Moving eye equals moving light: the firebrand test
A whirled firebrand appears as a continuous fiery line whether the brand or the eye moves
This note argues that moving the eye while a luminous object stays still is equivalent to moving the object while the eye stays still, and it offers a proof by experiment. Carrying a burning firebrand around in a circle before a fixed eye makes it look like a fiery line rising upward, though the brand occupies only one point at a time. Conversely, holding the brand still and sweeping the eye makes the same brand seem to climb in a continuous line, the image lingering on the eye. The folio is entirely text, with no diagram.
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Moving the eye equals moving the luminous object
To move the eye while the luminous object stays still is the same as to move the object while the eye stays still. Leonardo says the first part is proved by the preceding proposition, and promises to prove the second part with the help of that same proposition.
The whirled firebrand seen as a continuous fiery line
With the eye still and a firebrand carried in a circle or upward, the brand appears to be a fiery line rising from below, though it is only ever in one place at a time. Holding the brand still and moving the eye from top to bottom makes the brand seem to climb in a continuous line from below upward.