On painting and perspective. Three are the parts of perspective of which painting makes use, of which the first extends to the diminution of the sizes of opaque bodies; the second is of the diminutions and losses of the boundaries of those opaque bodies; the third is of the diminution and loss of colors at long distance.

On the diminishing perspective of opaque bodies. Among opaque bodies of equal magnitude, the diminution of their figures in appearance will be as that of their distances from the eye that sees them. But such proportion is inverse, because where the distance is greater, the opaque body shows itself smaller, and where the distance is less, that body shows itself greater. And from this is born linear perspective, and it teaches, as a second rule: 'Every body at long distance first loses that part of itself which is in itself thinnest'; as to say, of a horse the legs will be lost before the head, because the legs are thinner than the head, and the neck will be lost before the trunk for the same reason. Thus it follows that the last part of the knowledge of the horse reserved to the eye will be the trunk, remaining in oval form, but rather tending toward the columnar; and the thickness will be lost before the length, by the aforesaid second conclusion, etc.

If the eye is immobile, perspective terminates its distance in a point. But if the eye moves along a straight line, perspective terminates in a line, because it is proved that the line is generated by the motion of the point, and our seeing is in a point; and from this it follows that whoever moves the seeing moves the point, and whoever moves the point generates the line, etc.