THE PROGRESS OF PHYSICS. 177 



If light consists of waves, the question naturally 

 arises : (i Waves in what ? " Especially when the 

 study of polarisation and double refraction showed 

 that the elastic properties of air or water which act 

 as media for sound, will not work when applied to 

 the interpretation of light-phenomena, the conception 

 of the ether forced itself upon physicists. 



At first it seems to have been thought of as an ex- 

 ceedingly rare form of matter pervading space and 

 composed of discrete particles; and it was of course 

 invested with the requisite elastic qualities. But 

 gradually the conception became subtler. 



Identification of Luminiferous and Electro-mag- 

 netic Ether. The luminiferous ether was invented 

 as a conception which fitted the facts known in re- 

 gard to light. Similarly Faraday and Clerk Max- 

 well postulated a special ether for electrical and 

 magnetic phenomena. But when Clerk Maxwell 

 made the further step of showing that one hypo- 

 thetical medium would suffice for the interpretation 

 of luminous, electric, and magnetic radiations, the 

 case for the ether became much stronger. 



That the ether is a necessary conception in modern 

 physics seems to be unanimously admitted by experts, 

 but how exactly the ether is to be conceived of re- 

 mains quite uncertain. 



For some imagine it as an elastic solid, others as 

 a labile fluid, others as a vortex sponge (a phrase 

 which we cannot pretend to explain), and others 

 otherwise. 



The modern conception of the ether is that of an 

 absolutely continuous medium, " without breaks or 

 gaps or spaces of any kind in it," " a universal con- 

 nector," permeating space whether otherwise occu- 

 pied or not, susceptible of stress, but not of locomo- 



