Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/84

 lesse forcible and offensive to the sight, than that primary and direct light of the Sun. And thus without trouble do we behold the face of the Moon; which were it as a Glasse, it appearing to us by reason of its vicinity, as big as the Sun it self, its splendor would be absolutely intollerable, and would seem as if we beheld another Sun.

Ascribe not, I beseech you Sagredus, more to my demonstration, than it produceth. I will oppose you with an instance, which I see not well how you can easily resolve. You insist upon it as a grand difference between the Moon and Glasse, that it emits its reflection towards all parts equally, as doth the Wall; whereas the Glasse casts it upon one onely determinate place; and from hence you conclude the Moon to be like to the Wall, and not to the Glasse: But I must tell you, that that same Glasse casts its reflection on one place onely, because its surface is flat, and the reflex rayes being to depart at angles equal to those of the rayes of incidence, it must follow that from a plane or flat superficies, they do depart unitedly towards the same place; but in regard that the superficies of the Moon is not plain, but spherical, and the incident rayes upon such a superficies, being to reflect themselves at angles equal to those of the incidence towards all parts, by means of the infinity of the inclinations which compose the spherical superficies, therefore the Moon may send forth its reflection every way; and there is no necessity for its repercussion upon one place onely, as that Glasse which is flat.

This is one of the very same objections, which I intended to have made against him.

If this be one, you had need have more of them; yet I tell you, that as to this first, it seems to me to make more against you, than for you.

You have pronounced as a thing manifest, that the reflection made by that Wall, is as cleer and lucid as that which the Moon sends forth, and I esteem it nothing in comparison thereto. For, in this businesse of the illumination, its requisite to respect, and to distinguish the Sphere of Activity; and who questions but the Cœlestial bodies have greater Spheres of activity, than these our elementary, frail, and mortal ones? and that Wall, finally, what else is it but a little obscure Earth, unapt to shine?

And here also I believe, that you very much deceive your felf. But I come to the first objection moved by Salviatus; and I consider, that to make a body appear unto us luminous, it sufficeth not that the rayes of the illuminating body fall upon it, but it is moreover requisite that the reflex rayes arrive to our eye; as is manifestly seen in the example of that Glasse, upon