On the Apparent Magnitude

It would be necessary to respond to the reasons you raised against my stated First Cause, except that you insist on the shadow seen upon the sighting vane: a small metal plate with a hole or slit used for lining up an observation. One could also argue about the species: the visual image or form of an object that travels to the eye existing within the eye. Therefore, as far as I can see, those arguments are related to three categories. One concerns the object seen, insofar as its image is transmitted through the pupil, which is sometimes narrower and sometimes wider. The second concerns the image itself, whether it is complete and strong, or small and weak. The third concerns the part of the eye in which vision is performed, as there is a controversy whether it is the crystalline humor: the lens of the eye, which early scientists often thought was the primary organ of sight, as the older thinkers preferred; or the retina: the light-sensitive inner lining of the back of the eye, as most of the younger ones believe.

IV. Therefore, to say something about these things, but in reverse order: I will not linger first on the controversy about the location of vision, because many have already considered it. I only observe that all the reasoning you bring in favor of the crystalline humor seems to me to support the retina. Furthermore, there are reasons why the retina should be preferred. For you say that the function of seeing must occur mainly in that part where the images of all visible things can be received and contained, and the images of all colors. This seems to apply to the retina as much as to the crystalline lens. In fact, it applies to the retina even more, since it is at the back of the eye. In that location, the images and forms can finally be received and contained after passing through the transparent media: the various clear parts of the eye, such as the cornea and lens, that light must travel through. You also assume that this part must be transparent and devoid of all color. This also seems to apply no less to the retina than to the crystalline lens. It is so smooth, thin, transparent, and colorless that, while clinging to the choroid: the dark, vascular layer of the eye situated between the retina and the outer white layer, it leaves the choroid clearly visible, and the retina itself cannot be seen...