Chap. CCCXLVIII.—Why the most perfect Imitation of Nature will not appear to have the same Relief as Nature itself.
If nature is seen with two eyes, it will be impossible to imitate it upon a picture so as to appear with the same relief, though the lines, the lights, shades, and colour, be perfectly imitated[94]. It is proved thus: let the eyes A B, look at the object C, with the concurrence of both the central visual rays A C and B C. I say, that the sides of the visual angles (which contain these central rays) will see the space G D, behind the object C. The eye A will see all the space FD, and the eye B all the space G E. Therefore the two eyes will see behind the object C all the space F E; for which reason that object C becomes as it were transparent, according to the definition of transparent bodies, behind which nothing is hidden. This cannot happen if an object were seen with one eye only, provided it be larger than the eye. From all that has been said, we may conclude, that a painted object, occupying all the space it has behind, leaves no possible way to see any part of the ground, which it covers entirely by its own circumference[95].
Chap. CCCXLIX.—Universality of Painting; a Precept.
A painter cannot be said to aim at universality in the art, unless he love equally every species of that art. For instance, if he delight only in landscape, his can be esteemed only as a simple investigation; and, as our friend Botticello [96] remarks, is but a vain study; since, by throwing a sponge impregnated with various colours against a wall, it leaves some spots upon it, which may appear like a landscape. It is true also, that a variety of compositions may be seen in such spots, according to the disposition of mind with which they are considered; such as heads of men, various animals, battles, rocky scenes, seas, clouds, woods, and the like. It may be compared to the sound of bells, which may seem to say whatever we choose to imagine. In the same manner also, those spots may furnish hints for compositions, though they do not teach us how to finish any particular part; and the imitators of them are but sorry landscape-painters.