A most important part of painting consists in the backgrounds of the
objects represented; against these backgrounds the outlines of
those natural objects which are convex are always visible, and also
the forms of these bodies against the background, even though the
colours of the bodies should be the same as that of the background.
This is caused by the convex edges of the objects not being
illuminated in the same way as, by the same light, the background is
illuminated, since these edges will often be lighter or darker than
the background. But if the edge is of the same colour as the
background, beyond a doubt it will in that part of the picture
interfere with your perception of the outline, and such a choice in
a picture ought to be rejected by the judgment of good painters,
inasmuch as the purpose of the painter is to make his figures appear
detached from the background; while in the case here described the
contrary occurs, not only in the picture, but in the objects
themselves.