1909] on Solar Vortices and Magnetic Fields. 617 



light, and that their apparent darkness is merely the result of contrast 

 with the intensely brilliant surface of the photosphere. We also know 

 that the sun is a gaseous globe, attaining a temperature of about 

 6000° at its surface, and perhaps millions of degrees at its centre. 

 If we examine a large-scale photograph of a sun-spot we see that it 

 consists of a dark central region, called the umbra, and a surrounding 

 area, decidedly less dark, called the penumbra. The structure of a 

 spot, as this admirable photograph by Janssen shows, is granular, 

 like that of the photosphere. In the penumbra these granulations 

 seem to group themselves more or less radially, as though under the 

 influence of some force directed toward or away from the umbra. 

 Unfortunately, direct photographs of the sun have not yet attained 

 such perfection as to show the most minute details of sun-spots. To 

 appreciate these, we must have recourse to the exquisite drawings of 

 Langley, the truthful quality of which is recognised by every astro- 

 nomer who has observed sun-spots under favourable conditions. We 

 shall see that the characteristic structure represented by these draw- 

 ings is repeated, on a far greater scale, in the higher regions of the 

 sotar atmosphere disclosed on recent spectroheliograph plates. 



Since the time of Sir John Herschel, many astronomers have 

 proposed vortex theories of sun-spots. One of the first of these is 

 the theory of Faye, who supposed the whirling motion to be the 

 direct result of the peculiar law of the sun's rotation. This law was 

 discovered by Carrington, who found from observations of spots near 

 the equator that the sun completes a rotation in about 25 days, while 

 the motion of spots at a latitude of 40° indicated the time of rotation 

 to be nearly two days longer. Thus, as the rotation period increases 

 toward the poles, the photosphere at the northern and southern 

 boundaries of a sun-spot must move at different velocities (assuming 

 the law of the sun's rotation to be the same as that of the spots). 

 This difference in velocity would tend to set up whirling motions, 

 clockwise in the southern hemisphere and counter-clockwise in the 

 northern hemisphere. Sun-spots, in Faye's opinion, are the visible 

 evidences of such whirls. 



This theory has had many supporters, but it is now generally 

 agreed that the difference in the rotational velocity of adjoining 

 regions of the photosphere is not nearly sufficient to account for the 

 observed phenomena. Secchi, one of the most assiduous observers 

 of solar phenomena, was strongly opposed to Faye's theory. He 

 pointed out that about 6 per cent, of the spots he observed gave some 

 evidence of cyclonic action, but in the vast majority of cases such 

 forms as Faye's theory seemed to demand were lacking. We never- 

 theless owe to Secchi a most striking drawing of a sun-spot vortex. 



When the spectroheliograph was first systematically applied to 

 solar research in 1892, many rival theories of sun-spots occupied the 

 field. Since the function of this instrument is to photograph the 

 phenomena of the invisible solar atmosphere, it might be hoped that 



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