Galilei, Galileo, The systems of the world, 1661

Table of figures

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              angle in the Starry Sphere, that inſiſteth upon the diameter of
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              the Star, and by the proportion of the ropes thickneſſe to the
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              diſtance from the eye to the rope, by the table of Arches and
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              Chords, I have immediately found the quantity of the angle;
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              ſing all the while the wonted caution that is obſerved in taking
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              angles ſo acute, not to forme the concourſe of the viſive rayes
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              in the centre of the eye, where they are onely refracted, but
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              beyond the eye, where really the pupils greatneſſe maketh them
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              to concur.</s>
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            <p type="margin">
              <s>
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                <emph type="italics"/>
              A way to
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              ſure the apparent
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              diameter of a ſtar.
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              * Rendred in
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              Latine
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              Corum,
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              that
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              is to ſay,
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              weſt.</s>
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              <s>
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              *
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              i.e.
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              Is
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              ded by.</s>
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              <s>SAGR. </s>
              <s>I apprehend this your cautelous procedure, albeit I
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              have a kind of hæſitancy touching the ſame, but that which moſt
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              puzzleth me is, that in this operation, if it be made in the dark
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              of night, methinks that you meaſure the diameter of the
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              ted
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              Diſcus,
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              and not the true and naked face of the Star.</s>
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              <s>SALV. </s>
              <s>Not ſo, Sir, for the rope in covering the naked body
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              of the Star, taketh away the rayes, which belong not to it, but
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              to our eye, of which it is deprived ſo ſoon as the true
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              Diſcus
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              thereof is hid; and in making the obſervation, you ſhall ſee, how
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              unexpectedly a little cord will cover that reaſonable big body of
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              light, which ſeemed impoſſible to be hid, unleſſe it were with a
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              much broader Screene: to meaſure, in the next place, and
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              ctly to find out, how many of thoſe thickneſſes of the rope
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              poſe in the diſtance between the ſaid rope and the eye, I take not
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              onely one diameter of the rope, but laying many pieces of the
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              ſame together upon a Table, ſo that they touch, I take with a
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              pair of Compaſſes the whole ſpace occupied by fifteen, or
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              ty of them, and with that meaſure I commenſurate the diſtance
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              before with another ſmaller cord taken from the rope to the
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              courſe of the viſive rayes. </s>
              <s>And with this ſufficiently-exact
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              ration I finde the apparent diameter of a fixed Star of the firſt
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              magnitude, commonly eſteemed to be 2
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              min. </s>
              <s>pri.
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              and alſo 3
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              min.
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              </s>
              <s>prim.
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              by
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              Tycho
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              in his
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              Aſtronomical Letters, cap.
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              167. to be no
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                <arrow.to.target n="marg611"/>
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              more than 5
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              ſeconds,
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              which is one of the 24. or 36. parts of what
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              they have held it: ſee now upon what groſſe errours their
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              ctrines are founded.</s>
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            <p type="margin">
              <s>
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                <emph type="italics"/>
              The diameter of
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              a fixed ſtar of the
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              firſt magnitude not
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              more than five ſec.
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              </s>
              <s>min.
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              </s>
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              <s>SAGR. </s>
              <s>I ſee and comprehend this very well, but before we
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              paſſe any further, I would propound the doubt that ariſeth in
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              me in the finding the concourſe [or interſection] of the viſual
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              rayes beyond the eye, when obſervation is made of objects
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              prehended between very acute angles; and my ſcruple proceeds
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              from thinking, that the ſaid concourſe may be ſometimes more
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              remote, and ſometimes leſſe; and this not ſo much, by meanes
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              of the greater or leſſer magnitude of the object that is beheld, as
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              becauſe that in obſerving objects of the ſame bigneſſe, it ſeems
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              to me that the concourſe of the rayes, for certain other </s>
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