THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 

 FEBRUARY 1901. 



XV. On the Stresses in Solid Bodies due to unequal Heating, 

 and on the Double Refraction resulting therefrom. By 

 Lord Rayleigr*. 



THE phenomena of light and colour exhibited in the 

 polariscope when strained glass is interposed between 

 crossed nicols are well known to every student of optics. 

 The strain may be of a permanent character, as in glass im- 

 perfectly annealed or specially unannealed, or it may be 

 temporary, due to variations of temperature or to mechanical 

 force applied from without. One of the best examples under 

 the last head is that of a rectangular bar subjected to flexure, 

 the plane of the flexure being perpendicular to the course of 

 the light. The full effect is obtained when the length of the 

 bar is at 45° to the direction of polarization. The revival of 

 light is a maximum at the edges, where the material tra- 

 versed is most stretched or compressed, while down the middle 

 a dark bar is seen representing the " neutral axis/'' It is 

 especially to be noted that the effect is due to the glass being 

 unequally stretched in the two directions perpendicular to the 

 line of vision. Thus in the case under discussion no force 

 is operative perpendicular to the length of the bar. Under 

 a purely hydrostatic pressure the singly refracting character 

 of the material would not be disturbed. 



When a piece of glass, previously in a state of ease, is 

 unequally heated, double refraction usually ensues. This is 

 due, not directly to the heat, but to the stresses, different in 

 different directions and at different places, caused by the 



* Communicated by the Author from the Lorentz Collection of 

 Memoirs. 



Phil. Mag. S. 6. Vol. 1. No. 2. Feb. 1901. N 



