nie ai Gs pis arp ps ; dai # A pig ip. is ar ti j fiely “Nt a HE ae ny Hi fly it AY § (Uy EE 4 Se iii Hey H Ay i it he va i ff is i ae | i us i; if Hi Bs a | f fish i ae " 4 es i si f ‘ir i fr 7 "ty igh | I f it i; ay pied fj f i Ar ie al a sai f Ae = fp ff, Hy ae { ff Ae io ' vet } ii fy i f f aie a in ii it ff I ies So ‘Ny iit Ar, q Ae | st ies tr ii f; 4 i e 4 ies He & ow 7 Hig y Se de a ea: He { ret aie Hi } te nt aud fa ip aie me 4 ave ae 7 tf os iris » % xt ey 4 Hf Si i v Ht uy ; Ny yi ie fai iH f "y in ye a tt Hh I & i iy rs i (| Yo ait pat : : Y 2 Hi uf i op GH f 7 tg ith ft “y i i but t id pe Nig fed ay! ak ne Bi 4} ; a 1 ht if Pp ' bi if f 4h \y, ay iti nt fj oe ie pi s id. \i Hy He ita i cf ay tf ee Bi 1 Wa it ait He Ht id af Tn Mr: on iy! yy pus Ty Ve % ye f te bs, fee i Ae | 7 Hi He ie Lg ? +) * f vl 4 ith iy Tf 7 oe Ca uf id i if i ts iy Y { sav ay £ Hee nay i % in Ht ; ee f i i Af i Be iit ' . i Hs ti He Le es i NY r i a 07 He if iy ff nee Ve itn i ii ii *y ) A ¥ 4 ff {i Hit he Hi io ae ee Hi i a if fi, Ha i yy Be Mt i ‘i ti Hi As He it f ie ii fig! r HH f , Att did ifn ff i fist Hi ‘i; i as s ey! VF a if ap itt Hit dy, a Ly { ve f iin! 4 He ic Ht | uy, af, Hit i) Hi ty a a i Mf Hy : He af ii a Af ig f “ay Hh i Aisi tr He S| f fe j Ht By \4 fy! His hy 7 itt! My HE r iit i i ty Hig b j tH a lf ty if fied ii re Hi ts Ht ft f a if Li f a i Le ee itt! A it ef) te I a Na i ir; it; iii! A ee fe iF ity! fi (it ile af ity a aut Re Hie ft _ Ati! iy iti cul i Aut! itn! ty CL | ii! il if i ae HIP pt} He ai 4 ti tt f Hi Ha ie ri | (iy! if od it i | tf i HH aid it Bt a { lay i} ae via ah a tt +. ny i dt ities ia it su itt ety aE fH tH f ie My Hip i. i a | He 7 f itt tnt ‘g Hi ay, Hit Ht iii} mie Ny iy nig , F } inns nit et j 5 ttt F ite! , If fi! Gui; ai ig f Ht 45 H tH ta Hi] t f j na f { f iit in ith % t Hi He ft iy! hei i i f I; itt} 4 ane i 1 i j f, Fi unt oF ! tet rh | } iti Hi 5 i H if at tt iti j fe tf in ii i ' i int (fi fy i! i hy f aii i iy! Vik ity _ fy f 1 ant Ay rt ; pi wi a iF t i wit Mtg fits Fr a ty Hi ee He iy i a 4h ia * it uid Hy Gi i t ce i ita 7 te He Sek isi a oF i if \, tat a Fi Aye ‘ae ike tt fil ne if AY ad 9, A iy f SS ii 'y ; Ny re Ab, ie ie i i iy it i ha ce c. ii if ne aN i mn ff " Ce Hi it “f 1H fi i ah At aS iin iy i ie uy A ut a itn 'y ‘N Mie tad Haye ae int iy ne af, a is ine Po iin 7 i hi Ai % ic i f, i) tip uh ii i sn oe ij ‘a iy De 7 ba iti; ci ity i Be & Mp i ity f ti! a itae ty ap hy ca ta { ate ef i i ane ae iit ty ip ee i iti! ' j it ity! 4 ne at oe hy sles uti ay! i it hi ot i i tina tate iy

Loe t Ribrary of the Museum

Or

COMPARATIVE ZOOLOGY,

AT HARVARD COLLEGE, CAMBRIDGE, MASS,

Founded by private subscription, in LSGL.

LDAIDO IO

Deposited by ALEX. AGASSIZ.

