an autobiography-第72部分

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＇Image＇　　〃Trunnion　Vision〃　Reflecting　telescope　of　20…inch　diameter　　　　　　　　　　mounted　on　a　turn…table。

The　observer；　when　seated；　could　direct　the　telescope　to　any　part　of　the　heavens　without　moving　from　his　seat。　　Although　this　arrangement　occasioned　some　loss　of　light；　that　objection　was　more　than　compensated　by　the　great　convenience　which　it　afforded　for　the　prosecution　of　the　special　class　of　observations　in　which　I　was　engaged　namely；　that　of　the　Sun；　Moon；　and　Planets。

I　wrote　to　my　old　friend　Sir　David　Brewster；　then　living　at　St。　Andrews；　in　1849；　about　this　improvement　and　he　duly　congratulated　me　upon　my　devotion　to　astronomical　science。　　In　his　letter　to　me　he　brought　to　mind　many　precious　memories。

〃I　recollect；〃　he　said；　〃with　much　pleasure　the　many　happy　hours　that　I　spent　in　your　father's　house；　and　ever　since　I　first　saw　you　in　your　little　workshop　at　Edinburgh；then　laying　the　foundation　of　your　future　fortunes；I　have　felt　a　deep　interest　in　your　success；　and　rejoiced　at　your　progress　to　wealth　and　reputation。

〃I　have　perused　with　much　pleasure　the　account　you　have　sent　me　of　your　plan　of　shortening　and　moving　large　telescopes；　and　I　shall　state　to　you　the　opinion　which　I　have　formed　of　it。　　If　you　will　look　into　the　article　'Optics'　in　the　Edinburgh　Encyclopedia　（vol。　xv。　p。　643）；　you　will　find　an　account　of　what　has　been　previously　done　to　reduce　by　one…half　the　length　of　reflecting　telescopes。　　The　advantage　of　substituting；　as　you　propose；　a　convex　for　a　plane　mirror　arises　from　two　causes　that　a　spherical　surface　is　more　easily　executed　than　a　plane　one；　and　that　the　spherical　aberration　of　the　larger　speculum；　if　it　be　spherical；　will　be　diminished　by　the　opposite　aberration　of　the　convex　one。　　This　advantage；　however；　will　disappear　if　the　plane　mirror　of　the　old　construction　is　accurately　plane；　and　in　your　case；　if　the　large　speculum　is　parabolic　and　the　small　one　elliptical　in　their　curvature。

〃The　only　objection　to　your　construction　is　the　loss　of　light；　first　of　one…fourth　of　the　whole　incident　light　by　obstruction；　and　then　one…half　of　the　remainder　by　reflection　from　the　convex　mirror；　thus　reducing　100　rays　of　incident　light　to　37　1/2　before　the　pencil　is　thrown　out　of　the　tube　by　a　prism　or　a　third　reflector。　　This　loss　of　light；　it　is　true；　may　be　compensated　by　an　additional　inch　or　two　to　the　margin　of　the　large　speculum；　but　still　it　is　the　best　part　of　the　large　speculum　that　is　made　unproductive　by　the　eclipse　of　it　by　the　convex　speculum。　　〃With　regard　to　the　mechanical　contrivance　which　you　propose　for　working　the　instrument；　I　think　it　is　singularly　ingenious　and　beautiful；　and　will　compensate　for　any　imperfection　in　the　optical　arrangements　which　are　rendered　necessary　for　its　adoption。　The　application　of　the　railway　turn…table　is　very　happy；　and　not　less　so　is　the　extraction　of　the　image　through　the　hollow　trunnions。

〃I　am　much　obliged　to　you　for　the　beautiful　drawing　of　the　apparatus　for　grinding　and　polishing　specula；　invented　by　Mr。　Lassell　and　constructed　by　yourself。　　I　shall　be　glad　to　hear　of　your　further　progress　in　the　construction　of　your　telescope；　and　I　trust　that　I　shall　have　the　pleasure　of　meeting　you　and　Mr。　Lassell　at　the　Birmingham　meeting　of　the　British　Association。

In　the　course　of　the　same　year　（1849）　I　sent　a　model　of　my　Trunnion　turn…table　telescope　for　exhibition　at　a　lecture　at　the　Royal　Institution；　given　by　my　old　friend　Edward　Cowper。　　In　the　model　I　had　placed　a　neat　little　figure　of　the　observer；　but　the　head　had　unfortunately　been　broken　off　during　its　carriage　to　London。　Mrs。　Nasmyth　had　made　the　wearing　apparel；　but　Edward　Cowper　wrote　to　her；　before　the　lecture；　that　he　had　put　〃Sir　Fireside　Brick〃　all　to　rights　in　respect　of　his　garb。　　His　letter　after　the　lecture　was　quite　characteristic。

〃The　lecture；〃　he　said；　〃went　off　very　well　last　night。　All　the　models　performed　their　duty；　and　were　duly　applauded　for　doing　so。　My　new　equatorial　was　approved　of　by　astronomers　and　by　instrument…makers。　The　last　gun　I　fired　was　a　howitzer；　but　mounted　swivel…gun　fashion；　on　a　sort　of　revolving　platform；　or　something　like　a　turn…table　proper　the　gunner　at　the　side　of　the　carriage。　　Do　you　know　anything　of　the　kind？　　Bang！　　Invented　by　one　Nasmyth。　　Bang！　　The　observer　is　sitting　at　ease；　the　stars　are　brought　down　to　you　instead　of　your　creeping　up　a　scaffolding　after　the　stars。　　Well；　the　folks　came　to　the　table　after　the　lecture；　and　'The　Nasmyth　Telescope'　kept　banging　away　for　a　quarter　of　an　hour；　and　was　admired　by　everybody。　　The　loss　of　light　was　not　much　insisted　on；　but　it　was　said　that　you　ran　the　risk　of　error　of　form　in　three　surfaces　instead　of　two。　　I　see　that　Sir　J。　South　states　that　Lord　Rosse　would　increase　the　light　of　his　telescope　from　five　to　seven　by　adopting　Herschel's　plan。

