The eye

The eye may be looked upon as an optical instrument, a sort of photographic camera, designed to produce by means of its refracting system a small and inverted picture of surrounding objects upon the,
retina; the stimulation produced by this picture on the retina is conveyed through the optical nerve; to the ganglion cells of the cortex, that part known as the optical area; this excitation of the ganglion cells becomes sensation, and thus it is here that the object seen comes within the domain of consciousness and so the brain is enabled to interpret correctly the impressions thus transmitted to it. Immediately behind the transparent retina is a layer of pigment, which absorbs some of the rays of light as soon as the image is formed; were this not so the rays would be reflected
to other parts of the retina, and cause much dazzling, considerably interfering with vision ; this is the case in those persons who have a congenital absence of pigment, and who are known as albinos.

The refracting system of the eye is so arranged that only little, if any, spherical or chromatic aberration takes place, as is the case with ordinary optical instruments.

For distinct and accurate binocular vision the following conditions are necessary:

1. That a well-defined inverted image be formed on the layer of rods and cones at tha yellow spot of each eye.
2. That the impression there received be conveyed to the brain.

In a work of this character the first of these conditions alone concerns as, and for the carrying out of this, the media being transparant. three important factors call for a separate description, namely.

Refraction
Accommodation
Convergence