190 



CARNEGIE INSTITUTION OF WASHINGTON. 



rule given in preceding reports. The data include all spots whose 

 polarities were observed during the calendar year 1917. It should 

 be noted that much uncertainty attaches to the determination of the 

 direction of the whirl on Ha spectroheliograms, especially in members 

 of bipolar groups. The polarities, on the contrary, can almost invari- 

 ably be depended upon, although in the case of very small spots the 

 estimates are sometimes uncertain on account of the minute displace- 

 ments. 



For the years 1913 to 1917, inclusive, the total number of "regular" 

 polarities is 872, with 33 exceptions and 99 undetermined cases. For 

 the year 1917, the 15 exceptions to the rule include 1 faint marking, 

 2 unipolar spots with no flocculi, 1 very complex spot, 5 spots for which 

 there are only single unchecked observations, and 6 well-observed 

 spots. 



INCLINATION OF THE LINES OF FORCE IN SUN-SPOTS. 



When a Zeeman triplet in the spectrum of a spot near the sun's 

 limb is examined with a Nicol and quarter-wave plate, a line can 

 generally be found across the spot along which the red and violet 

 components of the triplet are equal in intensity. In a symmetrical 

 spot this line is nearly parallel to the limb. It represents the regions 

 in the spot at which the lines of force are approximately normal to 

 the line of sight. 



In spots near the east limb this "neutral line" passes through the 

 umbra, not far from its center. As the spot is carried toward the 

 center of the disk by the sun's rotation, the line moves toward the 

 following edge of the penumbra, and finally disappears. In large 

 spots the neutral line can sometimes be detected just within the 

 following edge of the penumbra when the spot is only 20° or 25° from 

 the center of the disk. After the spot passes the sun's center, the 

 neutral line reappears near the preceding edge of the penumbra, and 

 moves toward the center of the umbra, which it would finally attain 

 at the extreme limb if the observations could be made at this point. 



Irregularities in the forms of spots, the presence of small com- 

 panions opposite in polarity to the chief umbra, and other magnetic 

 peculiarities such as are noted below, frequently complicate the 

 observations, and affect the position of the neutral line. But its 

 average change in position with longitude, determined from observa- 



