9 8 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
which the pestle fits, serves to confine the fine grains to the mortar during 
the tapping process. A small platinum spatula (prepared by hammering 
thin a short piece of platinum wire i mm. diameter) serves to convey the 
mineral powder from the mortar to the object glass, where it is mounted in 
a drop of the refractive liquid. In actual work care should be taken that 
everything is perfectly clean in order that no foreign particles are introduced 
which might lead the observer to false conclusions regarding the composi- 
tion of the preparation. The drop of refractive liquid should be placed to 
one side and not directly on the powder to be examined, in order that the 
dropper may not be contaminated ; and the powder can then be immersed 
by drawing it by means of the tilted cover-slip into the liquid. 
TABLB 4. 
Refractive indices. 
Liquids. 
.450-1.475 
.480-1.535 
.540-1.635 
.640-1 .655 
.66o-l .740 
.740-1.790 
.790-1.960 
Mixtures of petroleum and turpentine. 
Turpentine and ethylene bromide or clove oil. 
Clove oil and a-monobromnaphthalene. 
a-monobromnaphthalene and a-monochlornaphthalene. 
a-monobromnaphthalene and methylene iodide. 
Sulfur dissolved in methylene iodide. 
Methylene iodide, antimony iodide, arsenic sulfide (realgar), 
antimony sulfide (stibnite) and sulfur.* 
THE DIRECT MEASUREMENT OF REFRACTIVE INDICES. 
For this purpose special apparatus and specially prepared crystal plates 
or prisms are required. Many different methods have been suggested for 
measuring refractive indices and are described in detail in the text-books 
and need not be repeated here. For the petrologist, the total refractometer 
method is the simplest and most convenient, as it requires only a single 
polished surface of any orientation to furnish the three principal refractive 
indices (a, /3, 7) of the crystal. With this method the angles of total re- 
flection are observed for different azimuths of the crystal plate mounted on 
the glass hemisphere of the total refractometer in a liquid of higher refrac- 
tive index. In each position of the crystal plate there are, in general, two 
bands or limiting lines of total reflection ; if these lines be observed for all 
azimuths of the crystal plate, they form two continuous curves from which 
the refractive indices a, /3, y can be obtained directly, 7 being the maximum 
value of the upper curve, a the minimum of the lower curve, and /3 either 
the minimum value of the upper curve or the maximum of the lower. Of 
these two values for /3 the correct one is obtained either ( i ) by means of a 
second plate, in which case the correct value of /3 is common to both plates,f 
*Thii last croup of mixture* has been prepared by Dr. Merwin. of this Laboratory, whose results will be 
published in the near future in the American Journal of Science. Measurements by the writer on the highly 
refracting mixtures cited by P. Zirkel. Lehrbuch d. Petrojcrapbie. ad cd.. I. 40. 1893 (especially mercury 
sulfide dissolved in aniline and quiniline. with a refractive index about a. a), rave values considerably lower 
(- 1.80 or less) than those listed by Zirkel. The original source of these determinations is not given by 
Zirkel and the writer has not been able to ascertain the exact methods by which these mixtures were prepared 
so as to five such high refractive indices (see also A. Himmclbauer. Centralblatt Min. 306, 1009). 
tSoret. Ch. C. R.. 10*. 176. 479. 1888; Arch. Sc. phys. nat. Geneve (3). 30, 377. 1888; Zeitschr. Krist.. 
IS, 45. 1888; Also B. Hecht, Neues Jahrb. Beil. Bd. (6) 341. 1889; A. Brill. Math. Ann. 34, 297. 1889; 
Mtinchen. Sitzungsber.. 13, 433, 1883. L. Perrot. C. R.. 108, 137. 1889; Arch. sc. Phys. nat. Geneve (3). 
31, 113. 1889: C. Viola. Zeitsch. f. Krist.. 31, 40, 1890; A. Lavenir. Bull. Soc. Min.. 14, 100. 1891. 
