184 PHILOSOPHY OF ZOOI.OGV. 



rays of the inverted image, as our eye is, when looking on 

 the picture formed on the white paper, but by the direct 

 rays from the object passing through its substance. We do 

 not, therefore, see the picture of the object, but the object 

 itself. And as we see the object, or any part of it, in the 

 direction of those rays which proceed from them, and which 

 produce the sensation, it follows, that the eye really sees 

 objects in their natural and relative situation. 



It is well known that the eye discerns objects placed at 

 different distances. As the rays of light, which reach the 

 eye from a distant object, are nearly parallel, they will con- 

 verge into a focus nearer the humours than those rays 

 which proceed from near objects and which are more diverg- 

 ent, and, consequently, will unite in a focus still more remote. 

 In order to obtain distinct vision in these different circum- 

 stances, either the retina must recede or approach, accord- 

 ing to the focal distance ; or, if we suppose the retina sta- 

 tionary, the lens must move, or experience a change in its 

 refractive powers, by an alteration of its form or density ; 

 or, in viewing near objects, those rays only may be admit- 

 ted which are nearest to the axis, and which are conse- 

 quently the least diverging. But physiologists are by no 

 means agreed in their opinion of the means employed by 

 nature for this purpose. 



The sclerotic coat is considered by some as subservient to 

 this end. They suppose that the muscles compress it, and 

 that the humours are thus pressed forward, to encrease the 

 convexity of the cornea, and enable it more readily to con- 

 verge the rays. But while the sclerotic coat is flexible in 

 man, whose eye is globular, and easily retains that form by 

 the humours pressing equally in all directions, it is nearly 

 inflexible in many animals whose eyes possess this adjust- 

 ing power, but which are more or less removed from a* 

 spherical form. It is indeed pretty obvious, that the use 



