WRIGHT: DETERMINATION OF REFRINGENCE 389 



PHYSICS. — The determination of the relative refringence of mineral 

 grains under the petrographic microscope. Fred. Eugene 

 Wright, Geophysical Laboratory. 



Two standard methods are in common use by microscopists for 

 the determination of the relative refractivity of two adjacent 

 mineral grains in the thin section, and also of a single grain and the 

 liquid in which it is immersed; the first method is based on the 

 phenomena produced by central illumination with a narrow pencil 

 of incident rays, while in the second the phenomena obtained by 

 oblique illumination are observed. Both methods are convenient 

 and entirely satisfactory for ordinary purposes. Their accuracy is 

 the same, differences of 0.001 in refractive indices being recogniz- 

 able under favorable conditions. But in many instances, espe- 

 cially in the measurement of the refractive indices of fine grains 

 immersed in refractive liquids, it is extremely difficult to detect 

 ^the faint differences in light intensity which appear at the margins 

 of the grains and by means of which the differences in refractivity 

 are recognized. Under such conditions the eye of the observer is 

 subjected to severe strain and tires quickly. Fortunately, how- 

 ever, it is possible, by modifying the conditions of observation 

 slightly, to render the phenomena more easily visible and thus to 

 relieve the eye strain to a large extent and at the same time to 

 increase the accuracy of the determinations. These modifications 

 involve both the sources of light and a new method of two-fold 

 oblique illumination. They are not difficult to apply and may, 

 therefore, be described briefly. 



Sources of light. In place of the sodium flame ordinarily used 

 as source of monochromatic light, the following light sources have 

 been substituted: Mercury light, helium light, and either a cal- 

 cium-flame or a molybdenum- or tin-spark. With this array of 

 lights set up side by side on the dark room table, and in conjunc- 

 tion with a monochromatic illuminator or a dispersion prism or 

 suitable ray filters (Wratten mercury-line filters), the following 

 spectral line sources are available: X = 546 . 1, 558 to 561 (average 

 about 560), 577 and 579, (average 578) and 588^^. With these 

 lights it is not difRcult to determine between which two of the 



