The other shades that have been before spoken of, serve and help to the like purposes when properly opposed; but as in nature they are continually fleeting and changing their appearances, either by our or their situations, they sometimes oppose and relieve, and sometimes not, as for instance; I once observed the tower-part of a steeple so exactly the colour of a light cloud behind it, that, at the distance I stood, there was not the least distinction to be made, so that the spire (of a lead-colour) seemed suspended in the air; but had a cloud of the like tint with the steeple, supplied the place of the white one, the tower would then have been relieved and distinct, when the spire would have been lost to the view. Nor is it sufficient that objects are of different colours or shades, to shew their distances from the eye, if one does not in part hide or lay over the other, as in fig. 86. [Illustration: Fig. 90] For as fig. [90 T p. II] the two equal balls, tho' one were black and the other white, placed on the separate walls, supposed distant from each other twenty or thirty feet, nevertheless, may seem both to rest upon one, if the tops of the walls are level with the eye; but when one ball hides part of the other, as in the same figure, we begin to apprehend they are upon different walls, which is determin'd by the perspective[17]: hence you will see the reason, why the steeple of Bloomsbury-church, in coming from Hampstead, seems to stand upon Montague-house, tho' it is several hundred yards distant from it. [17] The knowledge of perspective is no small help to the seeing objects truly, for which purpose Dr. Brook Taylor's Linear perspective made easy to those who are unacquainted with geometry, proposed to be publish'd soon by Mr. Kirby of Ipswich, may be of most service. Since then the opposition of one prime tint or shade to another, hath so great a share in marking out the recessions, or distances in a prospect, by which the eye is led onward step by step, it becomes a principle of consequence enough to be further discussed, with regard to the management of it in compositions of nature, as well as art. As to the management of it, when seen only from one point, the artist hath the advantage over nature, because such fix'd dispositions of shades as he hath artfully put together, cannot be displaced by the alteration of light, for which reason, designs done in two prime tints only, will sufficiently represent all those recessions, and give a just keeping to the representation of a prospect, in a print; whereas, the oppositions in nature, depending, as has been before hinted, on accidental situations and uncertain incidents, do not always make such pleasing composition, and would therefore have been very often deficient, had nature worked in two colours only; for which reason she hath provided an infinite number of materials, not only by way of prevention, but to add lustre and beauty to her works.