THE EYE AND VISION 



209 



corresponding motion. From this experiment we learn that, a convex lens 

 will form an inverted image of a luminous object ; and that, the nearer the 

 object is to the lens, the greater is the distance between the lens and the 

 image. 



Fig. 194. 



Next, substitute for the lens just used another one with more convex 

 surfaces, and notice that the distance at which the distinct image is formed is 

 less than in the preceding experiment. And by repeating the experiment with 

 a variety of lenses we shall learn that, the more convex the lens, the shorter 

 the distance at which the sharp image is produced ; and vice versfi. 



Fig- 195. 



We will now turn our attention to a piece of apparatus by 

 which we can illustrate the uses of the various parts of the eye : 



In fig. 196 A is a water-tight box, the back, B, of which consists of a square 

 of glass, the rest being made of wood or metal blackened on the inside. In 

 the centre of the front, F, is a circular hole, over which is fitted a watch-glass 

 with its convex surface turned outward. G is a sheet of ground glass which 



Fig. 196. 



may be moved forward and backward in the box so as to receive images formed 

 at various distances from the watch-glass. D represents two ' diaphragms ' 

 with different apertures, which, being placed behind the watch-glass, may be 

 used to limit the amount of light admitted ; and L is a convex lens attached 



P 



