156 ANNUAL UErUKT SMITHSONIAN INSTITUTION, 1937 



visual light, a result quite in agreement with the assumption that the 

 coronal radiation is Rayleigh scattering from atoms or ions. At the 

 ecHpse of 1932, however, photographic measures by Dufay and 

 Grouiller proved that the amount of polari/.ation was entirely inde- 

 pendent of wave length and was a maximum of 26 percent in the 

 corona at 10' from the sun's edge. At the succeeding eclipse of 1934 

 Johnson found a maximmn of 28 percent polarization at 8'. 5 from the 

 sun's limb, a result in good agreement with the 1932 ccUpsc. Evi- 

 dently the scattering of Ught in the corona cannot be caused by 

 atoms or ions. 



THE SPECTRUM OF THE CORONA 



It may be said that the corona exhibits three separate spectra: 

 First, the emission spectrum of "coronium" existing in the inner 

 corona and extending on the average to 5', or 200,000 km from the 

 sun's edge; second, the continuous spectrum (without lines), of the 

 middle corona; and third, the Fraunhofer lines showing feebly in the 

 inner corona. 



In the Revised Rowland Tables (1928) are given a list of 40 bright 

 Unes attributed to the corona by Campbell and Moore. Of this list 

 of lines more than half, or 22, appear to take their origin in the high 

 chromosphere. Only 23 emission linos in the corona are known with 

 certainty at the following wave lengths: 3328, 3387.96, 3454.13, 

 3600.97,' 3642.87, 3800.77, 3986.88, 4086.29, 4231.4, 4311, 4359, 4567, 

 4586, 5116.0, 5302.90, 5536, 6374.30, 6701.8, 7059.6, 7891.9, 8024.2, 

 10746.8, and 10798.0. Other faint lines are suspected but not yet 

 verified. The green line of 5303 (discovered in 1869) and the ultra- 

 violet line at 3388 have the greatest strengths. 



In comparing the green and red coronal rings obtained in 1930 on 

 spectra taken with grating and without sUt, it was surprising to find 

 how httle they resembled each other in their structural details of 

 greatest strength and how little either resembled the liigh-level lines 

 K and Ha of the chromosphere. It was further found that the radia- 

 tion of 6374 always sticks close to the sun's edge and is more con- 

 centrated and more uniform in distribution than 5303. It is evident, 

 therefore, that these two lines cannot take their origin in the same 

 atom, or at least not in the same atom in the same state of ionization. 



In order to determine the atomic origin of "coronium," several 

 authorities have grouped coronal lines together depending on the sim- 

 ilarities in appearances or distribution of light in the spectral lines. 

 As the radiation of 5303 is very unevenly distributed around the sun, 

 it is e\ddent that spectra taken with sUt will be badly handicapped in 

 detecting smiilar structural details, especially when in many of the 

 coronal spectra the definition is none too good. The only groups to 

 which most authors agree are 5303 and 3388 put together, the strongest 



