- . 



ETHER. 



and rwwlsions of electrified and magnetic bodies, and the cohesive forces 

 i the interior of bodies, without attempting to account for these forces. 



Newton himself, boweTcr, endeavoured to account for gravitation by dif- 

 fojao,, ,/ puHHiro in an wther (see Art. ATTRACTION*, VoL m. p. 64); but 

 k* did not publish his theory, "because he was not able from experiment and 

 ubtm fit inn to give a satisfactory account of this medium, and the manner of 

 to opffmtfr" in producing the chief phenomena of nature." 



On the other hand, those who imagined aethers in order to explain phe- 

 nosMBa could not specify the nature of the motion of these media, and could 

 not prow that the media, as imagined by them, would produce the effects 

 UMT were meant to explain. The only {ether which has survived is that 

 which was invented by Huygens to explain the propagation of light. The 

 for the existence of the luminiferous aether has accumulated as addi- 

 fitnnm**** of light and other radiations have been discovered; and the 

 of this medium, as deduced from the phenomena of light, have been 

 to be precisely those required to explain electromagnetic phenomena. 



Function o/ the other in the propagation of radiation. The evidence for 

 the undulatory theory of light will be given in full, under the Article on 

 LIGHT, but we may here give a brief summary of it so far as it bears on 

 the existence of the nether. 



That light is not itself a substance may be proved from the phenomenon 

 of interference. A beam of light from a single source is divided by certain 

 optical methods into two parts, and these, after travelling by different paths, 

 are made to reunite and fall upon a screen. If either half of the beam is 

 stopped, the other falls on the screen and illuminates it, but if both are allowed 

 to pass, the screen in certain places becomes dark, and thus shews that the 

 two portions of light have destroyed each other. 



Now, we cannot suppose that two bodies when put together can annihilate 

 each other; therefore light cannot be a substance. What we have proved is 

 that one portion of light can be the exact opposite of another portion, just 

 f +o is the exact opposite of a, whatever a may be. Among physical 

 quantities we find some which are capable of having their signs reversed, and 

 others which are not. Thus a displacement in one direction is the exact 

 opposite of an equal displacement in the opposite direction. Such quantities 



* [p. 485 of the present vol.] 



