If the work is done at night, an Engraver’s glass is requisite in order to concentrate the light without glare upon the shell. There are two kinds of these glasses; one is filled with water in which sulphate of copper is dissolved, and clarified with oil of vitriol; the other, which costs 10s., consists of a large green glass eye, which moves up and down a brass rod, and is screwed to the required height. This is the better glass to use, as the oil of vitriol, however much diluted, would, by the accidental breakage of the globe, cause the destruction of any carpet over which the liquid ran. But no glass is required during the day-time, and no artificial light is equal to the natural light of day; work should therefore be confined to hours before dark.