a history of science-4-第38章

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men　undertook　the　novel　task　of　interrogating　the　relations　of　body　and　mind　from　the　standpoint　of　physics。　　They　sought　to　apply　the　vernier　and　the　balance；　as　far　as　might　be；　to　the　intangible　processes　of　mind。

The　movement　had　its　precursory　stages　in　the　early　part　of　the　century；　notably　in　the　mathematical　psychology　of　Herbart；　but　its　first　definite　output　to　attract　general　attention　came　from　the　master…hand　of　Hermann　Helmholtz　in　1851。　It　consisted　of　the　accurate　measurement　of　the　speed　of　transit　of　a　nervous　impulse　along　a　nerve　tract。　　To　make　such　measurement　had　been　regarded　as　impossible；　it　being　supposed　that　the　flight　of　the　nervous　impulse　was　practically　instantaneous。　But　Helmholtz　readily　demonstrated　the　contrary；　showing　that　the　nerve　cord　is　a　relatively　sluggish　message…bearer。　According　to　his　experiments；　first　performed　upon　the　frog；　the　nervous　〃current〃　travels　less　than　one　hundred　feet　per　second。　Other　experiments　performed　soon　afterwards　by　Helmholtz　himself；　and　by　various　followers；　chief　among　whom　was　Du　Bois…Reymond；　modified　somewhat　the　exact　figures　at　first　obtained；　but　did　not　change　the　general　bearings　of　the　early　results。　Thus　the　nervous　impulse　was　shown　to　be　something　far　different；　as　regards　speed　of　transit；　at　any　rate；　from　the　electric　current　to　which　it　had　been　so　often　likened。　An　electric　current　would　flash　halfway　round　the　globe　while　a　nervous　impulse　could　travel　the　length　of　the　human　bodyfrom　a　man's　foot　to　his　brain。

The　tendency　to　bridge　the　gulf　that　hitherto　had　separated　the　physical　from　the　psychical　world　was　further　evidenced　in　the　following　decade　by　Helmholtz's　remarkable　but　highly　technical　study　of　the　sensations　of　sound　and　of　color　in　connection　with　their　physical　causes；　in　the　course　of　which　he　revived　the　doctrine　of　color　vision　which　that　other　great　physiologist　and　physicist；　Thomas　Young；　had　advanced　half　a　century　before。　The　same　tendency　was　further　evidenced　by　the　appearance；　in　1852；　of　Dr。　Hermann　Lotze's　famous　Medizinische　Psychologie；　oder　Physiologie　der　Seele；　with　its　challenge　of　the　old　myth　of　a　〃vital　force。〃　　But　the　most　definite　expression　of　the　new　movement　was　signalized　in　1860；　when　Gustav　Fechner　published　his　classical　work　called　Psychophysik。　　That　title　introduced　a　new　word　into　the　vocabulary　of　science。　Fechner　explained　it　by　saying；　〃I　mean　by　psychophysics　an　exact　theory　of　the　relation　between　spirit　and　body；　and；　in　a　general　way；　between　the　physical　and　the　psychic　worlds。〃　The　title　became　famous　and　the　brunt　of　many　a　controversy。　So　also　did　another　phrase　which　Fechner　introduced　in　the　course　of　his　bookthe　phrase　〃physiological　psychology。〃　In　making　that　happy　collocation　of　words　Fechner　virtually　christened　a　new　science。


FECHNER　EXPOUNDS　WEBER'S　LAW

The　chief　purport　of　this　classical　book　of　the　German　psycho…physiologist　was　the　elaboration　and　explication　of　experiments　based　on　a　method　introduced　more　than　twenty　years　earlier　by　his　countryman　E。　H。　Weber；　but　which　hitherto　had　failed　to　attract　the　attention　it　deserved。　The　method　consisted　of　the　measurement　and　analysis　of　the　definite　relation　existing　between　external　stimuli　of　varying　degrees　of　intensity　（various　sounds；　for　example）　and　the　mental　states　they　induce。　Weber's　experiments　grew　out　of　the　familiar　observation　that　the　nicety　of　our　discriminations　of　various　sounds；　weights；　or　visual　images　depends　upon　the　magnitude　of　each　particular　cause　of　a　sensation　in　its　relation　with　other　similar　causes。　　Thus；　for　example；　we　cannot　see　the　stars　in　the　daytime；　though　they　shine　as　brightly　then　as　at　night。　Again；　we　seldom　notice　the　ticking　of　a　clock　in　the　daytime；　though　it　may　become　almost　painfully　audible　in　the　silence　of　the　night。　Yet　again；　the　difference　between　an　ounce　weight　and　a　two…ounce　weight　is　clearly　enough　appreciable　when　we　lift　the　two；　but　one　cannot　discriminate　in　the　same　way　between　a　five…pound　weight　and　a　weight　of　one　ounce　over　five　pounds。

