STUDY OF ORES AND METALS. 443 



retically it is thus possible by measurements in different immersion 

 liquids to determine the refractive indices w^, n^, and also the ab- 

 sorption indices k^, Ko even with normally incident light; but prac- 

 tically this method is of little value because the results are encum- 

 bered with a large probable error due chiefly to the relative insen- 

 sitiveness of the eye to slight changes in the observed phenomena. 

 In a transparent, birefracting substance the maximum effect is 

 obtained by the use of a liquid of refractive index equal either to 

 Wi or to ^2- The intensity of one of the reflected components is then 

 zero; the intensity of the second component for tiQ^iii is 



Ir \ Wo + Wi / ' 



which is a very small quantity for weekly birefracting substances ; 

 and even for strong birefringence such as that of calcite it amounts 

 only to one half of one per cent. With intense illumination, how- 

 ever, relatively weak birefringence can be detected by the use of 

 proper refractive liquids. Since clear refracting Hquids of index 

 above 2.2 are not available, it is not possible to obtain the most sen- 

 sitive conditions of observation with minerals of very high refractive 

 index, such as hematite and selenium. But with most minerals con- 

 ditions of observation can be improved somewhat by the use of im- 

 mersion liquids of proper refractive index. In practical work, how- 

 ever, it is doubtful if the improvement thereby gained is worth the 

 bother. 



Summary. 



In the foregoing pages the attempt has been made to present in 

 .connected form the electromagnetic theory of the reflection of light 

 by absorbing media and especially that part of the theory which 

 treats of the reflection phenomena resulting from vertically incident 

 light waves under the conditions usually encountered in the use of 

 the reflecting or metallographic microscope ; this outline of the 

 theory is given in order to indicate and also to emphasize the 

 fundamental principles on which all methods involving the appli- 

 cation of polarized light to the study of opaque substances are nec- 

 essarily based ; thus the possibilities and the limitations as well of 



