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 minutes when we are very busy. Science teaches us the value of short periods of time, because nature, which science deals with, is always busy, filling up every moment with some kind of work. We poor mortals count time according to our own wants, and we think a second is about as short a time as we need pay any attention to; but nature does work in much shorter periods of time than a second. A lightning flash occupies only the millionth part of a second, and a dragon-fly's wing, as it passes through the air on a summer day, is quivering many hundred times a second. The fact is, our appreciation of intervals of time by the senses is very limited, and when events happen that are not more than the tenth of a second apart, we are apt to think they are simultaneous. The reason of this is that we need time to think, or, in other words, time is occupied by the processes that go on in our brains. Time is needed for perception, and if two events follow each other so rapidly that during the time we are perceiving the first the second comes on, we blur the one perception into the other, and we fail to notice the interval of time between them. And yet during that