418 
ME. W. HUGGINS AND DR. W. A. MILLER ON 
be without lines. The stars admit of no such broad distinctions of classification. Star 
differs from star alone in the grouping and arrangement of the numerous fine lines by 
which their spectra are crossed. 
6. For the convenience of reference and comparison, a few of the more characteristic 
lines of twenty-nine of the elements were measured with the instrument. These were 
laid down to scale, in order to serve as a chart, for the purpose of suggesting, by a com- 
parison with the lines measured in the star, those elements the coincidence of the lines 
of which with stellar lines was probable. 
For the purpose of ensuring perfect accuracy in relative position in the instrument 
between the star-spectrum and the spectrum to be observed simultaneously with it, the 
following general method of observing was adopted: — The flame of a small lamp of 
alcohol, saturated with chloride of sodium, was placed centrally before the object-glass 
of the telescope, so as to furnish a sodium-spectrum. The sodium-spectrum was then 
obtained by the induction spark, and the mirror /was so adjusted that the components 
of the double line D, which is well divided in the instrument, should be severally coin- 
cident in the two spectra. The lamp was then removed, and the telescope directed to 
the sun, when Fraunhofer’s line D was satisfactorily observed to coincide perfectly with 
that of sodium in the induction-spark. Having thus ascertained that the sodium lines 
coincided in the instrument with the solar lines D, it was of importance to have assurance 
from experiment that the other parts of the solar spectrum would also accurately agree 
in position with those corresponding to them in the spectrum of comparison. When 
electrodes of magnesium were employed, the components of the triple group character- 
istic of this metal severally coincided with the corresponding lines of the group b. 
C and F also agreed exactly in position with the lines of hydrogen ; the coincidence of 
several of the principal lines of iron was also observed. The stronger of the Fraunhofer 
lines were measured in the spectra of the moon and of Venus, and these measures were 
found to be accordant with those of the same lines taken in the solar spectrum. 
Before commencing the examination of the spectrum of a star, the alcohol-lamp was 
again placed before the object-glass of the telescope, and the correct adjustment of the 
apparatus obtained with certainty. The first observation was whether the star contained 
a double line coincident with the sodium line D. When the presence of such a line 
had been satisfactorily determined, we considered it sufficient in subsequent observations 
of the same star to commence by ascertaining the exact agreement in position of this 
known stellar line with the sodium line D. 
Since from flexure of the parts of the spectrum apparatus the absolute reading of the 
micrometer might vary when the telescope was directed to stars differing greatly in alti- 
tude, the measure of the line in the star which was known to be coincident with that of 
sodium was always taken at the commencement and at the end of each set of measures. 
The distances of the other lines from this line, and not the readings of the micrometer, 
were then finally registered as the measures of their position ; and these form the numbers 
given in the Tables, from which the diagrams of the star-spectra have been laid down. 
