PSYCHOLOGY. 514 bit of sense-knowledge, a mental state of process that informs us of something in the outside world, we see what an advance in insight and scientific method is implied in Wundt's formula- tion. The sources from which experimental psychol- ogy has drawn are manifold. It is especially indebted, for fact and for inspiration, to physics, astronomy, and the physiology of the organs of sense. The photometric investigations of Bou- guer, Arago, and JIasson contained hints of the constancy of the relative sensible discrimination for light intensities. The observed relation be- tween the intensity of a star, as photometrically determined, and its apparent brightness or 'mag- nitude' was similarly significant. The 'error of observation' in pliysical measurements indicates, by its very name, the close connection of ])hysics and psychology, and the need of the former to be supplemented by the latter. The 'personal equa- tion' noticed by astronomers, as an inevitable source of error in their observations of stellar transits, formed the starting-point for the later elaborate researches into simple and compound reaction-times. On the other hand, a large number of professedly 'physiological' inqui- ries, inquiries carried out by physiologists with physiological interest, have been taken over bodi- ly by experimental psychology. Physiology can- not deal, in strictness, with the doctrine of sensa- tion and perception, but only with the mode of function of the sense-organs regarded as living structures. Where, however, ])sychological knowledge and method are in advance of physi- ological, the physiologist naturally seeks to till out the gaps in his own science by making an excursion into the other. This custom has led, at times, to the mistaken belief that psychology is only a department of physiology; but, on the whole, the countervailing gain has been well worth a little misunderstanding. The inquiries of E, H. 'eber into the cutaneous and muscular senses, and the spatial functions of eye and skin, have proved of fundamental imjiortance to the science. (See Webeb'.s Law.) The studies of A. W. Volkmann on visual space perception, of H. von Helmholtz on vision and audition, of E. Hering on vision, and of S. Exner on the dura- tion of mental processes, form some of the earlj' links of a chain of physiological research which is still wortliily continued ; in the work, e.g. of H. Zwaarilcmaker on smell, and of A. Gold- scheider on the cutaneous and muscular sensibili- ties. Until psychology has laboratories upon the scale of those devoted to physics and physiology, it must be in large measure dependent for exact investigations upon the representatives of these older disci]]lines ; while, in any case, the labors of men trained in general scientific method can- not fail to be of high value in this particular field. What, now, we may ask, are the ])rovinces of mind which the new jisycbolog^- has made pe- culiarly its own? In principle, there is no psychological problem that cannot be experimen- tally attacked. In actual fact, owing to the youth of the science, its lack of material means and of trained workers, and the extreme difliieulty of its subject-matter, there are very many prob- lems that .still await the experimenter. It we are to attempt a catalogue of what has been accom])lished, we must begin (1) with the fields of sensation and of sense-perception. The litera- ture of these subjects — of vision, audition, and PSYCHOMETRY. the I'est of the sense-qualities, of spatial and temporal perception (see Duration; Extek.sion) , and of qualitative perception (see Fusion)— has already attained very considerable proportions. When Hclmbultz published, in 18(i7, his great work on physiological optics, it seemed that he had exhausted the sulijcct, that its dilliculties were resolved, once and for all. But what Mas judged to be the end has |iroved to be only the beginning; the work of many men has accunm- lated and is still accumulating, bringing new facts and new questions. And as here, so else- where : experiment is taking us toward an exact doctrine of sensation and perception, whose com- plexity had, before its advent, been not so much as guessed at. ( 2 ) The psychology of attention (q.v.) may almost be termed a positive creation of the ex])erimental method. It is strange and instructive to turn from a modern system of psychology', in which the doctrine of attention looms .so large and important, to a German eighteenth-century work, or a volume of English associationism, where (except for a few scattered hints) we find no mention of it whatsoever. (3) The same thing may be said of the psychol- ogy of action. When F. C. Bonders, in 1806, proposed to use the method of reactit)n for the measurement of mental acts like choice, discrimi- nation, and judgment, he was building better than he knew; for the laboratory reaction, the exact type of a voluntary acticm, has been the chief aid toward a final analysis of the active consciousness. (4) H. Ehl)inghaus".s Diis Ge- ddchtniss (1884) brought the function of mem- ory under experimental control, and has been followed by many monographs upon recognition and the various conditions of the rcproduetory consciousness. (5) Finally, the feelings are gradually submitting themselves to experimental treatment. Fechner himself laid the foundations of an exact science of experimental a'sthetics (q.v.) ; and Mosso's researches into the bodily symptoms of affective processes have borne rich fruit. If we cannot say that experiment has given us a .settled psychology of feeling, we can at least assert that the issues are more clearly marked and the problems more definitely formu- lated than ever before; and this means that it is only a matter of time until our questions are adequately answered. There has been some dispute as to whether certain results of animal psychology (q.v.), of physiological experimentation on the brain cortex, of the treatment of brain disease, and of tests made upon hypnotized subjects (see Hypnotism) .should be included under the phrase 'experimen- tal psychology.' Such inclusion depends partly upon our detinition of the word 'experiment' — experiments on animals and on hypnotic subjects are of a dill'ercnt order from those descrilied above; and partly on the e.xtensibility of the word 'psychology'.' u])on the point, i.e.. whether all that furthers or contributes to a science is necessarily itself a part of the science. However, it is more important to note that psychologv' has gained or may gain from all these four sources than to find a single name for them. Consult: Titchcner, Experimentnl Psychohxjy (New York, 1901): Sanford, Course in Experimental Psy- clwlof/y (Boston, 1898). PSYCHOM'ETRY (from Ok. <pux-fi. psyche, breath, life, soul + fih-po", mctron, measure). A term used ( 1 ) for a supposed power, possessed
