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.
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.
By an infinite number of materials, I mean colours and shades of all kinds and degrees; some notion of which variety may be formed by supposing a piece of white silk by several dippings gradually dyed to a black; and carrying it in like manner through the prime tints of yellow, red, and blue; and then again, by making the like progress through all the mixtures that are to be made of these three original colours. So that when we survey this infinite and immense variety, it is no wonder, that, let the light or objects be situated or changed how they will, oppositions seldom miss; nor that even every incident of shade should sometimes be so completely disposed as to admit of no further beauty, as to composition; and from whence the artist hath by observation taken his principles of imitation, as in the following respect.
Those objects which are intended most to affect the eye, and come forwardest to the view, must have large, strong, and smart oppositions, like the fore-ground in fig. [89 p. II], and what are designed to be thrown further off, must be made still weaker and weaker, as expressed in figures 86, 92, and 93, which receding in order make a kind of gradation of oppositions; to which, and all the other circumstances already described, both for recession, and beauty, nature hath added what is known by the name of aerial perspective; being that interposition of air, which throws a general soft retiring tint over the whole prospect; to be seen in excess at the rising of a fog. All which again receives still more distinctness, as well as a greater degree of variety, when the sun shines bright, and casts broad shadows of one object upon another; which gives the skilful designer such hints for shewing broad and fine oppositions of shades, as give life and spirit to his performances.
Breadth of shade is a principle that assists in making distinction more conspicuous; thus fig. [87 p. I], is better distinguish'd by its breadth or quantity of shade, and view'd with more ease and pleasure at any distance, than fig. [88 p. I], which hath many, and these but narrow shades between the folds. And for one of the noblest instances of this, let Windsor-castle be viewed at the rising or setting of the sun.