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 nebulae 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 

 nebulffi 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, vvhich have thus pro- 

 duced a newer aud 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 m the early or middle 

 stage of evolution are by far the more numerous, and some indeed 

 are hardly advanced beyond the nebular stage. 8uch 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 can 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 



