However obscured the sun may be, this principle holds more or less; and it makes clear and helps you to observe and notice many facts in landscape light and shade which it is necessary to know.
Millet said that all the beauty of color and value, and the whole art of painting, rested on the comprehension and observance of these facts.
He said that as the planes of any form turned towards or away from the light and so got more or less of it, and as one form stood more or less far back of another and the atmosphere came between, the color and value changed; and in the observance of this, and its representation as applied to any and every object or group of objects, lay the whole of painting. All the possible beauties of the art rested on it. He showed a painting of a single pear in which these things were most subtly observed, and said that that painting was as complete and perfect as any painting he could do simply because in the observance of these relations was implied the observance of everything which was vital to painting.
=Short Sittings.=--This characteristic, and the steady change of position of the sun and its effects on all the objects which are directly lighted by it, make it necessary, whenever you are painting from nature out-of-doors, that you should not paint at one thing very long at a time. The light changes pretty rapidly; at high noon it only takes a few moments to exactly reverse the light. It is seldom that you can do any just study for more than an hour or an hour and a half at a sitting. Some men do work two or three hours, but they are not studying justly all that time; for that which was light is dark three hours later, and any true study of value and color is impossible under these conditions. Of course on gray days this is less marked, but you must suit your sittings to the time and facts.
It would be better if you had more canvases, and worked a short time on each, and many days on all. You would have the truest work.
Monet works never more than a half-hour on one canvas; but when he starts out he takes a half-dozen or more different canvases, and paints on each till the light has changed. Theodore Robinson seldom worked more than three-quarters of an hour, or at most an hour, on one canvas; but, he worked for twenty or thirty days on each canvas, and sometimes had a single canvas under way for successive seasons.