50 HILLEBRAND, MERWIN AND WRIGHT. [April 25, 



used, and minute air spaces therefore may have been present between 

 the grains. 



In the powder form the grains are usually irregular in shape and 

 colored more or less intensely in shades of orange, red, and yellow. 

 Pleochroism is noticeable; a = light cadmium-yellow; ;8 = cadmium 

 yellow; y = orange. Absorption y>;8>a. The refractive indices 

 were determined by the immersion method ; mixtures of methylene 

 iodide, arsenic tribromide and arsenic sulphide (realgar) being used 

 for the purpose. Owing to the color of the mineral, however, the 

 phenomena on which the refractive index determinations by this 

 method rest were less sharply marked than usual, and the probable 

 error is correspondingly larger. 



a =1.775 ±.005 

 ^=1.815 ±.005 

 7=1.825 ±.005 



The birefringence is strong and gives rise to high interference colors, 

 even in comparatively thin grains. By direct determination, y — a 

 was found roughly to be about .050. The optic axial angle was 

 measured by the double screw micrometer ocular^^ on sections show- 

 ing only one axial bar and also on a section normal to the acute bi- 

 sectrix. For sodium light 2V was found to be 50°. 5 ± 1° or 2E 

 is about 100° ; for lithium red light 2V is about 56° ±3°, or 2E, 

 about 115°. The determination in lithium light was much less satis- 

 factory and accurate than that in sodium light. The dispersion of 

 the optic axes is very considerable with 2VLi>2VNa and its effect is 

 clearly marked in the interference figure. The appearance of the 

 interference figure shows, moreover, remarkably strong crossed dis- 

 persion — so strong in fact that in white light a section normal to 

 the acute bisectrix never extinguishes completely, but near the 

 position of extinction for light of any wave-length shows abnormal 

 interference colors in characteristic tones, especially of green and 

 orange. On a section nearly normal to the acute bisectrix the posi- 

 tion of total extinction for sodium light made an angle of about 8° 

 with that for lithium light. This angle, yLi:yNa = 8°, is only ap- 



^^ Am. J. Sci., 24, 317-369, 1907. 



