VISION, 563 
Any act of perfect vision in a mammal may be shown to 
consist of the following: (1) The focusing of rays of light from 
6S 
ya 
Yn 
Fie. 407.—More advanced rie of development of eye (after Cardiat). a, epithelial cells 
forming lens, now much altered ; b, lens capsule ; c, cutaneous tissue about to form con- 
junctiva; d, e, two layers of optic vesicle, now folded back and forming retina; f. mucous 
tissue forming vitreous humors ; g, intercellular substance ; h, developing optic nerve ; 
i, nerve-fibers entering retina, 
an object on the retina, so as to form a well-defined image; (2) 
the conduction of the sensory impulses thus generated in the 
retina by the optic nerve inward to certain centers; and (3) the’ 
elaboration of these data in consciousness. 
We thus have the formation of an image—a physical pro- 
cess; sensation, perception, and judgment—physiological and 
psychical processes. 
In the natural order of things we must discuss first those 
arrangements which are concerned with the focusing of light 
—i. e., the formation of the image on the retinal screen. 
DIOPTRICS OF VISION. 
One of the most satisfactory methods of ascertaining that 
the eye does form images of the objects in the field of vision 
is to remove the eye of a recently killed albino rabbit. On 
