Nathan Goldstein quotes:

The local-tone is the intrinsic value of a thing - excluding any effects of light. The local-tone of a common pearl is very nearly white; that of a lump of coal, nearly black.

The inherent lightness or darkness of a form - its local-tone - is a given quality of all forms. It is affected by, but separate from, the light and dark areas that result from light falling on forms.

Value gives the artist the means of showing subtle changes in a form's surface-state, and it helps clarify the relative distance between forms.

Value, whether the result of massed lines, broad strokes, or washes, is the necessary and only tool for modelling form with light.

Perspective should be learned - and then forgotten. The residue - a sensitivity to perspective - helps perception, varying with each individual and determined by his responsive needs.