478 Mr Filon, On the variation with the wave-length 



On the variation with the wave-length of the double refraction in 

 strained glass. By L. N. G. Filon, B.A., King's College. 



[Received 17 June 1902.] 



1. It is well known that transparent isotropic substances 

 behave under strain like crystalline bodies. Attention was first 

 called to this by Fresnel (Annales de Chimie et de Physique, Vol. 

 XX.) and by Sir David Brewster (Phil. Trans. 1816). Since then 

 the phenomenon has been examined, theoretically and experimen- 

 tally, by M. Neumann (Abhandlungen der k. Acad. v. Wissen- 

 schaften zu Berlin, 1841, ir. ; see also Pogg. Ann. Vol. liv.), by 

 Clerk Maxwell (Trans. R. S. Edin. Vol. xx. Part I. ; or Collected 

 Papers, Vol. I.), by Wertheim (Annales de Chimie et de Physique, 

 Ser. 3, Vol. XL. p. 156) and by Kerr (Phil. Mag. Oct. 1888). 



Of these only Wertheim appears to have made any attempt at 

 determining experimentally how this effect varies with the kind 

 of light transmitted. If light passes through a plate of thickness 

 t, which is subjected to principal stresses P, Q in its plane, these 

 stresses being uniform throughout the thickness, then it is found 

 that the light in traversing the plate is broken up into two rays 

 polarized in the directions of principal stress, and the relative 

 retardation of these rays on emergence is given by 



r=Cx(P-Q)xr, 



where C is a coefficient depending only on the nature of the mate- 

 rial and on the wave-length of the light used. 



Wertheim, from observations of a uniformly compressed block 

 of glass through which he passed successively (i) sodium light, 

 (ii) white light, (iii) white light filtered through a red glass, stated 

 the following law : 



The relative retardation in air, measured in centimetres, is 

 constant for all the colours. In other words, if r in the above 

 equation be measured in centimetres in air, the coefficient C 

 should be independent of the wave-length. 



This leads to the conclusion that the difference of the two 

 refractive indices is independent of the colour, i.e. that the double 

 refraction due to strain exhibits no dispersion. 



