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 of the anatomy of the nervous system； gave tangible expression to the growth of the new movement in 1874； and four years later； with the opening of his laboratory of physiological psychology at the University of Leipzig； the new psychology may be said to have gained a permanent foothold and to have forced itself into official recognition。 From then on its conquest of the world was but a matter of time。
It should be noted； however； that there is one other method of strictly experimental examination of the mental field； latterly much in vogue； which had a different origin。 This is the scientific investigation of the phenomena of hypnotism。 This subject was rescued from the hands of charlatans； rechristened； and subjected to accurate investigation by Dr。 James Braid； of Manchester； as early as 1841。 But his results； after attracting momentary attention； fell from view； and； despite desultory efforts； the subject was not again accorded a general hearing from the scientific world until 1878； when Dr。 Charcot took it up at the Salpetriere； in Paris； followed soon afterwards by Dr。 Rudolf Heidenhain； of Breslau； and a host of other experimenters。 The value of the method in the study of mental states was soon apparent。 Most of Braid's experiments were repeated； and in the main his results were confirmed。 His explanation of hypnotism； or artificial somnambulism； as a self…induced state； independent of any occult or supersensible influence； soon gained general credence。 His belief that the initial stages are due to fatigue of nervous centres； usually from excessive stimulation； has not been supplanted； though supplemented by notions growing out of the new knowledge as to subconscious mentality in general； and the inhibitory influence of one centre over another in the central nervous mechanism。
THE BRAIN AS THE ORGAN OF MIND
These studies of the psychologists and pathologists bring the relations of mind and body into sharp relief。 But even more definite in this regard was the work of the brain physiologists。