Thedifferencesinthenumberofdimensionshavebeendeterminedwithcertaintyonlyinthecaseofcertainsensationalsystems。Thus，thetonalsystemisone-dimensional。Theordinarycolor-system，whichincludesthecolorsandtheirtransitionalqualitiestowhite，istwo-dimensional；
whilethecompletesystemoflight-sensations，whichincludesalsothedarkcolor-tonesandthetransitionalqualitiestoblack，isthree-dimensional。
6。Intherelationsdiscussedthusfar，sensationalandaffectiveelementsingeneralagree。Theydiffer，ontheotherhand，incertainessentialattributeswhichareconnectedwiththeimmediaterelationsofsensationstoobjectsandoffeelingstothesubject。
1)Whenvariedinasingledimension，sensationalelementsexhibitpurequalitativedifferences，whicharealwaysinthesamedirectionuntiltheyreachthepossiblelimitsofvariation，wheretheybecomemaximaldifferences。Thus，inthecolor-system，redandgreen，blueandyellow，orinthetonalsystem，thelowestandhighestaudibletones，arethemaximal，andatthe，sametimepurelyqualitative，differences。
Everyaffectiveelement，onthecontrary，whencontinuouslyvariedinthesuitabledirectionofquality，passesgraduallyintoafeelingofoppositequality。Thisismostobviousinthecaseoftheaffectiveelementsregularlyconnectedwithcertainsensationalelements，as，forexample，tonalfeelingsorcolor-feelings。Assensationsahighandlow-tonearedifferencesthatapproachmoreorlessthemaximaldifferencesoftonalsensation；thecorrespondingtonalfeelingsareopposites。Ingeneral，then，sensationalqualitiesarelimitedbymaximaldifferences，affectivequalitiesbymaximalopposites。Betweentheseopposites[p。34]isamiddlezone，wherethefeelingisnotnoticeableitall。Itis，however，frequentlyimpossibletodemonstratethisindifference-zone，because，whilecertainsimplefeelingsdisappear，otheraffectivequalitiesremain，ornewonesevenmayarise。Thelattercaseappearsespeciallywhenthetransitionofthefeelingintotheindifference-zonedependsonachangeinsensations。
Thus，inthemiddleofthemusicalscale，thosefeelingsdisappearwhichcorrespondtothehighandlowtones，butthemiddletoneshavestillother，independentaffectivequalitieswhichdonotdisappearwiththeseopposites。
Thisistobeexplainedbythefactthatafeelingwhichcorrespondstoacertainsensationalqualityis，asarule，acomponentofacomplexaffectivesystem，inwhichitbelongsatthesametimetovariousdimensions。Thus，theaffectivequalityofatoneofgivenpitchbelongsnotonlytothedimensionofpitch-feelings，butalsotothatoffeelingsofintensityandfinallytothedifferentdimensionsintheclang-qualitiesoftonesmaybearraigned。Atoneofmiddlepitchandintensitymay，lieintheindifference-zonesofarasfeelingsofpitchandintensityareconcerned，andyethaveaverymarkedclang-feeling。Thepassageofaffectiveelementsthroughtheindifference-zonecanbedirectlyobservedonlywhencareistakentoabstractfromotheraccompanyingaffectiveelements。Thecasesmostfavorableforthisobservationarethoseinwhichtheaccompanyingelementsdisappearentirelyoralmostentirely。Whereversuchanindifference-zoneappearswithoutcomplicationwithotheraffectiveelements，wespeakofthestateasfreefromfeelings，andofthesensationsandideaspresentinsuchastate，asindifferent。
2)Feelingsofspecific，andatthesametimesimpleandirreducible，qualityappearnotonlyasthesubjectivecomplementsofsimplesensations，butalsoasthecharacteristicattendantsofcompositeideasorevencomplexideational[p。35]processes。Thus，thereisasimpletonalfeelingwhichvarieswiththepitchandintensityoftones，andalsoafeelingofharmonywhich，regardedasafeeling，isjustasirreducible，butvarieswiththecharacterofcompoundclangs。Stillotherfeelings，whichmayinturnbeofthemostvariouskinds，arisefrommelodiousseriesofclangs。Here，again，eachsinglefeelingtakenbyitselfatagivenmoment，appearsasanirreducibleunit。Simplefeelingsare，then，muchmorevariousandnumerousthansimplesensations。
3)Thevariouspuresensationsmaybearrangedinanumberofseparatesystemis，betweenwhoseelementsthereisnoqualitativerelationwhatever。
Sensationsbelongingtodifferentsystemsarecalleddisparate。Thus，atoneandacolor，asensationofhotandoneofpressure，or，ingeneral，anytwosensationsbetweenwhichtherearenointermediatequalities，aredisparate。Accordingtothiscriterion，eachofthefourspecialsenses(smell，taste，hearing，andsight)hasaclosed，complexsensationalsystem，disparatefromtheothersenses；whilethegeneralsense(touch)containsfourhomogeneoussensationalsystems(sensationsofpressure，hot，cold，andpain)。Allsimplefeelings，onthecontrary，formasingleinterconnectedmanifold，forthereisnofeelingfromwhichitisnotpossibletopasstoanyotherthroughintermediateformsorthroughindifference-zones。
Butheretoowemaydistinguishcertainsystemswhoseelementsaremorecloselyrelated，as，forexample，feelingsfromcolors，tones，harmonies，andrhythms。Still，theyarenotabsolutelyclosedsystems，butthereareeverywhererelationseitheroflikenessorofoppositiontoothersystems。
Thus，suchfeelingsasthosefromsensationsofmoderatewarmth，fromtonalharmony，andfromsatisfiedexpectation，howevergreattheirqualitativedifferencesmaybe，areallrelatedinthattheybelongtothegeneralclassof“pleasurablefeelings“。Stillcloserrelations[p。36]existbetweencertainsingleaffectivesystems，as，forexample，betweentonalfeelingsandcolor-feelings，wheredeeptonesseemtoberelatedtodarkcolors，andbrightcolorstohightones。Wheninsuchcasesacertainrelationshipisascribedtothesensationsthemselves，itisprobablydueentirelytoaconfusionoftheaccompanyingfeelingswiththesensations。
Thisthirddistinguishingcharacteristicshowsconclusivelythattheoriginofthefeelingsismoreunitarythanthatofthesensations，whichdependonanumberofdifferentandinpartdistinguishableconditions。
Itisthesamedistinctionthatwefindinthecharacterizationofthesubject，whichstandsinimmediaterelationtothefeelings，asaunit，incontrastwiththepluralityoftheobjects，towhichthesensationsarerelated。
6a。Itisonlyinmodernpsychologythattheterms“sensation“and“feeling“
havegainedthemeaningsassignedtotheminthedefinitionsabovegiven。
Inolderpsychologicalliteraturetheyweresometimesusedindiscriminatingly，sometimesinterchanged。Evenyetsensationsoftouchandthosefromtheinternalorgansarecalledfeelingsbyphysiologists，andthesenseoftouchitselfisknownasthe“senseoffeeling“。Thiscorresponds，itistrue，totheoriginalsignificanceoftheword，wherefeelingisthesameastouching，still，aftertheveryusefuldifferentiationhasoncebeenmade，aconfusionofthetwotermsshouldbeavoided。Thenagain，theword“sensation“isusedevenbypsychologiststomeannotonlysimple，butalsocompositequalities，suchascompoundclangsandspacialandtemporalideas。Butsincewehavetheentirelyadequateword“idea“forsuchcompounds，itismoreadvantageoustolimitthewordsensationtopsychologicallysimplesense-qualities。Finally，theterm“sensation“hassometimesbeenrestrictedsoastomeanonlythosestimulationswhichcomedirectlyfromexternalsense-stimuli。Forthepsychologicalattributesofasensation，however，thiscircumstanceisentirelyirrelevant，andthereforesuchadefinitionofthetermisunjustifiable。[p。37]Thediscriminationbetweensensationalandaffectiveelementsinanyconcretecaseisverymuchfacilitatedbytheexistenceofindifference-zonesinthefeelings。Thenagain，fromthefactthatfeelingsrangebetweenoppositesratherthanmeredifferences，itfollowsthattheyaremuchthemorevariableelementsofourimmediateexperience。Thischangeablecharacter，whichrendersitalmostimpossibletoholdanaffectivestateconstantinqualityandintensity，isthecauseofthegreatdifficultiesthatstandinthewayoftheexactinvestigationoffeelings。
Sensationsarepresentinallimmediateexperiences，butfeelingsmaydisappearincertainspecialcases，becauseoftheiroscillationthroughanindifference-zone。Obviously，then，wecall，inthecaseofsensations，abstractfromtheaccompanyingfeelings，butneverviceversa。Inthiswaytwofalseviewsmayeasilyarise，eitherthatsensationsarethecausesoffeelings，orthatfeelingsareaparticularspeciesofsensations。
Thefirstoftheseopinionsisfalsebecauseaffectiveelementscanneverbederivedfromsensationsassuch，butonlyfromtheattitudeofthesubject，sothatunderdifferentsubjectiveconditionsthesamesensationmaybeaccompaniedbydifferentfeelings。Thesecondisuntenablebecausethetwoclassesofelementsaredistinguished，ontheonehandbytheimmediaterelationofsensationstoobjectsandoffeelingstothesubject，andontheotherbythefactthattheformerrangebetweenmaximaldifferences，thelatterbetweenmaximalopposites。Becauseoftheobjectiveandsubjectivefactorsbelongingtoallpsychicalexperience，sensationsandfeelingsaretobelookeduponasrealandequallyessential，thougheverywhereinterrelated，elementsofpsychicalphenomena。Inthisinterrelationthesensationalelementsappearasthemoreconstant；theyalonecanbeisolatedthroughabstraction，byreferringthemtoexternalobjects。Itfollows，therefore，ofnecessitythatininvestigatingtheattributesofboth，wemuststartwiththesensations。Simplesensations，intheconsiderationofwhichweabstractfromtheaccompanyingaffectiveelements，arecalledpuresensations。Obviously，wecanneverspeakof“purefeelings“
