Page:Light waves and their uses.djvu/121

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Light Waves as Standards of Length 103

Fig. 7(5 represents the actual instrument in perspective. In this the two microscopes, with their arrangement for pro- ducing 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 tempera- ture 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 Sevres. Many of the acces- sories that were required for the instrument which has just been described had to be made in this country, and were

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