Merwin and Larsen — Sulphur and Selenium. 43 



the refractive index of the substance under investigation with 

 the refractive index of a mixture by one of the well-known 

 microscopic methods.* The matching can be done as closely 

 as ±"001 in monochromatic light. It is, then, the variation in 

 the refractive index of the mixture that limits the accuracy to 

 =t "005. It is possible to exceed this accuracy by taking 

 account of the conditions as later described, but it is only in 

 exceptional cases that great accuracy is desired. A series of 

 mixtures suitable for approximate determination is given in 

 Table I. 



If the observations are made in white light the dispersion 

 and color of the mixtures introduce other factors of error. 



Table I. 



%Se 



n u 



n Na 



Equivalent 

 wave-length 



%Se 



n u 



5l Na 



Equivalent 



wave-length 



(ifi 



o-o 



1-978 



1-998 





57-0 



2-200 



2-248 



620 



9-0 



2-000 



2-022 







64-0 



2-250 



2307 



630 



17-6 



2-025 



2-050 



... 



70-0 



2-300 



2-365 



633 



250 



2-050 



2-078 





75-0 



2-350 



2-423 



636 



31-8 



2-075 



2-107 



... 



80-0 



2-400 



2-490 



640 



375 



2-100 



2-134 







87-7 



2-500 



2-624 



645 



43-2 



2-125 



2-163 



580 



93-8 



2-600 



2-755 



652 



48-2 



2-150 



2-193 



605 



99-2 



2-700 



2-90 



662 



53-0 



2-175 



2*220 



615 



100-0 



2-716 



2-92 



665 



The mixtures containing less than 45 per cent Se are yellow 

 or orange. The other mixtures have a deeper red color, and 

 their refractive indices must be expressed in certain wave- 

 lengths of red light. The simplest and best way is to observe 

 the preparation through a thin film of Se pressed out under a 

 cover-glass on a glass slide, and used as a screen over the eye- 

 piece of the microscope. Such a film transmits light which 

 is almost equivalent to Li-light.f This screen may be used 

 with any of the mixtures for determining approximate refrac- 

 tive indices for Li-light. The intensity of the light through 

 this screen, if the source of the light is the sky, is low but 

 sufficient for ordinary work, though an artificial light — an 



*See F. E. Wright, The Methods of Petroeraphic-Microscopic Eesearch, 

 pp. 83-98, 1911. 



f The film of Se transmits a narrow band of the spectrum in the red, of 

 which the chief intensity lies between wave-lengths 650 ftu and 680 /ifi. 

 This is equivalent to wave-length 665 fifi. The wave-length of Li-light is 

 671 /ifi. 



