MICHELSON'S KECENT RESEARCHES ON LICxHT.* 



By Joseph Lovering, President. 



For many generations it was assumed that no sensible tinie was 

 taken by light in moving over tlie hirgest 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 unmask 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 1(510 5 and the 

 eclipses of these satellites by the shadow of Jupiter became ;ui inter- 

 esting subject of observation. It was soon noticed that the interval 

 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 ex- 

 planation. A Danish astronomer, Romer, without the help of this 

 analogy, deciphered the problem in astronomy. The eclipse was tele- 

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

 or delayed in jtroportion to the distance from which it came. In this way 

 it was discovered that light took about eighteen 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 1075. The mathe- 

 matical astronomer Delambre, from a discussion of one thousand of 

 these eclipses observed between 1GG2 and 1802, found for the velocity 

 of light 193,350 miles a second. 



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

 substantially confirmed in an unexi)ected quarter. Dr. Bradley, of the 

 Greenwich Observatory, the greatest astronomical observer of his day, 

 was perplexed by certain periodical tluctuations, of small amount, in 

 the position of the stars. Suddenly the explanation was Hashed upon 

 him by something he observed while yachting on the River Thames. 

 He noticed that, whenever the boat turned about, the direction of the 



*An address delivered before tlio American Academy of Arts ;iiid Sciences, at the 

 meeting of April 10, 1889, wlienlho Riifiifonl medals wore presented to Prof. A. A. 

 MiCHELSON. (From tlie I'rocecdinfjs 0/ tkc Amcrwan Academy ; vol. xxiv (n. s. xvi), 

 pp. 380-401.) 



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