102
Light Waves and Their Uses
through an ordinary spectroscope. Thus the light from the tube Z is brought to a focus on the slit ix. It is then made parallel by means of the lens and passes through the prism TU, which is filled with bisulphide of carbon. The
lens x3 forms the spectral images of the slit tx in the plane of the slit /2* The arm Z W of the spectroscope can be moved so as to bring either the red, the green, or the blue spectral image upon this slit, from which it passes into the instrument.
Fig. 74 is a view of the plan of part of the instrument. The arrangement of surfaces shown diagramatically in Fig. 72 is readily recognized. All of the plates, I may state, instead of being rectangular, have a circular border, because in this form they can be worked true more readily.
Fig. 75 represents a vertical cross-section of the same instrument. It will be noted that the reference plane is divided into sections. This is done in order to enable us to determine very accurately the position of the interference fringes. The two intermediate standards will be recognized at the right.
fig. 7r>
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