234 ELEMENTARY CHEMICAL MICROSCOPY 



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 1 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 

 mixture can roughly be estimated. 2 



The immersion method above described permits an accuracy 

 in the determination of the refractive index within 0.005 =t but 

 with monochromatic light and more refined methods of illumi- 

 nation an accuracy of 0.002 ± or even 0.001 ± may sometimes 

 be reached. 



The Refractive Index of Anisotropic Substances. — Crys- 

 tals are either isotropic or anisotropic. In isotropic crystals 

 light rays are refracted to an equal degree, no matter in what 

 direction through the crystal the rays are sent, since the velocity 

 of transmission of light is the same in all directions through the 

 crystals, providing the crystals have not been subjected to 



1 For exceptionally complete lists of media for refractive index determinations 

 see Johannsen, Manual of Petrographic Methods. 



2 Formulas for calculating the refractive index of a mixture of two liquids each 

 of known index have been proposed, e.g., that of Van der Kolk n (Vi + V 2 ) = 

 niVi -\- 111V1. It is assumed in these formulas that the liquids are miscible in 

 all proportions, that in the final mixtures each component contributes equally its 

 own proportional part of the final index, and that no expansion nor contraction 

 of volume results when the two liquids are mixed. In the laboratory of the author 

 experiments have demonstrated that the results obtained by formulas of this sort 

 are unreliable. Only the first decimal is always correct. When it is necessary 

 to make a liquid of a given index from two liquids by mixing them, the above 

 formula may serve as a guide and the index of the liquid obtained should then be 

 determined with a refractometer or in the absence of sue h an instrument by means 

 of a cell under the microscope as described on page 240. 



