468 ASTROPHYSICS 



I incline strongly to the belief that this should be done. Let us con- 

 sider the case very briefly. Rowland's wave-lengths are based upon 

 spectrograms taken with high dispersion and resolving power. 

 Radial-velocity spectrograms are secured with instruments of much 

 lower power. Close solar and laboratory lines, of different intensities, 

 clearly separated on Rowland's plates, are blended on stellar plates. 

 For this and other reasons, the effective wave-lengths on the two 

 classes of plates are different. The difficulty of assigning correct 

 wave-lengths in the case of plates taken with a single-prism spectro- 

 graph is even greater : whole groups of separate lines are blended into 

 one apparent line, and lines actually single are very few indeed. It 

 is necessary to use blends, both in the stellar and comparison spectra. 

 Two methods at least are available to eliminate errors in velocity 

 due to errors in assumed wave-lengths. First: At the conclusion of 

 a long series of observations of stars of the same spectral type, the 

 velocity yielded by each line for each star should be tabulated. If 

 one line gives velocities consistently large or consistently small, the 

 conclusion is that its effective wave-length has been wrongly as- 

 sumed, and we should be justified in changing it arbitrarily. And so 

 on, for each line employed. This involves the assumption that the 

 comparison bright-lines and the corresponding stellar lines have the 

 same wave-lengths; and all the wave-lengths are reduced to one 

 system, true for the particular spectrograph employed. The method 

 is not entirely free from objection. Second: If the solar spectrum 

 and the comparison spectra are photographed on one and the same 

 plate, under precisely the usual observing conditions, measures of 

 this plate, corrected for the observer's very slight radial velocity 

 with reference to the sun, will form a reduction curve of zero velocity, 

 expressed in terms of micrometer readings. If a spectrogram of star 

 and comparison, made with the same instrument and measured in the 

 same manner, is compared with this reduction curve, measure for 

 measure, the speed of the star will be obtained directly, and irre- 

 spective of wave-length values; and many other fruitful sources 

 of systematic error will be eliminated at the same time. Mr. R. H. 

 Curtiss, of Mount Hamilton, formulated a method l on this basis last 

 year, and he has applied it to a spectroscopic-binary variable star. 

 The observations were made with a spectrograph whose dispersion 

 is but one fifth, and whose exposure-time for a given star is but one 

 tenth that of the Mills spectrograph. The probable error for a faint 

 star seems to be not more than twice as great as that for a bright 

 star with the Mills spectrograph. The method promises to be of 

 great utility, capable of application to several thousand stars be- 

 tween the fifth and eighth magnitudes. 



On account of the large proportion of spectroscopic binaries, stars 

 1 Lick Observatory Bulletin, no. 62. 



