PROFESSOR LOVERING'S ADDRESS. 211 



up in sidereal astronomy. The process of elimination requires the 

 mathematician to calculate the direction and velocity of the motion 

 of the sun which will leave behind it the smallest unexplained residu- 

 um ; and this remainder is the motion of the stars themselves. The 

 delicacy of the problem lies in the minuteness of the quantities to be 

 observed, and in the assumptions which must be made in regard to 

 the distances of the stars, only a few of which have been positively 

 computed from parallax. However, a result has been reached, highly 

 probable in the sun's case, but which can be converted into absolute 

 values for other stars only so fast as their individual distances are dis- 

 covered. Here, again, physics and chemistry, with the spectroscope in 

 hand, have come to the aid of astronomy and geometry. Should it 

 appear that the conclusions from spectrum analysis must be ques- 

 tioned, the attempt was brave, and even a defeat would be honorable. 



In 1675 a Danish astronomer observed the novel fact that the fre- 

 quency in the eclipses of Jupiter's satellites fluctuated with the motion 

 of 'the planet to or from the earth. He hit upon a happy explanation, 

 viz., that the swift light takes more or less time to telegraph the as- 

 tronomical news across the omnipresent lines of force. This early 

 observation is the avant-coureur of a host of others which have slowly 

 followed in close array. That of a blind musician comes next. He 

 noticed, in 1835, that the pitch of a steam-whistle, on the Lowell Rail- 

 road, fell suddenly as the locomotive passed him. Unfortunately, 

 Munroe's observation was never published, although he sought and 

 found an explanation of what was then a strange fact. In this case, 

 the whistle sends the message, the waves of sound transmit it, and the 

 ear is the register ; but the changing distance modifies the time. In 

 1842,Doppler, of Prague, was led, by theoretical considerations, to for- 

 mulate the proposition, now known in science as Doppler's principle : 

 that the color of light and the pitch of sound, as they tell upon the 

 senses, are changed by the relative velocity of the observer and the 

 origin of the disturbance. In 1845, Buy Ballot made experiments 

 upon the railroads in the Netherlands, and Scott Russell repeated 

 them on English railroads, which confirmed the theory in the case of 

 sound. In the application of the theory to color, few astronomers 

 will be willing to follow Doppler in all his extravagances. 



If it be true, theoretically, that the relative velocity of light, the 

 wave-length of transmission, and the period of oscillation in the ether, 

 are altered by the relative motion of the observer and the place from 

 which the undulation starts, it is obvious that all other velocities have 

 but a small chance in competition with the velocity of light, and that 

 slight changes of color, if physically real as Doppler supposed, would 

 fail of being recognized even by the eye of a painter. To interpose 

 the spectroscope, and observe the change of refrangibility by the dis- 

 placement of the sharp lines of the spectrum, was a lucky escape from 

 this embarrassment. After Huggins had tried his hand at this new 



