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1、Youstareatwaterfallforaminuteortwo,andthenshiftyourgazetoitssurroundings.Whatyounowseeappearstodriftupward.Theseopticalillusionsoccurbecausethebrainisconstantlymatchingitsmodelofrealitytosignalsfromthebody’ssensorsandinterpretingwhatmustbehappening—thatyour
2、brainmusthavemoved,nottheother;thatdownwardmotionsisnownormal,soachangefromitmustnowbeperceivedasupwardmotion.Thesensorsthatmakethismagicareoftwokinds.Eacheyecontainsabout120millionrods,whichprovidesomewhatblurryblackandwhitevision.Thesearethewindowsofnight
3、vision;onceadaptedtothedark,theycandetectacandleburningtenmilesaway.Colorvisionineacheyecomesfromsixtosevenmillionstructurescalledcones.Underidealconditions,everyconecan“see”theentirerainbowspectrumofvisiblecolors,butonetypeofconeismostsensitivetored,anothe
4、rtogreen,athirdtoblue.Rodsandconessendtheirmessagespulsinganaverage20to25timespersecondalongtheopticnerve.Weseeanimageforafractionofasecondlongerthanitactuallyappears.Inmovies,reelsofstillphotographsareprojectedontoscreensat24framespersecond,trickingoureyes
5、intoseeingacontinuousmovingpicture.Likeapparentmotion,colorvisionisalsosubjecttounusualeffects.Whendaygiveswaytonight,twilightbringswhatthepoetT.S.Eliotcalled“theviolethour.”Alightlevelsfall,therodsbecomeprogressivelylessresponsive.Rodsaremostsensitivetothe
6、shorterwavelengthsofblueandgreen,andtheyimpartastrangevividnesstothegarden’sblueflowers.However,lookatawhiteshirtduringthereddishlightofsunset,andyou’llstillseeitinits“true”color—white,notred.Oureyesareconstantlycomparinganobjectagainstitssurroundings.Theyt
7、hereforeobservetheeffectofashiftinthecolorofilluminatingonboth,andadjustaccordingly.Theeyescandistinguishseveralmilliongraduationsoflightandshadeofcolor.Eachwakingsecondtheyflashtensofmillionsofpiecesofinformationtothebrain,whichweavesthemincessantlyintoapi
8、ctureoftheworldaroundus.Yetallthisisdoneatthebackofeacheyebyafabricofsensors,calledtheretina,aboutaswideandasthickasapostagestamp.AstheRenaissanceinventorandartistLeonardodaVinciwroteinwonder,“Whowould