102 THE CHEMISTRY OF THE SUN. [CHAP. 



Angstrom, in extension of Forbes's observation of the spectrum 

 of the limb of the sun during an annular eclipse, repeated the 

 observation under conditions which were more likely to reveal 

 any differences, if such existed, between the spectra of the 

 centre and limb. He projected an image of the sun on the slit 

 of the spectroscope, and then at his leisure was able to see 

 whether the Fraunhofer lines underwent any change when 

 different parts of the image were allowed to fall on the open- 

 ing. The results of this observation are thus described by 

 Angstrom : 1 



" Any very remarkable change I could not discover ; all that I 

 fancied I could remark was, that the intensity of the spectrum light is 

 somewhat less when the ray comes from the edge, than when from 

 the centre of the disc ; and this is evidenced by the circumstance 

 that the fainter Fraunhofer lines show themselves in the latter case 

 comparatively stronger, whereas when the light comes from the 

 centre of the solar disc, the fainter lines will sometimes even totally 

 disappear, while the stronger lines, as for example, some of the iron 

 lines, appear with correspondingly increased brilliancy ; as we 

 know by Kirchhoff's experiments that an increased difference of 

 intensity between the source of light and the absorbing gas is 

 favourable to the distinctness of the lines in the gas spectrum, it 

 would seem that this observation, if confirmed, is not repugnant 

 to what we already know concerning the absorbing power of 



To facilitate the comparison of the spectrum of the limb with 

 that of the centre of the sun, I used a specially-constructed slit 

 plate. One half of the slit is covered by a totally reflecting 

 prism, and another similar prism actuated by a fine screw is fixed 

 in a slide, so that its distance from the former can be varied. 

 The centre of the sun's image is allowed to fall on the 

 uncovered part of the slit, and its edge on the sliding prism, 

 from which it is reflected to the fixed prism, and thence into the 



1 Phil. Mag. s. 4, vol. xxiv., July, 1862, p. 3, note. 



