trhitivr positions nf ih* /niioi ami bnW i-'-ifi- inuxt <m *io itcetrnni 
lif itlttreii, and it is undenlooii that the Ump was tu^usted to tlie 
piclui'e A in fig. 29G by inBpcction and without tho microscope. A 
very alight movement in n^.imnth, however, ts enough to effect the 
desired end (fig. :iH>f, andnll that now remains is to open the full aper- 
ture of the condenser nnd put in the smallest stop ; if thii) doeo not 
stop out all thf> light, n larger one must be tnAl ; but it is of the 
gr«at««t importmici? that thi? sinnllest stop pniwiblii br nsed, a very 
little difference in the siyi-of the utop making a remarkahlediHerenoe 
in the quality of the ptrtnrc Hrncrc the need of a Inrgv and varied 
supply of stops with nil conditnsersi. 

On account of Komc iit^idual spherical aberra- 
tion thn condeniu'^r will prolinlily have to l>o racked 
upxlijichtly to obtain thi' gmatmt intensity of light. 
In %. 3iS the cxpandnt "Ige n( tlie Same 
cnvcm (hi! fn'rwrt(iM«i. When the whole a[M>rl«re 
of the oondcniier is opened the nxx nf thni /li»r teilf 
nolle nJirrfiil, it* j'ii/™j<i/_v on/y will In- iHfmwH. 
Via Sin Wlien the rtnp in pincwl at the hack <)f the cnn- 

denser, i>nly in thnt |Mirt "if the field tviirewnted 
by tile disc <rf light will the "bi<«t Ih- iltnminntMl <m a dart ^rrnund. 
If, therefurc, the disc of light do<a not cover tliu obji-ct iw objects. 



y. Tl 

briiiii the Innip neitrvr Um- iiiirnir. TIu! su» uf the <IUc uf lig^t 
ileptnuis (>fi three thinKK : — 

a. TliP <lin(iii-lpr <tt the liuU'ii-eye. 

ff. Till' li^uhth "f the piitli uf tli« mj'B from tlic liuH'i-i-vf t4i the 
mlt'Ktaxir couik-nurr. 

"nw! mnnniiyitiii [ower f>t the condennvr. 
. iin'l y lire c'ciii»t«iilA, the mily way o( vnryiiiK the nia- of tlie 
(krk ti«l*l is by ^3. 

In tbr Asiiuo way the inUimitg o/lkt liijhl in the (lis; dependx on 
tbr«e liiiugo. 

A. The iniliAl iotAnsity uf the illumin&tioii. 

B. The Ati^lar a|)erture fA the huU'aeye, 

C. Tlie fttigular aperture of the Kiib-8t«ge cimdeiu^r. 
If the Ktwlent will thoroughly ami piuctically uudvi-aUtid the 

Klmve wries of sin;;le denioTutrntionii, and piuidei- hucIi inevitable 
vnriatioitf as pi-actioe will brin'f; in rv>g&rd to theiri, tJie ' dithcultica 
of iHnnUDAtioii ' will have pr&ctically pnased away. 

7Tt»r« aft tvc kindt of rnicroseopieal tnort — one, the iiioi« usual 
and (Munpnrativejy CAsy, is the ^xaminatioa of mi object tu see 
totiiHAiity itAifA in tuoH'tt. The otlM?r is the ex ami nation of an 
ubject in se«r«^h of the unknown. Thus some blood nmy be ex- 
luiiincd for the purpose of lindinif a. white corpuscle. It matters 
little what is tlie quality of either the lens or tlie illumination or 
tli« micniMOpe, or whether tite rooiu is darkened or not, becnuso the 
obeerver knows that there is such a thing lu a white corpuscle. It 
is quite immateriat as to whether the observer luid e\er seen oDe or 
noi; soU>a;(«ahe paaseuea the knowledge th/it there issuclm thing, 
tlie 6iu)inK ot it, even under unfavourable coiiditimis, »'ill l>e an 
easy task. 

But if tlie observer haa not tlutt knowledge, he may examine 
Uoud many times, nndN* fovourable con<litions, and yet n<'t notice 
the presence of a white corpuscle, an^l thnt, loo, with one iniinedintcly 
in the centre of the tleld ; this, moreover, is a Urge object. 

It is only th<«e in the hnbit of Hcarcliing for new things who can 
appreciate the eiioriiious difficulty in lirst recogniaine a new point. 
'nMrefoTf when i-riticjtl work is undertaken core Bhould be nxercised 
tn have the conditions as favourable lu piis^ihli', 

When wurking with artiticini light all naked liglil^ii in tlin room 
dufuld W avoideil. 

It ts quite unreasonable to expect the rntlna to rcDuiin hif^lilv 
MTiMtivo if, whenevtT tho eve is removed from the eye-piece, it i» 
expowd t" the glare of a naked gax tiame. 

At thr Mme time there Kliimid Im^ ample light mi the wiortiooupe 
t«bK u it ix not at all urc«iwary or (trurabin tluit tlie work iJiould 
Iw iiwufficiently illuminatnl. All tlint in m{uirntl i« that the lamps 
•hoitl<) have HhadMi and W plaeed at »uch a lu-ight that the direct 

if r»y« do mit enter the obw-rvrr's eye, 

f If thev precAuti'in.t are taken, w-veral iMiure oontinuod work 

J niay be carrinl tin withiiut any injuri«ux i-fft-ot. 

Clipners uw only the loft eye, wme the right> others the 
t indiacriiuinately. 

360 PiiACTiCAi, Mtcnoscon' 

II deems imBrnt^rial wliicli is used, it being merely a nuitter <if 
Iwbil, lis tlioso who are aoctuitnoiecl to Vf^e one piirticiilnr eye feH 
fivrkwanl willi the other. But in mntinuou^ work extenditiK over 
iiuui)' iiioikthN of long (Inilv obscn'ation it is a givnt g«in to use the 
hiuucular. especially with high powers. The cHec^ of years of work 
will] optical instcuiiiciilK ijn thniw poeMCsed with strong iionnal 
nijfht st^eIlls to hi> nii incrnn«i> in the (tofining pc^rveputw oceoinpanted 
with a decrease of tlie p'Ti?e|)iioii of hriglitnpHK. Tln>Bn iiciii««iiiei! 
to use one particalnr eye with niirrcwpojiitwl work, iinfl who liH\e 
done ninch work, winilcl, if thi?V looked nt, say, thn iimon with thnl 
eye, see mort' dplni! in it ihnii if the otlier t-ye witt uiird ; at the 
same timo it wmili! not nppr-nr a» lirijiiit, 

If there i» too niiich li(!ht, ns ihciv oftfin is, when InrRV-niigled 
itluininnting roiii's hit used, it is us wi-U to iiitt-rfHwe between the 
lamp nml the mipronciipe n, piecf or piert-* of bliu- ^tnss ; this softens 
the light, ami n-movi-s the iibjcietionuhle yellownws ; iv fttatiire of 
illumination mit due ti> llin liu^ht from thr (flge of a [laraHin litinp, 
which, an wr hnvi- itntod, is not partiiruliirly yellow. (tr*at tfri/oii-- 
■wn in a sigit tit iiii/irr/rrt ae/iromntinn in nii ciljjti-live. \Vb may 
with precisely the NUiiir eondilioiis tind llic iiiiiij,'*-.* yifl<liil by two 
objeetivps of thn snme |ii)wi'r (ind npi'i-tmi- ditl'i-r, iti so much iLt one 
is yellow and dim and the iithi-r whitr- mul bright ; other thtii^ 
being eqiuJ, Ihp while and briglit imiige is li> be preferrud. It in 
iieoeosary U> any "other things being e(]ual,' lieemise an obj^ctire 
which pveH » liriglit and a whiti? inmge may neverlliele™ 1« inferi<"r 
to the one leiving th<- yellow and dim pietun-. Thu« if the ptnneji 
oftJielenjira of which fhi' <ibjentive i» roniponi! an- not at right 
nnglea to the optit- iixis tlirii- will Ik- u-rioiis defect* in Uie imnp*, 
atthouKh it in bri^'ht and whiu*. ThiK fault is known in pnicti(.i* ni> 
an error of centring, which nliHi ineanx the error (rf not pliu:ing the 
axes of the leiintti in Ihe Kami' straight line ; au both faults an' 
described by the Mime term. 

It sliould be uuderilootl tlint neillier blue gla.-w nor tinimoiniK 
ouprie solution wUI yield nionochrtimntic illumination. If the light 
pusailig tln-ough tliese inedlft be examineil by the n|«i.-tniBeop^. rod, 
yellow, aud green will he ae«n. Srmie opeeimeiiii of blue gla*i piuei 
iiioi'e red than others. Out of a nomber of samples the betti result 
wiu) obtained by usitig two thickneaaeH of cobalt 'pot ' glow, which 
gftve two liands in the red. If a gi'enter thickness were uatwl tlie 
light became too dim. Tlie ammonio-cuprio solution merely dims 
dowu the whole specrrum. When n suthcient thickness of solution 
is used to cut out aII the re<l tight, tlien the light is too enfeebled tu 
be of any use visually, and niso, what otie would not expect, 
photographic exposure is grently ]irolorLged. 

it would seem that true monochmniatic illumination obtAined by 
absorbing media does not eiust. Neither does the manipulation lif 
the sourec of illuTniniition by means of chemical snbsl Alices 
Apparently give satisfactory result*. 

(floured light derived from a polariscop*- and a »elenit« is not 
m on och rom tttic. 

Tttero arc two w«ys of obtaining tnic monochromatic illoini- 


iMtinn. On« which would he itxtrnnaly simple wwuhl be to ctm- 
denae a pnwiH^il Ixxtii on » (tiflhtction derating niM <in a s)>fculuin. 
The cn«tlni<-» of lhi> npfmratUK U a Imr to tim nx'tlimt. 

G({tuillv piTfrct miiiiiHThnirantii- illuminntinn i-an he AblAilu^l l)V 
prUmiitir ili«|KTsJnii. E-lHrlnnok il<L>ijgn<id lAich an nppnmtue,' which 
WBH mi ordiiinry K]>p(Ttnisc<>pe. 

Hut it nrvr^r (■nine inta [lopulur uKn ImrnuKu witli any ordinary 
arliRrial light, such hk timt from » mrafBn lamp, the ti^lit u-iw ox- 
(wu-ivi-ly dim, iiltliuu^ih it ik jMiwi'rfiii (■noujult with nn idi^ctric arc. 
Th" only u»r it has Mtrrod is to gii-v obliqut! il I tinii nation, with xun- 
Kght froin u hHioKtat. Tlie followiiiK is a nxtthiid of iitipi'oxi muting 
to loonorhrDmatii; illutiiiniilioti uftfl bv Mr. X«lw>ii which itnit wen 
Mhninihly with an urdiiiary ^-inch wii'k pnniHiii lump. 

ilo phicod the mIk^ of the Hame tii the nrincipnl fi>au>i of n 

leiiaknownasa Wray A x i RR.worfchiKal / . In tlio [uirallfll 

bnoi from thi« kna niid clout! to it ho places an npiiUternl priiim of 
'knao Bint Dot nt ininimum du^-iiition. C1om> to tho priem he plnc<>« 

jfluthM- Wmy A x 4 RR, working at ^ . If a cardboonl screen 

be hehl at the prineipnl foetis of this lens, there will be aeeti tlireo 
iaa^ea of Die ed^-v of the thitiic, red, ignen, and blue, brilliantly illu- 
mtnated. A slit ,'„ inch in diaiuetar in cut in the cardboard screen, 
timugh which the m^uired colour ia allowed to paaH to the mirror 
of tlte inicToscope, thence to the sub-etage condenoer. For tisual 
work gTMti IK t1)e liest, but for pbott^^rapbic work blun woutil lie 
chocen nnlexx ortliochroiuatic w ork recjuired n colour lower down the 

/Vr erUim/ tcork, "hcA tm UAting ^^iiAiMor forcing init the ^reate&t 
remltition with the widest-angled oil-immersion lenses, daylight 
illumination ix inadmissible. 

Wlien dayli|;ht illumination is used a nortliem aai>eot,or at least 
one away from direct sunlight, is to l>e preferred. 

It is a ffxxi plan where it is pocwible, to arrange the table so that 
Um wiiulow is at the observer'e left band. The microscope should be 
|ilBc«d in a dincCion parallel U> th« window, and the light reflected 
by ttie mirror thraa),di a right angle A fici-cen may be placed parallel 
to tl»e window which Just allows the mirror of tho microscoiie to 
project beyond it. Thin cuts off ilin-ct light from tho stage and 
from the ohwrt-cr's eyes. 

A concave mirror with tho object in its principal focus is the brat 
for dtflusod daylight illumination. 'Die <tiaphmgiu should not Ih! 
doae to tho stage. When <lclicati^ niicmscopical work is earned <in, 
ft is important to n-memlirr tliat tho htimAti eye ran work Ixnt when 
the body is in a state of If there is any strain on the mundes 
of the t*ody, or if the oWr\'er ix in a cramped position, vision will 
he (mpairMl. Consequently, whrre jirrmissible, n mirmscoije should 
always Ifn itvliiifdy and the i>bL*erver seated in such a way that the 
eye con bt brought U> die eyepiece in a perfectly natural and CQEa- 

> Cluptcr IV. p, il% tg. tM. 


forUble inADiier. The body ahould Also be 8t«*di«d bv restiiif; the 
Anns on the table. 

