561 



TABLES 598-601.— MEDIA FOR DETERMINATIONS OF 

 REFRACTIVE INDICES WITH THE MICROSCOPE 



TABLE 598.— LIQUIDS, n D (0.589 M ) = 1.74 to 1.78 



In 100 parts of methylene iodide at 20° C the number of parts of the various substances 

 indicated in the following table form saturated solutions having the refractive indices 

 specified. When ready for use the liquids can be mixed to give intermediate refractions. 

 Commercial iodoform (CHI 3 ) powder is not suitable, but crystals from a solution of the 

 powder in ether may be used, or the crystallized product may be bought. A fragment of tin 

 in the liquids containing the SnL will prevent discoloration. 



TABLE 599.— RESINLIKE SUBSTANCES, n D (0.589,*) = 1.68 to 2.10 



Piperine, an inexpensive alkaloid, comes in very pure straw-colored crystals. Melted, it 

 dissolves the tri-iodides of Sb and As very freely. The solutions are fluid at slightly above 

 100° and when cold, resinlike. Three parts antimony iodide to one part of arsenic iodide 

 with varying proportions of piperine are easier to manipulate than one containing either 

 iodide alone. In preparing, the constituents, in powder of about 1 mm grain, should be 

 weighed out and then fused over, not in, a low flame. Three-inch test tubes are suitable. 



Percent iodides 00 



Index of refraction 1.683 



TABLE 600.— PERMANENT STANDARD RESINOUS MEDIA, 

 n D (0.589 M ) = 1.546 to 1.682 



Any proportions of piperine rosin form a homogeneous fusion which cools to a trans- 

 parent resinous mass. On account of the strong dispersion of piperine the refractive indices 

 of minerals apparently matched with those of mixtures rich in this constituent are 0.005 

 to 0.01 too low. To correct this error a screen made of a thin film of 7 percent antimony 

 iodide and 93 percent piperine should be used over the eyepiece. Any amber-colored rosin 

 in lumps is suitable. 



00 



10 



20 



30 



40 



50 



60 



70 



80 



90 



100 



Percent rosin 

 Index of 

 refraction 1.683 1.670 1.657 1.643 1.631 1.618 1.604 1.590 1.575 1.560 1.544 



TABLE 601.— SUBSTANCES, n u = 1.39 to 1.75 



SMITHSONIAN PHYSICAL TABLES 



