PRESIDENTIAL ADDRESS SECTION A. 43 



on the other hand, they may >onietimes bo regarded as expressing 

 physical truths whicli are more rea) and fundamental than the 

 simple princii)les from which they have been formed. 



I pass now to what I fear must be a very incomplete sketch 

 ot the new " principle of relativity/" which is a theory of 

 mechanics ba>ed on ideas nor consistent with some of Xewton's 

 theory. We must hrst consider the growth of the modern con- 

 ceptic^n of an " sether " tilling al^ space. * lalileo suspected, and 

 tried to tind, a finite \elocity for light. Roemer ( 1644 to 1710), 

 followed 1)}- Bradlex- ( 169J to 1702J. by observations on the 

 eclipses of Jujnter's satellites, obtained a value accepted to-day 

 after a long series of laboratory and astronomical experiments 

 — 186.000 miles a second, or 3.10'' cm. per second. 

 Huyghens suggested the wave theory of light. Newton 

 rejected this on the ground of two or three definite 

 conflicts with experiment. He could not decide on anv satisfac- 

 tory theory, but suggested as i)ossible a certain scheme of semi- 

 material " Corpuscles " shot out by the sun and by everv other 

 source of light. This corjniscular theory, backed hv some ac- 

 cordance with Descartes' theory of a space ])lenum, wa-^ elabor- 

 ated and stereotyped b\' New-ton'< disciples. The wave theory 

 was ultimately established by the phenomena of " diffraction " 

 and "interference" (the haziness of the edges of shadows — the 

 corpuscular theory w^ould require clean-cut shadow edges ) ; and 

 by 1800 Young's and Fresnel's work had convinced the scientific 

 world that light was propagated by means of vibrations in the 

 front of a s])herical w'ave with the source as centre, the radius 

 as the "ray." and the vilirations in the tano'ent plane: colours 

 being vibrations of different period — the red (and heat) rays 

 of long ]>eriod, the blue, \iolet and chemical rays short : all 

 analogous to the theory of sound. excei)t that the vibrations must 

 be transverse (at right angles) to the ray. This sj^herical w^ave 

 was modified in a material medium into an ellipsoid. But this 

 theory, while ultimately ex])laining completely such phenomena 

 as reflection, refraction, i^olarization. diffraction, and disi)ersion, 

 involved thinkers in a big difficulty — the conception of some 

 medium which should con\ ey such vibrations through space. 

 The medium could not be fluid, because the vibrations were trans- 

 verse, needing some kind of friction between adjacent portions. 

 ^luch labour was spent on the study of the mathematics of solid 

 and semi-solid elastic bodies which might satisfv the needs of 

 the " cether." 



A serious difficulty w-as the interaction of rether and matter: 

 experiment clearly proved that the rether must penetrate matter; 

 matter must in some wa\- consist of stufl' (presumably 

 particles) differentiated in some j^ermanent way from the 

 lether foundation. -\nv such conce|)tion implies disconiinidty ; 

 and the wdiole body of mechanical science, with its indubitably 

 correct results, has been built up on the mathematics of con- 

 tinuitv. The theory of electricitv and magnetism, which 

 had more slowlv reached the sta<>e of mathematical treat- 