It is fldvisable to tue il)e Ijull's-eye as little as possible ; *etti Hith 
dark-groHHtl iltuminntiur* the flat of the Sain«i%prafer»blp,r«<erviiig 
the buirs-«ye for lliose case« where tlie flat of tlio tinme will notforrt 
enough of the object. Oenernllj' Hpenkis};, if the whole ti«ld is re- 
quire<l to 1>e illuminated on a dark ground, n bull's-eye will bn nwM- 
Miry : but for au object such lu a single diAtom tlio lamp ftune will 
usually be larjie enough. 

1 1) nxnmining diatoms or other objecU^ such at tlio knr<,-okiii«tic 
Hsnres in very minute nuclei of inicrciscopie organism*, or othrrolwcttre 
und undot^rmined parts of such forms of life, it in miwt imjwrtant. 
aniQiigiit nthf^ mRans. to rraort to tliv iiso of lnrK>! i>olid vonrx ; what 
tlicy tnncli and suggest can scnrrf iy itr. nnglocttsl by tlie KMrcher fiw 
th(! unknown. Professor Ahbn doi^u iit>t adviM- their cnipIovmRnt nx 
in niiy way tinni ; he says tliat ' t)ip resulting itnimi' prtMlucnl bv 
mmns of il broad illuminating lii'nni is nlwnys a Tuixturn <>f » multi- 
tude of jMirtinl ima&TH whivli un^ mort; or Ii-kk didWi-iit nnd di.-wtmltar 
frotii the objtiet iUmli ; ' and he docs not 4;iini--rivc that ihiTt- i.i any 
grimnd fur i-xpcctation ' that Ibis mi.vtnre should coihl- uPart-r to » 
Mtriotly I'firrect pnijection <>i this object . . . than tho iniiL},tr wliith 
is prajectMl by a luirrow axial illuminating pencil.' This la a weighty 
judgmciit. anil Hhould rm:i^i\f full consideration. At the aauie tiiue 
the use of wide a-inX anlitl con«s is so full of auKgmtive re^uljU that 
we tav-ht employ tlteni, with all ])OSHibl« rontrol by other tueana uf Uie 
images they present. This is Uik more a nel^es!(ity since Mr. Nelaon 
has been able tonblnin the moxt wondrrful rtnult.t with narrow cones, 
'true ghosts' and ' false ghosts,' ihcpn-iiBncBof 'inlcrcosW murkiiigH' 
in the image nf n tly's eye (!), nnd many complex and fulde ttiMges 
with the coai^er diikloms. Jiut with wide corioi hi' has prnved tfiftt 
these false imager cannot be priKluce<I ; and that when the true iiaagv 
is reafheil by a wide cone, thp imiif.'r ia not altered liy aiiy i-haugeof 
focus, but simply fades in and nut of focus ' as & lUisy under n 4- 
iiidi objective,' 

Mr. Nelson hsa photogmphed all these results, and we hare smui 
them demonstrated. When theory and prarticeare thus at variance 
we must pause for further li;;ht. 

If it is required to accentuate a known structure, micA <■# ihe per- 
j'omtfd v\«mbrani i(f adialom,x%ca,\\ be done by annular illumination, 
which means the same MrrangeraeDt as for dark ground, but with a 
stop insutticiently large to shut out all the light. This method is not 
to l>e recommended when a structure is uukuowii, a« it is also liable 
to give false imitges. It must be remarked that diatom and other 
delicate structure when illuniinnted with a namm-'Hii^ii'd ■-«»■■ gi^TS 
on slight focal alterations a rttrKli/ of patterns like a kaleidoncope ; 
with a wide-angled cone a ungle structure gives a sinelc focus, i.e. 
it goes completely out of focus on focal alteration. When a large- 
nngW and a wide-angled objective are used a change of jiattera o<idy 
occum when the structure is fine. This practical obsenation has its 
value and must not he forgotten. 

To jii-QjTly tiuplaif nhjrctx inul-r a mierottiope is to a certain ex- 


ie»t nn art, for it not oiily (l»nianda dpxl«rity in llio miinipulation 
vt tli« inatruiuent ami its Applinnces, but it *bo recjutrwa koowl4!df[e 
o( wlikt BOrt o( illuintnution is beet Kuit^d to the particular object. 
For learning,' Itie tiuiiiipulittion of the in«tmineiit tio clam of objfvt^ 

uv us HuitableiLS diatoms ; they are alsoan excellent oi«aiiBO{tiutiiiiih' 
the «ye lo appreciate critical images. For a general vievr of the 
Urger dtatiimB take a spTMd did* in batnm ; a ^ of ^>0'', a, fptoA 
binocular, atid a dark-|{round illumination will giv^a fiueelfocl. Tliin 
is Dot merely a pretty objecrt, hat it is nlso a very instniciii'e oue 
because ve obtain a far clearer idea of th« contour of tnriotis iliatoniK 
than can lie obtAine<l in any other n-ay. The difttoni^ should be 
etuclie<l aitit worked Ht in (his manner most mrefully and for a long 
tiui?. Tlie same identical specimens should l>e then viewed witli 
iraLtiAinitted light Tbis lemon if conscitmtiousJy learnt will teach a 
student how to ftppr«ciat« form by focal alteration. Tliis is a most 
important lesson, and, if sevf^ml days urn sppiit in mastoril^ it> they 
viU be far from thrown nway. Diatoms, Psp4M»ntly the lar]|;or forms, 
Mf seen very well u-Aj-h inrmiilnt f/cjr on eov*r by means of n ^-incli 
objective and A Lirltrrkuhn ; tbt^liulIVcyeand the piano mirmr should 
1» used. Some objwt* nrn h> trnnNpnivnt.or become so tmnspurent. 
in iho medium in which thi-y am mounted, that they will not liwir a 
Ur^e illuminating cone, the brightness of tho illumination destroying 
Ibe oontmst. It will illustrato this whiii wi- mcMll that dirt on an 
eye-piaee which is (|uit«^ invisible in u .■'trong lig^ht l>r'ci>mi>s im* 
HMMtiittnly apparent in a fiit^ile light. Thus aninialcuirs rr<|uin- a 
hniall cone M illumination wh^n they atv l>?ing ttxiiminvd, particularly 
•iUi a J-inch objective ; for a gpiipral view nf ' pond lifo ' a 1 (i-iin-h 
olijective with a darlC'gmund illuminatinn, employing a biiuH-iilnr, in 
VVT\ 'lUit^ble, IStainwi bacteria in tiwiue ure i>r»t sxpn willi 11 litryf was pointed out by [>r. Kohirrt Kiwh, ami is dimclly supported 
by Y>T. Abbe as suitable in bis dimRtimis fi>r thi^ usi! of th« AbtHi 
eondenser' The hrillianey of the illumination oblitemti'd the thin 
tlsnte which is in a mfiiiuni whiise refnu-tivc index i* simitar to 
ttmlf. The iMtcteria, «-hi<-li are opntjue with pigment, then ^ttnnd 
onl lioldly. A Vuicteriuni nut in liiwue U alwiiy-s betti-r seen by 
ninint of a targe eime, provide"! tlint the objective it, j>r'>]ierly 
wwrecWsl. The very minute Iwin on the lining ni«nibnine ..f the 
l))ow-lly'K tongtie, if oxjimined by a I[ obj<-etivi- and a narrow cone, 
:>ppMr thickened, shorter, more blunted, uiul i>ften nplit i»l<i two 
porta. This is shown in tigx. '1 and 3 in tJie front ispiisie. Fig. 3 iii 
a critical imagt! niag»ilii'<l 510 diunieton. A lenn should l»e u.-<ed to 
iwamine this. 

Tt will be ceen tJint the hairK, esjiecially the long centrul one. are 
vcfy fine and spinous., Thoj- have n<it the ring wK-ket cunimun to 
■luM-t haini, i«it gniw direetly frimi a df bote meinbraiie. 

Tliic pbut^igmnh waa taken with an apocliromatic >( ot -dA N.A. 
imd No. :i projection eye-paecv ; nnJ it. vraa illumitiatetl with a Urge 
Hiilid cone of '615 K.A. from an odiroinutie condenser. 

Fig. 2 in an uncntical image, with nil the cunditiona o» above, 

■ nirwdniu far tb* a** ot Alibn'a illDinLnnllng kpp&rBtiw— « 1mI1*( iMupd by 

MiAcnCAL sncnoecopv 

>iave that k cone of small angle, i.«. of 0-1, waa used (or Ulamuuf 

The lint nltorntion which thrusls iUolf upon the pye is the 
lUiiihling of the hairs, which aivj in \\\a degree out of focns. 
ttut further, it will be noteii that there is a bright Hup with a ilark 
edge rouiit) the hairs wliich arc prcciwiy in focus ; tliii )>i a clilTraction 
fiflect, nlways, in our experience, present in objeci* ithiininatod hy 
vonos of insutGcient angle, axi<\ it cnn he ejujiy made to di«appear by 
widening tlio eone. An the illuniinnting cone is i^ntftrged th^ becoin* ' 
Nlini-per and lunger, nnil their edges beeoiua loore defiuif;. Bubj 
nothing w gained, but the rather a di«tincb lou i* incurred, by mnkini 
the illumiufttjng com- lai^r thnn thi^ objective L'one^ 

A-v nn cxiunpti' iif ■•i-i'oncmi.v intiTpri'tntion, the rvprawnt«tion <>l 
the pygidium of u flcA by Mcmic Ituuling Niniix-eH of infimnation of «^ 
firw yenn ngii may In> in.ituncN^i, It wnn n special li»t i>f many 
authont, iind hiLs Inth cnn-fully tigiired ; tlii* nlioux tliat it '\n not aa 
aecidiMital nri-or, wbirh it. niijiht have lit^n if it wimw uH-n-ly an 
wnliiiary obicci ; it is an i-rmrdqH-ndtng in all p rcil lability on iv faulty 
sysloiii uf illumination. MoreoviT, tlu- prmr ouitiut b« atlributrd to 
the obi^-t-Kliis.v» of thi- tiiui', iiH it is a l»w-powprobji«c;t, iiml the low 1 
puwi^rsiif ihiit duy writ? iiuite «h jpxjil aa tho^ lati-ly in u-ie. In the . 
(tmcriptiimw and in the dmwtii(t». "ften lieuutifully e\eciii*(l, thehuira^ 
proceeding; from the i-entn- of the wheel-like diws an- rqiroiented 
ai bnng ' stiil' and longiHli bristles,' thick at one end and tapering oflT 
to % point. Anil tin- numll luiiin R)Uiid are dencribwl ivh ' minute 
spines ' ; in i.lie drawing tliey arc like the spinous hair» of an Insirct 
and hnve the unuat aockt-t-joiiit m tlu* liiwe. In reitliiy tin- '»lifl' and 
loitgiaJi bristle ' ia iin extreoutly lonjj and delicate liliuncnt, t<itidly 
tuilike a bristle, Iwuig not lapensl but of neui'ly unifuriu thicknena. 
Die ' miiiuce spines' are ia reality very curious hiur». imd, aa fur as 
we at present know, unlike any oUiers. Tliey are tlelicate, himlienl, 
bulbouH hnim. Wlmt they mo^t rettemble iire the t<-»t»cle« of a seu- 
ajieinone, iind there Jire two tubes discoverable nhU'h are iin|>ortAiit 
and c'linpAratively liii-ge objects There appears to l>e coDsiderabJe 
prolubility that this intirreating object u]>on the last ring of tlie liotly 
of the flea^ mk) known as its ' pygidium,' actk as an uuditory instru- 
ment.' In the examination <>f ordinary Htaiued histological and 
pathologic-al sections by transmitted light, uoletw some very delicate 
point is sought, the condenser sljould have a slop so that when the 
bock of Uie objective is exandned the stop U seen cutting into the 
back of the objective by about a third. This in some instances may 
be inereoKetl to a, hntf by diminishing the cone, but it is not advisable 
to um: nnything lets than (v half unlens it is absolutely necessaiy. 
ThuK, to put it ill round numbers, an illuminating cone 3 N.A. is 
very milable fnr ordinary work with the apochromntic 1-inch and 
{ objpctivM, and one of 4 N.A. fnr the ^ and ^ an<l one of "6 N.A. 
for tlie I and ]._. It is n gixid plan to have one or two stops cut to 
[{ive npec-ial concu, the N.A. of which should be engraved on them. 
Tills subject ixnni'of great inijMjrtnnce, a* more than nine-tenths 


■nre rsK ov UBGE nj,nn>'ATn;r, roinis 365 

of kll mkroKcnpic ohjpcta ore extunined by nimiiH of traiiAinitted 

Let an now not'' tht- fSkmt of Iiir(i«! conm on tlw! simplrKt nhjoct. 

