When，however，theytouchsuchobjects，theyhavetheadvantageofgreaterpracticewiththeexternaltactalsenseandakeenerattentionforthesame。Theso-called“distance-senseoftheblind“isaproofofthis。Itconsistsintheabilitytoperceivefromsomedistance，withoutdirectcontact，aresistingobject，as，forexample，aneighboringwall。Now，itcanbeexperimentallydemonstratedthatthisdistance-senseismadeupoftwofactors:averyweaktactualstimulationoftheforeheadbytheatmosphericresistance，andachangeinthesoundofthestep。Thelatteractsasasignaltoconcentratetheattentionenoughsothattheweaktactualstimulationscanbeperceived。The“distance-sense“disappears，accordingly，whenthetactualstimulationsarepreventedbybindingaclotharoundtheforeheadorwhenthestepsarerenderedinaudible。
12。Besidesourideasofthepositionandmovementsofthevariouspartsofourbody，wehavealsoanideaofthepositionandmovementofourwholebody。Theformercanneverhaveanythingbutarelativesignificance；
itisonlywhenconsideredinconnectionwiththelatterthattheybecomeabsolute。Theorganoforientationforthisgeneralideaisthehead。Wealwayshaveadefiniteideaofthepositionofthehead；theotherorgansarelocalizedinourideas，generally，indeed，veryindefinitely，withreferencetoit，accordingtotheparticularcomplexesofinnerandoutertactualsensationsineachcase。Thespecificorganoforientationintheheadisthesystemofsemicircularcanals，towhichareaddedassecondaryaidstheinnerandoutertactualsensationsresultingfromtheactionofthemusclesofthehead。Thefunctionofthesecanalsasanorganoforientationcanbemosteasilyunderstoodbyassumingthat[p。114]innertactualsensationswithespeciallymarkeddifferencesinlocalsigns，ariseinthemthroughtheinfluenceofthechangingpressureofthefluidmedium，whichfillsthem。Itishighlyprobablethatdizziness，whichcomesfromrapidrotationofthehead，isduetothesensationscausedbytheviolentmovementsofthisfluid。Thisisinaccordwiththeobservationsthatpartialderangementsofthecanalsbringaboutconstantillusionsinlocalization，andcompletederangementofthesameisfollowedbyanalmosttotalsuspensionoftheabilitytolocalize。
12a。Theantagonistictheoriesinregardtothepsychicalformationofspacialideas，aregenerallycallednativismandempirism。Thenativistictheoryseekstoderivelocalizationinspacefromconnatepropertiesofthesense-organsandsense-centres，whiletheempiristictheoryseekstoderiveitfromtheinfluencesofexperience。
Thisdiscriminationdoesnotgiveproperexpressiontotheactualoppositionthatexists，fortheassumptionofconnatespacialideasmaybeattackedwithoutaffirmingthattheseideasarisethroughexperience。Thisisthecasewhen，asabove，space-perceptionsareregardedasproductsofpsychicalfusionsduebothtothephysiologicalpropertiesoftheorgansofsenseandofmovement，andtothegenerallawsfortheriseofpsychicalcompounds。
Suchprocessesoffusionandthearrangementsofsense-impressionsbaseduponthem，areeverywherethesubstrataofourexperience，butforthisveryreasonitisinadmissibletocallthem“experience“itself。Itismuchmorepropertopointouttheoppositionthatreallyexists，asthatofnativisticandgenetic，theories。Itistobenotedthatthewidespreadnativistictheoriescontainempiricalelements，while，ontheotherhand，empiricaltheoriescontainnativisticelements，sothatthedifferenceissometimesverysmall。Supportersofthenativisticviewassumethatthearrangementofimpressionsinspacecorrespondsdirectlytothearrangementofsensitivepointsintheskinandretina。Thespecialwayinwhichtheprojectionoutwardiseffected，especiallytheideasofthedistanceandmagnitudeofobjects，andthereferenceofapluralityofspecially[p。115]separatedimpressionstoasingleobject，areallregardedasdependentupon“attention“，“will“，oreven“experience“。Supportersoftheempiricaltheory，ontheotherhand，generallypresupposespaceasgiveninsomewayorother，andtheninterpreteachsingleideaasalocalizationinthisspaceduetosomeempiricalmotive。Inthetheoryofspacialideasfromsight，tactualspaceisgenerallyregardedasthisoriginallygivenspace；inthetheoryoftactualideas，originalspacialqualitieshavesometimesbeenattributedtomotorsensations。Thus，intheactualconcretetheoriesempirismandnativismareveryilldefinedconcepts。Theyagreeintheuseofthecomplexconceptsofpopularpsychology，suchas\'“attentions“，“will“，and“experience“，withoutanyexaminationoranalysis。Inthisrespecttheyaredifferentfromthegenetictheory，whichseekstoshowtheelementaryprocessesfromwhichtheideasrise，bymeansofapsychologicalanalysisoftheideas。Inspiteoftheirweaknesses，thenativististandempiristictheorieshaveservedtosetthepsychologicalproblemthatexistshere，clearlybeforeusandtobringtolightagreatnumberoffactsforitssolution。B。SPACIALSIGHT-IDEAS。
13。Thegeneralpropertiesofthetouch-sensearerepeatedinthevisualsense，butinamorehighlyorganizedform。Correspondingtothesensorysurfaceoftheouterskin，wehaveheretheretinawithitsrodsandconesarrangedinrowsandforminganextraordinarilyfinemosaicofsensitivepoints。Correspondingtothemovementsofthetactualorgans，wehavethemovementsofbotheyesinfixatingobjectsandfollowingtheirboundinglines。Still，whiletactualimpressionsareperceivedonlythroughimmediatecontactwiththeobjects，therefractivemediainfrontoftheretinathrowinverted，reducedimagesuponit。Theseimagesaresosmallthatspaceisallowedforalargenumberofsimultaneousimpressions，andtheabilityoflighttotraversespacemakesitpossibleforbothneighboringanddistantobjectstoyield[p。116]impressions。Visionthusbecomesadistance-senseinamuchhigherdegreethanhearing。Lightcanbeperceivedfromincomparablygreaterdistancesthansound。Furthermore，onlyvisualideasaredirectlylocalizedatdifferentdistancesfromthesubject；forauditoryideasthislocalizationisalwaysindirect，throughtheaidofvisualideasofspace。
14。Withregardtoitsspacialattributes，everyvisualideamayberesolvedintotwofactors:1)thelocationofthesingleelementsinrelationtooneanother，and2)theirlocationinrelationtotheideatingsubject。
Eventheideaofonesinglepointoflight，containsboththesefactors，forwemustalwaysrepresentapointinsomespacialenvironment，andalsoinsomedirectionandatsomedistancefromourselves。Thesefactorscanbeseparatedonlythroughdeliberateabstraction，neverinreality，fortherelationofanypointinspacetoitsenvironment，regularlydeterminesitsrelationtotheideatingsubject。Asaresultofthisdependence，theanalysisofvisualideasmaybetterstartwiththelocationoftheelementsinrelationtooneanother，andthentakeuplaterthelocationofthecompoundinregardtothesubject。
a。TheLocationoftheElementsofaVisualIdeainRelationtoOneAnother。
15。Intheapprehensionofthereciprocalrelationsbetweenelementsofavisualidea，theattributesofthetactualsenseareallrepeated，onlyinamuchmorehighlyorganizedform，andwithafewmodificationssignificantforthevisualideas。Here，too，weimmediatelyconnectwiththesimplestpossible，approximatelypunctiform，impressiontheideaofitsplaceinspace；thatis，wegiveitacertaindefinitepositioninrelationtothepartsofspaceaboutit。Thislocalizationisnoteffected，however，asintouch，bythe[p。117]directreferenceoftheimpressiontothecorrespondingpointofthesense-organitself，butweprojectitintoafieldofvision，whichliesatsomedistanceoutsideoftheideatingsubject。Heretoowehaveameasure，asinthecaseoftouch，fortheaccuracyoflocalization，inthedistanceatwhichtwoapproximatelypunctiformimpressionscanbejustdistinguishedasspaciallydifferent。
Thedistanceisnotgiveninthiscaseisadirectlymeasurablelinearextensiononthesensorysurfaceitself，butastheshortestperceptibleintervalbetweentwopointsinthefieldofvision。Thefieldofvisionmaybethoughtofasplacedatanydistancewhateverfromthesubject，sothatitisbesttouseasameasureforthefinenessoflocalization，notalinearextension，butanangle，theangleformedbytheintersectionofthelinespassingthroughthenodalpointoftheeye，fromthepointsinthefieldofvisiontothecorrespondingretinalpoints。Thisangleofvisionremainsconstantsolongasthesizeoftheretinalimageisunchanged，whilethedistancebetweenthepointsinthefieldofvisionincreasesinproportiontotheirdistancefromthesubject。Ifanequivalentlineardistanceissoughtinplaceoftheangleofvision，itcanbefoundinthediameteroftheretinalimage。Thismaybereckoneddirectlyfromtheangleandthedistanceoftheretinafromthenodalpointoftheeye。
16。Themeasurementsofthekeennessoflocalizationwiththeeye，madeaccordingtothisprinciple，showthatthereisagreatdifferencefordifferentpartsofthefieldofvision，justaswasfoundfordifferentregionsofthetactualorgans(p。105)。Still，thedistancesthatmeasurethesmallestperceptibleintervalshereareallverymuchsmaller。Then，again，therearemanyregionsoffinerdiscriminationscatteredoverthetactualorgan，butonlyoneregionoffinestdiscriminationinthefieldofvision。Thisisthemiddle，which[p。118]correspondstothecentreoftheretina。Fromthisregiontowardstheperipherythefinenessoflocalizationdiminishes，veryrapidly。Thewholefieldofvisionorthewholeretinalsurface，is，accordingly，analogoustoasingletactualregion，as，forexample，thatoftheindexfinger，exceptthatitverymuchsurpassesthelatterinfinenessoflocalization，especiallyatthecentre，wheretwoimpressionsatadistancecorrespondingto60“——90“intheangleofvision，arejustdistinguishable，whiletwodegreesandahalftowardtheperiphery，thesmallestperceptibleextensionis3\'30“，andateightdegreesitincreasesto1o。
