EYE AS A CAMERA OBSCURA. 345 



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 in- 

 crease 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 probably 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, f 



In the lower animals, then, the tapetum is probably serviceable — 

 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 carni- 

 vorous animals prey, of the neighbourhood of their enemies. 



In the discharge of those functions the retina more or less conspires, differing 

 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 reflection, 

 and of these probably the most important is the first, which cannot be attained 

 with light reflected from the retina. 



How far human vision is sensibly 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 cham- 

 bers, and which the astronomer sometimes purposely 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 



f 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." 