inicruBoofM' U «rt u[> luiviii;; iid nclirDiiinttc coiuli'iunr with an trw 
iBttphnwii ; Id thixr ;^k1 wkIisuhkIi^I olijeciivis Ih- dimrn, my I- 
inch, n r-inch,ntul j-inch ilry. IjA the ubjecl Inr the oim! wr hnvr 
alivoily Ntuilinl to nmrif extent in tliis relutioii, vie iinr »f thi> stifl' 
Iwirs on till- iiuixillary imtjius of ihv blou'-Sy'ii touKur ; plmv thn 1- 
inch on thi.' now-pii-cc, open llie full ai)erlureof cliiM-imiien.ipriiiKlnit 
tin' iiuitrunwnt into iMrrfpft aiijuaUu«nt. Now dose tlie iriii. Tho 
bur will !><> numiundiHl by n luminuus boixler, wliioii will girtt it 
n ii\nxv aiiiHiniice, Mid iu fine point will 1« IiIuitmI out. Now 
• •[ica tiw iris until the laat tnco of that (ftnzineAs diaupixMi-K. The 
hnir will ajiiwai- lu a (tifferent object, ite ouibne b^iii^ ijerfct-tly i-leu- 
nod sharp. If tli« eye-pioce is removed about iwo-iliii-ilN «( iln- ob- 
jectirebu-k will !<« full of li;<bt. Nowf. wiiliout distiirbiiii-aiiv of the 
iidjiiBtBiieuts, rejjlttoo the I -inch b}- the ^, and it will bo fuumi that the 
HUkiiwm or false li^tht will liave reluniod. Let the irit Iw further 
upnned unttt the la^l tface of it disupiMMrs ; now, on examination of 
the tiacic of iheobjective, two-thirds of it wilt lie found fullof lij;lil,iind 
on with ttie \. We call the attention of the student ti> tliene facta 
hftvinji a direct liearin^ upon theque«tionof ttiei.'oni|wrativeeflect!i 
Wluge and Hmall illuminating cones, and with no idea of offerinj; 
oppOMi^ opinions to thooe of Professor Abl*e, we have no diroul 
JnapnADt; but «e reroiil these facte as fnctors iu and for the 
thRUlAtioa of the duestioa. It is perhaps better tn test the ^ on 
HUD* of the mote minute liaii's which are studded over lli<- di-li<nito 
Uuing mnubfune. The uktue results will lie obtained. Tlmii it w<iuld 
■fiprur to su;tS*M itaelf tlutt (Iuh 'xbLzineas' dvpentbi on the rrlalioH 
of ihf a/ierlure t^' (A* mnminiitinff rime to ifml <•/' Of vbjtrlief cni*f. 
Apocbruntatic objectives beliave precisely aa achmiuatio objectives 
Hi tJvis rwpect. Of cuurae, if the liairbeoomea pale and iii<lidliu<jton 
the opening of the iriw, it sh'iwn that there in uncorrected spherioal 
aberration in the objective ; uiiutlierobjecti\e must therefore be used ; 
tbat paleness lias nothing whatever todo with tJie rIbm or false light 
iMntioned aliove. 

In jtkolo-m'ffijrai-h* ofhaftrrur one ireijucnlly sees a white balo 
round them. This in our hiincls lias always resulted from tlie photo- 
iiiicrogmph havini; l>ecn taken with too small a cone of illumtnatliin. 
Phnto-micniGTaiJiy with a small cone is (|uite easy, as great contrast 
can be serured. With a large rone tlip dilhcuttie« liegin — dil&culties 
"f adjuHtm^nl, dillicultieg of lens comdiou, ditticultics of exposure, 
and (lilficullieK of dcvfliipDient. If, so far as our oxperJencs go«S, a 
goodpltoto-mtcrnjmiph Ure'iuirc), tJieHo (lilIiciiltie<imHiitl)e moBtcred. 

It is hiiixlly unH-MNiiy !•■ n-niind thi- student that in micrumAtiy 
it is ft<«rnttiil that thir (■(!);>':« of the objo't iihoulcl lip dcfini-d ; (ionw- 
([Uently a \nrff.- rune »■■'■' thru In- cnipli'Vcd. 

For thri-xaniiniitiiin irf I'nlycysli'iiw, F-nnininifrm, A-r.ii bin<tcular 
in useful ; illuminiition may Ih- by » Lii'biTkuhii if mnuiitiil dry, and 
bjr daric ground by a i.-i>nclcnMT if numntcd in lialvim. Piii-tA ai 
ilIsoct« should Ih; UKUidly examined vith dark-gvi'iiud itlumiiuliun ; 



whole imects %n awn best with tlw LieWi-kuliu, fttid tlie bitiocuUr 
should ba used for both. 

Soniei of this cIas8 iif ob)pi-ts are beetseen under il/ruUr ifttimiua- 
lion ; thnl ts, n liark ^rvund irilA it ctntbiufr aiui ]if;ht lArtntn 
^fnm rifiove u-it/i « (t/iw nitif-rtfl^^itoi; on the Lielwrkiibii cttiinot he 
tiMid in conjunction with itii nchmmiitic condmsar. ]l in u pmd 
plan with low-powttr Lipliorkului work Ut ii)t«rpose h*>lweeri the slip 
niid the lodgd 11 ulrii) of pliiin gliiss ),-iiic]i widi.>: fhi* fnrtiv-nU llif 
/"//jr" ^tojijiiiiff out liijht fixiTii thr Lirhirkithn whmi it iHlM-^r ilMIiil' 
lnct(■^ than \.\w! sliji. 

Fiiltiri.wd light uk<hI with n mmdRiixcr is wry useful for iiu«ct 
u'lirk. For vpiy Uiw-p»wri- wurk — such us thu iiMinl lH)tniiiCAl mm-- 
tions — it ifi n ^'hhI pliin ti> kivi- up thf i-onc, niid pliicf » pi(>c9 of liue 
ground glnw iit the liiick of xXw cimdi'nsi-r ; iind with Inuip light it in 
lis wi'U U> coinbini- n piivt- iif Mui* ^lass with it. Wilh objectives of 
jtrivitt-r nnglc timn ■'! N.A. il is urviintly lUlhcult to jtni snti^-fiictory 
illuniiiiiition with n diirk ifiiiuml. The Ix-Nt Uint i-iui Ih> dour ix u> 
U!^i- nu oil-iiutui-niiiiii i-ondfn.wr will) it -luitJiblp ^llltl: this will give 
II K'""l ilurk H"^'"'"' "I' **' '"■'' N.A., Iiul it will Ciiil if ihi- objoct xn 
dry on llwoover. (lenerally uptHikiiin, llit-inily wny of lu-roinplishinu 
this with objecti»e«i'f wider ugienuiv is to redui-i* thi- apiTture of 
ibe olijeclive by » Ntop plm-ed nt the luick. 

when A coiideiiwr in united by a lUiii of oil to a sJiji, ■/' thr x'tii 
iV fJiiii, the oil invariofifi/ riiiit iltiiim irlien ihi- rnnrUnfr in /'imumnl, 
Tlie frdlowiiij; in ii method by wliich thi« inny 
Iw cntii-ely pi-e veil led. A piece of iJiit'k 

Uc(i\'«r-);]ii» idH>ut -02 ineh, una 1 inch MfUAre. 
hiiA II .ilrip of thicker fUnas j-inch brtiod, 
cuiiiiMited by (Jielliic to <nw fdjje. Thin piwi^ 
■ ■f kI""^ i" oil«l to the (dip, ihf Itnlgr- iH-inj^ 
hooked over the lop of the slidi-; this ii-it 
only prevents it^ slippin;; down, but also keeps 
the od frotii ci'eGping out at the JHittoin, wh'u^ 
wuuhl be the caw If the two wljies of th" 
"losa coincided.' This is illuHtnihHl in litf, 

Ill itf) jMi'per pliK'e we hiive deidt witli the 
^uttublf reliiltoii of n|)erture to [itmvr, nnd 
h&ve poiiited out the irre«islili1e niitun- «f thi' 
ContenlimM nnd tPHchin|>8of Ablie on the^ull 
jei-l. Elere ii direct practiod pi-ewnlntlim of 
the iiintter niiiy Iw of service to the atudonl. 
A iioriiiHl uiinided huninu eve iim divide' 

Thin dip ot sUm with 
■■dm to piMa gliH dip 
<fitE mI oontub, K> lu 
Id Tory Uio ibiclacM ot 

1 mtliie. 

Slide lu u'lii on thill (lip 
Willi liv])[i-. 

PlO. K17. 

J if, inch nt ten inches. CoiiAe^ueiiUy 

niiorubccipe wilh & power of 200 nhould be capable of showiin; 
^t^^(.■ture as flue as strAnn inch. Now ok thi» power can be made 
up by a ^-iuch objective and -i 1-inch eye-piece, il follows Uint 
Aulflcient aperture ought to be givrn to tlio li-inch to enable il to 
resolve 50,000 lines t>er inch. Thi«* will Ij^ -.VJ N.A. Thi- iitch 

' y. Jf. C. Jmirlml, Kutviahtrx IwA. 

■ In rwlllf it sill nquini mmv. Iwcauk ui ulal eona U (uhxiumI Io ba mod 
inMVBil ol wi ubl>i|ila baun. 


objr«t)vr nbuuld have b*If this apoH.viiv. Mid liie ^ duuble, nnd tli« 
jl fiiur tinWK ua iiiuoh, if pfrrftrcl vision is I'equirecl ; in oilier woitLs, 
■2fi S.A. for every 100 diimiotcrs, ' Those i<lptiU haw («s we linve 
bdore indicftted) lieeit realiMil, notiilily by ihc '/,cis» npochnimntics, 
the 1-inch uud the ^■ifK-h'resnlvtn^ ovcryUiing capnblw of being np- 
iirPcLut«d by the e}% wh«n thv I'J coinp^nMiting oye-piec« i^ \i»ed. 
Tbi- j-ini-h t» klui n ntwritppnHuih to thi? idriil, lut it niw been very 
triaely kept a dry kno. Tlic ]| with a 6 rye-piwe a1s» nctniiis tho 
ii)^. This relHtion <>f agit^rtim^ U> poirer in >iTy gtigiiilironi, and 
thuMdd be cni-efiilly pinwlerwl by tll<■M^ whu »titl (h«ire low npcrturrs 
ta iMiIy perfect fniin iif objcwtiw-x. 

H ia a« well (4> ini-iitiiiii thut obji-ctivrs nmy 1k> urrnii^i^l in twn 
ies — ^>i»e the 'i, I, i, j, nnd i, ihi- (.ther if, !j, J, J[, ,'j. Unr "f 
thenc !Wi'i<-« will fonii ii oiiniploti- luitt^-ry, lut it Lt uiiiieccxijiry to 
have objectives di(IWin>f froui thi- next in tbe M-ries by U^m« tliun 
tlouble tbe powfr. 

The iiiiwt ti>-u*l coii)binuti<tii in |>prhi>[Ki tlie 1 anil ihv i nf oiw 
(erxpn, IT th*' § mid the ^ of tbe other. Of ihrte two pn-fen-ni'i' 
inilfhi luthirrbn g\\en to the lu^t. Tb« only (•xfe]>tioii would bn thn 
nddiliun (if « H-inrh for ]>i>ckI life. 

Eye-pMi-i-s "sJioiild iib-> double the power thus: 0, 10, nnd 20 
(aii>'<>iii|'-ii-->iil*-<l), or C, Il>, nnd 'JT ((.>ij[|i{H-ii»uted), the tiiittt usirfii] of 
ihe ihiM^ treiii;; tbe 10 {tiiii.-oiii]ienMited) und the 12 ((.■(itn|H;n.'iuted). 
An ihfi* is no (i-power coiiipeiiMited eye-piece (or the lowj; lulx-, ii 4 
for tho Khort tube Adintnibly answers tbe purpose. 

In BtUlilioii til the expl»iuiiiftiis alivftdy Ki^'i*" ■>» t^c .Mibji-cl nf 
l««tiiig olijectivH, it may be uMful here to note thnt thi? (|tuilitie>t of 
ui iibJTvtivn am si-vcn in number: — 

1. MiifiiiifviuK powei- (inilini). 

2. Aiiertuiv or N.A. 

3. Resoh in;; power. 

4. Peuetisitii]({ piiueT- 
•V tlluiiiiiiutiii;; [H)wer. 
fi. FluliieM (if lield. 
I. Delbiing jMiwer. 

yniftfing fiifrr. —No tmt ih rwijuirwl, os tlir Initiitl magni- 
^inj; powivr cJin b«- tliri.'clly ineiuur«d. 

-'. Ait^flurr m N.A. c«ji lie ilirecitly meiiAured ; no toet is tliere- 

;i, l('iiui poHvr, biiin;; dii-ectly prn[iortinnnl lo N.A., ia ayno- 

(Its with it. \o t«st in iieedfut b(<c»uM! N.A. I'nn be meaBurcd. 

