184 PHILOSOPHY OF ZOOLOGY. 
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 cireum- 
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 
