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SCIENCE 



[N. S. Vol. XL. No. 1038 



The ether as Maxwell left it has two inde- 

 pendent properties, specific inductive capacity 

 and permeability, which may be regarded as 

 associated in the velocity of the electro- 

 magnetic wave passing through it. But the 

 equations apply only for a medium at rest or 

 at least approximately at rest, to a quasi- 

 stationary medium. It is fortunate that a 

 very coarse approximation to rest suffices; 

 otherwise the early workers would have lacked 

 encouragement. The new epoch, now about to 

 dawn, thus found its point of departure in the 

 motion of electrical systems. It has been in 

 the main an era of confusion and bewilder- 

 ment and one was to learn the hopelessness of 

 any fundamental proof in physics. Instead of 

 subjecting physics to the arbitrament of 

 dynamics, we see dynamics pleading at the 

 gates of electrical science, when electricity, 

 distraught within itself, has no fundamental 

 interpretations to offer. The troubles begin 

 with the study of the first-order effects of 

 moving optical systems, in the researches of 

 Pizeau (1851) ; they become grave in the 

 famous experiment of Michelson (1881) where 

 the effects to be observed are of the second 

 order. The speed of the earth, regarded optic- 

 ally from axes fixed in the ether, is zero. 

 The ether and the earth have no relative veloc- 

 ity. This is tantamount to a rejection of the 

 ether. Judge the consternation! As Max- 

 well's equation contained no direct reference 

 to the motion of the charged body, a first at- 

 tempt as I have already intimated was made 

 by Hertz (1890) to supply this deficiency; but 

 it was not of permanent value. The real inter- 

 pretative advance came from Lorentz, in 1892. 

 Although he fully realized and had endeavored 

 to explain away the Michelson difficulties, 

 Lorentz none the less boldly put his coordinates 

 in an absolutely fixed ether, penetrating all 

 bodies, even the atoms. He then went back to 

 the methods of Weber, but with this essential 

 difference that he included the whole dictum 

 of the Maxwellian electro-magnetics in his 

 postulates. The peculiar feature of the ether, 

 its permittance and permeability, were abol- 

 ished and in their place appears the velocity 

 and density of the electron, or charged particle. 



Electric fluid exists; magnetic fluid does not. 

 Lorentz then showed with consummate skill that 

 the equations of the classic electromagnetics of 

 Maxwell could be retained, that both the scaler 

 potential and the vector-potential would retain 

 their original form, would be invariant, so to 

 speak, if the time-variable were belated by the 

 interval consumed by light in passing from 

 the source to the point of application in ques- 

 tion. The profound originality and power of 

 this and the earlier Lorentz transformation 

 would perhaps not have been detected so soon, 

 but for the unexampled abundance of new re- 

 sources accruing to experimental physics at 

 this time. In 1892 Lenard had isolated the 

 cathode ray; Eontgen in 1895 discovered the 

 X-ray. As a sort of corollary of the X-ray 

 came the Becquerel-ray in 1896 ; the radium of 

 the Curies in 1897, soon to be interpreted as to 

 radiation by Thomson and Rutherford. The 

 year 1896 brought the Zeeman effect, virtually 

 predicted by Lorentz. The year 1898 brought 

 Thomson's electron. In these and similar re- 

 searches, bodies moving with a speed approxi- 

 mating that of light (easily exceeding c/10) 

 were for the first time in history, at the dis- 

 posal of the investigator. The new bodies, 

 showing an inertia or virtual mass depending 

 in a pronounced way on their sjjeed, made 

 havoc with Newton's laws and swept the classic 

 dynamics mercilessly out of the field, as an 

 arbiter of world phenomena. Theories such 

 as those of Lorentz, 1892, or of Larmor, 1894, 

 were now the only refuge. What could they 

 do, was the ardent question, to replace 

 dynamics ? 



Following the suggestion of Lorentz that 

 the moving system contracts in the direction 

 of motion, or at least apparently contracts to 

 the fixed witness, Einstein in 1905 was the 

 first to clearly perceive the iron logic of the 

 situation; and the logic of a desperate situa- 

 tion is aU there is in the theory of relativity. 

 Einstein saw that if systems were to be 

 interconsistent, time periods in the moving 

 system would have to expand in the same 

 second-order ratio to the ken of the fixed 

 observer, so that time specifications and time 

 frequencies may proportionately contract; or 



