SPECTROSCOPE AND STUFF OF COSMOS 349 



clues, a new study came into being, that of solar physics. 

 To-day we know more of the constitution, the temperature, and 

 physical characteristics of a body ninety millions of miles away, 

 and for ever inaccessible to man, than our forefathers three or 

 four generations ago knew of the earth itself. Not merely this, 

 but the spectroscope has reached out to tell the same story of 

 the stars ; to-day the stellar bodies are divided according to 

 their apparent temperature into different types solar or yellow 

 stars like our sun ; white or bluish stars like the great Dog-star, 

 Vega, and others ; red suns like the bright star of the constella- 

 tion of Hercules ; the subdivision is often carried much further. 



From out of solar physics has grown a yet broader study, 

 that of stellar, or, if one prefers, of cosmic physics. Amid the 

 multiplied interests of man there are surely none more distantly 

 removed from his primitive pre-occupations for food and shelter 

 than this. 



But the revelations of the spectroscope were not to stop 

 with chemical and physical conditions. By an extraordinary 

 circumstance, grounded in the very nature of light itself, it 

 was to do more ; it was to prove an independent method of 

 determining solar motion. 



There is much in the economy of science that resembles 

 that of our workaday and business life. Very often it has 

 happened that a loan of fact or theory from one branch of 

 scientific investigation has later been returned with heavy 

 interest. This was true in the present instance. It was the 

 motion of the earth which first revealed the finite velocity of 

 light. Later, as we have seen, this was turned about in ex- 

 planation of the annual aberration of the stars. The return 

 went further. 



If light be a form of wave motion, it follows that if the body 

 which sets up these light waves be itself in motion, in sufficient 

 velocity, this would occasion some disturbance in the behaviour 

 of the light it transmits. You find a familiar analogy in sound. 

 If while an express train is travelling at a high rate of speed 

 the engineer blows his whistle continuously, this will seem to 

 cause a change of pitch in the sound, accordingly as the train 

 is travelling towards or away from a bystander. The fact is 

 one of everyday experience. 



The case with light is a little different. Not only does it 



