a history of science-2-第39部分

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m　some　rude　observations。

　　　　　　　　　　〃PROPOSITION　VII。；　THEOREM　VII。　〃That　there　is　a　power　of　gravity　tending　to　all　bodies；　proportional　to　the　several　quantities　of　matter　which　they　contain。

That　all　the　planets　mutually　gravitate　one　towards　another　we　have　proved　before；　as　well　as　that　the　force　of　gravity　towards　every　one　of　them　considered　apart；　is　reciprocally　as　the　square　of　the　distance　of　places　from　the　centre　of　the　planet。　And　thence　it　follows；　that　the　gravity　tending　towards　all　the　planets　is　proportional　to　the　matter　which　they　contain。　〃Moreover；　since　all　the　parts　of　any　planet　A　gravitates　towards　any　other　planet　B；　and　the　gravity　of　every　part　is　to　the　gravity　of　the　whole　as　the　matter　of　the　part　is　to　the　matter　of　the　whole；　and　to　every　action　corresponds　a　reaction；　therefore　the　planet　B　will；　on　the　other　hand；　gravitate　towards　all　the　parts　of　planet　A；　and　its　gravity　towards　any　one　part　will　be　to　the　gravity　towards　the　whole　as　the　matter　of　the　part　to　the　matter　of　the　whole。　Q。E。D。

〃HENCE　IT　WOULD　APPEAR　THAT　the　force　of　the　whole　must　arise　from　the　force　of　the　component　parts。〃

Newton　closes　this　remarkable　Book　iii。　with　the　following　words：　〃Hitherto　we　have　explained　the　phenomena　of　the　heavens　and　of　our　sea　by　the　power　of　gravity；　but　have　not　yet　assigned　the　cause　of　this　power。　This　is　certain；　that　it　must　proceed　from　a　cause　that　penetrates　to　the　very　centre　of　the　sun　and　planets；　without　suffering　the　least　diminution　of　its　force；　that　operates　not　according　to　the　quantity　of　the　surfaces　of　the　particles　upon　which　it　acts　（as　mechanical　causes　used　to　do）；　but　according　to　the　quantity　of　solid　matter　which　they　contain；　and　propagates　its　virtue　on　all　sides　to　immense　distances；　decreasing　always　in　the　duplicate　proportions　of　the　distances。　Gravitation　towards　the　sun　is　made　up　out　of　the　gravitations　towards　the　several　particles　of　which　the　body　of　the　sun　is　composed；　and　in　receding　from　the　sun　decreases　accurately　in　the　duplicate　proportion　of　the　distances　as　far　as　the　orb　of　Saturn；　as　evidently　appears　from　the　quiescence　of　the　aphelions　of　the　planets；　nay；　and　even　to　the　remotest　aphelions　of　the　comets；　if　those　aphelions　are　also　quiescent。　But　hitherto　I　have　not　been　able　to　discover　the　cause　of　those　properties　of　gravity　from　phenomena；　and　I　frame　no　hypothesis；　for　whatever　is　not　deduced　from　the　phenomena　is　to　be　called　an　hypothesis；　and　hypotheses；　whether　metaphysical　or　physical；　whether　of　occult　qualities　or　mechanical；　have　no　place　in　experimental　philosophy。　。　。　。　And　to　us　it　is　enough　that　gravity　does　really　exist；　and　act　according　to　the　laws　which　we　have　explained；　and　abundantly　serves　to　account　for　all　the　motions　of　the　celestial　bodies　and　of　our　sea。〃＇2＇

