Pinhole vision: straight-line sight and image reversal
How the pupil sees an object beyond a small aperture, reversed left for right
A full page of mirror-script optical theory, without a diagram, continuing the study of sight through a small hole. Leonardo argues that because vision travels in straight lines through a uniform medium, part a of the pupil sees point o beyond the aperture at S, and that lowering the object changes where it appears to the eye. He then states that a pupil looking through such an aperture sees the left of an object with its right side and the right with its left, while its centre would see the object's surface-centre. The figures for this reasoning are carried on the adjacent pages.
On this page
Straight-line sight fixes the apparent place of point o
Because every ray of vision is a straight line and the medium is uniform, part a of the pupil sees o beyond the hole at S, and it could not see it at q along the bent path a b q. If o is lowered to n, p sees it at r; and if o is lowered as far as m, it appears to the lower part c of the eye to have risen to the extremity q.
The pupil reverses the object, left for right
A pupil that sees, through an aperture smaller than and close to itself, some object, sees the left part of that object with the right part of the pupil and the right part with the left part. With the centre of the pupil it would see the surface-centre of the object, provided it were visible and the pupil's centre had visual power.