a history of science-2-第13部分

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t　any　rate；　to　the　fact　that　Germany　at　this　time　was　under　sway　of　the　Lutheran　revolt　against　the　papacy。　So　effective　was　the　opposition　that　the　Gregorian　calendar　did　not　come　into　vogue　in　Germany　until　the　year　1699。　It　may　be　added　that　England；　under　stress　of　the　same　manner　of　prejudice；　held　out　against　the　new　reckoning　until　the　year　1751；　while　Russia　does　not　accept　it　even　now。　As　the　Protestant　leaders　thus　opposed　the　papal　attitude　in　a　matter　of　so　practical　a　character　as　the　calendar；　it　might　perhaps　have　been　expected　that　the　Lutherans　would　have　had　a　leaning　towards　the　Copernican　theory　of　the　universe；　since　this　theory　was　opposed　by　the　papacy。　Such；　however；　was　not　the　case。　Luther　himself　pointed　out　with　great　strenuousness；　as　a　final　and　demonstrative　argument；　the　fact　that　Joshua　commanded　the　sun　and　not　the　earth　to　stand　still；　and　his　followers　were　quite　as　intolerant　towards　the　new　teaching　as　were　their　ultramontane　opponents。　Kepler　himself　was；　at　various　times；　to　feel　the　restraint　of　ecclesiastical　opposition；　though　he　was　never　subjected　to　direct　persecution；　as　was　his　friend　and　contemporary；　Galileo。　At　the　very　outset　of　Kepler's　career　there　was；　indeed；　question　as　to　the　publication　of　a　work　he　had　written；　because　that　work　took　for　granted　the　truth　of　the　Copernican　doctrine。　This　work　appeared；　however；　in　the　year　1596。　It　bore　the　title　Mysterium　Cosmographium；　and　it　attempted　to　explain　the　positions　of　the　various　planetary　bodies。　Copernicus　had　devoted　much　time　to　observation　of　the　planets　with　reference　to　measuring　their　distance；　and　his　efforts　had　been　attended　with　considerable　success。　He　did　not；　indeed；　know　the　actual　distance　of　the　sun；　and；　therefore；　was　quite　unable　to　fix　the　distance　of　any　planet；　but；　on　the　other　hand；　he　determined　the　relative　distance　of　all　the　planets　then　known；　as　measured　in　terms　of　the　sun's　distance；　with　remarkable　accuracy。　With　these　measurements　as　a　guide；　Kepler　was　led　to　a　very　fanciful　theory；　according　to　which　the　orbits　of　the　five　principal　planets　sustain　a　peculiar　relation　to　the　five　regular　solids　of　geometry。　His　theory　was　this：　〃Around　the　orbit　of　the　earth　describe　a　dodecahedronthe　circle　comprising　it　will　be　that　of　Mars；　around　Mars　describe　a　tetrahedronthe　circle　comprising　it　will　be　that　of　Jupiter；　around　Jupiter　describe　a　cubethe　circle　comprising　it　will　be　that　of　Saturn；　now　within　the　earth's　orbit　inscribe　an　icosahedronthe　inscribed　circle　will　be　that　of　Venus；　in　the　orbit　of　Venus　inscribe　an　octahedron　the　circle　inscribed　will　be　that　of　Mercury。〃＇3＇　Though　this　arrangement　was　a　fanciful　one；　which　no　one　would　now　recall　had　not　the　theorizer　obtained　subsequent　fame　on　more　substantial　grounds；　yet　it　evidenced　a　philosophical　spirit　on　the　part　of　the　astronomer　which；　misdirected　as　it　was　in　this　instance；　promised　well　for　the　future。　Tycho　Brahe；　to　whom　a　copy　of　the　work　was　sent；　had　the　acumen　to　recognize　it　as　a　work　of　genius。　He　summoned　the　young　astronomer　to　be　his　assistant　at　Prague；　and　no　doubt　the　association　thus　begun　was　instrumental　in　determining　the　character　of　Kepler's　future　work。　It　was　precisely　the　training　in　minute　observation　that　could　avail　most　for　a　mind　which；　like　Kepler's；　tended　instinctively　to　the　formulation　of　theories。　When　Tycho　Brahe　died；　in　1601；　Kepler　became　his　successor。　In　due　time　he　secured　access　to　all　the　unpublished　observations　of　his　great　predecessor；　and　these　were　of　inestimable　value　to　him　in　the　progress　of　his　own　studies。　Kepler　was　not　only　an　ardent　worker　and　an　enthusiastic　theorizer；　but　he　was　an　indefatigable　writer；　and　it　pleased　him　to　take　the　public　fully　into　his　confidence；　not　merely　as　to　his　successes；　but　as　to　his　failures。　Thus　his　works　elaborate　false　theories　as　well　as　correct　ones；　and　detail　the　observations　through　which　the　incorrect　guesses　were　refuted　by　their　originator。　Some　of　these　accounts　are　highly　interesting；　but　they　must　not　detain　us　here。　For　our　present　purpose　it　must　suffice　to　point　out　the　three　important　theories；　which；　as　culled　from　among　a　score　or　so　of　incorrect　ones；　Kepler　was　able　to　demonstrate　to　his　own　satisfaction　and　to　that　of　subsequent　observers。　Stated　in　a　few　words；　these　theories；　which　have　come　to　bear　the　name　of　Kepler's　Laws；　are　the　following：　1。　That　the　planetary　orbits　are　not　circular；　but　elliptical；　the　sun　occupying　one　focus　of　the　ellipses。　2。　That　the　speed　of　planetary　motion　varies　in　different　parts　of　the　orbit　in　such　a　way　that　an　imaginary　line　drawn　from　the　sun　to　the　planetthat　is　to　say；　the　radius　vector　of　the　planet's　orbitalways　sweeps　the　same　area　in　a　given　time。

