446 



merwin: dispersion of carborundum 



The dispersion and birefringence are both much less than those 

 calculated by L. E. Jewell 2 from interference phenomena. The 

 birefringence corresponds closely to Becke's* measurement, but 

 the refractive index, co Na , is less than his rough measurements 

 indicated. 



TABLE 1 



Indices of Refraction of Carbonrundum 



Microscopical study of several samples of granular carborun- 

 dum has revealed no definite variations in the refractive index 

 for red light, even in grains of different color. Coarse material 

 which is very black may be, when powdered — grains a few 

 hundredths of a millimeter in diameter — fairly transparent. 

 Bluish grains are notably pleochroic, some from light blue to 

 dark blue, some from olive green to greenish blue. The w is the 

 more strongly absorbed. Coarse black crystals intimately 

 associated with graphite appear to contain small amounts of 

 material which give small grains a grayish to bluish color by 

 transmitted light, but this material, in all the grains so far 

 observed, has been submicroscopic in size. 



The results for w in table 1 were obtained as averages from 

 the two prisms. The observed values for one prism were 0.003- 

 0.005 higher than for the other. But inasmuch as one face of 



2 Quoted by Tone, in Mineral Industry, 16: 153. 



3 Zeitschr. Kryst., 24: 524. 1895. 



1907. 



