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14 CHAPTER IV.

PHYSIOLOGY OF THE EYE.

would again trespass on the patience of our readers, and ask their consideration of another aspect of our subject. It is said the eye is a refracting medium, and is only distinguished from the brain by being a translucent, refractive substance. That light does not stream into the eye like water into the sponge, but it progresses gradually into it, and operates upon it; and that, in order to experience the sensation of light, the eye is placed in a similar tension to the air and water, and that the tension between it and the main body of brain is perceived by the latter, as illumination. Some say the eye is the brain's prism, in which it sees the world,—in which the brain sees its own tension or the production of colour. Sight is also characterized as deoxidation of light; and the eye by some is now approved as an actual part of the brain. We may refer to the first expression or lines of man's existence ere his birth, and then, remembering how combined all his parts once were,—how that the line of life (the vertebræ) did solve into itself all parts for a time (as the trunk of a tree),—we are not indisposed to admit all such philosophy. Some consider that the optic nerve is an organized ray of light; the brain an organized sun; the eye an organized chromatic sun or rainbow; the optic nerve perceives not the light in general, but its terrestrial formation in the chromatic image, which has been propagated into the eye. In contemplating