Innormalvisionweturntheeyetowardsobjectsofwhichwewishtogainmoreaccuratespacialideas，insuchawaythattheylieinthemiddleofthefieldofvision，theirimagesfalling，accordingly，onthecentreoftheretina。Wespeakofsuchobjectsasseendirectly，ofallothers，whichlieintheeccentricpartsofthefieldofvision，asseenindirectly。Thecentreoftheregionofdirectvisioniscalledthepointofregard，orthefixation-point。Thelinethatunitesthecentreoftheretinawiththatofthefieldofvisionisknownasthelineofregard。
Ifwereckonthedistanceontheretinathatcorrespondstothesmallestangleofvisionatwhichtwopointsinthecentreofthefieldofvisionmaybeperceivedasseparate，itwillbefoundtobe。004to。006mm。Thisdistanceisequaltothediameterofaretinalcone，andsincetheconesaresoclosetogetherinthecentreoftheretinathattheyareindirectcontact，itmaybeconcludedwithprobabilitythattwoimpressionsmustfalluponatleasttwodifferentretinalelementsiftheyaretobeperceivedasseparateinspace。Thisviewissupportedbythefactthatintheperipheralregionsoftheretinatherodsandcones，whicharethetwoformsofelementssensitivetolight，are[p。119]reallyseparatedbygreaterintervals。
Itmay，then，beassumedthatthekeennessofvision，ortheabilitytodistinguishtwodistinctpointsinthefieldofvision，isdirectlydependentontheproximityoftheretinalelementstooneanother，fortwoimpressionscanbedistinguishedasspeciallydifferentonlywhentheyactupondifferentelements。
16a。Becauseofthisinterrelationbetweenthekeennessofvisionandthearrangementofretinalelements，ithasoftenbeenconcludedthateverysuchelementhasfromthefirstthepropertyoflocalizinganystimulusthatactsuponit，inthatpositioninspacewhichcorrespondstoitsownprojectioninthefieldofvision。Inthiswaytheattempthasbeenmadetoreducethepropertyofthevisualsensebyvirtueofwhichitrepresentsitsobjectsinanexternalfieldofvisionatsomedistancefromthesubject，toacondateenergyoftheretinalelementsoroftheircentralconnectionsinthevisualcentreinthebrain。Therearecertainpathologicaldisturbancesofvisionthatseematfirstsighttoconfirmthisassumption。Whensomeregionoftheretinaispushedoutofplaceasaresultofinflammationunderneath，certaindistortionsintheimages，theso-calledmetamor-phopsia，arise。Theextentanddirectionofthesedistortionscanbefullyexplainedwhenitisassumedthatthedisplacedretinalelementscontinuetolocalizetheirimpressionsastheydidwhenintheirnormalpositions。Butitisobviousthatthesedistortionsoftheimageswhen，asinmostcases，theyappearascontinuallychangingphenomenaduringthegradualformationanddisappearanceoftheexcretion，furnishuswithnomoreevidenceforaconnateenergyoflocalizationintheretinathandoesthereadilyobservedfactthatdistortedimagesofobjectsareseenwhenonelooksthroughprismaticglasses。If，ontheotherhand，astationaryconditionisgraduallyreached，themetamorphopsiadisappear，andthat，too，notonlyincaseswhereitmaybeassumedthattheretinalelementsreturntotheiroriginalposition，buteveninthosecaseswheresuchareturnisentirelyimprobableonaccountoftheextentoftheaffection。Incaseslikethelatter，thedevelopmentofanewconnectionbetweenthesingleretinalelementsandtheircorrespondingpointsinthefield[p。120]ofvision，mustbeassumed。Thisconclusionissupportedbyobservationsmadewithnormaleyesonthegradualadaptationtodistortedimageswhichareproducedbyexternalopticalappliances。
Ifapairofprismaticglassesarewornbeforetheeyes，markedanddisturbingdistortionsoftheimagesaretheregularresults。Thestraightboundinglinesappearbentandtheformsoftheobjectsarethusdistorted。Thesedisturbancesgraduallydisappearentirelyiftheglassesarewornsometime。Whentheglassesareremovedthedistortionsmayappearintheoppositedirection。Allthesephenomenacanbeunderstoodifwepresupposethatthespaciallocalizationsofvisionarenotoriginal，butacquired。
17。Besidestheretinalsensationsthereareotherpsychicalelementsthatalwaystakepartinthereciprocalspacialarrangementoflight-impressions。
Thephysiologicalpropertiesofthedyepointaprioritothesensationsthataccompanyocularmovements，assuchelements。Thesemovementsobviouslyplaythesamepartintheestimationofdistancesinthefieldofvisionasthetactualmovementsdointheestimationoftactualimpressions。
Thegrosserconditionsoftouchare，however，herereproducedinamuchmoredelicateandhighlydevelopedform。Theeyecanbeturnedinalldirectionsaboutitscentreofrotation，whichisfixedinitsrelationtothehead，bymeansofamostadmirablyarrangedsystemofsixmuscles。Itisthuswellsuitedtofollowingcontinuouslytheboundinglinesofobjectsortopassingeach[p。121]timeintheshortestlinefromagivenfixation-pointtoanother。Themovementsinthedirectionswhichcorrespondtothepositionoftheobjectsmostfrequentlyandcloselyobserved，namely，downwardandinwardmovements，arefavoredabovetheothersbythearrangementofthemuscles。Furthermore，themovementsofthetwoeyesaresoadaptedtooneanotherthroughthesynergyoftheirinnervation，thatnormallythetwolinesofregardarealwaysturneduponthesamefixationpoint。Inthiswayacooperationofthetwoeyesismadepossiblewhichnotonlypermitamoreperfectapprehensionofthepositionofobjectsinrelationtooneanother，but，moreespecially，furnishesthemostessentialmeansforthedeterminationofthespacialrelationofobjectstothesubject(24seq)。
18。Thephenomenaofvisionteachthattheideaoftherelativedistanceoftwopointsfromeachotherisdependentonthemotorenergyoftheeyeemployedinpassingthroughthisdistance，justasthediscriminationoftwodistinctpointsinthefieldofvisiondependsonthearrangementoftheretinalelements。Themotorenergybecomesacomponentoftheideathroughitsconnectionwithasensationwhichcanbeperceived，especiallyinextensivemovementsandbycomparingocularmovementsinvariousdirections。Thus，forexample，theupwardmovementoftheeyesisclearlyaccompaniedbymoreintensesensationsthanthedownwardmovements；
andthesameistrueofoutwardmovementsofoneeyeascomparedwithitsinwardmovements。
Theinfluenceofsensationsofmovementonthelocalizationaremostapparentinthecasesofdisturbancearisingfrompartialparalysisofsingleocularmuscles。Thesedisturbancescorrespondexactlytothechangesintheamountofenergyrequiredtomovetheeye。Thegeneralprinciple[p。122]ofsuchdisordersisthatthedistancebetweentwopointsseemsgreaterwhentheylieinthedirectionofthemoredifficultmovement。
Themoredifficultmovementhasacorrespondinglymoreintensemotorsensation，whichundernormalconditionsaccompaniesamoreextensivemovement。Asaresult，thedistancepassedthroughappearsgreater。Furthermore，thesameillusionmayappearfordistancesthatlieinthesamedirection，baithavenotbeenactuallypassedthrough，forthestandardfoundduringa，movementdeterminesthemotorimpulseintheeyeevenwhenitisnotmoved。
19。Similardifferencesintheestimationofdistancescanbedemonstratedforthenormaleye。Althoughtheocularmusclesaresoarrangedthattheirmovementsinvariousdirectionsrequireaboutthesameamountofexertion，still，thisisnotexactlyso。Thereasonsareapparentlycloselyconnectedwiththeadaptationoftheeyetoitsfunctions。Theneighboringobjectsofourimmediateenvironment，onwhichthelinesofregardmustbeconverged，aretheonesmostoftenlookedit。Forthisreason，themusclesoftheeyehavesoadaptedthemselvesthatthemovementsfortheconvergenceofthelinesofregardaretheeasiest，particularlythosedirecteddownwardsascomparedwithotherpossiblemovementsofconvergence。Thisgeneralfacilitationofconvergencehasbeenacquiredbytheadditionofspecialauxiliaryandcompensatorymuscles(superiorandinferioroblique)tothemusclesthatmovetheeyeupwardsanddownwards(superiorandinferiorrecti)。Asaresultofthegreatercomplexityofmuscularactivitythusnecessaryfortheupwardanddownwardmovementsoftheeyes，theexertionisgreaterinthesedirectionsthantowardsthetwosides，whereonlytheinternalandexternalrectiact。Therelativecaseofdownwardmovementsofconvergenceshowsitselfpartlyinthedifferencesintheintensityofsensationsaccom-[p。123]panyingthemovements，asalreadyremarked(p。121)，andpartlyinthefactthatdownwardconvergenceisinvoluntarilytoogreatandupwardtoosmall。Therearecertainconstantopticalillusionsdependingonthedirectionoftheobjectinthefieldofvision，whichcorrespondtothesedifferencesinthemotormechanism。Theyareoftwokinds:illusionsofdirection，andthoseofmagnitude。
