BCIOPTICON MANUAL. 



15 



can publishers, Benerman & Wilson, Of course one may 

 successfully operate the Sciopticon, or even excel in 

 photography, without a critical knowledge of lenses; 

 but a very short, connected showing of their properties, 

 with diagrams, will doubtless prove acceptable to many 

 who use the Sciopticon, or who are interested in pho- 

 tography. 



THE FORM OF LENSES. 



The convex, or converging lenses. 1, 2, and 3 (Fig. 5), 

 called biconvex, plano-convex, and meniscus, are thicker 



Fig. 5. 



in the centre than on the margin. The concave, or dis- 

 persing lenses, 4, 5, and 6, called biconcave, plano-concave, 

 and concavo-convex, are thinner in the centre than on 

 the margin. A line through the centre of these lenses, 

 from side to side, would show the axis of each lens. 



PENCILS OF RATS AND THEIR ILLUSTRATIONS. 



A pencil of rays considered in reference to its direction 

 and the points in the image which it illuminates, may 

 be represented by a simple straight line, as in Fig. 1 ; 

 but in most cases, when the action of lenses on its rays is 

 considered, it must be shown as a bundle of rays, as in 

 Fig. 4. The pencil in Fig. 6 differs from df in Fig. 4, in 

 having middle rays represented as well as marginal, and 



