CH. IX] LENSES AND THEIR PRINCIPAL FORMS 283 



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FIG. 167. 



SPHERICAL LENSES WITH THEIR FORMS AND PRINCIPAL 

 FEATURES. 



(1) Double convex lens showing the two spheres from which it was derived. 

 c-c' the centers of the two spheres with the principal axis of the lens on the line 

 joining the centers. 



(2) Double concave lens and the two spheres from which it was derived, c-c' 

 centers of the spheres and axis of the lens. 



(3) Plano-convex lens with the sphere from which it was derived. In this case 

 the axis is on the radius dividing the lens into two equal parts. 



(4) Double convex lens showing the two spheres from which it was derived; 

 r r' parallel radii; / /' tangents at the ends of the radii; c c' centers of the two 

 spheres, on the connecting line of which is the principal axis of the lens, and the 

 optic center (d). 



(5) Double concave lens showing the same features as in (4). 



(6) Plano-convex lens showing the same as in (5). In this case the radius of 

 the curved face is determined as usual, but that of the plane face may be considered 

 infinity, so that any line perpendicular to the plane face is a part of that radius. 

 As shown in the figure the center of the lens must be then at the convex surface 

 of the lens. 



(7) Plano-concave lens the parts are practically like (6). 



(8) Thin edge or converging meniscus lens with the two spheres from which 

 it was derived. The inner, concave face is from the greater sphere, and the 

 optic center (d) is wholly outside the lens. 



(9) Thick edge or diverging meniscus lens. In this case the concave face is 

 from the smaller sphere, and the center of the lens (d) is on the concave side. 



