462 On Spectral Lines of Solar and Terrestrial Origin, 



The eye then becomes sensitive to the least oscillation of the 

 lines. 



The distinguishing of the lines becomes then entirely- 

 intuitive. We observe each spectral line, and the result is 

 such as would be produced by shaking it. If it remains im- 

 movable, it is a telluric line ; if it oscillates, it is a solar line. 



The study of the telluric lines of the solar spectrum becomes 

 therefore infinitely easier than heretofore. I have devoted to 

 this study the fine days of the last few years, and the results 

 have been very fruitful. Amongst the most interesting of the 



observations I may mention : — (1) the anatomy of Angstrom's 

 group a (see Plate VIII.), in which I have succeeded in 

 detecting a group possessing the same constitution as the 

 bands A and B, according to the beautiful observation of 

 Prof. Langley ; (2) the telluric nature of a certain number 

 of lines beyond the band 8 ; and (3) the solar origin of the 

 line 1474 of KirchhofF : this line is double under strong dis- 

 persion, and as it oscillates we may conclude that the vapour 

 which absorbs the radiations of which it takes the place is 

 carried round by the rotation of the sun *. 



* The oscillation of the lines is not the only means of distinguishing 

 the two kinds of lines. If (by means of a Wollaston's double refracting- 

 prism) we obtain two images of the solar disk in such a way that the 

 two opposite extremities of the equatorial diameter are tangent and normal 

 to the slit (fig. 3), the telluric lines T T' of the spectra of the two images 



Fie:. 3. 



are upon the same line, while the solar lines S S' are dislocated. This 

 dislocation is very sharp if the adjustments enumerated above have been 

 properly carried out ; otherwise we obtain the confused appearance of 

 fig. 4. If we employ a collecting-lens of very short focus, giving a very 

 small image of the solar disk, it is easy to show that the appearance of 



