On the Apparent Magnitude of the Sun when Low and High

II. On the Apparent Magnitude

since there is nothing opaque behind it to block its transmission and contain it like a mirror; rather, only the vitreous humor: the clear gel that fills the space between the lens and the retina is present, which is itself also transparent. Because of this, it appears the crystalline lens: the transparent structure behind the pupil is not placed and suspended there for any other purpose than to provide a passage for the image through its own transparency. By its double surface, whether spherical or parabolic, it performs that refraction: the bending of light as it passes through different materials which ensures the proper placement of the image on the fundus: the interior back surface of the eye.

When the crystalline lens is removed, the images of things are painted inversely on the back of the eye, such as on the concave surface. The top appears at the bottom, and the right appears on the left. The crystalline lens, by refracting the images, restores them so that the upper parts appear at the top, the right parts appear on the right, and all other parts appear in their proper position. Gassendi is describing a contemporary theory of optics. He believes the lens "re-flips" the image so it hits the retina upright. Modern science knows the image remains inverted on the retina and the brain interprets it as upright.

This has been established through observation. It is also established regarding the decussation: the crossing of light rays of the rays within the vitreous humor at a specific distance from the original: "speculo" mirror, so that things are clearly represented. Gassendi uses the word "mirror" to refer to the reflective surface at the back of the eye, which we now identify as the retina or choroid.

It is noteworthy that the center of the optic nerve: the nerve carrying visual signals to the brain is not located directly opposite the center of the pupil. It is placed a little lower. This is perhaps a matter of providence, so that the brightness directed along the central axis does not enter the nerve too easily and cause damage. Nevertheless, the "mirror" is visible with a certain metallic gleam. It is bounded by a circumference in such a way that its center is placed directly opposite the center of the pupil.

It is no less wonderful that the area remaining forward from that circumference is stained with a certain blackness. Because of this, the light that strikes the mirror with the image is reflected within that space full of vitreous humor. This creates a remarkable brightness, which makes the image appear much clearer. There are many things to say in confirmation of this matter...