BotheyesaresubjecttoanillusionastotheDirectionofverticallinesinthefieldofvision。Suchalinewhoseupperendisinclined1o——3ooutward，appearsvertical，andonereallyvertical，seemsinclinedinward。Sincetheillusionisinoppositedirectionsforthetwoeyes，itdisappearsinbinocularvision。itcanobviouslybeexplainedbythefactjustnoted，thatthedownwardmovementsoftheeyesareconnectedwithaninvoluntaryincrease，andtheupwardmovementswithadecrease，intheconvergence。Thisdeflectionofthemovementfromtheverticalisnotnoticed，butwereferittotheobjectasadeflectionintheoppositedirection。Anequallyregularillusioninmagnitudeappearswhenwecomparedistancesextendingindifferentdirectionsinthefieldofvision。This，too，isveryprobablytobereferredtotheasymmetryinthearrangementofthemuscleswhicharisesfromtheadaptationoftheeyetotheordinarypositionofobjectsinspace。Averticalstraightlineisjudgedontheaverage1/6toolongascomparedwithanequalhorizontalline。Asquare，accordingly，appearsasarectanglewhosebaseisshorterthanitssides，andasquaredrawnbytheeyeisalwaystoolow。Thisillusionisexplainedwhenwerememberthat，asaresultofthehighlydevelopedtendencytoconvergence，themuscularactivityforupwardanddownwardmovementsismuchmorecomplexthanforinward[p。124]andoutwardmovements。
Theconsequenceisthesameasinthecaseofpartiallyparalyzedeyes，distancesinthedirectionofthemoredifficultmovementappeargreater。
19a。Besidesthisdifferencebetweenverticalandhorizontaldistances，whichismostnoticeablebecauseitissolarge，therearelessmarkeddifferencesbetweenupwardanddownward，aswellasbetweenoutwardandinwarddistances。Theupperhalfofaverticallineisoverestimatedontheaverageby1/16Ofitslength，andtheouterhalfofahorizontallineby1/40。Thefirstwaybeduetoaslightasymmetryinthearrangementoftheupperandlowermuscles，oritmaybeduetotheinvoluntaryconvergenceofthelinesofregardindownwardmovement，or，finally，toacombinationofbothinfluences。Theeffectofconvergenceisduetothefactthatitcorrespondstoanapproachoftheobject，sothatwearegenerallyinclinedtoseethelowerhalfofthelinenearer。Inaccordancewithcertainconditionsofassociationtobedescribedlater(§；16，9)，whentheangleofvisionremainsconstant，whateverisjudgedasnearerisjudgedtobesmaller，sothatthelowerhalfofalineseemsshorterthantheupper。Thisexplanationbytheperspectivecannotbeappliedtothegreaterillusionintheoverestimationofverticalascomparedwithhorizontallines，forifitwereapplicable，theillusionwouldatmostbeaboutequaltothatfoundinthecomparisonofthetwohalvesofaverticalline，whileinrealityitisapproximatelythreetimesasgreat。Thefactthatthisgreaterillusionappearsonlywhenstraightdistancesarecompared，notinthecaseofobjectsboundedbycurvedlines，isalsoaproofagainsttheexplanationbyperspective。
Acircle，forexample，doesnotappearasanellipsewithalongerverticalaxis，butasarealcircle。Theslightoverestimationoftheouterhalfofahorizontallineisalsoduemostprobablytotheasymmetricalactivityofthemuscles，whicharisesfromtherelativeeaseofconvergence-movements。
20。Addedtothesetwoillusions，whicharisefromthespecialstructureofocularmusclesintheiradaptationtothepurposesofvision，therearecertainothervariableoptical[p。125]illusionsthatareduetocertainattributesofallvoluntarymovementsandhavetheiranalogainthemovementsofthetactualorgans。Theseillusionsmayalsobedividedintothoseofdirection，andthoseofmagnitude。Theformerfollowtherulethatacuteanglesareoverestimated，obtuseunderestimated，andthatthedirectionoftheintersectinglinesvariescorrespondingly。Fortheillusionsofmagnitudewehavetherule。forcedorinterruptedmovementsrequiremoreexertionthanfreeandcontinuousones。Anystraightlinethatnecessitatesfixationis，accordingly，overestimatedincomparisonwithanopendistancemarkedoffbytwopoints，andastraightlineinterruptedbyseveraldividinglinesisoverestimatedincomparisonwithanuninterruptedline。
Thetactualanalogonoftheillusioninanglesisthetendencytooverestimatesmallarticularmovementsandtounderestimatelargeones。Thiscomesunderthegeneralprinciplethatarelativelygreaterexpenditureofenergyisrequiredforashortmovementthanforamoreextensiveone，becauseitismoredifficulttobeginamovementthantocontinueitafteritisalreadystarted。Thetactualphenomenonanalogoustotheoverestimationofinterruptedlines，isthatadistanceestimatedbyamovementofoneofthelimbsalwaysseemsshorterwhenitispassedthroughinasinglecontinuousmovementthanitdoeswhenthemovementisseveraltimesinterrupted。Here，too，thesensationcorrespondstotheexpenditureofenergy，whichis，ofcourse，greaterforaninterruptedthanforacontinuousmovement。Theoverestimationofinterruptedlinesbytheeye，takesplace，aswecaneasilyunderstand，onlysolongasnomotivesarisefromthewayinwhichthedivisionismade，tohinderthemovementoftheeyeovertheinterruptedline。Suchahindranceispresent，forexample，whenthelineisinterruptedonlyonce。Thisonepointofdivisionmakesfixation[p。126]necessary。Ifwecomparesuchalinewithacontinuousone，wetendtoestimatethefirstwithoutanymovement，withthepointofdivisionasafixation-centre，whilethesecondisapprehendedbyamovementoftheeye。Asaresultthecontinuouslineseemslongerthantheinterruptedline。
21。Allthesephenomenapointtotheimmediatedependenceoftheapprehensionofspacialdirectionsandmagnitudesonocularmovements。Asfurtherevidencepointinginthesamedirection，wehavethenegativefactthatthearrangementoftheretinalelements，especiallytheirproximitytooneanother，normallyhasnoinfluenceontheideasofdirectionandmagnitude。Thisismoststrikinglyevidentinthefactthatthedistancebetweentwopointsappearsthesamewhetherservedindirectorindirectvision。Twopointsthatareclearlydistinguishedindirectvision，maybecomeoneintheeccentricpartsofthefieldofvision，butsosoonastheyaredistinguishedatall，theywillappearjustasfarapartinoneregionasintheother。
Thisindependenceoftheproximityoftheretinalelements，inourperceptionofmagnitude，holdsevenforapartoftheretinathatisnotsensitivetolightatall——fortheblindspot，wheretheopticnervecomesintotheeye。Objectswhoseimagesfallontheblindspotarenotseen。
Thesizeofthisspotisabout6o，anditislocated15oinwardfromthepointoffixation。Imagesofconsiderablesize，as，forexample，thatofahumanfaceatadistanceofsixfeet，maydisappearentirelyonit。Still，whenpointsappearattherightandleftorbelowandabovethisregion，welocalizethemjustasfarfromeachotherasweshouldinanyother，uninterruptedpartofthefieldofvision。Thesamefactisobservedwhensomepartoftheretinabecomesblindthroughpathologicalconditions。Theresultingbreakinthefieldofvisionshowsitselfonlyillthefactthatimagesfallingonitarenotseen，never[p。127]throughanychangesinthelocalizationofobjectslyingonoppositesidesoftheblindregion。[2]
22。Allthesephenomenateachthatthekeennessofvisionandtheapprehensionofdirectionsanddistancesinthefieldofvision，aretwodifferentfunctions，whichdependupondifferentconditions:
thefirstdependsontheproximityoftheretinalelementstooneanother，thesecondonocularmovements。Itfollowsdirectlythatspacialideasfromsightcannotberegardedasoriginalandgivenimmediatelyintheactionofimpressionsoflightwiththeirspacialarrangement，anymorethancanthespacialideasoftouch。Thespacialorderis，heretoo，developedfromthecombinationofcertainsensationalcomponentswhich，takenseparately，havenospacialattributeswhatever。Otherconditionsalsoindicatethatthesensationalelementsarerelatedhereinthesamewayasinthecaseoftouch，andthatthedevelopmentofvisualspaceundernormalconditionsrunsentirelyparalleltothedevelopmentofspaceincongenitalblindness，theonlyconditionunderwhichtouchattainsasimilarindependence。Retinalimpressionscorrespondtoimpressionsofcontact，andocularmovementstotouch-movements。Tactualimpressionscangainspacialqualitiesonlythroughthelocalcoloringofthesensationsconnectedwiththem——thelocalsignsandinlikemanner——wemustpresupposethesameforretinalimpressions。
22a。Tobesure，aqualitativegradationoflocalsignson[128]theretinacannotbedemonstratedwiththesameevidentnessasfortheskin。
Still，bytheuseofcolorsitcanbeestablishedingeneralthatforgreaterdistancesfromtheretinalcentrethesensationalqualitygraduallychanges。
Thecolorsarenotsosaturatedinindirectvision，andthecolor-tonealsochanges；forexample，yellowappearsorange。Thereis，indeed，inthesepropertiesoftheretinanostrictprooffortheexistenceofpurelocaldifferencesinthesensations，atleastnotinthefinegradationsthatmustbeassumedintheretinalcentre，forexample。Still，theyshowthatlocaldifferencesinsensationsdoexist，andthisseemstojustifytheassumptionofsuchevenbeyondthelimitsofdemonstration。Thisisallthemorejustifiablebecausehere，wherethegradationsaremuchfiner，thetendencytotranslatesensationaldifferencesdirectlyintolocaldifferences，whichhasalreadybeennoticedinthecaseoftouch，willbemuchmoreapttodestroytheirspecificallyqualitativecharacter。Asaconfirmationofthisviewwehavethefactthattheclearlydemonstrablesensationaldifferencesatgreaterdistancesfromtheretinalcentre，canbeobservedonlyunderfavorableconditionsoflimitedimpressions，anddisappearsentirelywhensurfacesofuniformcolorarelookedat。Thisdisappearanceofqualitativedifferenceswhichareinthemselvesconsiderable，mustbeattributedinpartatleasttotheirrelationtolocaldifferences。When，however，suchrelativelygreatdifferencesdisappearasaresultofthisrelation，sothatspecialmethodsarerequiredfortheirdemonstration，itcannotbeexpectedthatverysmalldifferenceswillbedemonstrableatall。
