SOME COS'ir.MI'Ok'.IRV .1l>r.lXCLS IN I'llYSlCS-ni 281 



ftuiiiiry in physirs or choniistr%'. Over .» rathi-r lonjj prridd <if yt-ars, 

 it was iiKlffil m'iu"rali>' rojianlrd as |)rrfi'cll\' iiili-lliv;il)ir. Tlu- 

 famous haltli- ln-lweon tlu- rorpusnilar tlu-ory adopitd 1)\ Xrwion, 

 and the vvavt-lhooiA' foundod In- Disrartrs and Hiiyniuns. dird out 

 in the earher \ears of the nineteenth century witli ilie ^radiial ex- 

 linrtion of the former. The history of optics in the nineteenth cc'h- 

 tur\-, from Fresnel and V'oiinu to Michelson and Ra\kMj;h, i> liie tale 

 of a brilliant series of beaiitiftii and strikinj; demonstrations of the 

 wave-theory, of experiments which were founded upon liie wave- 

 theory as their basis and would have failed if the basis had not been 

 firm, of instruments which were designed and com]X'tent to make 

 ditlicult and delicate measurements of all sorts — from the thickness 

 of a sheet of molecules to the diameter of a star — and would ha\e been 

 useless had the theory been fallacious. The details of the bending 

 of light around the sides of a slit or the edge of a screen, the intricate 

 pattern of light and shade formed where subdivisions of a beam of 

 light are reunited after separation, the complexities of refraction 

 through a curved surface, are represented by the theory with all 

 verifiable accuracy; and so are the incredibK' complicatefl phenomena 

 attending the progress of light through crystals, [jhenomena which 

 have slipped out of common knowledge because few are willing to 

 undertake the labour of mastering the theory. The wa\e-lheor\- of 

 light stands with Newton's in\erse-st|uare law of gra\itation, in respect 

 of the many extraordinarily precise tests which it has undergone with 

 triumph; I know of no other which can rival either of them in I his 

 regard. 



By the term "wave-theory of light" I have meant, in the foregoing 

 paragraph, the conception that light is a wave-motion, an undulation, 

 a perifxlic form ad\ancing through space without distorting its shape; 

 I have not meant to imply any particular answer to the question, 

 what is it of which //?/// is a wave-motion.^ It may seem surprising 

 that one can make and defend the conception, without having answered 

 the question beforehand; but as a matter of fact there are certain 

 properties common to all undulations, and these are the properties 

 which have been verified in the experiments on light. There are also 

 certain properties which are not shared by such waves as those of 

 sound, in which the vibration is confined to a single direction (that 

 normal to the wavefront) and may not vary otherwise than in ampli- 

 tude and phase, but are shared by transverse or distortional waves 

 in elastic solids, in which the \'ibrati(3n may lie in any of an infinity 

 of directions (any direction tangent to the wavefront). I-ight pos- 

 sesses these properties, and therefore the wave-motion which is 



