1909] on Solar Vortices and Magnetic Fields. 629 



and side lines of such a triplet, the angle between the lines of force 

 and the line of vision can be obtained. In the case of sun spots, the 

 data at present available are not sufficient for the accurate determina- 

 tion of this angle, but it seems to lie between 30 and 60 degrees, 

 when the spot is near the centre of the sun. On the hypothesis that 

 the magnetic field is produced by the spot vortex, it would then 

 follow that the axis of the vortex, instead of being radial, as we at first 

 assumed, makes an angle of much less than 90° with the surface of 

 the photosphere. 



The time at my disposal permits me to describe briefly only a few 

 other phases of this investigation. In the laboratory the central line 

 of triplets is polarised in a plane parallel to the magnetic field. 

 Hence, if the fight is passed through a Nicol prism, used without a 

 rhomb, it should be possible to extinguish this line at certain positions 

 of the Nicol, in which case a spot triplet would appear as a doublet. 

 This test has also been applied to the spot triplets, with the expected 

 result. In fact this method supplies a convenient means of recognising 

 close triplets, the components of which are too closely crowded to be 

 seen separately before the central line is cut out. Indications have 

 also been obtained of what may prove to be unequal rotation of the 

 plane of polarisation of this central line in different parts of spots. 

 The gradual decrease in the strength of the field from the umbra to 

 the outer limit of the penumbi-a has been studied, and magnetic 

 fields have been detected on the sun's disk in certain regions outside 

 of sun-spots. It is evident that many new phases of the subject are 

 likely to be developed in the future, especially if larger images of 

 the sun and more powerful spectrographs are employed. In this 

 connection it may be stated that a tower telescope of 150 feet focal 

 length, to be used on Mount Wilson, with a spectrograph of 75 feet 

 focal length, is now under construction. This will give a focal image 

 of the sun about 16 inches in diameter, in which small spots, as well 

 as large ones, can be studied. 



Although it now seems to be demonstrated that sun-spots are 

 electric vortices, judgment should be reserved as to the various 

 theories which have been advanced to account for their origin. 

 Many of the results I have described appear favourable to Emden's 

 solar theory, but it seems to be opposed by the important investiga- 

 tions of Evershed, who has found that the metallic vapours in sun- 

 spots flow radially outward from the umbra, parallel to the photo- 

 sphere. The further development of Evershed's work, and the 

 continued study of solar vortices and magnetic fields, should soon 

 permit a reliable theory of sun-spots to be formulated. 



It is evident that the rapid decrease upward of the strength of 

 the field in spots would prevent it from having an appreciable 

 influence on the higher solar atmosphere. At the distance of the 

 earth, as Schuster has shown, the combined magnetic effect of several 

 spots, all assumed to be of the same polarity, and not taking into 



