but
as quickly as you can. Go on until the color has become so pale that you
cannot see it; then wash your brush thoroughly in water, and carry the
wave down a little farther with that, and then absorb it with the dry
brush, and leave it to dry.

34. If you get to the bottom of your paper before your color gets pale,
you may either take longer paper, or begin, with the tint as it was when
you left off, on another sheet; but be sure to exhaust it to pure
whiteness at last. When all is quite dry, recommence at the top with
another similar mixture of color, and go down in the same way. Then
again, and then again, and so continually until the color at the top of
the paper is as dark as your cake of Prussian blue, and passes down into
pure white paper at the end of your column, with a perfectly smooth
gradation from one into the other.

35. You will find at first that the paper gets mottled or wavy, instead
of evenly gradated; this is because at some places you have taken up
more water in your brush than at others, or not mixed it thoroughly on
the plate, or led one tint too far before replenishing with the next.
Practice only will enable you to do it well; the best artists cannot
always get gradations of this kind quite to their minds; nor do they
ever leave them on their pictures without after-touching.

36. As you get more power, and can strike the color more quickly down,
you will be able to gradate in less compass;[7] beginning with a small
quantity of color, and adding a drop of water, instead of a brushful;
with finer brushes, also, you may gradate to a less scale. But slight
skill will enable you to test the relations of color to shade as far as
is necessary for your immediate progress, which is to be done thus:--

37. Take cakes of lake, of gamboge, of sepia, of blue-black, of cobalt,
and vermilion; and prepare gradated columns (exactly as you have done
with the Prussian blue) of the lake and blue-black.[8] Cut a narrow
slip, all the way down, of each gradated color, and set the three slips
side by side; fasten them down, and rule lines at equal distances across
all the three, so as to divide them into fifty degrees, and number the
degrees of each, from light to dark, 1, 2, 3, etc. If you have gradated
them rightly, the darkest part either of the red or blue will be nearly
equal in power to the darkest part of the blue-black, and any degree of
the black slip will also, accurately enough for our purpose, balance