inasimilarsense，sincesimplefeelingscanneverbethoughtofapartfromtheaccompanyingsensationsandcombinationsofsensations。Thisfactisdirectlyconnectedwiththeseconddistinguishingcharacteristicmentionedabove(p。34sq)。
ClassicsintheHistoryofPsychology——Wundt(1897)Section6OutlinesofPsychologyWilhelmMaxWundt(1897)TranslatedbyCharlesHubbardJudd(1897)
I。PSYCHICALELEMENTS§；6。PURESENSATIONS
1。Theconcept“puresensation“asshownin§；5istheproductofatwofoldabstraction:1)fromtheideasinwhichthesensationappears，and2)fromthesimplefeelingswithwhichitisunited。Wefindthatpuresensations，definedinthisway，formanumberofdisparatesystemsofquality；eachofthesesystems，suchasthatofsensationsofpressure，oftone，oroflight，iseitherahomogeneousoracomplexcontinuity(§；
5，5)fromwhichnotransitiontoanyothersystemcanbefound。
2。Theriseofsensations，asphysiologyteachesus，isregularlydependentoncertainphysicalprocessesthathavetheiroriginpartlyintheexternalworldsurroundingus，partlyincertainbodilyorgans。
Wedesignatetheseprocesseswithanameborrowedfromphysiologyassense-stimuliorsensation-stimuli。Ifthestimulusisaprocessintheouterworldwecallitphysical；ifitisaprocessinourownbodywecallitphysiological。Physiologicalstimulimaybedivided，inturn，intoperipheralandcentral，accordingastheyareprocessesinthevariousbodilyorgansoutsideofthebrain，orprocessesinthebrainitself。Inmanycasesasensationisattendedbyallthreeformsofstimuli。Thus，toillustrate，anexternalimpressionoflightactsasaphysical，stimulusontheeye；intheeyeandopticnervetherearisesaperipheralphysiologicalstimulation；finallyacentralphysiologicalstimulationtakesplaceinthecorporaquadrigeminaandintheoccipitalregionsofthecerebralcortex，wheretheopticnerveterminates。Inmanycasesthephysicalstimulusmaybewanting，whilebothformsofphysiologicalstimuliarepresent；as，whenweperceiveaflashoflightinconsequenceofaviolentocularmovement。Instillothercasesthecentralstimulusaloneispresent；as，whenwerecallalight[p。39]impressionpreviouslyexperienced。Thecentralstimulusis，accordingly，theonlyonethatalwaysaccompaniessensation。Whenaperipheralstimuluscausesasensation，itmustbeconnectedwithacentralstimulus，andaphysicalmustbeconnectedwithbothaperipheralandacentralstimulus。
3。Thephysiologicalstudyofdevelopmentrendersitprobablethatthedifferentiationofthevarioussensationalsystemshasbeeneffectedinpartinthecourseofgeneraldevelopment。Theoriginalorganofsenseistheouterskinwiththesensitiveinnerorgansadjoiningit。Theorgansoftaste，smell，hearing，andsight，ontheotherhand，arelaterdifferentiationsofit。Itmay，therefore，besurmisedthatthesensationalsystemscorrespondingtothesespecialsense-organs，havealsograduallyarisenthroughdifferentiationfromthesensationalsystemsofthegeneralsense，fromsensationsofpressure，hot，andcold。Itispossible，too，thatinloweranimalssomeofthesystemsnowsowidelydifferentiatedareevenyetmorealike。Fromaphysiologicalstandpointtheprimordealcharacterofthegeneralsenseisalsoapparentinthefactthatithasforthetransferofsense-stimulitothenerveseitherverysimpleorgansornoneatall。Pressure，temperature，andpain-stimulicanproducesensationsatpointsintheskinwhere，inspiteofthemostcarefulinvestigation，nospecialend-organscanbefound。Thereare，indeed，specialreceivingorgansintheregionsmostsensitivetopressure(touch-corpuscles，end-bulbs，andcorpusclesofVater)，buttheirstructurerendersitprobablethattheymerelyfavorthemechanicaltransferofthestimulustothenerve-endings。
Specialend-organsforhot，cold，andpain-stimulihavenotbeenfoundatall。
Inthelaterdevelopedspecialsense-organs，ontheotherhand，wefindeverywherestructureswhichnotonlyeffectthesuitabletransferofthestimulitothesensorynerves，butgenerallybringaboutaphysiologicaltransformationofthe[p。40]stimulationwhichisindispensablefortheriseofthepeculiarsensationalqualities。Butevenamongthespecialsensestherearedifferencesinthisrespect。
Thereceivingorganintheear，inparticular，appearstobeofacharacterdifferentfromthatoftheorgansofsmell，taste，andsight。Initsmostprimitiveformsitconsistsofavesiclefilledwithoneormoresolidparticles(otoliths)，andsuppliedwithnerve-bundlesdistributedinitswalls。Theparticlesaresetinmotionthroughsound-vibrations，andmustcausearapidsuccessionofweakpressure-stimulationsinthefibresofthenerve-bundles。Theauditoryorganofthehigheranimalsshowsanextraordinarycomplexity，still，initsessentialstructureitrecallsthisprimitivetype。Inthecochleaofmanandthehigheranimalstheauditorynervepassesatfirstthroughtheaxis，whichispiercedbyalargenumberoffinecanals，andthenemergesthroughtheporeswhichopenintothecavityofthecochlea。
Herethebranchesaredistributedonatightlystretchedmembrane，whichextendsthroughthespiralwindingsofthecochleaandisweightedwithspecialrigidarches(archesofCorti)。Thismembrane-thebasilarmembrane，asitiscalled-must，accordingtothelawsofacoustics，bethrownintosympatheticvibrationswheneversound-wavesstriketheear。Itseems，therefore，toplaythesameparthereastheotolithsdointhelowerformsoftheauditoryorgan。Atthesametimeoneotherchangehastakenplacewhichaccountsfortheenormousdifferentiationofthesensationalsystem。Thebasilarmembranehasadifferentbreadthinitsdifferentparts，foritgrowscontinuallywiderfromthebasetotheapexofthecochlea。Inthiswayitactslikeasystemofstretchedchordsofdifferentlengths。Andjustasinsuchasystem，otherconditionsremainingthesame，thelongerchordsaretunedtolowerandtheshortertohighertones，sowemayassumethesameto[p。41]betrueforthedifferentpartsofthebasilarmembrane。
Wemaysurmisethatthesimplestauditoryorganswiththeirotolithshaveahomogeneoussensationalsystem，analogousperhapstooursystemsofsensationsofpressure。Thespecialdevelopmentoftheorganasseeninthecochleaofhigheranimalsexplainstheevolutionofanextraordinarilycomplexsensationalsystemfromthisoriginallyhomogeneoussystem。Still，thestructureremainssimilarinthisrespect，thatitseemsadapted，inthelattercaseasintheformer，tothebestpossibletransferofthephysicalstimulustothesensorynerveratherthantoanytransformationofthestimulus。Thisviewagreeswiththeobservedfactthat，justassensationsofpressuremaybeperceived，onregionsoftheskinnotsuppliedwithspecialreceivingorgans，so，inthecaseofcertainanimals，suchasbirds，wheretheconditionsarespeciallyfavorablefortheirtransmission，sound-vibrationsaretransferredtotheauditorynerveandsensedevenaftertheremovalofthewholeauditoryorganwithitsspecialreceivingstructure。
Withsmell，taste，andsightthecaseisessentiallydifferent。
Organsarepresentwhichrenderdirectactionofthestimulionthesensorynervesimpossible。Theexternalstimuliareherereceivedthroughspecialorgansandmodifiedbeforetheyexcitethenerves。Theseorgansarespeciallymetamorphosedepithelialcellswithoneendexposedtothestimulusandtheotherpassingintoanervefibre。Everythinggoestoshowthatthereceivingorgansherearenotmerelyforthetransferofthestimuli，butratherfortheirtransformation。Inthethreecasesunderdiscussionitisprobablethatthetransformationisachemicalprocess。Insmellandtastewehaveexternalchemicalagencies，insightwehavelightasthecausesofchemicaldisintegrationsinthesensorycells；theseprocessesinthecellsthenserveastherealstimuli。[p。42]
Thesethreesensesmay，aschemicalsenses，bedistinguishedfromthemechanicalsensesofpressureandsound。Itisimpossibletosaywithanydegreeofcertainty，towhichofthesetwoclassessensationsofcoldandhotbelong。Oneindicationofthedirectrelationbetweenstimuliandsensationinmechanicalsenses，ascontrastedwiththeindirectrelationinchemicalsenses，isthatinthefirstcasethesensationlastsonlyaverylittlelongerthantheexternalstimulus，whileinthelattercaseitpersistsverymuchlonger。Thus，inaquicksuccessionofpressuresandmoreespeciallyofsounds，itispossibletodistinguishclearlythesinglestimulifromoneanother；lights，tastes，andsmells，ontheotherhand，runtogetherataverymoderaterateofsuccession。
4。Sinceperipheralandcentralstimuliareregularphysicalconcomitantsofelementarysensationalprocesses，theattempttodeterminetherelationbetweenstimuliandsensationsisverynatural。Inattemptingtosolvethisproblem，physiologygenerallyconsiderssensationsastheresultofphysiologicalstimuli，butassumesatthesametimethatinthiscaseanyproperexplanationoftheeffectfromitscauseisimpossible，andthatallthatcanbeundertakenistodeterminetheconstancyoftherelationsbetweenparticularstimuli，andtheresultingsensations。Now，itisfoundinmanycasesthatdifferentstimuliactingonthesameend-organproducethesamesensations；thus，forexample，mechanicalandelectricalstimulationsoftheeyeproducelightsensations。Thisresultwasgeneralizedintheprinciple，thateveryreceivingelementofasense-organandeverysimplesensorynerve-fibretogetherwithitscentralterminus，iscapableofonlyasinglesensationoffixedquality；thatthevariousqualitiesofsensationare，therefore，duetothevariousphysiologicalelementswithdifferentspecificenergies。
