like a relievo, and some of its parts detached from the ground; he who
excels all others in that part of the art deserves the greatest
praise,"[1] wrote Leonardo da Vinci, and the insistence on this
"standing out" quality, with its appeal to the touch sense as something
great in art, sounds very strange in these days. But it must be
remembered that the means of creating this illusion were new to all and
greatly wondered at.

[Footnote 1: Leonardo da Vinci, _Treatise on Painting_, paragraph 178.]

And again, in paragraph 176 of his treatise, Leonardo writes: "The
knowledge of the outline is of most consequence, and yet may be acquired
to great certainty by dint of study; as the outlines of the human
figure, particularly those which do not bend, are invariably the same.
But the knowledge of the situation, quality and quantity of shadows,
being infinite, requires the most extensive study."

The outlines of the human figure are "invariably the same"? What does
this mean? From the visual point of view we know that the space occupied
by figures in the field of our vision is by no means "invariably the
same," but of great variety. So it cannot be the visual appearance he is
speaking about. It can only refer to the mental idea of the shape of
the members of the human figure. The remark "particularly those that do
not bend" shows this also, for when the body is bent up even the mental
idea of its form must be altered. There is no hint yet of vision being
exploited for itself, but only in so far as it yielded material to
stimulate this mental idea of the exterior world.

[Illustration: Plate IX.