DUBATION. 538 we shall put their duration as high as ^j DURATION. will vary, further, with the intensity of the applied stimuli. In the sphere of vision, the duration of sensation tlepcmls partly upon the intensity of stimulus, partly uiion the char- acter of the visual interval which separates the given sensations. Kiilpe estimates it, omitting the negative after-image (q.v. ) at ^g of a second. Temperature sensations last, on the average, about half a second. The times re- quired for the cognition of tones vary con- siderably with practice. Maximal practice pre- supposed, they lie. apparently, between one and two vil)rations. and tlius decrease with increase of the pitch-number of the stimulus. Thus, a tone of 04 vibrations nuist be sounded for ^ of a second, while a tone, of IS" vibrations can be cognized in yi- of a second. There is, however, a point upon the scale at which the duration of stimulus necessary to the cognition of a tonal quality sinks to an absolute miiiinuim. This point lies at the G of ."liiS vilirations in the second, which can be heard if the tone sound only lor yj'jj'fjj of a second. (For the duration of a pulse or wave of attention, see Attextiox.) No investigation of alTective durations has as yet been undertaken. Psychologists have also made a study of dura- tion as such — i.e. of our capacity to perceive and to compaie emi>ty time-intervals. Thus, two electric sparks, seen in daylight, are clearly differentiated if the time between them amount to j'iy of a second : the snaps of two sparks are heard as separate, under favorable conditions, if the interval be as small as ^^^j of a second. When the impressions belong to two dilTerent sense - departnumts (touch - hearing, touch - sight, sight-hearinff) . the just noticeable interval varies with the order in which they come (touch-hearing or beariug-touch) , and with the direction of at- tention: the values most frequently obtained fall within the limits jV and Jy of a second. It need hardly be said that the judgment in these cases is a judgment simply of the separateness of the two sensations, and that the attention is directed upon the stimuli themselves and not upon the length of the time-interval between them. Our appreciation of the relative duration of short intervals — i.e. our judgment as to whether a given interval is longer or shorter than an- other — is somewhat astonishingly accurate. For psj'chologieal purposes, intervals fall into three groups: (1) •Short" times, ranging from the just noticeable duration to about 0..j or 0.6 of a second; (2) 'moderate' times, extending from this latter value to .3 or 4 seconds: and (3) 'large' times, from 4 seconds upward. Tn the case of (1) our temporal estimation is direct; the impressions which limit the intervals (sharp sounds) are all in consciousness together: we do not compare the intervals, but judge simply of the rapidity or rate of succession of tlio limiting stiunili. Our judorment is therefore strongly influenced by the quality, intensity, and dura- tion of these limiting impressions, as well as by their rhythmical form, by the direction of at- tention, etc. With maximal practice and under favor.ible conditions, an interval of 0..3 second may be discriminated from an immediately fol- lowing interval of 0.30.3 .second. In the cise of (2) and (3), the temporal estimation is indi- rect. Large times are judged predominantly in terms not of time, but, of its contents; the jjrealer the number of ideas, perceptions, or feelings in the interval, the longer is the int<nval estimated to be. Moderate times seem to furnish the ma- terial of true temporal comp;irison. The first limiting impression of the lirsl interval is just disa2)pearing from consciousness as the first im- l)ression of the second interval enters. The former impression is then re|U'oduced ; a definite direction is given to the strain of attention, and the closing impression of the second interval is 'expected' at a moment corresponding to that at which the first interval closed. The basis of judgment is thus given by strain and other or- ganic sensations, with their accompanying feel- ings. The work so far accomplished upon the course of temporal discrimination is incomplete, and the laws upon which it rests demand further con- firmation. It appears, however, that there is cimstanc.v of the relative differential sensitivit.v (see DiscEiMiXATiox, SEX.sini.TS) over a certain range of short and moderate times; that short intervals are overestimated and longer intervals underestimated : and that this error of estima- tion has the zero value at an interval of 0.5 or O.G second (indifference point of temporal dis- crimination! . Consult: Kiilpe. Oifllincs of Pfujcholog;/ (Lon- don, 1895) : Wundt, GrtindziU/e tier phi/siolo- gischeit Psycholorfie (Leipzig. 1803) : Meunuinn, in Philosophisclte Stiidieii. viii. (Leipzig, 1893) ; Wever, in Philosophisciie l^tudien, ix. (Leipzig, 1 894 ) . DURATION. In plants, the length of the vegetative ]ieriud. One of the chief ecological classifications is based on the duration of the various plants or their organs. Perhaps the )nost important classification of tliis kind is that into monocyclic, dicyclic, and i)leiocyclic plants; or. as they are more jiopularly called, aimuals. biennials, and peremiials. Wbetber or not these types may be regarded as the product of certain ecological conditions, it is certain tli:it they are often associated with particular environ- ments. Annuals and biennials, for example, are most common in new habitats, as in gardens allowed to run wild, and after fires, or in xcro- pbytic situations, as on shores and deserts. Perennials, on the other hand, comprise most of the world's vegetation, and dominate particu- larlv in habitats that are ecologically rich or that have been long established. It seems, there- fore, that the perennial habit is, on the whole, the most successful, and it is not difficult to see whv, since perennials commonly spread vege- tativel.v as well as bv seed. Each .vear, there- fore, the perennials are on the ground at the beginning of the season, while anniials and bien- nials are obliged to start from seed. A garden patch allowed to run wild shows, as a conse- quence, fewer and fewer annuaN and more and more perennials each .vear until finally the an- nuals are gone. In desert regions and along shores the vegetation is sparse, and annuals or biennials may exist almost indefinitely. An- nuals are scarce in Alpine and .Arctic regions, perhaps because the vegetation periods are too short for their development. -Another important classification based on dira- tion is that into deciduous and evergreen forms;