Thisprinciple，generallycalledthe“lawofspecificenergy[p。43]
ofnerves“，isuntenableforthreereasons，evenifweneglectforthemomentthefactthatitsimplyrefersthecausesofthevariousdifferencesinsensationstoaqualtalitasoccutltaofsensoryandnervouselements。
1)Itiscontradictorytothephysiologicaldoctrineofthedevelopmentofthesenses。If，aswemustassumeaccordingtothisdoctrine，thecomplexsensationalsystemsarederivedfromsystemsoriginallysimplerandmorehomogeneous，thephysiologicalsensoryelementsmusthaveundergoneachangealso。This，however，ispossibleonlyundertheconditionthatorgansmaybemodifiedbythestimuliwhichactuponthem。Thatistosay，thesensoryelementsdeterminethequalitiesofsensationsonlysecondarily，asaresultofthepropertieswhichtheyacquirethroughtheprocessesofstimulationarousedinthem。If，then，thesesensoryelementshaveundergone，inthecourseoftime，radicalchangesduetothenatureofthestimuliactinguponthem，suchchangescouldhavebeenpossibleonlyundertheconditionthatthephysiologicalstimulationsinthesensoryelementsvariedtosomeextentwiththequalityofthestimulus。
2)Theprincipleofspecificenergyiscontradictorytothefactthatinmanysensesthenumberofdifferentsensoryelementsdoesnotcorrespondatalltothatofdifferentsensationalqualities。Thus，fromasinglepointintheretinawecanreceiveallpossiblesensationsofbrightnessandcolor；intheorgansofsmellandtastewefindnoclearlydistinguishableformsofthesensoryelements，whileevenalimitedareaoftheirsensorysurfacescanreceiveavarietyofsensations，which，especiallyinthecaseoftheolfactoryorgan，isverylarge。Wherewehaveeveryreasontoassumethatqualitativelydifferentsensationsactuallydoariseindifferentsensoryelements，asinthecaseoftheauditoryorgan，thestructureoftheorgangoestoshowthatthisdifference[p。44]isnotduetoanyattributeofthenerve-fibresorofothersensoryelements，butthatitcomesoriginallyfromthewayinwhichtheyarearranged。Differentfibresoftheauditorynervewill，ofcourse，bestimulatedbydifferenttone-vibrations，becausethedifferentpartsofthebasilarmembranearetunedtodifferenttones；butthisisnotduetosomeoriginalandinexplicableattributeofthesingleauditorynerve-fibres，buttothewayinwhichtheyareconnectedwiththeend-organ。
3)Finally，thesensorynervesandcentralelementscanhavenooriginalspecificenergy，becausetheperipheralsense-organmustbeexposedtotheadequatestimuliforasufficientinterval，ormustatleasthavebeensoexposedatsomepreviousperiod，beforethecorrespondingsensationscanarisethroughtheirstimulation。Personscongenitallyblindanddeafdonothaveanysensationsoflightortonewhatever，sofarasweknow，evenwhenthesensorynervesandcentreswereoriginallypresent。
Everythinggoestoshowthatthedifferencesinthequalitiesofsensationsareconditionedbythedifferencesintheprocessesofstimulationthatariseinthesense-organs。Theseprocessesaredependent，primarilyonthecharacterofthephysicalstimuli，andonlysecondarilyonthepeculiaritiesofthereceivingorgan，whichareduetoitsadaptationtothesestimuli。Asaresultofthisadaptation，however，itmayhappenthatevenwhensomestimulusotherthanthatwhichhaseffectedtheoriginaladaptationofthesensoryelements，thatis，whenaninadequatestimulusacts，thesensationcorrespondingtotheadequatestimulusmayarise。Still，thisdoesnotholdforallstimuliorforallsensoryelements。Thus，hotandcoldstimulationscannotcausecutaneoussensationsofpressureorsensationsinthespecialsense-organs；chemicalandelectricalstimuliproducesensationsoflightonlywhentheyactupontheretina，notwhentheyacton[p。45]theopticnerve；and，finally，thesegeneralstimulicannotarousesensationsofsmellortaste。Whenanelectriccurrentcauseschemicaldisintegration，itmay，indeed，arousesuchsensations，butitisthroughtheadequatechemicalstimuliproduced。
5。Fromtheverynatureofthecase，itisimpossibletoexplainthecharacterofsensationsfromthecharacterofphysicalandphysiologicalstimuli。Stimuliandsensationscannotbecomparedwithoneanotheratall；thefirstbelongtothemediateexperienceofthenaturalsciences，thesecondtotheimmediateexperienceofpsychology。Aninterrelationbetweensensationsandphysiologicalstimulimustnecessarilyexist，however，inthesensethatdifferentkindsofstimulationalwayscorrespondtodifferentsensations。Thisprincipleoftheparallelismofchangesinsensationandinphysiologicalstimulationisanimportantsupplementaryprincipleinboththepsychologicalandphysiologicaldoctrinesofsensation。Inthefirstcaseitisusedinproducingdefinitechangesinthesensationbymeansofintentionalvariationofthestimulus；intheseconditisusedininferringtheidentityornon-identityofphysiologicalstimulationsfromtheidentityornon-identityofthesensations。Furthermore，thesameprincipleisthebasisofourpracticallifeandofourtheoreticalknowledgeoftheexternalworld。A。SENSATIONSOFTHEGENERALSENSE。
6。Thedefinitionofthe“generalsense“includestwofactors。
Inpointoftime，thegeneralsenseisthatwhichprecedesallothersandthereforebelongstoallbeings，endowedwithmind。Initsspacialattributes，thegeneralsenseisdistinguishedfromtheparticularsensesinhavingthemostextensivesensorysurfaceexposedtostimuli。Itincludesnotonlythewholeexternalskinandtheadjoiningareasof[p。46]themucousmembrane，butalargenumberofinternalorganssuppliedwithsensorynerves，suchasjoints，muscles，tendons，andbones，whichareaccessibletostimulieitheralways，oratcertaintimes，underspecialconditions，asisthecasewithbones。Thegeneralsenseincludesfourspecific，distinctsensationalsystems:sensationsofpressure，hot，cold，andpain。Notinfrequentlyasinglestimulusarousesmorethanone，ofthesesensations。Thesensationisthenimmediatelyrecognizedasmadeupofamixtureofcomponentsfromthedifferentsystems；forexample，fromsensationsofpressureandpain，orfromsensationsofhotandpain。Inasimilarmannerasaresultoftheextensionofthesense-organ，wemayoftenhavemixturesofthevariousqualitiesofoneandthesamesystem，forexample，qualitativelydifferentsensationsofpressure，whenanextendedregionoftheskinistouched。
Thefoursystemsofthegeneralsenseareallhomogenoussystems(§；5，5)。Thisshowsthatthesenseisgeneticallyearlierthantheothers，whosesystemsareallcomplex。Thesensationsofpressurefromtheexternalskin，andthoseduetothetensionsandmovementsofthemuscles，joints，andtendons，aregenerallygroupedtogetherunderthenametouch-sensations，anddistinguishedfromthecommonsensations，whichincludesensationsofhot，cold，andpain，andthosesensationsofpressurethatsometimesariseintheotherinternalorgans。
Thisdistinction，however，hasitssourceintherelationofthesensationstoideasandconcomitantfeelings，andhasnothingtodowiththequalitiesofthesensationsinthemselves。
7。Theabilityofthedifferentpartsofthegeneralsense-organtoreceivestimulationsandgiverisetosensations，canbetestedwithadequateexactnessonlyontheexternalskin。Theonlyfactsthatcanbedeterminedinregardto[p。47]theinternalparts，arethatthejointsareinahighdegreesensitivetopressures，whilethemusclesandtendonsaremuchlessso，andthatsensationsofhot，cold，andpainintheinternalorgansareexceptional，andnoticeableonlyunderabnormalconditions。
Ontheotherhand，thereisnopointoftheexternalskinandoftheimmediatelyadjoiningpartsofthemucousmembrane，whichisnotsensitiveatoncetostimulationsofpressure，hot，cold，andpain。Thedegreeofsensitivitymay，indeed，varyatdifferentpoints，insuchawaythatthepointsmostsensitivetopressure，tohot，andtocold，donot，ingenerally，coincide。Sensitivitytopainiseverywhereaboutthesame，varyingatmostinsuchawaythatinsomeplacesthepain-stimulusactsonthesurface，andinothersnotuntilithaspenetrateddeeper。Ontheotherhand，certainapproximatelypunctiformcutaneousregionsappeartobemostfavorableforstimulationsofpressure，hot，andcold。Thesepointsarecalledrespectively，pressure-spots，hot-spots，andcold-spots。Theyaredistributedindifferentpartsoftheskininvaryingnumbers。Spotsofdifferentmodalitynevercoincide；still，temperature-spotsalwaysreceivesensationsofpressureandpainaswell；andapointedhotstimulusappliedtoacoldspot，alwayscausesasensationofhot，whilehot-spotsdonotseemtobestimulatedbypointedcoldstimuli。Furthermore，hot-spotsandcold-spotsreactwiththeiradequatesensationstoproperlyappliedmechanicalandelectricalstimuli。
8。Ofthefourqualitiesmentionedsensationsofpressureandpainformclosedsystemswhichshownorelationseithertoeachotherortothetwosystemsoftemperature-sensations。Theselasttwo，ontheotherhand，standintherelationofopposites；weapprehendhotandcoldnotmerelyasdifferent，butascontrastedsensations。Itis，however，veryprobablethatthisisnotduetotheoriginalnatureofthesensations，[p。48]butpartlytotheconditionsoftheirrise，andpartlytotheaccompanyingfeelings。For，whiletheotherqualitiesmaybeunitedwithoutlimitationtoformmixedsensations——as，forexample，pressureandhot，pressureandpain，coldandpain-hotandcoldexcludeeachotherbecause，undertheconditionsoftheirrise，theonlypossibilitiesforagivencutaneousregionareasensationofhotoroneofcold，orelseanabsenceofboth。
