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1、PassageQuestions1to5arebasedonthefollowingpassage.Youstareatwaterfallforaminuteortwo,andthenshiftyourgazetoitssurroundings.Whatyounowseeappearstodriftupward.Theseopticalillusionsoccurbecausethebrainisconstantlymatchingitsmodelofrealitytosignalsfromthebody’ssensorsandinte
2、rpretingwhatmustbehappening—thatyourbrainmusthavemoved,nottheother;thatdownwardmotionsisnownormal,soachangefromitmustnowbeperceivedasupwardmotion.Thesensorsthatmakethismagicareoftwokinds.Eacheyecontainsabout120millionrods,whichprovidesomewhatblurryblackandwhitevision.The
3、searethewindowsofnightvision;onceadaptedtothedark,theycandetectacandleburningtenmilesaway.Colorvisionineacheyecomesfromsixtosevenmillionstructurescalledcones.Underidealconditions,everyconecan“see”theentirerainbowspectrumofvisiblecolors,butonetypeofconeismostsensitivetore
4、d,anothertogreen,athirdtoblue.Rodsandconessendtheirmessagespulsinganaverage20to25timespersecondalongtheopticnerve.Weseeanimageforafractionofasecondlongerthanitactuallyappears.Inmovies,reelsofstillphotographsareprojectedontoscreensat24framespersecond,trickingoureyesintose
5、eingacontinuousmovingpicture.Likeapparentmotion,colorvisionisalsosubjecttounusualeffects.Whendaygiveswaytonight,twilightbringswhatthepoetT.S.Eliotcalled“theviolethour.”Alightlevelsfall,therodsbecomeprogressivelylessresponsive.Rodsaremostsensitivetotheshorterwavelengthsof
6、blueandgreen,andtheyimpartastrangevividnesstothegarden’sblueflowers.However,lookatawhiteshirtduringthereddishlightofsunset,andyou’llstillseeitinits“true”color—white,notred.Oureyesareconstantlycomparinganobjectagainstitssurroundings.Theythereforeobservetheeffectofashiftin
7、thecolorofilluminatingonboth,andadjustaccordingly.Theeyescandistinguishseveralmilliongraduationsoflightandshadeofcolor.Eachwakingsecondtheyflashtensofmillionsofpiecesofinformationtothebrain,whichweavesthemincessantlyintoapictureoftheworldaroundus.Yetallthisisdoneatthebac
8、kofeacheyebyafabricofsensors,calledtheretina,aboutaswideandasthickasapostagestamp.AstheRenaissanceinven
