880 PKOCEEDINGS OF THE AMERICAN ACADEMY 



XXVII. 



AN ADDRESS 



DELIVERED AT THE MEETING OF APRIL 10, WHEN THE RUM- 

 FORD MEDALS WERE PRESENTED TO PROFESSOR A. A. 

 MICHELSON. 



By Joseph Lovering. 



For many generations it was assumed that no sensible time was 

 taken by light in moving over the largest distances. The velocity 

 of sound was found by noting the time which elapsed between seeing 

 the flash and hearing the report of an explosion. It was only in the 

 vast spaces of astronomy that distances existed large enough to un- 

 mask the finite velocity of light, and, in extreme cases, to make it 

 seem even to loiter on its way. 



The satellites of Jupiter were discovered by Galileo in 1610; and 

 the eclipses of these satellites by the shadow of Jupiter became an 

 interesting subject of observation. It was soon noticed that the in- 

 terval between successive eclipses of the same satellite was shorter 

 when the earth was approaching Jupiter, and longer when the earth 

 was receding from Jupiter. The change of pitch in the whistle of a 

 locomotive, under similar motions, would suggest to the modern mind 

 an easy explanation. A Danish astronomer, Romer, wijthout the help 

 of this analogy, deciphered the problem in astronomy. The eclipse 

 was telegraphed to the observer by a ray of light, and the news was 

 hastened or delayed in proportion to the distance from which it came. 

 In this way it was discovered that light took about 18 minutes to run 

 over the diameter of the earth's orbit. This discovery was published 

 by Romer in the Memoirs of the French Academy in 1675. The 

 mathematical astronomer Delambre, from a discussion of one thou- 

 sand of these eclipses observed between 1662 and 1802, found for the 

 velocity of light 193,350 miles a second. 



Meanwhile Romer's method, after fifty years of waiting, had been 

 substantially confirmed in an unexpected quarter. Dr. Bradley of 

 the Greenwich Observatory, the greatest astronomical observer of his 

 day, was perplexed by certain periodical fluctuations, of small amount, 



