When I come to treat of colouring, I shall particularly shew in what manner the gradating of prime tints serve to the making a beautiful complexion; in this place we shall only observe how nature hath by these gradating shades ornamented the surfaces of animals; fish generally have this kind of shade from their backs downward; birds have their feathers enriched with it; and many flowers, particularly the rose, shew it by the gradually-increasing colours of their leaves.
The sky always gradates one way or other, and the rising or setting sun exhibits it in great perfection, the imitating of which was Claud. de Loraine's peculiar excellence, and is now Mr. Lambert's: there is so much of what is called harmony to the eye to be produced by this shade, that I believe we may venture to say, in art it is the painter's gamut, which nature has sweetly pointed out to us in what we call the eyes of a peacock's tail: and the nicest needle-workers are taught to weave it into every flower and leaf, right or wrong, as if it was as constantly to be observed as it is seen in flames of fire; because it is always found to entertain the eye. There is a sort of needle-work called Irish-stitch, done in these shades only; which pleases still, tho' it has long been out of fashion.
There is so strict an analogy between shade and sound, that they may well serve to illustrate each other's qualities: for as sounds gradually decreasing and increasing give the idea of progression from, or to the ear, just so do retiring shades shew progression, by figuring it to the eye. Thus, as by objects growing still fainter, we judge of distances in prospects, so by the decreasing noise of thunder, we form the idea of its moving further from us. And with regard to their similitude in beauty, like as the gradating shade pleases the eye, so the increasing, or swelling note, delights the ear.
I have call'd it the retiring shade, because by the successive, or continual change in its appearance, it is equally instrumental with converging lines[13], in shewing how much objects, or any parts of them, retire or recede from the eye; without which, a floor, or horizontal-plane, would often seem to stand upright like a wall. And notwithstanding all the other ways by which we learn to know at what distances things are from us, frequent deceptions happen to the eye on account of deficiencies in this shade: for if the light chances to be so disposed on objects as not to give this shade its true gradating appearance, not only spaces are confounded, but round things appear flat, and flat ones round.
[13] See p. [17]. The two converging lines from the ship, to the point C, under fig. 47, plate I.
But although the retiring shade hath this property, when seen with converging lines, yet if it describes no particular form, as none of those do in fig. 94, on top of plate II, it can only appear as a flat-pencil'd shade; but being inclosed within some known boundary or out-line, such as may signify a wall, a road, a globe, or any other form in perspective where the parts retire, it will then shew its retiring quality: as for example, the retiring shade on the floor, in plate II, which gradates from the dog's feet to those of the dancer's, shews, that by this means a level appearance is given to the ground: so when a cube is put into true perspective on paper, with lines only, which do but barely hint the directions every face of it is meant to take, these shades make them seem to retire just as the perspective lines direct; thus mutually compleating the idea of those recessions which neither of them alone could do.
Moreover, the out-line of a globe is but a circle on the paper; yet, according to the manner of filling up the space within it, with this shade, it may be made to appear either flat, globular, or concave, in any of its positions with the eye; and as each manner of filling up the circle for those purposes must be very different, it evidently shews the necessity of distinguishing this shade into as many species or kinds, as there are classes or species of lines, with which they may have a correspondence.