1902.] Differential Equations of Fresnel's Polarisation-vector. 37 



X 



Shift in tenth-metres. 



Atmospheres. 



ACiA^ -QQ 



'009 





4045 -98 



0-020 





4383 -72 



0-016 



9! 



4383 -72 



0-026 



Ill 



We find then that the known direct effect of pressure on the radia- 

 tion or absorption lines is the same, in quality, in water as in air, that 

 is, we get displacements in the opposite direction to that we observe 

 the dark lines to occupy in the spectra of Novae, and we find further 

 that the amount of shift observed in the spectra of new stars differs 

 not only in this respect but also in degree, thus : — 



Spark in water. 



New stars. 



1. Absorption lines least shifted. 



2. Eadiation lines most shifted. 



3. Absorption sbift small. 



Absorption lines most shifted. 

 Radiation lines least shifted. 

 Absorption shift enormous. 



It would thus appear that the pairs of bright and dark lines shown 

 in the spectra of new stars do not arise from the cause which produces 

 the appearances presented in the spectrum of the spark in water. 



My thanks are due to Mr. C. P. Butler, who obtained and discussed 

 the photographs of the spark spectra, and who, together with Dr. 

 Lockyer, assisted me in the preparation of the paper, and to Mr. F. E. 

 Baxandall, who checked the wave-lengths of the lines discussed and 

 studied the behaviour of the lines representative of the different 

 phenomena. 



" The Differential Equations of Eresnel's Polarisation-vector, with 

 an Extension to the Case of Active Media." By James 

 Walker, M.A. Communicated by Professor Clifton, F.K.S. 

 Keceived February 8, — Eead March 6, 1902. 



1. In many problems of physical optics it becomes necessary to know 

 the differential equations that the polarisation-vector of a stream of light 

 has to satisfy, and the boundary conditions that subsist at the interface 

 of media possessing different optical properties. 



