3i2 PHYSIOLOGICAL PHYSICS. [Chap. xxv. 



faces are not parallel but form an angle with one 

 another, do not emerge parallel to one another. 

 A figure whose surfaces are inclined to one another 

 at an angle is called a PRISM. A principal section of 

 a prism is represented in Fig. 138, a section, that is, 



made by a plane perpendicular 

 to both surfaces. The appear- 

 ance is that of a wedge. The 

 line R, in which the faces PR 

 and SR meet, is called the edge 

 of the prism ; the line PS is the 

 base of the prism. 

 Fig. 138. Prism. Now let a ray AB be 



incident on the face PR of 



the prism making an angle of incidence with the 

 normal nn'. On entering the prism the ray is re- 

 fracted towards the normal, and takes the course BB'. 

 On emerging from the prism the ray is again refracted, 

 but this time, because passing into a rarer medium, 

 the refraction is away from the normal n"n", and 

 takes a course B". An eye placed at B" will see the 

 ray as if it proceeded from b. The ray of light is 

 thus refracted towards the base of the prism by the 

 action at both surfaces. The angle B"OA' formed by 

 the direction of the incident ray with the direction of 

 the emergent ray expresses the amount of deviation 

 the light has undergone in passing through the prism, 

 and is called the angle of deviation. Other things 

 being equal, it depends on the refractive index of the 

 material forming the prism. There is a value of this 

 angle in which the refracted ray BB' would not 

 emerge from the side of the prism, but would so fall 

 on the internal surface of RS as to be totally reflected, 

 in which case the ray would be directed by the reflec- 

 tion towards the base within the prism. 



Lenses are transparent media, which refract rays 

 of light passing through them. They have curved 



