Apeil 13, 1900.] 



SCmNGE. 



581 



throughout the discussion. In part III. the 

 theoretical deductions are verified by experi- 

 ment for such cases as admit of accurate numer- 

 ical computation. Much of this theory is em- 

 bodied in the second volume of ' Sound' (16th 

 Chapter). 



After this Lord Eayleigh's m.ind seems to 

 have been intenselj' attracted by optical phe- 

 nomena, and we find first a critical examination 

 of Verdet's diffraction theory of the solar corona, 

 followed by a long experimental paper confirm- 

 atory of Maxwell's theory of color perception. 

 The experiments aim at establishing the linear 

 equation between any four colors with a higher 

 degree of accuracy. They are made with 

 Maxwell's educationally now very familiar 

 color wheel. In 1871 came the two papers 

 dealing with the dynamics of the blue sky, 

 perhaps the most famous of Lord Rayleigh's 

 theoretical researches. It is needless to refer 

 to them at length here," since an easily intelligi- 

 ble presentation is given in Preston's theory of 

 light (Art. 162). If the long waves are ab- 

 sorbed on transmission a.nd the short waves 

 scattered, the blue color of the sky is naturally 

 a manifestation of the surviving mean wave 

 lengths. Clausius, it appears, had previously 

 developed an interferential theory to account 

 for the same phenomenon at considerable length, 

 but had subsequently rejected it chiefly because 

 in the case of particles small in all their dimen- 

 sions as compared with the wave length of light 

 the ordinary laws of reflection no longer hold 

 and an independent investigation is imperative. 

 Without being aware of these misgivings, Ray- 

 leigh took up and completed the subject from 

 the point where Clausius * left it. The former 

 has frequently recurred to the same investiga- 

 tion, showing in a recent paper, for instance, 

 * To me it seems probable that these researches o£ 

 Clausius may be resuscitated in relation to the colors 

 of cloudy condensation. As seen in the color tube by 

 transmitted light, the yellows, oranges, browns, of 

 the first order, ending eventually in opaque are un- 

 doubtedly Eayleiiih's colors. The jet in reflected 

 light is bluish. Beyond this, with increasing size 

 of particles, the transmitted colors are violet, blue, 

 green, yellowish, purple, etc., following Newton's 

 color series, and to these, it would seem, that 

 Claasius's investigations are applicable. I shall re- 

 tnrn to this interesting subject elsewhere. 



that scattering may even be promoted by the 

 molecules of air themselves. 



Of the two optical papers which follow, the 

 first, a theory of double refraction based oh the 

 hypothesis of difference of molecular inertia in 

 different directions (given for instance by the 

 case of a disc vibrating in a resisting fluid), 

 seems to have been disproved shortly after in 

 experiments of Stokes's. The other is an elabo- 

 rate contribution to Presuel's fundamental in- 

 vestigation on the intensity of light reflected 

 from transparent media. Fresnel's expressions 

 have been remarkably suggestive, and they are 

 approximately true. The method by which the 

 tangent formula is derived, however, is not 

 rigorous, and he was of course unaware of 

 Jamin's discovery of the change of phase which 

 accompanies reflection. Green's, Cauchy's, 

 MacCulIagh's, Neumann's, and Lorentz's theo- 

 ries are successively examined, but the nature of 

 the correction (Fresnel's result predicts extinc- 

 tion at the angle of polarization), or a derivation 

 which shall satisfactorily dispose of the longi- 

 tudinal wave is not ascertained. All this re- 

 calls Lord Kelvin's Baltimore lectures. A sub- 

 sequent paper on the reflection of light from 

 opaque matter is much along the same lines, 

 being critical rather than constructive. It is 

 curious that a man of Rayleigh's genius instead 

 of wrestling with these abstruse elastic theories 

 did not make an entirely new departure from 

 the basis of the electromagnetic theory* of 

 light, as did afterwards Helmholtz in his famous 

 paper on dispersion. 



At about this time the reproduction of diffrac- 

 tion gratings by photography, a fascinating sub- 

 ject engaged Lord Rayleigh's attention, and as 

 in most of his work, grew eventually into an 

 extended treatment of the degree of perfection 

 attainable in gratings. Transparent gratings of 

 Nobert, with 3000-6000 lines to the inch were 

 found directly reproducible when used as nega- 

 tives, and the copies proved nearly equal in 

 quality to the original, showing for instance, 

 the nickel line between the D's. Gelatine 

 reproductions (obtained by the photolitho- 

 graphic process with chromate of potassium) 



* Although Maxwell's electricity was not completed 

 until 1873, one would suppose that the contents 

 could not be quite unknown to Eayleigh. 



