REFRACTION OF LIGHT. 



401 



622. Lenses. Lenses are generally made of crown 

 glass which is free from lead, or of flint glass which con- 

 tains lead and has greater refractive power. The curved 

 surfaces are generally spherical. With respect to their 

 shape, lenses are of six kinds: 

 123 



Thinner at the middle than 

 at the edges. 



FIG. 304. 



(1.) Double-convex, 1 Thicker at tlie middle than 



(2.) Plano-convex, at the ed 



(3.) Concavo-convex, or meniscus, J 



The double-convex may be taken as the type of these. 



(4.) Double-concave, ~\ 



(5.) Plano-concave, I 



(6.) Convex-concave, or diverging \ 

 meniscus, J 



The double concave may be taken as the type of these. 

 (a.) The effect of convex lenses may be considered as produced by 

 two prisms with their bases in contact ; that of concave lenses, by 

 two prisms with their edges in contact. 



623. Centre of Curvature ; Principal Axis ; 

 Optical Centre. A double-convex lens may be de- 

 scribed as the part common to two spheres which intersect 

 each other. The centres of these spheres are the centres 

 of curvature of the lens. The straight line passing 

 through the centres of curvature is the principal axis of 

 the lens. In every lens there is a point on the principal 

 axis called the optical centre. When the lens is bounded 

 by spherical surfaces of equal curvature, as is generally the 

 case, the optical centre is at equal distances from the two 



