400 



PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



and extraordinary rays. The effect of this prism is 

 shown in Fig. 180, where the line IIC/H is the line in 

 which the cut was made. A ray of light ab falling 

 on the prism undergoes double refraction into the 

 extraordinary ray bd, and the ordinary 

 be. The extraordinary ray passes on 

 through the Canada balsam junction, and 

 emerges at c in a direction parallel to the 

 entering ray ab. The ordinary ray meets 

 the balsam at c and is totally reflected. 

 Only one of the two rays, therefore, tra- 

 verses the prism. This ray is, however, 

 found to be of a character different from 

 an ordinary beam of light. If two 

 ^i c l' s prisms be taken and the one 

 placed in a line with the other so that 

 the extraordinary ray which passes through the 

 first is able to enter the second, it would be expected 

 that the ray from the first prism would undergo 

 double refraction oil entering the second, that the 

 ordinary ray would be totally reflected as in the first, 

 but that the extraordinary ray would pass 011 and a 

 circle of light would appear on looking through the 

 second Nicol. In one position of the prisms, namely, 

 when they are in such a position that their principal 

 planes are parallel, the circle of light is seen, and at 

 its greatest intensity. If, however, one of the prisms 

 be rotated on the other, the circle of light becomes 

 less brilliant, and as the rotation is continued it 

 becomes more and more dim, till, when the prism has 

 passed through a right angle, the light is extinguished. 

 If the rotation be carried on the light returns slowly, 

 till, after going through another right angle the light 

 is a second time at its greatest intensity \ and, if one 

 continues turning, the light will again disappear, and 

 again be restored. In two positions opposite to one 

 another the light is most intense, and in other two 



