518 WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 



tempt to measure the wave-lengths of the nebular lines somewhat 

 more accurately than had been done before ; in its present stage it is 

 concerned chiefly with the distribution of the radiations through the 

 nebulae. There have been intermediate developments. The rather 

 meager accumulation of material that is available is somewhat 

 heterogeneous, and can hardly, in its entirety, be presented to an 

 audience of general scientific interests. I shall, therefore, with a 

 full sense of the limitations of the observations, undertake to con- 

 sider them from the point of view of the relationship of the nebulas 

 to the stars. Such a relationship can be regarded as an element of 

 the theory of stellar evolution, and it may be well to recall a few 

 of the ideas that at present form the substance of that theory. 



At the mention of the term " stellar evolution," in a general 

 scientific gathering, one frequently becomes aware of an atmospnere 

 of amused toleration, or tolerant skepticism. The raising of a broW, 

 or the birth of an indulgent smile, diffuses such an atmosphere with 

 the velocity of light. It is not my purpose to proselyte in the in- 

 terests of any particular scientific creed, but inasmuch as we are 

 approaching the observations from the point of view of stellar rela- 

 tionships, it may be well to recall the principal consideration that 

 has led astronomers to the belief that such a thing as stellar evolu- 

 tion exists. The conception of stellar evolution finds its justifica- 

 tion very largely in the principle of the conservation of energy. The 

 sun and the stars are continually pouring out into space a simply 

 inconceivable amount of energy in the form of radiation. We are 

 all familiar with comparisons designed to help us sense the prodi- 

 gious outflow. Perhaps as good a one as any is represented in 

 the statement that if the surface of the earth, land and sea, were 

 covered a mile deep with coal, the quantity of fuel represented 

 would supply the output of solar heat for about a minute. Some 

 of the stars radiate several thousand times as much heat as the sun. 

 Whatever the nature of the process by which the energy is at present 

 being replenished, it is impossible to conceive of the expenditure 

 going on forever. Sooner or later the star must cool, and, through 

 alteration in its temperature, sufifer a change in its physical state. 

 This process of change had been termed the evolution of the star. 



On account of the enormous distances of the stars the principal 



