ii2 THE POPULAR SCIENCE MONTHLY 



Einstein abandons the ether, which he declares to be the totally unneces- 

 sary conception. Einstein makes two postulates which are sufficient to 

 explain all phenomena now known. The first has been stated, the other 

 is that the velocity of light is the same when measured in any system. 

 By measures of this velocity, we can, therefore, not determine whether 

 the system is moving or at rest. Clothed in a more mathematical form, 

 such as has been given by Minkowski, we may state the principle as 

 follows : If instead of the distance x measured in the direction of the 

 motion of the system, and of the time t measured by a clock standing 

 still, we substitute a quantity x' denoting a new length and t' a new 

 time, then all the equations of electro-dynamics and presumably all 

 those of physics admit of a so-called linear transformation of the 

 variables x and t to the variables x' and V . Under this transformation, 

 the equations remain, therefore, absolutely unchanged. It is accord- 

 ingly impossible by any observations to determine whether the time 

 measured by the clock is t or V or whether the distance measured by the 

 scale is x or x' . As has already been said, this proposition is of the most 

 startling nature and results in connecting the notions of time and space 

 in a most unexpected manner. In fact we may briefly sum up by saying 

 that we can not tell where a point is until we know when, and we can 

 not tell the time when until we know the place where ! If we accept this 

 principle it may be necessary to totally abandon the hypothesis of the 

 ether. Certain writers, such as Eitz in France, have established a system 

 of electrodynamics in which the conceptions of the ether and of the 

 magnetic and electric fields have totally disappeared. Eitz, for instance, 

 bases his whole theory upon the so-called retarded potentials of Lorentz, 

 by means of which the action of any electric charge, fixed or in motion, 

 is calculated at any other time and place by means of definite integrals. 

 This conception has been vigorously maintained : in England I may 

 mention the name of Mr. Norman Campbell, who in a recent article in 

 the Philosophical Magazine, as in his excellent modern treatise on elec- 

 tromagnetic phenomena, has vigorously assailed and even ridiculed the 

 school of those whom he calls the " etherealists," as making use of a 

 totally useless and hindering conception. 



In 1900 Professor Poincare had already asked the question, " does 

 the ether exist ? " This I may characterize as now the question of the 

 hour. To sum up what I believe to be the state of the case, certain 

 phenomena concerning radiation and the distribution of energy in the 

 spectrum have led to the necessity of certain assumptions which seem 

 difficultly explained on the ether hypothesis. Sir Joseph Thomson also, 

 in order to explain certain phenomena connected with the emission of 

 electrons from metals under the action of ultra-violet light and other 

 phenomena with which he is particularly competent to deal, has pro- 

 pounded the hypothesis that a wave of light is not uniform but is some- 

 what of a fibrous nature. I find it difficult to see how such a hypothesis 



