OF SOME OF THE STAES AND NEBTJL^E. 
547 
great distinctness. The line from the spark appeared, in comparison, very narrow, not 
more than about one-fifth of the width of the line of Sirius. When the battery circuit 
was completed, the line of hydrogen could be seen distinctly upon the dark line of Sirius, 
and extending to some distance on both sides of the spectrum of Sirius. The observa- 
tion of the comparison of the lines was made many times, and I am certain that the 
narrow line of hydrogen, though it appeared projected upon the dark line in Sirius, did 
not coincide with the middle of the line, but crossed it at a distance from the middle, 
which may be represented by saying that the want of coincidence was apparently equal 
to about one-third or one-fourth of the interval separating the components of the 
double line If. I was unable to measure directly the distance between the centre of 
the line of hydrogen and that of the line in the spectrum of Sirius, but several very 
careful estimations by means of the micrometer give a value for that distance of 0040 
of the micrometer-head. This value is probably not in error by so much as its eighth part. 
Comparisons on many other nights were also made, sometimes with the vacuum-tube 
before the object-glass, and sometimes with the vacuum-tube placed over the small hole 
in the gutta-percha plate. On all these occasions the numerous comparisons which 
were made, gave for the line in Sirius a very slightly lower refrangibility than that of 
the line of hydrogen, but on no one occasion was the air steady enough for a satisfactory 
determination of the amount of difference of refrangibility. 
I have not been able to detect any probable source of error in this result, and it may 
therefore, I believe, be received as representing a relative motion of recession between 
Sirius and the earth. 
The probability that the substance in Sirius by which this line is produced is really 
hydrogen, is strengthened almost to certainty by the consideration that there is a strong 
line in the red part of the spectrum which is also coincident with a strong line of hydro- 
gen. There is a third line more refrangible than F, which appears to coincide with the 
line of hydrogen in that part of the spectrum. 
As the line in Sirius is more expanded than that of the vacuum-tube, it seemed of 
importance to have proof from experiment that this line of hydrogen, when it becomes 
broad, expands equally in both directions. I made the comparison of the narrow line 
of the vacuum-tube with the more expanded band which appears when denser hydrogen 
is employed. For this purpose the intersection of the wires of the eyepiece was brought, 
as nearly as could be estimated, upon the middle of the expanded line which is produced 
by dense hydrogen. The vacuum-tube was then arranged before the slit, when the 
narrow line which it gives was observed to fall exactly upon the point of intersection of 
the wire. Under these terrestrial conditions the expansion of the line may be consi- 
dered to take place to an equal amount in both directions. There is very great proba- 
bility that a similar equal expansion takes place under the conditions which determine 
the absorption of light by this gas in the atmosphere of Sirius, for the reason that the 
nebulosity at the edges of the line in the spectrum of that star is sensibly equal on both 
sides. 
