Light through apertures and the apparent size of bright bodies
Camera-obscura rays, a pinhole test for a light's true size, and why the sun seems to grow at sunset.
Leonardo studies how light passes through apertures: rays crossing a square opening intersect beyond its faces, and no aperture, however distant, fails to carry the image of its source, so a crescent moon shone through one appears crescent-shaped. He gives several proofs that luminous bodies look larger the farther off they are: two candles half a braccio apart merge into a single wide light at two hundred braccia, and a pinhole in a small board, brought to the eye and taken away, makes a flame seem to swell and shrink. He asks why the eye sees distant things as larger than their measure on the perspective screen, and rejects the mathematicians' claim that the sun grows in the west merely because it is seen through denser air, holding instead that every luminous body simply appears larger with distance.
On this page
Rays crossing a square aperture carry the image of their source
The intersection of luminous rays passing between the faces of a square aperture is made beyond those faces, and that made at its corners lies in the broad part of the corner. No aperture, over any distance, can so alter the convergence of rays that they do not refer to their object the image of their source.
A crescent moon shone through an aperture appears crescent
Rays proceeding through a parallel-sided aperture cannot demonstrate on their object the form of their cause. But since all effects of luminous bodies show their causes, the moon of crescent (navicular) form, shining through an aperture, will appear on its object as a crescent body.
A pinhole test for the true size of a light
To see the true size of a luminous body, make a hole the size of a string's tip in a thin board and hold it close to the eye so that you see a good margin of air around the light. Removing the board and putting it back rapidly, you will see the light grow and shrink.
Two candles at two hundred braccia merge into one light
Set two lighted candles half a braccio apart and go two hundred braccia away; because each grows, they become a single luminous body as bright as two lights, appearing as one light a braccio wide.
Why the sun grows larger in the west
Certain mathematicians say the sun grows in the west because the eye sees it through denser air, as things look bigger in fog or water. Leonardo answers that fog only makes distant things seem larger by not diminishing them, and that the true reason is that every luminous body appears larger the farther away it is.
Why the eye sees distant things larger than on the perspective screen
Why the eye sees distant things as being greater than their measures on the perspective screen.