VISION IN THE MOUSE 



583 



I experimented by standing in front of the mouse where, with one 

 eye closed, I could plainly see both eyes of the animal, and then 

 moved my head from side to side in order to discover how great 

 was the angular field from which the two eyes might be seen. I 

 found that I could move my head through an angle of 70 degrees 

 without losing sight of either eye. 



Although the possibility of seeing the ball of the eye in a human 

 being does not always mean that the eye sees the observer, yet 

 we know that when the eye is turned as far as it can go toward 

 the observer, he is seen on the extreme border of the field of vision. 



Fig. 9 Diagram showing relative size of lens in eye of mouse, ch, choroid; 

 /, lens; c, cornea; i, iris; o, optic nerve; r, retina. 



This of course, is made possible by means of the refractive power 

 of the lens and by the fact that the retina extends all the way 

 around the inside of the eye right up to the ciliary process. 



If the retina were of uniform acuteness throughout its area, 

 or if the lens were larger, it would not be necessary to move the 

 eye to its limit in order to see the observer who can just see the eye- 

 ball. These are just the conditions which we have good reason 

 to suppose exist in the mouse. In the first place the size of lens, 



