350 



THEOlir OF VISION. 



[1855. 



As I have discussed this question at length elsewhere, I shall 

 merely observe here that as the light emitted I'rom a oat's or a 

 shark's eye, ex. r/r., is veritable light, there is no room for 

 aifirmiiig that its illuminatiug powers are not, cceleris 2'>arilius, 

 equal to light of the same quality from any other source. If 

 we can see a cat in the apparent darkness, which otherwise 

 would render it iavisible, by the liglit which issues from its 

 eyes, it cannot be questioned that it will see us by so much of 

 that light as our persons reflect back into those eyes. The 

 tapetum lucidum is, for every creature which possesses it, a 

 lantern, by which it can guide itself in the dimmest twilight, 

 and make each ray of light do double or triple service, in 

 assisting it to steer its course, and to find its food or prey. 



But if the tapetum assists carnivorous animals in finding 

 their living prey, it must also give the latter warning of the 

 approach of the destroyer. I am not aware that this use of the 

 tapetum has hitherto attracted attention. But a lion or a shark 

 does not more certainly bring into view, by means of tapetal 

 light, the creature it would devour, than it betrays its own 

 presence to that creature, and the balance is thus mercifully 

 maintained between the preyer and the prey. That singular 

 "hypnotising" or "mesmerising" power which, in the case of 

 the serpent, is called " fascination," is probably largely possessed 

 by the glaring tapetal eye, which acts with all the advantage 

 of surrounding darkness to increase its impressiveness, and 

 prevent other objects from distracting the attention of the 

 subject of fascination. On the other hand, however, the 

 tapetal light is peculiarly startling to an observer, for it is 

 always coloured and unlike that of day, resembling in character 

 (in the case at least of the cat and the dog) those fluorescent 

 rays of the spectrum, which Mr. Stokes describes as "ghostly," 

 and of which it pr9bably largely consists. At all events, its 

 unfamiliar appearance specially qualifies it to alarm creatures 

 who suddenly perceive it, and are led by instinct to flee from 

 all strange lights. 



In the lower animals, then, the tapetum is probable service- 

 able — 



1°. By doubling within the eye the impression of each ray 

 upon the retina. 



2°. By reflecting light from the eye upon external objects, 

 so as to render food or prey more visible. 



3°. By warning, through the agency of that light, creatures 

 on which carnivorous animals prey, of the neighbourhood of 

 their enemies. 



In the discharge of those functions the retina more or less 

 conspires, difi'ering from the tapetum chiefly in reflecting a less 

 coloured light than the latter does. Further, in such of the 

 lower animals as have not tapeta, there must occur in most, 

 alike from the choroid and the retina, and in all at least from 

 the retina, reflection of light. In those whose eyes exhibit 

 choroidal reflection, the same good ends will be served by it, 

 though in a much less degree, as are secured by tapetal reflec- 

 tion, and of these probably the most important is the first, 

 which cannot be attained with light reflected from the retina. 



How far human wisdom is sensihfi/ influenced by the 

 choroido-retinal reflection which is continually occurring within 

 the living eye, it is difficult to decide; but it must be influenced 

 to some extent by it. It seems probable that the acute vision 

 in faint light which characterizes those who are imprisoned in 

 dark chambers, and which the astronomer sometimes pui'posely 

 induces by long shading of his eyes before making observations, 

 is in part due to the return of light from the choroid through 

 the retina; in part to the passage through the highly-dilated 

 pupil of light reflected both from the choroid and retina, which 



is thrown upon external objects. It may startle us at first to 

 be told that we see in part by light issuing from our eyes, but 

 it must be so; and those traditions of learned men who could 

 read by the light of their own eyes in what was darkness to 

 others, are only exaggerations of a power more or less exercised 

 by every human organ of vision. 



To one result of this choroido-retinal reflection in the human 

 eye, I would, in conclusion, refer. The light which is thus 

 reflected, is always coloured, being, as we have already seen, 

 red, yellowish-red, or brownish-red, and differing necessarily in 

 its tint, according to the abundance of pigment in different 

 eyes. Each of us thus adds to every object on which he looks 

 so much colour, but no two pairs of eyes the same amount, 

 and hence one great reason why no two persons, almost, will be 

 found to agree as to the matching of one colour with another 

 where the coloured substances compared consist of different 

 materials; and why very marked differences present themselves 

 in the judgments of persons equally practised in observing and 

 copying colours. 



Two artists, for example, paint from nature the same flower. 

 The pigments which they employ for this purpose, will, of 

 course, be as much affected by the colour communicated from 

 the eye, as the flower is, so that, could the latter be imitated 

 in its own materials, the copies might be identical. But as 

 these must be made with substances whose lustre, transparency, 

 and particular tint, difl'er from those of the body copied, the 

 added colour from the eye tells unequally on the original and 

 the copy, as compared together, and as seen by different eyes. 

 Each, accordingly, objects to the other's colouring, but neither 

 can induce his neighbour to. adopt his tints, and both appeal 

 confidently to third parties (who perhaps differ from both), 

 assured that the adjudication will be in favour of the appellant. 

 Here each may have been equally skilful and equally faithful : 

 and neither has any means of testing to what extent he sees 

 everything as if through coloured spectacles, which give all 

 objects a tint for him inseparable from their natural colour. A 

 "chromatic equation," thus originated, belongs, I believe, to 

 every eye. 



P.S. — From my friend Professor Goodsir, who recently 

 (June 27th) delivered a lecture of great interest and originality 

 on the retina to the anatomical students of the University of 

 Edinburgh, I learn that, from the observations and speculations 

 of the Continental Physiologists, it appears very probable that 

 onli/ the rays of light which are returned from behind through 

 the retina produce a luminous sensation, and that the objective 

 perception of light commences physically at the choroidal, not 

 the hyaloid extremity of the optically sensifie constituents of 

 the retina. According to Kblliker, this objective perception 

 begins at the extremities of the rods and bulbs which are in 

 contact with the pigment of the' choroid, — a view of matters 

 not readily reconcilable with the organization of the yelloio spot, 

 where vision is most perfect ; according to Briicke, luminous 

 sensation begins in a layer of gray nervous substance, situated 

 nearer the front of the retina ; but both observers agree in 

 ascribing entirely to that light, which is passing back from the 

 choroid, the power of initiating luminous perceptions. I have 

 argued, in the preceding paper, for such returned light being 

 accessory to vision, but according to this view it is the only 

 light by which it is exercised. If this doctrine (however 

 modified in details) be established, the reflection of light from 

 the choroid will prove to be essential to the function of seeing, 

 and the necessity for the living eye being a Camera Lucida, 

 will be based upon deeper grounds of proof than I have at- 

 tempted to offer. 



