XA TURE 



[November 4, 1909 



SOLAR VORTICES AND MAGSETIC FIELDS.' 

 T HEARTILY appreciate the privilege of describing in 

 -'■ thii lecture-room some of the recent work of the Mount 

 Wilson Solar Observatory. Like so much of the scientific 

 lesearch of the present day, it goes back for its origin 

 to the fundamental investigations of English men of 

 science. The spectroheliograph, which tells us of the 

 existence of solar vortices, is a natural outcome of the 

 application of the spectroscope in astronomy, where 

 Englishmen were foremost among the pioneers. The detec- 

 tion of a magnetic field within these vortices followed 

 directly from Zeeman's beautiful discovery of the influence 

 of magnetism on radiation — a logical extension of the 

 earlier work of Faraday — and from the classic investiga- 

 tions of Crookes and Thomson on the nature of electricity. 

 In reviewing these great advances, investigators in other 

 lands must again and again wonder at the exceptional 

 ability of the English mind to make fundamental dis- 

 coveries. When these discoveries have been made it is a 

 comparatively simple matter to utilise them in many depart- 

 rrents of science. Americans cannot fail to rejoice that 

 they may share in the traditions of a race which counts 

 among its members the men who have given the Royal 

 Institution its fame. 



It is customary to distinguish sharply between the 

 observational and experimental sciences, including astro- 

 nomy in the former. In physics or chemistry the investi- 

 gator has the immense advantage of being able to control 

 the conditions under which his observations are made. 

 The astronomer, on the other hand, must be content to 

 observe the phenomena presented to him by the heavenly 

 bodies, and interpret them as best he may. I wish to 

 emphasise the fact, however, that the distinction between 

 these two methods of research is not so fundamental as it 

 may at first sight appear. In i860 a laboratory, in which 

 experiments were conducted for the interpretation of astro- 

 nomical observations, was established by Sir William 

 Huggins on Upper Tulse Hill. The advantage of imitating 

 celestial phenomena under laboratory conditions was thus 

 appreciated half a century ago. I shall indicate later 

 how important a part such a laboratory plays in the 

 work of the Mount Wilson Solar Observatory. I shall 

 also show that in other ways the astronomer may 

 advantageously follow the physicist, particularly in the 

 choice of observational methods and in the design of instru- 

 ments of research. 



-Sim-spots were discovered as soon as Galileo and his 

 contemporaries directed their little telescopes to the sun. 

 In fact, ancient Chinese records indicate that spots of 

 exceptional size had been detected by the naked eye many 

 ■centuries before. Long after their discovery, the most 

 diverse views were held as to the nature of sun-spots. 

 Sir William Herschel mentioned the uncertainty which 

 had existed prior to his time, remarking that the spots 

 had been variously described as solid bodies revolving 

 about the sun, very near its surface ; the smoke of 

 volcanoes ; sinoke floating on a liquid surface ; clouds in 

 the solar atmosphere ; the summits of solar mountains, 

 uncovered from time to time by the ebb and flow of a fiery 

 liquid, S.C. In Herschel's own view the spots are to be 

 considered as the opaque body of the sun seen through 

 openings in the luminous atmosphere which envelops it. 

 Indeed, he considered that the sun should be regarded as 

 the primary planet of our system, and even suggested the 

 probability that it is inhabited. " Whatever fanciful poets 

 might say, in making the Sun the abode of blessed spirits, 

 or angry moralists devise, in pointing it out as a fit place 

 for the punishment of the wicked, it does not appear 

 th.it they had any other foundation for their assertions 

 than mere opinion and vague surmise ; but now I think 

 mvself authorised, ttbon astronomical principles, to pro- 

 pose the Sun as an inhabitable world, and am persuaded 

 that the foregoing observations, with the conclusions I 

 liaye drawn from them, are fully sufficient to answer every 

 obiectinn that may be made against it."^ 



Sir John Herschel did not abandon the idea of an opaque 

 solar globe, but suggested that hurricanes or tornadoes 



^ Tliscour?5e delivered at ihe Rov.ll Institution on Friday, May 14, by 

 Prof. Georee K. Hale, Fnr.Mem.R.S. 



2 William Herschel, "On the Nature and Construction of the Sun and 

 Tixed Stars," p. 20. 



NO. 2088, VOL. 82] 



might account for the piercing of the two strata of 

 luminous inatter which ordinarily conceal this globe. 

 " Such processes cannot be unaccompanied by vorticose 

 motions, which, left to themselves, die away by degrees 

 and dissipate — with this peculiarity, that their lower por- 

 tions come to rest more speedily than their upper, by reason 

 of the greater resistance below, as well as the remote- 

 ness from the point of action, which lies in a higher region, 

 so that their centre (as seen in our water-spouts, which 

 are nothing but small tornadoes) appears to retreat 

 upwards. \ow, this agrees perfectly with that which is 

 obser\'ed during the obliteration of the solar spots, which 

 appear as if filled in by the collapse of their sides, the 

 penumbra closing in upon the spot, and disappearing 

 after it." 



We now know that sun-spots are brighter than the 

 brightest arc light, and that their apparent darkness is 

 merely the result of the contrast with the intensely brilliant 

 surface of the photosphere. W'e also know that the sun 

 is a gaseous globe, attaining a temperature of about 6000° 

 at its surface, and perhaps iniUions 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 soine 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 

 ttuthful quality of which is recognised by every astronomer 

 who has observed sun-spots under favourable conditions. 

 We shall see that the characteristic structure represented 

 by these drawings is repeated, on a far greater scale, in 

 the higher regions of the solar atmosphere disclosed on 

 recent spectroheliograph plates. 



Since the tiine of Sir John Herschel, many astronomers 

 have proposed vortex theories of sun-spots. One of the 

 first of these is the theorv 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 twenty- 

 five 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 stronglv 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 

 rases such forms as F.aye's theory seemed to demand were 

 lacking. W'e nevertheless owe to Secchi a most striking 

 drawing of a sun-spot vortex. 



When the spectroheliograph was first svstematically 

 applied to solar research in 1802, 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 the results would 

 throw much light on the nature of sun-spots. For many 

 years, however, this hope was not realised. The first 

 monochromatic images of the sun were made with the K 

 lint of calcium. If we compare such an image with a 

 direct photograph of the sun. made in the ordinary wav, 

 we see that the sun-spots are surrounded and frequently 

 covered by vast clouds of luminous calcium vapour. These 

 attain elevations of several thousand miles above the sun's 

 surface, hut thev must not be confused with the promin- 

 ences, which ascend to much higher elevations. When 



