7U 



Prof. E. G. Coker on the 



stresses X and Y in the plate, and if therefore we write 



^o-^ = C(X-Y), (2) 



where C is an optical coefficient, the value of which can be 

 determined by experiment, we obtain 



R=C(X-Y)T. 



Wertheim's* experiments showed that the optical co- 

 efficient is independent of the wave-length of the light 

 used, but the later experiments of Pockelsf and FilonJ 

 show that a variation exists which in very accurate experi- 

 ments must be taken into account. For the purposes of this 

 paper the variation of the optical coefficient is neglected, and 

 the retardation is assumed to follow the law stated by 

 equation 2. 



A convenient arrangement for examining the effects of 

 stress is shown by tig. 1. Light from a point source A is 



Fig. 1. 



plane-polarized by its passage through a NicoFs prism B, and 

 is transmitted through a transparent plate C of the material 

 under examination. This transmitted light is analysed by a 

 second Nicol's prism D, and the image showing the colours 

 produced by the interference of the ordinary and extra- 

 ordinary rays is projected on a sheet E of squared paper, or 

 is photographed as may be convenient. Condensing lenses 

 F and G are also provided for focussing purposes. A very 



* Annates de Chimie et de Physique, Series 3, vol. xl. 

 t "tlber die Anderung des optisclien Verlialtens verschiedener Glaser 

 durch elastische Deformation," Annalen der Pki/sik, Series 4, vol. vii. 

 X Camb. Phil. Soc. Proc. vols. xi. and xii. 



