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.