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ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1910. 



these gases the spectrum line which indicates the greatest height is 

 the red one, Ha, of hydrogen; the other lines of hj^drogen show 

 lesser heights and diminish in brightness from the red to the violet 

 end of the spectrum. But reaching to the greatest heights of all are 

 the gasses corresponding to the very brilliant violet lines, H and K, 

 which are emitted by the compounds of calcium. As the atomic 

 weight and density of calcium vapor are relatively great, this seems 

 strange ; it is easily explained, hoAvever, following the suggestion of 

 Lockyer, by the dissociation of calcium in the sun and in the electric 

 spark in our laboratory. The H and K lines, in all respects excep- 

 tional, are very bright at the solar edge, assuring the easy photo- 

 graphing of the prominences with ordinary photographic plates. 



On the other hand, the heavy vapors which are much more numer- 

 ous extend up but a short distance into the atmosphere and are not 

 easily seen except at eclipses. They form the lower, relatively very 

 brilliant layer of the chromosphere, called the reversing layer. 



Kz.3 



Fig. 1. — Curve of the intensities in the solar spectrum in the neighborhood of the broad 

 dark K line. The cross-hatched sections show the positions of the slits of the different 

 spectroheliographs. 



2. THE CHROMOSPHERE PROJECTED ON THE DISK THE AVERAGE LAYER. 



Such are the principal results obtained by the method of Lockyer 

 and Janssen. They are truly wonderful, but in certain respects in- 

 complete. They tell us only of that part of the chromos]Dhere ex- 

 terior to the edge of the solar disk and even there only about the 

 lighter vapors at some distance from the limb. The part within the 

 edge, projected upon the disk, and fifty times more extended in area, 

 eluded our vision. But, since from 1892 to 1894, even this gap in 

 our knoAvledge has been covered by an absolutely general method 

 which reveals all the vapors, both heavy and light, and their suc- 

 cessive layers in the entire hemisphere turned toward the earth. 



At the border of the sun the lines due to these vapors stand out 

 bright upon the continuous spectrum of the sky; on the disk they 

 appear dark, and the continuous spectrum which then serves as their 

 background is that of the sun itself which is much more brilliant, 

 so that the difficulty of seeing these lines is far greater. 



