104 THE CHEMISTRY OF THE SUN. [CHAP. 



An increased selective absorption at the limb appeared espe- 

 cially probable for the violet region, because the difference be- 

 tween centre and limb is more marked photographically than 

 visually, a view which received support from Herr Vogel's 

 observations. 1 With minute search however, Mr. Hastings 2 

 obtained evidence of increased selective absorption not only in 

 the extreme violet, but also in the visible part of the spectrum. 



He found that the lines \> \, and b B , " become sharper and 

 less hazy near the limb ; " c and F also become sharper in the 

 same region, and D is affected ; while on the other hand a line 

 7681 (Kirchhoff), which is strongly marked in the centre of the 

 disc, disappears entirely within 16" to 20" from the limb. 

 Two other lines near F (1828-6 and 1830'9 Kirchhoff) which 

 are strong at the limb become much fainter at the centre. 

 "These latter lines also become greatly strengthened over the 

 penumbrse of spots," while "768-1 is not thus affected." 



Besides these differences which were constantly observed, 

 others were suspected, and Mr. Hastings thus discussed the 

 phenomena : 



" Since the light from the border of the sun undergoes a general 

 absorption, which reduces its intensity to much less than one-fourth 

 that at the centre, according to Secchi's measurements, and yet the 

 spectroscopic character is changed so slightly, it is impossible for 

 me to escape the conviction that the seat of the selective absorption, 

 which produces the Fraunhofer lines, is below the envelope which 

 exerts the general absorption. But the phenomena of the faculae 

 prove not only that this envelope rests upon the photosphere, but 

 also that it is very thin. The origin of the Fraunhofer lines, then, 

 must be in the photosphere itself, which is in accordance with 

 Lockyer's views. 



"Any effects which the chromosphere might produce, we would 

 anticipate finding most evident in the lines of those gases which are 

 readily detected there. A reference to the observations shows at 



1 Naturforscher, Jahrgang v. p. 321, Oct. 1872. 



2 Nature, vol. viii. p. 77, May 22, 1873. 