4, I'trtiftratittg /irifer is the reciprocal of the molving power of 

^~ . So t«ct neeiied. 

B Hlumiu<ttiiu/ jtoiiy.r is the »i|iiare of the nunK-riciil iiperture 
I.A.)^ No test is necewc^ry. 

• B»olM ifftlnniif, vnl. ii:iti>i. IrKI. No. |i»,— F.. M. NcUon. 

■ TUit Uui. wUk »u H DComieUiBtiu^ cjc-pUMm, will ruKuli* ■ /V'umiiiii'iii 

fHlaStim vilb Ml aial can* : tliiii in th* Icnrotl power vitU whicli it liu ent Mru 



fi, Flntttr.M fif Jield i.s, in tlie strirt uiiviiiiiitt iif thf- t^rrm, 
n[iticnl impiiwibilily. TIir btwl thin;; lliprtrfun- i» Ui oniiIriK't 
riiiibli' lii'Ifl fts in iltim? in itie c(>in{<i>n»niiiiK ojf-pii-cp^. (T)-*!*; Ko( 
low {mnci's II iiiion>-(>li(>t<>|jni|tli; for iiitHliuiii »ml liigli jiuwrrx n stag 

7. IhrfiniitQ jHiu*r li^jviiiU on (») ilie I'ecluctiuii <if Kphrncntl 
iiliMTiition, (A) the rwJiiolion of c-tii-iitiuLtii- itboriiitioii, (c) the pt-rfMtl 
cciiiti-in)j; uf tlic li-iiaea — by wbii'li ts uinkiil (i,) iht- iili^'niiMjnt (iT 
thi'ir optic? iixi-H, (ii.) the jutnilleliKui of their pUnn, (iii.) the M'ttingf 
<if their pljiiit^s Ut rl^ht ailglen tu the optii- axis. ' 

Dejxnin'j jKitrer cnii only be l«Bled by a i-rillcul iinuge, Thp 
Ciillov'itiK is « list of suitable obje>cts of which a critiiTiil unitice in Ut 
be iibtuiiK-il, UMiiK X !^i>lid axial coiieof iiluuiiiiAtioti equul lo at Imuct 
three- fourtliti uf the nperture of tho obJ«>ctive. 

Vrnj /nil- fitiiivrt (3-, 2', unci l^'inch). — Wing of Aj/rian /hiI' 

fheVitiii (f (tlr«;,tili-Bj'), 

L'»r juiirerH (1 nntl j), — Probiiscis of blow -fly. 
Jfivliiitii /iufivT« (ij, ,*A> h' "'"' !■>'*' -Kogled }). — Minute Imini <in 
priiliuflcis of blow. fly ; hiiir iif ppncibtjLil (i'olj/xrnut twjuriiii) ■ 
iliiitouiH oil Ik (lai'k grouniL Thiii Inut isn nnivt srnsitive test; tuiltt« r 
thf "bjeeti^'e i§ good tliprr: i» mii-p lo he ftilso light. 

Mrilinm imwr* (witji wido ajjcrtui-o). — i'l'-uiywi/jmn /onnmmm ; 
A'liritiitn If/m in ImIhiiii or Ktyrnx ; i'lrurmtu/iiit nn/iiilnlum dry 00 
cover ; liactt?'l'iti and niici'ociivci slninnl. 

Niijh /"invrii (wide ntM-rtuif* niid oil-ininicrsion i and -jl,.- — Th0 
nfpiiiirinri/ ittriictnrr nf ilial<iniM, curxn-intty the fmvturp thiiitiffh tlie 
pei'foratioiiB. A'ttfiriiln r/ion»/ioit/r* /mm eherryjifld in bulaam or 
iftyni.v ; Itftcterin «tuI mirrncocei ntAinrd. 

Te«t with a 10 or \'i rye-pifKM', ami tnkdintn acconnt tile ft«neral ^J 
wbiteiies^ and brillinncy uf iJic picturp. ^| 

The piiduiu »cnk is nut incniioiivil nn a. tiiit, n* it may be very ^^ 
mtslMidiii}: in un«killcd Imndx. One great point in Imting objectives 
ig 111 know your ohji-ct. Can- niti>l In- ejicn-iJind Ui a^certaiii by 
meiins of veitieid illuminntor if obji.'rtN wmh as diatcmsdry on the 
cover ftn? in optical coiitiirt with the covi«r-ftliiK*- Ti»»iing objoctivvs 
18 an iirt which win only lie •»i]uir.'d in time and witli exppri«nc« 1 
gained by Mvring l"r^ nunilK-m of obi<i-tivi^9i. | 

In the innnipuliitiun of tbe miciMsiropi? it in not uncmnmon to 
obopne the operator rolUiuj the milleil hvoA of the Jiiir nitjuMmetU 
iiistnul of tinnly ][[ntspiii(( it iH-twivn the tinKiT »nd thumb ami I 
etiveniiiig, tii tjie :niniiliBt friictiiin of arc. tht! niiiount of alteration 
he lUuiirx. It is iindi-simble and an entin-ly inexjK-rl pmeeiiure to 
nill the initli-d beiid, iind eaunot yield tht- tiiu' resultti whieh a deli- 
mte inimtery of this |«»rt of the iuKtruincnt neeesailnLcH iind iniplie*. 
To u«' itrij^hl thr line adjuntnimV of a. ^v^i-iAium miciivtcopc IH Dot 
the fiml luid eii*itwt ihiiiH nmati'n^l by the tini. 

Iteyinil the L-oi-i'i'i-t and jiitlieirms use of the niioitini'o[H> and all 
its upi>liiince«, ihelt- is ihe nmtler of the rliminalirin of error* o/tH- 
l^r/irrtitlinit to Ite (.MiK-fully e<>i)ftid«m]. 

The eorrectiieHs of thu eoneluiaons which the niicrosco|H«t will 
ditiw regardirtg the imture of any object fnxn llin f'Uual A]>ptuiranci.'s 



whicli it pn>t«iu u> Kim when exAiuiiiwI in the \-un')U» nxidM now 
■ppcificfl will mvoMirily «k|«rnd in n ;*ivat detpve uijiiti his [iroWnus 
i-XpriPDCv in >iiicnn<x>j>ii' <ihtterv&tia)ii unit ujuin his knowhttfftr nf 
tJM cl»3n '4 iiudiM lo whii-li lh« )mrti[-iiliit' ^ivi-itni-ii inny imoag. 
Not nn\y ire oliaeri'kUuiia of oiiy kind li;il)le tn L-erliiin fiilUciM 
lUTunK •«!( "f (he |in<viou.*i notions which the 'jlMervei- iiiuy ■•iil«rtMin 
in rPipird !<> tlieoutulitutioti of lhetibji?cl4<ir the niilurvof tin.' lurtionx 
t>> wtiioh his ittt«nt)oii U directed, but even llie inoai piuclisw] ub' 
j«r\'er ia ii]>L lu take no iiot« of sui-li phenomena iw his niind is not 
prapared tu appreciate. Emii-s mid ia)[>ei-fei-tion4 of thi» kind can 
only be co»Tect«d, it is obvious hy geneml advance in wientifio 
knuwled]^; but the hiitlory of them nd'ord^ a useful wiu-nin^' ii^iiinHt 
faaaty «niiclu8ions drairn froiu a !■>> cm'sory fxiiniiniition. If iho 
histury of nhnint <iriy (cieiitiftc i n veal ij^nt ion were fully made known 
it would getiemlly appear ihnt the stability and oomplelenesa of ih« 
eotwiUiUoiM tinatly arrived nt hud unly been attained after many 
inodiAcationfl, •>■* even entira allenttioTis "f doctrine. And it is 
tJwnfbra of nueh great iinpnrinnci> as lo be Almost essential to tLe 
(onvctnesa of <nir (^nnctiMonn that tht^y iJi<mld nut Ixi tinally fumied 
Mid nnnounoi'd tinti! they ha^v l)wn t«ste<l in every conceivable 
luode. It in due to sricnm that it xJiould be burdened with as fe-cr 
taiae fivrtM uimI fulii>^ itoclHm-K im piiwiblc. It, in due to other trutJi- 
■ntkere that thi-y showltl not Iw miKlctl, to the great waste of their 
time •nd piiin>, by oiii' itrrunt. And it Ik ilui" t" oiiitelves that we 
shoald t»i>t ■■oinniit -nir r«-put«tii)h to lh« rhwiw of imiminnent by 
the premature formnlion and publication of conclusions wliioh may 
be at once rcvei>u.>d by othei' ol>tu>n'ei'( l>etter infoimed thaii our- 
■dviH, or iiu»y l>e ]>ivivmI to Iw fnUR<-iou« at wine fiitui* time, per- 
hnpe «ven by our own more extended <uid I'Jkn-ful reseaivhes. The 

MUjirNOi'iii 'i/' thr jutlifini-nt, H-/ifKrivr tkrft niy/m i\ioin j'or ili/iilit is 

a MmMi inruli'-atcd by nil th^Ki* philoKoplnTM who have gained the 
hi([falMt r»pul<- for pracli'-al wialom ; and it iis one which the niicro- 
senput mnnot tiioaoiin learn or tiioooniitiintly pmctisi*. Itcudes tJiese 
fCBDcrkl wiimintrs however, orrtain special raution;s>^houl(l bo given to 
th« younK niieni^'Opixt with n-^ird to i-i-mrv into whii:h he is liable 
to tn! led uvrn wbi-n the \Try Ixvit initruniimt-s nrr eninlnyeil, 

Errum of inliTpii'liition arising from thr inij)i'rfi>ction of the 
/bcnV adjiinluirnt am not at alt uncoiniiion uin(inK>t inicruccopists, 
and wotav of the nio«it serious ariw^ from th« um- of small oonw of 
illumiiutltuit. With liMUMiK of high p»wiir, And p.i])ncialty with thom 
•it Wjp! numvritid a[M-Tlurp, it vrry siddoin hupjicns that nil the 
puta at An object, however minute and flat it way Ik-, can bf in 
ioeaii t^iRetJier; and henw, when tli« foeal adjustment 'm exactly 
made feir one i«iri, eierythinji that Is not in exact focuM is nut only 
wore or less indistincrt, liul is often wronijly reprewMit«*i The in- 
•diiftuivtnMa of outline will aouietimtrs pit-seut toe up[i«inince of a 
pellud'l l)onler, which like the diirract ion -band may l>e mistaken 
inr actual Kubstanoe. But the most cooimon eri'ur ia that whicli ia 
produmj by the reveiwd of tli« ligjhtaand shadows resulting from the 
Tefractive powers of the object itxelf : tJius, the bi'conuavity of the 
blood-dines o( liuuiati (uud other mauimalian) blood occasions their 

1; O 

phactical uicncwtcopv 

fientreB to appear <tfn-t vhmi in the focus of the iiiicru«eot>«, thmogh 
the divergenc«> of the rujs which it oocnsions ; but wbeu thrv aru 
Iwon^t » little within thn fociM Uy n Hlight npproxinLitiuu of i)i« 
ot:{ject-gla8s the cciitrcH app<!»r brighter tJian ths poriphenkl jmrUi of 
th« discs. The stu'lvitt Miould if w*nied agaiitst Btippusini; tluit iii 
all cases the ina«t futaitirr niid Mrikiiig ap|iau«noe ig tl>e trur-ri, for 
this in oititn not the ciuw. Mr. SUrk'H I'liitrul iUtmUm, or 'iJwarmrli 
aUd*,' illUHtnttcsan nror of thi.^ <]ean-i]>t ion. A drop of vat«r liolilin;; 
colloid silica in aolulion in allowMl to cvaponito on a glass sUd«, aiul 
wh«n quito drv Ik covrrred witli thin ft\ii» to IcMp it clean. Thi* 
itilica depoiiitoil in thin way ia curioiuly oracknl, and the Jinnri irf 
those orack* can lie miuht lo prt-wnt a very piaitivii and decrpiii<- 
appenranco of Ix-ing niiNod liodie^ like iilaxt thirad-t It is al<m easy 
to olitain ilitrrnction-liiuv at Htmr M^Knt, living an appeatunM of 
duplicity ta that whiih is realiy single. 

