R E F 



In the double concave lens, r' is negative, 



ID the double convex , r is negative, 



In thej?'ano-coKcww, either r' is infinite, or r is infinite and v* negs- 

 live j therefore putting r for the single radius 



F = 



In the i^ano-convex 



_L - m ~~ 1 

 ~y ~ ~ 



When in the double concave or double convex lens the radii are equal, 



F~ ~~ ' 7W ' ~r y r ~~ 2. (wz-i)* 



Since = 1 -f 



A" It A 



A" rr -^T-TT- Hence the fcllowing useful proper. 



tions, it being understood that F and /are the principal foci of rays com- 

 ing in a contrary direction. 

 A ? I A Q I : A F I A Q" -f- A F r7 



or QJ : / A : : o A : A g ^ }_ . , , 



The following- are corresponding values of A and A" for a concur 

 lens : 



GO ... 2 F ... F ... T j ... ... ~ ... F . 2 F 3 F ... 00 



2 F F ? 



F ... ~ F ... ,.. 3 ... ... F ... X ... 2 F ... ~ F ... F 



236 



