THE RADIATION OF THE SUN—ABBOT. 141 
When the chemical element sodium or any of its compounds, like 
common salt, for instance, is placed in the flame, and the light which 
is-given out is examined in the spectroscope, it is seen to consist 
of a couple of bright yellow lines. No general extension of the spec- 
trum to include the green or violet is seen. On the other hand, if one 
observes the spectrum of the limelight or the electric are from carbon 
poles, it is seen to give a long band of color much like the solar spec- 
trum, except that, whereas in the solar spectrum a great number of 
dark lines are seen under good conditions, in the spectra of the arc 
light or of the limelight these lines will generally be absent. I, 
however, the vapor of metallic sodium be caused to intervene between 
the source of light and the slit of the spectroscope, two dark lines 
will be seen in the yellow, corresponding in position to the two bright 
yellow lines which are found by observing the light from heated 
sodium, or heated common salt. In short, the yellow light is ab- 
sorbed by the sodium vapor at the very positions in the spectrum 
where that vapor would itself give off light if strongly heated. The 
same is true of iron and other metals. The spectrum of iron is very 
complicated, consisting of a great number of lines, many of them 
in the green. If the arc light be caused to play between iron poles, 
these bright green lines will be the main features of the light as ob- 
served in the spectroscope. Some of these lines are very strong, 
others quite weak, so that there is often a well-marked distinction 
between one line and another, not only as regards its place but also 
as regards its intensity in the spectrum. 
Now, it is found on observing the spectrum of the sunlight or star- 
light that the dark lines are found in the same relative positions, 
and generally of nearly the same relative intensity, as in the bright 
line spectrum of the chemical elements themselves. In this way it 
is possible to determine what elements are found in the sun and the 
stars, although these bodies are so immensely distant from us. In 
this way we know that more than 40 of the ordinary chemical ele- 
ments found upon, the earth exist also in the sun, and the existence 
of about 20 more is doubtfully indicated by the solar spectrum. Not 
only does the approximate correspondence in position and intensity 
of the spectrum lines of the sun and of the chemical elements as ob- 
served in the Jaboratory yield this significant result, but the slight 
deviations from exact correspondence in intensity and in position of 
the spectrum lines yield other facts not less remarkable. For instance, 
it was predicted by Doppler and observed in the laboratory by Prince 
Galitzen that the motion of a source of light toward the observer 
displaces its spectral lines toward the violet, and, contrariwise, the 
motion of the source of ight away from the observer displaces the 
spectral lines toward the red. This effect is very noticeable in the 
