Modern science, however, tells us that this mode of measurement is by no means the simple and accurate process which it at first seems to be.  In point of fact, there is something like a constant error in all such retrospective measurement.  Vierordt has proved experimentally, by making a person try to reproduce the varying time-intervals between the strokes of the pendulum of a metronome, that when the interval is a very small one, we uniformly tend to exaggerate it in retrospection; when a large one, to regard it, on the contrary, as less than it actually was.[117]

A mere act of reflection will convince any one that when he tries to conceive a very small interval, say a quarter of a second, he is likely to make it too great.  On the other hand, when we try to conceive a year, we do not fully grasp the whole extent of the duration.  This is proved by the fact that merely by spending more time over the attempt, and so recalling a larger number of the details of the period, we very considerably enlarge our first estimate of the duration.  And this leads to great discrepancies in the appreciation of the relative magnitudes of past sections of time.  Thus, as Wundt observes, though in retrospect both a month and a year seem too short, the latter is relatively much more shortened than the former.[118]

The cause of this constant error in the mode of reproducing durations seems to be connected with the very nature of the reproductive act.  It must be borne in mind that this act is itself, like the experience which it represents, a mental process, occupying time, and that consequently it may very possibly reflect its time-character on the resulting judgment.  Thus, since it certainly takes more than a quarter of a second to pass in imagination from one impression to another, it may be that we tend to confound this duration with that which we try to represent.  Similarly, the fact that in the act of reproductive imagination we under-estimate a longer interval between two impressions, say those of the slow beats of a colliery engine, may be accounted for by the supposition that the imagination tends to pass from the one impression to the succeeding one too rapidly.[119]

The gross misappreciation of duration of long periods of time, while it may illustrate the principle just touched on, clearly involves the effect of other and more powerful influences.  A mere glance at what is in our mind when we recall such a period as a month or a year, shows that there is no clear concrete representation at all.  Time, it has been often said, is known only so far as filled with concrete contents or conscious experiences, and a perfect imagination of any particular period of past time would involve a retracing of all the successive experiences which have gone to make up this section of our life.  This, I need not say, never happens, both because, on the one hand, memory does not allow of a complete reproduction of any segment of our experience, and because, on the other hand, such an imaginative reproduction, even if possible, would clearly occupy as much time as the experience itself.[120]