meteorology-第16章

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　　Haloes　are　formed　round　stars　for　the　same　reasons；　but　they　are　not



prognostic　in　the　same　way　because　the　condensation　they　imply　is　so



insignificant　as　to　be　barren。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　4







　　We　have　already　stated　that　the　rainbow　is　a　reflection：　we　have　now



to　explain　what　sort　of　reflection　it　is；　to　describe　its　various



concomitants；　and　to　assign　their　causes。



　　Sight　is　reflected　from　all　smooth　surfaces；　such　as　are　air　and



water　among　others。　Air　must　be　condensed　if　it　is　to　act　as　a　mirror；



though　it　often　gives　a　reflection　even　uncondensed　when　the　sight



is　weak。　Such　was　the　case　of　a　man　whose　sight　was　faint　and



indistinct。　He　always　saw　an　image　in　front　of　him　and　facing　him　as



he　walked。　This　was　because　his　sight　was　reflected　back　to　him。　Its



morbid　condition　made　it　so　weak　and　delicate　that　the　air　close　by



acted　as　a　mirror；　just　as　distant　and　condensed　air　normally　does；



and　his　sight　could　not　push　it　back。　So　promontories　in　the　sea



'loom'　when　there　is　a　south…east　wind；　and　everything　seems　bigger；



and　in　a　mist；　too；　things　seem　bigger：　so；　too；　the　sun　and　the　stars



seem　bigger　when　rising　and　setting　than　on　the　meridian。　But　things



are　best　reflected　from　water；　and　even　in　process　of　formation　it



is　a　better　mirror　than　air；　for　each　of　the　particles；　the　union　of



which　constitutes　a　raindrop；　is　necessarily　a　better　mirror　than



mist。　Now　it　is　obvious　and　has　already　been　stated　that　a　mirror　of



this　kind　renders　the　colour　of　an　object　only；　but　not　its　shape。



Hence　it　follows　that　when　it　is　on　the　point　of　raining　and　the　air



in　the　clouds　is　in　process　of　forming　into　raindrops　but　the　rain



is　not　yet　actually　there；　if　the　sun　is　opposite；　or　any　other　object



bright　enough　to　make　the　cloud　a　mirror　and　cause　the　sight　to　be



reflected　to　the　object　then　the　reflection　must　render　the　colour



of　the　object　without　its　shape。　Since　each　of　the　mirrors　is　so　small



as　to　be　invisible　and　what　we　see　is　the　continuous　magnitude　made　up



of　them　all；　the　reflection　necessarily　gives　us　a　continuous



magnitude　made　up　of　one　colour；　each　of　the　mirrors　contributing



the　same　colour　to　the　whole。　We　may　deduce　that　since　these



conditions　are　realizable　there　will　be　an　appearance　due　to



reflection　whenever　the　sun　and　the　cloud　are　related　in　the　way



described　and　we　are　between　them。　But　these　are　just　the　conditions



under　which　the　rainbow　appears。　So　it　is　clear　that　the　rainbow　is



a　reflection　of　sight　to　the　sun。



　　So　the　rainbow　always　appears　opposite　the　sun　whereas　the　halo　is



round　it。　They　are　both　reflections；　but　the　rainbow　is



distinguished　by　the　variety　of　its　colours。　The　reflection　in　the　one



case　is　from　water　which　is　dark　and　from　a　distance；　in　the　other



from　air　which　is　nearer　and　lighter　in　colour。　White　light　through



a　dark　medium　or　on　a　dark　surface　（it　makes　no　difference）　looks　red。



We　know　how　red　the　flame　of　green　wood　is：　this　is　because　so　much



smoke　is　mixed　with　the　bright　white　firelight：　so；　too；　the　sun



appears　red　through　smoke　and　mist。　That　is　why　in　the　rainbow



reflection　the　outer　circumference　is　red　（the　reflection　being　from



small　particles　of　water）；　but　not　in　the　case　of　the　halo。　The



other　colours　shall　be　explained　later。　Again；　a　condensation　of



this　kind　cannot　persist　in　the　neighbourhood　of　the　sun：　it　must



either　turn　to　rain　or　be　dissolved；　but　opposite　to　the　sun　there



