NO. 1865 OUR KNOWLEDGE OF THE SUN — HAEE 347 



merits for different lines agree fairly well in magnitude with those ob- 

 served for the same lines in the laboratory when a source of light 

 containing the vapor in question is observed under pressure. That 

 increased effective pressure near the limb is probably the cause of 

 the line-shifts is further illustrated by the fact that the lines in bands 

 or flutings, such as those of cyanogen (shown in Fig. i, Plate xxx), 

 which are not displaced by pressure in the laboratory, retain the 

 same relative positions at the center and limb. 



These changes, and many others which it would be tedious to 

 enumerate, have been observed on photographs taken by Adams 

 and myself for the purpose of extending and perfecting our interpre- 

 tation of Sun-spot spectra. Almost the entire extent of the spec- 

 trum has been photographed and a large scale-map showing the 

 differences between the spectra of the limb and center is now in 

 preparation. The work of measurement is necessarily long and 

 trying, since the positions of hundreds of lines must be determined 

 on many photographs with the extreme precision required to reveal 

 the minute displacements concerned. For the interpretation of the 

 results extensive laboratory investigations on the effect of pressure 

 must be carried out, and special apparatus for this purpose is now 

 being prepared. Moreover, the possibility that anomalous dispersion 

 and other physical phenomena are involved must not be overlooked ; 

 and here, again, much laboratory work must be done. 



The Solar Rotation 



In mentioning the cyanogen band, I remarked that it occupies the 

 same position at the center of the Sun and at the limb. This is true, 

 of course, only after the effect of the solar rotation has been cor- 

 rected. All the lines of the spectrum, when observed at the east 

 limb of the Sun, are displaced toward the violet, while at the west 

 limb they are displaced toward the red, with respect to their normal 

 place as given by the light of the center of the Sun. The displace- 

 ments here involved are due to the Sun's axial rotation, and afford 

 the most accurate means we possess of determining its velocity. The 

 east limb of the Sun, in the region of the equator, is moving toward 

 us at the rate of 2.08 km. per second. Such a motion of a luminous 

 source shifts the lines of its spectrum a small distance toward the 

 violet. At the west limb, the motion being away from the observer, 

 the displacement is toward the red. In practice, the spectrum of the 

 east limb is photographed side by side with that of the west limb, 

 so that the double displacements may be measured. 



