Mr Filon, On the variation, etc. 55 



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

 strained glass (Second paper). By L. N. G. Filon, B.A., King's 

 College. 



[Received 20 December 1902.] 



1. In a previous paper (Proc. Gamb. Phil. Soc, Vol. xi. 

 Part vi.) an account was given of experiments on glass beams 

 with the object of finding out how the double refraction induced 

 by strain varied with the wave-length of the light. The ex- 

 periments exhibited the variation continuously throughout the 

 spectrum. They showed that, as a rough first approximation, 

 Wertheim's law, that the double refraction exhibits no dispersion, 

 is correct ; a divergence was however shown to exist, the differ- 

 ence in the refractive indices being smaller in the red than in 

 the violet, even for the comparatively light glass examined 

 (density 2-50). 



The accuracy attainable under the given conditions, however, 

 was not sufficient to allow the law of this divergence to be 

 exhibited. 



It seemed necessary, therefore, to devise a totally new method 

 of investigation, which should allow of greater precision. The 

 experiments undertaken with this object are described in the 

 following pages. 



2. Light coming from a powerful arc lamp E (Fig. 1) was focused 



a 3 



$ JV 



Fig. 1. 



by a large lens L on to the slit S of a collimator 0. The pencils 

 of parallel rays, after emerging from the collimator, passed through 

 a polarizing Nicol N, whose axes were inclined at 45° to the vertical 

 and horizontal, then through a block of glass B, then through 

 another Nicol A. The rays were then broken up by two prisms 



