194 CARNEGIE INSTITUTION OF WASHINGTON. 



action is much greater, show normal displacements, while simultaneous 

 observations on the large image show great irregularities. 



An important result appearing in the course of the investigation is 

 that a short series of observations may lead to misleading conclusions, 

 and that in attacking such questions as possible differences between 

 the solar rotation in the two hemispheres, or the variation with lati- 

 tude, observations should cover extended periods of time. 



The differences in rotation depending upon line-intensity or level 

 shown by Ha and the strong magnesium lines have been confirmed by 

 observations upon the high-level H and K lines of calcium and the 

 low-level lines of the nitrogen (cyanogen) band in the same region. 

 These two groups of lines represent the most extreme differences in 

 level with which we are at present acquainted, and should give the 

 widest range in rotation. At the equator the H and K lines show 

 displacements 10 to 15 per cent greater than those of the weak band 

 lines, and for higher latitudes the differences increase, while the Ha 

 lines, according to the measures of Mr. Adams, give at the equator a 

 velocity only 3 per cent higher than that derived from the reversing 

 layer. 



WAVE-LENGTH INVESTIGATIONS. 



A number of problems are dependent upon a knowledge of precise 

 values of the wave-lengths of the lines of iron in the solar spectrum. 

 These are being measured in international units, both at the center 

 and the limb, with the aid of the iron standards derived with the 

 interferometer and an iron arc from which the pole effect is elimi- 

 nated, supplemented by intervening lines whose wave-lengths are 

 being concurrently determined with the spectrograph. 



THE GREEN [CORONAL LINE AT THE 1918 ECLIPSE. 



The observations by Mr. Adams and Mr. St. John for the deter- 

 mination of the absolute wave-length of the green coronal line and 

 the rotation of the corona were only partially successful, since, owing 

 to clouds, the spectrum of the corona was obtained only at the east 

 limb of the sun. 



The instrument used was a prismatic spectrograph of 40 inches 

 focus, having a dispersion of 6 A per millimeter at X 5300. The slit was 

 placed in coincidence with the sun's equator on an image 2 cm. in 

 diameter, and just before totality a comparison spectrum was obtained 

 by exposure on an iron arc, a simple occulting arrangement cutting 

 out the region of X 5303. The comparison spectrum has made it 

 possible to determine the absolute wave-length of the green line to a 

 high degree of accuracy, aside from the influences of rotation and 

 local movements. The spectrum of the line is very weak and gives 

 the impression that with a stronger exposure it might appear less 

 simple in structure. 



