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Behind this convex segment is the iris, a membranous partition positioned like a field and pierced by a rounded opening called the pupil, which dilates or contracts as the iris tightens or extends. Slightly behind the iris, opposite the pupil's opening, is a lens or magnifying glass called the crystalline lens.
Opposite the crystalline lens, the globe is pierced to allow the optic nerve to enter, which penetrates the eye globe to form the retina. When the retina is unpleasantly affected by the brightness of a too vivid light, the pupil contracts to let through only a small number of light rays: it dilates in contrast when the light diminishes, to still receive a sufficient quantity of light rays.
Light rays, starting from an illuminated object, form a cone whose tip strikes the observer's eye. The rays that the pupil allows to pass meet the crystalline lens and become convergent. They then diverge, due to the lens-like shape of this part of the eye, and strike the retina, where they form the image of the body that reflected these rays.
The meeting point of the converging rays is called the focus.

Names of the different parts that make up the eye.

RS (fig. 1), envelope or sclera.
C, the crystalline.
LMOP, the retina, applied on the choroid.
D, the cornea (transparent).
AB, the optic nerve.
EF, the iris. — K, the pupil.

How the eye receives the image of objects that appear to us.

Let's suppose the arrow ac is placed in front of the eye. The ray starting from point a comes to strike the retina at point b. The ray from point c arrives at d. In this position, the image of the arrow ac is, therefore, inverted.
It is now necessary to explain how it happens that this image, however, appears to us as it occurs in nature. One can refer to general physics texts and specific treaties on vision for this. We propose the following explanation to facilitate students' understanding of this phenomenon: The optic nerve, due to its anatomical construction, might lead one to believe that (assuming the sensation of vision is produced in the brain) the image, after traveling through the optic nerve, is indeed reversed upon reaching the brain.

Remark relative to drawing from nature.

It is essential to note that in the continuation of this work, we will always speak of the observer's eye and not the eyes. When one wants to see an object very distinctly, one should only use one eye. The simultaneous operation of both eyes would create two optical angles that would counteract and blacken each other reciprocally.
Let us assume A and B (fig. 5) two eyes open looking at the globe C. This globe will hide for the observer two points in space. Eye B will see this globe at D, and eye A will see it at E; consequently, the two eyes will counteract each other. But if eye A is closed, for example, eye B will see the globe more clearly, as this globe will hide only one point in space.
When drawing an object, especially one that presents angles and foreshortening, a head for example, care must be taken to remain in the same line and always look with the same eye.
Indeed, let's assume two eyes looking at solid cdef (fig. 6); when eye A is closed, B will see the side cf of the solid larger than side fc. Conversely, if B is closed, A will see side ef larger than fe.

Translation Notes

Pupil: The opening in the center of the iris that controls light entering the eye.
Crystalline lens: A transparent lens behind the iris that helps to focus light onto the retina.
Focus: The point where light rays converge.