Caverni, Raffaello, Storia del metodo sperimentale in Italia, 1891-1900
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              <s>Sieno Aa, Bb (fig. </s>
              <s>7) le due linee conterminanti il mezzo diafano at­
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              traversato dall'atomo di luce G nel punto H: se sia quel mezzo meno denso
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              dell'altro d'onde il raggio GH è venuto “ et si attractio vel impulsus po­
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                <figure id="id.020.01.575.1.jpg" xlink:href="020/01/575/1.jpg" number="72"/>
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              <s>Figura 7
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              natur uniformis, erit ex demon­
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              stratis Galilaei curva HP parabola
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              “ (Principia mathem., Genevae
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              1739, T. I, pag. </s>
              <s>534). Soggiaccia
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              allo strato etereo Ab, un altro si­
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              mile strato etereo Bc, ma alquanto
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              meno denso del primo, nel quale
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              entri, emergendo dal punto P il
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              raggio
                <expan abbr="Pq.">Pque</expan>
              Si dimostra con gran
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              facilità dal Newton che la velocità
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              del raggio avanti l'incidenza è alla
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              velocità dello stesso raggio dopo
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              l'emergenza, come il seno del­
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              l'emergenza al seno dell'incidenza
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              (Propositio XCV, ibi, pag. </s>
              <s>536),
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              e il detto raggio PQ procederà
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              per le stesse ragioni in arco pa­
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              rabolico; cosicchè, avendo in Q raggiunto l'angolo limite, subirà in R la
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              riflessione interna, e come i gravi proiettili attratti al centro della Terra si
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              troverà aver descritta la traiettoria HPQR semiparabolica. </s>
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              <s>“ Perveniat corpus (giacchè l'atomo luminoso è pel Newton un corpo
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              qualunque) ad hoc planum in puncto R et quoniam linea emergentiae coin­
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              cidit cum eodem plano, perspicuum est quod corpus non potest ultra per­
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              gere versus planum Ee. </s>
              <s>Sed nec potest idem pergere in linea emergentiae
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              Rd, propterea quod perpetuo attrahitur vel impellitur versus medium inci­
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              dentiae. </s>
              <s>Revertetur itaque inter plana Cc, Dd, describendo arcum parabolae
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              QRq cuius vertex principalis, iuxta demonstrata Galilaei, est in R; secabit
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              planum Cc in eodem angulo in q ac prius in
                <expan abbr="q;">que</expan>
              dein pergendo in arcubus
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              parabolicis qp, ph etc. </s>
              <s>arcubus prioribus QP, PH, similibus et aequalibus,
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              secabit reliqua plana in iisdem angulis in p, h etc. </s>
              <s>ac prius in P, H etc. </s>
              <s>
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              emergetque tandem eadem obliquitate in h, qua incidit in H ” (ibi, pag. </s>
              <s>538). </s>
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              <s>Il Newton, che nelle speculazioni sue era originale, procede per le vie
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              della Meccanica con passo più sicuro di quel che non facesse il Cartesio
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              imitator del Keplero. </s>
              <s>Ma il forte si è che non è questione di Meccanica pura. </s>
              <s>
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              Nessuno può revocare in dubbio i Teoremi XLVIII, XLIX e L del Tomo I
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              de'
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              Principii,
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              ne'quali nulla osta a supporre un proiettile qualunque che at­
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              traversi mezzi via via meno densi. </s>
              <s>Si può dubitar però se l'etere neuto­
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              niano si trovi in così fatte condizioni. </s>
              <s>Chi non direbbe piuttosto che le den­
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              sità di lui crescono via via perchè più fortemente attratto verso la superficie
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              del riflettente? </s>
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            <p type="main">
              <s>Ma lasciamo un po'da parte questo mezzo etereo, il quale non esiste </s>
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