Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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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.</
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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.</
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*
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i.e.
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Is
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ded by.</
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<
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>SAGR. </
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>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.</
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<
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>SALV. </
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>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. </
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<
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>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. </
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<
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>pri.
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and alſo 3
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min.
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<
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>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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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.</
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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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<
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>min.
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<
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>SAGR. </
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<
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>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 </
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