Spectra of some of the Fixed Stars. 423 



lines C and F are highly characteristic of the solar spectrum and 

 of the spectra of by far the larger number of the fixed stars to 

 which our observations have been extended. 



These exceptions are further interesting as they seem to prove 

 that the lines C and F are due to the luminous bodies them- 

 selves. Of this some doubt might be entertained, and it might be 

 suspected that they are in some way due to our own atmosphere, 

 if these lines were present in the spectra of all the stars without 

 exception. 



This absence of the lines corresponding to hydrogen is also 

 the more entitled to consideration since it is so rare to find them 

 wanting, amongst the considerable number of stellar spectra 

 which we have observed. 



14. Sirius. — The spectrum of this brilliant white star is very 

 intense; but owing to its low altitude, even when most favour- 

 ably situated, the observation of the finer lines is rendered very 

 difficult by the motions of the earth's atmosphere. For the 

 present we do not give any details of our measures. The lines 

 in the green and blue appeared, in the less perfect form of spec- 

 troscope which we employed in the early part of 1863, of very 

 great breadth, and were so figured in the diagram of the spec- 

 trum of this star given in our "Note" of February 1863. With 

 our present instrument, possessing much greater dispersive power 

 and a very narrow slit, these bands appear but little broader than 

 F and G are at times seen in the solar spectrum. In February 

 1863, the breadth of the band corresponding to F measured 1^ 

 unit of the scale we then adopted; each unit corresponded to 

 15 '5 units of our present scale. The micrometric measurement 

 of this line in Sirius, in terms of our present scale, is only 3*7 — 

 that is, only about one-seventh of the breadth as seen with the 

 wider slit and a dispersing arrangement having little more than 

 one-third of the power of the present apparatus. 



Three if not four elementary bodies have been found to furnish 

 spectra in which lines coincide with those of Sirius, viz. sodium, 

 magnesium, hydrogen, and probably iron. 



(1) Sodium. — A double line in the star, though faint, coincides 

 in position with the line of this metal. 



(2) Magnesium. — Three lines in the star-spectrum coincide 

 with the triple group of magnesium. 



(3) Hydrogen. — Both the lines corresponding to F and C have 

 intensely strong lines in the star-spectrum. 



(4) Iron. — No direct comparison with this metal was made; 

 but the cross wires having been set to a position corresponding 

 with E of the solar spectrum, a faint line in the star was seen 

 exactly to bisect the wires when the telescope was turned upon 

 Sirius. 



2 F 2 



