英语六级巅峰阅读附详解-第65期-视力研究.docx

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英语六级巅峰阅读附详解第65期:视力研究

Thehumancriterionforperfectvisionis20/20forreadingthestandardlinesonaSnelleneyechartwithoutahitch.Thescoreisdeterminedbyhowwellyoureadlinesoflettersofdifferentsizesfrom20feetaway.Butbeingabletoreadthebottomlineontheeyechartdoesnotapproximateperfectionasfarasotherspeciesareconcerned.Mostbirdswouldconsiderusveryvisuallyhandicapped.Thehawk.forinstance,hassuchsharpeyesthatitcanspotadimeonthesidewalkwhileperchedontopoftheEmpireStateBuilding+Itcanmakefinevisualdistinctionsbecauseitisblessedwithonemillionconespersquaremillimeterinitsretina(视网膜).Andinwater,humansarefarsighted.whilethekingfisher,swoopingdowntospearfish.canseewellinboththeairandwaterbecauseitisendowedwithtwofoveae.(凹窝)-areasoftheeye,consistingmostlyofcones.thatprovidevisualdistinctions.Onefoveapermitsthebird,whileintheair.toscanthewaterbelowwithoneeyeatatime.Thisiscalledmonocularvision.Onceithitsthewater,theotherfoveajoinsin,allowingthekingfishertofocusbotheyes,likebinoculars,onitspreyatthesametime.Afrog'svisionisdistinguishedbyitsabilitytoperceivethingsasaconstantmotionpicture.Knownas"bugdetectors",ahighlydevelopedsetofcellsinafrog'seyesrespondsmainlytomovingobjects.So,itissaidthatafrogsittinginafieldofdeadbugswouldn’tseethemasfoodandwouldstarve.Thebeehasa"compound"eye.whichisusedfornavigation.Ithas15,000facetsthatdividewhatitseesintoapatternofdots.ormosaic.Withthishindofvision,thebeeseesthesunonlyasasingledot,aconstantpointofreference.Thus.theeyeisasuperbnavigationalinstrumentthatconstantlymeasurestheangleofitslineofflightinrelationtothesun.Abee'seyealsogaugesflightspeed.Andifthatisnotenoughtoleaveour20/20"perfectvision”palingintoinsignificance.thebeeiscapableofseeingsomethingwecan't-ultravioletlight.Thus,whathumansconsidertobe"perfectvision”isinfactratherlimitedwhenwelookatotherspecies.However,thereisstillmuchtobesaidforthehumaneye.Ofallthemammals,onlyhumansandsomeprimatescanenjoythepleasuresofcolorvision.判断人的视力是否完好的标准是看他是否在看Snelle