The eye as a camera obscura: how images cross and refract inside it
Notebook D: species pass the pupil, refract through the vitreous sphere, and invert like a pinhole image
Leonardo traces how the species of external objects pass through the pupil to the vitreous sphere, intersecting so that the right ray strikes the left side and vice versa. As the rays cross from the rarer albugineous humour into the denser vitreous sphere they draw toward their perpendicular and narrow, then form a pyramid on emerging that intersects at point f and reaches the visual faculty g at the end of the optic nerve g S. He confirms the inversion with the pinhole experiment: images of sunlit objects entering a dark room through a small round aperture in a thin iron plate fall onto white paper smaller and upside down, appearing as if painted. Two right-hand diagrams show the vitreous sphere and the darkened-room set-up with objects a b c d e.
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Rays refract passing from rare to dense
The rays of the species contract as they cross the vitreous sphere, being much narrower on the far side than where they first struck. This happens because rays draw toward their perpendicular passing from the rare to the dense: the albugineous humour is rarer and thinner than the space enclosed by the vitreous sphere.
The pinhole experiment: inverted painted images
Let the species of sunlit objects enter a very dark room through a small round aperture. Received on thin white paper near the aperture, all the objects appear with their own shapes and colours, but smaller and upside down because of the intersection, as if truly painted; the aperture should be cut in a very thin iron plate.
Right becomes left at the crossing
Let a b c d e be objects lit by the sun, o r the wall of the dark room holding the aperture n m, and S t the paper where the rays are cut upside down. Since the rays run straight, the right becomes left at K and the left becomes right, and the same happens within the pupil.
Vitreous sphere, albugineous humour and optic nerve
The image passes the door of the pupil to the vitreous sphere, penetrates it, then makes a pyramid on emerging into the albugineous humour, intersecting its sides at point f and passing to the visual faculty g at the end of the optic nerve g S. Nature has the pupil contract and dilate for bright, dark, universal or particular things.