Along with these muscle-joint experiences, the eye is also being trained. The position of the image on the retina comes to stand for direction, and the eye finally develops so remarkable a power of perceiving direction that a picture hung a half inch out of plumb is a source of annoyance. The ear develops some skill in the perception of direction, but is less dependable than the eye. 4. THE PERCEPTION OF TIME The philosophers and psychologists agree little better about our sense of time than they do about our sense of space. Of this much, however, we may be certain, our perception of time is subject to development and training. NATURE OF THE TIME SENSE.--How we perceive time is not so well understood as our perception of space. It is evident, however, that our idea of time is simpler than our idea of space--it has less of content, less that we can describe. Probably the most fundamental part of our idea of time is _progression_, or change, without which it is difficult to think of time at all. The question then becomes, how do we perceive change, or succession? If one looks in upon his thought stream he finds that the movement of consciousness is not uniformly continuous, but that his thought moves in pulses, or short rushes, so to speak. When we are seeking for some fact or conclusion, there is a moment of expectancy, or poising, and then the leap forward to the desired point, or conclusion, from which an immediate start is taken for the next objective point of our thinking. It is probable that our sense of the few seconds of passing time that we call the _immediate present_ consists of the recognition of the succession of these pulsations of consciousness, together with certain organic rhythms, such as heart beat and breathing. NO PERCEPTION OF EMPTY TIME.--Our perception does not therefore act upon empty time. Time must be filled with a procession of events, whether these be within our own consciousness or in the objective world without. All longer periods of time, such as hours, days, or years, are measured by the events which they contain. Time filled with happenings that interest and attract us seems short while passing, but longer when looked back upon. On the other hand, time relatively empty of interesting experience hangs heavy on our hands in passing, but, viewed in retrospect, seems short. A fortnight of travel passes more quickly than a fortnight of illness, but yields many more events for the memory to review as the "filling" for time. Probably no one has any very accurate feeling of the length, that is, the actual _duration_ of a year--or even of a month! We therefore divide time into convenient units, as weeks, months, years and centuries. This allows us to think of time in mathematical terms where immediate perception fails in its grasp. 5. THE TRAINING OF PERCEPTION