MOUNT WILSON OBSERVATORY. 237 



SUN-SPOT HYPOTHESES. 



Such curiously mixed polarities as those of the large JVIay spot must 

 be carefully considered in the elaboration of any hypothesis of bipolar 

 spots. Each major member of the group was divided into two parts, 

 of opposite polarity, so that in a sense the entire spot might be regarded 

 as a combination of two bipolar groups. Important abnormalities 

 remain, however, and these will afford much opportunity for study. 

 The polarity rule holds with very few exceptions in both hemispheres 

 of the sun, but it is not surprising that it should be departed from ex- 

 actly on the equator, especially in a region remarkable as a seat of 

 violent eruptions. 



The origin of spots at considerable depths below the surface of the 

 photosphere is indicated by (1) the prior appearance of eruptions, 

 which probably originate in viscous regions far beneath the surface; 

 (2) the law of the spot variation in latitude during the sun-spot cycle 

 and the variable length of the period ; (3) the reversal of the magnetic 

 polarity of spots at the minimum; (4) the reappearance of spots in 

 the same region; (5) the 27-day magnetic period, possibly indicating 

 the presence of invisible spots; (6) the increase in strength of the mag- 

 netic field with decreasing level; (7) the observed inclination of the lines 

 of force in the umbra and penumbra; (8) the existence of small spots 

 with fields more intense than their area would warrant, perhaps con- 

 nected with a large vortex below; (9) the irregular form of the penum- 

 bra, indicating that it is at a considerable distance above the spot 

 vortex. If a single spot represents a long columnar vortex, its lower 

 extremity, especially if deflected toward the east by a lower velocity 

 of solar rotation within the photosphere, may turn up to the surface of 

 the photosphere, thus forming a spot which should follow (on the east) 

 the primary spot in most cases. A bipolar group, in which the fol- 

 lowing member appears and disappears, may represent such a case. 

 It should be noted in this connection that a magnetic field, of the 

 expected polarity, is sometimes observed in such a group when the 

 second spot is not visible. In many bipolar groups, however, it is 

 hardly conceivable that the two principal members, especially when 

 large in diameter and close together, can represent the opposite 

 extremities of a semicircular vortex-ring. The difficulty is increased 

 in cases of mixed polarity, such as that presented by the May spot. 



SUN-SPOT SPECTRUM. 



The study of the Zeenoan effect in the sun-spot spectrum is a long 

 and diflBcult task, partly because of the large number of lines involved 

 (over 5,000), and also because of the incomplete separation of the vast 

 majority and the lack of sufficient laboratory data for those that are 

 clearly resolved. The measures made by Dr. H. C. Wilson, during a 

 residence of several months at the Observatory, and by Miss Mayberry 



