158 ANNUAL KKrORT SMITHSONIAN INSTITUTION, 1937 



eter measures make clear that the dark lines in the coronal spectrum 

 are certainly much less distinct through lack of contrast than in the 

 solar spectrum and, moreover, the dark lines are more prominent in 

 the outer corona than in the inner, but nevertheless the widths of the 

 coronal lines are the same at all distances out from the sun's surface, 

 and witliin errors of measurement arc identical in width with the 

 corresponding lines in the solar spectrum. From similar measures 

 carried out on the 1922 and 1932 spectra, Moore confirms Grotrian's 

 measures of the 1923 spectra. 



WHAT IS THE CORONA? 



According to Rosseland, the corona has "stimulated speculation to 

 the breaking point, it being even suggested that there we witness our 

 recognized physical laws set at naught by nature itself." It is evident 

 that the cause of the coronal radiation cannot be found in the emission 

 spectrum of coronium which extends no more than 5' from the sun's 

 edge. As the coronal streamers have been detected to distances 300 

 times greater, the explanation for the mysterious Ught of the corona 

 must be found in the radiation causing the continuous spectrum of the 

 inner corona and the dark-line spectrum at greater distances from the 

 sun. The spectra taken in 1923 showed Fraunhofer lines on the face 

 of the dark moon. On the assumption that these were caused by 

 coronal light scattered either in the earth's atmosphere or in tiie 

 spectrograph, Grotrian was able to apply corrections to the measured 

 intensities of the dark lines in the coronal spectrum. These corrected 

 intensities show that near the sun's edge the continuous spectrum is 

 much stronger than the Fraunhofer spectrum so that no lines are 

 visible, owing to lack of contrast, in the inner corona, though no doubt 

 dark linea may be there. At increasing distances from the sun's 

 edge outward, the intensity of the continuous spectrum falls off more 

 rapidly than the dark-line spectrum. At an angular distance of 19'. 5 

 on the east side and 14'. 5 on the west side, or at an average of one 

 solar radius, the intensities of the continuous and the Fraunhofer 

 spectra are equal. At increasing distau'-es from the sun's surface, the 

 dark lines become gradually more readily visible owing to increased 

 contrast on account of the diminished intensity of the continuous 

 spectrum. 



Spectra taken at many ecUpses during totality have shown the H 

 and K lines and the stronger hydrogen lines in great intensity, even 

 across the face of the dark moon. These spectral lines take their 

 origin in light from the liigh cliromosphere scattered by the earth's 

 atmosphere, particularly near the beginning and ending of totality. 

 It, therefore, may be quite unsafe to assume that the lines of the spec- 

 trum on the face of the moon in 1923 were actually caused by scattered 

 light emanating from the corona rather than from sunhght reflected 



