While examining, in polarised light, some of the crystals which he had discovered in Topaz, the author observed certain optical phenomena, depending on a peculiarity of structure. This peculiarity is manifested either in the depolarisation of light, when it gives rise to four quadrants of light, separated by the radii of a black rectangular cross similar to the central portion, or the tints of the first order in the uniaxal system of polarised rings, or in the unequal refraction of common light, which gives rise to the mirage of a luminous point, in the form of concentric circles surrounding the centre of force. In every case there was found a quadrangular cavity in the centre of the intersection of the cross, generally dark and opaque, but in one case having a luminous spot in the centre. These cavities are from the to the of an inch in diameter.