No. [Se /

QUARTERLY JOURNAL

OF

MICROSCOPICAL SCIENCE:

EDITED BY OWEN DANK ES TER, MDS OR. Ss &e.;

AND

EK. RAY LANKESTER, B.A. Oxon., F.R.M.S.

VOLUME X.—New SERIEs.

With Allustrations on Wood and Stone.

LONDON JOHN CHURCHILL AND SONS, NEW BURLINGTON STREET ~. 1870;

WAMHUCL (USE

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| Pee AAG | ae ee Pi iessicocuy) AO TO0G Gam MipaO AAD ; - | > ei ae ¥ | See. Tet Gd ee @ VL Raw | A Aik vy A

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A ay E | | DIT «000 A AE i Sis 6

MEMOIRS.

Notes on Sponces.—l. On HyALonEMA MIRABILIS, Gray. —2. On APHROCALLISTES BocaGEI, sp. nov.—3. On @ new GENUs and species of Derr Sea Sponce. By Ep. PrercevaL Wricut, A.M., M.D., F.L.S., Professor of Botany, Dublin University. (Plates I, I, and IIL)

1. Hyalonema mirabilis, Gray. (Plate II1.)

So much is now known of a deep sea fauna that one is very apt to forget how little was known about it just eighteen months ago. It is quite true that here and there stray facts were to be met with that most distinctly showed that animal life was to be found at very great depths of the ocean, not to mention the important researches of Sir John and Sir James Ross, or of Dr. Wallich, there were also records of the occurrence of zoantharian corals, at depths of 300 to 400 fathoms, and of fishes taken at depths of 312 fathoms. In- deed, to collect all the scattered observations bearing on this subject would be a work requiring considerable research. But some way or other the true bearing and the extreme importance of all these facts were in a great measure, and by a great many, overlooked ; and the fact that great depths act as no bar to the existence of animal life, however known to a few, and however much it should have been known to all, has only during the last year and a half been fully recog- nised.

Accustomed to dredge in what will be now considered the trifling depths of from 80 to 100 fathoms, I learnt with some surprise of the deep sea fishing for sharks at Setubal in depths of from 300 to 400 fathoms ; and when my friend Professor Bocage, of Lisbon, told me of the discovery of quite recent specimens of Hyalonema mirabilis in these same depths, I had a great wish to go and investigate the fact for my- self. The prevalent opinion, however, at the time about the discovery of the Hyalonema off the coast of Portugal was, that the specimens taken by the Setubal fishermen were stray

VOL. X.— NEW SER. A

2

specimens thrown overboard from some vessel trading between Japan and Lisbon, and that it would be a perfectly hopeless task to look for living specimens off the west coast of Por- tugal; and one great friend of mine, whose opinion on the subject had very ¢ reat weight with ine, believed this so firmly that I yielded to his ar uments on the point, and abandoned the idea of going to Setubal in the spring of 1868. This same friend has since, by suggesting the deep sea expeditions of 1868 and 1869, and by the amountof work that he hasaccom- plished in connection with these expeditions, opened up to the student of nature quite a new world. Itso happened, too, that at the very time that I was dredging up specimens of Hy- alonema in the tranquil waters of Setubal he was dredging it, and a host of other glorious species in the more stormy seas of the nor th, for hearing again from Professor Bocage in July, 1868, that the Hyalonema had been actually taken in situ, I lost no time in going out to Lisbon in 1868, the time when the shark fishery season commenced. I have in another place! given a short account of my excursion in Portugal, and will only here refer to it for the purpose of stating my firm conviction that, though so many new and rare species have been taken by the several deep sea expeditions of the Swedish, British, and American Governments; yet I believe much more remains to be done, and I would suggest that the deep ground off Setubal is well worthy of investigation, as it les within a distance of from ten to thirty miles of the shore; and as the sea there is, as a rule, peculiarly tranquil during the months of August and September, it would be possible to make a very thorough investigation of 1t without even the assistance, most valuable though such assistance be, of a man-of-war or a Government survey vessel. ‘The present King of Portugal is in every way entitled to take his place in the ranks of science ; and the national museum at Lisbon is already indebted to him fur most important aid, and for many valuable collections; perhaps Professor Bocage might induce him to follow the example set by Britain, and per- suade him to allow a Government survey vessel to spend a fortnight or three weeks on the ground I refer to; the collec- tions that would surely be made would form a most desirable addition to the museum at Lisbon, as well as be most valu- able for exchanges. It was my object on my return home to make a report on the structure of Hyalonema mirabilis, of which I had succeeded in taking living specimens, I had also every opportunity given me by Professor Bocage of studying the magnificent. series of specimens preserved by

