On the low and high Sun. Letter II.

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it is? considered to be its surface. The crystalline humor: the lens of the eye, which ancient anatomists believed was the primary seat of vision is more of an obstacle and appears more distinct from its surroundings. I will not even mention that the retina, although I have examined it a hundred times, has never appeared to be what you describe: an opaque body, naturally white but turning red, as if filled with many arteries and veins full of bright blood.

You then add a point from original: "Aristotele" Aristotle about the sense of touch. He argues that because the sense has balanced qualities, it only perceives an excess of external qualities. You also mention the saying: "that which is inside prevents what is outside," and similar ideas. This is no obstacle if the retina: the light-sensitive inner lining of the back of the eye, no less than the crystalline lens, is stripped of the nature or sensible species: the visual form or image of an object that travels to the eye of the thing to be known.

I add that vision must occur through the impression of images. It is logical for vision to be triggered where the image appears sensibly impressed, and where the faculty of sight is present with its serving visual spirits: the subtle fluids once thought to carry sensory information through the nerves.

It is clear from observations that the back of the eye, or the choroid: the vascular layer of the eye between the retina and the sclera, is a concave mirror. The images of things are very clearly depicted in it. The black fluid within it contributes to this by providing opacity. Likewise, the retina is nothing but the optic nerve spread out into a very thin, smooth, and transparent membrane. It is laid directly over the choroid? to serve as the mirror surface. Finally, the visual spirits are carried through the optic nerve to animate the retina where the image is located. This supports the faculty of seeing.

Therefore, it is logical for vision to occur in the retina where it touches the choroid? at the back of the eye. Do not say that the image is also received in the crystalline lens. The image is not? so much received as it is passed through the crystalline lens.

Vocabulary: Aristotle, retina, crystalline lens, choroid, visual spirits, vision, back of the eye, optic nerve.