a history of science-4-第52章
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f； 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
