442 WRIGHT— POLARIZED LIGHT IN THE 



The Ejfect of Thin Surface Films on the Character of the Re- 

 flected Light. — Drude^^ was the first to emphasize the importance 

 of thin surface films in affecting the polarization relations of re- 

 flected and of transmitted light waves. In the case of opaque sub- 

 stances (ores and metals) the surface film effects may be serious 

 and cause an isotropic mineral to show evident phenomena of aniso- 

 tropism. Thus a sheet of copper or other isotropic metal shows 

 distinct anisotropic effects. The rolled surface of the metal is stri- 

 ated and part of the polarization effects may be of the nature of 

 grating polarization effects or may result from reflection at oblique 

 surfaces. Many ground surfaces of isotropic pyrite are decidedly 

 anisotropic. In certain cases this is probably due, as Koenigsberger 

 suggested^^ to grinding striae ; but in other pyrite sections the aniso- 

 tropism persists in spite of care taken to eliminate grinding striae. 

 This may be the result of surface film effects or of strain bire- 

 fringence in the pyrite. Its presence in random sections, even care- 

 fully ground, serves as a danger-signal, warning the observer against 

 the drawing of too positive conclusions from a single section. As a 

 general rule pyrite sections are isotropic and the conclusion is war- 

 ranted that it is isometric, as crystallographic measurements prove it 

 to be. The statement of Koenigsberger that surface films affect 

 only the phase difference and not the amplitudes of the reflected 

 beams is borne out neither by theory nor by practical experience. 

 In general, however, it may be stated that surface film effects are 

 not so pronounced as to affect seriously the validity of the deter- 

 minations. 



The Use of Refractive Liquids. — In case the medium surround- 

 ing the crystal be not air but a refractive liquid the refractive indices 

 appearing in the above equations have to be reduced in the ratio of 

 the reciprocal of the refractive index of the liquid ; thus if «„ be the 

 refractive index of the liquid, n^, Wo of the crystal plate must be re- 

 placed by Mi/wo, Wj/Wq- With each different refractive liquid em- 

 ployed both the intensity ratio of the reflected components and the 

 angular rotation of the plane of polarization are changed. Theo- 



"^^ Ann. d. Physik, 43, p. 146, i8t>i ; Lehrbuch d. Optik, 1906, pp. 2721-280; 

 Winkelmann's Handbuch d. Physik, Vol. 6, pp. 1278-1316, 1906. 

 15 Centralblatt fiir Miner., Geol. u. PaldontoL, 1908, p. 597. 



