|Galilei, Galileo Dialogues on two world systems 1661, tr. Salusbury, Thomas|
white, but not burnished; would this yet suffice to the making
of it visible, and apt for darting forth the light of the Sun?
SALV. 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 consist
ing in indivisibles, an hair doth as much detract from its perfection
as a mountain.
SAGR. Here I meet with two difficulties, one is to know the
reason why the greater inequality of superficies maketh the stron
ger reflection of light; the other is, why these Peripatetick Gen
tlemen are for this exact figure.
SALV. I will answer to the first; and leave to Simplieius 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 re
ceiveth 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
The more rough
of light, than the
more than the ob
lique, and why.
SAGR. I see the effect, but comprehend not the cause.
SALV. 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.
SAGR. The bare sight of this Figure hath fully satisfied me,
SIMPL. Pray you let me hear you out, for I am not of so
quick an apprehension.
SALV. 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,