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followed by any of the senses. Thus we learn that our senses are limited organs as regards the detection of movement. We can only follow periodic movements through a narrow Fig. 3.—steel cylinders emitting a high tone when struck, a, 12,000 vibrations, b, 20,000 vibrations, and c, 30,000 vibrations per second. range; and there are regions in which delicate movements occur, which we can only explore by indirect methods and by processes of thought.

Let me point out to you, in passing, that man's supremacy over the lower animals lies in the power he possesses of pushing his