A very ini(M>i'lant aiitl very fn-<fuc'iit luiuri-i- ttf prn>r, w|iii*h 
luiiiirtimiw iiii<!nitn> «v«ti on exjienenced iiu^ninciipiiitA, lies in (bf 
n!fraeti%f inAu«nc« rx(-rte<t liy certain |ioculiaritieii in the inter 
strunturi' of nbjfcin ujxui llie rays of liKht ttnmuiiUti^ thrai. 
tliein, tliii injlueiii^i- 1ietn|{ nf a iiatun* t*> tp^» nKt- U> anpcwntnc 
in I'hi- iniiiK**. whidi Mi;;f^t ti> tlie i>1i»erver an ith-a iif their cau 
tluit niiiy lje nJtoi^lher difffniit fnun tite reality. Of (hit fallaoj 
w« Imvo u '|>rei;naiit inKtanire' in the inisinteritfTdalion >< the natur 
■if the /iificiip mid nrrinlieHli of bune, wlii<;b wimv I<»i^ Mll>pn6ei 
t(i lie ficilid <N>ri>u8(.-l<b Willi iwliatin;; filnnienlH nf poculiar opacilyJ 
iiuttead of Ifeinj;, nit la iii>« uiiirersully iwlinitted, minute clianibcra 
witli diver^ii^ jsuvomiint exL-avaird in thn solid hkmhius mlistanoe. 
Wtien Cuuuda buUam HIIh up thi' excavntiimit, Ix-in^ nearly <if tiie 
MLnio refractive pownr with the bune itaelf, it obtit^^rates tlien 
alt<:^ther. Su, Eij^ain, if a person wlio u> unaccwttoraeil to the nw 
ut tlitt inicroncupe tjuiuld have his attention (Urnctod to n preparation 
iniiuntfd in li<)uid i>r in ImJimm tlwt inixbt cliaiic«i t<> contnin fiiV- 
&m6Um, be will lie abnost certain to be ho inuoh nuirr utrorigly 
inipreaHed by the appearaiioee of tlieHe tJuui by that nf th« nbjeet, 
tliat bift Brut mnarlc will be upon the nuinWr of Ktmngo-lonking 
black riiiKH which he wea, and bis firet uii[uiry will Ixt in rr^iairl to 
tbeir iii«Bninf;. 

Althou;;b no experienced luicitibcopiiil could jiow In- lul n«tmy 
by such obvinuH fallAci«s as thobe alluded li>, it in neeniMaty tii 
notice them as wariuiigti to tlinse who have still lu k" thmugh tlui 
MUie education. The beat method of learmnf! t<> iipi)riT!Ht<' tln> 
class of appearances in question ia the oomparuuu of thr lupect <>f 
giobules ui oil in water with that of globulea of water in oil, oi* <>f 
bubbles of air in wnter or Canada Ijalaain. Tbln e»in[wrvM>i) ntav 
be very readily made by »hakin^ up mmte oil with watrr t<i whion 
a little KUOl hu lieen lulded, wt a« (o (orni an eiuulMim, i>r by 
nniply placing a dr<k|> iif tiil <if turpentine (coloureil with nia^nt« 
or canuine) ami a dmji of water together upon a slide, laying n thin 
glass ooiTr over tbem, and then moving the cover iHtckwiii-ilH and 

I Uonthlti ItKnttvjatal /ohtm^ vA t. IDT! p. II. 



evenil tiinM on thp didv. l^ualk tnstrui'ttte ntc Hin 
\ vt nn iur-l>ubbli^ in wgibrr and Ctuiadu tiaUam. 
TIm! tiftunw which illu»U»t« the Kppeftruioe at various pnnU 
■if Uiir fiwua III an air-bubl>lp in wntrr km Cttiintla b«]Mij]i, and »f a, 
ht -globule ill wnlor, may be thus iltaHtrnted. \U. a dia|>hni)(iu of 
«buut f »f a min. bdng |>Uoe<l nl a (lislAnce of 5miD. beiMwUi the 
■tafcr. umI tilt' concave mirror i-xartty contiwl. 

Air-hnbblm m inU-r. — No. I (fig. 3ll*) ivpreeeiiU tlie dillei-enL 
nppeanuicen tit mi air-liubblp in wratiT. On fucueninti thv ol!Jei-tivi> 


Km. ■»,— AirlinbblMin(l) ■>l«r, W Cumin bntum: ft) fat-globuloi m ■•tcr. 

tu tlie m]d(U« of llic bubUn (iS), thr ccnti-f i<t the iiiiiif^ 1% mum to bn^ 
very brif^it — Sriglit^r tlmn thi- ntct of tJn- field. It in KUmiundtt) by 
* grpyifji iiini-. and a soini-whiit bnuut hinck rinjj interrupted by one 
<* hh'W brijrtiipr circhv*. Kuund (lie block rinR nn* ajtnin one or 
man eoiiMtJtrii.- circles (of dittriu-tion), bri^lit«r than Uiv )i«ld. 

On (oduainK to tin- bcittuni of lltu bublilr (A) the i-oiitrol white 
cinle dintiniiihca aix) liei-onm briKlKcr : it!< niai;;iii is sharper, and 
it is Barroaiidnl by a twy bnud blovlc riiii,', whieli lias on itn 
IfriplKTy uti* or luoro diAruHiim i^ircUra. 

When the objwtiv« is f>iciiw>iil to the upper surfiMie of tlw 

H ft 3 

372 nt.\mr.\L micboscopy 

bubbl<- (C) th« d-ntnil drcU- incrwACH in sok, iuwI ia anrroundcil hy 
n grrntcr or Ir-ss niiinlx!i- €>f riiigK <if vnriiiua shmlcn *it xiry, arouud 
which i* iigain t'liuml ii )>lai.-k Hiiji, but nnrmvrcr thiin ihtnnF in th«< 
prrvjoUH [HMitiiiiis of t)i« ohji^ctivL- (A iinil R). Thu ouUtr dre]«« o( 
tlitfnictiiiii arc iilui niuuh ni<it« iiumcniuK. 

Aif-bvblilm Ml Ciinada JIaUam. — Caiuitlii ImIsuid Ixttng gf n 
hichnr ri'frnctivi; index tbau vriiU-r, Uit liiuiling un((le iiiittiHul M 
l>Mng IS" 35' i* 41° tmly. wi ibat Iht- vays wbiuh are incidt-nt tuncJi 
)r«i iibliiiui-l}' on the nurfuL-e ><( fl(.-pa|-aUi>li uiider^ t'lUil rplliMUaii, 
mid it will 1h- imly iIkmc nij'e which £ttoe very cliwo to tbn hiwcr 
piilt! iif thu Imbble that will ivai-li the eye, uiid tlie black tniirxiiinl will then-toil! be much Ui>[er. Tliis is nJiown in fiR. 319, So ± 

When the objective in f<:)euH»Ml to the iHittotu of the bubljJn 
{A'), We h»v(- u nniiill cetiti'id circle, brighter thtin the rent iif the 
^dd, ull the wsl of the bubble being black, willi the exc«|>tion of 
wine (leripbeiTkl dilfniction rings. On focu.vting to tlie c«ii1tv {B') 
or U]>})er [hliC (C") of tli« bubble, we hitve subatanlinlly lite siiiie 
u|i[ie»nuicea ba in B and 0, wiUi tlie exception of the Bni*Uer nice 
o( the reiitrat circle. 

Fat-'jMulr* in KYKri- (% IllK, No. S), — "Dw^ ilhiuirate lli« 
«iisei>f A highly refmcting b<iily in n lucrliuni of h'ss rcfrndivc power. 

When the objoctivo tn odjuKtMl to dn- liottom of the glol>ule A", 
it appcivns on n grey di^! n. littln dnrker thim t)iu Held, and sepamt«l 
from the rost of the lii^ld bv n <ln^ki.^h I'ing. 

FMcnmint: lo thi* niiddlo of tlu- bubble (B")t the central tltac 
1>ecoDics!Min)cwhut bri^jhU'r. und is Nurnjundcd by* niirrow blnclc 
ring, Imixlerod within nnd witbout by diftrnction cirvlee. 

On further remoniig the objective the diirk ring incrcwNM in 
ftiite, and when the upper [Mirt of tJie objective is in focus, w« lutve 
(C") a small whitJ- centnil dine, bi-ighlcr thiin the n^t iif the field, 
nn<l Nharply limited by a brood, dark ring which is blkckcr towards 
the centre. 

These nppcamm'es nre tlie cnnvurse of thine pn^senlctl by tliv 
iiir-bubblc. Tluil, ux vre -uiw, has a blnck ring «nd a whito onntrc, 
which «n' the shni-per ns the objeclive is iipiin«iclie<l to tin- lowrr 
polo of the bublihi. The fal-;:l(tbule hiw. liowevi-r, r AaA ring 
which is the bmmh'r, and a. Cf-ntre wUiuh ia the sJinqwr, AOCi>nling 
i\A the objective is brought nesin.-!' to the up|)er pi>le. 

Thrae coiiKidenttiunB, apurt fruiii their ei:idiluig 1U to dii^tingiiliJi 
between air-bubbles and fal -({lobules, and preventing their bt-inj; 
CMiifounileil with the bialoli>giciil vleuientti, enable two general 

JirincipW t"i be establiBliwI, \u. btidies which are of greater re- 
mclive ]K)Wcr than the nurmunding inedtum baie a wliil*? eentn^ 
which JN hharper and suinller, an'l a black ling whitdi la Urger when 
thi! objective is wiilidrawn ; wliilsttliotte which are of le)a refractive 
jHiwer liuve a centiv «'hich i& whiter and Huialler, and a bUvk ring 
which in brooder and darker when the objective is lowered. 

Jfonoehromatie I,ifft>t. — Thv aume phenomena are obeterveU by 
yellow inuuochroTUAlic li^it, exnftt that the ditfraction friugee are 
more distinct, fui-thei- apart, and in greater nuiubeit) Uuui wiUt 
«irdinary light. 




A Ent-f;lobule, iiideMl, ae«m& lo be coiuimb^ of n series of cwn- 
cwnlric lAjerH like ii grain of March. With blu^ li^Iit th^ep tria^ea 
arc niHi niultiplilieil, l>ut are riuser t(K;et)i«r itnd tiitei', so that they 
Mv Dot Ml easily viaib)^ 

Yellow monoobronmtic light, therefwe, cunstituMa a jpxkI 
■i»eniia for d«t«muUDg whether Uie Htiii^ seen on an obji^ are 
ppcoliur til it, or an only ttiRhti-tiuii liuOH. In ihe fm-mOT ciute 
they > re not exaggent«d by niunochnnnalio light: lint if, on tlio 
ountntry, they are found to be doubled, or ((uiidmplr^l, with thi« 
li(;;fot, we ntay l>e c«rtain that they are diS'ivction fringpH. 

But ther« is no source of fidlftcy to a certain class of workerH so 
muA to be guardod against as that arising from rrrors in the inter- 
PRtetion eoncwning movement as such, nmi fniuvinUg mnrrmiit'T 
U^r ntoivrivnf *^rliihiu^ 6« r*rtain Vfry mimifr jnti-lirlfi of niaUrr in 
a nUtlf oj" iutjmtMoH iaJtuiflA. Tho fnovemenl wits first obwrveil in 
the fine t^'aniilar pnrticlM which exist in gn-nt ti.biinitanoo in thn 
content)) of pollen gmins of plants known ns the /nvUfn, and which 
ai» BOt fnw by crushing the polloii. ]i wan first stipposi'd that thn)- 
inHic*tc(l somo spfcini %-itnl inovrnient analogous to th<- inotioii of the 
spennatocKNiof animnU. Itutitwasducoven^lin I.'*i7, liy Dr. Robert 
llrown, that iikoi^nic substances in a stiit^^ of line- tritumtion would 
gtrt^ thi- *iixav rrKult ; and it is now known tluit all suhstuntNw in a 
•nfficicntly (iim* static of powder an^ ntft^ctinj in Itin mmn manner, one 
nf tlw< ino«t rctnarkahln lic-in^ the inoviMoent visible in tllI^ contents 
of tlut dnid cHvitJM in [|unrtx in t\w ohhwt rocks. Thcon hai-o 
protwbly retainctl thfrir dancing motion for ipoms, A good illustra- 
tion is gambogr, which can In- onxily nili'icd firim a wati^r- colour 
cake upm a gtoos slip and covi-ntl, imd will at onc<: Hlmw tlii^ 

tmctTistic movement; sri will cnmiine, indigo, and othi^siniitiirly 
(bi liodii'>. But the imrtaU which arc from seven to twenty liiues 
as lM«vy ns w.ntiT nijuiw to In- niluL-nd to n .itate of minutcneas 
many tiim-s gmnti-r ; but, triturate*! Hnely imou^fli, lhe^^p alio nhow 
the inoveni4!iit for a imig time known, from tlie name of its discoverer, 
OK /Imtt.'Hitn inoveiiient, hut now more i;i'ii«rolly calleil /mlrrit, 

Tlie uioieinent i» chieliy of an "nfMrltiri/ natui'C, but the particles 
also rolut« luickwarda anii forwunU on their axes, and gradually {If 
IHY^Stently walclwv!) ehanij* lf<eir pliif'* in tlie field of view. It is an 
extmnely dtanoterintio uio\«Dient, and I'ould not he mistaken for 
■ny »ital motion by an <iti»en-er acijuuiuted with l>oth; hut tho 
sUKleiit must familiarise bitUHflf with thia kind of motion or he will 
lie utterly unable to tlistinguinh certain kindii of motion in minute 
living; foniiN in certain stugf^ of their life from this moveiueiib, and 
will midce erroneous infereiuH-s. 

