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 Ghimie et de Physique , Vol. 
XX.) and b} r Sir David Brewster ( Phil . Tr'ans. 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 Ghimie 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 
r, 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 = G x (P — Q) x r, 
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 G 
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. 
