18 REPORT— 1891. 



in 1864), and have since shown by direct observation, that the spectrum, 

 of the nebula in Andromeda, and presumably of similar nebulae, is in 

 appearance only wanting in the red. 



The stage at which the maximum radiation is in the green, corre- 

 sponding to the eye's greatest sensitiveness, would be that in which it 

 could be most favourably measured by eye-photometry. As the maxi- 

 mum rose into the violet and beyond, the star would increase in visual 

 brightness, but not in proportion to the increase of energy radiated by it. 



The brightness of a star would be affected by the nature of the sub- 

 stance by which the light was chiefly emitted. In the laboratory solid 

 carbon exhibits the highest emissive power. A stellar stage in which 

 radiation comes, to a large extent, from a photosphere of the solid parti- 

 cles of this substance, would be favourable for great brilliancy. Though 

 the stars ai'e built up of matter essentially similar to that of the sun, it 

 does not follow that the proportion of the different elements is everywhere 

 the same. It may be that the substances condensed in the photospheres 

 of different stars may differ in their emissive powers, but probably 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 scai'cely infer the 

 evolutional stages of the stars from a simple comparison of their eye- 

 naagnitudes. 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 perhaps, 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 temperature 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 hue 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 thickness 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 differences- 

 as suggested by the character of the third line of hydrogen near G. He 



