100
Light Waves and Their Uses
within two microns or so. In the third place, since there will be slight displacements, owing to the impossibility of getting the ways absolutely true, it must be possible to correct these displacements. The adjustments for effecting this are shown in Fig. 71. Fourth, we must have a firm support for the longer of the two standards to be compared,
FIG. 73
and a movable support, which moves parallel with itself, for the shorter standard.
The last standard, the auxiliary meter, has to be compared with the standard meter itself, and, therefore, the two must be of similar construction. In other words, in this last comparison we have to resort to the microscope again. For the meter bar which we had in the interferometer itself had two lines upon it as nearly as possible one meter apart, as determined by a rough comparison with the prototype meter. The standard No. 9 had to be compared with this. For this purpose an arm which had a fine mark on it was rigidly fastened to the standard No. 9, and arranged to come in the focus of the microscope. In making this comparison, we must admit, the order of accuracy is not so great.
Light Waves as Standards of Length 101
But there are only two of these to make, so that the possible error is the same as that to which we are liable in comparing two meter bars. This error is unavoidable.
The whole instrument had to be placed in a box, which protected it from temperature changes and drafts of air, and had to be placed on a firm pier so as to keep it as free from
FIG. 74
vibration as possible. Finally, the conditions which have been mentioned above for producing a suitable source of light had to be fulfilled. We have thus a fair idea of what conditions had to be met in constructing the complete apparatus for making this comparison.
We shall now show how these conditions were actually fulfilled in the apparatus that wTas used for the experiment.
Fig. 73 gives a plan of the entire arrangement. It is easy to recognize the vacuum tube which serves as a source of light and the arrangement of the plates in the interferometer. This arrangement is the same as that shown in Fig. 72. In order to have but one radiation at a time in the instrument, the light from the tube is passed