52 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



served that if a train whistles as it passes us, the sound 

 appears to alter at the moment the engine goes by. This 

 arises, of course, not from any change in the whistle 

 itself, but because the number of vibrations which reach 

 the ear in a given time are increased by the speed of the 

 train as it approaches, and diminished as it recedes. 

 So, like the sound, the color would be affected by 

 such a movement ; but Doppler's method was practi- 

 cally inapplicable, because the amount of effect on the 

 color would be utterly insensible ; and even if it were 

 the method could not, for other reasons, be applied ; 

 indeed, as we did not know the true color of the stars, 

 we have no datum line by which to measure. 



A change of refrangibility of light, however, does 

 occur in consequence of relative motion, and Huggins 

 successfully applied the spectroscope to solve the pro- 

 blem. He took in the first place the spectroscope of 

 Sirius, and chose a line known as F, which is due to 

 hydrogen. Now, if Sirius was motionless, or rather if 

 it retained a constant distance from the earth, the line 

 F would occupy exactly the same position in the spec- 

 trum of Sirius as in that of the sun. On the contrary, 

 if Sirius were approaching or receding from us, this 

 line would be slightly shifted either towards the blue 

 or red end of the spectrum. He found that the line 

 had moved very slightly towards the red, indicating 

 that the distance between us and Sirius is increasing at 

 the rate of about twenty miles a second. So also 

 Betelgeux, Rigel, Castor, and Regulus are increasing 

 their distance, while, on the contrary, that of others, as 

 for instance of Vega, Arcturus, and Pollux, is dimin- 

 ishing. The results obtained by Huggins on about 



