Page:A Treatise on Painting.djvu/195

 and in darkness the quality of the shades of the objects seen in the looking-glass; but attribute this circumstance to their own ignorance, and not to the true cause, because they do not know it. It is impossible that objects in painting should appear with the same relief as those in the looking-glass, unless we look at them with only one eye.

The reason is this. The two eyes A B looking at objects one behind another, as M and N, see them both; because M cannot entirely occupy the space of N, by reason that the base of the visual rays is so broad, that the second object is seen behind the first. But if one eye be shut, and you look with the other S, the body F will entirely cover the body R, because the visual rays beginning at one point, form a triangle, of which the body F is the base, and being prolonged, they form two diverging tangents at the two extremities of F, which cannot touch the body R behind it, therefore can never see it. Rh