The movement of (he ftmnllent particles iu pedesis is always tlio 
most active, while in the majority of caaes particles greater than tho 
■ ,,'n^tJi of »D inch are wholly inactive. A drop of common ink 
which hail been exjxwed to llie air for some weeks, or a drop of finit 
clay (such as the prepared tnalin used by photogniphers) shaken up 
with w.-iler, is recommended by Professor JcvonB,' who Ii'is rciwntty 

Cis subject, 08 showing tlie iiiovenient (which lie ilr^ignatcs 
>niirf«r(y JotifTtal e/ Uirm. A'ciVikt, K.S. toI. (lii. IhT^ {<. ITS. 

i74 phacticai, ancRoscoPT 

yivi/'Tinc) rxtremoly woll. Dut iiont^ of thi; pArUclM ho hiw oxAniiti^M) 
are ho kctivn as ttiOHO of punitctKtoun timt hns l)fNMi grminil up innii 
ng&t« mortar ; fur them ara «wn undrr th« nurrowopr' In l<«p aiul 
«Wftrm with nn inccswnt qiiivuring movcmcDt, m nyiid thiit it » 
ituptihsiblo to follow the courw of n purticlf whicJi probably cImiisck 
ito dirrction of motiun liftocii oi' twenty timu in « »«c«im1. TIio 
iliEtnnco through which n pivrticli- moriw ut any ono bound is uKUiUly 
l«u thnn jn'nnth of Jiti inch. This ' Dmwniitn mavt-nw-jit' {«x it » 
coiniEionly tt^nuriil) is not duf! to (?\«pnmtion of iiif liquid, Cor it 
roiitiiiiK-j' witJiout thn \riuA nlialniiitit uf CMirgy in a ilrrifi of ni)tM!oUa 
fluid tImt in cnmpl«t<ily :tnrriiiiii<li'd by oil, nna in tiivn^orv' cut olT 
from nil ]iiis.vil>ility of evitponitioii ; and it haH Imm known ut cod- 
tiiiue fur nuuiy ymrs in a Miiiill <]unntjty of tluid pnt-lcoMl bntweeti 
two gtowcst in an nir-titfht cum- ; and fur tlio Hum« reaaon it can 
•unrwly Ix! cnnnuctiHl with tin- chMnioiil i-haii^. Bui the oboerva- 
tiona uf Pn)fc»uir J.-voii» ( show that it i* jrrently nffedeil 
by tbi- uiliiiixtiiri! uf viiriuuH Kulintaiioca with walor, )>eiii^, f'>r 
<>](ainj)li>, iiifitm»i-d by ii Kniul) nilniixturfc of K^m, wliile it u cheicknl 
by an vxiifmHy niinutt! iidmixture uf iiulplmrii.' atid ""riif \-ariuue 
salin« imupounils thetie (hh Fr^ifvssur Jevmia pointa out) beinf; all 
BUfli iiH iiiurL-asD the ccmductiiiK ["iw^r of wat«r for electricity. The 
mte of !iub«icleiic« of finely dividt^ cUiys or other [larticlea soapendfld 
in water tiiua greatly dependn ujioii the aetiviiy of their * Bfwtdan 
Btovemeiit/forwhenthliia brought lotiatHnd t}iepaitii:lea aKgrei;a(e 
and siMk, HO tliat the liquid clearn it-ielf.' 

Pedotic tiiutiim itiqit-ndH on, ihnt is i-'' allecte*! by— 

1. TliP itis? uf iJic piirlii'les. 

2, 3"A# ajireijic '/rnriri/ <•/ Ihr jinrtieU*. Metals, or partidea of 
i«nuUiun, uf sinnlar hixi- to [uirticlen uf idliea or jpinibo^, nioi'e inacli 
nion- sluwly and it-ss frw)Ueiilly. 

S. The wilurf <•/ Ihr. litjuiii. Xoli'|iiid vloftt pedenis, Init li<)Utilt< 
wbicli have » chfinitiil ai-liun \m the sidislanoe do. This uolimi may 
be very hIow, ntiU it ii^isds to n.;ii{loiiii^niit- the [tiirtiole.'>. For in- 
atuuce, liariuiu Huiphute. wliifii {'ri.*i.-i]iitat<vl fixiiii the vuld nolution, 
takes a luji;; tune tu settle : wbeiY-tiA, wlii-n Mai-iii and in pre-ience 
of liydi'tu'lilurii.- Hciil, a^^-^totiieration tiinn iieoui-!L Ii-ou pi-eeipitateit 
as hydrate in presence- uf hhIIh of nmiiioiiiuin, and mud in Aalt watt-r, 
ai<e other inalanres. The niottun doeH nut oen^ but the jwrtiden 
a<lhei« together and move very hIuwIv. 

But bfsideH the I'ight aj)precintion of the nature fif pedeaia, tlt«re 
is the utnuMt uautidii required in ihe tnler|>T«iatiuii of the ritpifllti/tif' 
iiwvfinfnl, iind kuu.1 ••/ iiitiit^ineiit wliiub livin^iuid motile furiii8 eSet-l. 

?'/«■ tibvri'itlion (if tht }i>iftvmifna <(/" motion ntul'i' lAt mti'rrmvfr * 
luui led to niniiy false viewn aa to the nature of these movements. 
If, for instance, swarai-spores are Men to traverse the field of view 
ill one second, it niifrht lie thuught that tliey nuM tliroUKli the water 
nt thir Milled of an arrow, wltereas they in reality traverse in that 
time only a third part i4 a niilliinetre, which in sumewhat ntore than 

' So bIho the Rfv. J. D(Ju>ulx 'On tlia Tlioniici-dyuiuuio Origin ol lliv Ufuiciijiui 
Matliin*' ill Mmlhig /onrn, n/ .Vti-nur, Sei. *dI. mii. 1WT, 
' Dm Mikntlnp, Ki«g«U uid SdiwesdouT, p. -iM lEog. «dltj. 




ill H tiiwr. It niuxt not, ihrn^itrr., Iw forgotten thnt tho 
ntpiiliiy >>f inntUin »f nik'r<MC<iiiioul iilij(K.-t« in only Kit itppurrnt on^ 
An4 iliiti il.t iKMTur.kti' Httniatioii i<i tmly (iiKwibU; \iy liikiiijf ns our 
iteiktnn) iIih iwtuiil mttii ))utwi?e!n tiuu! and spooc. If w>; wi«h, for 
Um atki; uf RSiwi n>nipurin>ii, tiii-Mtimitti; th(! nin^ittHli- <if tlu- lunv- 
in)( UmIIim, w(- nwy atwayH ilo mt; llii' awi'rtjiiiini<'!it iif tlii- mtl 
ntiiililv n'lnain.t, hnvfcu-r, with eofli Kucwwisivf !ni>li"iii, the jtrinci- 
pal in«tt«r. 

If a atcTFW-ahafiLiI ajnnil "lytwt, of itli;{ht thiflcni'ss, ruvnlvcs im 
itM Kxw in tin- fiH-iil |>liii)e, at Hiv aainu time iiiovint; forvrnni, il 
pramto i1h> «lwi!|>tiv« n|>|i(>iii-aiice <tt B, nerjwntiin! tnntion. Tlius it 
M thitl llie liiiruoiilal [>iitjtfliiiin of nii iiliJeL-t of this kind, i'orr»- 

niinK I"i the MUH-iwnive iiiiiiue«t!i iif lim*, njijieiir cxuctlj' ns if the 
ni(-nl wi'iva true 9er)M-iitiue ime, Aniin e.vnmplt! of flli Kp))enr- 

mi thU niLture. we uiu,v uieutioii the itlle){eil twrpentiiK! luDlion 
«(SniriIluiii niid Vibno. 

oiinilur illuMiiiiB iii-e aIhii pnxluoed liy swiinii-a|Rire8 and sperina- 
tiMMN ; lb»y itppettr to detii-nife'iei'jieiitiiiti Hiii^ while tn ivnlity they 
more iu « »|>Jral. It um* fm-iiiei-ly th>>U};!it thnl n iiiiiut)ei' uf clitTer- 
«nt «{>pmmiic«s o( lautiun muat Ije (liatiii^^iiUietl, wherens modem 
vinnven Iwv« recin)pii»ecl iiioet of itieni iu< ooriMntiiig of a forwnrd 
aKivHin«nl ciMiihiiied with roUition, where the revohitioii takn place 
•iiaH<iaH'§ niuiui h c«ntnil. and sometimes r>>titi(l nti eccentric, axis. 
T<i this category heluti<;. for iiistAnce, the siippoited o§cillntions of 
tbe ottillttrinr, whow cJiwige* of level, when thus in motion, vreri< 
fameriy unnaticeil. 

Id wl'tilioQ til these cliaracleriBtica »f ii spiml iiii>lji>ii it must, of 
onone, Ih- ascriiiin**! whether il U Hfiht- "i- lrft-hiiii<teil. To <ii«- 
tinjtnbh ihis iii npheritwl or cylliidrii-u] )HHlte«, wliidi levolve round 
a oMitra] axis, is l>y no mettns easy, and in miiny vtitma, if tlie object 
ta ve«y -iiiiall and th<* mnteiits homtiKeneous, it is 
t|1tite impiw^uble. T)ie Hlt;{hl< variatimiK from cyliii- 
dritaJ <» spherical form, im they •x.-i.^ur in encii eel), 
•r« tlterefor« just sufficient lo admit of oui- jieri^eif - 
ing wlKllier any rotAliuii does udte jilat-e, Ttio 
discovery of the directioit of tlie rutnlion u only 
puesihie when Axed points whose position Ui the 
axis of the spimi \» known cnn lie followed in 
tiieir ti>otion round the fixi». The sann'' hokk g>i<>d 
also, niM/d/tN mulattdi*, of Hpirnllv witmid thrrndK, 
Hpitnl vftnsnU, Ac. ; wp must he a^>le !>• distinguiHh 
rlcM-ly which are the niden of tlie windings turned 
towards or tum«l away frcmi na. 

If tlw ooniNW* irf the wintlingn is rpiy irroguhir, 
as in (i|[. 919, a Htth' pmcticn and cni* «re nnrditl 

dixtinifuiKh n ttpiral liiic aa such in Kmall n)>- 
ctn. Thi- tiiicriKcnpicid inings might «AMly liiul 
~il» to ihr (-imd union that wr wen^ examining a 
cylindriciil Ixidy oonipiiiu^d of WIU or funncln in»7rtvd one in 
anutlKTr. Thn .■^pinilly thickrnixl thrnvl.-c, for instance, n* tliajr 
<iri|^Miti? from thi; rpidcnniH ctIIh of mikny mmxLs were thuD inter- 

Flu. ain— A ipinj 

in motion. 



prot«dt nltliou;;!! li«n- iiiul th<-re )>y tbit nidt- <>f Uw im-gulnr spintli 
quite rriEulAi' 'nifS i>i>i- »!»■ ■■Ij«>vi-vmI. 

Moi'iHivfr, it inuitl not be for^tten Uwt in the miL'maco]>icftl 
imo^ II Npinil liti« ;iK'A}'s appeurB wuuni] in tbt> xninp mAXiDpr as 
wlion »<-fti witti ihc linked eje, wlii!« iti u, iuvrr<>r(llip itivrnudn being 
only n liiklf one) u li^-lit-hundei:! 8ci«w in uliviously tvpiiwonWd as 
lefl-liiinilml, uikI i-auv«i-8ely. If, tliei'efi>i«, the niirnHvupicol iniiigtt 
is oImcivmI ill » miiToi', as in drswin;- with tbt- 8i>inniiTing mirror, 
or if th« imi4;fl-fi>iTiiii>ti pencils sttv niiynhei-e turned tutiik^ l)v n 
Mng)<- ntilexioii, A similar iiivcrsion Ukes plaee fminriKbt-luimlnl to 
loft-hnn^ed, i>n<l this iiivei-siim is n^iii cancelled by a necund tvflex- 
itm, in jjitue microsi-opcs. All Uiis is, of oourae, vfell known, »nd t»i 
till- piucliHed obsei-vei- self'cvidvnt ; neveitbeleaa tmiiiy iuii^rfxioo|NKto 
biivi; liliowii tliat they arc still entiroly in the djirk aliout inattAnt 0^^ 
thi« kind. ^1 

One ot PiKfi'sKor Al>bnV osperimeiitE on difftnelHiu jJmwnttiM 
proves tliat when tlii* iliirmctJon apectrn of the first orderarastoppwl 
out, while those of tlin mftoiuI arc odniitted. the appeai-ancv of tlM^ 
alrut^ture will be (tuubli' thr tint^nrss of \\\v nctuiil Bti-ucturi- which i» 
caUHinK the iiilcrf<Ti;Tit;c. ' ^H 

Upon thi-« Inw then' apjMMirs to tlc|>ni<l n number of puHiiible tsl-^l 
lacies, errors w hid i iniiyitriwffroiiK-ithri-itsiniBappreheiiHioHoriniiiiii- 
terpretation. At U-iist tlicM- ttjipcar to us, from n praclicnl point of 
view, to be of nuflicifnt iMi]«irtriiic:i' in iicfid either CKutiuti or 
fuller exposition iif ihv Kn«it law tif Abhr in regnrd to them. 

