On painting.

On the darknesses of shadows, or if you will, the brightnesses of lights.
Although practitioners put into all dark things — such as trees, meadows, hair, beards and body-hair — 4 kinds of brightness in counterfeiting one and the same colour, that is: first a dark ground; second a blotch that shares the shape of the part; third a part more distinct and brighter; fourth the lights [highlights] brighter than the other parts of the figure — yet it seems to me that these varieties are infinite over a continuous quantity, which in itself is divisible to infinity. And thus I prove it:

d - (b)g a b c; n-o n r p o
Let a g be a continuous quantity, let d be the light that illuminates it. I say, by the fourth [proposition], which states that 'that part of the illuminated body will be more luminous which is nearer to the cause that illuminates it,' that therefore g is darker than c by as much as the line d g is longer than the line d c. And by the conclusion that such degrees of brightness, or let me say of darkness, are not only 4 but can be imagined infinitely — because c d is a continuous quantity and every continuous quantity is divisible to infinity — therefore infinite are the varieties of the lengths of the lines that extend from the luminous source to the illuminated body; and such will be the proportion of the lights as is that of the lengths of the lines among themselves that extend from the centre of the luminous source to the illuminated part of that object.

On study and its order.
I say that first one must learn the limbs and their workings, and, that knowledge finished, one must follow the actions according to the circumstances that befall man, and third compose the narratives, the study of which will be made from natural actions done by chance, by means of their circumstances; and observe them in the streets, squares and fields, and note them with a brief description of outlines — that is, for a head make an o, and for an arm a straight or bent line, and do the same for the legs and the trunk — and then, returning home, make such records in perfect form.
The adversary says that, to become practised and produce many works, it is better that the first period of study be spent in copying various compositions made on papers or walls by diverse masters, and that in these one acquires a quick practice and a good habit. To whom it is answered that this habit would be good if it were formed upon works of good composition and by studious masters; and because such masters are so rare that few of them are found, it is safer to go to natural things than to those which, imitated from nature with great worsening, make a bad habit — for he who can go to the fountain does not go to the water-jar.