416 REFRACTION BY LENSES:—FORMATION OF IMAGES* 
the curvature of the lens is more considerable. Thus a convex 
lens has a long focus or a short focus (that is, brings rays to a 
focus at a greater or less distance from itself) according as the 
curvature of its surface is less or more considerable. 
531. The rays issuing from every point in an object, and 
falling upon a convex lens, are brought to a focus on the 
other side of the lens; and thus a distinct image or picture 
is formed upon any screen placed at the proper distance to 
receive it (as is seen in a Camera Obscura or a Magic Lantern), 
every point in that image being the representative of the 
corresponding point in the object, hut this image being 
inverted. 
532. Now, the Eye, in its most perfect form—-such as it 
possesses in Man and the higher animals—is an optical 
instrument of wonderful completeness, designed to form an 
exact picture of surrounding objects upon the expanded sur¬ 
face of the optic nerve, by which its impression is conveyed 
to the brain. As it is in the most perfect form of this instru¬ 
ment that we are best able to judge of the uses of its different 
parts, it will be preferable to consider this in the first instance, 
and then to advert to the less complete forms which we meet 
with in the lower animals. 
533. The Eye of Man, like that of all Yertebrata, has a 
nearly globular form. The walls of the sphere are composed 
of three coats; whilst in its interior are found three humors 
of a more or less fluid character. The outer coat, named the 
Sclerotic (s s , fig. 208), is tough and fibrous, and is destined 
to support and protect the delicate parts which it contains. 
It does not cover the whole globe, however; but gives place 
in the front of the eye to a transparent lamina of cartilaginous 
