Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/87

 times bigger than the naked and real body: and a like or greater augmentation doth the image of the Sun make, which you see in that glass. I say greater, for that it is more lively than the star, as is manifest from our being able to behold the star with much less offence, than this reflection of the glass. The reverberation therefore which is to dispere it self all over this wall, cometh from a small part of that glass, and that which even now came from the whole flat glass dispersed and restrain'd it self to a very small part of the said wall. What wonder is it then, that the first reflection very lively illuminates, and that this other is almost imperceptible?

I find my self more perplexed than ever, and there presents it self unto me the other difficulty, how it can be that that wall, being of a matter so obscure, and of a superficies so unpolish'd, should be able to dart from it greater light, than a glass very smooth and polite.

Greater light it is not, but more universal; for as to the degree of brightness, you see that the reflection of that small flat glass, where it beamed forth yonder under the shadow of the penthouse, illuminateth very much; and the rest of the wall which receiveth the reflection of the wall on which the glass is placed, is not in any great measure illuminated, as was the small part on which the reflection of the glass fell. And if you would understand the whole of this business, you must consider that the superficies of that wall's being rough, is the same as if it were composed of innumerable small superficies, disposed according to innumerable diversities of inclinations: amongst which it necessarily happens, that there are many disposed to send forth their reflex rays from them into such a place, many others into another: and in sum, there is not any place to which there comes not very many rays, reflected from very many small superficies, dispersed throughout the whole superficies of the rugged body, upon which the rays of the Sun fall. From which it necessarily followeth, That upon any, whatsoever, part of any superficies, opposed to that which receiveth the primary incident rays, there is produced reflex rays, and consequently illumination. There doth also follow thereupon, That the same body upon which the illuminating rays fall, beheld from whatsoever place, appeareth all illuminated and shining: and therefore the Moon, as being of a superficies rugged and not smooth, beameth forth the light of the Sun on every side, and to all beholders appeareth equally lucid. But if the surface of it, being spherical, were also smooth as a glass, it would become wholly invisible; forasmuch as that small part, from which the image of the Sun should be reflected unto the eye