Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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the ſaid irradiation of four inches to a circle that hath but two
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ches of diameter onely, the diameter of the irradiation or
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land would be ten inches, and the ſuperficial content of the circle
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would be to the
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area
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of the naked body, as 100. to 4. for thoſe
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are the ſquares of 10. and of 2. the agrandizement would
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fore be 25. times ſo much; and laſtly, the four inches of hair or
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fringe, added to a ſmall circle of an inch in diameter, the ſame
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would be increaſed 81. times; and ſo continually the
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tions are made with a proportion greater and greater, according
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as the real objects that increaſe, are leſſer and leſſer.</
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Superficial
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gures encreaſing
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proportion double to
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their lines.
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>SAGR. </
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>The doubt which puzzled
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Simplicius
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never troubled
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me, but certain other things indeed there are, of which I deſire
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a more diſtinct underſtanding; and in particular, I would know
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on what ground you affirm that the ſaid agrandizement is alwayes
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equal in all viſible
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Objects the more
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vigorous they are
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in light, the more
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they do ſeem to
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creaſe.
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<
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>SALV. </
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<
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>I have already declared the ſame in part, when I ſaid,
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that onely lucid objects ſo increaſed, and not the obſcure; now I
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adde what remaines, that of the reſplendent objects thoſe that are
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of a more bright light, make the reflection greater and more
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ſplendent upon our pupil; whereupon they ſeem to augment
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much more than the leſſe lucid: and that I may no more inlarge
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my ſelf upon this particular, come we to that which the true
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ſtris of
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Astronomy,
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Experience, teacheth us. </
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>Let us this evening,
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when the air is very obſcure, obſerve the ſtar of
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Jupiter
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; we
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ſhall ſee it very glittering, and very great; let us afterwards look </
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through a tube, or elſe through a ſmall trunk, which clutching the
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hand cloſe, and accoſting it to the eye, we lean between the palm
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of the hands and the fingers, or elſe by an hole made with a ſmall
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needle in a paper; and we ſhall ſee the ſaid ſtar diveſted of its
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beams, but ſo ſmall, that we ſhall judge it leſſe, even than a
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eth part of its great glittering light ſeen with the eye at liberty:
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we may afterwards behold the
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Dog-ſtars
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beautiful and bigger than
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any of the other fixed ſtars, which ſeemeth to the bare eye no
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great matter leſſe than
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Jupiter
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; but taking from it, as before, the
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irradiation, its
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Diſcus
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will ſhew ſo little, that it will not be
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thought the twentieth part of that of
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Jupiter,
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nay, he that hath not
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very good eyes, will very hardly diſcern it; from whence it may
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be rationally inferred, that the ſaid ſtar, as having a much more
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lively light than
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Jupiter,
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maketh its irradiation greater than
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ter
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doth his. </
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>In the next place, as to the irradiation of the Sun
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and Moon, it is as nothing, by means of their magnitude, which
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poſſeſſeth of it ſelf alone ſo great a ſpace in our eye, that it
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veth no place for the adventitious rayes; ſo that their faces ſeem
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cloſe clipt, and terminate. </
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<
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>We may aſſure our ſelves of the ſame
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truth by another experiment which I have often made triall of; </
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