The reflected lights which strike upon the midst of shadows, will brighten up or lessen their obscurity in proportion to the strength of those lights, and their proximity to those shadows. Many painters neglect this observation, while others attend to and deduce their practice from it. This difference of opinion and practice divides the sentiments of artists, so that they blame each other for not thinking and acting as they themselves do. The best way is to steer a middle course, and not to admit of any reflected light, but when the cause of it is evident to every eye; and vice versâ, if you introduce none at all, let it appear evident that there was no reasonable cause for it. In doing so, you will neither be totally blamed nor praised by the variety of opinion, which, if not proceeding from entire ignorance, will ensure to you the approbation of both parties.
Chap. CCXVII.—Where Reflexes are to be most apparent.
Of all reflected lights, that is to be the most apparent, bold, and precise, which detaches from the darkest ground; and, on the contrary, that which is upon a lighter ground will be less apparent. And this proceeds from the contraste of shades, by which the faintest makes the dark ones appear still darker; so in contrasted lights, the brightest cause the others to appear less bright than they really are[49].
Chap. CCXVIII.—What Part of a Reflex is to be the lightest.
That part will be the brightest which receives the reflected light between angles the most nearly equal. For example, let N be the luminary, and A B the illuminated part of the object, reflecting the light over all the shady part of the concavity opposite to it. The light which reflects upon F will be placed between equal angles. But E at the base will not be reflected by equal angles, as it is evident that the angle E A B is more obtuse than the angle E B A. The angle A F B however, though it is between angles of less quality than the angle E, and has a common base B A, is between angles more nearly equal than E, therefore it will be lighter in F than in E; and it will also be brighter, because it is nearer to the part which gives them light. According to the 6th rule [50], which says, that part of the body is to be the lightest, which is nearest to the luminary.
Chap. CCXIX.—Of the Termination of Reflexes on their Grounds.
The termination of a reflected light on a ground lighter than that reflex, will not be perceivable; but if such a reflex terminates upon a ground darker than itself, it will be plainly seen; and the more so in proportion as that ground is darker, and vice versa[51].