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 lion guer, Arago„ and Masson 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 physical measurements indicates, by its very name. the close connection of physics 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, psychological 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. I-1. Weber into the cutaneous and muscular senses, and the spatial functions of eye and skin, have proved of fundamental importance to the science. (See WEBER'S LAW.) The studies of A. NV. 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 proeesses, form sonic of the early links of a chain of physiological research which is still worthily continued ; in the work, e.g. of H. Zwaardemaker on smell, and of A. Gold scheider on the cutaneous and muscular sensibili ties. Until psychology has laboratories upon the scale of thoAe devoted to physics and physiology, it must be in large measure dependent for exact investigations upon the representatives of these older disciplines; while, in any ease, 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 provinces of mind which the new psychology 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 difficulty of its subject-matter, there are very many prob lems that still await the experimenter. If we are to attempt a catalogue of what has been accomplished, we must begin (1) with the fields of sensation and of sense-perception. The litera ture of these subjects—of vision, audition, and the rest of the sense-qualities, of spatial and temporal perception (see DtiaanoN; EXTENSION), and of qualitative perception (see FUSION)—has already attained very considerable proportions.
When Helmholtz published, in 1867, his great work on physiological optics, it seemed that he had exhausted the subject, that its difficulties were resolved, once and for all. But. what was judged to be the end has proved to be only the beginning; the work of many men has aceumu 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 experimental 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. Donders, in 1866, proposed to use the method of reaction for the measurement of mental acts like choice, discrimi nation, and judgment, he was building better than lie knew; for the laboratory reaction, the exact type of a voluntary action, has been the chief aid toward a final analysis of the active consciousness. (4) 11. Elffiinghans's Dos Ge Jiiehtniss (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 reproductory consciousness. (5) Finally, the feelings are gradually submitting themselves to experimental treatment. Fechner himself laid the foundations of an exact science of experimental nstheties (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 'experinW11 tal psychology.' Such inclusion depends partly upon our definition of the word 'experiment'— experiments on animals and on hypnotic subjects are of a different order from those described above: and partly on the extensibility of the word `psychology,' upon 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 psychology has gained or may gain from all the-se four sources than to find a single name for them. Consult: Titchener, Experimental Ps-ychology (New York, 1901) : Sanford, Course in Experimental Psy chology (Boston, 1808).