ABSTRACT

A procedure for directly measuring the shortest axis of an individual particle by focusing successively on its mounting surface and then on its highest point is found to be feasible. This distance, as read from a calibrated fine-focus drum, is both highly accurate and sufficiently precise for most purposes, though an easily determined correction factor may be required for built-in bias of the focus calibration. Comparison of 200 optical height measurements on 40 grains with paired observations derived by Plumley's (1948) eyepiece micrometer method demonstrates an identity of mean values. Optical height data are nearly three times less precise; nevertheless, roughly 170 of 200 observations occur within six percent of their group mean values throughout the sand size range.

After initial calibration of the microscope, no preparatory stages in grain mounting or equipment manufacture are required; the routine petrographic microscope and accessories are sufficient for optical height determinations. The result is a procedure which permits essentially simultaneous measurement of all three principal grain diameters, giving immediate access to three-dimensional grain size and shape information and the advantage of direct measurement.