a history of science-2-第44部分

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e　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　to　the　Royal　Society　a　second　and　somewhat　larger　telescope；　which　he　had　made；　and　this　type　of　instrument　was　little　improved　upon　until　the　introduction　of　the　achromatic　telescope；　invented　by　Chester　Moor　Hall　in　1733。　As　is　generally　known；　the　element　of　accurate　measurements　of　time　plays　an　important　part　in　the　measurements　of　the　movements　of　the　heavenly　bodies。　In　fact；　one　was　scarcely　possible　without　the　other；　and　as　it　happened　it　was　the　same　man；　Huygens；　who　perfected　Kepler's　telescope　and　invented　the　pendulum　clock。　The　general　idea　had　been　suggested　by　Galileo；　or；　better　perhaps；　the　equal　time　occupied　by　the　successive　oscillations　of　the　pendulum　had　been　noted　by　him。　He　had　not　been　able；　however；　to　put　this　discovery　to　practical　account。　But　in　1656　Huygens　invented　the　necessary　machinery　for　maintaining　the　motion　of　the　pendulum　and　perfected　several　accurate　clocks。　These　clocks　were　of　invaluable　assistance　to　the　astronomers；　affording　as　they　did　a　means　of　keeping　time　〃more　accurate　than　the　sun　itself。〃　When　Picard　had　corrected　the　variation　caused　by　heat　and　cold　acting　upon　the　pendulum　rod　by　combining　metals　of　different　degrees　of　expansibility；　a　high　degree　of　accuracy　was　possible。　But　while　the　pendulum　clock　was　an　unequalled　stationary　time…piece；　it　was　useless　in　such　unstable　situations　as；　for　example；　on　shipboard。　But　here　again　Huygens　played　a　prominent　part　by　first　applying　the　coiled　balance…spring　for　regulating　watches　and　marine　clocks。　The　idea　of　applying　a　spring　to　the　balance…wheel　was　not　original　with　Huygens；　however；　as　it　had　been　first　conceived　by　Robert　Hooke；　but　Huygens's　application　made　practical　Hooke's　idea。　In　England　the　importance　of　securing　accurate　watches　or　marine　clocks　was　so　fully　appreciated　that　a　reward　of　L20；000　sterling　was　offered　by　Parliament　as　a　stimulus　to　the　inventor　of　such　a　time…piece。　The　immediate　incentive　for　this　offer　was　the　obvious　fact　that　with　such　an　instrument　the　determination　of　the　longitude　of　places　would　be　much　simplified。　Encouraged　by　these　offers；　a　certain　carpenter　named　Harrison　turned　his　attention　to　the　subject　of　watch…making；　and；　after　many　years　of　labor；　in　1758　produced　a　spring　time…keeper　which；　during　a　sea…voyage　occupying　one　hundred　and　sixty…one　days；　varied　only　one　minute　and　five　seconds。　This　gained　for　Harrison　a　reward　Of　L5000　sterling　at　once；　and　a　little　later　L10；000　more；　from　Parliament。　While　inventors　were　busy　with　the　problem　of　accurate　chronometers；　however；　another　instrument　for　taking　longitude　at　sea　had　been　invented。　This　was　the　reflecting　quadrant；　or　sextant；　as　the　improved　instrument　is　now　called；　invented　by　John　Hadley　in　1731；　and　independently　by　Thomas　Godfrey；　a　poor　glazier　of　Philadelphia；　in　1730。　Godfrey's　invention；　which　was　constructed　on　the　same　principle　as　that　of　the　Hadley　instrument；　was　not　generally　recognized　until　two　years　after　Hadley's　discovery；　although　the　instrument　was　finished　and　actually　in　use　on　a　sea…voyage　some　months　before　Hadley　reported　his　invention。　The　principle　of　the　sextant；　however；　seems　to　have　been　known　to　Newton；　who　constructed　an　instrument　not　very　unlike　that　of　Hadley；　but　this　invention　was　lost　sight　of　until　several　years　after　the　philosopher's　death　and　some　time　after　Hadley's　invention。　The　introduction　of　the　sextant　greatly　simplified　taking　reckonings　at　sea　as　well　as　facilitating　taking　the　correct　longitude　of　distant　places。　Before　that　time　the　mariner　was　obliged　to　depend　upon　his　compass；　a　cross…staff；　or　an　astrolabe；　a　table　of　the　sun's　declination　and　a　correction　for　the　altitude　of　the　polestar；　and　very　inadequate　and　incorrect　charts。　Such　were　the　instruments　used　by　Columbus　and　Vasco　da　Gama　and　their　immediate　successors。　During　the　Newtonian　period　the　microscopes　generally　in　use　were　those　constructed　of　simple　lenses；　for　although　compound　microscopes　were　known；　the　difficulties　of　correcting　aberration　had　not　been　surmounted；　and　a　much　clearer　field　was　given　by　the　simple　instrument。　The　results　obtained　by　the　use　of　such　instruments；　however；　were　very　satisfactory　in　many　ways。　By　referring　to　certain　plates　in　this　volume；　which　reproduce　illustrations　from　Robert　Hooke's　work　on　the　microscope；　it　will　be　seen　that　quite　a　high　degree　of　effectiveness　had　been　attained。　And　it　should　be　recalled　that　Antony　von　Leeuwenboek；　whose　death　took　place　shortly　before　Newton's；　had　discovered　such　micro…organisms　as　bacteria；　had　seen　the　blood　corpuscles　in　circulation；　and　examined　and　described　other　microscopic　structures　of　the　body。

XIV。　PROGRESS　IN　ELECTRICITY　FROM　GILBERT　AND　VON　GUERICKE　TO　FRANKLIN　We　have　seen　how　Gilbert；　by　his　experiments　with　magnets；　gave　an　impetus　to　the　study　of　magnetism　and　electricity。　Gilbert　himself　demonstrated　some　facts　and　advanced　some　theories；　but　the　system　of　general　laws　was　to　come　later。　To　this　end　the　discovery　of　electrical　repulsion；　as　well　as　attraction；　by　Von　Guericke；　with　his　sulphur　ball；　was　a　step　forward；　but　something　like　a　century　passed　after　Gilbert's　beginning　before　anything　of　much　importance　was　done　in　the　field　of　electricity。　In　1705；　however；　Francis　Hauksbee　began　a　series　of　experiments　that　resulted　in　some　startling　demonstrations。　For　many　years　it　had　been　observed　that　a　peculiar　light　was　seen　sometimes　in　the　mercurial　barometer；　but　Hauksbee　and　the　other　scientific　investigators　supposed　the　radiance　to　be　due　to　the　mercury　in　a　vacuum；　brought　about；　perhaps；　by　some　agitation。　That　this　light　might　have　any　connection　with　electricity　did　not；　at　first；　occur　to　Hauksbee　any　more　than　it　had　to　his　predecessors。　The　problem　that　interested　him　was　whether　the　vacuum　in　the　tube　of　the　barometer　was　essential　to　the　light；　and　in　experimenting　to　determine　this；　he　invented　his　〃mercurial　fountain。〃　Having　exhausted　the　air　in　a　receiver　containing　some　mercury；　he　found　that　by　allowing　air　to　rush　through　the　mercury　the　metal　became　a　jet　thrown