16 
The lecturer pointed out that this gradation — from shape- 
less filmy form, through nebulous nuclei, to starlike conden- 
sations related to a kind of central mass — would seem to 
suggest that we really have exemplified various stages in an 
evolutionary process. Just as an ordered series of relation- 
ships establishes the unity of our solar system, so the many 
examples of nebulous matter in all the various stages of a 
condensing process, would seem to establish an essential 
connection between the stars, as we know them, and the 
faint nebulous areas which only the high-power telescope can 
reveal. Spectroscopy confirms this connection for the stars 
and the nebulae give similar spectra — that is, though the 
materials are the same in the celestial spaces, there is a gradual 
complexity of arrangement and appearance as we pass from 
the nebulae to the white and yellow and on to the red stars — 
through helium, hydrogen, calcium, and metals to the chemical 
compounds. What differences there are in these spectra are 
due to differences in the “ age ” of the bodies, i.e., to differ- 
ences of temperature. For it should not be overlooked that, 
in the nebulae and in the stars, among which we must class 
the sun, there must be going on during the working out of 
this evolutionary process a tremendous expenditure of 
energy which is gradually dissipating itself in heat and 
luminosity. 
Apart from its main theme, the lecture was remarkable for 
the striking way in which the great immensities of space and 
distance were vividly brought home to the minds of the 
audience. The vast field of stellar photography and teles- 
copy was well illustrated by the fact that whereas about 
6,000 stars were visible to the naked eye, it was certain that 
not less than a hundred million stars could be distinguished 
by the great telescope of the Lick observatory. Again, 
something of the tremendous distance of the stars from the 
earth was revealed when the lecturer incidentally remarked 
that a sphere, of radius equal to a million times the distance 
of the eaith from the sun, would probably contain only about 
eighteen stars ; and las+ly, after reminding his audience that 
light travels at the rate of 186,000 miles per second, the 
lecturer said that, if by some universal catastrophe the whole 
of the stars were instantly rendered extinct, four and a half 
years would elapse before th\ inhabitants of earth missed 
the nearest of the stars, and 38,000 years before they would 
observe the disappearance of the last. 
