MOUNT WILSON SOLAR OBSERVATORY. 261 



connect widely different spectral regions for comparing displacements 

 and wave-length. Vortical motions of the calcium vapor, shown by the 

 displacements of the H and K lines, appear to be characteristic of spots, 

 while there is little evidence of such motion given by the Ha lines of 

 hydrogen. Particular attention is being given to the comparative 

 study of the spectrum of the limb and the center of the sun, and to the 

 identification of the great number of new lines appearing only at the 

 limb. 



Further observations bearing upon possible evidence of anomalous 

 dispersion and the Einstein effect in the solar atmosphere are being 

 carried on. It has become evident that, in any effort to push deter- 

 minations to the third decimal place in comparing solar and terrestrial 

 sources by the spectrograph, it is necessary to make the exposures 

 rigorously simultaneous. Both the 150-foot and the 60-foot tower tele- 

 scopes are now equipped for this work, and the completion of the 

 interferometer installation in the 60-foot tower furnishes additional 

 means for checking old and of obtaining new data. Notwithstanding 

 the massiveness (4.5 tons), the stability, and the constant-temperature 

 condition of the 75-foot spectrograph, it is necessary to leave the instru- 

 ment completely undisturbed during the comparison of different sources. 



SOLAR ROTATION. 



During the year four series of plates have been made with the first 

 order of the 75-foot spectrograph (scale 1 mm. = 0.7 a) for the investiga- 

 tion of the solar rotation by Mr. Adams and Mr. St. John. The light 

 has been taken from a point 3 mm. within the edge of an image 410 

 mm. in diameter. These include the regions: 



(a) In the violet, X 4123-X 4340. 



(b) In the green, X 5018-X 5316. 



(c) In the red, X 6300-X 6563. 



(d) The fourth series, X 501S-X 5316, contains spectra of the two limbs and the center for 



the purpose of comparing the rotation values for the northern and southern hemi- 

 spheres, and is also available for obtaining limb-center displacements. 



The plates of series (a) and (6) have been measured for zero lati- 

 tude. The measurements confirm the results previously obtained at 

 this Observatory in showing differences depending upon the intensi- 

 ties of the lines, the stronger lines yielding the higher relative values. 

 For the three magnesium lines of the b group (mean intensity 22), the 

 equatorial velocity is 2.043 km. per second, while for 22 lines of inten- 

 sity 1 to 7 the mean equatorial velocity is 1.934 km. per second. 

 These results depend upon filar micrometer measurements made by 

 three observers, the three series being in practical agreement. 



An effect depending upon the intensit}- of the lines is shown by the 

 observations of 1906-07, 1908, and 1914-15 at this Observatory, which 

 is not apparent in the determinations at other observatories. In order 

 to eliminate systematic errrors possibly inherent in filar micrometer 

 measurements of lines differing widely in solar intensity, the displace- 