〃De　La　Rue　was　quite　delighted。　　He　said；　'Well；　I　congratulate　you　on　a　most　splendid　lectureI　cannot　call　it　anything　else。'　My　father；　who　takes　very　little　interest　in　these　things；　said；　'Well；　Edward　has　made　me　understand　more　about　telescopes　than　I　ever　did　in　my　life。'　The　theatre　was　full；　gallery　and　all。　　They　were　very　attentive；　and　I　never　felt　more　comfortable　in　a　lecture。　　I　am　happy　to　say　that；　having　administered　a　dose　of　cement　to　Mrs。　Nasmyth's　friend；　Sir　Fireside　Brick　of　Green　Lanes；　he　is　now　in　a　convalescent　state。　The　lecture　is　to　be　repeated　in　another　fortnight。　　With　many　thanks　for　your　kind　assistance；　yours　very　sincerely；

〃EDWARD　COWPER。〃

In　the　course　of　my　astronomical　inquiries　I　had　occasion　to　consider　the　causes　of　the　sun's　light。　　I　observed　the　remarkable　phenomena　of　the　variable　and　some　times　transitory　brightness　of　the　stars。　　In　connection　with　geology；　there　was　the　evidence　of　an　arctic　or　glacial　climate　in　regions　where　such　cannot　now　naturally　exist；　thus　giving　evidence　of　the　existence　of　a　condition　of　climate；　for　the　explanation　of　which　we　look　in　vain　for　any　at　present　known　cause。　I　wrote　a　paper　on　the　subject；　which　I　sent　to　the　Astronomical　Society。　　It　was　read　in　May　1851。　　In　that　paper　I　wrote　as　follows：

〃A　course　of　observations　on　the　solar　spots；　and　on　the　remarkable　features　which　from　time　to　time　appear　on　the　sun's　surface；　which　I　have　examined　with　considerable　assiduity　for　several　years；　had　in　the　first　place　led　me　to　entertain　the　following　conclusion：　　namely；　that　whatever　be　the　nature　of　solar　light；　its　main　source　appears　to　result　from　an　action　induced　on　the　exterior　surface　of　solar　sphere；　a　conclusion　in　which　I　doubt　not　all　who　have　attentively　pursued　observations　on　the　structure　of　the　sun's　surface　will　agree。

〃Impressed　with　the　correctness　of　this　conclusion；　I　was　led　to　consider　whether　we　might　not　reasonably　consider　the　true　source　of　the　latent　element　of　light　to　reside；　not　in　the　solar　orb；　but　in　space　itself；　and　that　the　grand　function　and　duty　of　the　sun　was　to　act　as　an　agent　for　bringing　forth　into　vivid　existence　its　due　portion　of　the　illuminating　or　luciferous　element；　which　element　I　suppose　to　be　diffused　throughout　the　boundless　regions　of　space；　and　which　in　that　case　must　be　exhaustless。

Assuming；　therefore；　that　the　sun's　light　is　the　result　of　some　peculiar　action　by　which　it　brings　forth　into　visible　existence　the　element　of　light；　which　I　conceive　to　be　latent　in；　and　diffused　throughout　space；　we　have　but　to　imagine　the　existence　of　a　very　probable　condition；　namely；　the　unequal　diffusion　of　this　light…yielding　element；　to　catch　a　glimpse　of　a　reason　why　our　sun　may；　in　common　with　his　solar　brotherhood；　in　some　portions　of　his　vast　stellar　orbit；　have　passed；　and　may　yet　have　to　pass；　through　regions　of　space；　in　which　the　light…yielding　element　may　either　abound　or　be　deficient；　and　so　cause　him　to　beam　forth　with　increased　splendour；　or　fade　in　brilliancy；　just　in　proportion　to　the　richness　or　poverty　of　this　supposed　light…yielding　element　as　may　occur　in　those　regions　of　space　through　which　our　sun；　in　common　with　every　stellar　orb；　has　passed；　is　now　passing；　or　is　destined　to　pass；　in　following　up　their　mighty　orbits。

〃Once　admit　that　this　light…yielding　element　resides　in　space；　and　that　it　is　not　equally　diffused；　we　may　then　catch　a　glimpse　of　the　cause　of　the　variable　and　transitory　brightness　of　stars；and　more　especially　of　those　which　have　been　known　to　beam　forth　with　such　extraordinary　splendour；　and　have　again　so　mysteriously　faded　away；　many　instances　of　which　abound　in　historical　record。

〃Finally；　in　reference　to　such　a　state　of　change　having　come　over　our　sun；　as　indicated　by　the　existence　of　a　glacial　period；　as　is　now　placed　beyond　doubt　by　geological　research；　it　appears　to　me　no　very　wild　stretch　of　analogy　to　suppose　that　in　such　former　periods　of　the　earth's　history　our　sun　may　have　passed　through　portions　of　his　stellar　orbit　in　which　the　light…yielding　element　was　deficient；　and　in　which　case　his　brilliancy　would　have　suffered　the　while；　and　an　arctic　climate　in　consequence　spread　from　the　poles　towards　the　equator；　and　thus　leave　the　record　of　such　a　condition　in　glacial　handwriting　on　the　everlasting　walls　of　our　mountain　ravines；　of　which　there　is　such　abundant　an