446 WRIGHT— POLARIZED LIGHT IN THE 



introducing some contrast device such as the bi-quartz-wedge-plate, 

 or the Lippich biprism. In general it may be stated that for practical 

 purposes the methods of the second group are simpler in manipula- 

 tion than those of the first and more sensitive and therefore better. 

 A disturbing factor is the introduction of a small amount of ellip- 

 tically polarized light by the reflecting prism or plate. The phase 

 differences between the two components of the reflected waves are 

 in general small and do not seriously affect measurements by the 

 methods outlined above which are based on the amplitude differences 

 between the two components. The phase differences may be meas- 

 ured by one of several devices of which the Babinet compensator is 

 the best and most widely used ; but for the detection and measure- 

 ment of anisotropism, methods, which are based on phase differences 

 between the components after reflection of vertically incident plane- 

 polarized light, are not in most cases of practical value. 



The use of immersion liquids and a monochromatic light source 

 is suggested for obtaining the greatest differences in intensity be- 

 tween the reflected components and thereby increasing the sensitive- 

 ness of the methods for detecting anisotropism. 



The above equations show that it is in general not possible to 

 measure both the refractive indices and the absorption indices of an 

 opaque body by means of the phenomena resulting on the reflection 

 of vertically incident light waves ; but it is possible in special cases to 

 determine the degree of anisotropism and also the positions of the 

 polarization directions in a given crystal plate. These data taken in 

 conjunction with crystallographic data enable the observer, just as 

 do birefringence and extinction angles in transparent crystals, to 

 draw conclusions regarding the crystal system of the substance under 

 investigation. But in opaque substances the precision attainable is 

 relatively slight and the phenomena which can be observed are rela- 

 tively few and restricted in scope. As a result one cannot expect 

 from the application of polarized light to such substances the harvest 

 of optical data which have been gathered from transparent crystals. 

 For opaque bodies the possibilities are few, the limitations are great, 

 and recourse must be had in practical diagnosis to other methods of 

 determination such as behavior of the substance toward abrasives 



