THE DETERMINATION OF REFRACTIVE INDEX 191 



I 



indices for use in this immersion method, it is advantageous to 

 select those having a slightly greater color dispersion than will 

 be found in the solids to be tested. But highly dispersive liquids 

 must be avoided since the color bands or halos are then so marked 

 as to seriously interfere with the recognition of dark contours. 



Having ascertained as described above whether the crystal 

 fragment has a higher or a lower index than that of the liquid 

 first tried, and thus in which direction to proceed, a second liquid 

 whose index is probably very much nearer that of the solid is 

 chosen. The first liquid is carefully removed by absorbing it 

 with a bit of filter paper, a drop of the liquid next to be applied is 

 added and allowed to flow completely around the crystal; after 

 standing a few moments this is removed as before and a new por- 

 tion added. The preparation is tested by raising the objective 

 and by the half-shadow method to learn whether the solid or the 

 liquid has the higher index. The process is repeated until the 

 proper liquid has been found. In making the trials add first a 

 liquid of a higher then one of lower value. When sufficient solid 

 material is available it will be found that time will be saved and 

 much more reliable data obtained if an entirely new preparation 

 is made with each liquid. This also avoids wasting valuable 

 liquids. 



At the end of the chapter will be found tables l of liquids for 

 use in the determination of refractive indices. In Table IV will 

 be found the indices of isometric crystals useful in estimating 

 the refractive indices of liquids. 



If it is found that the index of no liquid in a series at hand cor- 

 responds to that of the crystal under observation, mixtures of 

 two liquids may be made and the index of refraction of the mix- 

 ture can roughly be estimated from the formula, 2 



*hVi + %V 2 = n (V, + V,), 

 in which Vi and 2 are the volumes of the two liquids, n\ and n^ 



1 For exceptionally complete lists of media for refractive index determinations 

 see Johannsen, Manual of Petrographic Methods. 



2 Schroeder \an der Kolk, Mikroskopische Krystallbestimmung, Weisbaden, 

 1898, p. 13. 



