328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



particles. Moreover, at liigb temperatures carbon and many other elements 

 wliich occur in the sun emit negatively charged corpuscles in great numbers; 

 the complementary positively charged particles must also be present, more or 

 loss completely separated from the negative corpuscles." Thus electro-magnetic 

 disturbances on a vast scale may result from the rapid motions of charged par- 

 ticles produced by eruptions or other solar disturbances. 



Soon after the discovery of the vortices associated with sun spots it occurred 

 to me that if a preponderance of positive or negative ions or corpuscles could 

 be supposed to exist in the rapidly revolving gases, a magnetic field, analogous 

 to that observed by Eowland in the laboratory, should be the result. An equal 

 number of positive and negative ions, when whirled in a vortex, would produce 

 no resultant field, ^ since the.effect of the positive charges would exactly offset 

 that of the negative charges. But Thomson's statement regarding the possible 

 copious emission of corpuscles by the photosphere, and the tendency of negative 

 ions to separate themselves, bj^ their greater velocity, from positive ions, led to 

 the belief that the conditions necessary for the production of a magnetic field 

 might be realized in the solar vortices. 



Thanks to Zeeman's discovery of the effect of magnetism on radiation it 

 appeared that the detection of such a magnetic field should offer no great diffi- 

 culty, provided it were sufficiently intense. When a luminous vapor is placed 

 between the poles of a powerful magnet the lines of its spectrum, if observed 

 along the lines of force, appear in most cases as doublets, having components 

 circularly polarized in opposite directions. The distance between the compo- 

 nents of a given doublet is directly proportional to the strength of the field. 

 As different lines in the spectrum of the same element are affected in different 

 degree, it follows that in a field of moderate strength many of the lines may be 

 simply widened, while others, which are exceptionally sensitive, may be sepa- 

 rated into doublets. 



THE SUN-SPOT SPECTRUM. 



It has long been known that the spectrum of a sun spot differs from the 

 ordinary solar spectrum in several particulars. If, for example, we examine 

 the iron lines in a spot we find that some of them are more intense than in 

 the solar spectrum, while others are weaker. Again, we perceive that many 

 of the spot lines are widened and that the degree of widening varies for different 

 lines. Finally, if the observations are made with an instrument of high dis- 

 persion it will be seen that some of the iron lines which are single in the solar 

 spectrum are double in the spot spectrum. Such double lines wei-e first seen 

 by Young in 1892 with a large spectroscope attached to the 23-inch Princeton 

 refractor. Walter M. Mitchell, who subsequently observed them with the same 

 instrument, described the doublets as " reversals," which they closely resemble. 

 Mitchell's papers contain a valuable series of observations of these '* reversals " 

 and other sun-spot phenomena." * * * 



Our investigations in this field on Mount Wilson liave given the first photo- 

 graphic records of the "reversals" or doublets seen visually by Young and 

 Mitchell, and reveal thousands of faint lines beyond the reach of visual 

 observation. * * * 



"J. J. Thomson, Conduction of Electricity through Gases, p. 165. 



^ Unless separated by centrifugal force, as suggested by Professor Nichols. 



''Walter M. Mitchell, "Reversals in the spectra of sun spots," Astrophysical 

 Journal, Vol. XIX, p. 357, 1904 ; " Researches in the sun-spot spectrum, region 

 F to a," ibid., Vol. XXII, p. 4, 1905; " Results of solar observations at Princeton, 

 1905-1906," ibid., Vol. XXIV, p. 78, 1906. 



