Sulphur and Selenium as Immersion Media. 47 



the ends by melting. The glass strips should be about 4 or 

 5 ram wide, 2 cm long, and l mm thick, and should include an angle 

 of 35° to 45°. Microscope object glass cut crosswise is suit- 

 able provided its sides are sufficiently nearly plane and parallel, 

 in which case the reflections of a wire or cord 3 n ' m in diam- 

 eter in front of a window at a distance of 10 feet are not dis- 

 torted nor entirely separated when the reflections are seen 

 parallel to the direction the glass is to be cut. 



If the refractive index of the mixture in the prism is to 

 equal that of the film in which the mineral is embedded, it is 

 essential that the mixture be heated to 250° or more before it 

 is put into the prism. This is especially true of a mixture 

 that has been standing some time and is partly crystallized. 



The refractive indices in Table I and on the chart were 

 found in this manner. The probable error does not exceed 

 ±*005 except in the case of wave-lengths less than 625/u./* in 

 mixtures containing more than 80 per cent Se, where absorp- 

 tion is very strong. Likewise, the probable error in the dis- 

 persion of a given mixture does not exceed ± "002. 



The mixtures containing less than 15 per cent Se crystallize 

 so readily that they are not well adapted to accurate work. 

 Certain liquids, having refractive indices from l - 80 to 2*10, 

 which fill the gap between the sulphur-selenium mixtures and 

 liquids already well-known will be considered in a subsequent 

 paper. 



Geophysical Laboratory and Geological Survey, 

 Washington, D. C, April 18, 1912. 



