194 Tables 201-203. 



MEDIA FOR DETERMINATIONS OF REFRACTIVE INDICES WITH 

 THE MICROSCOPE. 



TABLE 201. —Liquids, njj (0.689m) = 1.74 to 1.87. 



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

 dicated in the following table can be dissolved, forming saturated solutions having the permanent 

 refractive indices specified. When ready for use the liquids can be mixed by means of a dropper 

 to give intermediate refractions. Commercial iodoform (CHI3) powder is not suitable, but crys- 

 tals from a solution of the powder in ether may be used, or the crystalized product may be 

 bought. A fragment of tin in the liquids containing the Snl4 will prevent discoloration. 



TABLE 202. — Resln-Uke Sulistances, iXj) (0.589m) =1.68 to 2.10. 



Piperine, one of the least expensive of the alkaloids, can be obtained very pure in straw-colored 

 crystals. When melted it dissolves the tri-iodides of arsenic and antimony very freely. The 

 solutions are fluid at slightly above 100° and when cold, resin-like. A solution containing 3 parts 

 antimony iodide to one part of arsenic iodide with varying proportions of piperine is easier to 

 manipulate than one containing either iodide alone. The following table gives the necessary data 

 concerning the composition and refractive indices for sodium light. In preparing, the constituents, 

 in powder of about i mm. grain, should be weighed out and then fused oz'er, not hi, a low flame. 

 Three-inch test tubes are suitable. 



TABLE 203. —Permanent Standard Resinous Media, n^ (0.589m) = 1.546 to 1.682. 



Any proportions of piperine and rosin form a homogeneous fusion which cools to a transparent 

 resinous mass. The following table shows the refractive indices of various mixtures. 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 per cent antimony iodide and 93 per cent piperine should be 

 used over the eye-piece. Any amber-colored rosin in lumps is suitable. 



All taken from Merwin, Jour. Wash. Acad, of Sc. 3, p. 35, 1913. 

 Smithsonian Tabucs. 



