ATMOSPHERE OF THE SUN DESLANDRES. 347 



Now, the number of solar lines amounts to some 20,000; and ac- 

 cording to Jewell, all the solar lines show more or less the special 

 characteristics of the typical line of calcium. The new field open 

 to investigation is evidently very broad. 



4. RECENT RESEARCHES A GREAT SPECTROHELIOGRAPH OF NEW TYPE. 



The program of researches laid out in 1894 was accordingly very 

 extensive. It was carried out in part during the following years, 

 and the actual progress was marked, if not very rapid. 



In 1903 Hale and Ellermann took up the study in the black lines 

 with a spectroheliograph of greater dispersion, and after 1906 con- 

 tinued the work at Mount Wilson with yet more powerful instru- 

 ments. They have obtained beautiful pictures and a whole series of 

 new facts. AVith the lines of the reversing layer the results are 

 practically the same as those obtained in 1894. But the hydrogen 

 lines, and recently the Ha line especially, have shown new and very 

 curious phenomena, which we will describe in detail shortly. 



However, the dispersion employed has been only moderate ; though 

 they have isolated a much greater number of lines than in 1894, the 

 finer lines have not been used; and indeed in each case they have 

 used the entire lines, making no distinction of the separate portions 

 and therefore of the separate layers of the vapors. Their images 

 have resulted from the mixture of the several distinct ones due to the 

 several layers. 



I assumed the task of filling this gap and thus completing the 

 program of 1894, isolating the upper strata hitherto unrevealed. 

 Becoming director of the Observatory of Meudon in 1907, I was able 

 to apply to this task the resources of the observatory, and here the 

 special grant already mentioned proved very opportune. In short, 

 it became possible to construct a great spectroheliograph having a 

 dispersion as great as that of Eowland's large spectrograph and a 

 special building for its protection. 



This building consisted of a large chamber, 22 meters by 6 meters ; 

 its roof was of stone and earth, assuring the constancy of the tem- 

 perature within. It received the light from the sun by way of a 

 coelostat placed south of the building, and constructed from some 

 old transit-of- Venus apparatus and an old objective of 0.25 meter 

 aperture and 4 meters focus. These pieces, though mediocre, were 

 used for the sake of economy. The spectroheliograph, on the other 

 hand, was of a novel type and presents several interesting features. 

 It is somewhat complicated, at least in design, for it really consists 

 of four diiferent spectroheliographs grouped about the same colli- 

 mator. The first is of three prisms, two slits and a camera 3 meters 

 long, giving an image of the sun 85 millimeters in diameter; the 



