How every body fills the air with its image; camera obscura
On species filling the surrounding air, vision as exchange, sound, and the pinhole image
The text argues that every illuminated body is present by its 'similitude' throughout all the opposite air and wholly in each of its least parts, and weighs whether the eye sends its image to the object or the luminous air itself draws in the species of things like a magnet. A short interposed heading, 'On noise,' states that sound is caused by bodies through the blow produced by motion. Leonardo then gives the camera-obscura proof: a small round hole pierced in the dark wall of a room facing a sunlit façade throws the images of all the lit things, inverted, onto the opposite white wall, and many holes give the same effect in each. This proves the species of lit things are present throughout the whole surface and in its every least part, carrying the colours and shapes of their luminous causes.
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Every body wholly present throughout all the air
Each body appears by its similitude wholly in every part of the opposite air, and wholly in the whole of it — all in the part and all in the whole. The luminous air seems to have the nature and power to draw and take into itself the species of the things within it.
Does the eye send its image, or the air draw it in?
If the object opposite the eye sends its likeness to the eye, the eye must equally send its likeness to the object, so these species would be spiritual powers; yet then each thing would soon come to nothing. Leonardo prefers to believe it is the nature of the luminous air, like a magnet, to attract the similitudes of things placed within it.
On noise
A short interposed heading, 'On noise,' states that sound is caused by bodies by means of the blow produced by motion.
The camera-obscura proof
A sunlit façade facing a dark room, pierced by a small round hole, throws the images of all the lit things — inverted — onto the opposite white wall; many holes give the same effect in each. This proves that the species of lit things are present throughout the whole face and in its every least part, and that the colours and shapes of the rays match their luminous causes.