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100 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, 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.