LENSES. 



13 



Pig. 7. 



Light rays of different colors are bent by different amounts, 

 since the refractive indices for the various colors of light are 

 different. White light is a combination of numerous colors, 

 and if a beam of sunlight falls upon a prism it does not come 

 through as white light, but the constituent colors are refracted 

 by different amounts, giving rise to a band of light containing 

 all the colors of the rainbow, namely, red, orange, yellow, green, 

 blue, indigo, violet, red being least re- 

 fracted, violet most. Such a band of 

 colors is known as a spectrum. An in- 

 strument has been constructed for con- 

 veniently observing the spectrum of white 

 light which is known as the Spectroscope. 

 It has proved to be of the greatest value 

 in chemical analysis. Many new ele- 

 ments were discovered by its aid, for example, calcium, 

 rubidium, thallium, indium, gallium and others. 



The separation of the various colors, due to the unequal 

 refrangibility of the differently colored rays, is known as 

 dispersion. We shall speak again of the unequal refrangibility 

 of differently colored rays in connection with lenses and one 

 of their defects, known as chromatic aberration. 



LENSES. A lens is a piece of transparent substance bounded 

 by curved surfaces (one surface may be plane), and, according 

 to the curvature, it may be spherical, cylindrical, elliptical or 

 parabolic. Lenses used in optics are always spherical or ap- 

 proximately so. They are usually made either of crown glass, 

 which is free from lead, or of flint glass, which contains lead, 

 and is more refractive than crown glass. In virtue of the 

 spherical surfaces, a lens has the property of causing rays of 

 light which traverse it either to converge or diverge. There 

 are six types of lenses according to the manner of combining 

 concave, convex and plane surfaces. These are represented in 

 section in Fig. 8. 



They are named, in the order of the numerals, double convex, 

 plano-convex, converging concavo-convex, double concave, plano- 

 concave, diverging concavo-convex. 



Lenses 3 and 6 are also called meniscus lenses, from the 

 resemblance to the crescent-shaped moon. The first three are 

 thicker in the middle than on the edges and have the power of 



