If by accident it should happen, that when drawing or copying in
chapels, or colouring in other unfavourable places, you cannot have the
light on your left hand, or in your usual manner, be sure to give relief
to your figures or design according to the arrangement of the windows
which you find in these places, which have to give you light, and thus
accommodating yourself to the light on which side soever it may be, give
the proper lights and shadows. Or if it were to happen that the light
should enter or shine right opposite or full in your face, make your
lights and shades accordingly; or if the light should be favourable at a
window larger than the others in the above-mentioned places, adopt
always the best light, and try to understand and follow it carefully,
because, wanting this, your work would be without relief, a foolish
thing without mastery.
_Cennino Cennini._