Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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white, but not burniſhed; would this yet ſuffice to the making
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of it viſible, and apt for darting forth the light of the Sun?</
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<
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>SALV. </
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<
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>It would ſuffice in part; but would not give a light ſo
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ſtrong, as it doth being mountainous, and in ſum, full of
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eminencies and great cavities. </
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<
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>But theſe Philoſophers will never
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yield it to be leſſe polite than a glaſſe; but far more, if more it
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can be imagined; for they eſteeming that to perfect bodies perfect
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figures are moſt ſutable; it is neceſſary, that the ſphericity of thoſe
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Cœleſtial Globes be moſt exact; beſides, that if they ſhould
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grant me ſome inequality, though never ſo ſmall, I would not
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ſcruple to take any other greater; for that ſuch perfection
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ing in indiviſibles, an hair doth as much detract from its perfection
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as a mountain.</
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<
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>SAGR. </
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<
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>Here I meet with two difficulties, one is to know the
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reaſon why the greater inequality of ſuperficies maketh the
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ger reflection of light; the other is, why theſe
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Peripatetick
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tlemen are for this exact figure.</
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>SALV. </
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<
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>I will anſwer to the firſt; and leave to
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Simplieius
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the
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care of making reply to the ſecond. </
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<
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>You muſt know therefore,
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that the ſame ſuperficies happen to be by the ſame light more or leſs
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illuminated, according as the rayes of illumination fall upon them
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more or leſſe obliquely; ſo that the greateſt illumination is where
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the rayes are perpendicular. </
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<
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>And ſee, how I will prove it to your
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ſenſe. </
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<
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>I bend this paper, ſo, that one part of it makes an angle
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upon the other: and expoſing both theſe parts to the reflection of
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the light of that oppoſite Wall, you ſee how this ſide which
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ceiveth the rayes obliquely, is leſſe ſhining than this other, where
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the reflection fals at right angles; and obſerve, that as I by
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degrees receive the illumination more obliquely, it groweth
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weaker.</
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The more rough
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ſuperficies make
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greater reflection
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of light, than the
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leſs rough.
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Perpendicular
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rays illuminate
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more than the
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lique, and why.
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<
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>SAGR. </
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<
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>I ſee the effect, but comprehend not the cauſe.</
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<
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>SALV. </
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<
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>If you thought upon it but a minute of an hour, you
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would find it; but that I may not waſte the time, ſee a kind of
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demonſtration thereof in
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Fig.
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7.</
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<
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>SAGR. </
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>The bare ſight of this Figure hath fully ſatisfied me,
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therefore proceed.</
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<
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>SIMPL. </
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<
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>Pray you let me hear you out, for I am not of ſo
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quick an apprehenſion.</
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<
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>SALV. </
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<
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>Fancie to your ſelf, that all the paralel lines, which you
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ſee to depart from the terms A. B. are the rays which fall upon the
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line C. D. at right angles: then incline the ſaid C. D. till it hang
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as D. O. now do not you ſee that a great part of thoſe rays which
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peirce C. D. paſs by without touching D. O? </
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<
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>If therefore D. O.
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be illuminated by fewer rays, it is very reaſonable, that the light
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received by it be more weak. </
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<
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>Let us return now to the Moon, </
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