Caverni, Raffaello, Storia del metodo sperimentale in Italia, 1891-1900

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              <s>
                <pb xlink:href="020/01/372.jpg" pagenum="353"/>
              siero al cap. </s>
              <s>XI del libro XVII della Magia di Giov. </s>
              <s>Battista della Porta.
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              </s>
              <s>È passo di cui confessa a V. S. il Keplero che non l'intende, nè ho io sa­
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              puto giammai che matematico alcuno l'abbia saputo dichiarare, come so che
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              l'istesso Magini ha confessato, nè il Porta, per quante istanze li sia state
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              fatte da principi e letterati, si è potuto mai inchinare a dichiarare l'animo
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              suo. </s>
              <s>Solo che disse che maestro Paolo da Venezia servita, l'aveva capito, e
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              quanto a me pare assai difficile il credere che questo sia un sibilo di vento
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              bugiardo, poichè si vede che, nel capitolo precedente, aveva così bene in­
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              segnato il modo di accoppiar le due lenti, il che però parve tanto strano
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              per tanto tempo. </s>
              <s>Aggiungo che egli stesso protesta di volere nascondere
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              l'artificio al volgo, ma che ai Prospettivi era cosa manifesta, sicchè uno di­
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              visando che in quelle parole sia qualche scambio o svario, siccome egli con­
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              fessa nella prefazione del libro, e di più che tal cosa non sia tanto difficol­
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              tosa ad un dotto; per tanto prego V. S. a considerare se preso quel testo
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              e trasportando le parole sicchè cominci
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              Costituatur ....
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              oppure
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              Construi­
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              tur hoc modo speculum ....
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              e poi tornar da capo alle parole
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              Virtus costi­
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              tuatur ....
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              si potesse per la prima aver la lettera ordinata. </s>
              <s>Tanto più che
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              in questa parte, che è scritta innanzi, dice
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              praedicti speculi
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              non avendolo
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              ancora nominato. </s>
              <s>Inoltre quelle parole
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              sectionibus illis accomodetur
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              sve­
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              gliano la memoria delle sezioni coniche, tanto celebri, sicchè par che egli
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              voglia intendere di una di quelle, perchè dalle opere sue par che si possa
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              cavare che questa sia la sezione parab olica, e questa è la ragione che egli
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              nel cap. </s>
              <s>XIX, trattando della refrazione, insegna che con la lente parabolica
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              gagliardissimamente si accenda il fuoco, perchè tutti i raggi che passano si
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              uniscono in un punto. </s>
              <s>E nel canocchiale, secondo la dottrina del Keplero
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              e l'esperienza, non si richiede altro che quell'unione, tanto più bella nella
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              parabola, quanto che toglie tutte le altre coincidenze più lunghe e più corte,
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              che caggiono da diverse parti della linea sferica. </s>
              <s>Onde potrebbe il convesso
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              parabolico esser più grande di quantità della sferica, abbracciando più parti
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              in un tempo dell'oggetto, e riuscirebbe chiarissimo. </s>
              <s>E per quanto spetta
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              all'incavato, di cui par che intenda il Porta in quelle parole
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              ubi valentis­
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              sime universales solares radii disperguntur et coeunt minime,
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              vorrebbe la
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              ragione che fosse anch'egli incavato parabolico, il quale per forza disgre­
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              gherebbe i raggi, poichè fossero passati, per la contraria ragione del con­
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              cavo e del convesso, secondo la regola del Porta nel fine della 2a proposi­
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              zione del 2° libro
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              De refractione.
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              E dalla formazione, che egli insegna della
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              sezione parabolica, nel cap. </s>
              <s>XV della Magia XVII, per via del triangolo ret­
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              tangolo, similmente si ha qualche luce da intendere quelle parole, nelle quali
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              fa menzione del triangolo e delle linee trasversali. </s>
              <s>Or sarà fatica di V. S. giu­
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              dicar queste congetture, e quando pure stimasse che fosse molto lontano il
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              pensiero dal Porta, tornerei a pregarla che applicasse l'animo a questo ne­
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              gozio, speculando se potesse riuscir migliore un Canocchiale fatto di cri­
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              stalli parabolici, per le ragioni che si son ricordate dal Porta, poichè, seb­
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              bene il Keplero ha più fede nell'iperbola che nella parabola, nondimeno i </s>
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