a history of science-2-第52章

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which　'tis　illustrated；　but　yet　most　luminous　in　red；　and　so　Bise　appeareth　indifferently　of　any　color　with　which　'tis　illustrated；　but　yet　most　luminous　in　blue。　And　therefore　Minium　reflecteth　rays　of　any　color；　but　most　copiously　those　indued　with　red；　and　consequently；　when　illustrated　with　daylightthat　is；　with　all　sorts　of　rays　promiscuously　blendedthose　qualified　with　red　shall　abound　most　in　the　reflected　light；　and　by　their　prevalence　cause　it　to　appear　of　that　color。　And　for　the　same　reason；　Bise；　reflecting　blue　most　copiously；　shall　appear　blue　by　the　excess　of　those　rays　in　its　reflected　light；　and　the　like　of　other　bodies。　And　that　this　is　the　entire　and　adequate　cause　of　their　colors　is　manifest；　because　they　have　no　power　to　change　or　alter　the　colors　of　any　sort　of　rays　incident　apart；　but　put　on　all　colors　indifferently　with　which　they　are　enlightened。〃＇2＇　This　epoch…making　paper　aroused　a　storm　of　opposition。　Some　of　Newton's　opponents　criticised　his　methods；　others　even　doubted　the　truth　of　his　experiments。　There　was　one　slight　mistake　in　Newton's　belief　that　all　prisms　would　give　a　spectrum　of　exactly　the　same　length；　and　it　was　some　time　before　he　corrected　this　error。　Meanwhile　he　patiently　met　and　answered　the　arguments　of　his　opponents　until　he　began　to　feel　that　patience　was　no　longer　a　virtue。　At　one　time　he　even　went　so　far　as　to　declare　that；　once　he　was　〃free　of　this　business；〃　he　would　renounce　scientific　research　forever；　at　least　in　a　public　way。　Fortunately　for　the　world；　however；　he　did　not　adhere　to　this　determination；　but　went　on　to　even　greater　discoverieswhich；　it　may　be　added；　involved　still　greater　controversies。　In　commenting　on　Newton's　discovery　of　the　composition　of　light；　Voltaire　said：　〃Sir　Isaac　Newton　has　demonstrated　to　the　eye；　by　the　bare　assistance　of　a　prism；　that　light　is　a　composition　of　colored　rays；　which；　being　united；　form　white　color。　A　single　ray　is　by　him　divided　into　seven；　which　all　fall　upon　a　piece　of　linen　or　a　sheet　of　white　paper；　in　their　order　one　above　the　other；　and　at　equal　distances。　The　first　is　red；　the　second　orange；　the　third　yellow；　the　fourth　green；　the　fifth　blue；　the　sixth　indigo；　the　seventh　a　violet　purple。　Each　of　these　rays　transmitted　afterwards　by　a　hundred　other　prisms　will　never　change　the　color　it　bears；　in　like　manner　as　gold；　when　completely　purged　from　its　dross；　will　never　change　afterwards　in　the　crucible。〃＇3＇

XII。　NEWTON　AND　THE　LAW　OF　GRAVITATION　We　come　now　to　the　story　of　what　is　by　common　consent　the　greatest　of　scientific　achievements。　The　law　of　universal　gravitation　is　the　most　far…reaching　principle　as　yet　discovered。　It　has　application　equally　to　the　minutest　particle　of　matter　and　to　the　most　distant　suns　in　the　universe；　yet　it　is　amazing　in　its　very　simplicity。　As　usually　phrased；　the　law　is　this：　That　every　particle　of　matter　in　the　universe　attracts　every　other　particle　with　a　force　that　varies　directly　with　the　mass　of　the　particles　and　inversely　as　the　squares　of　their　mutual　distance。　Newton　did　not　vault　at　once　to　the　full　expression　of　this　law；　though　he　had　formulated　it　fully　before　he　gave　the　results　of　his　investigations　to　the　world。　We　have　now　to　follow　the　steps　by　which　he　reached　this　culminating　achievement。　At　the　very　beginning　we　must　understand　that　the　idea　of　universal　gravitation　was　not　absolutely　original　with　Newton。　Away　back　in　the　old　Greek　days；　as　we　have　seen；　Anaxagoras　conceived　and　clearly　expressed　the　idea　that　the　force　which　holds　the　heavenly　bodies　in　their　orbits　may　be　the　same　that　operates　upon　substances　at　the　surface　of　the　earth。　