THE WAVE THEORY OF LIGHT. 331 



which the spar has of double refraction, and 

 produces the phenomenon which I now show 

 you. I turn one prism round in a certain direc- 

 tion and you get light a maximum of light. I 

 turn it through a right angle and you get black- 

 ness. I turn it one quarter round again, and 

 get maximum light ; one quarter more, maximum 

 blackness ; one quarter more, and bright light. 

 We rarely have such a grand specimen of a 

 Nicol prism as this. 



There is another way of producing polarised 

 light. I stand before that light and look at its 

 reflection in a plate of glass on the table through 

 one of the Nicol prisms, which I turn round, so. 

 Now if I incline that plate of glass at a par- 

 ticular angle rather more than fifty-five degrees 

 I find a particular position in which, if I look at it 

 and then turn the prism round in the hand, the 

 effect is absolutely to extinguish the light in one 

 position of the prism and to give it maximum 

 brightness in another position. I use the term 

 " absolute " somewhat rashly. It is only a reduc- 

 tion to a very small quantity of light, not an 



