The colours of the striae in mica

Abstract

On examining even the most regularly split and transparent pieces of mica by diffuse reflected light, a few fine hair-like and rather irregular lines may generally be seen running along the surface. We have found that these lines or striae show some very interesting effects when mica is examined in a Töpler "Schlieren" apparatus. The sheet as a whole, being optically good, remains invisible, but the striae shine out as brilliant and vividly coloured lines of light, the colours being different for different striae, and changing in a remarkable manner as the inclination of the mica relatively to the direction of the light in the apparatus is altered. For instance, a stria at normal incidence may appear crimson and, as the mica is rotated about an axis in its own plane, become successively purple, green, yellowish-green, yellow, orange, scarlet-red, green, yellow, and red.