56 M. J.-L. Soret on Polarization 



substance does not realize absolutely the necessary conditions for 

 complete polarization, but above all because multiple reflections 

 must be produced. It is evident, on any theory, that a certain 

 portion of the light diffused by a roughness of the surface must 

 reach the neighbouring roughnesses, and there undergo diffusion 

 a second time ; and as the plane of polarization of the light after 

 its first diffusion is generally different from that of the incident 

 radiation, the twice diffused rays will be polarized in a different 

 plane from that of the rays diffused once only. 



In the case of the illumination of transparent bodies where 

 particles are scantily disseminated in the medium, one can 

 understand that the effect of these multiple reflections may be 

 insensible ; but on a smoked surface, where all the particles are 

 contiguous, there is nothing surprising that the effect should 

 become very appreciable. 



Here again I regret being in discord with M. Lallemand, who 

 explains this residue by a phenomenon of isochromatic fluores- 

 cence. I do not contest that certain bodies, as sulphide of car- 

 bon, may possess this faculty — that is to say, may emit by fluo- 

 rescence rays of the same refrangibility as the incident light ; I 

 see no theoretic objection to admitting the existence of this pro- 

 perty, which would be analogous to that of membranes vibrating 

 in unison with all the sonorous undulations which strike them ; 

 but M. Lallemand gives it a generality the necessity of which 

 does not appear to me to be proved. Indeed he attributes to it 

 not merely the residue of neutral light observed in the diffusion 

 produced by a smoked surface .or other dull black bodies, but 

 also the much more considerable proportion of neutral light dif- 

 fused by white or coloured bodies. Isochromatic fluorescence 

 is for him the cause of the proper colours of bodies. 



The theory of the colours proper to substances appears to me 

 well established, although many treatises on physics present 

 large gaps in this respect : several do not mention it ; others 

 give it in an incomplete or erroneous manner*. It will, perhaps, 

 not be needless to explain it briefly, as I conceive it, taking into 

 consideration the laws of polarization by diffusion, the study of 

 which has recently made great progress. 



Let us lay aside the phenomena of coloration which proceed 

 from ordinary fluorescence (with change of refrangibility), of 

 coloration by superficial reflection (metallic lustre), and of inter- 



* Verdet's Cours de Physique a VEcole Polytechnique explains it, in my 

 opinion, very accurately but with sufficient brevity : vol. ii. p. 268. Dove 

 gives it with many more details (Darstellung der Farbenlehre, Berlin, 

 1853). I have not made extensive bibliographic researches ou the subject; 

 but I remember that the theory in question is found in a great number of 

 publications. 