This　last　example；　and　similar　ones　for　the　other　senses；　gave　Weber　the　clew　to　his　novel　experiments。　　Reflection　upon　every…day　experiences　made　it　clear　to　him　that　whenever　we　consider　two　visual　sensations；　or　two　auditory　sensations；　or　two　sensations　of　weight；　in　comparison　one　with　another；　there　is　always　a　limit　to　the　keenness　of　our　discrimination；　and　that　this　degree　of　keenness　varies；　as　in　the　case　of　the　weights　just　cited；　with　the　magnitude　of　the　exciting　cause。

Weber　determined　to　see　whether　these　common　experiences　could　be　brought　within　the　pale　of　a　general　law。　His　method　consisted　of　making　long　series　of　experiments　aimed　at　the　determination；　in　each　case；　of　what　came　to　be　spoken　of　as　the　least　observable　difference　between　the　stimuli。　Thus　if　one　holds　an　ounce　weight　in　each　hand；　and　has　tiny　weights　added　to　one　of　them；　grain　by　grain；　one　does　not　at　first　perceive　a　difference；　but　presently；　on　the　addition　of　a　certain　grain；　he　does　become　aware　of　the　difference。　Noting　now　how　many　grains　have　been　added　to　produce　this　effect；　we　have　the　weight　which　represents　the　least　appreciable　difference　when　the　standard　is　one　ounce。

Now　repeat　the　experiment；　but　let　the　weights　be　each　of　five　pounds。　Clearly　in　this　case　we　shall　be　obliged　to　add　not　grains；　but　drachms；　before　a　difference　between　the　two　heavy　weights　is　perceived。　　But　whatever　the　exact　amount　added；　that　amount　represents　the　stimulus　producing　a　just…perceivable　sensation　of　difference　when　the　standard　is　five　pounds。　And　so　on　for　indefinite　series　of　weights　of　varying　magnitudes。　Now　came　Weber's　curious　discovery。　　Not　only　did　he　find　that　in　repeated　experiments　with　the　same　pair　of　weights　the　measure　of　〃just…｛p｝erceivable　difference〃　remained　approximately　fixed；　but　he　found；　further；　that　a　remarkable　fixed　relation　exists　between　the　stimuli　of　different　magnitude。　　If；　for　example；　he　had　found　it　necessary；　in　the　case　of　the　ounce　weights；　to　add　one…fiftieth　of　an　ounce　to　the　one　before　a　difference　was　detected；　he　found　also；　in　the　case　of　the　five…pound　weights；　that　one…fiftieth　of　five　pounds　must　be　added　before　producing　the　same　result。　　And　so　of　all　other　weights；　the　amount　added　to　produce　the　stimulus　of　〃least…appreciable　difference〃　always　bore　the　same　mathematical　relation　to　the　magnitude　of　the　weight　used；　be　that　magnitude　great　or　small。

Weber　found　that　the　same　thing　holds　good　for　the　stimuli　of　the　sensations　of　sight　and　of　hearing；　the　differential　stimulus　bearing　always　a　fixed　ratio　to　the　total　magnitude　of　the　stimuli。　Here；　then；　was　the　law　he　had　sought。

Weber's　results　were　definite　enough　and　striking　enough；　yet　they　failed　to　attract　any　considerable　measure　of　attention　until　they　were　revived　and　extended　by　Fechner　and　brought　before　the　world　in　the　famous　work　on　psycho…physics。　Then　they　precipitated　a　veritable　melee。　Fechner　had　not　alone　verified　the　earlier　results　（with　certain　limitations　not　essential　to　the　present　consideration）；　but　had　invented　new　methods　of　making　similar　tests；　and　had　reduced　the　whole　question　to　mathematical　treatment。　　He　pronounced　Weber's　discovery　the　fundamental　law　of　psycho…physics。　In　honor　of　the　discoverer；　he　christened　it　Weber's　Law。　　He　clothed　the　law　in　words　and　in　mathematical　formulae；　and；　so　to　say；　launched　it　full　tilt　at　the　heads　of　the　psychological　world。　　It　made　a　fine　commotion；　be　assured；　for　it　was　the　first　widely　heralded　bulletin　of　the　new　psychology　in　its　march　upon　the　strongholds　of　the　time…honored　metaphysics。　The　accomplishments　of　the　microscopists　and　the　nerve　physiologists　had　been　but　preliminarymere　border　skirmishes　of　uncertain　import。　But　here　was　proof　that　the　iconoclastic　movement　meant　to　invade　the　very　heart　of　the　sacred　territory　of　minda　territory　from　which　tangible　objective　fact　had　been　supposed　to　be　forever　barred。


