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 2V) 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 = 6o° to 70 . 

 The salt therefore has 2E > muscovite. It must therefore be 

 either NaH 2 P0 4 -2 H 2 where 2E = 150 32' or Na 2 HP0 4 - 

 12 H2O where 2E = 86° i'. 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 n = 1.44: if the salt appears to 

 have an index equal to or greater than 1.44 it must be NaH 2 PO4 • 

 2 H 2 0, 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 

 cryst al lography . l 



1 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. 



