TWO WOXBURFUL INSTRUMENTS. 



53 



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 in- 

 distinct 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. 



Fi3. 2.— A Verticai, Section op the Eye.— ^, the cornea ; E, the crystalline lens ; I. the choroid ; 

 K, the retina; M, the optic nerve leading to the brain. 



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. Xow, 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 impres- 

 sion 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. 



