NOTES. 



425 



when the two prisms are placed with their refracting angles in opposite 

 directions, as in fig. 54, they nearly neutralize each other's effects, and 

 refract a beam of light without resolving it into its elementary colored 

 rays. Sir David Brewster has come to the conclusion, that there may be 

 refraction without color by means of two prisms, or two lenses, when 

 properly adjusted, even though they be made of the same kind of glass. 



KOTS 192, p. 159. The object glass of the achromatic 

 telescope consists of a convex lens A B, fig. 55, of crown-glass, 

 placed on the outside toward the object, and of a concavo- 

 convex lens C D of flint-glass placed toward the eye. The 

 focal length of a lens is the distance of its center from the 

 point in which the rays converge, as F, fig. 60. If, then, the 

 lenses A B and CD be so constructed that their focal lengths 

 are in the same proportion as their dispersive powers, they 

 will refract rays of light without color. 



NOTE 193, p. 162. When a sunbeam, after having passed through a 



Fig. 56. 



JVjr.57. 



colored glass V V, fig. 56, enters a dark room by two small slit* OCX in 

 a card, or piece of tin, they produce alternate bright and black bands on 



