“Every study made from nature should be made rigorously within an interval of two hours at the most, and if it is a rising or setting sun effect, no more than a half-hour should be allowed. It is good to paint the same view at different times of day in order to observe the differences produced on forms by light. The changes are so sensitive and so astonishing that one can hardly recognize the same objects.”

Pierre Henri de Valenciennes, Reflexions et Conseils, 1796