_Sitzungsberichte der k. k. Akad. der Wiss._, Wien, 1893, I Abt., p. 358. Translation by L. McI. Luquer in _School of Mines Quarterly_, Vol. XXIII., Jan., 1902, No. 2, p. 127. Review by Viola in _Min. Pet. Mitt._, Vol. 14, p. 554. _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 95. Footnote 36: Most of the petrographical microscopes carry over the polarizer a convex lens the effect of which is to widen the illuminating cone and hence make less visible this phenomenon. Footnote 37: In Seibert student microscope, No. II _a_, use next to smallest light-stop. Some of the Fuess microscopes are supplied with an iris-blende for limiting the cone of light. Footnote 38: In Seibert microscope use No. V, not sufficiently marked results being obtained with No. II. Footnote 39: See Viola’s diagram, _Minn. Pet. Mitt._, Vol. XIV., p. 556. Footnote 40: Seibert, No. II. Fuess, No. 4. Footnote 41: _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. Footnote 42: Crystals that have two good cleavages often develop so that the direction of elongation is parallel to the intersection of the two cleavages, while in the case of crystals with one good cleavage the tendency seems to be towards a tabular habit parallel to the cleavage. Footnote 43: Harker’s _Petrology for Students_, p. 306. Footnote 44: