There is often a vicious predilection of some artists in favour of a particular colour, from which many of our best colourists have not been totally free, and which arises from organic defect, or mental association. Such predilection is greatly to be guarded against by the colourist, who is every way surrounded by dangers. On the one hand, there is fear lest he fall into whiteness or chalkiness; on the other, into blackness or gloom: in front he may run into fire and foxiness, or he may slide backward into cold and leaden dulness: all of which are extremes he must avoid. There are also other important prejudices to which the eye is liable in regard to colours individually, that demand his particular attention. These are occasioned by the various specific powers of single colours acting on the eye according to their masses and the activity of light, or the length of time they are viewed. By consequence, vision becomes over-stimulated, unequally exhausted, and endued, even before it is fatigued, with a spectrum which not only clouds the colour itself, but gives a false brilliancy by contrast to surrounding hues, so as totally or partially to throw the eye off its balance, and mislead the judgment. This derangement of the organ may be caused by a powerful tint on the palette, a mass of drapery, the colour of a wall, the light of a room, or other accidental circumstance; and the remedy is to refresh the eye with a new object--of nature, if possible--or to give it rest. The powers of colours in these respects, as well as of pigments individually, together with their reciprocal action and influence chemically, will be adverted to under their distinct heads. The attention of the artist to the individual powers of pigments, although it may be of less concern than the attention to general effect in colouring, is by no means less necessary in practice. For he who would excel in colouring must study it from several points of view, in respect to the whole and the parts of a picture, as regards mind and body, and concerning itself alone. To this end, is needed a knowledge of his pigments individually.