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* PSYCHOMETRY. 515 PTAH. by certain "sensitive' persons, of reading char- acter from handwriting, or passing the fingers over the head of the subject, etc., etc. More re- cently, it has been used (2) as the equivalent of mental chronouietry, to cover all experimental in- vestigations into the temporal course of mental processes ; but the usage is lapsing. PSY'CHOPATHOL'OGY (from Gk. fvxfl, psyche, breath, life, soul -p trdOo^, pathos, disea.se + -oyia, -logia, account, from M-^uv. legein, to say ) . A synonym for mental patholog5 The term is wider than psychiatry (q.v. ), since it covers all study of the mental symptoms of dis- ease, without special reference to therapeutics. It is narrower than abnormal psychology, since (at least in current usage) it deals onlj' with serious mental derangement, and not with such comparatively normal abnormalities as dreaming and hypnosis. PSYCHOPHYS'ICS (from Gk. 4,vxv, psyche, breath, life, soul -f- tpviriKSs. pliysikus, physical, from ipva-is, physi.i, natuie. from (puuv, phyein, to produce: connected with Lat. fiii, I was, Skt. ihu, to become, and ultimately with Eng. be). The science of the inter-relations of mind and body. The term was coined by Fechner to desig- nate an exact science of the relations of depend- ence between the physical and psychical worlds. He discriminated an internal and an external psychophysics : in the former, sensation is con- sidered in its direct relation to the brain and nervous system; in the latter, sensaticm is in- directly studied in its dejiendence upon external physical stimuli. Xo definition could be more general. We have to ascertain the facts and laws of mind, and the physio- logical facts and laws (of brain or of sense-organs) with which they are connected: we have to parallel the two series of events, noting how variation in the one is related to variation in the other; we have to express the functional interdependences exactly, i.e. in terms of measure- ment, of mathematical formuL-e; and from our whole inquiry we .shall attain a 'philosophical' standpoint, a theory of the general relation be- tween the physical and psychical worlds. Fechner is concerned, primarily, with the prob- lem of mental measurement. And when we turn to the special portion of his Elemeiite der Psycho- physik, we find that he is occupied, for the most part, with the relation between stimulus and sensation that is formulated in Weber's law (q.v.). In the quantitative expression of this relation he sees the one universal psychophysical unifarmity. A regrettable result of this general- ization is that the term psychophysics has been narrowed in meaning. Thus iliinsterberg. finding his justification in historical usage, identifies it with the "question of the relation between stimu- lus and sensation;" and the Century Dictionary defines it as "the science of the relations between stimuli and the sensations which they evoke." Some writers go further still, and — in view of the fact that Weber's law has proved to be a law of sensation inten.sities — restrict psychophysics to the intensive relations between stimulus and sensation. Thus Baldwin states that "investiga- tions in intensity constitute psychophysics." In the light of modern developments, however, we have a full right to hold to the general definition laid down by Fechner. A word must be said in detail of the psycho- physical measurement methods, the elaboration of which owes much to Fechner, and the under- standing of which is an integral part of training in psychophysics. We cannot measure a mental process, sensation or other, as we can measure physical magnitudes; but we can measure func- tion, sensitivity, and sensible discrimination; and we can measure the differences or distances that separate term from term within a sensation scale. There are two groups of methods by aid of which such measurements can be made. The first in- cludes the gradation methods, the second the error methods. In the former, we approach the object of measurement by slow degrees, in order to arrive at a single accurate result; in the latter, we approach the object to a certain distance over and over again, and submit the whole number of results to mathematical treatment, in order to arrive at the required determination. The grada- tion methods imply, in general, a "procedure with knowledge;' the observer knows the nature and the direction (increasing or decreasing) of the differences submitted to him for estimation; the error methods imply the 'procedure without knowledge' — the observer is left in ignorance of the conditions of tlte experiment. Typical of the latter class of methods is Vierordt's method of right and urong cases. Two slightly different stinuili are laid before the observer, in a long series of experiments; and he must say. in each case, whether the sensation aroused by the sec- ond stimulus is greater ( louder, brighter, longer ) tlian, equal to, or I'ess than the sensation aroused bj- the first. He does not know either the amount of difference or the order of presentation of the two impressions in the separate tests. From the distribution of the 'right,' "wrong,' and "equal' cases (right and wrong being determined by the objective relations of the stimuli), that stimulus difference is calculated which is able to evoke a just noticeable difference of sensation. Typical of the former class is Wuudt's method of minimal changes. Two subjectively equal stimuli are taken, and the one gradually increased luitil it is just noticeably different from the standard. This point determined, the stronger stimulus is gradually decreased, until the point of subjective equality is reached. The differences from the standard found in these two determinations are averaged. The procedure is then repeated in the reverse direction ; the one stimulus is decreased, to the point at which it is just noticeably less than the standard, and then increased again to subjective equality. . The two differences are averaged as Before. From the average of the two averages (the upper and lower difference limcns), we obtain the value of the difference limen. BiBLiOGRAPHT. Fechner, Elemente der Psycho- physik (Leipzig, 1860) ; G. E. Miiller, Zur Grund- legung der Psychophysik (Berlin, 1878); Miin- sterberg, Beitrage zur expcrimentcllen Psychol- ogic (Freiburg, 1890) ; Kiilpe, Outlines of Psy- chology (trans.. London, ISnS) : Archiv fiir (le- schichte der Philosophie, vol. vi. (Berlin, 1893) ; Wundt, Physiologische Psychologic (Leipzig, 1893) : Sanford, Course in Experintcntal Psychol- ogy (Boston, 1898). See Disckimination, Sen- sible; Limen. PSYCHROM'ETEK. See Hygrometer. liii. An Egyptian deity. See Mem- PTAH PHIS.