104 weight: measurement of refractive indices 



(c) Experience has proved that the effects produced by 

 central illumination are best observed under relatively high 

 magnifications (300 to 75 diameters; 4 mm. or 16 nmi. objective) 

 while in obhque illumination lower magnifications are more 

 favorable (75 to 30 diameters; 16 mm. or 32 mm. objective). 



Refractive index measurements on birefracting grains. The 

 phenomena obtained by central illumination are different from 

 those resulting from oblique illumination; but the principle 

 underlying both types of illumination, namely that of the devia- 

 tion of transmitted hght waves by prismatic refraction, ^ is the 

 same. It is of interest to compare the results obtained by the 

 two kinds of illumination on irregular, birefracting crystal 

 particles. In a birefracting mineral the refractive indices 

 of the transmitted light waves vary with the direction of trans- 

 mission; the two methods of illumination are therefore subject 

 to correction factors because in both methods more or less 

 obliquely incident light is employed. In crystals of medium to 

 weak birefringence these factors are practically negligible and 

 experience has shown that the degree of accuracy attainable 

 by either method is about the same. But in strongly birefract- 

 ing minerals a slight change in the direction of transmission of 

 the • light waves may produce a decided change in refractive 

 index, and then the actual angle of inclination and the direction 

 of the waves may become an important factor in the accuracy 



2 The method of central illumination is frequently called the Becke-line 

 method while that of oblique illumination is designated the Schroeder van der 

 Kolk method. The papers by Becke and Schroeder van der Kolk were of great 

 importance in emphasizing the significance of these methods in microscopical 

 petrography; but in view of the fact that both methods had been described 

 and applied by Maschke and Thoulet one or two decades earlier and also that in 

 ordinary microscope work (biological, etc.) these methods have long been used 

 and called by the above descriptive terms — central and oblique illumination — 

 it would seem better than these terms be employed in preference to the above. 

 The term Becke-line should be retained, however, for some of the relative re- 

 fringence determinations in rock thin sections; but in refractive index deter- 

 minations of crystal grains by central illumination the Becke-line constitutes 

 only part of the phenomena observed, the determination being based primarily 

 on the relative convergence or divergence of the light transmitted through the 

 mineral grain. 



