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The US Bureau of Mines has developed an automated reflectance scanning microscope system, primarily for determination of volume proportions, size distribution, and mode of association of the physical (petrographic) components of coals. The process, which is potentially more rapid and objective and less tedious than a visual microscopic procedure, involves magnetic tape recording of several million reflectance readings on micrometer sized areas of polished coal surface, and computer analysis and interpretation of some characteristics of the coal components. Results of petrographic analysis by automated reflectance scanning microscopy and conventional visual microscopy have been compared for four coals on the basis of a partially arbitrary interpretation of reflectance distributions. Such comparisons indicate fairly good agreement of results by the two methods for at least three of the coals, particularly in showing the considerably higher proportions of exinite and lower amounts of vitrinite in dull coal samples contrasted with bright coals. Despite the interpretation being arbitrary, the interest of other laboratories in such research, and possible advantages of greater objectivity and rapidity of analysis and correlation with coal utilization, encourage further study along these lines.

The nature of the granular micrinite component of coal and relevant evidence offered by other investigators are discussed. Electron micrographs are presented that clearly show rounded discrete particles in the size range 100–1000 A. Ambiguous evidence offered by various investigators concerning the reactivity of this component is also discussed.

The utility of reflectance of the vitrinite component of coals as a criterion of degree of coalification has been investigated. Present standard systems for classification of coals by rank are intended mainly as guides for utilization behaviour, and there is a need for more widespread acceptance and usage of an objective rank criterion based more firmly on physical/chemical characteristics of a coal. The relations of reflectance of vitrinite to other commonly used or proposed rank parameters have been studied, using voluminous data supplied by United States and European laboratories. These relations were found to be excellent for high-rank bituminous coals, good for anthracitic coals, fair for low-rank bituminous coals, and rather poor for subbituminous coals, lignites, and brown coals. The data for the last category are least numerous, and work is continuing on their reflectance—rank characteristics, especially for the International Commission on Coal Petrology. The reflectance ranges established for present rank categories are listed and it is proposed that they be included in standard classification systems as supplementary criteria of rank.

Refractive and absorptive indexes and thicknesses of ultra-thin fragments of graphite have been determined at a wavelength of 5460 A by direct transmission and phase retardation measurements. A graphical procedure has been developed for this calculation. Values of 2.22 and 0.51 for indexes of refraction and absorption of graphite at 5460 A were obtained by these direct measurements as compared to 2.15 and 0.66 by reflectance. Extinction coefficients have also been determined in the wavelength region 2100—5500 A. In the visible spectrum the values of the coefficient indicate strong electronic absorption by charge carriers. A strong absorption peak has been observed near 2600 A; its profile is explained well by the theory of anomalous dispersion. The peak position (4.8 eV) apparently corresponds to the energy difference between the states of maximum density in the conduction band and the adjacent valence band of graphite. The peak value of the extinction coefficient is also shown to be predictable by the theory of anomalous dispersion.