If, for examgile, fig.i. :120, 32 1 , and A'l'i may be taken to rqireaent 
a square gratinshnving 'J'>,OOU holes per linear inch at the focus of an 
oluecti\-eat P, PI) the dioptrif l)«tm, P' P'diffraetioii npectfwof the 
fiwt Older, and P' P* thciw of tin- Ki^^und oi-der, then if the objective 
is aplaiiittie nil th«iic sp<!clni will l)c bn>ii;i()it t<i nn ideutieal foonl 

coiijugattr ; and the iinai^ of the 
gmtingwtll be a countei-jiart of the 
Ktrueture, chamctenBtie of hucb a 
griiup of sipectrii. I>>t ua suppose 
iiur objective to l>e '■vi'i'.c<>riwctfld, 
as in tig. .121, then when the gmt- 
injE is focUMH-d at P th< fjxvlnt of' 
thfjimt urdrr o»li/ trill hr hroto/M 
to Ifiif/acal rnHJinjnlf ; the image* 
however, will n<>t tw innterially 
nfr«cted nn that aecnunt, fik llw iliHhit-tioii rliimiit* <"f the &mA onler 
ant alone suHieient to give a truthful rppresttntatinn of the 2ri,000 
per inch grating. If, however, the nbjcetive Ui raiw^l so tlwt tho 
emting lies at P' //(■■ liij^t-arlion rlmitnU o/lhr tmind oitlrr tmlg arr 
bmut/lil Iff thr fiiriil roitjiii/iitf ; L'iiiiHei]Urntlv by the hvjiotbrsis the 
iniitge will hnve ■'>U,000 holes iier linenr iiicli, or double that of the 
original. In other words, pWui^ agnitinj; at the longerfocuK of an 
orer-correctnt objeetivi- is ap|mrciitly tntitnniount to cuttino; out the 
diffraction ipectm of thr first »ixl<-r by n Ktnp at the Imck of tlw 

Fl" 930. 

Fl->. IHI. 




Tlu) «ffinct of this is to givr< bii iniprvKsioit llint tli«ro is » Strang 
mttDg with :;r>,000 holm i>cr linmr inch ; nixl nver it nxrifAn- gr»t- 
mg witli r>0,OoO boles pei' linciir iiicli. The mininjt thn fncus mi ns 
to brtDg P to f nccMMirily g;iv«s llio Uh-n cif the Iiik^ Ktru<-tufi> Ix-ine 
MpcrimpoMxI on Ih^ coBnir>. Tlinii-fcirc the inicro>«i>]>iiit kIiouM 
hvwsn-' wlwixrviM- he initio's n Klrurlun* ot (ii>ulitc Hdimivss ovit 
uotbrr <«<■ l»t hn hiiB acomlitinn (if ihin^'XHtuiitiir tolif;. 'i'2\. The 
(nllowin;^ ix a Imt which uinv Ih- niiplii-d to cuiitirni thi' f[i-tiuiiii-ncM of 
my such rtnjclure. FiiM niiii.'.urt- hy iiii-nnH of th(* tliviilt'd hr-iid of 
tb<^ tiitft iulju.->tmciit sCivw iuk aot^uniwly ilh {xiMJbte, 
the luovtrinciit roi]uiritl to liriiif,' I' t*i P' in liK- S2I ; 
w*xt by inruiii-i iif the drawtulie iiiereiwe the dis- 
tanco bt?t«'«'ti thit tiyt-'-juece iinil the olijcctive : this 
irill Iwve ili«HTMt<>( inctwaitiiif; thi-over-iori-wtioH 
«( the ohjeictiv«, And n alutv nf thin^n will be uh- 
tftifted OB in Bg. ^'J'J. Henre it will rv<)uiiv it lar^r 
nwveneiit of the liue-adjustiu«ut screw to hriiit; 
P t» P'. Thia will make the diatance between 
Uie 50,000 grating miiI tlie 25,000 fating appenr 
{renter tfaan ii waa before. // thU hiim fhtet llir 30,000 yeatiug U 
9 met* it^ftMction ffhott. 

A preciselj- (umiliir ootulUion of things i>x\t,U witJi aii uudei-- 
i-orrected objective, only iu that ca>ie the false finer grating will 
Appear below tlie original coai'ne gmting, and to incrpiuc Ihf diHtniico 
mtweeD tliem the draw-tul>e must Ite idioilened. ' 

It iniiy tliAreCon b« <il M•r^ieo tii give an example of the URO of 
the iiiiiii«ncul aperture table as ji check in the inteTprtrtAtioii of 

Fig. 333 giv«s six illtiiitratiiiiiK .rf the bock of mi objpclive (the 
nye-pH-ce being n»iiov«d) of M N.A.. or 112' in air; I) si'iiidft for 

ilioptric lienm ; I for dilfriiction upcctrum of tlie fint order ; 3 for 
dilTraction spm-trum of tite Mcond [mUT. 

< ll in ovll (n BOla Ihtt* Dial or linvp u^t^ ■ pliot»mipmicnph bf Mr. romlM'r of 
t -lUbun "iiriiferr whjrii i« iin«(*(i- la tlicxr jiarln in o^rnwi Foca* IIia Umcture in 
ung]*, IfUt Ui Uie pArt* tthorp tlw ttvn* u vriihdiBim lb i« iloublod. 

> I Bill I 



When tlt« liuck of aii objedive <>f -83 N.A. ftlii>wt an )UT»ogo>] 
mont as in 

No. 1 thoD.altbunicli tho «trHcinrewilllM itiTitiblc, 

iicADDol b«ooar«erlliMi . . . (0,000 dot ineli. 
So. 3 „ „ „ 80.000 ,. 

Nv. 3 tlieatliaBtruclur>>dotnnol((ilTpr(rrtAt1yfrumlOtOOO „ 
No. * „ ,. „ 80.003 

No. S .. .... awoo 

Ko. It „ .. « 40.00U 

It will l>ciin(lereto.Kl by thfi -ttmlfiit thnt tbi> firfrrjiiian a/tlte 
mif.n>iif(ip« uid it* nppnrntiu is n iiintt<'i- ihnt must Inra^-ly <1imm^^ 
upon btKnwn action. The stnnd KhmiM Xn; tt^pt from <lu>l, K<>n<^^^^| 
wIjuhI witb n Biift i;)i&Ti»iis lr«t)K<r ivftf-r iisr, and wbi-Ji ntHtdlH^^R 
mintitt! iiiinntily of WAtchmnkrr'K oil tnity Iw put to u joint vr<irlcil^ 
iitiffly. TluM'ii is no iH-titr wny t<» piTs«rvu this stiinil than to ki^ 
it citliiT iinili;r n licll j^'lo-ss or in a. onbini-t witicb in nuiily lu-caudbHriai 

All olijiictivps shi>ul<l be oxn.inin»Ml iiftor nsi-, nni) all oils o^W 
vtlirr fluids carefully wiped awav ftniii tlii-tn with old (ounlirio whldi 
hits bnpn tluiroughly wAsboil witl^ hoAa, wi'll rinited and nut 'Jropad' ,, 
or liniKli<^ ill nny wuy, but »iiupty drind. Hj 

If 4ibeiniL-iil ivrtficiit* an- i^nplojod the oe«nti(in o( their uW 
nhould beoonat- the moment for wiping the leineit employed vr itii care ; 
nnd n,ll pnn'e.i«e» ijiKdvinR tlie u^e tti the \-apuura of voUtile ncids, 
or wliirh deTelui> nulpliurelted hydrogen, chlorine, Jtc. inoM never 
Uike place in ti room ni wliicb a niiaroscope <>f any vtlue in placed. ^ 

Dry elder-pith <uid Jnjuuicse jmper are by some workers !>ug'^|| 
jjCestNl for cleaning tlip front le:isfsof bomoKeneoua objectives ; but 
while them- anf i-xc(?i!ciit, eajii'ciiiUy the furiMcr, we find ntitliiuR 
1>etier than the Miiiple eambrit; we suggest. ^J 

Two or lliree piod chumoiii leathers should lie kept by tM^R 
woiker fur mn-oiCi- purposi-a and not Intercl lunged. Cleautin(aa,(«r% 
delicacy of l«>u<-b, find a puruKU! to be accui-ate in all that l>e ftoea or 
seekn Ut do, are cAtentials of tho sucoewiful niiuroscapifit. 

It may be noteil Ihat dust ou the eye-piece can be delected in ft 
dim li^ht, nnd i-nxi W dixcuvercd by cloaiug tlie iria diaplira^n. The 
l<>n>i of the eye-piece on which ttie dut>t appears may lie toc«liHcd by 
rotAtion : and thiit should l>e <1one before wiping. In reference to 
daiit on the iMick of the objective, it should be observed that if the eyo- 
pieoe be removed, dust Bomotimes ai>pears to be npwn it which comm 
nully fron) the focus of the sub-sta<^ condenser, and is. in fact, not 
on lh<^ Iwck of the objective at all. To find IhiK condition, remove the 
liglit tiioditier (if in use), fur the duNt may Im* on it, and rol4te tlic 
oondeiurer ; else there will be needles and ii^urious rubbing of the 
liack-lcnx of the objective. 

With nil-innnorsion ubJecti^eH dust or air-bubble« in the oil 
must be carrfuUy avoided. 

If <!hamoiii liiitlior be used for dcnninc the lenses, it should be 
previciuKly wirll Ijcaten and shaken, and uien kept constantly in ■■ 
vrellnmdn box. 



UsMOi iliJA iMAd it IB intentiixl tO||^v»nnnocountof tho«- niAWriiUx, 
iutmntentoT) niid tip|>liai)ec« of rarioux kinds, whicli hnvi- Iksiii foutitt 
most liei^ icoable to iiiknMcoputs onjgn^/sA in geupral biotti;n<Ml n^- 
■Mn-h, aod fi dmcribe tJtc most K^pranA methods of trnipUijiiiff 
tkeoi ill thi> [irviKirntinii nnrl moiniMng of objects for Uw dispU,v 'd 
ttw tnitiuto sti-uctniTs thus brouglu to oitr knowledw'. Not oidy i--« 
it uf the grattMt Bnlv«i>tJip' tliiit ihi'dinuDveriM niAdeby iiiicnx'>L'0|>ic 
rmurcli should— n« £nr ns pmailjln— Iw enhodicd {so to spcuk) in 
' prepMTKtiODH,' which Khali rnnblc tlicni to lie Ktudicd by fvi-ryoni' 
«Im> Mfty deNiro (o d» so, but it is now univornnlly lulinitttil ilint 
nctl ' prepAKtions ' often show m inucU mnm tlian <-jin Im »»en in 
Um fraui os^niui) thnt nn cxaniiiintirin nf it mn lir^ (Hmnidvml »» 
con)p1(*t« in whiHi tJir nictUniU nioxt nuitidito ti> viw\i particular 
oue ba^e not been put in prnclioc. It niuut Im^ obvimin tlint< in i» 
tomprdteti&ive (rmtise liltP tlit- pn-wiit »ndi n y^wwof trrrttm^nt o( 
this Mibject i» ntl that can bi; nttnmpt'il, rrxci'pting in » f'tw instances 
nf peculiar iiiton-Bt ; and as (hr liintoliiKJcnl student ciin liml all 
tlie gnid*nc« h« xk^» in thi> nuni'^rou* nianunU now pmparMl for 
his instruction, tbc Autlmr will not fi-wl it n-qnixitv to fiimiadi hitn 
with the tprciat iKn«tioii» tlml nn- rradily McoccsiblR to him ^In- 

