May 29, 1873] 
NATURE 
89 
ON THE SPECTROSCOPE AND ITS 
APPLICATIONS 
IX, 
New let me state to you how the discovery 
mentioned on p. 12 was finally established by 
Kirchhoff. In my notice of the spectroscope in the 
earlier articles, I had so much to say that there were 
several details it was absolutely essential I should 
curtail. One of these details was the scale by which the 
positions of the different bright or dark lines which are 
Fic. 50.—A sun-spot (Secchi), showing the ‘‘straws” in the penumbra, and 
the irregular masses on the general surface. 
seen in the different spectra are registered, so that we 
may say that such a line occupies such and such a posi- 
tion, and such another line occupies such another posi- 
tion, with regard to something else. When Kirchhoff 
and Bunsen, two German chemists, were engaged in 
mapping the spectra of the elements—a research which 
at its commencement had nothing whatever to do with 
the sun—they came across this difficulty of a scale. How 
could they get a good scale? I have already referred to 
some very obvious arrangements that might determine 
the actual position ; for instance, the observing telescope 
may be made to move along a graduated arc, so that by 
moving the telescope for the different rays and fixing it 
when in a proper position to see a particular ray, you 
might read off the index placed on the arc to a great 
nicety by means of a graduated vernzer working on the 
D3 > 
Di De 
Fic. 51.—Spectrum of sun-spot. (Young.) 
curve of the arc; or you may, by a modification of the 
instrument, use a reduced photographic picture of a scale, 
so that the thing to be measured and the actual scale 
would appear in the field of view at the same time. 
Kirchhoff and Bunsen tried these methods, but they did 
not like them. Then it suddenly struck them that, as 
they made their experiments in the day-time, they might 
use as a scale the black lines in the solar spectrum, which 
had not been known to change since the time of Wollas- 
ton, who discovered them. When working in the day- 
time, they had thus the solar spectrum visible in one half 
of the field of view of the telescope, which was easily 
managed by placing a reflecting prism over one half of 
the slit, as is shown in the enlarged slit in Fig. 46, so as 
to light one half of the slit by the sun, and the other half 
by whatever substance was under examination. With 
this arrangement they set to work with infinite care, and 
made a map of the solar spectrum. Such was their pro- 
roth J 
Fic.°52.—Spectrum of r Coronz. (Huggins.) 
posal : first to map the unchangeable solar spectrum, and 
then, having this unchangeable scale, about which there 
could be no mistake, always visible, they would be able to 
refer to the dark lines in it all the unknown phenomena 
they were about to investigate in the bright lines of diffe- 
rent vapours and gases. Having got this idea of the 
scale well into their minds, they were exceedingly anxious 
to test this question, which, as I have told you, was raised 
by Fraunhofer and many other men before them, of the 
asserted coincidence of the bright sodium line with the 
dark solar sodium lines; with a very delicate instrument, 
Prof. Kirchhoff made the following remarkable experi- 
ment :—“ In order,” says Kirchhoff, for these are his own 
words, “to test in the most direct manner possible the 
frequently asserted fact of the coincidence of the sodium 
lines with the lines D”—(that is to say, of the bright 
double lines of sodium in the yellow part of the spectrum, 
with the double line D of the solar spectrum)—“I ob- 
tained a tolerably bright solar spectrum, and brought a 
flame coloured by sodium vapour in front of the slit. I 
then saw the dark lines D change into bright ones.” 
That is to say, in the spectrum of the sodium which was 
burning in the flame were lines so exactly coincident with 
the two dark lines in the solar spectrum, that the bright 
lines of the sodium spectrum put these dark lines out al- 
together, so that they seemed to vanish, as it were, from 
the solar spectrum. He goes on :—“ In order to find out 
the extent to which the intensity of the solar spectrum 
could be reduced without impairing the distinctness of 
the sodium lines, I allowed the full sunlight to shine 
Fic. 53.—Alteration of wave-length of the hydrogen in the atmosphere of 
Sirius. 1, Hydrogen at atmospheric pressure ; 2, Solar Spectrum Line 
F ; 3, Spectrum of Sirius ; 4, Hydrogen in vacuum tube, 
through the sodium flame.” Here he varies the experi- 
ment. In the first instance he used a very feeble beam 
of sunlight, but he now allows the whole glare of the sun 
to enter the slit. What was the result ? “ To my astonish- 
ment, I saw that the dark lines D appeared with an ex- 
traordinary degree of clearness.” That is to say, the 
lines which came from the sodium in the first instance, 
were sufficiently bright to entirely eradicate the dark lines 
from the solar spectrum, but the two lines D were now 
