THE DETERMINATION OF REFRACTIVE INDEX 237 
In uniaxial crystals the determination of which index is co 
and which e is comparatively simple since e coincides with the 
crystallographic c axis; but in the case of biaxial crystals it 
is seldom that a chemist possesses either the knowledge or a 
microscope sufficiently well equipped to definitely locate the 
different axes of elasticity, since their directions are indicated by 
neither the crystallographic nor the optical axes. For this 
reason it is wiser for the chemist-analyst to follow the methods 
of Kley,^ Bolland ^ and others, and record values as obtained 
in the method given below. 
Swing the polarizer in place, having first removed all condens¬ 
ing lenses. Place upon the stage an object slide carrying the 
crystals or crystal fragments to be examined immersed in a 
liquid of known refractive index and covered with a tiny thin 
cover glass. Place the analyzer over the eyepiece (or slide it 
into the tube if an instrument of this type is used) and set the 
graduated circles of both prisms at zero so that their planes of 
vibration are crossed. Turn the stage of the microscope until 
the crystal selected for observation extinguishes; remove the 
analyzer. Ascertain by raising the objective whether the index 
of the crystal is greater or less than the liquid; check results by 
oblique light by placing the finger part way across the opening 
of the polarizer. Substitute one liquid after another until the 
refractive index of the crystal is ascertained, being very careful 
not to alter the position of the crystal. If the crystal is moved 
replace the analyzer and readjust the crystal to the position of 
extinction. Read the position of the crystal as indicated on the 
circumference of the stage and rotate the stage so as to turn the 
crystal exactly 90 degrees to its position of extinction and pro¬ 
ceed with the determination of the refractive index just as before. 
The two values obtained will, in the case of uniaxial crystals, be 
the indices e and w. When dealing with biaxial crystals in order 
to use the values in Bolland’s tables first set the crystal so that 
its prism edge lies parallel to a plane passing through the short 
diagonal of the polarizing nicol. Next determine the index for a 
1 Kley, Zeit. anal. Chem., 43 (1904), 160. 
2 Bolland, Monats., 29 (1908), 991; 31 (1910), 387. 
