AEE THEBE ANY FIXED STABS? 
367 
forming what is commonly called the double line D of sodium. 
An idea, therefore, may be formed of the difficulty of estimating 
the stellar motions of recess or approach, unless in those excep- 
tional cases where the star’s real motion is much greater than 
the above-mentioned average. 
Of course, the whole question is one of the dispersive power 
of the spectroscope ; and inasmuch as a telescope of large aper- 
ture will permit us to use a higher dispersive power than we 
could apply to a smaller instrument, the size of our telescopes 
enters into this as into so many other questions of astronomical 
interest. 
To secure the greatest dispersive power possible, without 
inconvenience, Mr. Huggins used the form of spectroscope 
exhibited in fig. 7, Plate XLIII. of my paper on the spectro- 
scope, in the “Populab Science Eeview” for April last. The 
reader is referred to that paper for a description of the qualities 
of this arrangement. 
The star selected for the first application of the new method 
of research was Sirius, on account of its great brilliancy. It 
was necessary to consider some one recognised line of his spec- 
trum, and the line corresponding to the solar line P (the blue- 
green hydrogen-line) was the one selected. 
Fig. 5 shows the result of the experiment. The two upper 
spectra are not directly concerned in the method applied ; but 
it is well to notice the perfect coincidence in position between 
the sharp dark line in the solar spectrum and the middle of the 
diffused line obtained from hydrogen at ordinary atmospheric 
pressure. Any want of coincidence here would have thrown 
doubt on the result of the experiment. 
The hydrogen-line, actually compared with the dark and 
somewhat diffused F-line of the spectrum of Sirius, was 
obtained from hydrogen in the so-called vacuum-tube. Mr. 
Huggins made it fall side by side with the diffused dark line F 
in the spectrum of Sirius, and in some experiments he brought 
the bright line upon the Sirius-line. It will be seen from fig. 5 
that the bright line fell sensibly away from the middle of the 
dark line. It became obvious from this that Sirius has a 
motion in the direction of the line of sight, and since the dark 
line was shifted towards the red end of the spectrum, it followed 
that the motion was one of recession. 
From a careful measurement of the discordance between the 
two lower spectra of fig. 5, Mr. Huggins calculated that at the 
epoch of the observation Sirius was moving from the earth 
at the rate of 41*4 miles per second. But a part of this 
motion was due to the earth’s motion in her orbit, and having 
made due reduction on account of this consideration, IMr. 
Huggins found that there remained a motion of recession from 
VOL. VIII. — NO. XXXIII. B B 
