404 



PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



the ray reflected from A being received by B. B is 

 capable of rotation on cc. In the position shown on 

 the right-hand side of the figure the ray will be re- 

 flected by B ; in the position of the left-hand figure it 

 will be extinguished. 



Fig. 183. Apparatus for Polarisation by Reflection. 



The angle which the incident ray must make with 

 the normal to the reflecting surface in order to be 

 completely polarised, is the ANGLE OF POLARISATION. 

 For glass the angle is 54 35', for water, 52 45', for 

 quartz 57 32', and for diamonds 68. The PLANE 

 OF POLARISATION is the plane in which the light be- 

 comes polarised. 



Doubly refractive substances may be 

 detected by means of a polarising apparatus. Let 

 two Nicol prisms be placed in line with their 

 principal planes at right angles to one another, the 

 extraordinary ray, transmitted by the first prism, will 

 not be transmitted by the second, because it is at 

 right angles ; no light will, therefore, be visible on 

 looking through the second prism. In this condition 



