[CHAPTER XIII.]
Of Composition with regard to Light, Shade and Colours.
Under this head I shall attempt shewing what it is that gives the appearance of that hollow or vacant space in which all things move so freely; and in what manner light, shade and colours, mark or point out the distances of one object from another, and occasion an agreeable play upon the eye, called by the painters a fine keeping, and pleasing composition of light and shade. Herein my design is to consider this matter as a performance of nature without, or before the eye; I mean, as if the objects with their shades, &c. were in fact circumstanced as they appear, and as the unskill'd in optics take them to be. And let it be remarked throughout this chapter, that the pleasure arising from composition, as in a fine landskip, &c. is chiefly owing to the dispositions and assemblages of light and shades, which are so order'd by the principles called opposition, breadth and simplicity, as to produce a just and distinct perception of the objects before us.
Experience teaches us that the eye may be subdued and forced into forming and disposing of objects even quite contrary to what it would naturally see them, by the prejudgment of the mind from the better authority of feeling, or some other persuasive motive. But surely this extraordinary perversion of the sight would not have been suffer'd, did it not tend to great and necessary purposes, in rectifying some deficiencies which it would otherwise be subject to (tho' we must own at the same time, that the mind itself may be so imposed upon as to make the eye see falsely as well as truly) for example, were it not for this controul over the sight, it is well known, that we should not only see things double, but upside down, as they are painted upon the retina, and as each eye has a distinct sight. And then as to distances; a fly upon a pane of glass is sometimes imagined a crow, or larger bird afar off, till some circumstance hath rectified the mistake, and convinced us of its real size and place.
Hence I would infer, that the eye generally gives its assent to such space and distances as have been first measured by the feeling, or otherwise calculated in the mind: which measurements and calculations are equally, if not more, in the power of a blind man, as was fully experienced by that incomparable mathematician and wonder of his age, the late professor Sanderson.
By pursuing this observation on the faculties of the mind, an idea may be formed of the means by which we attain to the perception or appearance of an immense space surrounding us; which cavity, being subject to divisions and subdivisions in the mind, is afterwards fashioned by the limited power of the eye, first into a hemisphere, and then into the appearance of different distances, which are pictured to it by means of such dispositions of light and shade as shall next be described. And these I now desire may be looked upon, but as so many marks or types set upon these distances, and which are remember'd and learnt by degrees, and when learnt, are recurred to upon all occasions.
If permitted then to consider light and shades as types of distinction, they become, as it were, our materials, of which prime tints are the principal; by these, I mean the fixed and permanent colours of each object, as the green of trees, &c. which serve the purposes of separating and relieving the several objects by the different strengths or shades of them being opposed to each other [Fig. 86 T p. II].
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.