REFRACTION THROUGH PRISMS. 



459 



the edge, and the angle between them is the i refracting 

 angle.' 



In fig. 261 J., let a b c be the section of a prism, of 

 which a is the edge, a b and a c the refracting surfaces. 

 For a ray of light d e the perpendicular at the point of 



A 



Fia, 261. 



incidence is feg. The angle of the ray with the 

 perpendicular is smaller in glass than in air. The ray 

 proceeds therefore within the prism in the direction 

 e A, the angle h e g being less than d ef. At h the ray is 

 refracted a second time ; g hi is now the perpendicular 

 at the point of incidence, and ghe the angle of in- 