Whenoneofthesesensationspassescontinuouslyintotheother，thechangeregularlytakesplaceinsuchawaythateitherthesensationofhotgraduallydisappearsandacontinuallyincreasingsensationofcoldarises，orviceversathesensationofcolddisappearsandthatofhotgraduallyarises。
Then，too，elementaryfeelingsofoppositecharacterareconnectedwithhotandcold，thepointwherebothsensationsareabsentcorrespondingtotheirindifferentzone。
Instillanotherrespectthetwosystemsoftemperature-sensationsarepeculiar。Theyaretoagreatextentdependentonthevaryingconditionsunderwhichthestimuliactuponthesense-organ。Aconsiderableincreaseabovethetemperatureoftheskinisperceivedashot，whileaconsiderabledecreasebelowthesameisperceivedascold，butthetemperatureoftheskinitself，whichistheindifferencezonebetweenthetwo，canadaptitselfrapidlytotheexisting，externaltemperaturewithinfairlywidelimits。Thefactthatinthisrespecttoo，bothsystemsarealike，favorstheviewthattheyareinterconnectedandalsoantagonistic。B。SENSATIONSOFSOUND。
9。Wepossesstwoindependentsystemsofsimpleauditorysensations，whicharegenerally，however，connectedasaresultofthemixtureofthetwokindsofimpressions。Theyare[p。49]thehomogeneoussystemofsimplenoise-sensationsandthecomplexsystemofsimpletone-sensations。
Simplenoise-sensationscanbeproducedonlyunderconditionsthatexcludethesimultaneousriseoftonalsensations，aswhenair-vibrationsareproducedwhoserateiseithertoorapidortooslowfortone-sensationstoarise，orwhenthesound-wavesactupontheearfortooshortaperiod。
Simplesensationsofnoise，thusproduced，mayvaryinintensityandduration，butapartfromthesedifferencestheyarequalitativelyalike。Itispossiblethatsmallqualitativedifferencesalsoexistamongthem，duetotheconditionsoftheirrise，butsuchdifferencesaretoosmalltobemarkedbydistinguishingnames。Thenoisescommonlysocalledarecompoundideasmadeupofsuchsimplenoise-sensationsandofagreatmanyirregulartonalsensations(cf。§；9，7)。Thehomogeneoussystemofsimplenoise-sensationsisprobablythefirsttodevelop。Theauditoryvesiclesoftheloweranimals，withtheirsimpleotoliths，couldhardlyproduceanythingbutthese。Inthecaseofmanandthehigheranimalsitmaybesurmisedthatthestructuresfoundinthevestibuleofthelabyrinthreceiveonlyhomogeneousstimulations，correspondingtosimplesensationsofnoise。Finally，experimentswithanimalsdeprivedoftheirlabyrinths，makeitprobablethatevendirectstimulationsoftheauditorynervecanproducesuchsensations(p。41)。
Intheembryonicdevelopmentofthehigheranimals，thecochleadevelopsfromanoriginal，vestibularvesicle，whichcorrespondsexactlytoaprimitiveauditoryorgan。Weare，therefore，justifiedinsupposingthatthecomplexsystemoftonalsensationsisaproductofthedifferentiationofthehomogeneoussystemofsimplenoise-sensations，butthatineverycasewherethisdevelopment，hastakenplace，thesimplesystemhasremainedalongwiththehigher。
[p。50]
10。Thesystemofsimpletone-sensationisacontinuityofonedimension。Wecallthequalityofthesinglesimpletonespitch。Theone-dimensionalcharacterofthesystemfindsexpressioninthefactthat，startingwithagivenpitch，wecanvarythequalityonlyintwooppositedirections:onewecallraisingthepitch，theotherloweringit。Inactualexperiencesimplesensationsoftoneareneverpresentedalone，butalwaysunitedwithothertonalsensationsandwithaccompanyingsimplesensationsofnoise。Butsince，accordingtotheschemegivenabove(§；5，1)，theseconcomitantelementscanbevariedindefinitely，andsinceinmanycasestheyarerelativelyweakincomparisonwithoneofthetones，theabstractionofsimpletoneswasearlyreachedthroughthepracticaluseoftonalsensationsintheartofmusic。Thenamesc，c#，d#，anddstandforsimpletones，thoughtheclangsofmusicalinstrumentsorofthehumanvoicebymeansofwhichweproducethesedifferentpitches，arealwaysaccompaniedbyother，weakertonesandoften，too，bynoises。Butsincetheconditionsfortheriseofsuchconcomitanttonescanbesovariedthattheybecomeveryweak，ithasbeenpossibletoproducereallysimpletonesofnearlyperfectpurity。
Thesimplestmeansofdoingthisisbyusingatuning-fork，andaresonatortunedtoitsfundamentaltone。Sincetheresonatorincreasestheintensityofthefundamentalonly，theother，accompanyingtonesaresoweakwhentheforksounds，thatthesensationisgenerallyapprehendedassimpleandirreducible。Ifthesound-vibrationscorrespondingtosuchatonalsensationareexamined，theywillbefoundtocorrespondtothesimplestpossibleformofvibration，thependulumoscillation，socalledbecausethevibrationsoftheatmosphericparticlesfollowthesamelawsasapendulumoscillatingin[p。51]averysmallamplitude。[1]Thattheserelativelysimplesound-vibrationscorrespondtosensationsofsimpletones，andthatwecanevendistinguishtheseparatetonesincompounds，canbeexplained，onthebasisofthephysicallawsofsympatheticvibration，fromthestructureoftheorgansinthecochlea。Thebasilarmembraneinthecochleaisinitsdifferentpartstunedtotonesofdifferentpitch，sothatwhenasimpleoscillatorysound-vibrationstrikestheear，onlytheparttunedtothatparticularpitchwillvibrateinsympathy。Ifthesamerateofoscillationcomesinacompoundsound-vibration，againonlytheparttunedtoitwillbeaffectedbyit，whiletheothercomponentsofthewavewillsetinvibrationothersectionsofthemembrane，whichcorrespondinthesamewaytotheirpitch。
11。Thesystemoftonalsensationsshowsitscharacterasacontinuousseriesinthefactthatitisalwayspossibletopassfromagivenpitchtoanyotherthroughcontinuouschangesinsensation。Musichasselectedatoptionfromthiscontinuitysinglesensationsseparatedbyconsiderableintervals，thussubstitutingatonalscaleforthetonalline。Thisselection，however，isbasedontherelationsoftonalsensationsthemselves。Weshallreturntothediscussionoftheserelationslater，intakinguptheideationalcompoundsarisingfromthesesensations(§；
9)。Thenaturaltonallinehastwoextremities，whichareconditionedbythephysiologicalcapacityoftheearforreceivingsounds。Theseextremitiesarethelowestandhighesttones；theformercorrespondsto8-10doublevibrationspersecond，thelatterto40，000-50，000。[p。52]C。SENSATIONSOFSMELLANDTASTE。
12。Sensationsofsmellformacomplexsystemwhosearrangementisstillunknown。Allweknowisthatthereisaverygreatnumberofolfactoryqualities，betweenwhichthereareallpossibletransitionalforms。Therecan，then，benodoubtthatthesystemisacontinuityofmanydimensions。
12a。Olfactoryqualitiesmaybegroupedincertainclasses，eachofwhichcontainsclassessensationswhicharemoreorlessrelated。
Thisfactmayberegardedasanindicationofhowthesesensationsmayperhapsbereducedtoasmallnumberofprincipalqualities。Suchclassesareforexamples，sensationslikethosefromether，balsam，musk，benzine，thoseknownasaromatic，etc。
Ithasbeenobservedinafewcasesthatcertainolfactorysensationswhichcomefromdefinitesubstances，canalsobeproducedbymixingothers。
Buttheseobservationsarestillinsufficienttoreducethegreatnumberofsimple，qualitiescontainedineachoftheclassesmentioned，toalimitednumberofprimaryqualitiesandtheirmixtures。Finally，ithasbeenobservedthatmanyodorsneutralizeeachother，sofarasthesensationisconcerned，whentheyaremixedintheproperintensities。Thisistruenotonlyofsubstancesthatneutralizeeachotherchemically，asaceticacidandammonia，butalsoofothers，suchascaoutchoueandwaxortolu-balsam，whichdonotactoneachotherchemicallyoutsideoftheolfactorycells。Sincethisneutralizationtakesplacewhenthetwostimuliactonentirelydiffererentolfactorysurfaces，oneontherightandtheotherontheleftmucousmembraneofthenose，itisprobablethatwearedealing，notwithphenomenaanalogoustothoseexhibitedbycomplementarycolors(22)，butwithareciprocalcentralinhibitionofsensations。Anotherobservedfacttellsagainstthenotionthattheyarecomplementary。Oneandthesameolfactoryqualitycanneutralizeseveralentirelydifferentqualities，sometimeseventhosewhichinturnneutralizeoneanother，whileamongcolorsitisalwaysonlytwofixedqualitiesthatarecomplementary。[p。53]
13。Sensationsoftastehavebeensomewhatmorethoroughlyinvestigated，andwecanheredistinguishfour，distinctprimaryqualities。Betweenthesethere，areallpossibletransitionaltastes，whicharetoberegardedismixedsensations。Theprimaryqualitiesaresour，sweet，bitter，andsaline。Besidesthese，alkalineandmetallicaresometimesregardedasindependentqualities。Butalkalinequalitiesshowanunmistakeablerelationshipwithsaline，andmetallicwithsour，sothatbothareprobablymixedsensations，(alkalinemadeupperhapsofsalineandsweet，metallicofsourandsaline)。Sweetandsalineareoppositequalities。Whenthesetwosensationsareunitedinproperintensifies，theresultisamixedsensation(commonlyknownas“insipid“)，eventhoughthestimulithatherereciprocallyneutalizeeachotherdonotenterintoachemicalcombination。Thesystemoftaste-sensationsis，accordingly，inallprobabilitytoberegardedasatwo-dimensionalcontinuity，whichmaybegeometricallyrepresentedbyacircularsurfaceonwhosecircumference，thefourprimary，andtheirintermediate，qualitiesarearranged，whiletheneutralmixedsensationisinthemiddle，andtheothertransitionaltaste-qualitiesonthesurface，betweenthismiddlepointandthesaturatedqualitiesonthecircumference。
