a history of science-4-第46节

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ed　to　be　able　to　cause　a　muscular　contraction　and　to　do　nothing　else；　yet　here　such　an　impulse　had　exactly　the　opposite　effect。　The　only　tenable　explanation　seemed　to　be　that　this　particular　impulse　must　arrest　or　inhibit　the　action　of　the　impulses　that　ordinarily　cause　the　heart　muscles　to　contract。　But　the　idea　of　such　inhibition　of　one　impulse　by　another　was　utterly　novel　and　at　first　difficult　to　comprehend。　Gradually；　however；　the　idea　took　its　place　in　the　current　knowledge　of　nerve　physiology；　and　in　time　it　came　to　be　understood　that　what　happens　in　the　case　of　the　heart　nerve…supply　is　only　a　particular　case　under　a　very　general；　indeed　universal；　form　of　nervous　action。　　Growing　out　of　Bernard's　initial　discovery　came　the　final　understanding　that　the　entire　nervous　system　is　a　mechanism　of　centres　subordinate　and　centres　superior；　the　action　of　the　one　of　which　may　be　counteracted　and　annulled　in　effect　by　the　action　of　the　other。　　This　applies　not　merely　to　such　physical　processes　as　heart…beats　and　arterial　contraction　and　relaxing；　but　to　the　most　intricate　functionings　which　have　their　counterpart　in　psychical　processes　as　well。　Thus　the　observation　of　the　inhibition　of　the　heart's　action　by　a　nervous　impulse　furnished　the　point　of　departure　for　studies　that　led　to　a　better　understanding　of　the　modus　operandi　of　the　mind's　activities　than　had　ever　previously　been　attained　by　the　most　subtle　of　psychologists。


PSYCHO…PHYSICS

The　work　of　the　nerve　physiologists　had　thus　an　important　bearing　on　questions　of　the　mind。　　But　there　was　another　company　of　workers　of　this　period　who　made　an　even　more　direct　assault　upon　the　〃citadel　of　thought。〃　A　remarkable　school　of　workers　had　been　developed　in　Germany；　the　leaders　being　men　who；　having　more　or　less　of　innate　metaphysical　bias　as　a　national　birthright；　had　also　the　instincts　of　the　empirical　scientist；　and　whose　educational　equipment　included　a　profound　knowledge　not　alone　of　physiology　and　psychology；　but　of　physics　and　mathematics　as　well。　These　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　ha