494 FRED. EUGENE WRIGHT 
a or ¥ (the optic ellipsity of the section) can then be ascertained 
by use of the sensitive tint plate or by the quartz wedge in parallel 
polarized light. 
MEASUREMENT OF THE OPTICAL PROPERTIES OF THE SECOND CLASS 
A number of new methods have been suggested in recent years 
for the measurement of the optical properties of this class, with 
the result that satisfactory methods are now available for use even 
on minute mineral grains; the accuracy of the results attainable 
by these methods under the different conditions is known and the 
application of these methods is now a matter of routine. 
Refractive indices.—For the measurement of refractive indices 
of minerals in the mounted thin section no accurate method has 
yet been devised; but on unmounted mineral grains and plates, 
measuring o.o1 mm. and over in diameter, the refractive indices 
can be readily determined by the immersion method with an 
accuracy of +o.oo1 for sodium light on isolated favorable sections. 
For this method a set of liquids of known refractivity is essential. 
The following set is used at present in the Geophysical Laboratory: 
Refractive Indices Mixtures of 
Te VAS Om Ao ieres ce Kerosene and turpentine 
TACO TAGS 5 sce Turpentine and ethylene bromide or clove oil 
Te SAO— OIG Ne Clove oil and a-monobromnaphthalene 
T:640-1.655.:.... a-monobromnaphthalene and a-monochlornaphthalene 
EAOOOsT A 7AOn a a-monobromnaphthalene and methylene iodide 
TA TAO— Ue TOG ee tere Sulfur dissolved in methylene iodide 
Ta7OO=2 0500 ae Methylene iodide, arsenic sulfide, sulfur and tin iodide.t 
DROSS oe 7 5Ous ae Glass produced by melting amorphous sulfur and selenium 
in different proportions. The mineral grains to be 
tested are immersed in the molten liquid but are examined 
after it has cooled and hardened to a red-colored glass.? 
«Dr. H. E. Merwin of this laboratory has recently made a detailed study of 
these highly refracting mixtures, their preparation, and their permanency. The 
results of his work are to appear shortly in the Amer. Jour. Sci. 
See also O. Maschke, Pogg. Ann. CXLV, 565, 1872; Wiedemann’s Ann., XI, 
722, 1880; J. Thoulet, Bull. Soc. Min. France, III, 62, 1880; H. Ambronn, Ber. 
Sachs. Gesell. d. Wissen. Math. Phys. Kl., 1-8, 1896; J. L. C. Schroeder van der Kolk, 
Zeitschr. f. Wissen. Mikrosk. VIIL., 458, 1898; F. E. Wright, T.M@.P.M., XX, 239, 
1901; Amer. Jour. Sci. (4), XVII, 385, 19004; Carnegie Inst. Wash. Pub. 158, p. 98, I91I. 
2 The refractive indices of these glasses were measured in lithium-light and not 
in sodium-light, as is the case with all the liquid mixtures in this list. 
