THE SPECTRA OF SOME OF THE FIXED STARS. 
427 
The spectra of iron and manganese were also compared with that of the star, but the 
state of the atmosphere prevented any certain conclusion. 
The lines in the spectra of nitrogen , tin , and mercury were not coincident with any 
definite lines in the star-spectrum. Neither of the hydrogen lines corresponding to C 
and F was present. 
At the end of the paper we have given a Table of such measures of the lines in the 
spectrum of this star as we can depend upon. Although it appears to be as full of lines 
as either of the preceding stars, the observations are attended with great difficulty, 
owing to the insufficient amount of light. 
The absence in the spectrum of a Orionis, and also in the spectrum of /3 Pegasi, which 
so closely resembles it in character, of lines corresponding to those of hydrogen, is an 
observation of considerable interest. It is of the more importance since the 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 themselves. 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 con- 
sideration 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 favourably 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 spectroscope which we employed in the early part of 1863, of 
very great breadth, and were so figured in the diagram of the spectrum 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. 
