THE WAVE THEORY OP LIGHT 277 



bullets on the upper side. It is brittle like the Trinidad 

 pitch or Burgundy pitch which I have in my hand you 

 can see how hard it is but when left to itself it flows like 

 a fluid. The shoemakers' wax breaks with a brittle fracture, 

 but it is viscous and gradually yields. 



What we know of the luminiferous ether is that it has the 

 rigidity of a solid and gradually yields. Whether or not it 

 is brittle and cracks we cannot yet tell, but I believe the dis- 

 coveries in electricity and the motions of comets and the 

 marvellous spurts of light from them, tend to show cracks 

 in the luminiferous ether show a correspondence between 

 the electric flash and the aurora borealis and cracks in the 

 luminiferous ether. Do not take this as an assertion, it is 

 hardly more than a vague scientific dream: but you may re- 

 gard the existence of the luminiferous ether as a reality 

 of science; that is, we have an all-pervading medium, an 

 elastic solid, with a great degree of rigidity a rigidity so pro- 

 digious in proportion to its density that the vibrations of light 

 in it have the frequencies I have mentioned, with the wave- 

 lengths I have mentioned. The fundamental question as to 

 whether or not luminiferous ether has gravity has not been 

 answered. We have no knowledge that the luminiferous 

 ether is attracted by gravity; it is sometimes called im- 

 ponderable because some people vainly imagine that it has 

 no weight: I call it matter with the same kind of rigidity 

 that this elastic jelly has. 



Here are two tourmalines; if you look through them to- 

 ward the light you see the white light all round, *. e. they are 

 transparent. If I turn round one of these tourmalines the 

 light is 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 polaris- 

 ing 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 crystal- 