The　very　magnitude　of　the　importance　of　the　theory　of　universal　gravitation　made　its　general　acceptance　a　matter　of　considerable　time　after　the　actual　discovery。　This　opposition　had　of　course　been　foreseen　by　Newton；　and；　much　as　be　dreaded　controversy；　he　was　prepared　to　face　it　and　combat　it　to　the　bitter　end。　He　knew　that　his　theory　was　right；　it　remained　for　him　to　convince　the　world　of　its　truth。　He　knew　that　some　of　his　contemporary　philosophers　would　accept　it　at　once；　others　would　at　first　doubt；　question；　and　dispute；　but　finally　accept；　while　still　others　would　doubt　and　dispute　until　the　end　of　their　days。　This　had　been　the　history　of　other　great　discoveries；　and　this　will　probably　be　the　history　of　most　great　discoveries　for　all　time。　But　in　this　case　the　discoverer　lived　to　see　his　theory　accepted　by　practically　all　the　great　minds　of　his　time。　Delambre　is　authority　for　the　following　estimate　of　Newton　by　Lagrange。　〃The　celebrated　Lagrange；〃　he　says；　〃who　frequently　asserted　that　Newton　was　the　greatest　genius　that　ever　existed；　used　to　add'and　the　most　fortunate；　for　we　cannot　find　MORE　THAN　ONCE　a　system　of　the　world　to　establish。'　〃　With　pardonable　exaggeration　the　admiring　followers　of　the　great　generalizer　pronounced　this　epitaph：　　〃Nature　and　Nature's　laws　lay　hid　in　night；　　　God　said　‘Let　Newton　be！'　and　all　was　light。〃