23。Weassume，accordingly，qualitativelocalsigns，which，judgingfromthedataderivedfromthekeennessofvision，aregradedinthefineststagesattheretinalcentreandmoreslowlyintheeccentricparts。Theformationofvisualspacemaythenbedescribedasacombinationofthissystemoflocalsignsarrangedintwodimensions，withasystemofintensivesensationsofmovement。Foranytwolocalsignsaandbtherewillbeacorrespondingsensationofmovementaarisingfromthemovementthroughthedistanceaband[p。129]servingasameasureofthesame。Alongerdistanceacwillhaveamoreintensesensationofmovement，g。Justasthepointoffinestdiscriminationonthefingeristhecentreofreference，sointhesamewaytheretinalcentreissuchapointofreferencefortheeye。
Infact，thisisfromthelawsofocularmovementsmoreobviousfortheeyethanitisforthetactualorgan。Anyluminouspointinthefieldofvisionisastimulusforthecentreofoccularinnervation，andtendstoturnthelineofregardreflexivelyuponitself。Thisreflexrelationofeccentricstimulitotheretinalcentreisprobablyanessentialconditionforthedevelopmentofthesynergyofocularmovementsmentionedabove，andis，atthesametime，anexplanationofthegreatdifficultyofobservingobjectsinindirectvision。Thisdifficultyisevidentlyduetothegreaterrefleximpulsetowardapointinindirectvisionwhentheattentionisconcentrateduponit，thantowardlessfavoredpoints。Asaresultofthepreeminentimportancewhichtheretinalcentrehasforocularmovements，thepointoffixationnecessarilybecomesthecentreofreferenceinthefieldofvision，andalldistancesinthisfieldarebroughtunderaunitarystandardbybeingdeterminedwithreferencetothefixation-point。Theexcitationoflocalsignisduetotheactionofexternalimpressions，andbothtogethercausethemovementtowardstheretinalcentre。Thewholeprocessofvisualspace-arrangementisthusduetothefusionofthreedifferentsensationalelements:first，thesensationalqualitiesdependinguponthecharacteroftheexternalstimulus，secondthequalitativelocalsignsdependingontheplaceswherethestimuliact，andthird，theintensivemotorsensationsdeterminedbytherelationofthestimulatedpointstothecentreoftheretina。Thelatterelementsmayeitheraccompanyactualmovements——thisistheoriginalcase——or，whentheeyeremainsatrest，are[p。130]meremotorimpulsesofaparticularintensity。Becauseoftheregularconnectionbetweenqualitativelocalsignsandintensivesensationsofmovement，theymaybothtogetherberegardedasasinglesystemofcomplexlocalsigns。Thespaciallocalizationofasimplevisualimpression，isaproductofacompletefusionofthesensationcausedbytheexternalstimuluswiththetwointerconnectedelementsbelongingtothissystemofcomplexlocalsigns。Thearrangementofanumberofsimpleimpressionsinspaceconsistsinthecombinationofagreatnumberofsuchfusions，whicharegradedinqualityandintensityaccordingtotheelementsofthesystemoflocalsigns。Thepredominatingelementsinthesefusionsarethesensationsduetotheexternalstimulation。Incomparisonwiththese，theelementsofthesystemoflocalsignsaresoobscure，evenintheiroriginalqualityandintensity，thatfortheimmediateapprehensionofobjectstheyareentirelylostexceptasspacialqualities。
Connectedwiththiscomplexprocessoffusion，whichdeterminestheorderoftheelementsinthefieldofvision，isstillanother。Thislatterprocess，whichtakesplaceintheformationofeveryspacialidea，arisesfromtherelationoftheobjectseentothesubject。Wepassnowtotheconsiderationofthissecondprocess。
b。TheLocationofVisualIdeasinRelationtotheIdeatingSubject。
24。Thesimplestcaseofarelationbetweenanimpressionandthesubject，thatcanappearinavisualidea，isevidentlythatinwhichtheimpressionisreducedtoasinglepoint。Ifasinglepointoflightispresentedinthefieldofvision，bothlinesofregardareturneduponitasaresultoftherefleximpulseexertedbythestimulus(p。129)，insuch[p。131]
awaythatinbotheyestheimagesfallupontheretinalcentres。Atthesametimetheorgansofaccommodationareadaptedtothedistanceofthepoint。Thepointthusrepresentedonthecentresofbothretinasisseenassingleandassituatedinacertainparticulardirectionandatacertainparticulardistancefromtheideatingsubject。
Thesubjectisrepresented，asarule，byapointthatmaybedefinedasthemiddlepointofthestraightlineconnectingthecentresofrotationofthetwoeyes。Wewillcallthisthepointoforientationforthefieldofvision，andthestraightlinedrawnfromittotheintersectionofthetwolinesofregard，thatistotheexternalfixation-point，thelineoforientation。Whenapointinspaceisfixated，thereisalwaysafairlyexactideaofthedirectionofthelineoforientation。
Thisideaisproducedbytheinnertactualsensationsconnectedwiththepositionofthetwoeyes。Suchsensationsareverynoticeablebecauseoftheirintensity，whentheeyesarerotatedmuchoutofthecentralposition。
Theyarejustasperceptibleforasingleeye，sothatlocalizationindirectionisasperfectinmonocularasinbinocularvision。Intheformercase，however，thelineoforientationgenerallycoincideswiththelineofregard。[3]
25。Theideaofthedistanceoftheobjectsfromthesubject，oroftheabsolutelengthofthelineoforientation，ismuchmoreindefinitethanthatofitsdirection。Wearealwaysinclinedtoideatethisdistanceshorterthanitreallyis，asmaybeshownbycomparingitwithastandard[p。132]placedsomewhereinthefieldofvisionperpendiculartothelineoforientation。Inthiswaywefindthatthedistanceonthestandardjudgedtobeequaltothelineoforientation，isalwaysmuchshorterthanthereallengthofthisline。Thedifferencebetweenthetwoincreasesfurtherawaythepointoffixationmoves，thatis，thelongerthelineoforientationbecomes。Theonlysensationalcomponentsthatcanproducethisideaofdistance，arethoseinnertactualsensationsconnectedwiththepositionofthetwoeyes，thatariseparticularlyfromtheconvergenceofthelinesofregardandgivesomewhatofameasurefortheabsoluteextentofthisconvergence。Infact，itispossibletoobservesensationswhentheconvergenceischanged；mainlyfromtheinnerangleoftheeyewhenthedegreeofconvergenceisincreased，fromtheouter，whenitisdecreased。Thesumofallthesensationscorrespondingtoagivenpositionofconvergencedistinguishesitcompletelyfromallotherpositions。
26。Itfollowsthatanideaofadefinite，absolutelengthofthelineoforientationcanbedevelopedonlythroughtheinfluencesofexperience，whereinadditiontothesensationalelementsagreatmanyassociationsalsohaveapart。Thisexplainswhytheseideasalwaysremainindefiniteandwhytheyaresometimesaided，sometimesinterferedwithbyothercomponentsofvisualideas，especiallybythesizeoftheretinalimagesoffamiliarobjects。Ontheotherhand，wehavearelativelyfinemeasureinthesensationsofconvergencefordifferencesinthedistancesofobjectsseen，thatis，fortherelativechangeswhichthelengthofthelineoforientationundergoeswhenthefixation-pointapproachesorrecedes。Forpositionsinwhichthelinesofregardarenearlyparallel，changesinconvergencemaybeperceivedthatcorrespondtoanangleofvisionof60“
or70“。Whentheconvergenceincreases，thisleastperceptiblechangeinconvergencealso[p。133]increasesconsiderably，but，inspiteofthat，thecorrespondingdifferencesinthelengthofthelineoforientationbecomesmallerandsmaller。Thusthepurelyintensivesensationswhichaccompanymovementsofconvergence，aretranslateddirectlyintoideasofchangesinthedistancebetweenthefixation-pointandthepointoforientationofthesubject。
Thistranslationofacertainparticularsensationalcomplexintoanideaofdistance，isnotduetoanyconnateenergy，buttoaparticularpsychicaldevelopment，asisshownbyagreatnumberofexperienceswhichpointtosuchadevelopment。Amongtheseisthefactthattheapprehensionbothofabsolutedistancesandofdifferencesindistance，isgreatlyimprovedbypractice。Childrenaregenerallyinclinedtolocalizeverydistantobjectsintheimmediateneighborhood:theygraspatthemoon，attheslateronthetower，etc。Inthesameway，ithasbeenobservedthatthecongenitallyblindare，immediatelyafteranoperation，entirelyunabletodistinguishnearandfar。
27。Itisofimportanceforthedevelopmentofthisdiscriminationbetweenfarandnear，thatunderthenaturalconditionsofvisionnotmereisolatedpointsarepresented，butextendedthree-dimensionalobjects，oratleastanumberofpointsatdifferentdepths，towhichweassignrelativelydifferentdistancestheirrespectivelinesoforientation。
