The moment we pass beyond a very few seconds our consciousness of duration ceases to be an immediate perception and becomes a construction more or less symbolic. To realize even an hour, we must count 'now! now! now! now!' indefinitely. Each 'now' is the feeling of a separate _bit_ of time, and the exact sum of the bits never makes a clear impression on our mind. The _longest bit of duration_ which we can apprehend at once so as to discriminate it from longer and shorter bits of time would seem (from experiments made for another purpose in Wundt's laboratory) to be about 12 seconds. _The shortest interval_ which we can feel as time at all would seem to be 1/500 of a second. That is, Exner recognized two electric sparks to be successive when the second followed the first at that interval. =We have no sense for empty time.= Let one sit with closed eyes and, abstracting entirely from the outer world, attend exclusively to the passage of time, like one who wakes, as the poet says, "to hear time flowing in the middle of the night, and all things moving to a day of doom." There seems under such circumstances as these no variety in the material content of our thought, and what we notice appears, if anything, to be the pure series of durations budding, as it were, and growing beneath our indrawn gaze. Is this really so or not? The question is important; for, if the experience be what it roughly seems, we have a sort of special sense for pure time--a sense to which empty duration is an adequate stimulus; while if it be an illusion, it must be that our perception of time's flight, in the experiences quoted, is due to the _filling_ of the time, and to our _memory_ of a content which it had a moment previous, and which we feel to agree or disagree with its content now.