is　an　interval　during　which　the　water　is　formed。　If　there　were　not



this　distinction　haloes　would　be　coloured　like　the　rainbow。　Actually



no　complete　or　circular　halo　presents　this　colour；　only　small　and



fragmentary　appearances　called　'rods'。　But　if　a　haze　due　to　water　or



any　other　dark　substance　formed　there　we　should　have　had；　as　we



maintain；　a　complete　rainbow　like　that　which　we　do　find　lamps。　A



rainbow　appears　round　these　in　winter；　generally　with　southerly　winds。



Persons　whose　eyes　are　moist　see　it　most　clearly　because　their　sight



is　weak　and　easily　reflected。　It　is　due　to　the　moistness　of　the　air



and　the　soot　which　the　flame　gives　off　and　which　mixes　with　the　air



and　makes　it　a　mirror；　and　to　the　blackness　which　that　mirror



derives　from　the　smoky　nature　of　the　soot。　The　light　of　the　lamp



appears　as　a　circle　which　is　not　white　but　purple。　It　shows　the



colours　of　the　rainbow；　but　because　the　sight　that　is　reflected　is　too



weak　and　the　mirror　too　dark；　red　is　absent。　The　rainbow　that　is



seen　when　oars　are　raised　out　of　the　sea　involves　the　same　relative



positions　as　that　in　the　sky；　but　its　colour　is　more　like　that　round



the　lamps；　being　purple　rather　than　red。　The　reflection　is　from　very



small　particles　continuous　with　one　another；　and　in　this　case　the



particles　are　fully　formed　water。　We　get　a　rainbow；　too；　if　a　man



sprinkles　fine　drops　in　a　room　turned　to　the　sun　so　that　the　sun　is



shining　in　part　of　the　room　and　throwing　a　shadow　in　the　rest。　Then　if



one　man　sprinkles　in　the　room；　another；　standing　outside；　sees　a



rainbow　where　the　sun's　rays　cease　and　make　the　shadow。　Its　nature　and



colour　is　like　that　from　the　oars　and　its　cause　is　the　same；　for　the



sprinkling　hand　corresponds　to　the　oar。



　　That　the　colours　of　the　rainbow　are　those　we　described　and　how　the



other　colours　come　to　appear　in　it　will　be　clear　from　the　following



considerations。　We　must　recognize；　as　we　have　said；　and　lay　down：



first；　that　white　colour　on　a　black　surface　or　seen　through　a　black



medium　gives　red；　second；　that　sight　when　strained　to　a　distance



becomes　weaker　and　less；　third；　that　black　is　in　a　sort　the　negation



of　sight：　an　object　is　black　because　sight　fails；　so　everything　at　a



distance　looks　blacker；　because　sight　does　not　reach　it。　The　theory　of



these　matters　belongs　to　the　account　of　the　senses；　which　are　the



proper　subjects　of　such　an　inquiry；　we　need　only　state　about　them　what



is　necessary　for　us。　At　all　events；　that　is　the　reason　why　distant



objects　and　objects　seen　in　a　mirror　look　darker　and　smaller　and



smoother；　why　the　reflection　of　clouds　in　water　is　darker　than　the



clouds　themselves。　This　latter　is　clearly　the　case：　the　reflection



diminishes　the　sight　that　reaches　them。　It　makes　no　difference　whether



the　change　is　in　the　object　seen　or。　in　the　sight；　the　result　being　in



either　case　the　same。　The　following　fact　further　is　worth　noticing。



When　there　is　a　cloud　near　the　sun　and　we　look　at　it　does　not　look



coloured　at　all　but　white；　but　when　we　look　at　the　same　cloud　in　water



it　shows　a　trace　of　rainbow　colouring。　Clearly；　then；　when　sight　is



reflected　it　is　weakened　and；　as　it　makes　dark　look　darker；　so　it



makes　white　look　less　white；　changing　it　and　bringing　it　nearer　to



black。　When　the　sight　is　relatively　strong　the　change　is　to　red；　the



next　stage　is　green；　and　a　further　degree　of　weakness　gives　violet。　No



further　change　is　visible；　but　three　completes　the　series　of　colours



（as　we　find　three　does　in　most　other　things）；　and　the　change　into



the　rest　is　imperceptible　to　sense。　Hence　also　the　rainbow　appears



