DURATION or PERSISTENCE IN TIME, one of the attributes of a sensory ex perience or an emotional state. It •is only with difficulty subject to further analysis, if at all. The flash or snap of a spark seems to carry with its place in the scale, °longer or shorter,° as directly as in the quality scale of redder or yellower, higher or lower in pitch, or in the intensity scale of brighter or duller, louder or more quiet. One of the cardinal problems of sensory psychology, then, is the study of the properties of this intrinsic temporal relation be tween experiences.
In the first place, by a historical development with which we are not here concerned, we have come into the possession of instruments which, through the mediation of what we know as periodic processes, enable us to assign a certain number as a temporal measure to the phenomena which we observe. This number stands in a very close relationship to our experience of duration: in general that datum which we expe rience as more lasting will have a greater tem poral measure. The first task of the investigator of duration is to determine to what extent this rule holds good, ann what the exceptions to it are. It is found that the situation here is in most respects parallel to that in other series. There is in each sense an initial limen (see LAMINA) below which a decrease in the tem poral measure produces no decrease in the sen sory duration. In the case of light, at any rate, although apparently not of sound, unlike the situation in the intensity series this lower limen of duration does not approximately coincide with the total evanescence of all sensation: thus an electrical spark of just noticeable intensity lasting 1/10,000 of a second, though still visible, cannot be distinguished from a spark likewise of just noticeable intensity lasting 1/100,000 of a second. In addition to the initial limen, dura tion in each sense has a difference limen, or just noticeable difference in duration and there is a terminal stimulus beyond which increases in the temporal measure although they may be correlated with experiences which enable us to place them in our temporal system (see TIME), are no longer registered by increases in the duration of a single sense-datum. The initial stimulus for a tone of 64 vibrations is 1/40 second of 187 vibrations, 1/125 second; of 3,168 vibrations or over, 0.0063 second. Obser vations on the initial limen, difference-limen and terminal stimulus for the duration of data are relatively scanty, but these same sorts of quantity exist for temporal intervals and here the literature is much more rich. If the
intervals are measured between sharp sounds, the initial limen, which here nearly coincides with the lower limit of observable intervals, is about 1/400 of a second. The difference limen is about 1/100 of the temporal measure of the stimulus when the latter is in the neighborhood of 0.3 second. For moderate stimuli Weber's Law (q.v.) is approximately obeyed. The ter minal stimulus is about 0.5 or 0.6 second. The initial limen of visual intervals is about 0.005 second, of tactile (pressure) intervals about 0.0002 second. For intervals between sensa tions of two kinds, the initial limen, which is between 0.1 and 6.02 second, varies with the order of the stimuli. While it is natural to suppose that the initial limen for sensation dif ferences of a given sort is about the same as for sensations of the sort, it should be noted that this is not an a priori law. Another quan tity which is also of much the same size is the length of the liminal experience qua experience. This is the measure by the chronometer of the interval from the instant at which the subject is aware of the presence of the stimulus to the instant at which he is aware it is no longer pres ent. For light Kiilpe has estimated that this period is about 1/20 second.
All the numerical values here are based on measurements which it is difficult, in fact almost impossible, to make with any high degree of ac curacy. A rapid succession of like sensations as the speed is increased, does not at once fuse into a single sensation, but goes through the very complicated phases of °flicker° or
or uvibration,>> where there seems to be superim posed upon a sequence of experiences a new single experience. Just when the sequence com pletely disappears is not easy to ascertain. For this reason, estimates as to least noticeable in tervals are likely to have a probable error bigger than the measurement itself. (See BERCs SON, HENRI ; TIME). Consult Kiilpe,
of Psychology> (tr. London 1901) ; Meumann, in Philosophische studien (VIII Leipzig 1893); Stern,