THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



SEPTEMBER 1888. 



XXV. On the Reflexion of Light at a Ttcin Plane of a Crystal. 

 By Lord Rayleigh, Sec. R.S.* 



MY object in the present paper is to calculate a priori the 

 reflexion of light at the surface between twin crystals, 

 and to obtain formulae analogous to those discovered by 

 Fresnel for the case where both media are isotropic. It is 

 evident that success can only be attained upon the basis of a 

 theory capable of explaining at once FresneFs laws of double 

 refraction in crystals and those just referred to, governing the 

 intensity of reflexion when light passes from one isotropic 

 medium to another. So far as I am aware the electric theory 

 of Maxwell is the only one satisfying these conditions f ; and 

 I have accordingly employed the equations of this theory. It 

 will be remembered that the electric theory of double refrac- 

 tion was worked out by Maxwell himself, and that the 

 application to the problem of reflexion was successfully 

 effected by von Helmholtz and LorentzJ. The present 

 investigation starts, however, independently from the funda- 

 mental equations, as given in Maxwell's * Electricity and 

 Magnetism/ 



* Communicated by the Author. 



t See Prof. Willard Gibbs's excellent " Comparison of the Elastic and 

 the Electrical Theories of Light with respect to the Law of Double Re- 

 fraction and the Dispersion of Colours'' (Am. Joum. Sci. June 1888), 

 which reaches me while revising the present investigation for the press. 



X References to the works of previous writers will be found in Glaze- 

 brook's Report on Optical Theories, Brit. Assoc. Rep. 1886. 



Phil. Mag. S. 5. Vol. 26. No. 1G0. Sept. 1888. R 



