It is to be added that this constant error in time-reproduction is greater in the case of remote periods than of near ones of the same length. Thus, the retrospective estimate of a duration far removed from the present, say the length of time passed at a particular school, is much more superficial and fragmentary than that of a recent corresponding period. So that the time-vista of the past is seen to answer pretty closely to a visible perspective in which the amount of apparent error due to foreshortening increases with the distance. In practice, however, this defect in the imagination of duration leads to no error. Although, as a concrete image answering to some definite succession of experiences a year is a gross misrepresentation, as a general concept implying a collection of a certain number of similar successions of experience it is sufficiently exact. That is to say, though we cannot imagine the _absolute_ duration of any such cycle of experience, we can, by the simple device of conceiving certain durations as multiples of others, perfectly well compare different periods of times, and so appreciate their _relative_ magnitudes. Leaving, then, this constant error in time-appreciation, we will pass to the variable and more palpable errors in the retrospective measurement of time. Each person's experience will have told him that in estimating the distance of a past event by a mere retrospective sense of duration, he is liable to extraordinary fluctuations of judgment. Sometimes when the clock strikes we are surprised at the rapidity of the hour. At other times the timepiece seems rather to have lagged behind its usual pace. And what is true of a short interval is still more true of longer intervals, as months and years. The understanding of these fluctuations will be promoted by our brief glance at the constant errors in retrospective time-appreciation. And here it is necessary to distinguish between the sense of duration which we have during any period, and the retrospective sense which survives the period, for these do not necessarily agree. The former rests mainly on our prospective sense of time, whereas the latter must be altogether retrospective.[121] Our estimate of time as it passes is commonly said to depend on the amount of consciousness which we are giving to the fact of its transition. Thus, when the mind is unoccupied and suffering from _ennui_, we feel time to move sluggishly. On the other hand, interesting employment, by diverting the thoughts from time, makes it appear to move at a more rapid pace. This fact is shown in the common expressions which we employ, such as "to kill time," and the German _Langweile_. Similarly, it is said that when we are eagerly anticipating an event, as the arrival of a friend, the mere fact of dwelling on the interval makes it appear to swell out.[122]