Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/90

 white, but not burnished; would this yet suffice to the making of it visible, and apt for darting forth the light of the Sun?

It would suffice in part; but would not give a light so strong, as it doth being mountainous, and in sum, full of eminencies and great cavities. But these Philosophers will never yield it to be lesse polite than a glasse; but far more, if more it can be imagined; for they esteeming that to perfect bodies perfect figures are most sutable; it is necessary, that the sphericity of those Cœlestial Globes be most exact; besides, that if they should grant me some inequality, though never so small, I would not scruple to take any other greater; for that such perfection consisting in indivisibles, an hair doth as much detract from its perfection as a mountain.

Here I meet with two difficulties, one is to know the reason why the greater inequality of superficies maketh the stronger reflection of light; the other is, why these Peripatetick Gentlemen are for this exact figure.

I will answer to the first; and leave to Simplicius the care of making reply to the second. You must know therefore, that the same superficies happen to be by the same light more or less illuminated, according as the rayes of illumination fall upon them more or lesse obliquely; so that the greatest illumination is where the rayes are perpendicular. And see, how I will prove it to your sense. I bend this paper, so, that one part of it makes an angle upon the other: and exposing both these parts to the reflection of the light of that opposite Wall, you see how this side which receiveth the rayes obliquely, is lesse shining than this other, where the reflection fals at right angles; and observe, that as I by degrees receive the illumination more obliquely, it groweth weaker.

I see the effect, but comprehend not the cause.

If you thought upon it but a minute of an hour, you would find it; but that I may not waste the time, see a kind of demonstration thereof in Fig. 7.

The bare sight of this Figure hath fully satisfied me, therefore proceed.

Pray you let me hear you out, for I am not of so quick an apprehension.

Fancie to your self, that all the paralel lines, which you see to depart from the terms A. B. are the rays which fall upon the line C. D. at right angles: then incline the said C. D. till it hang as D. O. now do not you see that a great part of those rays which peirce C. D. pass by without touching D. O? If therefore D. O. be illuminated by fewer rays, it is very reasonable, that the light received by it be more weak. Let us return now to the Moon,