344 DB, GEOEGE WILSON ON THE 



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 eye. 



The greater part of this light, however, after traversing 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, especially in recent 

 times, to acknowledge this. The supposed necessity of maintaining the chamber 

 of the eye dark, the apparent impossibility of the eye reflecting and receiving 

 light simultaneously, and the faintness of the light emitted from tapetal 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 reality of no value, 

 and were not entertained by the older writers, such as Hunter and Monro, who 

 not only regarded the tapetum as casting light on external objects, but, in the 

 case of graminivorous animals, as affording them, by the green colour of the light 

 which is reflected, an assistance in discovering their food ; an opinion which Cuvier 

 in part countenances. 



As I have discussed this question at length elsewhere,* I shall merely observe 

 here that as the light emitted from a cat's or a shark's eye, ex. gr., is veritable 

 light, there is no room for affirming that its illuminating powers are not, cceteris 

 paribus, 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 invisible, by the 

 light 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 luci- 

 dum 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 ser- 

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



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, 



* Researches on Colour-Blindness, pp. 88-100. 



f John Hunter fully recognises this function of the tapetum, hut (unless I misunderstand his 

 meaning) regards it as useful to its possessor solely hy 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 func- 

 tion of the tapetum as deepening the visual impression of the same object. 



J I suggested it last autumn in Researches on Colour-Blindness, Edin. Monthly Med. Journal, 

 September 1854, p. 234. 