Letusconsiderfirstthesimplestcase，wheretwopointscandbarepresented，lyingatdifferentdepths，andconnectedbyastraightline。Achangeinthefixationfromatobisalwaysaccompaniedbyachangeinconvergence，andbringsabout，first，thepassagethroughacontinuousseriesofretinallocalsignscorrespondingtothelineab，and，secondly，asensationofmovement，[alp]，correspondingtothedifferenceinconvergencebetweenthesetwopoints。Thisgivesushere，too，theelementsofaspacialfusion。The[p。134]productofthisfusionis，however，peculiarinkind；itdiffersinbothitscomponents，inthesuccessiveseriesoflocalsignsandintheconcomitantsensationsofmovement，fromthefusionsthatarisewhenalineinthefieldofvisionispassedover(p。128)。Inthelattercasethechangesinlocalsignsandsensationsofmovementarealikeforbotheyes，whileinchangingthepointoffixationfromfartonearorthereversetheyareoppositeinthetwoeyes。Forwhentherighteyearotationtowardstheleftconvergencegivesitwillproducearotationtowardstherightinthelefteye，andviceversa。Thesamemustalsoholdforthemovementoftheretinalimages:
whentheimageofthepointasitleavesthepointoffixation，movestowardstherightintherighteye，itmovestowardstheleftinthelefteye，andviceversa。Thefirsttakesplacewhentheeyesturnfromanearertoamoredistantpoint，thelatter，whentheymoveintheoppositedirection。
Suchfusionsarisingfrommovementsofconvergencehave，sofarastheirqualitativeandintensivecomponentsareconcerned，acompositionanalogoustothatonwhichthearrangementoftheelementsinthefieldofvisionwithregardtooneanotherdepends；butthespecialwayinwhichtheseelementsareunitedisentirelydifferentinthetwocases。
28。Thus，thefusionsbetweenlocalsignsandsensationsofconvergenceformasystemofcomplexlocalsignswhichisanalogoustothatdeducedabove(p。130)，butstillpeculiarinitscomposition。Thissystem，differingincompositionfromthesystemoflocalsignsinthefieldofvision，issupplementarytothelatterinthatitaddstothereciprocalrelationbetweentheobjectiveelementsarelationbetweentheideatingsubjectandtheseelements。Therelationtothesubjectdividesintotwoideationalelements，characterizedbypeculiarsensationalelements:theideaofdirectionand[p。135]thatofdistance。Bothreferprimarilytothepointoforientationintheheadoftheideatingsubject，andarethensecondarilyappliedtotherelationsofexternalobjectsinregardtooneanother。Thus，wecometoassigntotwopointswhichlieatdifferentdistancesalongthelineoforientationadirectionanddistanceinrelationtoeachother。Allsuchideasofspacialdistancereferringtovariouspositionsalongthelineoforientation，whentakentogether，arecalledideasofdepth，orwhentheyarealsoideasofparticularsingleobjectsideasofthreedimensions。
29。Anideaofdeptharisinginthewaydescribedvariesaccordingtoobjectiveandsubjectiveconditions。Thedeterminationoftheabsolutedistanceofanisolatedpointinthefieldofvision，isalwaysveryuncertain。
Even，thedeterminationoftherelativedistancebetweentwopointsaandblyingatdifferentdepthsisgenerallycertainonlyundertheconditionassumedabove，thattheyareconnectedbyalinealongwhichthepointsoffixationforthetwoeyescanmoveinchangingtheconvergencefromatob。Wemaycallsuchlineswhichconnectdifferentpointsinspacewithoneanotherlinesoffixation。Theprinciplemaythenbeformulated:pointsinspaceareapprehendedintheirtruerelationsonlywhentheyareconnectedbylinesoffixation，alongwhichthepointoffixationmaymove。Thisprincipleisexplicableonthegroundthattheconditionofaregularlyconnectedchangeinthelocalsignsoftheretinaandintheaccompanyingsensationsofconvergence，thatis，theconditionfortheriseofideasofdepthaswefoundbefore(p。133)，isobviouslyfulfilledonlywhenimpressionsarepresentedwhichcanarousetheappropriatelocalsigns。
30。Whentheconditionmentionedisnotfulfilled，thereariseseitheranimperfectandindefiniteideaofthedifferentrelativedistancesofthetwopointsfromthesubject，orelse[p。136]thetwopointsseemtotheequallydistant——aphenomenonwhichcanappearonlywhenoneofthepointsisrigidlyfixated。Underthelatterconditionstillanothercharge，alwaysarisesintheidea；onlythefixatedpointisseenassingle，theotherasdouble。Thesamethinghappenswhilelookingatobjectswhentheyarenotconnectedwiththebinocularfixation-pointbymeansoflinesoffixation。Doubleimagesthatariseinthiswayareuncrossed——i。e。，therightbelongstotherighteye，thelefttothelefteye——whenthecrossedfixatedpointisnearerthantheobservedobjectandcrossedwhenthepointisbeyondtheobject。
Binocularlocalizationindepthandbinoculardoubleimagesare，accordingly，phenomenadirectlyinterrelated；wheretheformerisindefiniteandimperfect，wehavedoubleimages，andwhere，ontheotherhand，thelatterareabsent，the，localizationindepthisdefiniteandexact。Thetwophenomenastandinsucharelationtothelineoffixationthat，whenitispresent，localizationisaidedanddoubleimagesremoved。Still，thisruleisnotwithoutexception，forwhenapointisridgidlyfixatedwithbotheyes，doubleimagesariseeasilyinspiteofanylinesoffixationthatmaybepresent。Thisisexplainedbythenecessaryconditions，forbothofdepthasmentionedabove(p。133)。
Justastheabsenceoflinesoffixationresultsinthelackoftherequiredsuccessionofthelocalsigns，soinasigularmannerthesensationsofconferenceconnectedwithmovement，areabsentinrigidfixation。
c。RelationsbetweentheLocationoftheElementsinregardtooneanotherandtheLocationinRegardtotheSubject。
31。Whenthefieldofvisionisthoughtofasmerelyalocationofimpressionsinrelationtooneanother，werepresentittoourselvesisasurface，andcallthesingleob-[p。137]jectslyinginthissurfaceideasoftwodimensions，incontrasttotheideasofdepth。Butevenanideaoftwodimensionsmustalwaysberelatedtotheseeingsubjectintwoways。
First，everypointinthefieldofvisionisseenillaparticulardirectiononthesubjectivelineoforientationmentionedabove(p。131)。Secondly，thewholefieldofvisionislocalizedatsomedistanceorotherfromthesubject，thoughthisdistancemaybeveryindefinite。
Thelocationinaparticulardirectionresultsinanerectideationalobjectcorrespondingtoaninvertedretinalimage。Thisrelationbetweentheobjectivelocalizationindirectionandtheretinalimageisasnecessaryaresultofocularmovementsastheinversionoftheimageitselfisaresultoftheopticalpropertiesoftheeye。Ourlineoforientationinspaceistheexternallineofregardor，forbinocularvision，themiddlelineresultingfromthecombinedeffectsofmovementsoffixation。
Adirectionupwardonthislineoforientationinexternalspacecorrespondstoadirectiondownwardinthespacewheretheretinalimagelies，behindthecentreofocularrotation，andviceversa。Itfollowsthattheretinalimagemustbeinvertedifwearetoseetheobjecterect。
32。Thelocationatsomedistanceorother，whichisalsoneverabsent，bringsabouttheresultthatallthepointsofthefieldofvisionseemtobearrangedonthesurfaceofaconcavehemispherewhosecentreisthepointoforientation，or，inmonocularvision，thepointoftheeye\'srotation。Now，smallareasofalargecurvedsurfaceappearplane，sothatthetwo-dimensionalideasofsingleobjectsareasaruleplane；
thus，forexample，figuresdrawnupontplane，asthoseofplanegeometry。
Butassoonassomepartsofthegeneralfieldofvisionseparatefromitinsuchawaythattheyarelocalizedbeforeorbehind，thatisindifferentplanes，theideaoftwodimensionsgivesplacetooneofthree。[p。138]
32a。Thefusionsformedbetweenqualitativelocalsignsandsensationsofconvergencewhenwechangefromthefixationofamoredistantpointtoanearer，orthereverse，maybecalledcomplexlocalsignsofdepth。Suchlocalsignsformforeveryseriesofpointslyingbeforeorbehindthefixation-point，orforanextendedbodywhichisnothingbutaseriesofsuchpoints，aregularlyarrangedsysteminwhichastereometricformlocatedataparticulardistanceisalwaysunequivocallyrepresentedbyaparticularfusion。Whenoneoftwopointslyingatdifferentdistancesisfixated，theotherischaracterizedbythedifferentpositionofitsimagesinthetwoeyes，andbythecorrespondinglydifferentdirectionofthecomplexlocalsignsinthetwocases。Thesameistrueforconnectedseriesofpointsorextendedbodies。Whenwelookatasolidobject，itthrowsimagesinthetwoeyesthataredifferentfromeachotheronaccountofthedifferentrelativepositionoftheobjectwithregardtothetwoeyes。Wemaydesignatethedifferencebetweenthepositionsofcertainpointintheimageinthetwoeyesasthebinocularparallax。Thisparallaxiszeroforthepointfixatedandforthosepointswhichareequallydistantonthelineoforientation；forallotherpointsithassomerealpositiveornegativevalueaccordingly，astheyaremoreorlessdistantthanthefixation-point。Ifwefixatesolidobjectswithbotheyes，onlythepointfixated，togetherwiththosepointswhichareequidistantandinitsneighborhoodinthefieldofvision，willgiverisetoimagescorrespondinginpositioninthetwoeyes。Allpointsoftheobjectlocatedatdifferentdistances，giveimagesvaryinginpositionandsize。Thesedifferencesintheimagesarejustwhatproducetheideaofthesolidityoftheobjectwhentheproperlinesoffixationarepresent。Forinthewayabovedescribed，theangleofbinocularparallaxfortheimageofanypointlyingbeforeorbehindthepointoffixationandconnectedwiththesamebyalineoffixation，furnishes，accordingtoitsdirectionandmagnitude，ameasurefortherelativedistanceofthispointindepththroughthecomplexlocalsignsconnectedwiththeangleofparallax。Thisangleofparallaxforagivenobjectivedepth，decreasesproportionallytothedistanceofthesolidobject，sothattheimpressionofsoliditydiminishes，thefurtheroftheobjectsare，andwhenthedistanceissogreatthatallanglesofparallax[p。139]disappear，thebodywillappearflat，unlesstheassociationstobediscussedlater(§；16，9)produceanideaofdepth。
