Focal Lines of Cylindrical Lenses. 73 



deviation due to the second lens being = ^(«i — 8), since -I 



is the deviation per unit length along the line ^r/, and v 1 -—B 

 is the distance /gu/. 



\ 



Let Z, be the semi-length of the line. 

 From fig. 11 we have 



h = 



Ji l v 1 h 



h 



and 



or 



h—h _ ft 

 6 ~~ 



(*i-«) , 



(11) 



h 



Vi 



-0(KW 



Substituting this value of // 3 in (11) and simplifying, 

 we get 



If #!=/!, or the incident line is parallel, 



7i=/ii/i (I4 + m)- 



(b) The line jmrallel to the axis of the second lens which 

 is horizontal. 



Produce the refracted ray h z d' (fig. 11) to meet the 

 horizontal plane in the point v. 2 , v 2 l . 



Then v 2 e = l 2 is the semi-length of the focal line. 



