OPTICAL INSTRUMENTS. 



179 



Fig. 088. 



Pig. 689. 



Lenses. A plane glass (Fig. 686) 

 has two plane surfaces parallel to 

 one another. 



A double convex lens (Fig. 687) 

 is bounded by two convex spherical 

 surfaces, whose centres are on op- Fig. 687. 



posite sides of the lens. It is 

 equally convex when the radii of 

 both surfaces (that is, the distances 

 from the centres to the circumfer- 

 ences of the circle they belong to) 

 are equal, and unequally convex, 

 when their radii or distances are 

 unequal. 



A plano-convex lens (Fig. 688) 

 is bounded by a plane surface on 

 one side, and by a convex one on 

 the other. 



A double concave lens (Fig. 689) 

 is bounded by two concave spheri- 

 cal surfaces, whose centres are on 

 opposite sides of the lens. Fig. 690. 



A plano-concave lens (Fig. 690) 

 is bounded by a plane surface on 

 one side, and a concave one on the 

 other. 



A meniscus (Fig. 691) is bound- Fig. 691. 



ed by a concave and a convex 

 spherical surface ; and these two 

 surfaces meet, if continued. 



Price, plane glass, - 12 cts. 



" double convex lens, - - 38 cts. 



" double concave " 38 cts. to 62 cts. 



" plano-convex " 50 cts. to 62 cts. 



" plano-concave " 50 cts. to 62 cts. 



" meniscus, 50 cts. 



" set of six lenses, - - - $1.50. 



Models to show the focus of a Lens. This is an arrange- 

 ment of wires as represented in the cuts (Figs. 692 and 693, 

 next page), as having a wooden disc in the form of a lens, 

 through which the wires pass, representing the rays of light 

 converging to a focus ; the whole handsomely japanned. 



