320 



"WELLS'S NATURAL PHILOSOPHY. 



bounded by a plane surface, and the other by a conv'ex 

 surface. It is represented at B, Fig. 259. 



A meniscus, or concavo-convex lens is convex on one 

 side and concave on the other, as at C, Fig. 259. 



To this kind of lens the term "periscopic" baa recently been applied, from 

 the Greek, signifying to view on all sides. 



A double concave lens is concave upon both sides, as 

 at D, Fig. 259. 



A plano-concave, or single concave lens, is bounded on 

 one side by a plane, and on the other by a concave sur- 

 face, as at E, Fig. 259. 



A concavo-convex lens is bounded on one side by a 

 concave, and on the other by a convex surface, as at F, 

 Fig. 259. 



The six varieties of simple lenses are divided 

 into two classes, which are denominated con- 

 ver2;in2r and diverf]^infi; lenses, since the one 

 class renders parallel rays of light falling upon them con- 

 vergent, and the other class renders them divergent. 



In Fig. 259 ABC are converging, or collecting lenses, and D E F diverg- 

 ing, or dispersing lenses. The former are thickest at the center ; the latter 

 are thinner at the center than at the edges. 



In the first class it is sufficient to consider only the double-convex lens, 

 and in the second class only the double-concave lens, since the properties of 

 each of these lenses apply to all the others of the same claSs. 



For optical purposes lenses are generally made of glass, but in soma 

 instances other substances are employed, such as rock-crystal, the dia- 

 mond, etc. 



What is the III al^ ^liG various kinds of ienses there must 



•faTcMf"'" be a point through which rays of light passing 



experience no deviation ; or in other words, 



Into how many 

 classes may- 

 lenses be di- 

 vided? 



