80 CELESTIAL SPECTROSCOPY. 
short period of increasing temperature, which culminates in the white 
stage, and a second time during a more prolonged stage of gradual 
cooling. He suggested that the two groups of banded stars may cor- 
respond to these different periods, the young stars being those in which 
the stronger edge of the dark band is towards the blue, the other banded 
stars, which are relatively less luminous and few in number, being those 
which are approaching extinction through age. 
Recently a similar evolutional order has been suggested, which is 
based upon the hypothesis that the nebule and stars consist of collid- 
ing meteoric stones in different stages of condensation. 
More recently the view has been put forward that the diversified 
spectra of the stars do not represent the stages of an evolutional 
progress, but are due for the most part to differences of original con- 
stitution. 
The few minutes which can be given to this part of the address are 
insufficient for a discussion of these different views. I purpose, there- 
fore, to state briefly, and with reserve, as the subject is obscure, some of 
the considerations from the characters of their spectra which appeared 
to me to be in favor of the evolutional order in which I arranged the 
stars from their photographie spectra in 1879. This order is essentially 
the same as Vogel had previously proposed in his classification of the 
stars in 1874, in which the white stars, which are most numerous, 
represent the early adult and most persistent stage of stellar life; the 
solar condition that of full maturity and of commencing age; while in 
the orange and red stars with banded spectra we see the setting in and 
advance of old age. But this statement must be taken broadly, and 
not as asserting that all stars, however different in mass and possibly 
to some small extent in original constitution, exhibit one invariable suc- 
cession of spectra. 
In the spectra of the white stars the dark metallic lines are relatively 
inconspicuous, and occasionally absent, at the same time that the dark 
lines of hydrogen are usually strong, and more or less broad, upon a 
continuous spectrum, which is remarkable for its brilliancy at the blue 
end. In some of these stars the hydrogen and some other lines are 
bright, and sometimes variable. 
As the greater or less prominence of the hydrogen lines, dark or 
bright, is characteristic of the white stars as a class, and diminishes 
gradually with the incoming and increase in strength of the other 
lines, we are probably justified in regarding it as due to some condi- 
tions which occur naturally during the progress of stellar life, and not 
to a peculiarity of original constitution. 
To produce a strong absorption-spectrum a substance must be at the 
particular temperature at which it is notably absorptive; and further, 
this temperature must be sufficiently below that of the region behind 
from which the light comes for the gas to appear, so far as its special 
rays are concerned, as darkness upon it. Considering the high tem- 
