104 THE WAVE THEORY OF LIGHT. 



tionsof .sLiiTouiiding-niodia the appuroiit action at a distanco of olcctrieal 

 and magnetic s3'stoms. Faraday was recompensed for his boldness b}'^ 

 the discoveiy of induction. 



And since induction acts even across a space void of ponderable 

 matter, one is forced to admit that the active medium is precisely that 

 which transmits the luminous waves — the ether. 



The transmission of a moveuient by an elastic medium can not be 

 instantaneous; if it is truh' luminous ether that is the transmitting 

 medium, ought not the induction to be propagated with the velocity of 

 luminous waves ? 



The verification was dilficult. Von llelmholtz, who tried the direct 

 measurement of this velocity, found, as Galileo formerly, for the 

 velocity of light a value practically infinite. 



But the attention of physicists was attracted by a singular numer- 

 ical coincidence. The relation between the unity of electrostatic quan- 

 tity to the electro-magnetic unit is represented by a number precisely 

 equal to the velocity of light. 



The illustrious Clerk Maxwell, following the ideas of Faraday, did 

 not hesitate to see in the relationship the indirect measure of the 

 velocity of induction, and by a series of remarkal)le deductions he 

 built up this celebrated electro-magnetic theory of light, which idiMiti- 

 fies in one mechanism three groups of phenomena completely distinct 

 in appearance — light, electricity, and magnetism. 



But the abstract theories of natural phenomena arc nothing without 

 the control of experiment. 



The theory of ^laxwell was submitted to proof, and the success sur- 

 passed all expectation. The results are too recent and too well known, 

 especiall}^ here, for it to be necessary to insist upon them. 



A 3'oung German physicist, Henry Hertz, prematurely lost to sci- 

 ence, starting from the beautiful analysis of oscillatory discharges of 

 Von Helmholtz and Lord Kelvin, so perfectly produced electric and 

 electro-magnetic waves that these waACs possess all the properties 

 of luminous waves. The onh^ distinguishing peculiarity is that their 

 vibrations are less rapid than those of light. 



It follows that one can reproduce with electric discharges the most 

 delicate experiments of modern optics — reflection, refraction, diftrac- 

 tion, rectilinear, circular, elliptic polarization, etc. But I must stop, 

 gentlemen. I feel that I have assumed too weighty a task in endeav- 

 oring to enumerate the whole wealth which waves of transverse vibra- 

 tions have to-day placed in our hands. 



I said at the beginning that optics appeared to me to be the direct- 

 ing science in modern physics. 



If any doubt can have arisen in your minds, I trust this impression 

 has been effaced to give place to a sentiment of surprise and admira- 



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