burning-glass. It is held in its position by a very delicate membrane which suspends it in its place in front and behind. If it were not for this crystalline lens of the eye, we should be able only to have an indistinct impression of light. This lens enables us to see the forms of things; defining them in the same manner as the lens of spectacles, or the lenses of the telescope or opera-glass. Now, in the artificial eye which we are considering, we must place, in the front part, glass lenses through which the picture or view can pass into its interior.
In the human eye the entire inner surface of the eyeball is covered with a brownish-black membrane called the choroid coat. Its use is to absorb light which reaches it and to prevent reflections. Now, in our artificial imitation, we must cover the entire interior of the box with black paint, so as to absorb every ray of light, except that for which we have a use.
In the back part of the human eye is the termination of the optic nerve called the retina. It is that part of the eye which is especially sensitive to light; it receives the rays entering through the front window, forms a picture of the scene, and communicates the impression through the fibers of the optic nerve to the brain behind it. How it does this we do not know. It is certain, however, that an exact picture of anything we see is created upon this membrane in the back part of the eyeball.
Doubtless the reader has already guessed the name of the artificial invention I have been describing — the photographic camera. But what shall take the place of the nerve or retina of the eye? What shall stand in place of the mysterious cells of gray matter in the brain, which receive and retain the visual impressions? After all, this is the only really wonderful part of either instrument.