CHEMICALS, CRYSTALS, AND MINERALS 225 



the phase microscope. Water-in-oil emulsions are generally revealed 

 Avith 0.2A+0.25X and 0.14A — 0.25X diffraction plates in the oil im- 

 mersion objective, and oil-in-\vater emulsions are often seen better 

 with 0.2A+0.25X and 2.5B— 0.25X diffraction plates. In some cases, 

 especially when measurement is to be made, the 0.14A— 0.25X plate is 

 preferable. To arrest the Brownian and other movements usually 

 encountered with these preparations, photomicrographs are reciuired 

 and the measurements made on the photographs. Suggestions for mak- 

 ing preparations are given in Sections 3.3 and 3.6 of Chapter IV (see also 

 Section 3 of this chapter). Phase identification is possible and sol-gel 

 reversals may be examined with the phase microscope. 



Refractive index determination is more sensitive when a phase ob- 

 jective is used. Differential visual contrast may be obtained by mount- 

 ing the specimen in a medium of proper index (see Section 3.2 of Chap- 

 ter IV). 



Smithson (1946, 1948) found phase microscopy to have the following 

 advantages in mineralogy: index differences of less size were visible, 

 relief was improved, and heterogeneity was shown. The phase micro- 

 scope did not show strain. Adding a Polaroid polarizer to the condenser 

 showed "twinkling" with less birefringence, inclusions, and intergroAvths, 

 such as one feldspar with another. Using a polarizer and an analyzer 

 with phase sometimes was helpful and other times not, depending on the 

 nature of the specimen. Zoning was shown better in plagioclase feldspar, 

 as was the relation of the zoned regions to the sun'ounding feldspar. 

 Smithson points out that path differences arising from differences in 

 thickness, differences in compensation, and, with polarized light, dif- 

 ferences in refractive index arising from optical orientation are made 

 visible with the phase microscope; but it may be difficult to determine 

 which are involved, especially when roughness of the specimen is also 

 part of the problem. 



Stereophotomicrographs should assist in evaluating the roughness 

 factor, and replica technics also should be helpful. When two colorless 

 isotropic substances with only slight differences in refractive index 

 are intermingled, the phase system is the only effective microscopical 

 method for observation. For example, a phyllite of silica and feldspar 

 clearly showed boundaries with a 0.2A+0.25X diffraction plate when the 

 index difference was only 1 in the fourth decimal place. 



The mathematical analysis of the combined effects of phase and polar- 



FiG. VI. 1. A-D, paraffin crystals supporting a thin tissue section, 135 X: A, 



brightfield. B, dark-contrast (B — ) phase. C, dark-contrast (A — ) phase. D, 



bright-contrast (A + ) phase. E-F, paraffin microcrystals from isopropyl alcohol, 



1250 X: E, brightfield. F, bright-contrast phase. 



