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 Our arrangements for studying muscle are not yet complete. We have already seen that a muscular contraction is so rapid as to make it impossible to follow all its phases with the unaided eye. The questions at once occur to us: how long a time does it take to contract? if it does not contract at the same rate throughout its contraction, for how long a time does it contract quickly, and for how long a time does it contract more slowly? These questions lead us at once into another department of inquiry of immense importance in science—the measurement of time, and more especially the measurement of minute periods of time. We all recognise more or less the value of time, and the busier we are the more we value what we call fragments of time. On a long summer day in the holidays, when we have not much to do except gratefully to enjoy the beauties of nature and the sense of physical well-being, a quarter of an hour, or even an hour, is not appreciated as of much value; but when we have a great deal to do, as in winter, when every moment seems to be occupied, five or ten minutes are felt to be precious. We can put a good deal into ten