On the Apparent Magnitude of the Sun when Low and High

On the Sun Near the Horizon and High in the Sky. Letter II.

The left edge of the original Latin text is partially cropped, but the missing letters have been reconstructed based on the context of Gassendi's known arguments regarding optics.

...they create a very weak light there, being hindered by the interposed small bodies of vapors. Do you not see that it will follow that the space, which previously received that very weak light, is now deprived of light and cast into shadow? Thus, the shadow that existed before is considered to be increased by that same amount. You see, therefore, how much I have striven to distinguish the causes of distinct effects, even if I have not carried the matter out with all the clarity that was required.

For this reason, you state at the start that it is difficult for you to understand how a wider pupil helps one perceive a wider shadow at the base of the Sun. Conversely, you ask how a more contracted pupil causes the shadow of the same opaque body: an object that does not let light pass through, such as a sighting vane to be seen as narrower when the Sun is high, due to the smaller quantity of the visible species: the image or form of an object as it was thought to travel from the source to the eye entering through the narrower opening of the pupil when the Sun is already high, and so on.

Finally, you say that I disagree with my own first reasoning. In that reasoning, it was stated that the cause for the shadow appearing wider when the Sun is near the horizon: the boundary where the sky meets the earth, and narrower when the Sun is far from the horizon and displaying a smaller path to the sight, should be attributed to two factors. First, to the dilation of the pupil due to the weaker light of the lower Sun and the contraction of the same opening due to the more vivid light of the higher Sun. Second, it should be attributed to the greater or lesser magnitude of the visible species representing the shadow on the retina: the light-sensitive membrane at the back of the eye.

However, I either did not say, or did not intend to say, that the cause of the expanded or narrowed shadow must be attributed to the expansion or contraction of the pupil. Rather, I meant that the expanded or narrowed shadow is the effect of one cause, while the expansion or contraction of the pupil is the cause of a different effect. Therefore, there is no reason for me to justify my thoughts as you assume them to be. Furthermore, nothing...