ATMOSPHERE OF THE SUN DESLANDRES. 



345 



That the H and K lines of calcium are an exception to this rule 

 was announced simultaneously in February, 1892, by Hale and Des- 

 landres. These dark lines are very broad, indeed the broadest in the 

 solar spectrum; but wherever on the solar surface there is a facula, 

 they are reversed ; in other words, there appears a bright line through 

 the center of the broad dark line, and this bright line is itself double 

 and stands out therefore as the lines of the prominences do outside 

 of the limb. (See fig. 1, which shows the K line and its components 

 K lV , K 2V , K 3 , K 2R , K lR .) 



This result was obtained by Hale with the spectroheliograph, a 

 new, somewhat complex contrivance that isolates a certain radiation 

 with a second slit and by the movement of the first slit over the sun's 

 image furnishes a monochromatic image of the sun. I, myself, have 



Fig. 2. — (Schematic) ss, section of the sun made by the slit of the spectroscope; the 

 chromosphere and the spot are very much enlarged ; bright K 2 line, attributed to the 

 calcium vapor and which appears in the middle of the broad dark K line of the ordinary 

 spectrum ; it is single and narrow above spots and at the upper level of the chromosphere 

 and double at other places, being then divided into two parts by the dark central Kg line. 



employed an ordinary simple spectroscope giving successive sections, 

 though fully recognizing the use of the spectroheliograph. 



Meanwhile these two observers were at variance upon an impor- 

 tant point. Hale placed these vapors thus revealed in the facula 

 itself, below the surface, while I placed them, on the contrary, above 

 in the atmosphere. The ordinary spectroscope furnishes all the data 

 necessary for the solution of this question. Accordingly, in this re- 

 spect it is superior to the spectroheliograph. 



The double K 2 line is bright not only over the faculse but at all 

 other points over the disk where it is present — weaker, it is true, 

 and more difficult to detect. The bright, double K 2 line is always 

 sharp just within the limb, and is prolonged beyond the edge of the 

 disk as a double, bright line. (See fig. 2, which shows plainly the 



