20 THE ART OF PROJECTING. 



double convex 

 lens c d when 

 they are con- 

 verged to a 

 focus, and aft- 

 erward they 

 Fig. 11. separate and 



fall upon the screen s. It may be noticed that this 

 lens, used next to the orifice, is represented as a 

 double convex lens in nearly every place in the book. 

 Such a lens, when properly constructed, has less 

 spherical aberration than any other form. To be thus 

 properly constructed one side should have a greater 

 convexity than the other, the radii of curvature for 

 the two sides being as one to six, but such lenses are 

 not enough better than the more common form of hav- 

 ing the two sides of equal convexity, to make any dif- 

 ference except for special work, if the lens is used as a 

 condenser : so that for this purpose a common double 

 convex lens, with the faces of equal curvature, will be 

 found to answer for most purposes. But a plano-con- 

 vex lens with the same focal length as the convex lens 

 is nearly as good. 



Here it may be remarked that whenever a plano- 

 convex lens is used it 

 should have its convex 

 side turned toward the 

 rays which are parallel, 

 or most nearly parallel. 

 Thus the lens (Fig. 12) 

 has its convex side to- 

 ward a, where the rays 

 are parallel, and should 

 Fig. 12. be so whether the rays 