with　three　colours；　this　is　true　of　each　of　the　two；　but　in　a　contrary



way。　The　outer　band　of　the　primary　rainbow　is　red：　for　the　largest



band　reflects　most　sight　to　the　sun；　and　the　outer　band　is　largest。



The　middle　band　and　the　third　go　on　the　same　principle。　So　if　the



principles　we　laid　down　about　the　appearance　of　colours　are　true　the



rainbow　necessarily　has　three　colours；　and　these　three　and　no



others。　The　appearance　of　yellow　is　due　to　contrast；　for　the　red　is



whitened　by　its　juxtaposition　with　green。　We　can　see　this　from　the



fact　that　the　rainbow　is　purest　when　the　cloud　is　blackest；　and　then



the　red　shows　most　yellow。　（Yellow　in　the　rainbow　comes　between　red



and　green。）　So　the　whole　of　the　red　shows　white　by　contrast　with　the



blackness　of　the　cloud　around：　for　it　is　white　compared　to　the　cloud



and　the　green。　Again；　when　the　rainbow　is　fading　away　and　the　red　is



dissolving；　the　white　cloud　is　brought　into　contact　with　the　green　and



becomes　yellow。　But　the　moon　rainbow　affords　the　best　instance　of　this



colour　contrast。　It　looks　quite　white：　this　is　because　it　appears　on



the　dark　cloud　and　at　night。　So；　just　as　fire　is　intensified　by



added　fire；　black　beside　black　makes　that　which　is　in　some　degree



white　look　quite　white。　Bright　dyes　too　show　the　effect　of　contrast。



In　woven　and　embroidered　stuffs　the　appearance　of　colours　is



profoundly　affected　by　their　juxtaposition　with　one　another　（purple；



for　instance；　appears　different　on　white　and　on　black　wool）；　and



also　by　differences　of　illumination。　Thus　embroiderers　say　that　they



often　make　mistakes　in　their　colours　when　they　work　by　lamplight；



and　use　the　wrong　ones。



　　We　have　now　shown　why　the　rainbow　has　three　colours　and　that　these



are　its　only　colours。　The　same　cause　explains　the　double　rainbow　and



the　faintness　of　the　colours　in　the　outer　one　and　their　inverted



order。　When　sight　is　strained　to　a　great　distance　the　appearance　of



the　distant　object　is　affected　in　a　certain　way：　and　the　same　thing



holds　good　here。　So　the　reflection　from　the　outer　rainbow　is　weaker



because　it　takes　place　from　a　greater　distance　and　less　of　it



reaches　the　sun；　and　so　the　colours　seen　are　fainter。　Their　order　is



reversed　because　more　reflection　reaches　the　sun　from　the　smaller；



inner　band。　For　that　reflection　is　nearer　to　our　sight　which　is



reflected　from　the　band　which　is　nearest　to　the　primary　rainbow。　Now



the　smallest　band　in　the　outer　rainbow　is　that　which　is　nearest；　and



so　it　will　be　red；　and　the　second　and　the　third　will　follow　the　same



principle。　Let　B　be　the　outer　rainbow；　A　the　inner　one；　let　R　stand



for　the　red　colour；　G　for　green；　V　for　violet；　yellow　appears　at　the



point　Y。　Three　rainbows　or　more　are　not　found　because　even　the



second　is　fainter；　so　that　the　third　reflection　can　have　no　strength



whatever　and　cannot　reach　the　sun　at　all。　（See　diagram。）







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　5







　　The　rainbow　can　never　be　a　circle　nor　a　segment　of　a　circle



greater　than　a　semicircle。　The　consideration　of　the　diagram　will　prove



this　and　the　other　properties　of　the　rainbow。　（See　diagram。）



　　Let　A　be　a　hemisphere　resting　on　the　circle　of　the　horizon；　let



its　centre　be　K　and　let　H　be　another　point　appearing　on　the　horizon。



Then；　if　the　lines　that　fall　in　a　cone　from　K　have　HK　as　their　axis；



and；　K　and　M　being　joined；　the　lines　KM　are　reflected　from　the



hemisphere　to　H　over　the　greater　angle；　the　lines　from　K　will　fall



on　the　circumference　of　a　circle。　If　the　reflection　takes　place　when



the　luminous　body　is　rising　or　setting　the　segment　of　the　circ