332 POPULAR LECTURES AND ADDRESSES. 



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 talking 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 transmission. 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-vibrations are 

 to and fro in, this is transverse to, the line of pro- 

 pagation. Here is another, perpendicular to the 

 diagram, still following the law of transverse 

 vibration ; here is another, circular vibration. 

 Imagine a long rope, you whirl one end of it and 

 you see a screw-like motion running along, and 

 you can get this circular motion in one direction 

 or in the opposite. 







