42 Loomis and Yowiig — Shell Heaps of Maine. 



185)8 Berry, G. S., New England Magazine, vol. xix, p. 178-188. Dainaris- 

 cotta heap. 



1908 Holmes, W. H., Twentieth Ann. Rep. Bureau Amer. Ethnology, vol. 



xx, p. 107-183. On pottery of Eastern U. S. 

 1903 Prentiss, D. \V\, Proc. U. S. Nat. Museum, vol. xxvi, p. 887-888. On 



big mink. 

 1903 Hardy, M., Forest and Stream, vol. lxi, p. 125. On big mink. 

 1907 Townsend, C. W., Jour. Maine Orn. Soc, vol. ix, p. 82. On great 



auk. 



1909 Skinner, A., Amer. Museum Nat. Hist., Guide Leaflet No. 29. 



Indians of Manhattan Island. 

 1911 Loomis, F. B., this Journal, vol. xxxi, p. 227. On big mink. 



Amherst College. 



Art. IY. — Mixtures of Amorphous Sulphur and Selen- 

 ium as Immersion Media for the Determination of High 

 Refractive Indices with the Microscope ; by H. E. Merwin 

 and E. S. Larsen. 



The difficulty of identifying transparent minerals and other 

 solids the refractive indices of which can not at present be 

 determined under the microscope because of the lack of suit- 

 able immersion media, has led to the following experimental 

 studies. 



In a molten condition sulphur and selenium are miscible in 

 all proportions. The mixtures can be melted readily on a 

 glass slip held over a low flame, and when melted readily 

 adhere to pulverized minerals. The cooled mixtures rich in 

 selenium remain amorphous for months, those very rich in 

 sulphur may crystallize almost immediately on cooling or after 

 some hours or days, according to the conditions of heating and 

 cooling. With increasing selenium the refractive indices of 

 the mixtures increase from about 1*98 to 2 - 92 for sodium 

 light and from 1*96 to 2 - 72 for lithium light. 



The mixtures containing less than 50 per cent Se by weight, 

 if heated on a glass slip to near boiling and then cooled in air, 

 crystallize more slowly and have slightly higher ('010) indices 

 of refraction than those which, after having crystallized, are 

 barely melted and similarly cooled.* This is probably due to 

 the presence of varying amounts of the two allotropic forms 

 of molten sulphur. With increasing Se these effects of differ- 

 ences in heating and cooling diminish. 



Having given the refractive indices of such mixtures for 

 light of various wave-lengths (Chart I) it only remains to match 



* By quenching some of the highly heated mixtures in water, the refrac- 

 tive index has been raised "05 above the values obtained by cooling in air. 



