1855.] 



THEORY OF VISION. 



349 



4. The ej-es of vertebrate animals are only to a limited 

 extent camerse ohscura;, and internally are least dark in the 

 portions most directly exposed to the action of light, and where 

 the seat of perfect vision is placed. 



5. The back of the iris, over which the retina does not 

 pass, is the darkest internal portion of the eye in vertebrates ; 

 and next to it, in the majority of these, are the ciliary processes 

 of the choroid, and its anterior lateral portions. 



From those premises the conclusion is deducible that in 

 vertebrates much light is reflected from the bottom of the ej-c- 

 chamber during the exercise of vision without disturbing it ; 

 but that little is reflected again, .so as to return to the bottom 

 of the eye, in consequence partly of its absorption by the pig- 

 ment of the anterior portions of the choroid, partly of its 

 escape through the pupil. 



It may seem to some that this reasoning proves too much, 

 for why is there in man and many other animals a pigment at 

 the bottom of the eye, if reflection from the membrane there 

 is so free to take place ? To this I reply that the pigment, 

 which is never altogether inoperative, comes into special action 

 when the eye is exposed to very bright light, and saves the 

 retina from the paralyzing influence of intensely luminous 

 rays. Vision, however, cannotbecoutinuously exercised under 

 such exposure, even where the light is not excessively bril- 

 liant, in consequence of the instinctive closure of the eyelids, 

 and the abundant secretion of tears which then take place. 

 The pigment at the bottom of the eye is thus, I apprehend, a 

 safeguard against sudden exposure to intense light ; but dur- 

 ing continuous vision under an illumination which does not 

 dazzle the eye, its action is secondary as an absorber of light, 

 and it always acts as a reflector. 



Hitherto I have been arguing almost solely for the nega- 

 tive conclusion, that the vertebrate, and especially the human 

 eye, is not the kind of darkened chamber which it has been 

 supposed to be. It is impossible, however, to regard the deep 

 iotra-ocular reflection which so certainly occurs in most animals, 

 as an incidental or useless phenomenon. That it has a direct 

 and beneficial influence over vision I cannot doubt, and I pro- 

 ceed briefly to indicate where the proof of this is to be found. 



Intra-ooular reflection, as a normal phenomenon, is at a 

 maximum in the tapetal or mirrored ej'e of the lower animals. 

 It is desirable, accordingly, to study it first as occurring in 

 them ; nor can a better example of a mirrored eye be found 

 than that presented by the shark. In it the tapalum hirldicm 

 occupies the whole of the bottom, and one-half or more of the 

 lateral surface of the choroid, which is covered by pigment 

 only in front. The iris, as in other fishes, is incontractile, so 

 that the diameter of the pupil never varies ; and the tapetum, 

 which is colourless and very brilliant, is thus always in action 

 as a reflector. The shark, however, swims near the surface of 

 the sea, where the amount of light is considerable, and the 

 acuteness of its vision is proverbial. I have selected it, rather 

 than a mammal, with eyelids and a contractile iris, because in 

 the shark luminous reflection never ceases unless in absolute 

 darkness ; and when light is shining occurs the more, the 

 brighter the light is. Its eye is thus always in the condition 

 in which that of a cat, or dog, or ox is, when subdued light 

 causes the iris to expand, and allows the reflecting tapetum to 

 come into play, so that the considerations which I have to urge 

 apply to the mammal as much as to the fish, provided they are 

 taken with pupils ci|ua]iy dilated; but as the tapetum in the 

 shark is very large, very brilliant, and always in action, I sliall 

 restrict myself for the present to it. 



The light, which penetrates to the bottom of a shark's eye, 

 will, in part, bo reflected from the retina (a phenomenon which 

 for the present I disregard), in part traverse it, and reach the 

 tapetum, where a portion will be lost by absorption and irregu- 

 lar reflection or dispersion, and (what alone concerns us here) 

 in part undergo direct reflection, return through the retina, 

 and escape by the pupil. This returned light will impress the 

 retina in traversing it, and illuminate external objects on 

 leaving the eye. 



The first question, then, is, "How will this light impress 

 the retina i"' According to J. Miiller and "\V. 3Iackcnzie, as 

 we have already seen, only injuriously, so far as freedom from 

 the sensation of dazzling, or distinctness of visual perception, 

 are concerned; according to Todd and Bowman "probably" 

 by "increasing the visual power, particularly when the quan- 

 tity of light admitted into the eye is small." I have urged 

 elsewhere that "what is equivalent to two rays of light fallTn" 

 upon the retina will produce two impressions. We send a 

 capillaiy sunbeam through the retina in one direction, and 

 instantly return it through that membrane, a little diminished 

 in intensity, in the opposite direction; if it determined a sen- 

 sation in its first passage, what is there to prevent its doing so 

 in its second? If, for simplicity's sake, we suppose exactly the 

 same points of the retina to be traversed by the incident and 

 the reflected ray, then (unless the luminous intensity of the 

 incident ray was so great as by its passage to exhaust the sen- 

 sibility of the retina), the reflected ray will repeat somewhat 

 less powerfully the impres.sion made by the incident one. The 

 diff'erence will be as great as there is between a sound and its 

 echo, but not greater. 



On this view of matters, the tapetum, especially in twilight, 

 will serve the important purpose of making every perceived ray 

 of light tell tirire upon the retina, so that the sensation it pro- 

 duces will either be increased in distinctness or in duration, 

 and probably in both. 



I will not deny that we are not entitled at once to infer that 

 because a molecular change (modulation, vibration, polariza- 

 tion?) transmitted through a special sti-ucture in one direction 

 produces a peculiar sensation, it will certainly produce the 

 same sensation on being transmitted through that structure in 

 the opposite direction ; but there are strong analogies in favour 

 of such a view, and it is entitled to be regarded as a likely 

 hypothesis 



The first probable use of the tapetum, then, is to double 

 the impression which light produces upon the retina, whilst 

 that light is within the ej-o. 



The greater part of this light, however, after travci-sing the 

 retina with little diminution by absorption, passes outwards 

 through the pupil, and, along with the light reflected from the 

 retina, is thrown upon external objects, and illuminates them. 

 A singular reluctance has been .shown by physiologists, especi- 

 ally in recent times, to acknowledge this. The supposed 

 necessity of maintaining the chamber of the eye dark, the 

 apparent imiio-ssibility of the eye reflecting and receiving light 

 simultaneously, and the faintness of the light emitted from 

 ta]>ctal eyes, have led most writers to contemn the doctrine 

 that the tapetum is a serviceable reflector of light. But the 

 objections to this doctrine are in realitj- of no value, and were 

 not entertained by the older writers, such as Hunter and 

 Jlonro, who not only regarded the tapetum as casting light ou 

 cxterii.il objcct.s, but, in the ca.se of gniminivorous animals, as 

 affording them, bj' the green colour of the light which is 

 rcflictcd, an a.ssistance in discovering their food; an opinion 

 which Cuvier in part countenances. 