XIII。　INSTRUMENTS　OF　PRECISION　IN　THE　AGE　OF　NEWTON　During　the　Newtonian　epoch　there　were　numerous　important　inventions　of　scientific　instruments；　as　well　as　many　improvements　made　upon　the　older　ones。　Some　of　these　discoveries　have　been　referred　to　briefly　in　other　places；　but　their　importance　in　promoting　scientific　investigation　warrants　a　fuller　treatment　of　some　of　the　more　significant。　Many　of　the　errors　that　had　arisen　in　various　scientific　calculations　before　the　seventeenth　century　may　be　ascribed　to　the　crudeness　and　inaccuracy　in　the　construction　of　most　scientific　instruments。　Scientists　had　not　as　yet　learned　that　an　approach　to　absolute　accuracy　was　necessary　in　every　investigation　in　the　field　of　science；　and　that　such　accuracy　must　be　extended　to　the　construction　of　the　instruments　used　in　these　investigations　and　observations。　In　astronomy　it　is　obvious　that　instruments　of　delicate　exactness　are　most　essential；　yet　Tycho　Brahe；　who　lived　in　the　sixteenth　century；　is　credited　with　being　the　first　astronomer　whose　instruments　show　extreme　care　in　construction。　It　seems　practically　settled　that　the　first　telescope　was　invented　in　Holland　in　1608；　but　three　men；　Hans　Lippershey；　James　Metius；　and　Zacharias　Jansen；　have　been　given　the　credit　of　the　invention　at　different　times。　It　would　seem　from　certain　papers；　now　in　the　library　of　the　University　of　Leyden；　and　included　in　Huygens's　papers；　that　Lippershey　was　probably　the　first　to　invent　a　telescope　and　to　describe　his　invention。　The　story　is　told　that　Lippershey；　who　was　a　spectacle…maker；　stumbled　by　accident　upon　the　discovery　that　when　two　lenses　are　held　at　a　certain　distance　apart；　objects　at　a　distance　appear　nearer　and　larger。　Having　made　this　discovery；　be　fitted　two　lenses　with　a　tube　so　as　to　maintain　them　at　the　proper　distance；　and　thus　constructed　the　first　telescope。　It　was　Galileo；　however；　as　referred　to　in　a　preceding　chapter；　who　first　constructed　a　telescope　based　on　his　knowledge　of　the　laws　of　refraction。　In　1609；　having　heard　that　an　instrument　had　been　invented；　consisting　of　two　lenses　fixed　in　a　tube；　whereby　objects　were　made　to　appear　larger　and　nearer；　he　set　about　constructing　such　an　instrument　that　should　follow　out　the　known　effects　of　refraction。　His　first　telescope；　made　of　two　lenses　fixed　in　a　lead　pipe；　was　soon　followed　by　others　of　improved　types；　Galileo　devoting　much　time　and　labor　to　perfecting　lenses　and　correcting　errors。　In　fact；　his　work　in　developing　the　instrument　was　so　important　that　the　telescope　came　gradually　to　be　known　as　the　〃Galilean　telescope。〃　In　the　construction　of　his　telescope　Galileo　made　use　of　a　convex　and　a　concave　lens；　but　shortly　after　this　Kepler　invented　an　instrument　in　which　both　the　lenses　used　were　convex。　This　telescope　gave　a　much　larger　field　of　view　than　the　Galilean　telescope；　but　did　not　give　as　clear　an　image；　and　in　consequence　did　not　come　into　general　use　until　the　middle　of　the　seventeenth　century。　The　first　powerful　telescope　of　this　type　was　made　by　Huygens　and　his　brother。　It　was　of　twelve　feet　focal　length；　and　enabled　Huygens　to　discover　a　new　satellite　of　Saturn；　and　to　determine　also　the　true　explanation　of　Saturn's　ring。　It　was　Huygens；　together　with　Malvasia　and　Auzout；　who　first　applied　the　micrometer　to　the　telescope；　although　the　inventor　of　the　first　micrometer　was　William　Gascoigne；　of　Yorkshire；　about　1636。　The　micrometer　as　used　in　telescopes　enables　the　observer　to　measure　accurately　small　angular　distances。　Before　the　invention　of　the　telescope　such　measurements　were　limited　to　the　angle　that　could　be　distinguished　by　the　naked　eye；　and　were；　of　course；　only　approximately　accurate。　Even　very　careful　observers；　such　as　Tycho　Brahe；　were　able　to　obtain　only　fairly　accurate　results。　But　by　applying　Gascoigne's　invention　to　the　telescope　almost　absolute　accuracy　became　at　once　possible。　The　principle　of　Gascoigne's　micrometer　was　that　of　two　pointers　lying　parallel；　and　in　this　position　pointing　to　zero。　These　were　arranged　so　that　the　turning　of　a　single　screw　separated　or　approximated　them　at　will；　and　the　angle　thus　formed　could　be　determined　with　absolute　accuracy。　Huygens's　micrometer　was　a　slip　of　metal　of　variable　breadth　inserted　at　the　focus　of　the　telescope。　By　observing　at　what　point　this　exactly　covered　an　object　under　examination；　and　knowing　the　focal　length　of　the　telescope　and　the　width　of　the　metal；　he　could　then　deduce　the　apparent　angular　breadth　of　the　object。　Huygens　discovered　also　that　an　object　placed　in　the　common　focus　of　the　two　lenses　of　a　Kepler　telescope　appears　distinct　and　clearly　defined。　The　micrometers　of　Malvasia；　and　later　of　Auzout　and　Picard；　are　the　development　of　this　discovery。　Malvasia's　micrometer；　which　he　described　in　1662；　consisted　of　fine　silver　wires　placed　at　right…angles　at　the　focus　of　his　telescope。　As　telescopes　increased　in　power；　however；　it　was　found　that　even　the　finest　wire；　or　silk　filaments；　were　much　too　thick　for　astronomical　observations；　as　they　obliterated　the　image；　and　so；　finally；　the　spider…web　came　into　use　and　is　still　used　in　micrometers　and　other　similar　instruments。　Before　that　time；　however；　the　fine　crossed　wires　had　revolutionized　astronomical　observations。　〃We　may　judge　how　great　was　the　improvement　which　these　contrivances　introduced　into　the　art　of　observing；〃　says　Whewell；　〃by　finding　that　Hevelius　refused　to　adopt　them　because　they　would　make　all　the　old　observations　of　no　value。　He　had　spent　a　laborious　and　active　life　in　the　exercise　of　the　old　methods；　and　could　not　bear　to　think　that　all　the　treasures　which　he　had　accumulated　had　lost　their　worth　by　the　discovery　of　a　new　mine　of　richer　ones。〃＇1＇　Until　the　time　of　Newton；　all　the　telescopes　in　use　were　either　of　the　Galilean　or　Keplerian　type；　that　is；　refractors。　But　about　the　year　1670　Newton　constructed　his　first　reflecting　telescope；　which　was　greatly　superior　to；　although　much　smaller　than；　the　telescopes　then　in　use。　He　was　led　to　this　invention　by　his　experiments　with　light　and　colors。　In　1671　he　presented