Colour, and what in painting is called transparency, belong chiefly to shade. It has been a common error to ascribe those properties to light only, and hence many have employed a uniform shade tint, regarding shadows as simply darkness, blackness, or the mere absence of light; when, in truth, shadows are infinitely varied by colour, and always so by the colours of the lights which produce them. But while we incline attention toward the relation of colour to shade, both light and shade being strictly co-essential to colour, a vicious extreme must be avoided. For although, as transparent, colour inclines to shade, and, as opaque, it partakes of light; yet the general tendency of colour is to transparency and shade, all colour being a departure from light. Hence it becomes a maxim, which he who aspires to good colouring must never lose sight of, that _the colour of shadow is always transparent, and only that of extreme light objects opaque_. It follows, that white is to be kept as much as possible out of shadow, and black, for the same reason, out of colour. In their stead, whenever it is necessary to cover, opaque tints may be employed, glazed over with transparent colours. Such practice would also be more favourable to durability of the tones of pictures, than the shades and tints produced with black and white. The hues and shadows of nature are in no ordinary case either black or white, which, except as local colours, are always poor and frigid. The perfection of colouring is to combine harmony with brilliancy, unity with variety, and freshness with force, without violating the laws of nature. With regard to the _perspective of colours_, or the manner in which they affect the eye, according to position and distance, it is a branch of aërial perspective or the perspective of light and shade. This is distinguished from linear perspective, or the perspective of drawing, as drawing is from colouring; and they have progressed alike in the art. The most ancient painters seem to have known little of either; and linear perspective was established as science before the aërial, as drawing and composition preceded colouring.