A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
A North Light will be the most proper for a Painters purpose, as being the Steadiest. If his Chamber be open towards the South, it will be convenient to place an oil'd Sash before it; to the End that the Light of the Sun which will be upon it all Day, being moderated, may spread it self equally, and without any Sensible alteration. The Light by which he Designs from the Life, ought to come from such an Altitude, as that the Shadows of Bodies projected on the Plane, may be equal to their heights.