230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



tion of the principle of relativity in the case of acceleration. Then 

 considering that as a whole the principle of relativity has failed, he 

 keeps the Lorentz transformation only for very small changes in 

 the variables. Considerable discussion has passed between him and 

 Einstein, but we will not follow the details.^ 



Admitting that these theories will have a lasting effect upon 

 science, in the future new experiments will be required and a more 

 powerful theoretical effort than that of the past. We will close our 

 exposition of this question by citing the opinions of several skeptical 

 physicists who, from the beginning, have found the postulates upon 

 which the theory of relativity rests too absolute and to whose voices 

 we are now beginning to listen. 



The ether in the principle of relativity has been emptied little by 

 little of all its physical properties; it is represented now only by a 

 system of mathematical equations, those of Maxwell-Lorentz, and a 

 number, the velocity of light. It remains as the vehicle of radiant 

 energy without our questioning how. Ritz,^ following to the logical 

 conclusions such notions, proposes to renounce wholly the hypothesis 

 of an ether and to return to a theory very close to the old one of 

 emission. According to him, we need not speak of electric and mag- 

 netic fields, but only of electric charges acting upon each other. 

 We thus return to action at a distance but taking into account the 

 finite velocit}^ with which such action takes place. Consequently it 

 is necessary to throw away the partial differential equations of the 

 electric field and replace them with integrals (retarded potentials). 

 There is thus introduced an irreversibility of which the former equa- 

 tions could not take account. Mass at great velocities will remain 

 constant, but the force will vary. We thus arrive at another system 

 of mechanics. Against these new conceptions, the development of 

 which was unfortunately interrupted by the death of the author, 

 there are grave objections which have so far kept the majority of 

 theorists from adopting them, although they are perfectlj^ consistent 

 among themselves. 



Brillouin,'' on the other hand, makes the ether more substantial 

 than has been customary. There must be, according to him, a drastic 

 revision of the hypotheses relative to it. For example, its absolute 

 immobility, perfect permeability, homogeneity, isotropy, and the in- 

 variability of the velocity of light. Those upholding the principle of 

 relativity have themselves commenced to attack the last postulate, as 

 we have just seen. Now it will be the turn of the other properties. 

 We may come, through the increasing acuteness of our powers of 



1 Einstein, Annalen der Physik, vol. 38, p. 355 and 1059, 1912 ; vol. 39, p. 704, 1912. 



2 Ritz, Annales de chimle et de physique, vol. 13, p. 145, 1908, 



3 Brillouin, Scientia, vol, 13, p. 10, 1913. See the Revue g^p^rale des Sciences, Mar. 

 SO, 1913, p. 214. 



