The centre of light: shadows, the moon, and vision
Rays from a window, why the moon and a cannon-ball do not spin, and how images enter the eye
A page of optical and physical arguments built around lettered ray diagrams. Leonardo reasons that it is the centre of a light source, not its whole face, that casts a body's shadow, and traces on a diagram where a window's light on a standing stick begins to weaken into penumbra. He then argues that a uniform bombard ball flies without rotation, using the moon as an example: since the dark spots on its spherical body never change place, it too does not turn. A further note on afterimages, seeing the sun with the eyes shut, is offered as proof that images (species) enter the eye.
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It is the centre of the light that casts the shadow
If the whole face of a light cast the shadow, a body much smaller than the light would throw a pyramidal shadow behind itself. Since experience does not show this, Leonardo concludes that the centre of the light performs that office.
Window light on a standing stick and the onset of penumbra
With a b the width of a window's light falling on an upright stick, the lines from a c and a d bound where the whole window lights the ground. At c the point cannot see the part of the window beyond b, and likewise d f cannot see a m, so at these two places the light begins to diminish. Diagram letters: a b c d e f m.
A uniform bombard ball flies without rotation
If the ball is of even material with its surface equidistant from its centre and the powder strikes it squarely in the middle, its course must be without any rotation, because the fire that drives it is of even force and equally pushes and meets the air on every side.
The moon's dark spots never shift, proving it does not turn
As an example, the moon is likewise a spherical body meeting equal resistance and, though far swifter than the ball, its dark spots never change their place. Since no such change appears, this plainly confirms that the moon moves without rotation.
Images enter the eye: the afterimage of the sun
Looking at the sun or another luminous thing and then shutting the eyes, you will still see it within the eye for a long stretch of time. Leonardo takes this as a sign that the images (species) enter within the eye.
Two windows and the shadows they throw
A second pair of ray diagrams shows the luminous window a b, with d e casting the shadow g h and e f casting the shadow i K, while the triangle m K remains wholly lit. Diagram letters: a b d e f m i K g h.