. Carnegie Institution of Washington publication. The surprising success obtained with the film grating at short distancesinduced me to test similar methods at long distances. Figure 19 is an appa-ratus of this kind, in which L is the white beam incident from a collimator,G and G are the transmitting gratings, M, N, m, n, pairs of opaque mirrors,T the telescope. The undeviated ray, d, is screened off. The componentpaths a+b+c, a-f-fr-f-c were each about 4 meters long. The method ofadjustment again consisted in bringing the shadow of the thin wire acrossthe slit into the same position of the sp
. Carnegie Institution of Washington publication. The surprising success obtained with the film grating at short distancesinduced me to test similar methods at long distances. Figure 19 is an appa-ratus of this kind, in which L is the white beam incident from a collimator,G and G are the transmitting gratings, M, N, m, n, pairs of opaque mirrors,T the telescope. The undeviated ray, d, is screened off. The componentpaths a+b+c, a-f-fr-f-c were each about 4 meters long. The method ofadjustment again consisted in bringing the shadow of the thin wire acrossthe slit into the same position of the spectra seen in the telescope when thespectra coincide. For this purpose the adjustment screws for horizontal andvertical axes on M, N, m, n must be actuated together. To facilitate thistiresome work, with the observer at T, long levers brought from m and n,with their ends near his hands, as well as a lever from G (fore-and-aft motion),were very useful. Since the adjustment screws at M and N are alreadywithin reach, it is thus easy to bring any Fr
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