THAT THE LIGHT FOR DRAWING FROM NATURE SHOULD BE HIGH UP.
The light for drawing from nature should come from the North in order that it may not vary. And if you have it from the South, keep the window screened with cloth, so that with the sun shining the whole day the light may not vary. The height of the light should be so arranged as that every object shall cast a shadow on the ground of the same length as itself.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.