^{page 327 note *} Researches on Colour-Blindness. Sutherland and Knox, Edinburgh, 1855.Google Scholar

^{page 328 note *} Müller's, Elements of Physiology, vol. ii., p. 1133. 1842. Translated by Baly.Google Scholar

^{page 328 note †} I do not intend by the words “covered” or “replaced,” to imply any opinion on the anatomical relation of the tapetum lucidum to the other structures of the eye. In an optical point of view, it is the substitution of a highly reflective, for a partially absorptive surface.

^{page 329 note *} Cumming's observations are contained in Medico-Chir. Trans., Lond., vol. xxix., p. 283; Brücke's, in Müller's Archiv., 1847, p. 225.

Helmholtz's description of his speculum occurs in a little work, entitled, “Beschreibung eines. Augen-Spiegels zur Untersuchung der Netzhaut in lebenden Auge. Berlin, 1851.” An excellent abstract of this paper by Dr W. R. Sanders, accompanied by comments of his own, will be found in the Edinburgh Monthly Journal of Medical Science, July 1852, p. 40. I am indebted to this gentleman for my knowledge of Helmholtz's instrument, and for the opportunity of using it.

The eye-specula of Ruete and Coccius, as well as of Helmholtz and others, are described in a work, entitled, “Bildliche Darstellung der Krankheiten des Menschlichen Auges, von Dr C. Gr. T. Ruete; 1 and 2 Lieferung: Leipzig, 1854.” Professor Ruete's beautiful work contains a series of coloured drawings, representing the internal structures of the eye, as seen under the speculum.

Since this paper was read to the Society, a valuable communication on the medical employment of eye-specula has appeared in the British and Foreign Medico-Chir. Review for April 1855 (p. 501 ) It is entitled, “On the Means of Diagnosing the Internal Diseases of the Eye. By C. Bader, M.D., and Bransby Roberts, Esq., Resident Medical Officer, Royal London Ophthalmic Hospital, Moorfields,” and contains the fullest and most recent account of eye-specula accessible to English readers, with a record of observations made on healthy and diseased eyes. From this paper, I have borrowed the use of the word Ophthalmoscope, used occasionally in the text.

^{page 330 note *} See Helmholtz's “Beschreibung ernes Augen-Spiegels,” &c.; and Dr Sanders' excellent abstract of this Memoir, from which I have borrowed in the text; also Ruete's Preliminary Chapters in his Bildliche Darstellung.

^{page 330 note †} On the Luminousness Observed in the Eyes of Human Beings. Edinburgh Phil. Journal, 1827, p. 164.

^{page 331 note *} Edinburgh Monthly Med. Journal, July 1852, p. 41.

^{page 331 note †} Op. cit., p. 42.

^{page 332 note *} I am indebted to my pupil, Mr James Wardrop, an accomplished theologian and naturalist, for an abstract of the views of Kölliker and H. Müller. Their opinions are contained in Kölliker's Micr. Anatomie, B. 2, Zweite Hälfte 606–720. See also H. Müller's Remarks on the Structure and Function of the Retina, translated in the Quarterly Journal of Micr. Science, July 1853, pp. 269–273. Without entering into the minute discussion of anatomical questions which I am not competent to decide, it may be noticed, that both the skilful observers referred to, agree in denying to the fibres or expansion of the optic nerve, the function of perceiving “objective light.” This function belongs, according to them, to the deepest of the five layers of which they regard the retina of vertebrate animals as composed. This layer, which is immediately in front of the choroid, is thus described by Kölliker:—“The external or bacillar layer consists of the ‘rods and bulbs’ (bacilli et coni), whose ends evenly terminated, form a sort of mosaic pavement towards the black pigment, and which internally are continued by fibres (Müller's fibres) through the three succeeding retinal layers, to abut abruptly, and with radiating terminations on the external aspect of the fifth layer, which, however, they do not penetrate.” This fifth layer is a very delicate membrane, investing the entire internal or anterior aspect of the retina.