+ * Annals and Magazine of Natural History,’ December, 1868,

3

him in alcohol in his museum. Finding, however, that Professor Wyville ‘Thomson had taken a large number of specimens during his cruise in the Lightning,” and that he intended publishing a memoir on the genus, I contented myself with a simple record of its occurrence in a living state; of its mode of growth, viz. as Loven suggested, with its siliceous stem anchored in the mud, and with expressing my opinion that the stem was truly a part of the sponge- mass, and that the Polythoa was simply parasitic upon the stem. Nor do I here intend to do more than call attention to one or two peculiarities which it strikes me are to be met with in the specimens that I have examined from Portugal, and which do not seem to exist; at least, not exactly after the same fashion in the specimens taken in the Light- ning” and Porcupine” expeditions. My knowledge of these latter is based upon a very casual examination of the specimens taken by Professor Wyville Thomson; and upon a more careful examination of a beautiful little specimen, about an inch and a half in length, most kindly given to me by Professor Thomson. And in these remarks I do not mean to anticipate at all the memoir on this genus which is so impatiently expected, but rather to state what I know about the differences between the specimens taken off Por- tugal, and those off the west coast of Great Britain and Ireland.

Some of the Setubal specimens are of very great size; the stems of several measuring nearly two feet in length. In one very perfect specimen the head consists of a large somewhat oval mass, about eight inches broad in its long diameter, and four inches across in its short diameter ; it is cup-shaped, resembling somewhat the ordinary shape of a common toilet sponge, and, like it, it is hollow on the inner surface or om that portion where the glass rope” ends. The outer sur- face has been somewhat worn off by either lying on the mud or from rough handling, and presents that appearance of wet brown paper that must be familiar to all who have exa- mined specimens of Hyalonema with the sponge mass attached from Japan. On opening out the sponge, the interior con- cave surface appears to have remained uninjured, and here will be seen a delicate network of spicules and _ sarcode, lining the concavity and passing into the texture of the sponge. A number of irregular large openings (oscula) are also seen, and these are covered over with a delicate open sarcode network, the edges of the meshes of which are thickly lined by the spicules called spiculate cruciform spicules’ by Dr. Bowerbank. These spicules are met with all through

4

the sponge, but almost always lining the cavities or hollow passages of the mass. They are likewise to be found as a lining all over the surface of the sponge, but in no place are they to be met with arranged in so regular a fashion as on the meshes of the network covering the oscula. From the peculiar way in which they are placed on the edges of the meshes, and from the fact that the barbs on the stem of the spicules all point in the one direction, it is possible that while it would be easy to glide over the slimy sarcode down into an osculum, return would be no easy task, as any solid body would be at once caught and retained by the barbs. From the manner in which the cruciform basal portion of these spicules is inserted in or attached to the sarcode, I make no doubt but that they are subject to being moved up and down and to and fro, and that on the contraction of an osculum, and on the consequent discharge of water from the oscular cavity, the spicules are pushed outwards and upwards, falling down again on the expansion of the osculum. In all the numerous writings on the structure of Hyalonema, I cannot find that the exact position of these spicules in the living sponge has been determined. I have, therefore, thought it.advisable to give the accompanying illustration (Plate III), for which I am indebted to Mr. Lens Aldous. It represents one of the oscula removed from a specimen of H. mirabilis in the Lisbon Museum. ‘The spiculate cruciform spicules which line the edges of the sarcode network are very easily displaced, and but comparatively few of them were on the specimen drawn by Mr. Aldous, but in a living state they line, packed in a close row, the edges of the sarcode mesh; they differ slightly from any of those figured by Dr. Bowerbank or Max Schultze. One other subject I should like here to allude to. The oscula of H. mirabilis being now dis- covered and described, and they being found to be just those that one would have expected and just in the position in which one would have looked for them, it scarcely requires my state- ment that I saw the little parasitic Polythoa in a living state on the siliceous axis of the Hyalonema, and that I watched them expand their tentacles, after the fashion of any other zoan- tharian, to prove that though they have mouths these mouths are their own, and not at the service directly or indirectly of the Hyalonema. Is it too much to expect to settle the last lingering doubt that may still exist in some minds as to the nature of these independent though parasitic organisms ¢