PHYSIOLOGICAL　PSYCHOLOGY

Hardly　had　the　alarm　been　sounded；　however；　before　a　new　movement　was　made。　　While　Fechner's　book　was　fresh　from　the　press；　steps　were　being　taken　to　extend　the　methods　of　the　physicist　in　yet　another　way　to　the　intimate　processes　of　the　mind。　As　Helmholtz　had　shown　the　rate　of　nervous　impulsion　along　the　nerve　tract　to　be　measurable；　it　was　now　sought　to　measure　also　the　time　required　for　the　central　nervous　mechanism　to　perform　its　work　of　receiving　a　message　and　sending　out　a　response。　This　was　coming　down　to　the　very　threshold　of　mind。　The　attempt　was　first　made　by　Professor　Donders　in　1861；　but　definitive　results　were　only　obtained　after　many　years　of　experiment　on　the　part　of　a　host　of　observers。　The　chief　of　these；　and　the　man　who　has　stood　in　the　forefront　of　the　new　movement　and　has　been　its　recognized　leader　throughout　the　remainder　of　the　century；　is　Dr。　Wilhelm　Wundt；　of　Leipzig。

The　task　was　not　easy；　but；　in　the　long　run；　it　was　accomplished。　Not　alone　was　it　shown　that　the　nerve　centre　requires　a　measurable　time　for　its　operations；　but　much　was　learned　as　to　conditions　that　modify　this　time。　　Thus　it　was　found　that　different　persons　vary　in　the　rate　of　their　central　nervous　activitywhich　explained　the　〃personal　equation〃　that　the　astronomer　Bessel　had　noted　a　half…century　before。　It　was　found；　too；　that　the　rate　of　activity　varies　also　for　the　same　person　under　different　conditions；　becoming　retarded；　for　example；　under　influence　of　fatigue；　or　in　case　of　certain　diseases　of　the　brain。　　All　details　aside；　the　essential　fact　emerges；　as　an　experimental　demonstration；　that　the　intellectual　processessensation；　apperception；　volitionare　linked　irrevocably　with　the　activities　of　the　central　nervous　tissues；　and　that　these　activities；　like　all　other　physical　processes；　have　a　time　element。　　To　that　old　school　of　psychologists；　who　scarcely　cared　more　for　the　human　head　than　for　the　heelsbeing　interested　only　in　the　mindsuch　a　linking　of　mind　and　body　as　was　thus　demonstrated　was　naturally　disquieting。　But　whatever　the　inferences；　there　was　no　escaping　the　facts。

Of　course　this　new　movement　has　not　been　confined　to　Germany。　　Indeed；　it　had　long　had　exponents　elsewhere。　Thus　in　England；　a　full　century　earlier；　Dr。　Hartley　had　championed　the　theory　of　the　close　and　indissoluble　dependence　of　the　mind　upon　the　brain；　and　formulated　a　famous　vibration　theory　of　association　that　still　merits　careful　consideration。　Then；　too；　in　France；　at　the　beginning　of　the　century；　there　was　Dr。　Cabanis　with　his　tangible；　if　crudely　phrased；　doctrine　that　the　brain　digests　impressions　and　secretes　thought　as　the　stomach　digests　food　and　the　liver　secretes　bile。　Moreover；　Herbert　Spencer's　Principles　of　Psychology；　with　its　avowed　co…ordination　of　mind　and　body　and　its　vitalizing　theory　of　evolution；　appeared　in　1855；　half　a　decade　before　the　work　of　Fechner。　　But　these　influences；　though　of　vast　educational　value；　were　theoretical　rather　than　demonstrative；　and　the　fact　remains　that　the　experimental　work　which　first　attempted　to　gauge　mental　operations　by　physical　principles　was　mainly　done　in　Germany。　　Wundt's　Physiological　Psychology；　with　its　full　preliminary　descriptions