meteorology-第21章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！




never　appeared　by　night　as　a　moon　rainbow。　This　opinion　was　due　to　the



rarity　of　the　occurrence：　it　was　not　observed；　for　though　it　does



happen　it　does　so　rarely。　The　reason　is　that　the　colours　are　not　so



easy　to　see　in　the　dark　and　that　many　other　conditions　must



coincide；　and　all　that　in　a　single　day　in　the　month。　For　if　there　is



to　be　one　it　must　be　at　full　moon；　and　then　as　the　moon　is　either



rising　or　setting。　So　we　have　only　met　with　two　instances　of　a　moon



rainbow　in　more　than　fifty　years。



　　We　must　accept　from　the　theory　of　optics　the　fact　that　sight　is



reflected　from　air　and　any　object　with　a　smooth　surface　just　as　it



is　from　water；　also　that　in　some　mirrors　the　forms　of　things　are



reflected；　in　others　only　their　colours。　Of　the　latter　kind　are



those　mirrors　which　are　so　small　as　to　be　indivisible　for　sense。　It　is



impossible　that　the　figure　of　a　thing　should　be　reflected　in　them；　for



if　it　is　the　mirror　will　be　sensibly　divisible　since　divisibility　is



involved　in　the　notion　of　figure。　But　since　something　must　be



reflected　in　them　and　figure　cannot　be；　it　remains　that　colour　alone



should　be　reflected。　The　colour　of　a　bright　object　sometimes　appears



bright　in　the　reflection；　but　it　sometimes；　either　owing　to　the



admixture　of　the　colour　of　the　mirror　or　to　weakness　of　sight；　gives



rise　to　the　appearance　of　another　colour。



　　However；　we　must　accept　the　account　we　have　given　of　these　things　in



the　theory　of　sensation；　and　take　some　things　for　granted　while　we



explain　others。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　3







　　Let　us　begin　by　explaining　the　shape　of　the　halo；　why　it　is　a　circle



and　why　it　appears　round　the　sun　or　the　moon　or　one　of　the　other



stars：　the　explanation　being　in　all　these　cases　the　same。



　　Sight　is　reflected　in　this　way　when　air　and　vapour　are　condensed



into　a　cloud　and　the　condensed　matter　is　uniform　and　consists　of　small



parts。　Hence　in　itself　it　is　a　sign　of　rain；　but　if　it　fades　away；



of　fine　weather；　if　it　is　broken　up；　of　wind。　For　if　it　does　not



fade　away　and　is　not　broken　up　but　is　allowed　to　attain　its　normal



state；　it　is　naturally　a　sign　of　rain　since　it　shows　that　a　process　of



condensation　is　proceeding　which　must；　when　it　is　carried　to　an　end；



result　in　rain。　For　the　same　reason　these　haloes　are　the　darkest。　It



is　a　sign　of　wind　when　it　is　broken　up　because　its　breaking　up　is



due　to　a　wind　which　exists　there　but　has　not　reached　us。　This　view



finds　support　in　the　fact　that　the　wind　blows　from　the　quarter　in



which　the　main　division　appears　in　the　halo。　Its　fading　away　is　a　sign



of　fine　weather　because　if　the　air　is　not　yet　in　a　state　to　get　the



better　of　the　heat　it　contains　and　proceed　to　condense　into　water；



this　shows　that　the　moist　vapour　has　not　yet　separated　from　the　dry



and　firelike　exhalation：　and　this　is　the　cause　of　fine　weather。



　　So　much　for　the　atmospheric　conditions　under　which　the　reflection



takes　place。　The　reflection　is　from　the　mist　that　forms　round　the



sun　or　the　moon；　and　that　is　why　the　halo　is　not　seen　opposite　the　sun



like　the　rainbow。



　　Since　the　reflection　takes　place　in　the　same　way　from　every　point



the　result　is　necessarily　a　circle　or　a　segment　of　a　circle：　for　if



the　lines　start　from　the　same　point　and　end　at　the　same　point　and



are　equal；　the　points　where　they　form　an　angle　will　always　lie　on　a



circle。



　　Let　AGB　and　AZB　and　ADB　be　lines　each　of　which　goes　from　the　point　A



to　the　point　B　and　forms　an　angle。　Let　the　lines　AG；　AZ；　AD　be　equal



and　those　at　B；　GB；　ZB；　DB　equal　too。　（See　diagram。）



　　Draw　the　line　AEB。　Then　the　triangles　are　equal；　for　their　base



AEB　is　equal。　Draw　perpendiculars　to　AEB　from　the　angles；　GE　from　G；



ZE　from　Z；　DE　from　D。　Then　these　perpendiculars　are　equal；　being　in



equal　triangles。　And　they　are　all　in　one　plane；　being　all　at　right



angles　to　AEB　and　meeting　at　a　single　point　E。　So　if　you　draw　the　line



it　will　be　a　circle　and　E　its　centre。　Now　B　is　the　sun；　A　the　eye；　and



the　circumference　passing　through　the　points　GZD　the　cloud　from



which　the　line　of　sight　is　reflected　to　the　sun。



　　The　mirrors　must　be　thought　of　as　contiguous：　each　of　them　is　too



small　to　be　visible；　but　their　contiguity　makes　the　whole　made　up　of



them　all　to　seem　one。　The　bright　band　is　the　sun；　which　is　seen　as　a



circle；　appearing　successively　in　each　of　the　mirrors　as　a　point



indivisible　to　sense。　The　band　of　cloud　next　to　it　is　black；　its



colour　being　intensified　by　contrast　with　the　brightness　of　the



halo。　The　halo　is　formed　rather　near　the　earth　because　that　is　calmer：



for　where　there　is　wind　it　is　clear　that　no　halo　can　maintain　its



position。



　　Haloes　are　commoner　round　the　moon　because　the　greater　heat　of　the



sun　dissolves　the　condensations　of　the　air　more　rapidly。



　　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