V.] NORMAL VARIABILITY. 63
through the formidable thicket of difficulties that bars the path of those who pursue the Science of man.
Mechanical Illustration of the Cause of the Curve of Frequency.-The Curve of Frequency, and that of Distribution, are convertible : therefore if the genesis of either of them can be made clear, that of the other becomes also intelligible. I shall now illustrate the origin of the Curve of Frequency, by means of an apparatus shown in Fig. 7, that mimics in a very pretty way the conditions
on which Deviation depends. It is a frame glazed in front, leaving a depth of about a quarter of an inch behind the glass. Strips are placed in. the upper part to act as a funnel. Below the outlet of the funnel stand a