This may be done variously in water and oils. A simple way is to rub in slowly with a small camel-hair brush a minute piece of dry colour laid upon the part, as of flesh tint for the face, etc. When properly distributed, the paper may be breathed upon, and the tint will not easily be rubbed off. Or it may be carefully coated with gelatin. M. Minotto has described a plan of colouring on the back of [the paper. The picture, being held up to the light, is first faintly outlined, on the reverse side; colours are then laid on, of water or oil, as preferred, on this side. When dry, the paper is rendered transparent by a varnish, and the colours will then appear through it with all the delicacy and effect of a miniature on ivory. Good strong writing paper is best for this purpose; the colours must be vivid; and the varnish may consist of Canada balsam dissolved in turpentine, or a mastic varnish may be used, or turpentine and wax, or oil. Pigments. A few of these have been noticed before; see Indigo, Lakes, Prussian Blue, Purple of Cassius. They generally constitute a distinct branch of manufacture, but a brief account of the composition of some of them may be useful. Those of which the colouring matter is derived from the animal and vegetable kingdoms will first be noticed; then the mineral colours.