OPTICAL IMAGES. 



26. A plano-convex lens, fig. 15, has one side, A c, flat, and 

 the other convex. 



A plano-concave lens, fig. 



16, has one side, A' c', flat, 

 and the other concave. 



A double convex lens, fig. 



17, has both sides convex, 

 and a double concave lens, fig. 



18, both sides concave. 



It is not necessary that the 

 convexities of the sides in the 

 one, or the concavities in the 

 other, should be equal. The degree of convexity or concavity 

 will depend on the radius o B or o' B' of the sphere of which 



the lenticular surface is a part. The less that radius is, the 

 greater will be the curvature of the surface. Thus, if o B be greater 



than o' B', the sur- 



Fig ' 1? ' face A' c' will be 



more convex (fig. 

 17), or more con- 

 cave (fig. 18), than 



AC. 



o/ A concavo-convex 

 lens has one side, 

 A c, fig. 19, concave 

 and the other con- 

 vex, the concavity, however, being greater than the convexity. 

 A meniscus has also one side, A c, fig. 20, concave, and the 

 other convex, but, on the contrary, the convexity is greater than 

 the concavity. 



27. A line, o o', which joins the centres of the two lenticular 

 surfaces in figs. 17, 18, 19, and 20, and which passes through the 

 centre of the lenses, and one which, in figs. 15 and 16, is drawn 

 from the centre at right angles to the flat surface, and passing 

 through the centre of the lens, is called the AXIS OF THE LENS. 

 94 



