W. B. CartmeJ 



A formula was therefore used which may be deduced as fol- 

 lows : If a piece of mica of thickness t is traversed by a ray 

 which meets it at an angle of incidence i as shown in the sketch, 

 the length of the path of the ray will be increased by (/a/— -^), if the 

 index of refraction of the mica is represented by /a. 



Also 



/=f sec r=- 



l/i- 



sin'r 



«'=:/ COS (z — r) 

 fii — d=/\_ix — COS {z — ;■)] 



1 08 



