How the eye sees: the crystalline humour and binocular vision
Experiments proving that images come to the eye, and that two eyes refer one thing to a single point
A page on the theory of vision illustrated by two sets of lettered eye diagrams. Leonardo reports the mathematicians' view that the eyeball is divided into quarters, one filled with crystalline humour that refers what appears in the field of view to a single angle, but counters with an experiment: leaning against the middle of a hall wall and staring straight ahead, one still sees every motion on the left, right, and facing walls, which shows the humour sits differently. He then argues from binocular vision that the same thing seen by two concordant eyes is referred inside the head to one point, and that pressing one eye with the finger doubles the image because you move the likeness in the eye, not the thing itself.
On this page
The crystalline humour and the quarter of the eyeball
The mathematicians hold that the ball of the eye is divided into quarters, and that the quarter b e f is full of crystalline humour that refers to the angle b whatever appears in the triangle b c d. Leonardo's hall experiment, seeing all motion on three walls at once, argues that the humour instead sits as t S r. Diagram letters: K i e f b c d g h t S r.
The final proof that the thing comes to the eye
Leonardo announces the last experiment that gives the true verdict that the thing comes to the eye and not the eye to the thing, framing the vision studies that follow as decisive on the direction of sight.
Two concordant eyes refer one thing to a single point
A thing seen by two concordant eyes is referred inside the head to one and the same point, as at m n o p. If you push one eye aside with your finger the single thing becomes two, because you have not moved the thing but the likeness of it that is in your eye; were the eye to send out a visual power, moving it would make the letters you read seem to shift place.
Lights mirrored in the eyes and referred to the common sense
Set two or three lights in a hall and look into the eyes of those gazing at them, and you will see their likeness impressed in the eyes; with three lights each person has six impressed across two eyes, which referred to the common sense remain three.