JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV JUNE 4, 1914 No. 11 



PHYSICS. — The measurement of the refractive index of a drop of 

 liquid. Fred. Eugene Wright, Geophysical Laboratory. 



Of the many optical characteristics of a substance, the re- 

 fractive indices are the most important and fundamental, and 

 from them a number of the other optical constants can be de- 

 rived. In determinative work, moreover, substances are classi- 

 fied and distinguished within narrow limits by their refractive 

 indices. In modern petrographic microscope analysis great 

 emphasis is placed on this optical propert}^, and tables have 

 been prepared in which the minerals are arranged according to 

 the refractive indices. Simple methods are available by means 

 of which the refractive indices of an irregular mineral grain, 0.01 

 mm. in diameter, can be readily measured with an accuracy of 

 ±0.001, a degree of precision which is sufficient for allpractical 

 pm'poses. In these methods, liquids of known refractive index 

 are used; finely powdered grains of the mineral are immersed 

 in a liquid and the refractive indices of the grains compared 

 with that of the liquid. For routine work, sets of such liquids 

 are usually prepared by mixing liquids of known refractive 

 index and then measuring the refractive index of the liquid 

 mixture, either on a total refractometer or a spectrometer. 

 In many instances, only a drop or two of the liquid is available, 

 and it is essential that its refractive index be measured accm'ately. 

 In this paper, the more important methods for this purpose 

 (except those based on the Abbe refractometer) are considered 

 with respect to their accuracy, and ease and range of application, 



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