MOUNT WILSON OBSERVATORY. 



215 



Solar Wave-Lengths. 



The question whether the wave-lengths of the lines at the center of the sun 

 are constant is a vital one in studies of the relative displacements of arc and 

 solar lines. The Mount Wilson observations indicate that in the direct com- 

 parisons of these sources the more carefully conditions are controlled and 

 instrumental errors are eliminated, the smaller become the deviations between 

 separate determinations. The following table, prepared by Mr. St. John, 

 shows the accordance of the wave-lengths of lines at the center of the sun 

 when observed on different dates: 



These measurements were made on grating spectrograms. The results 

 with the interferometer are of about the same order of accuracy. 



In view of the probable existence of convection currents in the solar atmos- 

 phere, this constancy of wave-length at the sun's center is rather surprising. 

 It appears to find its explanation in the fact that at any given level these 

 currents reach a comparatively steady state, although at different levels they 

 may differ both in amount and direction. 



Mr. Babcock has continued his measurements of the wave-length of lines 

 at the sun's center with an interferometer attached to the Snow telescope. 

 These observations extend toward the violet as far as X4000, but are incom- 

 plete for wave-lengths shorter than X5000. In the interval between X4000 

 and X6923 about 900 lines have now been measured on from 1 to 15 photo- 

 graphs. The wave-lengths are determined by reference to the iron arc, and 

 in the region of wave-length longer than X5000 may be compared with 

 those obtained by Mr. St. John with the high-dispersion plane-grating spectro- 

 graph. For 184 lines common to both lists, the difference, interferometer 

 minus grating, is zero for 33 lines, positive for 88, and negative for 63. The 

 mean deviation is 0.002 a and the mean difference taken with regard to sign is 

 +0.0004 a. Measures are now in progress which, with the addition of a 

 comparatively small number of new photographs, will extend the work as 

 far as is practicable into the ultra-violet. 



The data which are being collected in this investigation, when combined 

 with the corresponding measures made with the grating spectrograph, will 

 provide, on the one hand, the basis for a new table of standard solar wave- 

 lengths, and on the other the direct observational material for a discussion 

 of the gravitational shift of the solar lines required by the theory of relativity. 



Center and Limb Displacements in the Sun. 



One of the phenomena, the study of which is well adapted to throw light 

 upon the complex question of the sources of the displacements of solar lines, 

 is that of the relative shift between the lines at the center and the limb. 

 First discovered by Halm in 1907 and later studied more extensively by 

 Adams, this effect requires a thorough reinvestigation with the aid of powerful 

 modern spectroscopic apparatus, a large image of the sun, and direct com- 



