38 ORGANIC EVOLUTION CONSIDERED 



light, — refraction, diffraction, polarization, colors 

 shown by films, — can only be fully explained by the 

 undulatory theory which necessarily involves a medi- 

 um. Besides, this theory easily explains the differ- 

 ent colors of light, the differences in the qualities of 

 heat, and the ultra violet invisible rays. 



In order to explain the polarization of light, it is 

 necessary to assume that the ether vibrates trans- 

 versely to the line of radiation, instead of longitudi- 

 nally, as in the case of sound. 



What, then, must be the properties of a medium 

 which can transmit radiant energy at the rate of 

 186,000 miles per second by means of transverse 

 vibrations? 



Among the various authors who have expressed 

 their views as to the nature of ether, no one stands 

 higher than Lord Kelvin. He expresses his views 

 in the following language: 



" 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 discoveries in electricity 

 and the motions of comets and the marvelous 

 spurts of light from them, tend to show cracks in 

 the luminiferous ether — show a correspondence be- 

 tween 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 scien- 

 tific 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 prodig- 

 ious in proportion to its density that the vibrations 

 of light in it have the frequencies I have mentioned, 

 with the wave-lengths 1 have mentioned. The fun- 

 damental question as to whether or not luminifer- 



