But it will be proper to premise, that when it is quite shut, and flat or parallel to the eye and window, it will have only 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 perspective 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 species of shade would then be accompanied by its corresponding form, a circle[14]. [14] 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. Note also, that when planes are seen parallel to the eye in open day-light, they have scarce any round gradating or penciling shade at all, but appear merely as uniform prime tints, because the rays of light are equally diffused upon them. Nevertheless, give them but obliquity, they will more or less exhibit the retiring shade. But to return; we observ'd 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 should come in at the door-way, instead of the window, the gradation then would be reversed but still the effect of recession would be just the same, as this shade ever complies with the perspective lines. In the next place, let us observe the _ovolo_, or quarter-round in a cornice, fronting the eye in like manner, by which may be seen an example of the second species; where, on its most projecting part, a line of light is seen, from whence these shades retire contrary ways, by which the curvature is understood.