CLAY AND DUST 227 



ization with various shaped anisotropic specimens is very complicated 

 and incompletely known at present. With such specimens the addition 

 of a polarizer, an analyzer, or both may give either appreciably better 

 detail or may lessen the contrast. This can be done easily with pieces 

 of Polaroid (sextant grade H is recommended) and is worth trying on an 

 empirical basis. The pictures of microgranite (Fig. VI. 2) illustrate 

 some of the possibilities of combined phase and polarization microscopy. 



2. CLAY AND DUST 



Samples of clays usually contain a considerable range of particle sizes, 

 and more than one diffraction plate may be necessary for the complete 

 observation of the sample. The larger particles usually show better 

 with 0.2A±0.25X and the smaller ones with 0.07A±0.25X diffraction 

 plates. When the range of sizes is too great, the particles may be 

 separated by sedimentation. Some specimens may be examined to 

 advantage with B— diffraction plates. By altering the refractive index 

 of the mounting medium, contrast may be changed to give optimal 

 visibility. In fact, it is possible to make some components invisible 

 with a mounting medium of the same refractive index as the unwanted 

 grains, thereby facilitating the examination of the other particles, 

 ^'isibility is improved with counting chambers having the lines ruled into 

 the glass (Kohler and Loos, 1941; Magiiozzi, 19-48). 



By varying the dispersion difference between the specimen and its 

 surround by means of an appropriate mounting medium, it is possible 

 to add color contrast to the densiphase contrast from the phase micro- 

 scope. Dust containing silica mounted in nitrobenzene (no = 1.55) 

 reveals the silica as violet-colored particles when a 0.2A + 0.255X or a 

 0.2A+0.35X phase objective is used. This simplifies finding the smaller 

 particles and provides differentiation when counts are made to estimate 

 the relative composition of a sample. Bright-contrast (A+) phase 

 objectives are preferable, because they show the colors better than the 

 dark contrasts, and the eye sees small bright objects on a darker field 

 more easily than dark ones on a bright field (see Section 3.3 of Chapter 



IV). 



Dust counts made with a phase microscope will be more accurate 

 because of the increased visibility of the particles and more complete 

 because smaller particles will be seen. These difTerences should be kept 

 in mind when counts made with the phase microscope are compared 

 with those made with a brightfield microscope. With proper choice of 

 mounting or dispersing medium, differential dust counting will be pos- 

 sible for components of the sample that look alike with the brightfield 

 microscope. 



