Chap, xxv.] LENSES. 313 



surfaces, and the direction which the rays take on 

 emerging from the lens depends on the nature of the 

 curvature. The CHIEF FORMS OF LENSES are shown 

 in Fig. 139. They are convex or concave. In the 

 figure, A is doubly convex, B plano- 

 convex, c concavo-convex, while D 

 is doubly concave, E plano-concave, 

 and F convexo-concave. 



Convex lenses, owing to the 

 nature of their curvature, cause Fig. lou. Lenses. 

 the rays of light issuing from them 

 to converge to one another. They are, therefore, 

 called converging lenses. Supposing the surfaces of 

 the lens to be parts of spheres, the centres from which 

 the sphere would be described in each case are called 

 the centres of curvature of the lens, and the straight line 

 joining the two centres is the principal axis of the lens. 



Each lens has also what is called its OPTICAL 

 CENTRE or simply its CENTRE, which lies on the prin- 

 cipal axis, and is such that every ray passing through 

 it emerges from the lens in a direction parallel to that 

 in which it entered the lens. In a doubly convex or 

 concave lens the centre is in the interior of the lens ; 

 in plano-convex or plano-concave lenses it is on the 

 convex or concave surface ; in a concavo-convex lens 

 it is outside the lens. Any straight line passing 

 through the centre is a SECONDARY AXIS. 



1. Let parallel rays of light fall on a convex lens, 

 they are so refracted as to meet in a point on the 

 other side, and this point is called the principal focus, 

 the distance from it and the lens being the focal dis- 

 tance of the lens. In Fig. 140 LL' is a convex lens. 

 The ray 5 which falls perpendicularly on the surfaces 

 passes straight through unaffected. The other rays 

 123 and 4, fall obliquely, and are, therefore, 

 refracted. In each case the refraction occurs twice, 

 first on entering the lens, and secondly on issuing from 



