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.

The neck, or any other part which is raised straight upwards, and has a projection over it, will be darker than the perpendicular front of that pro-