In compositions of many figures and animals, observe, that the parts of these different objects ought to be darker in proportion as they are lower, and as they are nearer the middle of the groups, though they are all of an uniform colour. This is necessary, because a smaller portion of the sky (from which all ​bodies are illuminated) can give light to the lower spaces between these different figures, than to the upper parts of the spaces. It is proved thus: A B C D is that portion of the sky which gives light to all the objects beneath; M and N are the bodies which occupy the space S T R H, in which it is evidently perceived, that the point F, receiving the light only from the portion of the sky C D, has a smaller quantity of it than the point E which receives it from the whole space A B (a larger portion than C D); therefore it will be lighter in E than in F.