THE WAVE THEORY OF LIGHT. 329 



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 regard 

 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 prodigious 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 at- 

 tracted 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 



ihem toward the light you see the white light 

 til round, i.e. they are transparent. If I turn 

 ound one of these tourmalines the light is 



