SCIENTIFIC COMMUNIC.4 TIONS 7 3 7 CHECKING OF OPTICAL DATA DERIVED FROM

Complete optical data are at present available for only a limited number of ore minerals. Reflectivity, bireflectance, phase difference and the rotation of the azimuth of vibration of reflected light have been systematically inves-tigated (1, 2 and others) and reliable published data are now available. Refractive and absorption indices have not, however, received the same attention and relatively little precise information is to be found in the litera-ture. This is principally because these properties are not as readily or conveniently determined with the ore microscope as those listed above. Polarimetric methods that examine the nature of light reflected at oblique incidence (e.g., Tool, 8) will yield the required constants for refractive index (n) and absorption index (K) but the apparatus necessary is not in common use. Furthermore, the area of polished surface required is con-siderably greater than that needed for the measurement of reflectivity and rotation angles. An adaptation of these polarimetric methods for determina-tion of n and K of small grains under the ore microscope has been describe