R E F 



Let D = distance of the foci of incident and emergent rays, T th 

 thickness of the medium, then 



3. To determine the refraction which a ray experiences in passing 1 

 through a medium bounded by planes not parallel ; for example a tri- 

 angular prism_of glass, 



Let i =. vertical angle of the prism. 

 $ / of incidence. 

 ^ {_ of emergence. 

 of deviation of the incident and emergent rays j then 



S = <p 4. ^ ,, 



or if the / of the prism and the /'s. of incidence and emergence be ex- 

 ceedingly small, 



$ = (OT-1) ;. 



Cor. The / of deviation is a minimum when the incident and emer- 

 gent rays make equal /'s. with the sides of the prism. 



II. Refraction at spherical surfaces. 



1. A ray of light is refracted at a spherical surface bounding two dif- 

 ferent media ; given the point where it meets the axis, required the 

 point where the refracted ray meets the axis. 



Let r radius of the spherical surface, 



A and A' distances of Q and q from the refracting surface ; 

 then we may tabulate the results as follows : 



N4 



