330 POPULAR LECTURES AND ADDRESSES. 



extinguished, it is absolutely black, as though the 

 tourmalines were opaque. This is an illustration 

 of what is called polarisation of light. I cannot 

 speak to you about qualities of light without 

 speaking of the polarisation of light. I want to 

 show you a most beautiful effect of polarising 

 light, before illustrating a little further by means 

 of this large mechanical illustration which you 

 have in the bowl of jelly. What you saw first 

 were two plates of the crystal tourmaline (which 

 came from Brazil, I believe) having the property 

 of letting light pass when both plates are placed 

 in one particular direction as regards their axes 

 of crystallisation, and extinguishing it when it 

 passes through them with one of the plates 

 held in another direction. Now I put in the 

 lantern an instrument called a " Nicol prism," 

 which also gives rays of polarised light. A Nicol 

 prism is a piece of Iceland spar, cut in two 

 and turned one part relatively to the other in 

 a very ingenious way, . and put together again 

 and cemented into one by Canada balsam. The 

 Nicol prism takes advantage of the property 