33。Theinfluenceofbinocularvisionontheideaofdepthmaybeinvestigatedexperimentallybymeansofastereoscope。Thisinstrumentconsistsoftwoprismswiththeiranglesofrefractionturnedtowardeachotherinsuchawaythatitrenderspossibleabinocularcombinationoftwoplaindrawingswhichcorrespondtotheretinalimagesfromathree-dimensionalobject。Theinfluenceofthevariousconditionsthatunderlietheformationofideasofdepths，may，inthisway，bestudiedmuchbetterthanbylookingatactualthree-dimensionalobjects，forherewemayvarytheconditionsatwill。
Togiveaconcreteillustration，itisobservedthatcomplexstereoscopicpicturesgenerallyrequireseveralmovementsofconvergencebackandforthbeforeaclearplasticideaarises。Furthermore，theeffectoftheparallaxappearsinlookingatstereoscopicpictureswhosepartsaremovableinrespecttoeachother。Suchmovementsareaccompaniedbychangesinthereliefwhichanswerexactlytothecorrespondingchangesinbinocularparallax。
Thisparallaxisdependentonthedistanceofthetwoeyesfromeachother，sothatideasofdepthcanbeproducedeveninthecaseofobjectstoodistantinrealitytogiveaplasticeffect，bycombininginthestereoscopepicturestakenfrompositionsmuchfurtherapartthanthetwoeyesare。
Thisisdone，forexample，inmakingstereoscopicphotographsoflandscapes。
Theresultisthatthesephotographswhencombineddonotlooklikereallandscapes，butlikeplasticmodelsregardedfromashortdistance。
34。Inmonocularvisionalltheconditionsareabsentwhichareconnectedwithmovementsofconvergence，andwithbinoculardifferencesintheretinalimages，andwhichmaybe[p。140]artificiallyreproducedwiththestereoscope。
Still，notalltheinfluencesarewantingevenheretoproducealocalizationinthethirddimension，althoughthislocalizationismoreimperfect。
Thedirectinfluenceofmovementsofaccomodationhaveincomparisonwithotherconditionsarelativelysmall，perhapsentirelyinsignificantinfluence。Still，likemovementsofconvergence，theytooareaccompaniedbysensationswhichcanbeclearlyperceivedintheelseofgreaterchangesofaccommodationfromdistanttoneighboringpoints。Forsmallerchangesindepththesesensationsareveryuncertain。Asaresultthemovementofapointinthedirectionofthelineofregard，whenitislookedatwithonlyoneeye，isgenerallynotclearlyobserveduntilachangeinthesizeoftheretinalimageappears。
35。Forthedevelopmentofmonocularideasofdepththeinfluenceswhichthecomponentsoftheso-calledperpectiveexercise，areofthegreatestimportance。Thesearetherelativemagnitudeoftheofvision，thetrendoflimitinglines，thedirectionofshadows，thechangeincolorsduetoatmosphericabsorption，etc。Alltheseinfluences，whichactinexactlythesamewayinmonocularandbinocularvision，dependonassociationsofideas，andwill，therefore，betreatedinalaterchapter(§；
16)。
35a。Wehaveingeneralthesameopposingtheoriesfortheexplanationofvisualideasasfortactualideas(p。114)。Theempiricaltheoryhassometimescommittedthefallacyoflimitingitselftoopticsandturningtherealproblemofspaceperceptionovertotouch。Insuchcasesithastriedtoexplainonlyhowalocalizationofvisualideascantakeplacewiththeaidofexperience，onthebasisofalreadyexistingspacialideasfromtouch。Suchaninterpretationis，however，notonlyself-contradictory，butitalsoconflictswithexperience，whichshowsthatnormalpersonswithvision，visualspace-perceptiondeterminestactual，notthereverse(p。104)。Thefactofgeneraldevelopment，thattouch[p。141]isthemoreprimitivesense，cannotbeappliedtothedevelopmentoftheindividual。
Thechiefevidencesinsupportofnativistictheoriesare，first，themetamorphopsiaafterdislocationofretinalelements(p。119)and，secondly，thepositionofthelineoforientation(p。131)，whichindicatesunitedfunctioningofthetwoeyesfromthefirst。Ithasbeennotedalready(p。120)thatthemetamorphopsiaandotherrelatedphenomenaprovetheexactoppositeassoonasthechancestowhichtheyareduebecomestationary。Furthermore，thefactthatinlongcontinueduseofonlyoneeyethelineoforientationcomestocoincidewiththelineofregard(p。131)，provesthatthepositionofthislineisnotgivenfromthefirst，butthatithasarisenundertheinfluenceoftheconditionsofvision。Stillanotherfactagainstnativisticandinfavorofthegenetictheoryisthedevelopmentinthechildofthesynergyofocularmovementsundertheinfluenceofexternalstimuliandtheorganizationofspace-perceptionswhichapparentlyaccompaniesit。
Hereasinmanyotherrespectsthedevelopmentofmostanimalsisdifferent。
Inthelattercasesthereflexconnectionsofretinalimpressionswithmovementsoftheeyesandheadfunctionperfectlyimmediatelyafterbirth。
(v。inf。§；9，2)。
Thegenetictheoryhasgainedtheascendencyoveroldernativisticandempiricalviewsprimarilythroughthemorethoroughinvestigationofthephenomenaofbinocularvision。Nativismhasdifficultywiththequestionwhywegenerallyseeobjectssinglealthoughtheyproduceimagesineachofthetwoeyes。Theeffortismadetoavoidthedifficultybyassumingthattwoidenticalretinalpointsareconnectedwiththesameopticfibrewhichdividesinthechiasma，andthatinthiswaytheyrepresentinthesensoriumonlyasinglepoint。Thisdoctrineofthe“identitvofthetworetinas“was，however，untenableassoonastheactualconditionsofbinocularvisioninthree-dimensionsbegantobeinvestigated。Especiallytheinventionofthestereoscopethusbroughtwithitaneweraforthegenetictheoryofvision。
[1]Aprocessanalogoustothiseliminationofthemetamophopsiaissometimesobservedinbinocularvisionwhenthedisturbancesarisingfromsquintingaregraduallyovercome。Whenthesquintingbegins，thetwolinesofregardnolongermeetinthefieldofvision，sothatdoubleimagesofobjectsarise。Thesemaygraduallydisappear，however，iftheconditionoftheeyesremainsperfectlystationary；anewsetofrelationsisdevelopedfortheretinalelementsofthesquintingeye。
[2]Inthisconnection，wehavethefactthattheblindspotdoesnotappearasabreakinthefieldofvision，withoutsensationalcontents，butasacontinuationofthegeneralbrightnessandcolorofthewholefield；forexample，aswhitewhenwearelookingatawhitesurface，asblackwhenwelookatablackone。Thisfillingoutoftheblindspotispossibleonlythroughreproducedsensations，andistobeconsideredasoneofthephenomenaofassociationtobediscussedlater(§；16)。
[3]Thehabitofseeingwithtwoeyesresultsinexceptionstothisrule。Oftenwhenoneeyeisclosed，thelineoforientationremainsthesameasinbinocularvisionanddoesnotcoincidewiththelineofregard。Insuchcasestheclosedeyeusuallymakesthecorrespondingmovementsofconvergenceuponacommonfixation-pointwiththeopeneye。
ClassicsintheHistoryofPsychology——Wundt(1897)Section11OutlinesofPsychologyWilhelmMaxWundt(1897)TranslatedbyCharlesHubbardJudd(1897)II。PSYCHICALCOMPOUNDS§；11。TEMPORALIDEAS
1。Allourideasareatoncespacialandtemporal。Justastheconditionsforthespacialarrangementofimpressionsbelongoriginallyonlytothetactualandvisualsenses，andjustasspacialrelationsareonlysecondarilycarriedoverfromthesetoallothersensations，sothereareonlytwokindsofsensations，namely，theinnertactualsensationsfrommovementsandtheauditorysensations，whicharetheprimarysourcesoftemporalideas。Still，thereisacharacteristicdifferencebetweenspacialandtemporalideasinthefactthatinthefirstthetwosensesmentionedaretheonlyoneswhichcandevelopanindependentspacialorder，whileinthesecondthetwomostimportantkindsofsensationaremerelythoseinwhichtheconditionsaremostfavorablefortheriseoftemporalideas。Theseconditionsarenotentirelywanting，however，foranysensations。Thisindicatesthatthepsychologicalbasesoftemporalideasaremoregeneral，andthattheyarenotdeterminedbythespecialstructuresofparticularsense-organs。Itfollowsfromthisfactthatevenwhenweabstractfromtheideasthatenterintoanyseriesofpsychicalprocesses，andtakeaccountonlyofthesubjectivephenomenaaccompanyingtheideas，suchasfeelingsandemotions，westillascribetotheaffectiveprocessesthusisolatedthroughabstractionexactlythesametemporalattributesastotheideas。Inphilosophytheconclusionhasgenerallybeendrawnfromthisfactthattimeisa“universalformofperception“，thatis，thereisabsolutelynopsychicalcontentthatdoesnothaveapositionintime，thoughsuchcontentmayexistwithoutspacialattributes。Thisconclusionthattime-perceptionismoreuniversal，arising，asitdoes，fromthegreateruniversalityoftheconditionsofsuchperception，iserroneousandisnotconfirmedbypsychologicalobservation。
[p。143]
