EYE AS A CAMERA OBSCURA. 339 



B. Prevost satirizes the belief, that a mirror within an animal's eye can assist it 

 in seeing.* Dr W. Mackenzie agrees with him, and observes, that " Reasoning 

 a priori, we should say that the tapetum would render the eyes weak, and impa- 

 tient of light."f 



It is needless to discuss those opinions in detail. If in the eyes of those ani- 

 mals which have not tapeta, there is, as we may phrase it, simply permission for 

 internal reflection of light to occur ; in those which have tapeta, there is plainly 

 direct provision for its occurrence ; and it is astonishing to find writers of such 

 ability as those I have quoted, finding no function for the tapetum but that of dis- 

 turbing the vision of its possessor. We are often compelled to acknowledge our 

 ignorance of the function of an animal organ, but surely never to confess that it 

 was given to its owner solely to incommode or injure it ! Only those whose 

 thoughts were preoccupied by the theory that the eye must be preserved a 

 camera obscura, could have brought themselves to credit and affirm that the horse, 

 the ox, the lion, the dog, the seal, or the shark, are animals that see imperfectly. 

 With the exception of birds, they are probably excelled by no animals in the 

 quality of their vision. 



3. The last objection I have to notice raises the difficulty, that the known 

 laws of luminous reflection render it impossible that perfect vision can be per- 

 formed by an eye, across the chamber of which light is continually passing and 

 repassing. But if it be the case, as I have sought to show, that the most perfect 

 vision which comes under our notice is performed in spite of such reflection con- 

 tinually occurring, it is manifest that there must be some misconception regard- 

 ing the evil influence of cross lights within the chamber of the eye. 



A little consideration will show, that as all the light which enters a normal 

 eye enters it by the pupil, and is refracted to a focus on the retina at a point 

 more or less directly opposite its place of entrance, if it is reflected from the re- 

 tina or choroid, it will in larger part simply retrace its course, and pass out 

 through the pupil as it entered by it. This escape of the reflected light through 

 the pupil, carries it clear of the internal walls of the eye-chamber, and, as we 

 have already seen, renders possible the construction of ophthalmoscopes. 



Two circumstances may interfere with the exit through the pupil of the reflected 

 rays. 1st, The pupil suddenly exposed to bright light, may contract and diminish 

 its area before the rays which entered by it have had time to undergo reflection. 

 In that case the rays furthest from the centre will fall upon the back of the iris, 

 and undergo in part a second reflection from it. But the reflection thus occurring 

 will not be great, for special provision is made in all but Albino eyes for the stop- 

 page of such rays, by the great thickness of the uvea or pigment on the posterior 

 surface of the iris, which has no highly reflective retinal covering like the choroid. 



* Edin. Phil. Journal, 1827, p. 302. f Physiology of Vision, p. 220. 



VOL. XXI. PART II. 4 Y 



