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 was 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
