. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. The D lines placed in coincidence are obviously opposites, each line being paired with the mate of the other. A fine wire must be drawn across the slit of the collimator, in order that the vertical coincidence may be tested. One should expect the interferences to appear between the D lines on gradually moving the micrometer mirror M, parallel to itself, into the required posi- tion. As stated above, I did not at first succeed in finding the interferences, but the experiment is a delicate one. In a repetition w
. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. The D lines placed in coincidence are obviously opposites, each line being paired with the mate of the other. A fine wire must be drawn across the slit of the collimator, in order that the vertical coincidence may be tested. One should expect the interferences to appear between the D lines on gradually moving the micrometer mirror M, parallel to itself, into the required posi- tion. As stated above, I did not at first succeed in finding the interferences, but the experiment is a delicate one. In a repetition with first-order spectra, it would be advisable to replace the plane mirrors m, n, by slightly concave mirrors, about 2 meters in focal distance, and to replace the telescope T by a strong eyepiece. This is the method used in the next paragraph, and it was more easily successful. Later I returned to the experiment with the same adjustment, except that the plane mirrors m, n, were placed beyond the grating, with the object of using the equivalent of second-order spectra to get more dispersion. This plan did not fail, and, having once obtained the interferences, the reproduction seemed quite easy, as they remained visible while the micrometer M was moved over about 5 mm. or more, a very important observation. Their appearance with a small telescope was that of a single fine line, alternately flaming yellow (very bright on the yellow background of the surrounding part of the spec- trum) and jet black as compared with the D lines, between which the interferential line was situated, and on an enhanced yellow ground. The flicker is referable to the tremor of the laboratory, which makes it im- possible to keep these interferences quiet. Shutting off the light from either mirror, M or N, naturally quenches the interferences, but leaves the yellow part of the spectrum behind. Obviously, coincidence of the longitudinal axes of the spectra alone is needed. Therefore, upon moving the two do
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