a history of science-2-第56章

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ate　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　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　t