globular room. The window is so contrived that it can be made small or large, as the light is strong or feeble. From the wall in the rear upon which the picture is made, a nerve carries the impression backward
Fig. 1.
to the brain, and by means of that impression we perceive. This is the mystery, how the brain gets its impression; not how the eye gets its image.
In the present article I shall not describe the structure and functions of the eye, except to show how human ingenuity has contrived an instrument almost exactly resembling it, and capable in some respects of doing far more wonderful work. Man has invented in reality an artificial eye which sees farther, with infinitely greater distinctness, and in a very much shorter space of time, nearly everything which lies before it. Almost every particular in the structure of the human eye must be imitated by this instrument. When in its most perfect condition its work is quite as wonderful as the eye of an animal.
In the first place, we must have a perfectly dark box, say about a foot high, a foot wide, and about eighteen inches long. This is the dark chamber, and corresponds to the eyeball. In one end is an opening in which is inserted a peculiar arrangement of optical glasses. These will correspond to that part of the human eye which is called the crystalline lens.
What is this? Just in front of the main body of the eyeball, behind the curtain which we see, is a transparent, circular and flattened body, thicker in the middle and thinner at its edges, the exact shape of a