With　Anaxagoras　this　was　scarcely　more　than　a　guess。　After　his　day　the　idea　seems　not　to　have　been　expressed　by　any　one　until　the　seventeenth　century's　awakening　of　science。　Then　the　consideration　of　Kepler's　Third　Law　of　planetary　motion　suggested　to　many　minds　perhaps　independently　the　probability　that　the　force　hitherto　mentioned　merely　as　centripetal；　through　the　operation　of　which　the　planets　are　held　in　their　orbits　is　a　force　varying　inversely　as　the　square　of　the　distance　from　the　sun。　This　idea　had　come　to　Robert　Hooke；　to　Wren；　and　perhaps　to　Halley；　as　well　as　to　Newton；　but　as　yet　no　one　had　conceived　a　method　by　which　the　validity　of　the　suggestion　might　be　tested。　It　was　claimed　later　on　by　Hooke　that　he　had　discovered　a　method　demonstrating　the　truth　of　the　theory　of　inverse　squares；　and　after　the　full　announcement　of　Newton's　discovery　a　heated　controversy　was　precipitated　in　which　Hooke　put　forward　his　claims　with　accustomed　acrimony。　Hooke；　however；　never　produced　his　demonstration；　and　it　may　well　be　doubted　whether　he　had　found　a　method　which　did　more　than　vaguely　suggest　the　law　which　the　observations　of　Kepler　had　partially　revealed。　Newton's　great　merit　lay　not　so　much　in　conceiving　the　law　of　inverse　squares　as　in　the　demonstration　of　the　law。　He　was　led　to　this　demonstration　through　considering　the　orbital　motion　of　the　moon。　According　to　the　familiar　story；　which　has　become　one　of　the　classic　myths　of　science；　Newton　was　led　to　take　up　the　problem　through　observing　the　fall　of　an　apple。　Voltaire　is　responsible　for　the　story；　which　serves　as　well　as　another；　its　truth　or　falsity　need　not　in　the　least　concern　us。　Suffice　it　that　through　pondering　on　the　familiar　fact　of　terrestrial　gravitation；　Newton　was　led　to　question　whether　this　force　which　operates　so　tangibly　here　at　the　earth's　surface　may　not　extend　its　influence　out　into　the　depths　of　space；　so　as　to　include；　for　example；　the　moon。　Obviously　some　force　pulls　the　moon　constantly　towards　the　earth；　otherwise　that　body　would　fly　off　at　a　tangent　and　never　return。　May　not　this　so…called　centripetal　force　be　identical　with　terrestrial　gravitation？　Such　was　Newton's　query。　Probably　many　another　man　since　Anaxagoras　had　asked　the　same　question；　but　assuredly　Newton　was　the　first　man　to　find　an　answer。　The　thought　that　suggested　itself　to　Newton's　mind　was　this：　If　we　make　a　diagram　illustrating　the　orbital　course　of　the　moon　for　any　given　period；　say　one　minute；　we　shall　find　that　the　course　of　the　moon　departs　from　a　straight　line　during　that　period　by　a　measurable　distancethat：　is　to　say；　the　moon　has　been　virtually　pulled　towards　the　earth　by　an　amount　that　is　represented　by　the　difference　between　its　actual　position　at　the　end　of　the　minute　under　observation　and　the　position　it　would　occupy　had　its　course　been　tangential；　as；　according　to　the　first　law　of　motion；　it　must　have　been　had　not　some　force　deflected　it　towards　the　earth。　Measuring　the　deflection　in　questionwhich　is　equivalent　to　the　so…called　versed　sine　of　the　arc　traversedwe　have　a　basis　for　determining　the　strength　of　the　deflecting　force。　Newton　constructed　such　a　diagram；　and；　measuring　the　amount　of　the　moon's　departure　from　a　tangential　rectilinear　course　in　one　minute；　determined　this　to　be；　by　his　calculation；　thirteen　feet。　Obviously；　then；　the　force　acting　upon　the　moon　is　one　that　would　cause　that　body　to　fall　towards　the　earth　to　the　distance　of　thirteen　feet　in　the　first　minute　of　its　fall。　Would　such　be　the　force　of　gravitation　acting　at　the　distance　of　the　moon　if　the　power　of　gravita