If you cannot arrange that those who look at your work should stand at one particular point, when constructing your work, stand back until your eye is at least 20 times as far off as the greatest height and width of your work. This will make so little difference when the eye of the spectator moves, that it will be hardly appreciable, and it will look very good.
If the point of sight is at t you would make the figures on the circle d b e all of one size, as each of them bears the same relation to the point t. But consider the diagram given below and you will see that this is wrong, and why I shall make b smaller than d e [Footnote 8: The second diagram of this chapter stands in the original between lines 8 and 9.].
It is easy to understand that if 2 objects equal to each other are placed side by side the one at 3 braccia distance looks smaller than that placed at 2 braccia. This however is rather theoretical than for practice, because you stand close by [Footnote 11: Instead of ‘se preso’ (=sie presso) M. RAVAISSON reads ‘sempre se’ which gives rise to the unmeaning rendering: ‘parceque toujours ...’].
All the objects in the foreground, whether large or small, are to be drawn of their proper size, and if you see them from a distance they will appear just as they ought, and if you see them close they will diminish of themselves.
[Footnote 15: Compare No. 526 line 18.] Take care that the vertical plan on which you work out the perspective of the objects seen is of the same form as the wall on which the work is to be executed.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.