Inthesamewaythatwecarryoverspacialattributesfromthetwosensesthatgiveusspace-pereeptiontootherkindsofsensations，wealsogivethemsecondarilytofeelingsandaffectiveprocessesthroughthesensationsandideasinseparablyconnectedwiththem。Itmaywithequalrightbedoubtedwhetheraffectiveprocessesinthemselves，withouttheirrelatedideas，wouldhavetemporalattributes，foramongtheconditionsofatemporalorderarecertainattributesofthesensationalelementsofideas。Therealfactsinthecasearethatourideasand，therefore，sinceideasenterintoeverypsychicalexperience，allpsychicalcontentsareatoncespacialandtemporal。Thespacialorderarisesfromcertainparticularsensationalelements:innormalcaseswherevisionispresentfromvisual，inblindness，fromtactualimpressions；
whiletime-ideascanarisefromallpossiblesensations。
2。Temporalcompoundslikespacialandincontrasttointensiveideas，arecharacterizedbythedefinite，unchangeableorderoftheircomponentelements。Ifthisorderischanged，thegivencompoundbecomesanother，eventhoughthequalityofitscomponentsremainsthesame。Inspecialcompounds，however，thisunchangeablenessoftheorderrefersonlytotherelationoftheelementstooneanother，nottotherelationoftheelementstotheideatingsubject。Intemporalcompounds，ontheotherhand，whentherelationofoneelementischangedwithregardtootherelements，itisatthesametimechangedwithregardtotheideatingsubject。Thereisnochangeofpositionintimeanalogoustothatpossibleinthecaseofspace-compounds。
2a。Thispropertyoftheabsolute，strictlyspeakingunchangeable，relationofeverytemporalcompoundandeverytime-element，howevershort，totheideatingsubject，iswhatwecalltheflowoftime。Everymomentintimefilledbyanycontentwhateverhas，onaccountofthisflow，arelationto[p。144]theideatingsubjectthatnoothermomentcanbesubstitutedforit。Withspacethecaseisjustreversed:theverypossibilityofsubstitutinganyspacialelementinitsrelationtothesubjectforanyotherelementwhatever，iswhatgivesrisetotheideaofconstance，orabsoluteduration，asweexpressit，byapplyingatime-ideatoaspace-idea。Theideaofabsoluteduration，thatisoftimeinwhichnochangetakesplace，isstrictlyspeakingimpossibleintime-perceptionitself。Therelationtothesubjectmustchangecontinually。Wespeakofanimpressionaslasting，whenitssingleperiodsintimeareexactlyalikesofarastheirsensationalcontentsareconcerned，sothattheydifferonlyintheirrelationtothesubject。Theconceptofdurationwhenappliedtotimeis，therefore，amerelyrelativeconcept。Onetime-ideamaybemorelastingthananother，butnotime-ideacanhaveabsoluteduration，forwithoutthedoublerelationofdifferentsensationstooneanotherandtotheideatingsubject，nosuchideasatallcouldarise。Evenanunusuallylongunchangingsensationcannotberetained。Weinterruptitcontinuallywithothersensationalcontents。
Wemay，however，separatethetwotemporalrelationsalwaysunitedinactualexperience，thatoftheelementstooneanotherandtotheideatingsubject，sinceeachisconnectedwithcertainparticularattributesoftime-ideas。Infact，thisseparationofthetworelationsfounditsexpressioninparticularwordsforcertainformsofoccurrenceintimeevenpriortoanexactpsychologicalanalysisoftime-ideas。Iftherelationoftheelementstooneanotherisaloneattendedto，withoutregardtotheirrelationtothesubject，temporalmodescometobediscriminated，such，forexample，asbrief，long，regularlyrepeating，irregularlychanging，etc。If，onthecontrary，therelationofthesubjectisattendedtoandtheobjectiveformsofoccurrenceabstractedfrom，wehaveasthechiefformsofthisrelationthetemporalstagespast，present，andfuture。A。TEMPORALTOUCH-IDEAS。
3。Theorginaldevelopmentoftemporalideasbelongstotouch。Tactualsensations，accordingly，furnishthegeneralsubstratumfortheriseofboththespacialandtemporal[p。145]arrangementsofideationalelements(p。104，3)。Thespacialfunctionsoftouch，however，comefromtheoutertactualsensations，whiletheinnersensationswhichaccompanymovementsaretheprimarycontentsoftheearliesttemporalideas。
Themechanicalpropertiesofthelimbsareimportantphysiologicalbasesfortheriseoftheseideas。
Thearmsandlegscanbemovedintheshoulder-jointsandhip-jointsbytheirmuscles，andareatthesametimesubjecttotheactionofgravitationdrawingthemdownward。Asaresulttherearetwokindsofmovementspossibleforthem。First，wehavethosewhicharecontinuallyregulatedbyvoluntaryactivityofthemusclesandmay，therefore，beindefinitelyvariedandaccommodatedateverymomenttotheexistingneeds——wewillcallthesethearhythmicalmovements。Secondly，wehavethoseinwhichthevoluntaryenergyofthemusclesisoperativeonlysofarasitisrequiredtosetthelimbsoscillatingintheirjointsandtomaintainthismovement——rhythmicalmovements。Wemayneglectforourpresentconsiderationthearhythmicalmovementsexhibitedinthevarioususesofthelimbs。Theirtemporalattributesareinallprobabilityderivedfromtherhythmicalmovements，andonlyaveryindefinitecomparisonofthedurationofirregularmovementsispossible。
4。Withrhythmicalmovementsthecaseisdifferent。Theirsignificanceforthepsychologicaldevelopmentoftime-ideasisduetothesameprinciplewhichgivesthemtheirimportanceasphysiologicalorgans，namely，theprincipleoftheisochronismofoscillationsoflikeamplitude。Inwalking，theregularoscillationsofourlegsinthehip-jointsnotonlymakethemuscularenergyexpendedless，butreducetoaminimumthecontinualvoluntarycontrolofthemovements。
Furthermore，innaturalwalkingthearmsaresupplementaryaids。[p。146]
Theiroscillationisnotinterruptedateverysteplikethatofthelegsbytheplacingofthefootontheground，sothattheyfurnishbecauseoftheircontinuityameansforthemoreuniformregulationoftheirmovements。
Everysuiteperiodofoscillationinsuchamovementismadeupofacontinuoussuccessionofsensationsthatarerepeatedinthefollowingperiodinexactlythesameorder。Thetwolimitsoftheperiodaremarkedbyacomplexofoutertactualsensations:thebeginningbytheimpressionaccompanyingtheremovalofthefootfromtheground，theendbythataccompanyingitsreturntotheground。Betweenthesethereisacontinuousseriesofweakinnertactualsensationsfromthejointsandmuscles。Thebeginningandendofthisseriesofinnersensationscoincidewiththeoutersensationsandaremoreintensethanthosebetweenthem。Theyarisefromtheimpulseofmovementcomingtothemusclesandjointsandfromthesuddeninhibitionofthesame，andservealsotomarkofftheperiods。
Connectedwiththisregularsuccessionofsensationsisaregularandexactlyparallelseriesoffeelings。Ifweconsiderasingleperiodinaseriesofrhythmicalmovements，thereisalwaysatitsbeginningandendafeelingoffulfilledexpectation。
Betweenthetwolimitsoftheperiod，beginningwiththefirstmovement，isagraduallygrowing，feelingofstrainedexpectation，whichsuddenlysinksatthelastmomentfromitsmaximumtozero，tomakeplacefortherapidlyrisingandsinkingfeelingoffulfillment。Fromthispointonthesameseriesisagainrepeated。Thus，thewholeprocessofarhythmicalofatouch-environmentconsists，onitsaffectiveside，oftwoqualitativelyantagonisticfeelings。Intheirgeneralcharacterthesefeelingsbelongtothedirectionofstraining。andrelaxingfeelings(p。83)。Oneisamomentaryfeeling，thatis，onethatrisesveryrapidlytoitsmaximumandthen[p。147]sinkswithequalrapidity；theotherisafeelingoflongdurationwhichgraduallyreachesamaximumandthensuddenlydisappears。
Asaresult，themostintenseaffectiveprocessesarecrowdedtogetherattheextremitiesoftheperiods，andaremadeallthemoreintensethroughthecontrastbetweenthefeelingofsatisfactionandtheprecedingfeelingofexpectation。Justinthesamewaythatthissharplymarkedlimitbetweenthesingleperiodshasitssensationalsubstratuminthestrongouterandinnertactualimpressionsthatariseatthisinstant，asabovementioned，sowehaveacompletecorrespondencebetweenthegradualriseofthefeelingofexpectationandthecontinuousseriesofweakerinnertactualsensationsaccompanyingtheoscillatorymovementsofthelimbs。
5。Thesimplesttemporalideasoftoucharemadeupoftherhythmicallyarrangedsensationsthatfollowoneanotherwithperfectuniformityinthemannerdescribed，whenlikeoscillatorymovementsarerepeatedlycarriedout。Buteveninordinarywalkingaslighttendencytowardasomewhatgreatercomplicationarises；thebeginningofthefirstoftwosuccessiveperiodsisemphasized，bothinthesensationandintheaccompanyingfeeling，morethanthebeginningofthesecond。
Inthiscasetherhythmofmovementbeginstobemetrical。Infact，sucharegularsuccessionofaccentedandunaccentedideascorrespondstothesimplestmeasure，2/8time。Itariseseasilyinordinarywalkingbecauseofthephysiologicalsuperiorityoftherightside，andappearsveryregularlywhenseveralpersonsarewalkingtogetherinmarching。
Inthelattercaseevenmorethantwoperiodsmaybeunitedintoonerhythmicalunit。Thesameistrueofthecomplicatedrhythmicalmovementsofthedance。
Butinsuchcompositetactualrhythmstheauditorytemporalideashaveadecidedinfluence。[p。148]B。TEMPORALAUDITORYIDEAS。
6。Theattributeoftheauditorysensewhichmostofalladaptsittothemoreaccurateapprehensionofthetemporalrelationsinexternalprocesses，istheexceedinglyshortpersistenceofitssensationsaftertheexternalstimulation；sothatanytemporalsuccessionofsoundsisreproducedwithalmostperfectfidelityinthecorrespondingsuccessionofsensations。Incloseconnectionwiththiswehavecertainpsychologicalpropertiesoftemporalauditoryideas。
