SUMMARYAutomated reflectance microscopy (ARM) systems employ a microscope photometer in conjunction with a computer which controls a motor‐driven stage drive and records and categorizes the photomultiplier output. In a system being developed at The Pennsylvania State University, the object stage moves continuously under the microscope objective at a constant speed and in a predetermined raster. With a scan speed of 1600 μm/s and the computer recording the photo‐multiplier output every 1.25 ms, each recording represents a scan of 2 μm. An aperture below the photomultiplier is set to give an effective area of measurement of 2 μm square, so that a continuous series of readings is obtained on adjacent fields of this size. The difference in reflectance between coal and pyrite particles in polished coal pellets enables pyrite chord‐size distributions to be measured; consecutive recordings on pyrite grains are summed to give a measure of chord length. Size distributions determined by ARM compare well with values measured by visual microscopy. In the course of analysis of a single coal pellet, approximately 400,000 individual readings are recorded, the entire operation of scanning and printout of data taking approximately 18 min. Coal reflectograms obtained by ARM may be used for rank measurements and to indicate petrographic comp