Light Waves as Standards of Length 103
Fig. 7(> represents the actual instrument in perspective. In this the two microscopes, with their arrangement for producing an illumination on the meter bar by means of reflected light, are shown. On the left are the handles which turn the two screws. One of these moves the intermediate standard
FIG. 76
and the other moves the reference plane. The complete instrument in the case which protects it against temperature changes is shown in Fig. 77.
This investigation was reported in the spring of 1892 to Dr. Gould, who at that time represented the United States in the International Committee of Weights and Measures. It was principally through his goodness that I was asked to carry out the actual experiments at the International Bureau of Weights and Measures at S&vres. Many of the accessories that were required for the instrument which has just been described had to be made in this country, and were
104
Light Waves and Their Uses
taken crver 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 than
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.