On the backgrounds of the figures of painted bodies.

The background that surrounds the figures of any painted thing must be darker than the illuminated part of those figures, and lighter than their shadowed part, etc.

A case of perspective.

first - r a o - t b p - n m - f

second - p r q - S t h

If the parallel wall of 4 sides and 4 angles, of long extension, shows itself to the eye with upper and lower boundaries that are straight or curved: by the second [proposition] of this, such parallel sides will show themselves as a hexagonal figure, that is of six sides and six angles, while in its reality it has only 4 angles and four sides. And this is proved by that same second, which says: 'Of things of equal size, the more remote will show itself the smaller, the farther it is.' It follows that o p, the width of the wall r t o p, is smaller than the width a b by as much as the line p f is longer than the line b f, which line p f exceeds the line b f by the space p m, which is a third of the space p f. Therefore o p will be smaller than a b, that is by 1/3; and this proportion is converse, because the greater distance makes the thing seen smaller, and the lesser distance increases the thing seen.

Conclusion.

By what is proved above, we may conclude in the second figure how the two straight lines r o, t p divide into 4 straight lines: r p and t S, r g and t h. These are straight lines, and thus it is proved that the eye, which stands in the middle of the rectilinear wall of 4 straight sides and four right angles, sees six straight sides and six angles, of which 2 are obtuse and 4 are right.