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 ^ 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.^ 
The immersion method above described permits an accuracy 
in the determination of the refractive index within 0.005 ± 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 
* 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 V2) = 
wiFi + ihV2- 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 such an instrument by means 
of a cell under the microscope as described on page 240. 
