ADAMS AND WILLIAMSON: BIREFRINGENCE AND STRESS 



615 



barium crown, containing 43 per cent barium oxide (BaO), and 

 to a still greater extent the extra heavy flint, containing 69 per 

 cent lead oxide (PbO), show a much smaller ratio of birefrin- 

 gence to stress. The variation of this ratio with lead content in 

 the flint glass has been investigated by Pockels,^ who has ex- 

 pressed his results in a form very different from the one we have 

 used. Before comparing his results with ours it is therefore 

 necessary to give a short discussion of the principles involved. ^ 



THE 



BY 



ELEMENTARY THEORY OF OPTICAL EFFECTS PRODUCED 



STRESS 



Since the stress at any point in a solid may be resolved into 

 three components at right angles to each other, it is sufficient 



Fis. 3- 

 Drawing to accompany elementary discussion of optical effects of stress. The 

 thrust P is applied in the direction Y. The ray of light enters the cube of glass in 

 the direction OX, becomes elliptically polarized and is treated as two rays vibrating, 

 respectively, in the directions OY and OZ. Ordinarily the ray vibrating along OY 

 travels with the higher velocity, that is, ordinary glass under uni-directional com- 

 pression behaves like an optically negative uniaxial crystal. 



to consider what happens to light passing through a cube of 

 glass subjected to forces normal to its faces. In Fig. 3 let OX, 

 OY, OZ represent the coordinate axes, and consider a beam of 

 Hght passing through the cube in a direction parallel to OX. 

 Now if the glass be acted upon by a vertical thrust {i. e., parallel 

 to OY) the cube will be compressed in that direction and ex- 



^ F. PocKELS. Ann. Phys. (4) 7: 745. 1902. 