13a。Intheseattributesoftaste-qualitiesweseemtohavethefundamentaltypeofachemicalsense。Inthisrespecttasteisperhapstheantecedentofsight。Theobviousinterconnectionwiththechemicalnatureofthestimulation，makesitprobableevenherethatthereciprocalneutralizationofcertainsensations，withwhichthetwo-dimensionalcharacterofthesensationalsystemisperhapsconnected，depends，notonthesensationsinthemselves，butontherelationsbetweenthephysiologicalstimulations，justasinthecaseofsensationsofhotandcold(p。48)。Itiswellknownthatverycommonlythechemicaleffectofcertainsubstancescanbeneutralizedthroughtheactionofcertainothersubstances。Now，wedonotknowwhatthechemical[p。54]changesarethatareproducedbythegustatorystimuliinthetaste-cells。Butfromtheneutralizationofsensationsofsweetandsalinewewayconclude，inaccordancewiththeprincipleoftheparallelismofchangesinsensationandinstimuli(p。45)，thatthechemicalreactionswhichsweetandsalinesubstancesproduceinthesensorycells，alsocounteracteachother。Thesamewouldholdfortheirsensationsforwhichsimilarrelationscouldbedemonstrated。Inregardtothephysiologicalconditionsforgustatorystimulations，wecandrawonlythisoneconclusionfromthefactsmentioned，namely，thatthechemicalprocessesofstimulationcorrespondingtothesensationswhichneutralizeeachotherinthisway，probablytakeplaceinthesamecells。Ofcourse，thepossibilityisnotexcludedthatseveraldifferentprocessesliabletoneutralizationthroughoppositereactions，couldariseinthesamecells。Theknownanatomicalfactsandtheexperimentsofphysiologyinstimulatingsinglepapillaeseparately，giveliecertainconclusionsinthismatter。Whetherweareheredealingwithphenomenathatarereallyanalogoustothoseexhibitedbycomplementarycolors(v。
inf。22)isstillaquestion。D。SENSATIONSOFLIGHT。
14。Thesystemoflight-sensationsismadeupoftwopartialsystems:thatofsensationofachromaticlightandthatofsensationsofchromaticlight。Betweenthequalitiesinthesetwo，allpossibletransitionalformsexist。
Sensationsofachromaticlight，whenconsideredalone，formacomplexsystemofonedimension，whichextends，likethetonalline，betweentwolimitingqualities。Thesensationsintheneighborhoodofoneoftheselimitswecallblack；intheneighborhoodoftheotherwhite，whilebetweenthetwoweinsertgreyinitsdifferentshades(darkgrey，grey，andlightgrey)。Thisone-dimensionalsystemofachromaticsensationsdiffersfromthatoftonesinbeingatonceasystemofqualityandofintensityforeveryqualitativechangeinthedirectionfromblacktowhiteisseenatthe[p。55]sametimeasanincreaseinintensity，aneveryqualitativechangeinthedirectionfromwhitetoblackisseenasadecreaseintensity。Eachpointintheseries，whichthushasadefinitequalityandintensity，iscalledadegreeofbrightnessoftheachromaticsensations。Thewholesystemmay，accordingly，bedesignatedasthesensationsofpurebrightness。Theuseoftheword“pure“indicatestheabsenceofallsensationsofcolor。Thesystemofpurebrightnessisabsolutelyone-dimensionalfor，boththevariationsinqualityandthoseinintensitybelongtooneandthesamedimension。Itdiffersessentially，inthisrespect，fromthetonalline，inwhicheachpointismerelyadegreeofquality，andhasalsoawholeseriesofgradationsinintensity。Simpletone-sensationsthusformatwo-dimensionalcontinuitysosoonaswetakeintoaccountbothdeterminants，qualityandintensity，whilethesystemofpurebrightnessisalwaysone-dimensional，evenwhenweattendtobothdeterminants。
Thewholesystemmay，therefore，beregardedasacontinuousseriesofgradesofbrightness，inwhichthelowergradesaredesignatedblacksofarasqualityisconcerned，andweakinpointofintensity，whilethehighergradesarecalledwhiteandstrong。
15。Sensationsofcoloralsoformaone-dimensionalsystemwhentheirqualitiesalonearetakenintoaccount。Unlikethesystemofsensationsofpurebrightness，thissystemreturnsuponitselffromwhateverpointwestart，foratfirst，afterleavingagivenquality，wepassgraduallytoaqualitythatshowsthegreatestdifference，andgoingstillfurtherwefindthatthequalitativedifferencesbecomesmalleragain，untilfinallywereachthestartingpointoncemore。Thecolor-spectrumobtainedbyrefractingsunlightthroughaprism，orthatseenintherainbow，showsthischaracteristic，thoughnotcompletely。Ifinthesecaseswestartfromtheredendofthespectrum，wecomefirsttoorange，thentoyellow，yellow-[p。56]green，green-blue，blue，indigo-blue，andfinallytovioletwhichismorelikeredthananyoftheothercolorsexceptorange，whichliesnexttored。
Thelineofcolorsinthespectrumdoesnotreturnquitetoitsstarting-point，becauseitdoesnotcontainallofcolorsthatwehaveinsensation。Purple-redshades，whichcanbeobtainedbytheobjectivemixtureofredandvioletrays，arewantinginthespectrum。Onlywhenwefilloutthespectralserieswiththem，isthesystemofactualcolor-sensationscomplete，andthenthesystemisaclosedcircle。Thischaracteristicisnottobeattributedtothecircumstancethatthespectrumactuallypresentsforourobservationaseriesreturningnearlytoitsbeginning。Thesameorderofsensationscanbefoundbyarrangingaccordingtotheirsubjectiverelationship，coloredobjectspresentedinanyirregularorder。Evenchildrenwhohaveneverobservedattentivelyasolarspectrumorarainbow，andcan，therefore，begintheserieswithanyothercolorjustaswellaswithred，alwaysarrangetheminthesameorder。
Thesystemofpurecolorsis，then，tobedefinedasone-dimensional。
Itdoesnotextendinastraightline，however，butreturnsuponitself。Itssimplestgeometricalrepresentationwouldbeacircle。Fromagivenpointinthissystemwepass，whenthesensationisgraduallyvaried，firsttosensations，thentothosemostmarkedlydifferent，andfinallytootherssimilartothefirstquality，butintheoppositedirection。
Everycolormust，accordingly，berelatedtooneotherparticularcolorasamaximumofdifferenceinsensation。Thiscolormaybecalledtheoppositecolor，andintherepresentationofthecolor-systembyacircle，twooppositecolorsaretobeplacedatthetwoextremitiesofthesamediameter。Thus，forexample，purple-redandgreen，yellowandblue，lightgreenandviolet，areoppositecolors，thatis，colorswhichexhibitthegreatestqualitativedifferences。[p。57]
Thequalitydeterminedbythepositionofasensationinthecolor-system，indistinctiontootherqualitativedeterminations，iscalledcolor-tone，afigurativenameborrowedfromtonalsensations。Inthissensethesimplenamesofcolors；suchasred，orange，yellow，etc。，denotemerelycolortones。Thecolor-circleisarepresentationofthesystemofcolor-tonesabstractedfromalltheotherattributesbelongingtothesensations。Inreality，everycolor-sensationhastwootherattributes，onewecallitssaturation，theotheritsbrightness。Saturationispeculiartochromaticsensations，whilebrightnessbelongstoachromaticsensationsaswell。
16。Bysaturationwemeantheattributeofcolor-sensationsbyvirtueofwhichtheyappearinallpossiblestagesoftransitiontosensationsofpurebrightness，sothatacontinuouspassageispossiblefromeverycolortoanypointintheseriesofwhites，greys，andblacks。Theterm“saturation“isborrowedfromthecommonmethodofproducingthesetransitionalcolorsobjectively，thatis，bythemoreorlessintensesaturationofsomecolorlesssolublewithcolor-pigment。Acolormaybeeversosaturated，yetitispossibletothinkofastillgreatersaturationofthesamecolor-tone，and，ontheotherhand，purebrightnessalwaysdenotestheendoftheseriesofdiminishinggradesofsaturationforanycolorwhatever。Adegreeofsaturationmay，therefore，bethoughtofasanattributeofallcolor-sensations，and，atthesametime，astheattributebywhichthesystemofcolor-sensationsisdirectlyunitedwiththatofsensationsofpurebrightness。If，now，werepresentsomeparticularsensationofwhite，grey，orblackbythecentralpointofthecolor-circle，allthegradesofsaturationthatcanariseastransitionalstagesfromanyparticularcolortothisparticularsensationofpurebrightness，willobviouslyberepresentedbythatradiusofthecirclewhichconnectsthecentrewith[p。58]thecolorinquestion。Iftheshadesofsaturationcorrespondingtothecontinuoustransitionalstage，fromallthecolorstoaparticularsensationofpurebrightnessarethusgeometricallyrepresented，wehavethesystemofsaturation-gradesasacircularsurfacewhosecircumferenceisasystemofsimplecolor-tones，andwhosecentreisthesensationofpurebrightness，correspondingtotheabsenceofallsaturation。Fortheformationofsuchasystemofsaturation-gradesanypointwhateverintheseriesofsensationsofpurebrightnessmaybetaken，solongastheconditionisfulfilledthatwhiteisnottoobrightortheblacktoodark，forinsuchdifferencesinbothsaturationandcolordisappear。Systemsofsaturationwhicharearrangedaboutdifferentpointsintheseriesofpurebrightness，alwayshavedifferentgradesofbrightness。Apuresystemofsaturation，accordingly，callbemadeforonlyoneparticulargradeofbrightnessatatime，thatis，foronlyonepointintheseriesofsensationsofpurebrightness。Whensuchsystemsaremadeforallpossiblepoints，thesystemofsaturationwillbesupplementedbythatofgradesofbrightness。