Accepting this description as well-founded (and it has received the approbation of the majority of recent physiologists), it appears that light must more or less traverse the anterior layers of the retina, till it reaches the “rods and bulbs,” with their connecting radiating fibres, before it can excite a luminous sensation. It must, however, in part be reflected from those anterior layers before reaching the deepest one. The internal or anterior surface of the bacillary layer is further described, as smooth and brilliant, the “bacilli and coni” appearing like the polished surfaces of crystals, so that they reflect light powerfully, and the greater number of the rays which are returned from the retina, have probably been reflected from its deeper layers, whilst a portion has been thrown back from the anterior layers, without contributing to the perception of light. The only point, however, which I am much concerned to urge is, that the retina, as a whole, reflects much of the light which reaches it.

^{page 333 note *} Observations on certain parts of the Animal Economy. By John Hunter, F.R.S., vol iv., p. 278.

^{page 333 note †} Catalogue of Museum, R. C. S., Lond., vol. iii., p. 133.

^{page 334 note *} Bildliche Darstellung, Tab. II.

^{page 334 note †} Brit, and For. Medico-Chir. Rev., April 1855, p. 509.

^{page 334 note ‡} Since reading this paper to the Society, I have learned from Mr Wardrop's Abstract of Kölliker's paper, that some of the latter's countrymen suppose that the bacillar layer of the retina reflects light on the optic fibres, and thus enhances vision. Kölliker's words are “This bacillar layer does not act as a catoptrical apparatus, as Hannover, Brücke, and Helmholtz think, for reflecting the light back to the optic fibres, but as a true nervous apparatus for itself receiving the impression.”— (Micr. Anat.. B. ii., Zweite Hälfte, p. 691.) Even, however, were the view objected to by Kölliker well-founded, it would not dispose of the question discussed in this paper, for the main direction of the light reflected from the bacillar layer of the retina must be forwards and ontwards, so as to illuminate more or less the entire chamber of the eye.

^{page 337 note *} A comparison, also, of pink-eyed with dark-eyed rabbits appeared to show that in diffuse day-light the average size of the pupil is the same in both; but in direct sunlight the albino pupil is smaller.

^{page 337 note †} In truth, in the case even of casual human albinoes, vision, however painful in full daylight, is not to any marked extent optically imperfect; and in the diffuse or moderate light by which they see correctly, reflection of its rays within the eye is occurring as certainly as when the light is direct and intense. I hesitated to urge thus much when the text of this paper was read to the Society, although the conclusion in question seemed justified by the accounts on record of albinism, especially by Dr Sachs' very interesting description of his own case and his sister's. (Hist. Nat. duorum Leucaethiopum auctoris ipsius et sororis ejus, à G. Sachs, M.D. See also my Researches on Colour-Blindness, p. 102). But since this paper was read, I had an opportunity, through the kindness of Dr James Sidey, of testing, along with Mr James C. Maxwell, the vision of an albino girl, aged 18 or 19. She was born in India, of Scotch parents, in humble life, and is, in all respects, a well-marked case of albinism. She sees with much less suffering in this country, than she did in that of her birth, but bright sunlight still distresses her. There is occasional strabismus of one eye, and both eyes exhibit, under exposure to light, the tremulous oscillation characteristic of albinism; not, however, to a very great degree. She thinks that her vision has improved within the last few years, but how far this is the result merely of removal from the influence of a tropical sun, it would be difficult to determine.

We found her quick and intelligent. By diffuse daylight, she distinguished the forms of objects rapidly and accurately. Many trials also were made as to her perception of colours, which Mr Maxwell's Colour-Top allowed us to test carefully, and with the result, that her sense of colour was acute, precise, and normal. In short, her vision by diffuse light was optically as perfect as that of the majority of mankind, and to appearance, a diminution in the sensitiveness of the retina was all that was required to make vision equally perfect in direct light.

