270 WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



especially in petrographic microscope work. Several new meth- 

 ods are described; also certain details of manipulation of the 

 standard methods which the writer has found convenient and 

 satisfactory in practice. 



Sources of light. For most refractive index work with min- 

 erals, the practice among mineralogists has been to use as mono- 

 chromatic light sources the salts of lithium (X = 671mm), sodium 

 (X = 589mm), and thallium (X = 535^^) in the Bunsen flame. 

 In addition to these the lines of a mercury lamp (X = 435.8, 

 491.6, 546.1, 576.9, 579 hij.) are convenient, especially if used in 

 conjunction with a carbon bisulfide prism or the Wratten ray 

 filters^ which isolate effectively the green line, 546.1 (mercury 

 green filter), and also the two yellow lines, 577 and 579 (Wratten 

 E filter No. 22). Other useful light sources are helium and 

 hydrogen tubes and cadmium and iron arcs, but ordinarily the 

 mercury light is a sufficient addition to the salts mentioned. 



Uniform illumination from all these light sources can be readily 

 obtained by means of a finely ground glass plate (frosted glass), 

 placed between the source of light and the measuring instru- 

 ment. The glass plate enables the observer to overcome, without 

 further inconvenience, the ordinary troubles of a flickering flame, 

 which quickly tires the eyes. In this Laboratory, a ground glass 

 plate (25x35 cm.), mounted in a simple upright wood frame is 

 placed directly in front of' the Meker burners in which the salts 

 are volatilized. The salt is contained in a platinum crucible 

 into which a wick of fine platinum wires extends. This wick 

 carries the melted salt (kept melted by a Meker bm*ner) into 

 the hot flame of a second Meker burner. ^ A hood carries off 

 the vapors and thus shields the instruments from attack by the 

 salts. By this method an intense and lasting flame is produced 

 which burns for hours without further attention. In routine 

 work, too much stress cannot be placed on the satisfactory per- 

 formance of such details; the above light sources and glass diffus- 

 ing screen have been proved by long usage in this Laboratory 

 to be convenient and entirely satisfactory. 



1 F. E. Wright, Am. J. Sci. (4), 27: 195. 1909; 31: 185. 1911; Carnegie Insti- 

 tution of Washington, Pub. 158: 8S. 1911. 



