⚠The left margin of the page is tightly cropped, resulting in the loss of the initial characters of almost every line.

Of the Low and High Sun. Letter I.

...are subject to the same conditions as the organ of sight. Truly, if you ever look toward the border of light and shadow, you can observe that the light there is very weak. This is because that part of the space is illuminated only by the outermost edge of the Sun. However, the light is stronger where it is more remote, because the space there is seen by a more abundant portion of the Sun. Finally, the light that is quite far removed is the most brilliant, because the space there receives light from the whole Sun, since no opaque object is now blocking it. Furthermore, if it happens that the rays, which are emitted individually from the very edge of the Sun and create that weak light, are blocked by the tiny particles of intervening vapors: atmospheric moisture or particles, do you not see what will happen? The space that was in the weakest light is now deprived of light and sits in shadow. The shadow itself, which existed before, would then be considered increased by just as much.

VI. Although I propose these things to you quite modestly, because I distrust my own reasoning, I would prefer the matter to be judged by the excellent Fortunio Liceti (1577–1657) was a prominent Italian physician and philosopher who corresponded with Galileo and wrote extensively on astronomy. Licetus, if you think this difficulty should be shared with him. For me it is enough if I have provided a reliable account of the observed facts. To confirm this for you with a consistent experiment, I want you to know that I observed the Moon during the darkness of night. Its diameter: the width of a circular object appeared larger then than it did later during the aurora: the period of dawn just before sunrise. During the dawn, it appeared larger than when the Sun first rose. Even then, it was still larger than during the full diurnal: belonging to the daytime brightness.

I have no other explanation for this other than the aforementioned pupil. The more the pupil is illuminated, the more it becomes contracted. As a smaller original: "specie," referring to the visual image or "species" that ancient and early modern optics believed traveled from an object to the eye image is transmitted through it, it creates a smaller appearance. I would not have even considered this continuous decrease in size, but a chance observation almost forced it on my attention. At that time...