OF THE WAY TO LEARN TO COMPOSE FIGURES [IN GROUPS] IN HISTORICAL
PICTURES.

When you have well learnt perspective and have by heart the parts
and forms of objects, you must go about, and constantly, as you go,
observe, note and consider the circumstances and behaviour of men in
talking, quarrelling or laughing or fighting together: the action of
the men themselves and the actions of the bystanders, who separate
them or who look on. And take a note of them with slight strokes
thus, in a little book which you should always carry with you. And
it should be of tinted paper, that it may not be rubbed out, but
change the old [when full] for a new one; since these things should
not be rubbed out but preserved with great care; for the forms, and
positions of objects are so infinite that the memory is incapable of
retaining them, wherefore keep these [sketches] as your guides and
masters.

[Footnote: Among Leonardo's numerous note books of pocket size not
one has coloured paper, so no sketches answering to this description
can be pointed out. The fact that most of the notes are written in
ink, militates against the supposition that they were made in the
open air.]