MOUNT WILSON SOLAR OBSERVATORY. 207 



INCLINATION OF THE LINES OF FORCE IN SUN-SPOTS. 



In the last report reference was made to the differences in the polari- 

 zation of Zeeman triplets observed in various parts of a spot near the 

 sun's limb. These correspond with what would be expected on the 

 a^umption that the lines of force at the center of the spot are nearly- 

 parallel to a solar radius, and that the angle they make with the surface 

 decreases in going outward to the edge of the penumbra. In the case 

 of small spots very close to the limb it is difficult to see the lines of 

 force (directed toward the observer) on the edge of the penumbra 

 lying nearest the center of the sun, but in large spots this difficulty is 

 not experienced. It goes without saying that a single symmetrical 

 spot is needed to bring out the effect properly, for if companions are 

 present the lines of force are distorted, with corresponding changes in 

 the polarization phenomena. 



If a single symmetrical spot is watched as it crosses the sun's disk, 

 the changes in the polarization phenomena are generally such as would 

 be expected from the above characteristics of the lines of force. When 

 the spot is near the center of the disk, plane polarization is best seen 

 near the edges of the penumbra, where the lines of force are nearly 

 nonnal to the line of sight. It sometimes happens, however, that the 

 central component of triplets remains visible in spots near the center 

 of the sun, where its intensity would be very small if the lines of force 

 were really radial. Very interesting polarization phenomena are 

 observed in complex sun-spots, especially' in regions between closely 

 adjacent spots of opposite polarity. 



SUN-SPOT POLARITIES. 



Sun-spots have been so numerous that the magnetic polarities of 

 nearly 40, on an average, have been observed each day with the 

 75-foot spectrograph of the 150-foot tower telescope. These of course 

 include the small members of large spot-gToups. With the aid of a 

 parallel-plate micrometer it has also been possible to measure visually 

 the strength of the magnetic fields in a large percentage of these spots. 



Throughout the year a new system of recording has been employed. 

 All spots of the 16-inch solar image at the focus of the 150-foot tower 

 telescope are traced on a large sheet of paper, and their polarities and 

 field-strengths are also indicated on the drawing. Thus our present 

 records are much more complete than those obtained formerly. 



In spite of the peculiarities of complex spot-groups, in which the 

 simple features of bipolar spots are frequently lacking, the fact remains 

 that bipolar structure is almost invariably observed. That is to say, 

 most spot-groups, however complex, can be divided into two regions 

 of opposite magnetic polarity, though it sometimes happens that 

 minor companion spots are not of the expected sign. Cases are not 

 uncommon in which a second bipolar group, or one of its members, 



