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The use of those immersion methods for determining refractive index in which a regulation of t emperature is employed has been seriously limited by the impossibility of obtaining interference figures. A thin cell is described which overcomes this obstacle, so that it is possible to know the exact orientation of the crystal grains which are chosen for identification. A distinct jmpediment in applying the double-variation method of Emmons 1 for determining refractive indices has been the impossibility of obtaining conoscopic interference figures. As it is only in special cases, therefore, that the optical orientation of a crystal grain can be accurately known, the peculiar advantages of the method are almost lost. Emmons endeavors to remedy this situation in a later publication/ in which he describes an application of the universal stage to the problem, but this method is cumbersome and slow, and a large proportion of particles are too small to be studied with it.3 Emmons mounts the preparation upon a cell with glass windows, between which water of controlled t emperature is circulated. The temperature of the liquid mounting of the crystal fragments quicldy becomes almost the same as that of the water. The refractive index of the liquid is modified accordingly and can be made to match an index of the immersed crystal fragments. But this cell is so thick that the preparation is raised above the region traversed by the more oblique rays. Oonsequently, satisfactory interference figures are impossible. To avoid this difficulty, the writer, three years ago, constructed a cell of different form. It has been successful, and during three years of use no reason has appeared for modifying the design shown in figure 1. Three pieces of brass A, B, and C, which are 0.5, 0.9, and 0.2 mm thick, respectively, were cut along the full lines with a jeweler's jig saw. Two holes were drilled in A, as shown. From B, clamped upon a plane surface, half the thickness of the parts indicated by stippling was removed with a chisel, and, in the same manner but from the reverse side, half the thickness was removed from those parts indicated by crosshatching. The bottom of A, the top and bottom of B, and the top of C were thinly coated with solder; the 1 R. C. Emmons, Am. Mineral. 13, 504 (1928); H , 414 (1929). • A. N. Winchell, The Microscopic Characters of Artificial Inorganic Solid Substances or Artificial Minerals, (John Wiley & Sons, Inc., 1931). Chapter VIII, on the Universal Stage, was written by Emmons. 3 These difficulties are clearly set forth in a review of Winchell's book by O. Tunell and G. W. Morey, J. Am. Chern. Soc. 5!, 2574-7 (1932). 97 98 Journal of Research of the National Bureau of Standards [Vot.16

A thin cell for use in determining the refractive indices of crystal grains