The perspective of colours depends upon their powers to reflect the elements of light, powers which are by no means uniform. Accordingly, blue is lost in the distance before red, and yellow is seen at a point at which red would disappear; yet blue preserves its hue better than yellow, because colours are cooled in the distance. In this respect, the compound colours partake of the powers of their components, in obedience to a general rule, by which local colours closely connected with black are first lost in the distance, and those nearly related to white disappear last. The same may be said of local light and shade, the latter of which is totally lost at great distances; and it is for this reason the shadowed side of the moon is not generally seen. These powers of colours are, however, varied by mist, air, altitude, and mixture, which produce evanescence; and by contrast, which preserves the force of colours by distinguishing them. Colours do not decline in force so much by height as by horizontal distance, because the upper atmosphere is less dense and clouded with vapour: and hence it is that mountains of great elevation appear much nearer than they really are. From all these circumstances, it is evident that a simple scumbling or uniform degradation of local colours will not effect a true perspective--for this will be the aërial of light and shade only--but such a subordination of hues and tints, as the various powers of colours require, and as is always observable in nature. In furnishing or _setting the palette_ philosophically and upon principle, it is necessary to supply it with pure blue, red, and yellow; to oppose to these an orange, of a hue that will neutralise the blue--green, of a hue that will neutralise the red--and purple, of a hue that will neutralise the yellow; and so on to black and white, which will neutralise each other. As in nature, the general colour of the sky is blue, and the colour of light is always opposite to that of the sky and shade, so the white which is to represent light should be tinged with the orange of the palette sufficiently to neutralise the predominant coldness of black. Pure neutral white may thus be reserved as a "local" colour, which is a technical term for the _natural_ colour of an object, unvaried by distance, reflection, or anything interfering with distinct vision; although, properly speaking, local colours are subject to all the relations and effects of the places they occupy in a composition--whether of light, shade, reflection, or distance.