LECT. 

 \'lt 



242 



FERGUSON'S LECTURES. 



the bottom of the eye, but towards the side next the nose ; 

 so that whatever part of the image falls upon the optic 

 nerve of one eye, may not fall upon the optic nerve of 

 the other. Thus the point a of the image c b a falls upon 

 the optic nerve of the eye D, but not of the eye E ; 

 and the point e falls upon the optic nerve of the eye E, 

 but not of the eye D : and therefore to both eyes taken 

 together, the whole object A B C is visible. 



The nearer that any object is to the eye, the larger 

 is the angle under which it is seen, and the magnitude 

 under which it appears. Thus to the eye D, the ob- 

 ject A B C is seen under the angle A P C ; and its 

 image c b a is very large upon the retina : but to the 

 eye E, at a double distance, the same object is seen un- 

 der the angle A p C, which is equal only to half the an- 

 gle A P C, as is evident by the figure. The image cba 

 is likewise twice as large in the eye D, as the other 

 image c b a, is in the eye E. In both these representa- 

 tions, a part of the image falls on the optic nerve, and 

 the object in the corresponding part is invisible. 



As the sense >f seeing is allowed to be occasioned by 



