Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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ſpects ought to be made more and leſſe remote from the eye.</
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<
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>SALV. </
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<
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>I ſee already, whither the apprehenſion of
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Sagredus,
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a moſt diligent obſerver of Natures ſecrets, tendeth; and I
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would lay any wager, that amongſt the thouſands that have
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ſerved Cats to contract and inlarge the pupils of their eyes very
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much, there are not two, nor haply one that hath obſerved the
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like effect to be wrought by the pupils of men in ſeeing, whilſt
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the
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medium
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is much or little illumin'd, and that in the open light
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the circlet of the pupil diminiſheth conſiderably: ſo that in
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king upon the face or
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Diſcus
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of the Sun, it is reduced to a
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neſſe leſſer than a grain of ^{*}
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Panick,
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and in beholding objects
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that do not ſhine, and are in a leſſe luminous
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medium,
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it is
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god to the bigneſſe of a Lintel or more; and in ſumme this
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expanſion and contraction differeth in more than decuple
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portion: From whence it is manifeſt, that when the pupil is
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much dilated, it is neceſſary that the angle of the rayes
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courſe be more remote from the eye; which happeneth in
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holding objects little luminated. </
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<
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>This is a Doctrine which
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gredus
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hath, juſt now, given me the hint of, whereby, if we
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were to make a very exact obſervation, and of great
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quence, we are advertized to make the obſervation of that
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courſe in the act of the ſame, or juſt ſuch another operation; but
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in this our caſe, wherein we are to ſhew the errour of
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mers,
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this accurateneſſe is not neceſſary: for though we ſhould,
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in favour of the contrary party, ſuppoſe the ſaid concourſe to be
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made upon the pupil it ſelf, it would import little, their miſtake
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being ſo great. </
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<
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>I am not certain,
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Sagredus,
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that this would have
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been your objection.</
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The circle of the
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pupil of the eye
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largeth and
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tracteth.
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+
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Panicum,
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a
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ſmall grain like to
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Mill, I take it to be
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the ſame with that
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called Bird Seed.</
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<
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>SAGR. </
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<
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>It is the very ſame, and I am glad that it was not
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together without reaſon, as your concurrence in the ſame
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reth me; but yet upon this occaſion I would willingly hear what
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way may be taken to finde out the diſtance of the concourſe of
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the viſual rayes.</
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<
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>SALV. </
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<
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>The method is very eaſie, and this it is, I take two
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long^{*} labels of paper, one black, and the other white, and make
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the black half as broad as the white; then I ſtick up the white
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gainſt a wall, and far from that I place the other upon a ſtick, or
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other ſupport, at a diſtance of fifteen or twenty yards, and
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ding from this, ſecond another ſuch a ſpace in the ſame right line,
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it is very manifeſt, that at the ſaid diſtance the right lines will
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concur, that departing from the termes of the breadth of the
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white piece, ſhall paſſe cloſe by the edges of the other label
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ced in the mid-way; whence it followeth, that in caſe the eye
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were placed in the point of the ſaid concourſe or interſection,
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the black ſlip of paper in the midſt would preciſely hide the </
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