THE DETERMINATION OF REFRACTIVE INDEX 
227 
In like manner if we have a series of crystals, or fragments of 
transparent solids whose indices of refraction we know, it is 
possible to roughly ascertain the index of a given liquid. 
The index of refraction is a constant for any given substance 
of definite composition. Its determination often affords a ready 
means of identification or differentiation and in many instances 
is in fact the only simple means at our command for the recogni¬ 
tion of a compound. By the index of refraction is meant the 
ratio of the sine of the angle of incidence to the sine of the angle 
of refraction. It is customary to refer indices of refraction to 
that of air, which is taken as unity n ^ i. 
Although the identification of compounds through determina¬ 
tions of their refractive indices by the immersion method and 
the microscope has long been practiced by mineralogists, petrol- 
ogists and microscopists, it is only within the last few years 
that chemists have awakened to the value ot the data so easily 
obtained. 
A determination of the refractive index is of special value in 
the qualitative analysis of soils, sands, mineral fragments, etc., 
in the examination of plant and animal fibers,^ in the study of 
crystalline residues, in the differentiation of isomeric compounds 
and in the study of materials which, although pure, cannot 
properly be separated from foreign matter. 
In order to determine the refractive index of crystalline solids 
we may proceed as described below: 
Determination of the Refractive Index of Isotropic Substances. 
— One or more tiny fragments or crystals of the material are 
placed upon a clean slide, a small drop of a liquid of known 
refractive index is placed upon a small, scrupulously clean cover- 
glass and the cover with its drop is inverted and laid upon the 
solid under investigation, care being observed in lowering the 
glass with its drop of liquid to avoid the formation of air bubbles. 
Place the preparation under the microscope with the Abbe con¬ 
denser 2 raised as high as it will go. Focus with a 32-millimeter 
or 16-millimeter objective. Under these conditions the prepa- 
^ See Herzog: Chem. Zeit. 40 (1916), 528. J. Soc. Ch. Ind. 36 (1916), 832, Abs. 
* These directions refer specifically to the Chemical Microscope described on p. 19. 