2. Aphrocallistes Bocaget, sp.nov. (Plate I.) Sponge fistulous, erect, branching somewhat irregularly ;

5

skeleton siliceo-fibrous, more or less symmetrically radial ; radii short and stout on the outer surface, and somewhat longer and thinner on the inner surface of the skeleton, forming a series of hexagonal spaces, which are nearly all of the same dimensions, central umbo of the ray giving origin on its inner surface, often on both surfaces, to a long spine. These spines, generally long, sharp-pointed, sometimes knob- headed. Spicules, acerate; retentive verticillately spined ; attenuated rectangulated hexradiate, and subfusiformi cylin- drical entirely spinous. Main tube closed by an irregular siliceous network, which is deeply concave. Pores and dermal system unknown.

Habitat.—Cape de Verde Islands, in museum of Lisbon, in British Museum, London, and in my own collection (Sept., 1868) also off south-west coast of Ireland in deep water ; Professor Thomson, Porcupine Expedition, 1869.

Dr. J. E. Gray established the genus Aphrocallistes in 1858 for a very beautiful sponge from Malacca (‘ Proc. Zool. Soc.,’ London, 1858, p. 115, Pl. XI, Radiata), A. beatriz. Dr. Bowerbank having identified the Iphiteon panicea of Valen- cienne in the museum of the Jardin des Plantes, Paris, as be- longing to the same genus as A. beatrix adopts Valenciennes’s name. As, however there were never any descriptive characters of the genus Iphiteon published until 1869 (it was affixed to the specimen in the Paris Museum in 1800), and was described’ as Aphrocallistes in 1858, I have no hesitation in assigning the priority to Dr. Gray’s name and in adopt- ing it for those sponges, with a siliceo-fibrous skeleton in which the reticulations are symmetrical. It is true that by an accident Dr. Gray described the genus as having cal- careous instead of siliceous spicules—an error which he afterwards corrected. But this mistake could not for a moment mislead when the rest of the diagnosis and the beautiful figure by Mr. Ford were taken into account; in- deed, such a figure with a name attached would amount to a publication.

It is confessedly unsatisfactory to describe a sponge from a dead and bleached specimen ; for if in any group of the animal kingdom, surely here we require all the assistance it is possible to have from an examination of all the structures of the organism. It is, therefore, not without an apology that I publish the above beautiful form as a new species. When examining the very interesting collection at the Museum of Lisbon in September, 1868, I discovered three or four specimens of this sponge, which I immediately re- garded as a new species of this genus. Professor Bocage,

6

with his accustomed liberality, at once gave me the specimen figured on Plate I, and accorded me permission to describe it. In dedicating it to my friend I take this opportunity of

thanking him for the many kindnesses which he showed me while in Portugal. The memory of a delightful Sunday spent with him at charming Cintra will ever remain with me. The museum under his care is one of the most interesting in Europe ; the more especially interesting on account of the fine collection of native species brought together by the per- severing energy of Professor Bocage and his admirable assist- ant Sig. Capello. It is also rich in species from the Portu- guese ‘settlements abroad, and this sponge formed but one of a fine series of invertebrata from the Cape de Verd Islands. The nearest ally of this species is undoubtedly A. beatriz,

Gray, andit is quite possible when we know more about both forms, and when they have both been examined in a living state, that they may prove but varieties of the same species. This is is possible, but for the moment I think not probable. There is a certain regularity of form in the sponges which have a non-elastic siliceo-fibrous skeleton, which I venture to think will be found to be in a measure characteristic of the species. But apart even from this consideration, the areas forming the skeleton in A. Bocagei are much more regularly hexagonal than those in A. beatriz. The spines on the bosses are very much longer in the former than in the latter species; in it too the central cavity is larger. The reticulated network-lke lid is much more radial in its com- position than in A. beatriz. ‘The bosses of the rays of the body-skeleton are often knobbed, and there is an apparent absence of porrecto multiradiate spicules so characteristic of A. beatriz, This fact I do not lay much stress upon, as it may arise from an error of observation. I have, however, met with these spicules in every specimen that I have examined of A. beatriz, and never in the many specimens examined of A. Bocagei. 'This latter, too, isa much more erect form than the former, and I should expect that when the sarcode layer of both species is known that the spicules of this layer may be somewhat different in both. I am in- debted to Mr. Ford for the accompanying drawing which he made for me in January, 1869. Circumstances have pre- vented me from publishing a description of it sooner. Professor Wyville Thomson has kindly forwarded to me portions of this sponge taken in the recent cruise of H.M.S. Porcupine.”

