due to the observer’s error are sufficient, I think, to explain all of the low values of e obtained by Dr. Ehrenhaft by the Brownian-movement method. Indeed, I have myself repeatedly found
coming out less than half of its proper value until I corrected for the evaporation of the drop, and this was true when the evaporation was so slow that its rate of fall changed but 1 or 2 per cent in a half-hour. But it is not merely evaporation which introduces an error of this sort. The running down of the batteries, the drifting of the drop out of focus, or anything which causes changes in the times of passage across the equally spaced cross-hairs tends to decrease the apparent value of
. There is, then, so far as I can see, no evidence at all in any of the data published to date that the Brownian-movement method actually does yield too low a value of “
”, and very much positive evidence that it does not was given in the [preceding chapter].
Indeed, the same type of Brownian-movement work which Fletcher and I did upon oil-drops ten years ago (see [preceding chapter]) has recently been done in Vienna with the use of particles of selenium, and with results which are in complete harmony with our own. The observer, E. Schmid,[123] takes as many as 1,500 “times of fall” upon a given particle, the radius of which is in one case as low as
—quite as minute as any used by Dr. Ehrenhaft—and obtains in all cases values of