in
weight the degree similarly numbered in the red or the blue slip. Then,
when you are drawing from objects of a crimson or blue color, if you
can match their color by any compartment of the crimson or blue in your
scales, the gray in the compartment of the gray scale marked with the
same number is the gray which must represent that crimson or blue in
your light and shade drawing.

38. Next, prepare scales with gamboge, cobalt, and vermilion. You will
find that you cannot darken these beyond a certain point;[9] for yellow
and scarlet, so long as they remain yellow and scarlet, cannot approach
to black; we cannot have, properly speaking, a dark yellow or dark
scarlet. Make your scales of full yellow, blue, and scarlet, half-way
down; passing _then_ gradually to white. Afterwards use lake to darken
the upper half of the vermilion and gamboge; and Prussian blue to darken
the cobalt. You will thus have three more scales, passing from white
nearly to black, through yellow and orange, through sky-blue, and
through scarlet. By mixing the gamboge and Prussian blue you may make
another with green; mixing the cobalt and lake, another with violet; the
sepia alone will make a forcible brown one; and so on, until you have as
many scales as you like, passing from black to white through different
colors. Then, supposing your scales properly gradated and equally
divided, the compartment or degree No. 1 of the gray will represent in
chiaroscuro the No. 1 of all the other colors; No. 2 of gray the No. 2
of the other colors, and so on.

39. It is only necessary, however, in this matter that you should
understand the principle; for it would never be possible for you to
gradate your scales so truly as to make them practically accurate and
serviceable; and even if you could, unless you had about ten thousand
scales, and were able to change them faster than ever juggler changed
cards, you could not in a day measure the tints on so much as one side
of a frost-bitten apple. But when once you fully understand the
principle, and see how all colors contain as it were a certain quantity
of darkness, or power of dark relief from white--some more, some less;
and how this pitch or power of each may be represented by equivalent
values of gray, you will soon be able to arrive shrewdly at an
approximation by a glance of the eye, without any measuring scale at
all.

40. You must now go on, again with the pen, drawing patterns, and any
shapes of shade