17。Brightnessisjustasanattributeofcolor-sensationasitisofachromaticsensations，andisinthiscase，too，atonceaqualityanddegreeofintensity。Startingfromagivengrade，ifthebrightnessincreases，everycolorapproacheswhite，inquality，whileatthesametimetheintensityincreases；ifthebrightnessdecreases，thecolorsapproachblackinquality，andtheintensitydiminishes。Thegradesofbrightnessforanysinglecolorthusformasystemofintensivequalities，analogoustothatofpurebrightness，onlyinplaceoftheachromaticgradationsbetweenwhiteandblack，wehavethecorrespondinggradesofsaturation。
Fromthepointofgreatestsaturationtherearetwooppositeforvariationinsaturation:onepositive，towards[p。59]white，accompaniedbyanincreaseintheintensityofthesensation，andtheothernegative，towardsblack，withacorrespondingdecreaseinintensity。Aslimitsforthesetwodirectionswehave，ontheonehand，thepuresensationwhite，ontheother，thepuresensationblack；thefirstisatthesametimethemaximum，thesecondtheminimumofintensity。Whiteandblackareinthiswayoppositeextremitiesofthesystemofsensationsofpurebrightness，andalsoofthesystemofcolor-sensationsarrangedaccordingtogradesofbrightness。Itfollowsobviouslythatthereisacertainmediumbrightnessforeverycolor，atwhichitssaturationisgreatest。Fromthispoint，thesaturationdiminishesinthepositivedirectionwhenthebrightnessincreases，andinthenegativedirectionwhenthebrightnessdecreases。
Thegradeofbrightnessmostfavorableforthesaturationisnotthesameforallcolors，butvariesfromredtoblue，insuchawaythatitismostintenseforredandleastintenseforblue。Thisaccountsforthefamiliarphenomenonthatintwilight，whenthedegreeofbrightnessissmall，thebluecolor-tones——ofpaintings，forexample——arestillclearlyvisible，whiletheredcolor-tonesappearblack。
18。Ifweneglectthesomewhatdifferentpositionofthemaximalsaturationofthevariouscolorsinthelineofbrightness，therelationthatexistsbetweensensationsofchromaticbrightnessandthoseofpure，orachromatic，brightness，byvirtueofthegradualtransitionofcolorsintowhiteontheonehand，andintoblackontheother，mayberepresentedinthesimplestmannerasfollows。First，wemayrepresentthesystemofpurecolor-tones，thatis，ofthecolorsattheirmaximalsaturation，byacircle，asabove。Thenwemaydrawthroughthecentreofthiscircle，perpendiculartoitsplane，thestraightlineofpurebrightness，insuchawaythatwhereitcutstheplaneofthecircularsurface，[p。
59]itrepresentsthesensationofpurebrightnesscorrespondingtotheminimumofsaturationforthecolorswithwhichwestarted。Inlikemanner，theothercolor-circlesforincreasinganddecreasinggradesofbrightness，maybearrangedperpendicularlyalongthisline，aboveandbelowthecircleofgreatestsaturation。Butthedecreasingsaturationofthecolorsintheselattercirclesmustbeexpressedintheshorteningoftheirradii；
justasinthefirstcircle，theshorterthedistancefromthecentre，thelessthesaturation。Theseradiigrowcontinuallyshorter，untilfinally，atthetwoextremitiesoftheline，thecirclesdisappearentirely。Thiscorrespondstothefactthatforeverycolorthemaximumofbrightnesscorrespondstothesensationwhite，whileitsminimumcorrespondstoblack[2]
19。Thewholesystemofsensationsofchromaticbrightnessmay，accordingly，bemostsimplyrepresentedbyasphericalsurfacewhoseequatorrepresentsthesystemofpurecolor-tones，orcolorsofgreatestsaturation，whilethetwopolescorrespondtowhiteandblack，theextremitiesofthesensationsofchromaticbrightness。Ofcourse，anyothergeometricalfigurewithsimilarattributes，as，forexample，twoconeswithacommonbaseandapexespointingindifferentdirections，wouldservethesamepurpose。Theonlythingessentialfortherepresentation，isthegradualtransitiontowhiteandblack，andthecorrespondingdecreaseinthevarietyofthecolor-tones，whichfindsitsexpressioninthecontinualdecreaseinthelengthoftheradiiofthecolor-circles。Now，asaboveshown，thesystemofsaturationscorrespondingto[p。61]aparticularsensationofpurebrightness，mayberepresentedbyacircularsurfacewhichcontainsallthesensationsoflightbelongingtoonegradeofbrightness。
Whenweunitegradesofsaturationandbrightnesstoasinglesystem，thetotalsystemoflightsensationsmayberepresentedbyasolidsphere。Theequatoristhesystemofpurecolor-tones；thepolaraxisisthesystemofpurebrightnesses；thesurfacerepresentsthesystemofchromaticbrightnesses，and，finally，everycircularplaneperpendiculartothepolaraxis，correspondstoasystemofsaturationsofequalbrightness。
Thisrepresentationbymeansofasphereisindeedarbitrary，inthesensethatanyothersolidfigurewithanalogousattributesmaybechoseninitsplace；still，itpresentstoviewthepsychologicalfactthatthetotalsystemoflightsensationsisaclosedcontinuityofthreedimensions。Thethree-dimensionalcharacterofthesystemarisesfromthefactthateveryconcretesensationoflighthasthreedeterminants:color-tone，saturation，andbrightness。Pure，orachromatic，brightnessandpure，orsaturated，colorsaretoberegardedasthetwoextremecasesintheseriesofsaturations。Theclosedformofthesystemcomesfromthecircularcharacterofthecolor-line，ontheonehand，and，ontheother，fromtheterminationofthesystemofchromaticbrightnessintheextremesofpurebrightness。Aspecialcharacteristicofthesystemisthatonlythechangesinthetwodimensions，orthoseofcolor-tonesandsaturations，arepurevariationsinquality，whileeverymovementinthethirddimension，orthatofbrightness，isatonceamodificationofbothqualityandintensity。Asaconsequenceofthiscircumstance，thewholethree-dimensionalsystemisrequiredtorepresentthequalitiesoflight-sensations，butitincludesalsotheintensitiesofthesesensations。
20。Certainprincipalsensesareprominentinthis[p。62]system，becauseweusethemaspointsofreferenceforthearrangementofalltheothers。Theseare，whiteandblack，intheachromaticseries，andthefourprincipalcolors，red，yellow，green，andblue，inthechromatic。Onlythesesixsensationshaveclearlydistinguishednamesintheearlydevelopmentoflanguage。Allothersensationsarethennamedeiherwithreferencetotheseorwithmodificationsofthenamesthemselves。Thus，weregardgreyasastageintheachromaticserieslyingbetweenwhiteandblack，Wedesignatethedifferentgradesofsaturationaccordingtotheirbrightness，aswhitishorblackish，lightordarkcolor-tones；
anwegenerallychoosecompoundnamesforthecolorsbetweenthefourprincipalones，as，forexample，purple-red，orange-yellow，yellow-green，etc。Theseallshowtheirrelativelylateoriginbytheirvercomposition。
20a。Fromtheearlyoriginofthenamesforthesixqualitiesmentioned，theconclusionhasbeendrawnthattheyarefundamentalqualitiesofvision，andthattheothersarecompoundedfromthem。Greyisdeclaredtobeamixtureofblackandwhite，violetandpurple-redtobemixturesofblueandred，etc。Psychologicallythereisnojustificationforcallinganylight-sensationscompoundincomparisonwithothers。Greyisasimplesensationjustasmuchaswhiteorblack；suchcolorsasorangeandpurple-redarejustasmuchsimplecolorsasredandyellow；andanygradeofsaturationwhichwehaveplacedinthesystembetweenapurecolorandwhite，isbynomeans，forthatreason，acompoundsensation。Theclosed，continuouscharacterofthesystemmakesitnecessaryforlanguagetopickoutcertainespeciallymarkeddifferencesinreferencetowhichallothersensationsarethenarranged，forthesimplereasonthatitisimpossibletohaveanunlimitednumberofnames。Itismostnaturalthatwhiteandblackshouldbechosenassuchpointsofreferencefortheachromaticseries，sincetheydesignatethegreatestdifferences。Whenoncethesetwoaregiven，however，allotherachromaticsensationswillbeconsideredastransitional[p。63]sensationsbetweenthem，sincetheextremedifferencesareconnectedbyaseriesofallpossiblegradesofbrightness。Thecaseofcolor-sensationsissimilar；onlyhere，onaccountofthecircularformofthecolor-line，itisimpossibletochoosedirectlytwoabsolutelygreatestdifferences。
Othermotivesbesidesthenecessaryqualitativedifference，aredecisiveinthechoiceoftheprincipalcolors。Wemayregardassuchmotives，thefrequencyandaffectiveintensityofcertainlight-impressionsduetothenaturalconditionsofhumanexistence。Theredcolorofblood，thegreenofvegetation，theblueofthesky，andtheyellowoftheheavenlybodiesincontrastwiththeblueofthesky，andtheyellowheavenlybodiesmaywellhavefurnishedtheearliestoccasionsforthechoiceofcertaincolorsasthosetoreceivenames。Languagegenerallynamesthesensationfromtheobjectthatproducedit，nottheobjectfromthesensation。Inthiscasetoo，whencertainprincipalqualitieswereoncedetermined，allothersmust，onaccountofthecontinuityoftheseriesofsensations，seemtobeintermediatecolor-tones。Thedifferencebetweenprincipalcolorsandtransitionalcolorsis，therefore，veryprobablydueentirelytoexternalconditions。Iftheseconditionshadbeenother，redmighthavebeenregardedasatransitionalcolorbetweenpurpleandorange，justasorangeisnowplacedbetweenredandyellows[3]
