346 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



appearance of the double K2 line at the border of the sun as well 

 as over a spot.) 



Since the K2 line, exterior to the limb of the sun, corresponds to 

 just what we have actually defined as the chromosphere, then we 

 must conclude: The photograph of the sun taken with the K, line 

 with the spectroheliograph represents the whole solar chromosphere 

 projected on the disk. 



Besides these images of calcium made in Paris in 1894, which were 

 the first exact images of this vapor, show the bright regions of faculae 

 larger than on the ordinary photograph of the surface, as well as 

 smaller bright regions now called flocculi. These flocculi are pres- 

 ent at the pole as well as at the equator. I have confirmed their 

 jDresence at the pole during the years of a sunspot minimum as well 

 as during the whole 11-year period. 



The bright Ko line remains double beyond the edge for some 4" 

 or 5" of arc, and as the chromosphere at the edge has a height of 

 10", we may say that our photograph represents the mean chro- 

 mosphere. 



Finally, while the first spectroheliographs were obtained in 

 America, in France was discovered for the first time the whole chro- 

 mosphere of the sun. 



3. LOWER CHROMOSPHERE. 



But we may proceed still further. In 1893, I stated that this 

 isolation with the spectroheliograph of an ordinary dark line pro- 

 duced an image of the corresponding vapor ; and in 1894, I isolated, 

 with a small spectroheliograph of low dispersion constructed at 

 Paris, the fading edges of the K line called K^r and K^v and the 

 neighboring strongest dark lines due to aluminum, to iron and to 

 carbon. The spectroheliogram obtained differed from those taken 

 with the K2 line. The spots, at times hidden in the K2 image, have 

 here their umbrae and penumbrse perfectly sharp and the regions of 

 faculse both at the edge and at the center though less extensive 

 than in the IC image. Indeed, this new image is intermediate be- 

 tween that of the surface and that of the mean chromospheric layer 

 as shown by the Ko photograph. It gives a picture of the entire 

 reversing layer now obtained for the first time. 



I showed, also, that a still greater dispersion would allow the iso- 

 lation of the much more numerous finer lines, and especially the 

 narrow black central Kg between the two components of K.. The 

 K3 line corresponds to the upper layer of the chromosphere. This 

 method becomes thus absolutely general; it will furnish views of 

 all the solar vapors indicating as well the successive superposed 

 strata whenever the spectrum lines can be divided into distinct parts 

 as in the case of the broad K line. 



