18 Professor Airy on the Spherical Aberration 



K' = -a-(|.»-+im|+?|5»0. 



«-"'a-"-f.»M'"=) 



1st. R + R + R" may be made = o, or the distortion may be 

 entirely destroyed. 



2d. If the first lens be plano-convex, its plane side toward the 

 object-glass, ^ = ^ • ^ • ^^ equi-convex, -^ = g ]g^2 • If Plano- 

 convex, with its convexity toward the object-glass, i? = g"- j^' 



3d. The same values apply, without alteration, to the third lens. 

 For the second, their order must be reversed, and they must be made 

 negative. 



4th. The best combination of common lenses is, all plano- 

 convex, with their convexity toward the object-glass : then 



If we make either the first or the third equi-convex, then 



R + R' + R = --^^. 

 If both be equi-convex, 



a* 



5th. A single lens to produce the same power must have a focal 



227 a" 



length M. and, therefore, its smallest value of Q is -^ . -^ . 



