52 



THE POPULAR SCIENCE MONTHLY. 



globular room. The window is so contrived tbat 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 back- 



FlG. 1. 



ward 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 func- 

 tions of the eye, except to show how human ingenuity has contrived 

 an instrument almost exactly resembling it, and capable in some re- 

 spects 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 con- 

 dition 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 open- 

 ing 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 



