DEVICES FOR SEEING 



305 



7 



plano-concave lens of low refractive but high dispersive power, 

 an image may still be formed that is free from the color fringe 

 because the second lens will not overcome the refraction of the 

 first lens completely, while it will undo the dispersive effect 

 of the first lens. Now to grind 

 and combine two or more 

 lenses so as to correct their 

 defects is a laborious process 

 that requires great skill, hence 

 the superior photograph, mi- 

 croscope, or telescope lens 

 must be costly. 



In the human eye there is 

 such a combination of lenses. 



The aqueous humor in the front of the eye is in the shape of a 

 convex meniscus; then comes the double convex crystalline 

 lens; then the vitreous humor making a plano-concave lens, 

 plane on its posterior side because the retina is imbedded in it 

 so that no refraction occurs as the light passes from it to the retina 

 (Fig. 135, p. 288). 



According to the still generally accepted theory, light is due 

 to waves in the ether or in other substances through which it is 

 passing. The wave form advances, but each molecule moves 

 in a tiny orbit somewhat as do the particles of water when a 



FIG. 152. Correction of chromatic 

 aberration of a convex lens by a concave 

 lens. 



w v v - 



FIG. 153. Diagram showing wave motion 



water- wave forms. Thus in Figure 153 molecule i is struck by 

 an impulse that makes it vibrate or revolve in the orbit represented 

 by the dotted line. It has just completed such a revolution. 

 It takes an appreciable, though very short, time for the impulse 