He informs me that it was dredged living off the south-west coast of Ireland at a great depth. The portion sent to me is a fragment of a dead specimen. But Professor Thomson

7

thinks that living specimens were met with, and that they are somewhere among the vast stores of good things collected during the expedition. Professor Alexander Agassiz also recognises Mr. Ford’s drawing as that of a species taken by Count Pourtales in his last expedition, and informs me that all the sponges taken in the course of the coast survey expeditions of America have been forwarded to Professor Oscar Schmidt for description ; perhaps, therefore, we may expect still another supplement to that most important and useful work ‘‘ Die Spongien des Adriatischen Meeres.” If so, I hope he will agree with me in considering this species a good one, and that from better specimens and with his great powers of drawing he will still further describe and illus- trate it.

3. Onanew Genus and Species of Sponge from the Deep Sea. (Plate II).

In March, 1869, my friend Dr. Wallich, so well known by his botanical and zoological writings, as well as by his researches into the deep sea fauna, gave me a small portion of a minute sponge, of which three specimens had been brought up from the great depth of 1913 fathoms, with the request that I should describe it. I have to apologise to Dr. Wallich for letting the summer pass over without fulfilling the promise that I made to him. But there were two difficulties in my way. One was to have the most perfect of the three speci- mens discovered drawn. ‘This specimen had been presented by Dr. Wallich along with a vast collection of Foraminifera, Polycystina, Diatomacee, and Desmidiacez, to the Royal Microscopical Society of London. ‘The other difficulty was to find out where to place the species when described. My first difficulty has been surmounted—thanks to the Council of the Royal Microscopical Society and their assistant-secretary Mr. Reeves—by Mr. C. Stewart, F.L.S., of St. Thomas’ Hos- pital, to whose friendship I am indebted for the accompanying very characteristic, faithful, and beautiful drawing. My second difficulty Iam not so sure of having as yet clearly seen my way through. But to this I will allude more par- ticularly a little further on. By the help of the enlarged figure on Plate II, and the following description, I hope this earliest discovered (October, 1860) of all the deep sea sponges will be easily recognised.

Wyvillethomsonia, gen. nov.

Sponge body subspherical, attached by a stem. In the summit of the sponge, é.e. the end farthest from the stem, there is one large osculum, which is fringed by long, delicate, biacerate spicules. The interior of the sponge body consists of several cavities which open into the osculum. ‘The stem is prolonged through the body as an axis, and consists of numerous biacerate spicules somewhat more robust than those fringing the osculum, and mixed with these are a number of anchoring spicules (fusiformi-recurvo-ternate of Bowerbank), the recurved end being always directed to the point of attachment (which in this case is a small stone). The body is composed of a large number of spicules (furcated attenuato-patento-ternate of Dr. Bowerbank), the radu of the ternate spicules meeting each other as they lie on the surface of the sponge, form a remarkable loose network-like pattern ; the long pointed process from the central boss projecting inwards towards the axis of the sponge.

The whole of the body of the sponge and of the stem is covered by a thin sarcode layer which abounds in stellate spicules varying much in size. One remarkable spicule (bifurcated expando-ternate) seems to terminate the axis in the centre of the large osculum.

W. Wallichii, sp. nov. (Plate II, figs. 1 to 6.)

Habitat.—Dredged from a depth of 1913 fathoms, October, 1860, in lat. 58° 23’ N., long. 48° 50’ W., by Dr. Wallich, who was then Acting Naturalist to H.M.S. Bulldog,” Sir F.L. McClintock, R.N., commander.

I name this genus and species after my friends Professor Wyville Thomson and Dr. Wallich. The name of Wallich has been long since employed in botany as a generic term, other- wise I should have employed it as such here. Those who may object to the length of my generic name I may remind of the precedent I have in Vaughanthomsonia. I could not com- memorate two more original workers than Drs. Wallich and Thomson. ‘There can be little doubt but that the three spe- cimens dredged of this species are in a very young condition, but from what we know of sponges generally I think it is fair to assume that a mere question of size of specimen is of very little consequence in determining a species; indeed, once the sponge arrives at that stage of its existence that it forms all its characteristic spicules, neither the form of

