C. S. Hastings—Constitution of the Sun. 35 
lines of the ordinary spectrum are strengthened. A careful 
examination has persuaded me that the spectrum of a spot 
differs from that of the unbroken photosphere, just as the 
spectrum of the limb differs from that of the center of the 
disk, save that the variations are more pronounced. Indeed, I 
could have considerably extended the list of lines strengthened 
at limb by an examination of the spot spectrum, where the 
variations appeal to the eye more clearly. 
The accepted theory of the spots attributes the phenomenon 
to the absorption of the solar light by cooler, denser gases of 
the same nature as those producing the Fraunhofer lines. 
Familiar experiments teach, however, that as the density of a 
gas increases, the change in the character of its radiation is 
,Shown in its spectrum by the broadening of its distinctive 
fc lines, which at the same time grow more ill defined. 
herefore it follows that, according to the law connecting 
radiation and absorption, dark lines produced by such a gas 
must also, under similar conditions, show increased breadt 
and diminished sharpness. That no such changes are to be 
recognized is a fatal objection to the theory. 
Another class of unexplained phenomena is the duplicity of 
certain lines of the solar spectrum, lines which are single in 
the spectra of terrestrial sources. Of these Prof. Young has 
discovered E,, b, and 6, with others. 
own observations can be arranged very simply in classes, 
and will then better lend themselves to theoretical discussion. 
. The most important fact of all is that the differences in 
the two spectra of center and limb are extremely minute, 
escaping all but the most perfect instruments, and all methods 
which do not place them in close juxtaposition. 
I. Certain lines, the thickest and darkest in the spectrum, 
notably those of hydrogen, magnesium and sodium, whic 
appear with haze on either side, in the spectrum of the center 
of the solar disk, are deprived of this accompaniment in that 
of the limb. 
ns Certain very fine lines (four observed) are stronger at 
imb. 
IV. Other very fine lines (two or three observed) are 
stronger at center. 
atmosphere, then the absorption must be greater at the limb 
