REVIEWS 543 



enunciated to deal with the problems connected with the propagation of electro- 

 magnetic disturbances— light — in moving media, which had formed one of the 

 chief fields for experimental and theoretical researches at the end of the nineteenth 

 and the beginning of this century. Neither the equations of Maxwell, as modified 

 by him for a moving ponderable medium, nor those of Hertz and Heaviside, gave 

 results in agreement with experiment ; for the work of Fizeau, and later Michelson 

 and'Morley, showed that a moving medium imparted in virtue of its motion to 

 the light travelling through it— or to the contained aether, on the aether hypothesis 

 —not the full velocity of the medium, as demanded by the Hertz-Heaviside 

 equations, nor a velocity about half that of the medium, as required by Maxwell's 

 modifications, but one determined by the index of refraction of the medium, 

 according to Fussel's well-known expression, which could not be deduced on the 

 basis of the older electromagnetic theory. H. A. Lorentz, in his famous essay, 

 with the aid of his hypothesis of " ions," or electrons as we should now say, fixed 

 in the ponderable body, deduced the right value of this coefficient ; but to explain 

 the results of the other experiments of Michelson and Morley, which showed that 

 the measured velocity of light from a terrestrial source was independent of the 

 direction of propagation with respect to the earth's motion— that is, independent 

 of the motion of the source — Lorentz had to introduce the ad hoc hypothesis of a 

 slight contraction of a solid body in the direction of its motion through the aether, 

 the so-called Lorentz-Fitzgerald hypothesis. This most artificial assumption was 

 accepted for want of a better : it is a merit of the relativity theory that, even if its 

 fundamental concepts be very difficult of acceptance, once they are accepted it 

 gives an account of all positive and negative experimental results, without special 

 assumptions for special cases, and it introduces symmetrical transformations of 

 the widest application. 



With a historical introduction on these lines, tracing the origin and growth of 

 the problems of the propagation of light in moving media, and the various attempts 

 to solve them, and showing the relativity theory to be the outcome of attempts to 

 explain experimental results, Dr. Silberstein occupies the first chapters of this 

 Theory of Relativity. This introductory treatment goes far to make the theory 

 comprehensible, and to show the need of it ; it is excellently carried out. He then 

 proceeds to deal with Einstein's conception of simultaneity, based on the trans- 

 mission of light signals for co-ordinating time systems in different places. Two 

 events in different bodies, in general moving relatively to one another, are defined 

 as simultaneous by the aid of the postulate that the time of passage of the light 

 from one body to the other, and of its return, shall be equal, time of arrival and 

 departure from the second body being measured at that body. This principle is 

 of fundamental importance; as the author says, "To have initiated a critical 

 analysis of the conception of simultaneity at all is certainly a great merit of 

 Einstein's." From it, together with the second postulate that the measured 

 velocity of light is independent of the relative motion of the source, can be deduced 

 the so-called " Lorentz transformations," which transform the laws of physical 

 phenomena invariantly from one system to a second moving with any relative 

 translation. One of the first results of the theory is the reduction of the aether 

 to nothingness. For as no system has any preference over any other with respect 

 to the propagation of light, we are left with a choice between no aether at all or, 

 since every body may be considered as having an equal right to its own aether, a 

 triple infinity of aethers — one supposition being as comforting as the other. 



Passing on to consider various representations of the Lorentz transformation, 

 the author gives an account of Minkowski's celebrated work, showing that the 