9

these nor their general arrangement in the sponge structure is very much altered by growth; hence the diminutive size of the specimens examined by me, seeing that they appear complete in all their parts, is not a sufficient reason for this species remaining undescribed. At the next meeting (15th April, 1869) of the Dublin Microscopical Club, after Dr. Wallich had given me the small portions of the third specimen above referred to, I exhibited a series of the spicules, and stated it as my impression that the species belonged to the section of sponges with siliceo-fibrous skeleton and hexradiate spicules called Vitrea by Wyville Thomson. In this I was led astray by some siliceous network, like that met with in Aphrocallistes which was entangled by the body spicules of the little sponge, and I have now little hesitation in referring it to the Corticatee of Oscar Schmidt, suggesting that its affinities are to the genus Stellata, Sdt. I do this for the following reasons: The number of stellate spicules in the outer sarcode layer, which on some portions of the sponge body are so tightly packed together as to form quite a hard layer of silex; the prevalence of the large furcate ternate spicules, which are certainly most important in the structure of the sponge mass—such spicules (No. 850—51 of Bower- bank) are to be met with in Pachymatisma Listeri, Bowk. MS. in Stellata discophora, Sdt., S. Helleri, Sdt., S.mamillaris, Sat., and S. mucronata, Sdt. So far as I know these two forms of spicules are only met within the same sponge when that sponge belongs to the division Corticate of O. Schmidt. The genus, however, cannot be easily confounded with any of those placed among the corticates. In some specimens of mud, taken from the same locality by Dr. Wallich, spicules (furcate ternate) occur seven eighths of an inch in length, proving the existence of some enormous specimens of some sponge of this group. Professor Wyville Thomson, who was present at the meeting of our Dublin Microscopical Club at which I ex- hibited this species, stated that he had taken this species, or at least one very closely allied to it, in the same ground that he had taken Holtenia Carpenteri, W. Thomson.

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On CERTAIN ImpERFEcTIONS and Txsts of OBJECT-GLASSES. By G. W. Royston-Picotr, M.D., M.R.C.P., M.A., F.R.A.S., late Fellow of St. Peter’s College, Cam- bridge. (Received Sept., 1869.)

THE actual diameter of the least circle of aberration caused by lenticular vision is the real gist of the much- debated question of ‘‘ aberration” and imperfect definition. The performance of the eye-picce is altogether secondary and inferior to that of the objective, whose errors are multiplied by it, as well as the ratio of the distances of the final con- jugate foci from the posterior lenses of the objective.

This circle or ring, being the smallest space through which the focal pencil passes, is seldom so reduced as to leave no traces in the highly developed image presented to the eye.

Omitting here to dilate upon the terms now so vaguely used, as ‘‘resolution,”’ ‘‘ penetration,” and “definition,” I may be permitted to enumerate a few points worthy of con- sideration, as they have occurred to me during the last twenty-five years.

Under the use of very high power every free edge of an object, and every isolated point, exhibits an umbra and penumbra exactly representing the diameter of the least circle of aberration generated by the final objective re- fractions.

Every object being an assemblage of such points exhibits, under high power, “similar aberrating shadow, principally visible at the sharp borders and edges ; this shadow or penumbra depending upon the aberration, and being inde- pendent of the size of the object considered as an assemblage of points. After a multitude of experiments, I conclude that—

This shadow can be considerably diminished—

(a) By limiting the aperture of the illuminating pencil.

(6) By reducing the aperture of the objective.

(c) By further correction and better approximation to aplanatism of the objective itself.

(d) By viewing objects directly, without a covering glass, properly adjusting the position of the front lenses.

(e) By the use of immersion lenses, destroying the aber- rating effects of a plate of air.

(f) By carefully searching the axis of the instrument for a position of minimum aberration of the conjugate foci.

(g) By employing direct rays, from a radiant point, free

ila

from circles of confusion and chromatic dispersion, such as a fine pencil of rays admitted through an exceedingly fine aperture from the direct solar beam.

(h) By analysing the aberration of the annular surfaces of the objective, and selecting such areas and annuli as are the more perfect in their operation.

(2) Lastly, by abstention from pressing the powers of the objective beyond distinct vision, a canon universally adopted by astronomers, but too much neglected by microscopists. In telescopes every inch of diameter is generally considered to barely admit a power of 100, a 10-inch objective scarcely allowing a power of 1000. In microscopes the power may be similarly estimated for useful effects by taking 100 times the reciprocal focal length, one eighth giving 800 diameters. There are two other points to which I beg to direct the attention of microscopists, as worthy of their best efforts and scientific research.

