721 



We luxve therefore tlien tried to see the phenomenon directly by 

 means of tlie spectrometer, which served for provisional dispersion 

 in the experiments with the éciielon spectroscope. We were of 

 opinion that, the dissolving power of the prism system amounting 

 to 60.000, it must be possible to observe in this way a displacement 

 of the above given amount. Nothing was to be detected of this, 

 however. It is true that the at first narrow absorption line (which 

 has originated by self-reversal in the sodium flame, widened considerably 

 on the heating of the Wood tube up to about half the distance of 

 the two Z)-lines, but a displacement could not be perceived. 



On account of Dr. d?: Haas' return to P>erlin we had to stop our 

 joint observations at this point, and I continued the research alone. 



First I tried to obtain comparatively narrow emission lines, which 

 should be intensive enough to superpose the absorption lines on 

 them in the investigation with the échelon spectroscope by electric 

 discharges in a heated evacuated Wood tube with sodium. This, 

 however, appeared impossible, the width remained considerable, and 

 the self-reversal troublesome. 



Then I took another course, and used an arclamp as source of 

 light. To obtain monochromatic light 1 used the above mentioned 

 spectrometer as monochromator, diminishing the widths of the slits 

 so that the issuing light comprised a range of only about 0,5 A. Ü., 

 for some observations 0,4 A. U. In this case the adjacent spectra 

 will overlap only for a small part, as the distance between two 

 successive orders amounts to 0,39 A. U. The dissolving power of 

 the échelon spectroscope, which consists of 30 plates of 14,45 m.m., 

 amounts to 450.000 ') for D-\ight. 



The absorption lines of the arc of light, which are ' caused by 

 the sodium vapour present in it, appear, observed in this way, to 

 be double. Both for D^ and for D.^ there are two maxima of absorp- 

 tion, which are somewhat sharper for JD^ than for D^, and the 

 distance of which is smaller for D^ than for D^ under for the rest 

 the same circumstances. These distances are, how^ever, variable. The 

 lamp burning normally, the distance generally amounted to 0.053 

 A. U- for Z>i, to 0.078 A. Ü. for D^ for a point lying in the middle 

 of the arc, when Siemens' S A-carbons were used. It sometimes 

 occurred, however, for a certain pair of carbons that the distance 

 was much smaller, down to half the value ; sometimes too that it 

 was much larger, up to almost double the said amount. When new 



^) The results of the observations made about the components of the mercury 

 lines by the aid of this échelon speclroscope, were in agreement with those of 

 most of the other observers. 



48* 



