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

Table of figures

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            <p type="main">
              <s>
                <pb xlink:href="040/01/311.jpg" pagenum="291"/>
              ed; therefore tell me how he diſ-ingageth himſelf in the next place
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              from that particular of the Stars having conſtantly kept the ſame
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              diſtance from the fixed Stars circumjacent to it.</s>
            </p>
            <p type="main">
              <s>SALV. </s>
              <s>He betakes himſelf, in like manner, to two threads, yet
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              more unable to uphold him than the former: one of which is
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              wiſe faſtened to refraction, but ſo much leſs firmly, in that he
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              ſaith, that refraction operating upon the new Star, and ſublimating
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              it higher than its true ſituation, maketh the ſeeming diſtances
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              tain to be diſtinguiſhed from the true, when compared to the
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              cumpoſed fixed Stars that environ it. </s>
              <s>Nor can I ſufficiently
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              mire how he can diſſemble his knowing how that the ſame
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              ction will work alike upon the new Star, as upon the antient one
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              its neighbour, elevating both equally, ſo as that ſuch a like
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              dent altereth not the ſpace betwixt them. </s>
              <s>His other ſubterfuge is
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              yet more unhappy, and carryeth with it much of ridiculous, it
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              ing founded upon the errour that may ariſe in the inſtrumen
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              peration it ſelf; whilſt that the Obſervator not being able to
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              conſtitute the centre of the eyes pupil in the centre of the
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              tant (an Inſtrument imployed in obſerving the diſtance between
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              two Stars) but holding it elevated above that centre, as much as
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              the ſaid pupil is diſtant from I know not what bone of the cheek,
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              againſt which the end of the Inſtrument reſteth, there is formed
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              in the eye an angle more acute than that which is made by the ſides
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              of the Inſtrument; which angle of rayes differeth alſo from it
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              ſelf, at ſuch time as a man looketh upon Stars, not much elevated
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              above the Horizon, and the ſame being afterwards placed at a
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              great height; that angle, ſaith he, is made different, while the
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              ſtrument goeth aſcending, the head ſtanding ſtill: but if in
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              ting the Inſtrument, the neck ſhould bend backwards, and the
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              head go riſing, together with the Inſtrument, the angle would then
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              continue the ſame. </s>
              <s>So that the Authours anſwer ſuppoſeth that
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              the Obſervators in uſing the Inſtrument have not raiſed the head,
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              as they ought to have done; a thing which hath nothing of
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              hood in it. </s>
              <s>But granting that ſo it had been, I leave you to judge
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              what difference can be between two acute angles of two
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              ral triangles, the ſides of one of which triangles are each four
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              [
                <emph type="italics"/>
              Italian] Braces
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              [
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              i.e.
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              about three Engliſh yards] and thoſe of the
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              other, four braces within the quantity of the diameter of a Pea;
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              for the differences cannot be abſolutely greater between the length
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              of the two viſive rayes, whilſt the line is drawn perpendicularly
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              from the centre of the pupil, upon the plain of the Rule of the
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              Sextant (which line is no bigger than the breath of the thumb)
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              and the length of the ſame rayes, whilſt elevating the Sextant,
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              without raiſing the head together with it, that ſame line no longer
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              falleth perpendicularly upon the ſaid plane, but inclineth, making </s>
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          </chap>
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