As a means of practising this perspective of the variation and loss or
diminution of the proper essence of colours, take at distances, a
hundred braccia apart, objects standing in the landscape, such as trees,
houses, men, and places, and in front of the first tree fix a piece of
glass so that it is quite steady, and then let your eye rest upon it and
trace out a tree upon the glass above the outline of the tree; and
afterwards remove the glass so far to one side that the actual tree
seems almost to touch the one that you have drawn. Then colour your
drawing in such a way that the two are alike in colour and form, and
that if you close one eye both seem painted on the glass and the same
distance away. Then proceed in the same way with a second and a third
tree, at distances of a hundred braccia from each other. And these will
always serve as your standards and teachers when you are at work on
pictures where they can be applied, and they will cause the work to be
successful in its distance. But I find it is a rule that the second is
reduced to four-fifths the size of the first when it is twenty braccia
distant from it.
_Leonardo._