— FOLIO 4 (recto) —

A manuscript page featuring three short paragraphs of text and a central diagram. The diagram consists of vertical parallel lines with some intersecting horizontal strokes, likely representing optical rays or a coordinate system for measuring light transmission.

It will be seen how the diameter of the earth
and the sphere of fire In the medieval cosmology Leonardo inherited, the sphere of fire was a region of the atmosphere located between the air and the moon. with the moon and sun.
It will be seen how opaque bodies send out [images] to the world and the air.
It will be seen how the rays of the sun pass through the air.
It will be seen how the air distributes the rays everywhere.
It will be seen how the air is full of all colors,
sending them through its light.

The diagram shows vertical lines representing rays of light or lines of perspective.

— FOLIO 4 (verso) —

The pupil of the eye, in which in the middle of the air it will be seen?
because the air receives within itself infinite points
of every length and every color.
Every point fills with a [likeness]
sent, returned, sent, sent, sent, sent,
filling everything with images.

The entire surface of every opaque body is full of its likenesses similitudini: the visual forms or "species" that Leonardo believed objects emitted into the air and its whole face. For the eye to be in a point, it must be in every point of the surrounding air. This confirms the reasoning of the luminous rays passing through the point of a very narrow window. These rays send their species spetie: the formal appearance or image of an object as it travels through a medium into a dark place inside a room original: "camera che in tenebre" — This is an early description of the camera obscura principle. without any change in the order of height, length, or width, except for that which is sent by a diagonal line.
Because where the point descends, they send marre?
through? the point of a window renders? the eye? renders? they send?
Because one species sends the other, it does not point? renders?
They send point? of the eye for they send? renders?
They send species to every point? of the air and they do not hinder
one another. They send species of all the bodies?. They send
through rays they send? their images? through luminous rays,
and they do not hinder? each other. The rays? do not confuse as they send?
into the point of the eye?. One does not hinder? the other.
They send, they send through the air, they send to the eye?, they do not hinder each other.
They send, they send? to the eye because? from every? point? they send?
the species of the body into the point? of the eye? and they do not hinder
one another, as they send? species into the point? of the eye?.
They send species and colors?. The points? of the air do not hinder each other.

A densely written manuscript page with Leonardo's mirror script. The text discusses the anatomy of the eye and the transmission of images through space. Leonardo theorizes that every point in the air is filled with 'similitudes' or images of all objects, which enter the pupil through a single point without interference.