21。Theattributesofthesystemoflight-sensationsabovedescribed，aresopeculiarastoleadustoexpectapriorithattherelationbetweenthesepsychologicalattributesandtheobjectiveprocessesofstimulation，isessentiallydifferentfromthatinthecasesofthesensationalsystemsdiscussedbefore，especiallythoseofthegeneralandauditorysenses。
Most[p。64]striking，inthisrespect，isthedifferencebetweenthesysteminquestionandthatoftones。Inthelattercase，theprincipleofparallelismbetweensensationandstimulus(p。45)，holdsnotonlyforthephysiologicalprocessesof。stimulation，buttoagreatextentforthephysicalprocessesaswell。Asimplesensationcorrespondstoasimpleformofsound-vibration，andapluralityofsimplesensationstocompoundform。Furthermore，theintensityofthesensationvariesinproportiontotheamplitudeofthevibrations，anditsqualitywiththeirform，sothatinbothdirectionsthesubjectivedifferencebetweensensationsincreaseswiththegrowingdifferencebetweentheobjectivephysicalstimuli。Therelationinthecaseoflight-sensationsisentirelydifferent。Likeobjectivesound，objectivelightalsoconsistsofvibrationsinsomemedium。Tobesure，theactualformofthesevibrationsisstillaquestion，butfromphysicalexperimentsonthephenomenaofinterferenceweknowthattheconsistofveryshortandrapidwaves。Thoseseenaslightvaryinwave-lengthfrom688to393
millionthsofamillimetre，andinratefrom450to790billionvibrationspersecond。Inthiscase，too，simplesensationscorrespondtosimplevibrations，thatis，vibrationsoflikewave-length；andthequalityofthesensationvariescontinuouslywiththerate:redcorrespondstothelongestandslowestwives，andviolettotheshortestandmostrapid，whiletheothercolor-tonesformacontinuousseriesbetweenthese，varyingwiththechangesinwave-length。
Evenhere，however，anessentialdifferenceappears，forthecolorsredandviolet，whicharethemostdifferentinwave-length，aremoresimilarinsensationthanthosewhichliebetween[4]
Therearealsootherdifferences。1)Everychangeintheamplitudeofthephysicalvibrationscorrespondstoasubjectivechangeinbothintensityandquality，aswenotedaboveinthediscussionofsensationsofbrightness。
2)Alllight，eventhoughitbemadeupofallthedifferentkindsofvibration，issimpleinsensation，justasmuchasobjectivelysimplelight，whichismadeupofonlyonekindofwaves，asisimmediatelyapparentifwemakeasubjectivecomparisonofsensationsofchromaticlightwiththoseofachromaticlight。Fromthefirstofthesefactsitfollowsthatlightwhichisphysicallysimplemayproducenotonlychromatic，butalsoachromaticsensations，foritapproacheswhitewhentheamplitudeofitsvibrationsincreases，andblackwhentheamplitudedecreases。Thequalityofanachromaticsensationdoesnot，therefore，determineunequivocallyitssource；itmaybeproducedeitherthroughachangeintheamplitudeofobjectivelight-vibrationsorthroughamixtureofsimplevibrationsofdifferentwave-lengths。Inthefirstcase，however，thereisalwaysconnectedwiththechangeinamplitudeachangeinthegradeofbrightness，whichdoesnotnecessarilytakeplacewhenamixtureismade。
22。Evenwhenthegradeofbrightnessremainsconstant，thisachromaticsensationmayhaveoneofseveralsources。Asensationofpurebrightnessofagivenintensitymayresultnotonlyfromamixtureofalltheratesofvibrationcontainedinsolarlight，as，forexample，inordinarydaylight，butitmayalsoresultwhenonlytwokindsoflight-waves，namelythosewhichcorrespondtosensationssub-[p。66]jectivelythemostdifferent，thatis，tooppositecolors，aremixedinproperproportions。Sinceoppositecolors，whenmixedobjectively，producewhite，theyarecalledcomplementarycolors。Asexamplesofsuchoppositeorcomplementarycolorswemaymentionspectralredandgreen-blue，orangeandsky-blue，yellowandindigo-blue。
Likeachromaticsensations，eachofthecolor-sensationsmayalso，thoughtoamorelimitedextent，haveoneofseveralsources。Whentwoobjectivecolorswhichlienearereachotherinthecolor-circlethanopposites，aremixed，themixtureappears，notwhite，butofacolorwhichintheseriesofobjectivelysimplequalitiesliesbetweenthetwowithwhichwestarted。Thesaturationoftheresultingcoloris，indeed，verymuchdiminishedwhenthecomponentsofthemixtureapproachoppositecolors；
butwhentheyareneareachother，thediminutionisnolongerperceptible，andthemixtureandthecorrespondingsimplecoloraregenerallysubjectivelyalike。Thus，theorangeofthespectrumisabsolutelyindistinguishablefromamixtureofredandyellowrays。Inthisway，，allthecolorsinthecolor-circlebetweenredandgreencanbeobtainedbymixingredandgreen，allbetweengreenandvioletbymixinggreenandviolet，and，finally，purple，whichisnotinthesolarspectrum，canbeproducedbymixingredandviolet。Thewholeseriesofcolor-tonespossibleinsensationcan，accordingly，beobtainedfromthreeobjectivecolors。Bymeansofthesamethreecolorswecanalsoproducewhitewithitsintermediatestages。
Themixtureofredandvioletgivespurple，andthisisthecomplementarycolorofgreen；andthewhitesecuredbymixingthesecomplementarycolors，whenmixedindifferentproportionswiththevariouscolors，givesthedifferentgradesofsaturation。
23。Thethreeobjectivecolorsthatmaybeusedinthiswaytoproducethewholesystemoflight-sensations，are[p。67]calledfundamentalcolors。Inordertoindicatetheirsignificance，atriangularsurfaceischosentorepresentthesystemofsaturation，ratherthanthecircularsurfacewhichisderivedfromthepsychologicalrelationsalone。Thespecialsignificanceofthefundamentalcolorsisthenexpressedbyplacingthenattheanglesofthetriangle。Alongthesidesarearrangedthecolor-tonesintheirmaximalsaturation，justasonthecircumferenceofthecolor-circle，whiletheothergradesofsaturationintheirtransitionstowhite，whichliesinthecentre，areonthetriangularsurface。Theoretically，anysetofthreecolorscouldbechosenasfundamentalcolors，providedtheyweresuitablydistantfromoneanother。Practically，thosementioned，red，green，andviolet，arepreferablefortworeasons。First，byusingthemweavoidhavingasoneofthethree，purple，whichcannotbeproducedbyobjectivelysimplelight。Secondly，atthetwoendsofthespectrumsensationsvarymostslowlyinproportiontotheperiodofvibration，sothatwhentheextremecolorsofthespectrumareusedasfundamentalcolors，theresultobtainedbymixingtwoneighboringonesismostliketheintermediate，objectivelysimplecolor。[5]
24。Thesephenomenashowthatinthesystemoflightsensationsasimplerelationdoesnotexistbetweenthephysicalstimuliandthesensations。
Thiscanbeunderstoodfromwhathasbeensaidabove(3)astothe，characterofthephysiologicalstimulation。Thevisualsenseistobe[p。
68]reckonedamongthechemicalsenses，andwecanexpectasimplerelationonlybetweenthephotochemicalprocessesilltheretinaandthesensations。Now，weknowfromexperiencethatdifferentkindsofphysicallightproducelikechemicaldisintegrations，andthisexplainsingeneralthepossibilitymentionedabove，ofhavingthesamesensationfrommanydifferentkindsofobjectivelight。Accordingtotheprincipleofparallelismbetweenchangesinsensationandinthephysiologicalstimulation(p。45)，itmaybeassumedthatthevariousphysicalstimuliwhichcausethesamesensationallproducethesamephotochemicalstimulationintheretina，andthataltogethertherearejustasmanykindsandvarietiesofthephotochemicalprocessesaskindsandvarietiesofdistinguishablesensations。Infact，allthatweknow，uptothepresenttime，aboutthephysiologicalsubstratumoflight-sensationsisbaseduponthisassumption。Theinvestigationofthephysiologicalprocessesofstimulationthroughlight，hasnotyetgivenanyfurtherresultthanthatthestimulationisinallprobabilityachemicalprocess。
25。Therelativelylongpersistenceofthesensationafterthestimulationthatoriginatedit，isexplicableontheassumptionthatthelight-stimulationsareduetochemicalprocessesintheretina(3，p。42)。
Thispersistenceiscalled，withreferencetotheobjectusedasstimulus，theafter-imageoftheimpression。Atfirstthisafter-imageappearsinthesamebrightnessandcolorastheobject:whitewhentheobjectiswhite，blackwhenitisblack，andifitiscolored，inthesamecolor。
Thesearethepositiveandlike-coloredafter-images。Afterashorttimeitpasses，inthecaseofachromaticimpressions，intotheoppositegradeofbrightness，whiteintoblack，orblackintowhite；inthecaseofcolors，itpassesintotheoppositeorcomplementarycolor。Thesearethenegativeandcomplementaryafter-images。Iflight-stimuliofshortduration[p。
69]actupontheeyeindarkness，thistransitionmayberepeatedseveraltimes。Asecondpositiveafter-imagefollowsthenegative，andsoon，sothatanoscillationbetweenthetwophasestakesplace。Thepositiveafter-imagemaybereadilyexplainedbythefactthatthephotochemicaldisintegrationcausedbyanykindoflight，listsashorttimeaftertheactionofthelight。Thenegativeandcomplementaryafter-imagescanbeexplainedbythefactthatdisintegrationinagivendirectioncausesapartialconsumptionofthephotochemicalsubstancemostdirectlyconcerned，andthisresultsinacorrespondingmodificationofthephotochemicalprocesseswhenthestimulationoftheretinacontinues。