(k) The spherical aberration, both lateral and longitudinal ; which will be improved—

(7) By extending the visual focal distance of deep ob- jectives, and so withdrawing the face-glass from its extreme and dangerous proximity to the ‘‘ covering glass.”

(m) By greatly increasing the depth of focal vision, and calculating its amount. With a 3-inch objective it is possible to view, at one and the same instance, both surfaces of a thin covering glass; and the marvellous and delightful perspective view into deeper parts of insects given by Ross’s 4-inch objective illustrates the same principle of the ad- vantages of increasing focal depth. ‘These points having occupied my attention for many years, I hope shortly to communicate the methods adopted to accomplish these desiderata.

There is a great deal of interest in the subject of definition, because it is one common to the sister sciences of the astro- nomer and the microscopist. To the former dividing power,” *‘ definition,” giving fixed stars a round disc, resembling a bright spangle placed upon black velvet, and diffraction rings,” and lastly nebulosity” and haze, have all their spe- cial, though humbler, representatives in the microscopic field. And in order to obtain precise ideas, it is wise to proceed from the known to the unknown; I therefore beg to suggest the study of these points in the images formed of given and known objects by minute lenses and high microscopic power. For the information of those interested in this point I have calculated the diameters of the circles of least aberration for parallel rays for minute lenses of the following dimensions,

12

the index of refraction being taken at 1:50 = yw for plate- glass :

Focallength . . gs thinch = “05” Aperture . 5 ee ee UP

The diameters of the aberrating penumbra of a point—

Diameter of ees aberration. Plano-convex 0009" riinth of an inch nearly.

Equiconvex . 00033”

Convexo-plane . 00023” = ath “4 Crossed lens - 000214" = ZAth 5

If an equiconvex lens be used where the diameter of circle of

cube of 3 aperture square of focal length»

a minimum when the object and image are equidistant from the lens, and each at twice its focal length from it, or v= 2f and u =

Now, by a combination of many glasses, objectives may be corrected to show scarcely any penumbral aberration, but, unfortunately, nearly all opticians using the Podura markings as an unequalled standard, all the best glasses are corrected to show what is absolutely false and dela! and the result is a misplaced belief. Till the explosion of this creed, every one, being taught to look upon the Podura spectral markings as the ne plus ultra of objective accuracy, is satisfied with that ignorance which is bliss. The glasses are constructed on purpose to show this supposed standard appearance; but opticians will be obliged, at some future day, to elect a new standard. ‘The same glasses which show this admirable note (!!!) fail in the higher tests, such as will now be described.

I have found in the best objectives a residuary aberration, and some of it, when the glasses are pressed with too high an amplification by deep DD eye-pieces, will in some degree always remain to put a stop to further research, except with deeper objectives still; and these objectives, rising already to the 50th of an inch, will probably soon reach their limit owing to manipulative defects in their manufacture. The precious stones, such as the topaz and sapphire, and lenses formed by accident or fortunate working, may perhaps assist a further development of power or detection of error.

For forming beautiful images of bright discs, to imitate close double stars, I recommend the use of two minute crossed lenses, set with their deepest convexities in contact; but a fine objective analyser may be formed of two lenses in contact,

least aberration = °835 x

the aberration will be

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made of plate glass of 1°5 index of refraction ; the first lens being equiconvex, and the second concavo-convex, whose tadii are as 1 to 101—which combination for forming an image of a distant object—as a silvered-glass ball or illu- minated globe or lamp—is absolutely aplanatic for plate glass of refractive power 1°5. I propose to name this instrument the aplanatometer.

Armed with such an analyser amateurs will experience little difficulty in examining the performance of their objectives. ‘The appearance of black points on a white disc, or a white disc upon a black ground, as imaged by minute lenses, affords one of the most instructive lessons in microscopic aberration to students of this difficult subject. Familiarity with these optical phenomena of known objects enables the observer to detect the errors of observation with fidelity.

When a thorough acquaintance has been made with these signs of defective correction, objectives required to be ex- amined will be found either to increase or diminish the glaring aberration of these minute images, and increase, or even destroy, the penumbra of circular aberration.

Still more difficult of definition is the minute lenticular image of a brilliant point, or of a minute brilliant aperture, or reflecting mercurial globule, placed sufficiently far from the lens to produce a minute image. Thus, if the lens is the 20th of an inch in focal length, the image of the globule will become smaller and smaller as it recedes.

