_In this case the stimuli are presented together in space; they may also be presented in time._ If you listen to a metronome beating, say, 15 in the minute, you will be able with practice to hold _six successive strokes_ in the focus of attention, but not more; if you try to group the seventh stroke with the preceding six you become confused; the series breaks, and cannot be welded together again. As the speed of the metronome is increased, the beats fall of themselves into groups of twos and threes; and you can still grasp and hold six of these rhythmical impressions. When the speed has reached some 200 in the minute, the rhythmical grouping becomes more complicated; as many as eight single beats may be bound together in a rhythmical unit; and the attention is adequate, again after practice, to five of these complex groups; the focus comprises no less than forty separate strokes of the pendulum. _This result, we may note, agrees very well with the canons of musical and poetic composition._ The musical phrase never contains more than six measures, and the poetical line or verse never contains more than six feet; a seven-measured phrase or a seven-footed line falls to pieces, ceases to be unitary. The rhythmical wholes of a higher order, the period in music and the stanza or strophe in poetry, never contain more than five phrases or verses; as a rule, neither contains more than four. So much for range; we turn to consider =duration=; how long can a sensation maintain itself at the focus? how long can we attend to a single simple impression? The early experiments on this question were most ingenious. The observer was required to look steadily at a little disc of very light grey, shown against a white background, or to listen intently to the very faint sound of a stream of fine sand; and the theory was that, since these stimuli were barely distinguishable at the outset, any lapse of attention, any decline in the vividness of the sensations, would blot them out altogether; they would disappear. The sensations did disappear, after a few seconds; and then, after another few seconds, came back; and so the conclusion was drawn that attention fluctuates, that we can attend to a single simple impression only for a few seconds at a time. No doubt attention fluctuates; but these experiments, unfortunately, are not to the point; for the disappearance and reappearance of the sensations can be accounted for by changes in the sense-organ, by adaptation, by twitching of the eyes, and so on. Other experiments have therefore been suggested. If we have recourse to smell and touch, we find that the course of adaptation to an odour, or to the pressure of a small weight laid upon the skin, may be followed attentively, without noticeable fluctuation, for two or three minutes; and the observers report that they could have kept up their attention still longer. Again, however, objection may be raised; for