The polished surface technique is used to study the nature and occurrence of opaque matter in coal. The appearance of the opaque matter under reflected light was determined from a polished, thin section, in which the opaque constituents were first established under transmitted light.Two distinctly different forms of opaque matter are recognized, namely, fusain lenses and opaque attritus. Based upon the presence or absence of a botanical structure, the opaque attritus is subdivided into two groups of opaque ingredients. For those devoid of structure the term micrinite is used. Micrinite occurs in three forms: finely divided granular, massive, and as groundmass. Opaque attritus with structure consists of fusinite, semifusinite, and sclerotinite (fungal matter).Evidence for the origin of the opaque matter during the early stages of coal formation is given. A distinction is made between the opaque matter of "primary" origin, and the opacity of high-rank coals, which is due to increased coalification. It is shown that in anthracite the opaque matter of "primary" origin can be recognized under reflected light with the aid of etched, polished sections. No increase in the amount of "primary" opaque matter with increase in rank has been observed.The percentage of opaque matter is determined with an integrating stage, using polished specimens of ground coal, mounted in lucite. The results are compared with the proximate analyses of the coal investigated. They show that a high percentage of opaque matter corresponds with a high percentage of fixed carbon and usually a high percentage of ash.Photomicrographs illustrate the paper.