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 double D lines apart, by aid of the adjustment screws on the mirror M 

 and N, symmetrically to the ends of the yellow field in the telescope, the 

 interferences were isolated and located midway between the D doublets of 

 each spectrum, i.e., in the center of the field of the telescope. They could 

 now be observed to better advantage. In the small telescope there is appar- 

 ently but one dark line. If stationary, its ultimate character, when centered, 

 would be surmised to be given by the intersection of a vertical diameter with 

 a series of confocal ellipses, successively bright and dark, as indicated in 

 figure 2. The light and dark parts alternate or flicker. On moving the 

 micrometer, the vertical intersector A takes a more and more lateral position 

 like B, so that the trembling interferences would soon be invisible, as they 

 rapidly become finer and hair-like (not shown). 

 'On using higher magnification (larger telescope), two black lines bordering 



3) 



