IX. The Practice of Painting: 547.
547.
WHERE A SPECTATOR SHOULD STAND TO LOOK AT A PICTURE.
Supposing a b to be the picture and d to be the light, I say that if you place yourself between c and e you will not
understand the picture well and particularly if it is done in oils, or still more if it is varnished, because it will be lustrous and somewhat of the nature of a mirror. And for this reason the nearer you go towards the point c, the less you will see, because the rays of light falling from the window on the picture are reflected to that point. But if you place yourself between e and d you will get a good view of it, and the more so as you approach the point d, because that spot is least exposed to these reflected rays of light.