424 



WRIGHT— POLARIZED LIGHT IN THE 



positions of extinction have been discussed in detail by the writer* 

 and the conclusion reached that the bi-quartz-wedge-plate is the most 

 sensitive device available for the purpose. With it the sensitiveness 

 can be varied to meet the conditions of illumination. M. Berek^ has 

 shown that with the bi-quartz-wedge-plate a precision of 0.5' can be 

 attained under the most favorable conditions of observation. 



Absorbing Media. 



In the foregoing section the phenomena produced on reflection 

 from transparent crystal plates are treated in some detail and the 

 factors underlying the methods for the detection of anisotropism and 

 for the measurement of the birefringence and the refractive indices 

 are discussed. The introduction of the absorption index into the 

 equations carries with it a series of complications which render the 

 relations less easy to follow, but which are, as a result, the more 

 interesting. 



TABLE 6. 



In this table the intensity is given of the light reflected, at vertical inci- 

 dence, from the surface of an isotropic substance of refractive index n and 

 absorption index k. Thus a plate of refractive index 2.0 and absorption index 

 1.32 reflects 50 per cent, of the incident light. 



^Aiii. Jour. Scl. (4), 26, 377-37S, 1908. Carnegie Institution of Wash- 

 ington, Publication 158, p. 139, 191 1. 



^ Neues Jahrbuch fiir Mineralogie etc., Beilage Band jj, pp. 583-661, 1912. 