Inthefirstplace，theydifferfromtemporalideasoftouchinthatoftenonlytheextremitiesofthesingleintervalsthatgotomakeupthetotalidea，aremarkedbysensations。Insuchacasetherelationsofsuchintervalstooneanotherareestimatedessentiallybytheapparentlyemptyorheterogeneouslyfilledintervalsthatliebetweenthelimitingsensations。
Thisisespeciallynoticeableinthecaseofrhythmicalauditoryideas。Thereareingeneraltwopossibleformsofsuchideas:continuousoronlyrarelyinterruptedsuccessionsofrelativelylastingsensations，anddiscontinuoussuccessionsofstrokes，inwhichonlytheextremitiesoftherhythmicalperiodsaremarkedbyexternalsounds。Foradiscontinuoussuccessionofentirelyuniformsoundsthetemporalattributesoftheideasareingeneralmoreapparentthanforlastingimpressions，sinceintheformercasetheinfluencesofthetonalqualitiesareentirelywanting。Wemayconfineourconsiderationtodiscontinuousseries，becausetheprinciplesthatapplyhereholdforcontinuoussuccessionsalso。Infact，therhythmicaldivisioninthelattercase，asmaybeeasilyobserved，ismadebymeansofcertainsingleaccentswhichareeithergivenintheexternalimpressionorabitrarilyappliedtoit。[p。149]
7。Aseriesofregularstrokesmadeinthiswayasthesimplestformoftemporalauditoryideas，isdistinguishedfromthesimplestformoftemporaltouch-ideas，describedabove(p。147)，mainlybytheabsenceofallobjectivesensationalcontentintheintervals。
Theexternalimpressionsheredonothingbutdividetheseparateintervalsfromoneanother。Still，theintervalsofsuchaseriesarenotentirelyempty，butarefilledbysubjectiveaffectiveandsensationalcontentswhichcorrespondfullytothoseobservedintactualideas。Mostemphaticofallaretheaffectivecontentsoftheintervals。Thesefeelingsintheirsuccessiveperiodsofgraduallyrisingandsuddenlysatisfiedexpectation，arethesameasinthecourseofarhythmicaltactualmovement。
Eventhesensationalsubstratumforthesefeelingisnotentirelyabsent；
itismerelymorevariable。Sometimesitisnothingbutthesensationsoftensionofthetympanumintheirvariousintensifies。Thenagainitistheaccompanyingsensationsoftensionfromotherorgans，orfinallyothersensationsofmovementincaseswhereaninvoluntaryrhythmicalmovementisconnectedwiththeauditoryseries。Butonaccountofthechangeablecharacterandgenerallysmallintensityofthesemotorsensations，theaffectiveprocessesinauditoryideas\'areverymuchmoreclearlyperceptible。
Itfollowsfromtheconditionsdescribedthattheinfluenceofthesubjectiveelementsonthecharacteroftime-ideasistheeasiesttodemonstrate。Firstofall，thisshowsitselfintheeffectwhichdifferentratesofthesensationshaveontheformationoftemporalideas。Itisfoundthatthereisacertainmediumrateofabout0。2sec。
whichismostfavorablefortheunionofanumberofsuccessiveauditoryimpressions。Now，itiseasytoobservethatthisistherateatwhichtheabovementionedsubjectivesensationsandfeelingsaremostemphaticintheiralternation。Iftherateis[p。150]mademuchslower，thestrainofexpectationistoogreatandpassesintoanunpleasurablefeelingwhichbecomesmoreandmoreunendurable。If，onthecontrary，therateisaccelerated，therapidalternationoffeelingsbecomesfatiguing。Thus，inbothdirectionslimitsareapproachedwherethesynthesisoftheimpressionsintoarhythmicaltime-ideaisnolongerpossible。Theupperlimitisaboutonesecond，thelowerabout0。1sec。
8。Thenagain，thisinfluenceofthecourseofoursensationsandfeelingsuponourapprehensionoftemporalintervals，showsitselfjustasclearlyillthechangesthatourideaofsuchanintervalundergoeswhentheconditionsofitsapprehensionarevariedwithoutchangingitsobjectivelength。Thus，ithasbeenobservedthatingeneralaperioddividedintointervalsisestimatedaslongerthanonenotsodivided。Wehavehereaphenomenonanalogoustothatobservedintheillusionwithinterruptedlines(p。125)。Theoverestimationisgenerallymuchgreaterfortemporalintervals。Thisisobviouslyduetothefactthattheoftrepeatedalternationinsensationsandfeelingsinanintervaloftimehaveamuchgreaterinfluencethantheinterruptionofthemovementthroughpointsofdivisioninthecaseofthesimilarspacialillusion。Furthermore，ifinalongseriesofregularbeatssingleimpressionsareemphasizedbytheirgreaterintensity，orbysomequalitativepeculiarity，theuniformresultisoverestimationoftheintervalsprecedingandfollowingtheemphasizedimpression，incomparisonwiththeotherintervalsofthesameseries。If，however，acertainrhythmisproducedsuccessivelywithweakandthenwithstrongbeats，therateappearsslowerinthefirstcasethaninthesecond。
Thesephenomenaarealsoexplicablefromtheinfluenceofthesensationalandaffectivechanges。Animpressiondistinguishedfromtherest，demandsachangeinthe[p。151]courseofthesensations，andespeciallyofthefeelings，precedingitsapprehension，fortheremustbeamoreintensestrainofexpectationanda，correspondinglystrongerfeelingofrelieforsatisfaction。Thefeelingofexpectationlengthenstheintervalprecedingtheimpression，thefeelingofreliefthatfollowing。Thecaseisdifferentwhenthewholeseriesismadeupatonetimeofweakimpressions，andatanotherofstrongones。Inordertoperceiveaweakimpressionwemustconcentrateourattentionuponitsnore。Thesensationsoftensionandtheaccompanyingfeelingsare，accordingly，moreintense，asmaybeeasilyobserved，forweakerbeatsthanforstrongerones。Heretoo，then，thedifferentintensifiesofthesubjectiveelementsthatgiverisetothemarereflectedinthedifferencesbetweentemporalideas。Theeffectis，therefore，notonlylost，butevenreversed，whenwecomparenotweakwithstrongbutstrongwithstillstrongerbeats。
9。Thetendencyfoundinthecaseofrhythmicaltouch-ideasforatleasttwolikeperiodstouniteandformaregularmetricalunitshowsitselfinauditoryideasalso，onlyinamuchmoremarkeddegree。Intactualmovements，wherethesensationsthatlimitthesingleperiodsareundertheinfluenceofthewill，thistendencytoformarhythmicalseriesshowsitselfintheactualalternationofweakerandstrongerimpressions。Withauditorysensations，ontheotherhand，wherethesingleimpressionscanbedependentonlyonexternalconditions，andare，therefore，objectivelyexactlyalike，thistendencymayleadtothefollowingcharacteristicillusion。Inaseriesofbeatswhichareexactlyalikeinintensityandareseparatedbyequalperiodsoftime，certainsinglebeats，occurringatregularintervals，arealwaysheardasstrongerthantheothers。Thetimethatmostfrequentlyariseswhenthereisnothingtodetermineit，isthe2/8-time，thatis，theregularalternationofarsesandtheses。Aslight[p。152]modificationofthis，the3/8-time，wheretwounaccentedfollowoneaccentedbeat，isalsoverycommon。Thistendencytomarktimecanbeovercomeonlybyaneffortofthewill，andthenonlyforveryfastorveryslowrates，where，fromtheverynatureoftheseries，thelimitsofrhythmicalperceptionarenearlyreached。
Formediumrates，whichareespeciallyfavorabletotheriseofrhythmicalideas，asuppressionofthistendencyforanylengthoftimeishardlypossible。Iftheeffortismadetouniteasmanyimpressionsaspossibleinaunitarytime-idea，thephenomenabecomemorecomplicated。Wehaveaccentsofdifferentdegreeswhichalternateinregularsuccessionwithunaccentedmembersoftheseriesandthus，throughtheresultingdivisionsofthewholeintogroups，umberofimpressionsthatmaybecomprehendedinasingleideaisconsiderablyincreased。Thepresenceoftwodifferentgradesofaccentgives3/4-timeand5/8-time，thepresenceofthreegradesgives4/4-timeand6/4-time，andasformswiththreefeetwehave9/8-timeand12/8-time。Morethanthreegradesofaccentuationor，whentheunaccentednoteiscounted，morethanfourgradesofintensity，arenottobefoundineithermusicalorpoeticalrhythms，norcanweproducemorebyvoluntarilyformationof\'rhythmicalideas。Obviously，thesethreegradesofaccentuationmarkthelimitsofthepossiblecomplexityoftemporalideas，inawayanalogoustothatinwhichthemaximalnumberofincludedbeats(§；15，6)marksthelimitsoftheirlength。
Thephenomenonofsubjectiveaccentuationanditsinfluenceonthesensationofrhythms，showsclearlythattemporalideas，likespacialideas，arenotderivedfromobjectiveimpressionsalone，butthatthereareconnectedwiththese，subjectiveelements，whosecharacterdeterminestheapprehensionoftheobjectiveimpressions。Theprimarycause[p。153]oftheaccentuationofaparticularbeatisalwaystobefoundintheincreasedintensityoftheprecedingandconcomitantfeelingsandsensationsofmovement。Thisincreaseintheintensityofthesubjectiveelementsisthencarriedovertotheobjectiveimpression，andmakesthelatteralsoseemmoreintense。Thestrengtheningofthesubjectiveelementsmaybevoluntary，throughtheincreaseofthemuscularstrainwhichproducessensationsofmovement，andinthisway，finallyresultsinacorrespondingincreaseinthefeelingsofexpectation；orthisstrengtheningmaytakeplacewithoutvolition，whentheefforttoperceiveanumberofimpressionstogetherbringsaboutanimmediatearticulationofthetemporalideathroughthecorrespondingsubjectivesensationalandaffectivevariations。C。GENERALCONDITIONSFORTEMPORAL
IDEAS。
