278 KJEJLVIN 



lisation, 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 which the 

 spar has of double refraction, and produces the phenom- 

 enon which I now show you. I turn one prism round in a 

 certain direction and you get light a maximum of light. 

 I turn it through a right angle and you get blackness. 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 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 

 particular 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 reduction to a 

 very small quantity of light, not an absolute annulment as 

 we have in the case of the two Nicol prisms used conjointly. 

 As to the mechanics of the thing, those of you who have 

 never heard of this before would not know what I was talk- 

 , ing about ; it could only be explained to you by a course of 

 lectures in physical optics. The thing is this, vibrations of 

 light must be in a definite direction relatively to the line in 

 which the light travels. 



Look at this diagram, the light goes from left to right; 

 we have vibrations perpendicular to the line of transmis- 

 sion. There is a line up and down which is the line of 

 vibration. Imagine here a source of light, violet light, and 

 here in front of it is the line of propagation. Sound-vibra- 

 tions are to and fro in, this is transverse to, the line of prop" 



