THE EYE. 



Fig. 11. 

 li 



drawn from the right eye to it shall be perpendicular to the screen. 

 Let the left eye be now closed, and let the right eye be directed to 

 the extreme wafer on the left, but so that all three wafers may still 

 be perceived. Let another person now slowly move the screen, so 

 as to bring it nearer to the observer, maintaining, however, the 

 middle wafer in the direction of the eye at c. It will be found 

 that the screen being so moved to a distance of 10 feet from 

 the eye, the middle wafer will appear to be suddenly extinguished, 

 and the extreme wafers on the right and left will be seen. 



74. This remarkable phenomenon is explained by showing that 

 in this particular position of the eye and the screen the image of 

 the middle wafer falls upon the base of the optic nerve when the 

 choroid coat is not under it. 



This will be rendered more intelligible by reference to fig. 11. 

 where B is the middle wafer, A the left-hand, and c the right- 

 hand wafer. The image of 

 A is formed at a, to the 

 right of the optic nerve ; 

 and the image of c is 

 formed at c, to the left of 

 that nerve. In both these 

 positions the choroid coat 

 is behind the retina. But 

 the image of B is formed at 

 b, directly upon the point 

 where the optic nerve 

 issues from the eye-ball, 

 and where the choroid does 

 not extend behind it. 



75. Sir Pavid Brewster 

 gives the following expe- 

 riment as a further argu- 

 ment in support of this 

 hypothesis. In the eye 

 of the Sepia loligo, or 

 cuttle-fish, an opaque membranous pigment is interposed between 

 the retina and the vitreous humour, so that if the retina 

 were essential to vision, the impression of the image on this 

 black membrane must be conveyed to it by the vibration of 

 this membrane in front oi it. Sir David Brewster also mentions 

 that in young persons the choroid coat, instead of being covered 

 with a black pigment, reflects a brilliant crimson, like that of dogs 

 and some other animals ; and imagines that if the retina were 

 affected by the rays which pass through it, this crimson light 

 ought to excite a corresponding sensation, which is not the case. 

 80 