At a distance of forty inches the image of a globule of mercury 100th of an inch in diameter will be 80,000th of an inch + 64,000,000th, very nearly. By this plan, there- fore, we possess a means of forming images of known dia- meter, and with more or less aberration, according to the kind of lenses employed.

The Podura is, according to the high-class objectives made by Messrs. Powell and Lealand, a scale covered with ribbing on each side of the basic membrane ; each ribbing is separated by a clear interval, through which the second set of beads placed on the other side are visible. When the scale is folded over, so as to expose the underneath side, a similar ribbing is seen. Mr. Wenham states! that he cannot see any signs of structure at the folded edge, but the ribs and intervals are clearly displayed when the objective corrections are exquisitely performed. ‘These ribs are composed of beads, varying from the 30,000th to the 150,000th of an inch in diameter, according to the size and character of the scale.

' Microscopical Trausac.,’ July, 1869.

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The upper and lower sets of beaded ribbing cross each other at an acute angle, and their general direction is somewhat wavy. ‘lhe markings are caused by the cross intersections of the ribbing, and exactly imitate the effect of watered silk caused by pressing two pieces powerfully together.

Minute aplanatic lenses being employed on the stage—

A. Image of a watch, 8 feet Tienes: Some objectives show the image enveloped 1 in yellow fog; the time cannot be distinguished. No focusing or ‘collar. adjustment gets rid of the nebulosity.

A small bright aperture appears nebulous and radiating ; either the brightest part of the radiant cone is before or behind the most distinctimage. ‘The images of two contiguous bright apertures coalesce and cannot be divided as double stars.

B. The divisions of a micrometer imaged by the minute lens can scarcely be discerned in a haze of nebulous light: the micrometer was placed on the plane mirror illuminated ee a bright cloud.

Rows of beads appear cylindrical bodies unless the sealants is finely corrected.

p. The image of a church clock, 200 feet distance, can be formed by the plane mirror, and the stage lenses will give the details of the face, the small dots for the minute hand, and, of course, the figures are beautifully displayed, sharply cut and defined or lost in mist according to the size of the stage lenses and the power employed, and the corrections of the objectives.

Other Tests.

Podura scales. As I had the honour of communicating to the Royal Microscopical Society in May last an account of the podura beads existing on both sides of the basic membrane, I may be permitted to draw attention to the nearest appr oach made to the definition of this difficult object as given by Mr. Richard Beck, who has figured the cylindrical bands of the Podura, but described them as out of focus and a false appear- ance, whereas I always find these bands to present their pro- visional appearance as heralding the development of their beading.

Lepisma saccharina. The exceedingly beautiful striz of this scale are also shown, in the same work, as perfectly transparent and clear, like threads of glass. These cylindrical vibs are also composed of spherical beads, and they exist on both sides of the scale, the lower set radiating from the quill being much smaller than the upper set.

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The finest and most resplendent definition of the diatoms may certainly be seen with the immersion lens and a 1-16th, which it converts into a 1-20th objective.. The structure of the P. formosum has been an object of care- ful study for many years, as one of the easiest forms for definition. Separated spherules generally characterise this diatom. The beading of this object is brilliant in the extreme, never grey. ‘The exquisite beauty of these minute gems of nature rival the most glorious tints of the diamond, ruby, or sapphire ; but a power of 7000 diameters begins to develop shadow and haze. Between the spaces of the upper beading another struc- ture is discernible, but whether the interspaces are crossed by a deeper set of beading or the upper set are superimposed upon the lower I cannot at present decide, but I strongly incline to the latter supposition.

P. strigosum. Here the upper set appear to hide a parallel lower set of beads, like row upon row of cannon shot. But always do I perceive the two sets of different colours, one row pink-red, the intervening row violet or blue; probably the colours are produced by the dispersion of position, and may be good evidence of the sets existing in different planes.

P. hippocampus. Similar phenomena are observed. Rows of beading appear to cross in different planes at right angles to each other.

A severe test is the appearance of minute hairs 1-50,000th of an inch diameter. <A fine definition shows a hair to bear two black borders and a central line of light, with scarcely any penumbra under the 1-16th and immersion lens. Hairs of antennee of male gnat were employed.

Glittering particles of gold leaf. Some of these may be found 1-50,000th of an inch in diameter; brilliant illumination, if the corrections be not good, shows four to five diffraction rings. I have seen them diminish to one.

Crystalline surface of metal recently broken. The glare is universal unless the