46 NATIONAL ACADEMY OF SCIENCES 



opportune formation of two large spots, which appeared on the 

 spectroheliograph plates to be rotating in opposite directions 

 (Fig. 1 1 ) , permitted a still more exacting experiment to be tried. 

 In the laboratory, where the polarizing apparatus is so adjusted 

 as to transmit one component of a line doubled by a magnetic 

 field, this disappears and is replaced by the other component 

 when the direction of the current is reversed. In other words, 

 one component is visible alone when the observer looks toward 

 the north pole of the magnet, while the other appears alone when 

 he looks toward the south pole. If electrons of the same kind 

 are rotating in opposite directions in two sun-spot vortices, the 

 observer should be looking toward a north pole in one spot and 

 toward a south pole in the other. Hence the opposite compo- 

 nents of a magnetic double line should appear in two such spots. 

 As our photographs show, the result of the test was in harmony 

 with my anticipation. 



I may not pause to describe the later developments of this in- 

 vestigation, though two or three points must be mentioned. The 

 intensity of the magnetic field in sun-spots is sometimes as high 

 as 4500 gausses, or nine thousand times the intensity of the earth's 

 field. In passing upward from the sun's surface, the magnetic 

 intensity decreases very rapidly so rapidly, in fact, as to suggest 

 the existence of an opposing field. It is probable that the vortex 

 which produces the observed field is not the one that appears on 

 our photograph, but lies at a lower level. In fact, the vortex 

 structure shown on spectroheliograph plates may represent the 

 effect, rather than the cause of the sun-spot field. We may have, 

 as Brester and Deslandres suggest, a condition analogous to that 

 illustrated in the aurora: electrons, falling in the solar at- 

 mosphere, move along the lines of force of the magnetic field into 

 spots. In this way we may perhaps account for the structure sur- 

 rounding pairs of spots, of opposite polarity, which constitute the 

 typical sun-spot group. The resemblance of the structure near 

 these two bipolar groups to the lines of force about a bar magnet 

 is very striking, especially when the disturbed condition of the 

 solar atmosphere, which tends to mask the effect, is borne in 



