In a paper which I had the honour of submitting to the Society, at the end of last session, I gave a detailed account of the effects of mechanical compression and dilatation, in communicating to glass and other uncrystallised bodies, all the properties of doubly refracting crystals. I had at that time attempted, without success, to alter the polarising structure of doubly refracting crystals, although I applied the force of powerful screws to Topaz, Rock-crystal, and Calcareous spar. All the specimens which I employed were crushed to pieces by the pressure, but exhibited no traces of the coloured rings when exposed to polarised light.