488 C. Barus — Interferences of Crossed 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 position. As stated above, I did not at first succeed 

 in finding the interferences, but the experiment is a delicate 



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one. In a repetition with first order spectra, it would be 

 advisable to replace the plane mirrors ?n, n by slightly concave 

 mirrors, about 2 meters in focal distance and to replace the 

 telescope Thy 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 adjust- 

 ment, 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 micro- 

 meter M was moved over about *50 cm. or more. Their 

 appearance with a small telescope was that of a single fine 

 line, alternately flaming yellow (very bright on the yellow back- 

 ground of the surrounding part of the spectrum) and jet black 

 as compared with the D lines, between which the interferential 

 line was situated. This flicker is referable to the tremor of 

 the laboratory, which makes it impossible to keep these 



