taken over and installed in one of the laboratories of the Bureau.

The standard meter itself is kept in a vault underground and under double lock and key, and is inspected only once in ten years, and even then it is not handled any more thanFIG. 77 is absolutely necessary. It took the better part of an entire year to accomplish the work as it has been described. The final result of the investigation was that the number of light waves in a standard meter was found to be, for the red radiation of cadmium 1,553,163.5, for the green 1,966,249.7, for the blue 2,083,372.1—all in air at 15° C. and at normal atmospheric pressure.

It is also worth noting that the fractions of a wave are important, because, while the absolute accuracy of this measurement may be roughly stated as about one part in two million, the relative accuracy is much greater, and is probably about one part in twenty million.