44 PRESIDENTIAL ADDRESS SECTION A. 



ment, was found to ncetl this same <ether as a vehicle of trans- 

 mission. At length Clefk Maxwell (1831-1879), an exceptional 

 mathematician and experimenter combined, finally summed up 

 a mass of subtle work (of which Faraday's was chief) by 

 ])roving that light consists of a comljination of electrical and 

 magnetic vibrations in the c'ether, the electric vibration Ijeing at 

 rig'ht angles to the magnetic. To explain this state of affairs 

 physically he had to de\ise an iether consisting of rotating tubes 

 (sort of vortices) conAexing the magnetic disturbance, surroimded 

 by particles, which are displaced by the tubular rotations, and 

 so give rise to electric currents, the resulting strain producing 

 electrostatic ])henomena. These conclusions rest on a set of fairly 

 simple differential equatirins, which describe the electro-magnetic 

 state of the aether. 



Maxwell's model has not carried conviction, and has been 

 variously modified by later thinkers, the last form ]>eing known 

 as the electron theory, an electron l)eing a minute something 

 charged with electricit}-, sometimes bound up in the constitution 

 of the atoms of matter, sometimes conveying its charges through 

 matter* Lord Kehin, whose mathematical and experimental 

 genius threw light on every branch of physical science during 

 his long and strenuous life i 1824 to 1908), made many attempts 

 at a " model " of the fether, and at the end of his life confessed 

 to the unsatisfactory results of everyone's efforts, adding: " I 

 fear the electro-magnetic theory does not help us much." How- 

 ever, more recentlv Hertz confirmed Maxwell's theorv bv effect- 

 ing the reflection and refraction of electric and magnetic waves 

 experimentally, and showed that these waves obey the laws of 

 light in these resi)ects, and in interference, and in other respects : 

 so that to-day, though Maxwell's " model " is not considered to 

 represent the actual state of the aether, his ec|uations are accepted 

 universally. But one (|uestion has continually eluded discovery : 

 Does a mo\ ing body move relative to the c'ether, or does it carry 

 the ?ether with it in its course? Every experiment which is 

 exj)ected to show motion of the earth, e.g., relative to the aether, 

 gives a )iuU result; and yet theory seems to need such relative 

 motion. Three crucial experiments nill show the state of affairs, 

 and will introduce the new Principle of Relativity: — 



E.vp. I. — Arago (1818) deduced from theory that since the 

 deviation of a ray of light by a prism depends on the ratio of 

 the velocity of light in space to its velocity in the material of 

 the ])rism, any motion of the prism ought to betray itself bv a 

 change of angle well within his powers of measurement. The 

 experiment showed no change whatever. Arago concluded that 

 the aether was dragged along by the earth. Fresnel showed that 

 there was another inter|)retation of the null result, viz., the prism 

 might carry the sether with it partially (so as to i)reserve the 



*■' An election may be treated dynamically as a ri«id body : it? 

 motion of translation is electricity, and its rotatory motions arc nia',;- 

 netism." 



