How That Diligence [Accuracy] Should First Be Learnt Rather than Rapid Execution.
If you, who draw, desire to study well and to good purpose, always go slowly to work in your drawing; and discriminate in. the lights, which have the highest degree of brightness, and to what extent and likewise in the shadows, which are those that are darker than the others and in what way they intermingle; then their masses and the relative proportions of one to the other. And note in their outlines, which way they tend; and which part of the lines is curved to one side or the other, and where they are more or less conspicuous and consequently broad or fine; and finally, that your light and shade blend without strokes and borders [but] looking like smoke. And when you have thus schooled your hand and your judgment by such diligence, you will acquire rapidity before you are aware.
The artist's private life and choice of company (493-494).
493.