November ii, 1909J 



A'A TURE 



5! 



doublet could be cut off at will by rotating the Nicol, 

 precisely as Zeeman had done in the laboratory. On 

 account of the unique character of the Zeeman doublets, 

 this test alone was alnnost sufficient to prove the existence 

 of a magnetic field in sun-spots. But one of the great 

 beauties of the Zeeman effect is its many-sided character, 

 which permitted the test to be multiplied and extended. 

 I'Vom Zeeman's first experiments it was known, for ex- 

 ample, that if the strength of the magnetic field is in- 

 suflicient to separate completely the components of a 

 doublet, the edges of the resulting widened line should be 

 circularly polarised in opposite directions. Thus those 

 lines which are w'idened, but not doubled, in spots might 

 be expected to shift in position when the Nicol is rotated. 

 This was found to be the case. Again, the lines which 

 constitute the flutings of the spectra of compounds are not, 

 in general, affected by a magnetic field. Hence such lines 

 in the spectrum of a sun-spot should not be shifted when 

 the Nicol is rotated. This, also, was found to be true. 

 But a still more satisfactory test was suggested by another 

 laboratory phenomenon. When a doublet is observed along 

 the lines of force, with one of the components extinguished 



K 5940-87 



Fig. 6— (i) Souihern .Spot, showing Red Components of Doublets. Nic, 

 Spot, showing Violet Components of Doublets. Nicol, 29' VV. (3) 

 Red Components of Doublets. Nicol, 61° E. (4) Spot Spectrum w: 

 showing both Components of Doublets. 



by the Nicol, reversal of the current through the magnet 

 should extinguish the visible component and cause the in- 

 visible one to appear. In the sun, according to our hvpo- 

 thesis, reversal of the direction of revolution in a vortex 

 should correspond to reversal of the current through the 

 coils of a magnet. Hence the red component of a doublet 

 should appear in the spectrum of a vortex rotating in one 

 direction, the violet component in that of a vortex rotating 

 in the reverse direction. Fortunately, the appearance, on 

 opposite sides of the solar equator, of two spot vortices 

 rotating in opposite directions (Fig. 4) made this test 

 possible. The results were perfectly in accord with the 

 hypothesis. 



So far we have been considering only such phenomena 

 as are observed parallel to the lines of force of a magnetic 

 field; but a spectral line which, in such circumstances, 

 appears as a doublet is usually transformed into a triplet 

 when the observation is made at right angles to the lines 

 of force. The circularly polarised side components of the 

 NO. 2089, VOL. 82] 



doublet give place to plane polarised components, occupy- 

 ing the satne position, while another line appears centrally 

 between them. The light of this line is also plane 

 polarised, the direction of the vibrations being parallel to. 

 the field, while the vibrations of the side components are 

 in a plane at right angles to the field. Thus when a spot 

 is carried by the solar rotation to a point near the limb 

 we might expect the double lines in its spectrum to be 

 transformed into triplets if produced by a magnetic field. 

 The failure of the central line to appear seemed to raise 

 an important argument against the magnetic hypothesis. 



At this point the necessity of conducting laboratory in- 

 vestigations in immediate conjunction with astronomical 

 observations is well illustrated. Fortunately, our labora- 

 tory was already well equipped for work of this nature 

 (Fig. 7). In anticipation of the possibility that observa- 

 tions of the Zeeman effect would be needed in the inter- 

 pretation of solar and stellar phenomena, a powerfuf 

 electromagnet, with suitable accessory apparatus, had been 

 provided. \ brilliant spark, produced between metallic 

 electrodes in the field of the magnet, furnished the source 

 of light. .As many of the double lines in sun-spot spectra 

 are due to iron, this metal was 

 selected for the first experiments. 

 The spectrum was photographed, 

 at various angles with the lines 

 of force, with a powerful spectro- 

 graph, like the one used witli- 

 the tower telescope, similarly 

 mounted in an underground 

 chamber. 



The difficulty of accounting for 

 the behaviour of the iron doublets 

 in the sun was removed by these 

 investigations. It appears that 

 these lines do not become trip- 

 lets when observed across the 

 lines of force. In reality they 

 are changed to quadruplets, or 

 doublets in which each of the 

 components is a close double line. 

 In the magnetic field of sun- 

 spots, which is much weaker 

 than the field used in the labora- 

 tory, the closely adjoining lines- 

 which constitute the components 

 of the doublets cannot be separ- 

 ated. Thus these sun-spot lines 

 should appear double at what- 

 ever position the spot may occupy 

 on the sun's surface. 



The distance between the com- 

 ponents of doublets or triplets 

 separated in the magnetic field' 

 varies greatly for different lines. 

 Some exceptional lines are not 

 affected in the least, others arf> 

 merely widened, and others an? 

 clearly and sometimes greatly 

 separated. It is therefore im- 

 portant to compare the widen- 

 ing and the separation of lines in a sun-spot spectrum 

 with the corresponding phenomena in the magnetic 

 field. With few exceptions, most of which may b^? 

 accounted for by the presence in the spot spectrum 

 of closely adjoining lines of other elements, the solar ancT 

 laboratory results were found to be in good agreement. 

 The following table gives a comparison of certain iron 

 lines in the spot and laboratory : — 



.1. 29" W. (2) Norihern 

 Northern Spot, showing 

 thout Rhomb or Nicol, 



:-length .iA, spark 





The column headed " A\, Spark " gives the distance- 

 between the components of the lines as observed in 

 the laboratory. As the strength ef the magnetic field used' 



