I12 THE POPULAR SCIENCE MONTHLY 
Kinstein abandons the ether, which he declares to be the totally unneces- 
sary conception. Hinstein 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 ¢ measured by a clock standing 
still, we substitute a quantity wv’ denoting a new length and 7’ 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 z and ¢ to the variables z’ and z’. 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 ¢ or ¢’ or whether the distance measured by the 
scale is z ora’. 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 Ritz 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. Ritz, 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 Poincaré 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 
