Salusbury, Thomas, Mathematical collections and translations (Tome I), 1667

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            <p type="main">
              <s>
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              ſpects ought to be made more and leſſe remote from the eye.</s>
            </p>
            <p type="main">
              <s>SALV. </s>
              <s>I ſee already, whither the apprehenſion of
                <emph type="italics"/>
              Sagredus,
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              a moſt diligent obſerver of Natures ſecrets, tendeth; and I
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                <arrow.to.target n="marg612"/>
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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
                <emph type="italics"/>
              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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                <arrow.to.target n="marg613"/>
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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. </s>
              <s>This is a Doctrine which
                <emph type="italics"/>
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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. </s>
              <s>I am not certain,
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              Sagredus,
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              that this would have
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              been your objection.</s>
            </p>
            <p type="margin">
              <s>
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                <emph type="italics"/>
              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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              </s>
            </p>
            <p type="margin">
              <s>
                <margin.target id="marg613"/>
              +
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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.</s>
            </p>
            <p type="main">
              <s>SAGR. </s>
              <s>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.</s>
            </p>
            <p type="main">
              <s>SALV. </s>
              <s>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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                <arrow.to.target n="marg614"/>
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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 </s>
            </p>
          </chap>
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      </text>
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