R E F 



The following are for a convex one : 



If it be not thought proper to neglect t the thickness of the lens, then 

 in the case of parallel incident rays the equation is 



3. If the lens be a sphere. 



P 



of focus 

 then 



. If the lens be a sphere. 



ut q distance of the focus of incident rays from the centre ; q' =do. 

 ocus of rays after the first refraction ; q" = do. after two refractions j 



Cor. 1. To find the principal focus, suppose q infinite, or = o ; 



.'. F -r m ^r-' TIl n? ative si n meaning that the focus 

 .is on the opposite side from whence the light proceeds. 



o Q 



Cor. 2. If the sphere be glass, m = , and F = r. If water, m = 



4ri and F - 2 r. 



3 



4. If there be a conapoxiu-l lens, or a system of lenses placed close to 

 each other, whose principal focal distances are F, F', F" ...... lE^ n \ and F 



^ 



is the distance of the focus after ali the refractions ; then 



] _ m I m' 1 m" 1 in\ n ) I 



~~ ~~ " 7 " 



Hence if, with Mr Herschel, we call the reciprocal quantity -^ th 

 ytower of a lens, we have the following" enunciation, 



" The power of any system of lenses is ths sum of tfie powers of the c<?f- 

 ponent lenses. 1 * 



swr 



