140 RUMFOilD SPECTROHEtlOGRAl^fi. 



sphere lies beneath the lowest level that can be observed at the limb. 

 On the basis of Kirclihoff's law, the comparative darkness of these 

 bands in the solar spectrum woidd be ascribed to the fact that the 

 calcium vaj^or which they represent is cooler than the photosphere 

 below it. With increasing elevation, in a region of lower pressure, 

 the density of the vapor decreases, and to this decrease of density 

 there corresponds a decrease in the width of the bands. In the lowest 

 portion of the chromosphere that can be observed at the sun's limb 

 the density of the vapor is so far reduced that the broad and diffuse 

 bands are replaced by fairly well-defined lines (Ho, K,), which main- 

 tain their width up to a certain elevation in the chromosphere and 

 then grow narrower, thinning out to much narrower lines (H^, K3) 

 in the upper chromosphere and prominences (fig. 2a, pi. 11). On 

 the disk H.. and K^ appear as fairly narrow dark lines at the center 

 of the broad H^ and K^ bands. They occur in practically all parts 

 of the disk, but differ greatly in intensity in different regions. Every 

 bright calcium flocculus on the disk is characterized by the presence 

 of bright II. and K. lines at the center of Hj and K^, with narrow 

 dark H3 and K3 lines, due to the absorption of the cooler and rarer 

 vapor in the upper chromosphere superposed upon them. Intensity 

 curves showing these peculiarities of the H and K lines are given by 

 Jewell in the Astrophysical Journal, iii (1896), page 100, Avhere the 

 displacements of the lines are also discussed. They are also illus- 

 trated in the photographs reproduced in plates 11 and iii. 



From a strict application of Kirchhoff's law it would appear that 

 the calcium vapor in the lower chromosphere is actually hotter than 

 the calcium vapor which lies above and below it. It seems imj^rob- 

 able that the law can be rigorously applied in this case, and hence it 

 may be necessary to attribute the strong radiation of the intermediate 

 layer to causes other than temperature alone. 



In view of the composite character of the calcium lines, it should 

 be possible with the spectroheliograph to photograj^h sections of 

 the calcium flocculi at levels corresponding to their several elements." 

 If, for example, the second slit were set at the extreme edge of K^, 

 the resulting photograph should show only that calcium vapor 

 which is dense enough to produce a line of tliis breadth, i. e., a sec- 

 tion across the base of the calcium flocculus, should be obtained. 

 Under no circumstances could the upper and rarer portions of the 



a Experiments of this kind were not undertalcen with the Kenwood spectro- 

 lieliograph, since the instrnnient was not well adapted for work with dark lines. 

 Some of the photoijraphs, however, nj)parentl.v show low-level (Ki) phenomena, 

 and Mr. Evershwl informs mo that his plates do likewise. M. Deslandres 

 made photographs with K, and K. in 1S04, hnt I have seen no statements of 

 conclusions derived from a study of the K, plates, and do not know whether 

 the method lias since been employed at Meudou. 



