August 20 1908] 



NATURE 



Manv visual observations of the drawing of photo- 

 spheric matter into spots have been made, anions;- 

 others bv Sir Norman Lockyer more than forty years 

 ago, who described the apparent drawing" of a willow' 

 leaf into a dark spot,' and the late Father Secchi, 

 who was convinced that a swirl and a kind of suction 

 existed in them. 



A notable feature of a photograph taken after this 

 indrawing, on June 5, is the amount of " bright 

 eruptive hvdrogen in the region surrounding the two 

 spots." These opposite phenomena together strongly 

 suggest Sir Norman Lockyer's sun-spot theory of a 

 cooler central downrush with the later encircling 

 splash of dissociated matter.- A gyratory motion in 

 spots has been suggested bv many observers. A num- 

 ber of problems as to the levels at which the activi- 

 ties recorded take place are suggested by the photo- 

 graphs. The remarkable fact of the non-participation 

 of the high-level hydrogen in the difl'erential rotation 

 with latitude would at once follow if the equatorial 

 acceleration were due to a fall of material from greater 

 equatorial heights in the solar atmosphere. If the 

 hvdrogen is ascending in consequence of dissociation 

 from the densef materials of the fall, it cannot show 

 the same forward velocity as the falling material, to 

 which velocity I,ockver ascribes the more rapid equa- 

 torial movement of the photosphere.' 



The known correlation of the meridian passage of 

 laige spots or disturbed areas and the sudden changes 

 in the magnetic elements has given currency to 

 theories of the magnetic nature of this spot influence. 



6J0271 



Fli.. 2. — widening of spot line due to Zeeinan effect. 



.\ direct magnetic effect was negatived by Lord Kelvin 

 on account of the e.Kcessive amount of energy re- 

 quiied. A causal relation of this nature, whatever 

 the intermediate step or steps, has been almost 

 undoubted. 



It is suggested by Prof. Hale that a segregation 

 of positively or negatively charged particles caught 

 into the stream of a solar vortex would give rise to 

 magnetic lines of force at right angles to the plane 

 of the swirl. 



If such a field of force existed in a spot, the analogy 

 of terrestrial experiment would suggest that the period 

 of vibration of the ions emitting light in such a field 

 should suffer modification. The light emitted from a 

 spot near the sun's centre would be along these lines 

 of force. The doubling of spectruin lines with the 

 components circularly, and oppositely, polarised would 

 be expected. Prof. Hale promised to make spectro- 

 scopic search for this Zeeman effect on the first 

 opportunitv. In a more recent letter he announces 

 the complete success of his search. 



Spectra of the light of a spot and of the photo- 

 sphere were taken with the tower telescope and 30-foot 

 spectrograph, having a Fresnel rhomb and Nicol 

 prism mounted in front of the slit. The Zeeman 

 doublets in the spot spectrum photographed under 



1 J/n»//t^v Notices^ vol. xxv., Tune.'i865. p. 236. 



- " The Chemistry of the Sun," p. 412. (Macmillan and Co., 18S7.) 



y Ibid., pp. 422 and 424. 



NO. 2025, VOL. 78] 



these conditions should change in relative intensity 

 as the Nicol is rotated. When the spot was near the 

 limb of the sf ■ the results were uncertain, but when 

 about 45° from the centre the characteristic changes 

 were observed. The accompanying figure shows a 

 pair of photographs in the region of the iron line 

 A. 6302'7i taken on June 27. They are enlargements, 

 the resulting scale being i Angstrom = 9 mm. The 

 widening in the central spot bands of the line marked 

 represents a true doubling. In the reproduction it 

 will probably only be seen with difficulty that the 

 relative intensities of the components are reversed in 

 the two photographs due to the turning of the Nicol 

 through 45°. 



The separation of the components ranges from 

 0018 to o'2i6 Angstroms. In several cases a tripling 

 of lines was observed. 



Every care w'as taken, of course, that the variations 

 were not instrumental. .Similar effects were found 

 with other lines, though a remarkable and unex- 

 plained paucity ' of them occurred in the blue and 

 violet regions. Further details are promised in a 

 foithcoming paper of these exceedingly interesting 

 discoveries. 



The systematic recording of the solar magnetic 

 fields for comparison with simultaneous records of 

 terrestrial magnetism is suggested as desirable. 



The ease, apparently, with which these varied 

 records of solar activity can be obtained gives hope 

 that soon will exist ample material for the discussion, 

 and it is to be hoped the solution, of many of the 

 outstanding solar problems. 



From the perfect equipment at Mount Wilson much 

 is expected, but no one will think of minimising the 

 gieat credit due to Prof. Hale for the progress 

 towards more intimate knowledge of the stupendous 

 activities in the neighbourhood of the sun. 



T. F. C. 



.SOL.4R MAGNETIC FIELDS AND SPECTRUM 

 ANALYSIS. 



PROF. GEORGE E. HALE, of Mount Wilson 

 Solar Observatory, with great kindness has sent 

 me a letter of date July 6, together with a copy of a 

 manuscript destined for publication in Nature, on 

 " Solar Vortices and the Zeeman Effect." Prof. Hale's 

 paper is accompanied by two photographs on glass o-f 

 the double lines in the spot spectrum between two 

 comparison spectra of penumbra and photosphere of 

 the region A 6250-A. 6360. The position of the Nicol 

 in the arrangement used was changed 45° between the 

 first and the second of these photographs. 



Prof. Hale asks me to examine the photographs of 

 i^pectra, and to send a note to N.mure expressing my 

 opinion as to the interpretation of the results. I can 

 sav at once that I have come to the conclusion that 

 Prof. Hale has given what appears to be decisive 

 evidence that sun-spots are strong magnetic fields, the 

 direction of these fields being mainly pei'pendicular to 

 the sun's surface. Light received from a spot at the 

 centre of the sun would in this case be parallel to the 

 lines of force. 



A source of light in the laboratory, and placed in a 

 uniform magnetic field, emits, in the most simple case 

 covered bv the elementary Lorentz theory, parallel to 

 the lines of force, two rays circularly polarised in oppo- 

 site directions. Each spectral line is split up into a 

 doublet of two circularly polarised lines, the one 

 polarised clockwise, the other anti-clockwise. We 

 may imagine that for further analysis a Fresnel 

 rhomb and Nicol ai'e mounted before the slit of the 

 spectroscope, the arrangement actually used by Dr. 

 Hale. The two circular vibrations of the doublet are 



