8 
—the rings or spiral streams disclosed by the photograph suggesting 
precisely such a succession of events, as the nebular hypothesis 
requires. The luminosity or “fiery mist’ of the nebule is, so 
Helmholtz considers, accounted for by their contraction or shrinkage 
the energy exerted by the natural gravitation of matter being 
amply sufficient to generate the high temperature of the stars. and 
even the sun himself. 
It is a curious fact that while so many suns have grown dim 
from age, and their light has gradually faded away, other 
nebule are still in their youth, not having yet passed into the star 
or sun like stage of their existence. Is it possible that they have 
originated from the collision of dark suns, which have thus pro- 
duced a newer and later generation of heavenly bodies? During 
the comparatively short period of astronomical observations, no such 
collision has been recorded, but it appears the only possible solution 
of the temporary rejuvenescence of the Heavens which we have 
every reason to believe is going on. For although the number of 
dark stars is continually increasing, the stars in the early or middle 
stage of evolution are by far the more numerous, and some indeed 
are hardly advanced beyond the nebular stage. Such at least is 
the opinion of Professor Higgins, who may fairly be considered our 
highest authority on all matters connected with spectrum analysis. 
One of the most practical results obtained by means of the 
spectroscope is the power of determining the spproach or recession 
of a star in the line of sight; its velocity also may thus be 
accurately measured, independently of its distance from us, The 
scientific value of this novel method of calculation ean hardly be 
over-estimated ; by no other means has it been possible even to 
determine whether the so-called fixed stars were moving or 
stationary, still less could we measure their rate of advance or 
retreat ; they appeared indeed to us to be standing still. So great 
is the distance of the stars that no clue to the problem is afforded 
by the waxing or waning of their brilliance; it is calculated that 
if the nearest star were approaching us at the rate of one hundred 
miles per second, its brightness would only be increased one fortieth 
part, supposing the same rate of progress were maintained for one 
century! Now, however, thanks to the spectroscope, all this is 
changed, and the astronomer can, by its aid, determine whether 
the star is approaching or receding, and can measure accurately its 
rate of motion, and this moreover independently of its distance, for 
the rules of measurement apply equally to the nearest planet or 
the most distant star. This system of measurement was suggested 
by a well known theory in another branch of science, namely 
acoustics. I allude to Doppler’s famous discovery in 1841, that a 
sound becomes flatter or sharper as it recedes from, or approaches 
