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needle-work called Irish-stitch, done in these shades only; which pleases still, though 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 called it the retiring shade, because by the successive, or continual change in its appearance, it is equally instrumental with converging lines', in shewing how much objects, or any parts of them, retire or recede from the eye; without which, a floor, or hori
zontal-plane, would often seem to stand upright like P.98 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.
1 See p. 7. The two converging lines from the ship, to the point C, under fig. 47, plate 1.
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 2, it can only appear as a flat penciled shade; but being inclosed within some known boundary or outline, 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 2, 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 cu spective 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 completing the idea of those recessions which neither of them alone could do.
Moreover, the outline 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 P. 99 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,
In doing which, it will be found, that, by their correspondency with, and conformity to objects, either composed of straight, curved, waving or serpentine lines, they of course take such appearances of variety
as are adequate to the variety made by those lines ; and by this conformity of shades we have the same ideas of any of the objects composed of the above lines in their front aspects, as we have of them by their profiles; which otherwise could not be without feel. ing them.
Now instead of giving engraved examples of each species of shade, as I have done of lines, I have found that they may be more satisfactorily pointed out and described by having recourse to the life.
But in order to the better and more precisely fixing upon what may be there seen, as the distinct species, of which all the shades of the retiring kind in nature partake, in some degree or other, the following scheme is offered, and intended as an additional means of making such simple impressions in the mind, as may be thought adequate to the four species of lines described in chapter 27. Wherein we are to
suppose imperceptible degrees of shade gradating P. 100 from one figure to another. The first species to be
represented by, 1, 2, 3, 4, 5.
As the first species varies or gradates but one way, it is therefore least ornamental, and equal only to straight lines.
The second gradating contrary ways, doubling the others variety, is consequently twice as pleasing, and thereby equal to curved lines.
The third species gradating doubly contrary ways, is thereby still more pleasing in proportion to that quadruple variety which makes it become capable of conveying to the mind an equivalent in shade, which expresses the beauty of the waving line, when it cannot be seen as a line.
The retiring shade, adequate to the serpentine line, now should follow; but as the line itself could not be expressed on paper, without the figure of a cone *, so neither can this shade be described without the assistance of a proper form, and therefore must be deferred a little longer.
When only the ornamental quality of shades is spoken of, for the sake of distinguishing them from retiring shades, let them be considered as pencilings only; whence another advantage will arise, which is, that then all the intervening mixtures, with their de- P. 101 grees of beauty between each species, may be as easily conceived, as those have been between each class of lines.
And now let us have recourse to the experiments in life, for such examples as may explain the retiring power of each species; since, as has been before observed, they must be considered together with their proper forms, or else their properties cannot be well distinguished.
All the degrees of obliquity that planes, or flat surfaces are capable of moving into, have their appearances of recession perfected by the first species of retiring shades, which may evidently be seen by setting
opposite a door, as it is opening outwards from the eye, and fronting one light.
But it will be proper to premise, that when it is quite shut, and flat or parallel to the eye and window, it will only have a penciling shade gradating upon it, and spreading all around from the middle, but which will not have the power of giving the idea of recession any way, as when it opens, and the lines run in per-. spective to a point; because the square figure or parallel lines of the door, do not correspond with such shade; but let a door be circular in the same situation, and all without side, or round about it, painted of any other colour, to make its figure more distinctly seen, and it will immediately appear concave like a bason,
the shade continually retiring; because this circular P. 102 species of shade would then be accompanied by it's corresponding form, a circle '.
But to return; we observed that all the degrees of obliquity in the moving of planes or flat surfaces, have the appearances of their recession perfected to the eye by the first species of retiring shade. For example, then; when the door opens, and goes from its parallel situation with the eye, the shade last spoken of, may be observed to alter and change its round gradating appearance into that of gradating one way only; as when a standing water takes a current upon the least power given it to descend.
Note, if the light were to come in at a very little hole not far from the door, so as to make the gradation sudden and strong, like what may be made with a small candle held near a wall or a wainscot, the bason would appear the deeper for it.