26。Theoriginofapartofthephenomenaincludedunderthenamelightcontrastsandcolor-contrastsisveryprobablythesameasthatofthenegativeandcomplementaryafter-images。Thesephenomenaconsistintheappearanceofsimultaneoussensationsofoppositebrightnessandcolorintheneighborhoodofanylight-impression。Thus，awhitesurfaceappearstobesurroundedbyadarkmargin，ablacksurfacebyabrightmargin，andacoloredsurfacebyamarginofthecomplementarycolor。Thesephenomena，whicharecalled“marginalcontrasts“whentheyarelimitedtotheimmediateneighborhoodoftheobject，areinpartatleastnothingbutnegativeorcomplementaryafter-imagesthataresimultaneouslyvisibleintheimmediateneighborhoodoftheimpressionasaresultofcontinualweakocularmovements。Whetherthereisalsoanirradiationofthestimulationisaquestion；itsexistencestillwantscertainproof。Thefactthatthesecontrastsincreaseasthelightbecomesmoreintense，justasafter-imagesdo，speaksfortheirinterconnectionwiththelatter。Inthisrespect，thisphysioloicalcontrastdiffersessentiallyfromcertainpsychologicalcontrast-phenomena，withwhichitisgenerallyconfused。Thelatterarecloselyconnectedin[p。70]theirrise，withnumerousotherformsofpsychologicalcontrast，sothatwewillnotdiscussthemuntillater，whenweenterintothegeneraltreatment(§；17，9)ofsuchphenomena。
26a。Ifwetakethepricipleofparallelismbetweensensationandphysiologicalstimulationasthebasisofoursuppositionsinregardtotheprocessesthatoccurintheretina，wemayconcludethatanalogousindependenceinthephotochemicalprocessescorrespondstotherelativeindependencewhichappearsbetweenachromaticandchromaticsensations。Twofacts，onebelongingtothesubjectivesensationalsystem，theothertotheobjectivephenomenaofcolor-sensationcanbemostnaturallyexplainedonthisbasis。
Thefirstisthe，tendencythateverycolor-sensationshows，ofpassingintooneofpurebrightnesswhenthegradeofitsbrightnessdecreasesorincreases。Thistendencyismostsimplyinterpretedontheassumptionthateverycolor-stimulationismadeupoftwophysiologicalcomponents，onecorrespondingtothechromatic，theothertotheachromaticstimulation。
Tothisassumptionwemayeasilyaddthefurthercondition，thatforcertainmediumintensifiesofthestimulithechromaticcomponentsarethestrongest，whileforgreaterandsmallerintensifiestheachromaticcomponentscomemoreandmoretothefront。Thesecondfact，isthatanytwooppositecolorsarecomplementary；thatis，whenmixedinsuitableproportions，theyproduceanachromaticsensation。Thisphenomenonismosteasilyunderstoodwhenweassumethatoppositecolors，whicharesubjectivelythegreatestpossibledifferences，representobjectivephotochemicalprocessesthatneutralizeeachother。Thefactthatasaresultofthisneutralizationanachromaticstimulationarises，isveryreadilyexplainedbythepresuppositionthatsuchastimulationaccompanieseverychromaticstimulationfromthefirst，andisthereforeallthatisleftwhenantagonisticchromaticstimulationscounteracteachother。Thisassumptionofarelativeindependencebetweenthechromaticandachromaticphotochemicalprocesses，issupportedinaverystrikingwaybytheexistenceofanabnormityofvision，sometimescongenital，sometimesacquiredthroughpathologicalchangesintheretina，namelytotalcolor-blindness。Insuchcasesallstimulationsare，eitheronthewhole[p。71]retinaoroncertainpartsofit，seenaspurebrightness，withoutanyadmixtureofcolor。Thisisanincontrovertibleproofthatthechromaticandachromaticstimulationsareseparablephysiologicalprocesses。
Ifweapplytheprincipleofparallelismtothechromaticstimulation，twofactspresentthemselves。Thefirstisthattwocolorsseparatedbylimited，shortdistance，whenmixedgiveacolorthatisliketheintermediatesimplecolor。Thisindicatesthatcolor-stimulationisaprocessthatvarieswiththephysicalstimulus，notcontinuously，asthetonalstimulation，butinshortstages，andinsuchawaythatthestagesinredandvioletarelongerthaningreen，wherethemixtureofcolorsfairlyneareachother，showstheeffectsofcomplementaryaction。Suchanon-continuousvariationoftheprocesscorrespondsentirelywithitschemicalnature，forchemicaldisintegrationandsynthesismustalwayshavetodowithqroupsofatomsormolecules。Thesecondfactisthatcertaindefinitecolors，whichcorrespondtoratherlargedifferencesinthestimuli，aresubjectivelyoppositecolors，thatis，aremaximaldifferences，andthesamecolorsareobjectivelycomplementary，thatis，mutuallyneutralizing，processes。
Chemicalprocesses，however，canneutralizeeachotheronlywhentheyareinsomewayoppositeincharacter。Anytwocomplementarycolor-stimulationsmust，therefore，standinarelationtoeachothersimilartothatwhichexistsbetweentheneutralizingprocessesoperativeinthecaseofantagonisticachromaticstimulations。Still，therearetwoveryessentialdifferenceshere。First，thisoppositioninthecharacterofcolor-stimulationsisnotlimitedtoonecase，butappearsforeverycolordistinguishableinsensation，sothatwemustconclude，accordingtoourpresupposition，thatforeverystageofthephotochemicalprocessof。chromaticstimulationwhichistobeassumedonthegroundoftheresultsobtainedbymixingneighboringcolors，thereisacertaincomplementaryprocess。Secondly，thedifferencebetweentwooppositecolors，whichissubjectivelythegreatestpossibledifference，ismediatedbytransitionalforms，notmerelyinonedirectionfromeachcolor，asinthecaseofblackandwhite，butintwooppositedirections。Inasimilarway，theobjectivecomplementaryactionoftwocolorsgraduallydiminishesas，startingfromoppositecolors，theyapproacheachotherineitherof[p。72]thesetwodirections。Wemay，then，inferfromthistwofoldeliminationofcomplementaryactionthatthereturnofthecolor-linetoitsstartingpointcorrespondstoarepetitionofrelatedphotochemicalprocesses，onthesamegroundsthatledustoinfertheoppositecharacteroftheprocessescorrespondingtooppositecolors，fromthefactthattheyarecomplementary。Thewholeprocessofchromaticstimulation，beginningwithredandpassingbeyondvioletthroughpurplemixturestoitsstartingpoint，runningparallel，asitdoes。，withcontinuouschangesinthewavelengthofobjectivelight，istoberegardedasanindefinitelylongsuccessionofphotochemicalprocesses。Alltheseprocessestogether，formaclosedcircleinwhich，foreverystagethereisaneutralizingoppositeandapossibletransitiontothisoppositeintwodifferentdirections。
Weknownothingaboutthetotalnumberofphotochemicalstagesinthiscircleofprocesses。Thenumerousattemptsmadetoreduceallcolor-sensationstothesmallestpossiblenumberofsuchstages，lackadequatefoundation。
Sometimestheyindiscriminatelytranslatetheresultsofphysicalcolor-mixingintophysiologicalprocesses，asintheassumptionofthreefundamentalcolors，red，green，andviolet，fromthedifferentmixturesofwhichallsensationsoflight，eventheachromatic，aretobederived(Young-Helmholtzhypothesis)。Sometimestheystartwiththepsychologicallyuntenableassumptionthatthenamingofcolorsisnotduetotheinfluenceofcertainexternalobjects，buttotherealsignificanceofthecorrespondingsensations(v。
sup。p。63)，andassumeaccordinglyfourfundamentalcolorsasthesourcesofallcolor-sensations。Thefourfundamentalcolorshereassumedarethetwopairsredandgreen，yellowandblue，towhichareaddedthesimilarpairofsensationsofpurebrightness，blackandwhite。Allotherlight-sensationssuchasgrey，orange，violet，etc。，areregardedassubjectivelyandobjectivelymixedcolors(Hering\'shypothesis)。Theevidenceinsupportofthefirstasofthesecondofthesehypotheseshasbeenderivedforthemostpartfromtheriotinfrequentcasesofpartialcolor-blindness。Thosewhoacceptthreefundamentalcolors，assertthatallthesecasesaretobeexplainedasalackoftheredorgreensensations，orelseasalackofboth。Thosewhoacceptfour，holdthatpartialcolor-blindnessalwaysincludestwofundamentalcolorsthatbelongtogetherasopposites，andis，therefore，either[p。73]red-green-blindnessoryellow-blue-blindness。
Anunprejudicedexaminationofcolor-blindnessdoesnotjustifyeitheroftheseassertions。Thethree-colortheorycannotexplaintotalcolorblindness，andthefour-colortheoryisincontradictiontocasesofpurered-blindnessandpuregreen-blindness。Finally，boththeoriesareoverthrownbythecasesthatunquestionablyoccur，inwhichsuchpartsofthespectrumasdonotcorrespondtoanyofthethreeorfourfundamentalcolors，appearcolorless。Theonlythingthatourpresentknowledgejustifiesusinsaying，isthateverysimplesensationoflightisconditionedphysiologicallybyacombinationoftwophotochemicalprocesses，amonochromaticandachromatic。Thefirstismadeup，inturn，ofaprocessmainlyofdisintegration，whenthelightismoreintense，andaprocessofrestitution，whenthelightisweaker。Thechromaticprocessvariesbystagesinsuchawaythatthewholeseriesofphotochemicalcolor-disintegrationsformsacircleofprocessesinwhichtheproductsofthedisintegrationforanytworelativelymostdistantstages，neutralizeeachother。[6]
