CHAPTER IX. 



OPTICAL INSTRUMENTS RAINBOW AND COLOR. 



LENSES are transparent bodies, usually glass, one or both of 

 whose surfaces are curved . The principal forms are the double 

 convex lens A, bounded by two convex surfaces; the piano con- 

 vex lens B, bounded by a plane surface and a convex surface; 

 the concavo convex lens C, in which one surface is convex and 

 the other concave, the concavity being less than the convexity ; 



the double concave lens 

 D, which has two con- 

 cave surfaces ; the piano 

 concave lens E, with one 

 plane surface and the 

 other concave, and the 

 concavo convex lens F, 

 in which there is one 

 concave and one convex 

 surface, the convexity being less than the concavity. The con- 

 vex lenses diminish in width towards the edge, while the concave 

 lenses increase in width towards the edge. (See Fig. 12.) 



The effect of convex lenses on rays of light may be shown by 

 tracing them through a lens according to the law of refraction. 

 Suppose MN, Fig. 13, represents a double convex lens, C and C' 

 representing the centers of curvature, and the line C C' the prin- 

 cipal axis of the lens. Now, if a ray of light from C fall upon the 

 lens at A it will be bent toward the perpendicular C'A at the 

 point A towards the point B, on emerging from the lens the ray 

 will be bent from the perpendicular CB at the point B towards 

 C'; and the ray of light from C' in the same way would be re- 

 fracted to the point C, so that C and C' are called the conjugate- 

 (78) 



