84 CELESTIAL SPECTROSCOPY. 
tospheres of different stars may differ in their emissive powers, but prob- 
ably not to a great extent. 
All the heavenly bodies are seen by us through the tinted medium of 
our atmosphere. According to Langley the solar stage of stars is not 
really yellow, but, even as gauged by our imperfect eyes, would appear 
bluish-white if we could free ourselves from the deceptive influences of 
our surroundings. 
From these considerations it follows that we can scarcely infer the 
evolutional stages of the stars from a simple comparison of their eye 
magnitudes. We should expect the white stars to be, as a class, less 
dense than the stars in the solar stage. As great mass might bring in 
the solar type of spectrum at a relatively earlier time, some of the 
brightest of these stars may be very massive, and brighter than the 
sun—for example, the brilliant star Arcturus. For these reasons the 
solar stars should not only be denser than the white stars, but per- 
haps, as a Class, Surpass them in mass and eye brightness. 
It has been shown by Lane that, so long as a condensing gaseous 
mass remains subject to the laws of a purely gaseous body its tempera- 
ture will continue to rise. 
The greater or less breadth of the lines of absorption of hydrogen in 
the white stars may be due to variations of the depth of the hydrogen 
in the line of sight, arising from the causes which have been discussed. 
At the sides of the lines the absorption and emission are feebler than 
in the middle, and would come out more strongly with a greater thick- 
ness of gas. 
The diversities among the white stars are nearly as numerous as the 
individuals of the class. Time does not permit me to do more than 
to record that, in addition to the three sub-classes into which they have 
been divided by Vogel, Scheiner has recently investigated minor differ- 
ences aS suggested by the character of the third line of hydrogen near 
G. He has pointed out, too, that so far as his observations go the 
white stars in the constellation of Orion stand alone, with the exception 
of Algol, in possessing a dark line in the blue which has apparently 
the same position as a bright line in the great nebula of the same con- 
stellation; and Pickering finds in his photographs of the spectra of 
these stars dark lines corresponding to the principal lines of the bright- 
line stars, and the planetary nebule with the exception of the chief 
nebular line. The association of white stars with nebular matter in 
Orion, in the Pleiades, in the region of the Milky Way, and in other 
parts of the heavens, may be regarded as falling in with the view that 
I have taken. 
In the stars possibly farther removed from the white class than our 
sun, belonging to the first division of Vogel’s third class, which are 
distinguished by absorption bands with their stronger edge toward 
the blue, the hydrogen tines are narrower than in the solar spectrum, 
In these stars the density gradient is probably still more rapid, the 
