Page:Light waves and their uses.djvu/117

Rh obtainable were placed in the instrument, and the thermometers themselves were carefully tested, their errors determined, and other well-known precautions taken. In this way the temperature at which the intermediate standard No. 9 contains the number of waves given above was determined to within one-hundredth of a degree.

The final step in the process is the comparison of the decimeter standard with the standard meter. This is a comparatively simple affair. In fact, it is exactly the same as the comparison of the first intermediate standard with the second, except that the second standard is now ten times as long—which necessitates going through the process ten times instead of twice.

Since in this case also we use the fringes for determining when one end of the standard and the reference plane are in the same plane, the error, as before stated, may be as small as one-twentieth of a wave; so that all the errors added together would be of the order of one-half of a wave, or one quarter of a micron.

The conditions which had to be fulfilled by the instrument which was used for this purpose are, then, these: We have, in the first place, to provide for the displacement of the intermediate standard and of the reference plane in such a way that the parallelism of the mirrors is not disturbed. This necessitates that the ways along which the carriage supporting the mirrors moves be exceedingly true. It took a whole month to perform this part of the work—to get the ways so nearly true that there should be no change in the position of the fringes as the mirrors were moved back and forth. In the second place, we must be able to know the position of the mirrors inside of the box which is placed over the instrument to protect it from temperature changes. To secure this, the carriage which holds the mirrors must be moved by means of a long screw carefully calibrated to