Note 41 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.
Reading edition
Kurze Anleitung zur mikrosk. Krystallbestimmung, Wiesbaden, 1898, and Tabellen zur mikrosk. Bestimmung, etc., Wiesbaden, 1900. Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 93.
This method is particularly favorable for the accurate determination of the refractive indices of small isolated fragments or grains, by using liquids of known indices. In this case a bright line appears on the “near” edge and a dark line on the “far” edge if the grain has a higher index than the liquid. The reverse occurs when the index is lower than the liquid. When the index is the same (using white light) the “far” edge is blue and the “near” edge red and also the contours about disappear.
F. Krantz, of Bonn, furnishes a series of 21 liquids in small bottles, with indices from 1.447 to 1.83.
The indices of a few convenient liquids are: water, 1.34; alcohol, 1.36; glycerine, 1.41; olive oil, 1.47; nut oil, 1.50; clove oil, 1.54; aniseed oil, 1.58; almond oil, 1.60; cassia oil, 1.63; monobromnapthalene, 1.65; methylene iodide, 1.75.