52 



NA TURE 



[November ii, 1909 



in the laboratory was about 5-1 times tliat of tlie spot, the 

 quantities obtained by dividing the separations in the 

 second column by 51 are given in the third column. 

 These separations are directly comparable with the separa- 

 tions of the corresponding lines in the spot, which are 

 given in the fourth column. The fifth column shows that 

 the differences between the solar and laboratory results are 

 very small. As the strength of the field in the laboratory 

 was about 15,000 gausses, the strength of the field in this 

 spot would be about 15,000-^5-1=2900 gausses. The 

 strongest field hitherto measured on our photographs of 

 spot spectra is about 4500 gausses, corresponding to a 

 considerably greater separation of the lines (Fig. 8). 



When a similar comparison was made for various lines 

 of titanium and chromium, a much less perfect agreement 

 between the spot and laboratory results was found. It had 

 already been observed that such lines as D of sodium and 

 of magnesium, which undoubtedly represent a much 

 higher level than the great majority of lines in the spot 

 spectrum, are but very slightly widened. As these lines 

 are strongly affected by a magnetic field in the laboratory, 

 it appeared evident that the strength of the field in spots 

 must fall off rapidly in passing outward through the spot 



line crowds the components , so closely together that they ^ 

 are not. readily separated with the resolving power available, 

 .^s these triplets are photographed even when the spot is 

 very near the middle of the sun, it is evident that the spot 

 always sends out light which makes a considerable angle 

 with the lines of force. In a normal triplet the central line is 

 of twice the intensity of the side components, when observed 

 at right angles to the lines of force, and disappears 

 altogether when observed parallel to the lines of force. 

 Thus, by determining the relative intensities of the central 

 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 determination of this angle, 

 but -it seems to lie between 30° and 60° w'hen 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 



Fig. 7. — Interior of Pa.sadena Laboratory, showing .Slit-end of Vertical Spectro,[;raph and Magnet used in study of Zeeman effect. 



vapours. In these circumstances lines of other elements, 

 which represent levels higher than the average, should 

 show small separations in the magnetic field of the spot. 

 It seems probable that in this way the lack of perfect 

 agreement between the laboratory and solar results 

 observed in the case of titanium and chromium can be 

 accounted for. 



A further important test was afforded by the well-known 

 phenomenon exeinplified in Preston's law. According to 

 this law, the distance between the components of the lines 

 split up by a magnetic field varies directly as the square 

 of the wave-length. This we found to be true even in 

 the case of a metal like iron, the lines of which cannot 

 be grouped into series, if the average separations of a 

 sufficient number of lines were considered. We should 

 therefore expect that the widening of lines in spots would 

 rapidly decrease toward the violet, and that the separation 

 of spot doublets should diminish in a similar way. A study 

 of the spot spectrum shows that this actually occurs. 



It soon appeared that the normal spot spectrum always 

 contains triplets as well as doublets (Fig. 8). These are 

 less easily recognised, because the presence of the central 

 NO. 2089; VOL. 82] 



plane parallel to the magnetic field. Hence, if the light 

 is passed through a Is'icol 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 

 apiiear as a doublet. This test has also been applied to 

 the spot triplets, wjth 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 de- 

 crease in the strength of the field from the umbra to the 

 outer limit of the penumbra has been studied, and magnetic 

 fields have been detected on the sun's disc, 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 con- 

 nection it may be stated that a tower telescope of 150-feet 

 focal length, to be used on Mount Wilson with a spectio- 

 graph of 7S-feet focal length, is now under construction. 



