OF SITUATION.
Remember [to note] the situation of your figures; for the light and shade will be one thing if the object is in a dark place with a particular light, and another thing if it is in a light place with direct sunlight; one thing in a dark place with a diffused evening light or a cloudy sky, and another in the diffused light of the atmosphere lighted by the sun.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.