On Circular Crystals

Extract

The author, after mentioning Mr Fox Talbot's observation, in 1836, of circular crystals from a solution of borax in phosphoric acid, stated, that about twenty years before Mr Talbot's paper was published, he had obtained circular crystals from oil of mace, and from a mixture of that oil with tallow or rosin. These circular crystals are groups of radiating prisms, in optical contact, so as to appear like individual crystals. Viewed by polarized light, they exhibit four luminous sectors, separated by a rectangular black cross, which often has its arms so divergent, as to form four dark sectors. The arms of the cross are parallel and perpendicular to the plane of primitive polarization. When a bright disc of ordinary light was looked at through these circular spots, there was seen a halo, or two halos, produced by the crystals of the oil of mace.