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]. [Illustration: Fig. 86]