Page:Popular Science Monthly Volume 21.djvu/447

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AUGUST, 1882.


BETWEEN the external sensation and the internal perception stands the time-sense, adapted to the distinction and estimation of the succession; it is really a grosser hearing and sight, for the cochlea and retina do nothing more than distinguish the more or less rapid succession of impressions. The time-sense is susceptible of a high degree of training, as may be learned from intercourse with astronomers and watch-makers. The later chronoscopy has warranted the possibility of determining the educability of the nervous system to a punctual obedience. In the experiments which Herr Donders first made tentatively, the mean of the time which the same observer required to execute a determined movement in accordance with some signal which he saw, heard, or perceived, sunk from day to day to a limit which was soon reached. The result was the same in a small degree which the drill-master enjoys in a large degree, when to his command a single sound responds, hardly longer protracted than to twice the difference in the time required for the sound-wave to pass from him to the nearest and to the farthest man.

Finally, the internal sense also, which has already entered into our considerations, is susceptible of exercise. Before all, the memory is strengthened by practice to a certain degree, and according to its employment in different directions. It may be recorded here that, as I have heard Schleiden relate, Robert Brown was able to distinguish twenty-five thousand species, Kunth only twenty thousand species of plants; and, if Kunth undertook to impress more than that number

  1. An address at the anniversary of the Institute for Military Surgeons, Berlin, August 2, 1881.