546 
ME. W. HUGGINS ON THE SPECTEA 
§ IV. Observations of Stars. 
The chief difficulties which I have had to encounter have arisen from the unsteadiness 
of our atmosphere. There is sufficient light from stars of the first and second magni- 
tude for the large spectroscope described in this paper, and so far as the adjustments of 
the instrument are concerned, the lines in the spectra of the stars would be well defined. 
Unless, however, the air is very steady, the lines are seen too fitfully to permit of any 
certainty in the determination of coincidences of the degree of delicacy which is 
attempted in the present investigation. I have passed hours in the attempt to deter- 
mine the position of a single line, and have then not considered that the numerous 
observations which I had obtained were possessed, even collectively, of sufficient weight 
to establish with any certainty the coincidence of the line with the one compared with it. 
I prefer, therefore, to reject a large number of observations which appear unsatis- 
factory from this cause, and to give in this place a very few of the most trustworthy of 
the observations which I have made. 
Sirius . — The brilliant light of this star and the great intensity of the four strong lines 
of its spectrum, make it especially suitable for such an examination. The low altitude 
of this star in our latitude limits the period in which it can be successfully observed 
to about one hour on each side of the meridian. 
T have confined myself to comparisons of the strong line in the position of E, with the 
corresponding line of the spectrum of hydrogen. My first trials were made with hydrogen 
at the ordinary atmospheric pressure ; the width of the band of hydrogen, under these 
circumstances, was greater than the line of Sirius. This line in Sirius, from some cause, 
is narrower relatively to the length of the spectrum, when considerable dispersion and 
a narrow slit are employed, than when the image of the star, rendered linear by a cylin- 
drical lens, is observed with a single prism*. 
When the large spectroscope was employed I estimated the breadth of the line to he 
about equal to that of the double line D. In Kirchhoff’s map the line F of the solar 
spectrum is represented as a little more than one-fourth of the interval separating the 
lines D. When the spectroscope attached to the telescope was directed to the moon, 
the line F appeared even narrower than it is represented in Kirchhoff’s map; I 
estimated it at about one-sixth of the apparent breadth of the corresponding line in the 
spectrum of Sirius. The character of the line agrees precisely with Kirchhoff’s repre- 
sentation of the solar line F. It appears, as in the diagram, to be equally nebulous at 
both edges, and agrees in this respect precisely with the line of hydrogen under certain 
conditions of tension and temperature. 
As it was obviously impossible to determine with the required accuracy the coincidence 
of the line of Sirius when the much broader band of hydrogen at the ordinary pressure 
was compared with it, I employed a vacuum-tube fixed before the object-glass. In all 
these observations the slit used was as narrow as possible. The air at the time of the 
present observations was more favourable than usual, and the line in Sirius was seen with 
* See Philosophical Transactions, 1864, p. 42. 