Matkruia Isi»Tm:jiitsT>, axi» Appi-iAxret 

6Im< Slide*.— Tlir kind iif ^•hissbiwt Kuiti^l for mount in(E ot>jecta 
i* thdtului'h i% known an ' [mt^'nt plat<','iui<l it In now hI must in- 
TRruibly CTit, by tiif foinmoii conjuint of iiiicroM'<>pi«ta in tlii-H cuunlry. 
iflto jalijut itirusuriii^ 't iii. by I inch. Pur oV>ir<!ti tix' larf:;« to be 
nonntrd im thi-»' tlif- nizc <>f rt in. by I .V in. may hn iulopl«d. Sucli 
ib|K( may ■» punhaMMl, accunitf^ly col' to »t»p, and Kt^uud *t Uie 
nip's, for «■ liitit? luori! than tJie cost »( thu siaa thnt fi-w ifersons 
to whom lini^ if< uti obiccl would trouble tbeiaselt'en to prepare 
tlum ; it Will); only wlirn i;!naA slidea of soiae unusuiil <limenBionn 
Am required, or when it in denirod to construct ' built-up («11>>' that 
• £>ctlity in cutliii);; i-Uss n'ttU n (jloj^ier'x diamond 1]«comn iwffal- 
Tlie ;;laA« ftlidei^ prejMred for u»e sliould be free from^-cini^ mir-bubblea, 
or otiier flawa. at leaiit in the central pitrt on which the oltjecl is 
placed : And any wtuMe deferts render them unsuitnble for ordinnry 
furpu8es should be aelecteil mkI laid aiiide for usi^ to which llie 


working niicrosoopHst will tinil nn difKciilly in ])utUng thi^m. As- 
the Nli|w vnry coiisidombl}'' in thinkncjis it will ho A(li*untaf(eoua %<>■ 
(l^tpmiino "It n Knugo tor t/iin, l/iirri, oiid mnlium gUus. Th« fil^t- 
niay Ijo nnployiKl foi" tniniiituijj (l(-lic«tr objcMTti to hf viewwl \>y tliB 
high {lowc^irt with whit-'h tht! iijMiohroniAtio nndiU'lintnuit4cci>i)(ltfU.->em 
nift to hf UMv), so ivit ti) ftlluw plenty of room for tlii' focal iHiInt uf nci 
■iptiwil coniliiniition with gnat aperture ti> In- lixiil n>«Mlily upOD tJie 
plane lit' thi- object ; the h(M.-oii(1 Hhoulil )x> wrt itaide for the attAch- 
inont of ottji-ctn wliifh art' lu W }(r<iunil down, niiil for wliich, thera- 
fore, n stnin^frr niountiit;; tluin unuiil !k ilMinilili- : and the third ar« 
to W u-ii'd for iiiountinK iiitlinarj' objects, Oroiit cure should bo 
taken in wiiahin;; Oifi iilideB, and in reiiiuviii); fi-om ihMM every trace 
of grciuiiifna l>y thv line of ii litllei sixtii nr polatis solution. If this 
dxiuld not tmllice they may lie iiuTuerftod in tb<- solution ivcomntmxlod 
by Dr. Seilvr. coiilpoaod of '2 ok. of bichiatiiiilv of |M)tn»4, 3 11. ox. of 
sulphuric add. and 2bot~of w»i4-r. iind aftervai'dH tliorou);hly riiiMNl. 
(Thi! nam^ solution niay \)e advanla^eoualy used for clewnang covor- 
ffliLNiies,) Before they nje put away the slides should l>o wipaxl 
pei-fectly diy, Urst with an ordinary 'glflts cloth,' and nftorwanU 
with an oL<) cambric handkerchief; and tiefotti Iieing ugpiI, fluh' 
Hlide should bo wimhed in methyhited spirit to ensure ftmlom fmm' 
greasinost. Where Klid«K that have been nlwidy i-niploj-pd for 
mounting prepnmtions Are again brought inbi Wie, grrat care Nhould 
be taken in completely iwnoving all ti-aw of ndherent %-amiith or 
oement -lirxt by KCinpio^ (caii- being taken not ti>9iNnt4-li thi-ghi^s), 
then by using an appniprinte solvent, and then by rwbbin;.' the ulittif 
nith H. mixture of equal partA of alcohol, beuxole, and liquor codM-, 
linisbing with clean water. 

Thin Glass, -The older microucopiHt^ were obliged to employ thin 
laminii' uf iitli- fur covering objeets to l>e ncved with lenses (if alitirt 
focus ; but this inaterini, wbirh was in many respects object innabl*, 
in. entiri'ly supemediil by tlii' thin glitM mamifnclured by MeMrK. 
Chnnci", of Binninghani, wbii'b may Ix! obtained of various itiyeces nf 
thickiii^Hs, down til the ^^^.thiif an int^b. Tbis^da^)>eiiignnaiuii'al><d, 
i« very hni\l and brittle, and much can^ and siinc dexterity an- re- 
quired in eultinif it ; lienee cuvpis should lir puivhiisiHl, as iviiuinHl, 
from ihn doali-nt, whti unudly ki-ep them in several sires and Kupply 
any othfrs 111 iinler. Sitvi- the fact that ' eover-jtlnas ' is made by 
Meitini. Chance, there is no definite informatiun aa t4i the mode of its 
manufacture and the oon<)itionH ii]H)n which it is ii)<ist oatisfuctonly 
produced, tt VI oiild t)e an iMlvuntjitp- to the utic^roscopiat to pcuMM- 
iiifoi-ntation on this {>oinL The ditlerent tliicknes*™ aiv luroally 
ranked as l,2,»iid 3; the first, which should not exceed intliiekneas 
the 'OOC in., beiiif* itaed frir eo^ering objeets t<i lie viewed with loto 
powers ; the seeond, which slwuld not exceetl ■OO-'i in. in tbicknem, 
for objects to be \'ieweil with mnliHiii powersi : and tlie ihinl, which 
ought never to exceed '001 in. in thickness, fr>r objects vi'hich either 
raqniiv nr may lie Ciipable of lieiiig used with Itu/li powen. It iniut, 
however, bo i-emeitiliei-ed that the acbroiiiuliL- objectives of great 
power and preat aperture (I'S) will require much thinner ooiiers 
than even tlii«. The lhiiiiie«t glass is i>f cnui'se miAt difficult to- 




liftiKlli* Mifily, atitt in rii<«t tuibl« to fmclure from iu-ciil«nt»of vuHous 
JdtKis ; iinil Ix-ticc it .■&li»alil mdy lie employed for the pui-pose [ni- 
wht«K it ia itbM>luli'ly ih-mIoiI. Tlie lliifkest pi«oo!i, ngMiti. inoy be 
iDo«t lulvatituKOouKly wnpli>ye<I as i-nvera («r uiif^ cells, iii wliK-b 
«tijciotK are m(r^llt4^cl in HukI ti> lie tiewrcl hy the kiw nowera wltuae 
Mnanuttnoe U iu)i aviutiUly »fli.i:te<l by the abeirntioii uius produced. 
Tb" workini; microHmpliil will Hiid it de^irnblc lo prnvido )iiiD>H?lf 
with itotae lueanti nf ini-iLNui-iti'; Itie thicknras of hi^ cover-gUss ; nnd 
ihia b especial ly iiei-ili-il if be is in tho ha)>i( of omploying objectives 
vilbuut udjiistini-iit, wliicb nre »>nv«tod to n pnniruinr stAndard. 
A aioall mc'i-ew g^*")!^ "f ^levl, itiiMie for inrAsuring the thicktiMii <>f 
taUni plaleeof l)rm.-m,aDd sold at ilii' timbshops, an»wi<ii; tliUnui'pme 
wry well ; liut R<r«'s I'-r'r of' •■onin'-t (fig. 324), lievisMl for this 
•Xp rao purpcMU, b in many nttj-et-t- pmfiTaWc, This conw.stJs "f n 
aiMll Itorixontal taMe of bi'a«s, niuiiritttd upon a Einnil, mid having 
at ooe end aii aiv ;inulu»lMl iiit" twenty divUions, rach t4 which iv- 
|irMenta tli« , n'nn''i '"^ ''" inch, B<> Ihnt thi- entire arc nicJMure-K iJie 
i^xh ol an inch ; at tbeother end is* pivot on which inovma long nnd 

Vm. Ml.— BaH*«l«iracoloonU«t. 

«MicNto l«ver of Ktvnl, wliom^ oxtfeniity points to tlie gradualnl arv. 
vliilMt it ban ver^' ttmr it.i pivot » Hurt uf prajecling tottth, which 
liOBra it againxt a vi-rttcal ]>lut(' of Bt«el that is screwed tn tlu' 
iKirivontal (alilr. Tie- pi>-cc ••{ thin ^lauto be moMurwd hdng in- 
wrlwi Iwiwei-ii tlif v«itioal jdiitu and lh» projecting tnoth of the 
li'tiT, itfl tJtickneu in tliounaudlJia of an inch is givr-n by llui iiuiidxT 
■ m ti»e gnulaatoil ar« to whicli tJw extremity of the htvrr points. 
TliuK, if (!)<■ numlfei' )n< 8, the ihickneas <it the glass IS '008, or the tIj t^> 
of au inch. U will lie found oiin^etueiit to sort th« covm ncconliiig 
to tJi«ir tliicknesses, and t<> ki-e)> (he sortings kpnrt, so thnt there 
May be a suitable thifkneas uf i-uver f»i- each objei't. Rut it is well 
lo renteinWr that, with Uie i-xteplion of ohj'^'ts to whirh ftotn tlioir 
kite or nature it is impijcsiblu Xfi apply high powi>n, it vx bi-tlcr to 
mount llie object m ibnt if it be r«quir«d or acMrabli! high powers 
may t>o U(«d upon it. 

Another simple and verj' eHicient coror-ghuw teati'r i» made by 
v!eiss of Jena, aitd illustrated in tig. iih. It will bo nru-n tliat tho 
measurement is effected by a clip pi-ojocting from » box, lietwecn th« 
lows of which the cover to be mntsured is pUced : the reAding is 
given by ui indicator moving owr a divided circle on ilie upper facr 

" the box. The tlivliioiu show hundrcdtlu of a uii!limi'tr<.', ami the 

strumeilt me«8tirM to upwartU of h mm. 


Tt iii well to k«<!p I 



i-„. M,',,-/, 

^ln»» l<>nlpr. 

lit well to k«i!p luuix'ti^l iiicjk»uiTil nnd cl<-iincil ciircr-glaxKos in 
uuilII )i^itirat« wi(K--!>lii|i[H-n-d liiittlt« of inelhylatM] npirit, mch 

bolt 1 1' liein^ biMlnl 
with tilt; gitugp "f thick- 

ritas <tt thf I'livi'iK it 

•.iiiiIaIiis. What U theo 
rei]uire(l i.>. a luiupto «p- 
[Mrntus for cUuiniofc t)i« 
•)«>licut« cuieni with lh« 
le»sl rifck of )>i>«ikitge. 
Thia can bt- nell accum- 
pllslied by haviii;; two 
blocks of IhiswmoiI, 
nhup^ !>o OS lo b«r en^ilr 
hcl<l oiip i» enrh liiknil, 
turnod with perfect true- 
■Mm tmthpfiUTB opposite to th^ rcspfctiiT hnndlctt, so that when tlie 
■Hr&tOM Ml tlAttj'nnil aTi: luirl upon «n<l piT'ssi^l towards Midi other 
thev wo everj-wluT" in i»-i-fcct tniitiict. They »lioiil<l be from two 
l» four inches in tlininotcr, iind tlu^se fliittciirfl sur&iccK iihould mch 
liiivo very tightly Hlrutohcd upon (lipin a. tirm, «ren-t«xtiinHl, 
niotlcratrdy tliiok pii-CR uf choinois liMither. [f covert \xs slightly 
iiioisti^ned — even liriutht^il upun^&ncl hiid on one nf thfw blocks 
und jiivssed <tiiwn willi thr other, bituitli, or luoiKtum applied by ft 
mniill cfUiii-1-luur bruidi bu the uppnr ■urfiu.v of tlir eovrr, niajr bo 
»[)I)lie<l, nnil ti fi*w twiat« of lhfj«" blix-ks upon tuwh otiwr when 
tirinly pn-sseil togi'thcc will cH'octuHllv chnn without breaking the 
thinniT roviTS. It will lie often niiilfnl to tivjit both rifles 01 thp 
i-oviTs thus, lis one nidc gf^ncrally itdhi-m whilt; tbi? ollwr is subject 
to tilt f riot ion. 

For rliutning xlipK luu) cove rx by luuxl, finUliiiiK shoald txi doRO 
with iild tint- onmbric hnnilkurchicEii. Thinr sJiould not b<< woihed 
with !":ukp, but with (.■mninciii suclu nnd hot wittt-r, pli^ntyof the Inttor 
U.'ing xui»ii[ui:utly ('tuployed to get rid of «very truce of tlit^ nlknli. 
But when dry lliiwf clollu must not be ■ Iront^l ' or Mnonthrxl in nay 
way, thi- ' ri>u;;h dry ' ^urfiu^c- avtliig adiuirnbly for wiping ditlicAtA 

Vunishes and Cementa.— Ther« are tliret! irry dlitinct purpom-s 
Rir wlii(!h cenu'niH which jjasaess the power of huldiiic finnly in gliuw, 
iind i)f iTwisting not merely water but ollifr i>it»erv«tive licjuidit, 
uiw retjuiitsl by the odcnwoopist, thenn hein^j (1) the atiuohiiirnt of 
the lianas covem to tlie aUdea or eells i-imtuinin;; the ubjecl, (2) the 
fornuitiiiii of lliin 'cel!n' of oeiuent only, and (3) the «Uuehiut?nt of 
iho 'gliisM jilate' or "tube-cells' to the wUdiii. The twu furm«r of 
these pur^HML-B ufu answered by liquid c^uicnLs or vamuhm, which 
may be applied without heat ; the last requireit a nolid tfitunl of 
great«r tenacity, wlitch can only be used in the inelteU stute. Among 
the ninny such cenienU that hiivo beeit recuimueiided by diftei'eiit 
workei-H, two or t.lii-ee will l>e seJected by the workei' for Renend 
purjHvte^, and {lerliups three or four for i^]>eciAl purposes, and tfa« re- 
maiiulti' will be iu praclice neglected. We do not liesitat« to say 


VARMK^nBR AMD CE3CEirrfl 383 

tlut lllr twii L-riiitjiili oci wliit'li tile lU'vit ouui'li-li* ti-u-vt xttny Iw rr- 
pcaral am jti/ianruT'» i