These　two　laws　Kepler　published　as　early　as　1609。　Many　years　more　of　patient　investigation　were　required　before　he　found　out　the　secret　of　the　relation　between　planetary　distances　and　times　of　revolution　which　his　third　law　expresses。　In　1618；　however；　he　was　able　to　formulate　this　relation　also；　as　follows：　3。　The　squares　of　the　distance　of　the　various　planets　from　the　sun　are　proportional　to　the　cubes　of　their　periods　of　revolution　about　the　sun。

All　these　laws；　it　will　be　observed；　take　for　granted　the　fact　that　the　sun　is　the　centre　of　the　planetary　orbits。　It　must　be　understood；　too；　that　the　earth　is　constantly　regarded；　in　accordance　with　the　Copernican　system；　as　being　itself　a　member　of　the　planetary　system；　subject　to　precisely　the　same　laws　as　the　other　planets。　Long　familiarity　has　made　these　wonderful　laws　of　Kepler　seem　such　a　matter　of　course　that　it　is　difficult　now　to　appreciate　them　at　their　full　value。　Yet；　as　has　been　already　pointed　out；　it　was　the　knowledge　of　these　marvellously　simple　relations　between　the　planetary　orbits　that　laid　the　foundation　for　the　Newtonian　law　of　universal　gravitation。　Contemporary　judgment　could　not；　of　course；　anticipate　this　culmination　of　a　later　generation。　What　it　could　understand　was　that　the　first　law　of　Kepler　attacked　one　of　the　most　time…honored　of　metaphysical　conceptionsnamely；　the　Aristotelian　idea　that　the　circle　is　the　perfect　figure；　and　hence　that　the　planetary　orbits　must　be　circular。　Not　even　Copernicus　had　doubted　the　validity　of　this　assumption。　That　Kepler　dared　dispute　so　firmly　fixed　a　belief；　and　one　that　seemingly　had　so　sound　a　philosophical　basis；　evidenced　the　iconoclastic　nature　of　his　genius。　That　he　did　not　rest　content　until　he　had　demonstrated　the　validity　of　his　revolutionary　assumption　shows　how　truly　this　great　theorizer　made　his　hypotheses　subservient　to　the　most　rigid　inductions。

GALILEO　GALILEI　While　Kepler　was　solving　these　riddles　of　planetary　motion；　there　was　an　even　more　famous　man　in　Italy　whose　championship　of　the　Copernican　doctrine　was　destined　to　give　the　greatest　possible　publicity　to　the　new　ideas。　This　was　Galileo　Galilei；　one　of　the　most　extraordinary　scientific　observers　of　any　age。　Galileo　was　born　at　Pisa；　on　the　18th　of　February　（old　style）；　1564。　The　day　of　his　birth　is　doubly　memorable；　since　on　the　same　day　the　greatest　Italian　of　the　preceding　epoch；　Michael　Angelo；　breathed　his　last。　Persons　fond　of　symbolism　have　found　in　the　coincidence　a　forecast　of　the　transit　from　the　artistic　to　the　scientific　epoch　of　the　later　Renaissance。　Galileo　came　of　an　impoverished　noble　family。　He　was　educated　for　the　profession　of　medicine；　but　did　not　progress　far　before　his　natural　proclivities　directed　him　towards　the　physical　sciences。　Meeting　with　opposition　in　Pisa；　he　early　accepted　a　call　to　the　chair　of　natural　philosophy　in　the　University　of　Padua；　and　later　in　life　he　made　his　home　at　Florence。　The　mechanical　and　physical　discoveries　of　Galileo　will　claim　our　attention　in　another　chapter。　Our　present　concern　is　with　his　contribution　to　the　Copernican　theory。　Galileo　himself　records　in　a　letter　to　Kepler　that　he　became　a　convert　to　this　theory　at　an　early　day。　He　was　not　enabled；　however；　to　make　any　marked　contribution　to　the　subject；　beyond　the　influence　of　his　general　teachings；　until　about　the　year　1610。　The　brilliant　contributions　which　he　made　were　due　largely　to　a　single　discoverynamely；　that　of　the　telescope。　Hitherto　the　astronomical　observations　had　been　made　with　the　unaided　eye。　Glass　lenses　had　been　known　since　the　thirteenth　century；　but；　until　now；　no　one　had　thought　of　their　possible　use　as　aids　to　distant　vision。　The　question　of　priority　of　discovery　has　never　been　settled。　It　is　admitted；　however；　that　the　chief　honors　belong　to　the　opticians　of　the　Netherlands。　As　early　as　the　year　1590　the　Dutch　optician　Zacharias　Jensen　placed　a　concave　and　a　convex　lens　respectively　at　the　ends　of　a　tube　about　eighteen　inches　long；　and　used　this　instrument　for　the　purpose　of　magnifying　small　objectsproducing；　in　short；　a　crude　microscope。　Some　years　later；　Johannes　Lippershey；　of　whom　not　much　is　known　except　that　he　died　in　1619；　experimented　with　a　somewhat　similar　combination　of　lenses；　and　made　the　startling　observation　that　the　weather…vane　on　a　distant　church…steeple　seemed　to　be　brought　much　nearer　when　viewed　through　the　lens。　The　combination　of　lenses　he　employed　is　that　still　used　in　the　construction　of　ope