Notebook on Mechanics and Natural Philosophy

If the eye is placed at the center of the circle of a wheel that turns through the air, it will see the entire wheel composed of circles that touch one another, provided this rotation is of infinite velocity. Leonardo is exploring the phenomenon of "persistence of vision," where an object moves so quickly that the eye perceives it as a continuous solid shape rather than a moving part.

Every part of the wheel causes a circle, of which it is either the diameter or the chord original: "corda"; a straight line segment whose endpoints both lie on a circular arc.. This circle will maintain a consistent height throughout the portion of the wheel’s circumference visible to the eye. If the wheel turns through the air, it will be observed by the eye between the endpoints of its diameter; the space between one endpoint and the other will appear filled with circles that touch one another across the diameter of the wheel. The circle will appear thicker the closer it is to the eye and thinner the further away it is from the eye.

If the wheel is viewed edge-on original: "per taglio"; literally "by the cut," meaning viewing the wheel from its side profile so it appears as a thin line., it will appear to the sight that the circles all align on a straight line. This line will appear thicker the closer it is to the eye and thinner the further away it is. It will possess varying thicknesses original: "grozze"; referring here to the perceived breadth or thickness of the lines in perspective. that converge toward the same point, depending on whether the edges of the wheel are nearer to or further from the eye.