258 
ELEMENTARY CHEMICAL MICROSCOPY 
phosphates the data given, are for light of medium wave-length 
yellow (X 5893) at room temperature. The axial angle is some¬ 
times greater for red than for violet or may be less for red than 
for violet. This variation is known as the Dispersion of the Optic 
Axes and is indicated by the formulas: p > v and p<v, where 
the Greek letter rho — p refers to red rays and v to violet rays. 
It is usually sufficient for most purposes in qualitative analysis 
to know whether the axial angles are large or small. A simple 
method is to compare the appearance of the interference figure 
(see page 259) obtained from the unknown with that given by 
a mineral (or other substance) of known 2E (or 2F) viewed 
under the same conditions. If for example it had been possible 
to observe a biaxial interference figure in the case of the sodium 
phosphates cited above — obviously the salt could not have 
been the trisodium phosphate (uniaxial). A plate of mica sub¬ 
stituted for the preparation gives an interference figure in which 
the distance between the optic axes as measured on an eyepiece 
micrometer (with Bertrand lens in place) is less than that of 
the crystal in question. In mica (muscovite) 2E = 60° to 70°. 
The salt therefore has 2E > muscovite. It must therefore be 
either NaH2P04-2 H2O where 2E = 150° 32' or Na2HP04- 
12 H2O where 2E = 86° 1'. In this case it will be quite safe to 
decide from simple inspection of the axial angle which salt the 
unknown is, for there is a very great difference in the magni¬ 
tudes of the angles. To further confirm our decision we may 
test the crystals with a liquid w = 1.44: if the salt appears to 
have an index equal to or greater than 1.44 it must be NaH2 PO4 • 
2 H2O, if it shows an index of less than 1.44 it is the salt with 
7H2O. 
Determinations of optic angles are complicated problems 
requiring great care and good working knowledge of optic 
crystallography.^ 
* For further information see: Wherry. The Application of Optical Methods 
of Identification to Alkaloids and other Organic Compounds. Bui. 679, Bur. 
Chem. U. S. Dept. Agric. (1918). Weinschenk-Clark: Petrographic Methods. 
Johannsen: Manual of Petrographic Methods. Wright, F. E.: Methods of 
Petrographic-Microscopic Reasearch; Bui. 158, Carnegie Inst., Washington. 
Peck: The Polarizing Microscope in Ceramics. J. Amer. Ceram. Soc. 1919, 695. 