The iris in this case was pale blue, and the pupil was not pink by diffuse daylight. It became sor however, in the neighbourhood of a gas flame, and her friends were familiar with the fact that by gas-light, her eyes often “flashed fire.”

We found it easy to observe this ocular luminousness at will, and the fact is important, as proving that the comparatively perfect vision which this Albino possessed, was exercised by eyes, within which a large amount of cross-reflection of light was continually occurring.

^{page 339 note *} Edin. Phil. Journal, 1827, p. 302.

^{page 339 note †} Physiology of Vision, p. 220.

^{page 340 note *} John Hunter, in Catal. of Museum of Royal College of Surgeons, London. Vol. iii., p. 165.

^{page 341 note *} Hunter states that in animals where the pigment of the choroid is light in colour, “the lightest part is always at the bottom of the eye, becoming darker gradually forwards, and in such it is often quite black; viz., from the termination of the retina to the pupil; or if not black, it is there much darker than anywhere else. This is generally the case in the eyes of the human subject.” Catal. Mus. R. C. S., London. Vol. iii., p. 133.

^{page 341 note †} Comparative anatomists must decide to what extent these observations demand qualification in reference to particular tribes of animals. The nocturnal lemurs, which have a uniformly coloured dark choroid, no tapetum, and a very sensitive retina, probably exhibit intra-ocular reflection to a small extent compared with other quadrupeds. A similar remark applies to birds, qualified bv the fact that the bottom of the eye-chamber is occupied in them by the marsupium or pecten, an organ the use of which has not been ascertained, so that we cannot be certain how it modifies vision. But as the researches of Kölliker and H. Müller demonstrate that the general structure of the retina is the same in all vertebrates, it appears certain that, however dark and absorptive of light the choroid may be in some of them, the retina in all will act as a mirror towards light incident upon it; and the very curious observations of the latter author on the eyes of the cephalopoda (Quart. Journal of Micr. Science. July 1853, p. 269), show that the retina of these invertebrates will act in the same way, a remark which, mutatis mutandis, may he applied to every creature, whatever its rank in the animal scale, which has shining or so-called phosphorescent eyes. A most interesting field is open to naturalists in the examination, by means of the ophthalmoscope, of the eyes of living animals of all grades.

^{page 343 note *} Physiology of Man, chap, xvii., p. 23.

^{page 343 note †} Researches on Colour-Blindness, p. 99.

^{page 344 note *} Researches on Colour-Blindness, pp. 88–100.

^{page 344 note †} John Hunter fully recognises this function of the tapetum, but (unless I misunderstand his meaning) regards it as useful to its possessor solely by reflecting rays of light “on the very object from which they came”—(On the Colour of the Pigmentum of the Eye, Hunter's Works, vol. iv., p. 285),—so that, on their return from this object, they “strike exactly, or nearly, on the same points in the retina through which they first passed” (Op. et loc. Cit.), and increase the visibility of the object in question.

This undoubtedly will be the result where the eye of the animal remains for some interval of time perfectly at rest; but the movements of a shark, ex. gr., are sufficiently rapid to enable its eye to receive light from one object and reflect it upon another, from which it receives it again, so that the rays sent from the first body enable it to see the second; and this, I apprehend, is as much the function of the tapetum as deepening the visual impression of the same object.

^{page 344 note †} I suggested it last autumn in Researches on Colour-Blindness, Edin. Monthly Med Journal, September 1854, p. 234.

^{page 345 note †} Colonel Madden, H.E.I.C.S., who was present when this paper was read, informed me afterwards, in reference to the subject discussed in the text, that in India, where he had served for many years, he had had occasion to verify the truth of the statement made above, so far as one animal is concerned. In a district at the foot of the Himalayas much infested by tigers, the natives, according to their own statement, were frequently afforded timely warning of the approach of these animals in the dusk, by the glare of their eyeballs, which the men compared to “yellow pumpkins.”

^{page 346 note *} Esser on the Luminousness observed in the Eyes of Human Beings, Edin. Phil. Journal, 1827, p. 164.