Page:The wonders of optics (1869).djvu/271

 made and filtered through blotting-paper, or at any rate allowed to settle, the room is made quite dark and a piece of common brimstone is ignited. The pale blue light given off is comparatively feeble, but it is very rich in the ultra-violet rays; consequently, when the infusion of horse-chestnut bark is poured into a tall jar of water, beautiful waves of phosphorescent light are seen flashing backwards and forwards as the two liquids mingle. The tincture of stramonium is also possessed of this property, and characters traced on paper with it, although nearly invisible by ordinary daylight, appear distinctly when examined by the light of burning sulphur.