ELliCTRO-MACiNETIC TllEOKV Oi- LIMIT. 285 



The Electromagnetic Theory of Light. 



" I have cleared the electrotiiagnetic theory of Hght from all 

 unwarrantable assumption, so that we may safely determine the 

 velocity of light by measuring the attraction between bodies kept 

 at a given difference of potential, the value of which is known in 

 electromagnetic measure." Thus Clerk Maxwell wrote in Sep- 

 tember, 1864. More than thirty years later the theory of light, 

 based upon the assumption of an elastically deformable sther 

 still held virtually unchallenged sway. While treatises on natural 

 philosophy could not fail to mention other i^hases of Maxwell's 

 views and work, they w^ere, as a rule, strangely silent on this 

 ]iarticular aspect thereof, and when theories were contrasted, it 

 was invariably the corpuscular against which the undulatory 

 theory was ranged. After Hertz's classical experiments. C. (j. 

 Knott and other text-book comj)ilers gave just due to Maxwell's 

 views. To-day and for many years past. Maxwell's electro- 

 magnetic theory has had universal acce])tance. With the latter 

 assertion Dr. L. Silberstein opens his little book on the elements 

 of the theory.* Maxwell seemed to express himself more 

 cautiouslv in his writings for publication than in the private letter 

 from which the first four lines of this paragraph are quoted. He 

 contented himself with the remark that his theory is not contra- 

 dicted by the results obtained for the velocity of light in air, and 

 interstellar space. Yet, says Dr. Silberstein, " there can be but 

 little doubt that the said agreement has had a decisive influence 

 upon the birth of the electromagnetic theory of light," and " Max- 

 well's predictions found a splendid corroboration a quarter of a 

 century later in the famous experiments of Hertz, who has not 

 only confirmed the existence of electromagnetic waves, but also 

 verified the approximate equality of their velocity of propagation 

 with that of light." The little book whence these extracts are 

 taken is an English version of a single chapter from the author's 

 three- volume treatise (written in the Polish language) and 

 originally published in Warsaw^) on " Electricity and Magnetism." 

 In concise form the essentials of the theory, as generally accepted 

 at the present time, are presented in such a manner as to afford 

 the reader both a complete and a relatively simple exposition of 

 the fundamentals of Maxwell's theory. The advantages of the 

 electromagnetic over the elastic theory are very lucidly set forth, 

 and the luminiferous aether, as originally conceived by Green, that 

 continuous elastic medium of which one has heard so much 

 during the last 80 years, becomes converted into an almost incom- 

 pressible jelly, for which a foam-a?ther was afterwards substi- 

 tuted b}' Lord Kelvin. All elastic theories of light, however, 

 are forced into somewhat strained assumptions in consequence 

 of the phen(^mena of polarisation, while the electromagnetic 

 theory, w ith its purely transversal waves, needs no such assump- 

 tions. 



Adopting the language of vectors, the author j)roceeds 



* 



L. Sillicrstein : " Elements of the electromag}tetic theory of light." 

 8vo.. pp. 4S. London : Longmans, Green & Co., 1918. 3s. 6d.. net. 



