Joan R. Clark using a two-circle optical goniometer to study a crystal of colemanite. Washington, D.C. 1960.

Album caption and index card: Morphology of a crystal of colemanite, CaB3O4 (OH) 3.H2O, being studied with a two-circle optical goniometer by Joan R. Clark. The crystal is mounted on a goniometer head on the vertical circle at the left, the horizontal circle is at the bottom of the instrument, a light source is at the back, right, and the optical system is in the center of the picture. By rotating the crystal around the axis of either the vertical or the horizontal circle it is brought into position to reflect light from the crystal faces at given angles. The reflections can be observed through the optical system, their angular readings recorded, and hence the angles between various crystal faces can be measured. Geologic Division, [...]