tree that you think pretty, which is nearly bare of leaves,
and which you can see against the sky, or against a pale wall, or other
light ground: it must not be against strong light, or you will find the
looking at it hurts your eyes; nor must it be in sunshine, or you will
be puzzled by the lights on the boughs. But the tree must be in shade;
and the sky blue, or grey, or dull white. A wholly grey or rainy day is
the best for this practice.

You will see that _all_ the boughs of the tree are _dark_ against the
sky. Consider them as so many dark rivers, to be laid down in a map with
absolute accuracy; and, without the least thought about the _roundness_
of the stems, map them all out in flat shade, scrawling them in with
pencil, just as you did the limbs of your letters; then correct and
alter them, rubbing out and out again, never minding how much your paper
is dirtied (only not destroying its surface), until every bough is
exactly, or as near as your utmost power can bring it, right in
curvature and in thickness. Look at the white interstices between them
with as much scrupulousness as if they were little estates which you had
to survey, and draw maps of, for some important lawsuit, involving heavy
penalties if you cut the least bit of a corner off any of them, or gave
the hedge anywhere too deep a curve; and try continually to fancy the
whole tree nothing but a flat ramification on a white ground. Do not
take any trouble about the little twigs, which look like a confused
network or mist; leave them all out,[204] drawing only the main branches
as far as you can see them distinctly, your object at present being not
to draw a tree, but to _learn how_ to do so. When you have got the thing
as nearly right as you can--and it is better to make one good study than
twenty left unnecessarily inaccurate--take your pen, and put a fine
outline to all the boughs, as you did to your letter, taking care, as
far as possible, to put the outline within the edge of the shade, so as
not to make the boughs thicker: the main use of the outline is to
_affirm_ the whole more clearly; to do away with little accidental
roughnesses and excrescences, and especially to mark where boughs cross,
or come in front of each other, as at such points their arrangement in
this kind of sketch is unintelligible without the outline. It may
perfectly well happen that in Nature it should be less distinct than
your outline will make it; but it is better in this