PROFESSOR LOVERING'S ADDRESS. 205 



The instrumental arrangements of these two experimentalists 

 agreed only in the part which each borrowed from Poggendorff; the 

 details differed so widely as to give to whatever agreement might ap- 

 pear in their results the force of an irresistible argument for their 

 accuracy. The velocity of light, as found by Fizeau in 1849 by the 

 artificial eclipses which the teeth of his revolving wheel produced, 

 exceeds by about six per cent, the velocity which Foucault obtained, 

 in 1862, with the moving mirror. The arithmetical mean of the tw^o 

 values comes very close to the astronomer's estimate of the velocity 

 of light. But this simple average is precluded unless it can be proved 

 that the two experiments are entitled to equal weight. The internal 

 evidence, expressed by what mathematicians call the probable error, 

 manifested a decisive preference for Foucault's result, and it has met 

 with general acceptance. The soundness of the scientific judgment in 

 this case has been placed beyond all cavil by Cornu, who has recently 

 repeated Fizeau's experiment, with additional precautions, and re- 

 solved the discord into a marvelous accord. Fizeau's experiment, in 

 spite of the numerical defect, was hailed as one of the grandest 

 triumphs of experimental skill. In 1856 he received the prize of 

 30,000 francs which the Emperor of France had founded, to be given 

 for the w r ork or the discovery which, in the opinion of the five acade- 

 mies of the Institute, has conferred the greatest honor and service 

 upon the nation. Hitherto, it had been supposed that nothing short 

 of an interstellar or an interplanetary space was a match for the 

 enormous velocity of light. And yet one physicist, by using a distance 

 of less than six miles, and another, without going outside of his labo- 

 ratory, have discovered what astronomers had searched heaven and 

 earth to find out. 



By these capital experiments the science of optics has achieved its 

 own independence. Let us see what they have done, at the same 

 time, for astronomy. The sequences in the eclipses of Jupiter's moons 

 are modified by the velocity of light. The aberration of starlight is a 

 measure of the ratio between the velocity of light and the velocity of 

 the earth. For nearly two centuries our knowledge of the velocity of 

 light leaned upon one or the other of these relations. If the velocity 

 of light can be known from experiment, the problem may be reversed 

 and the distance of the sun given to the astronomer. As soon as it 

 appeared that Foucault's estimate of the velocity of light fell short 

 of the astronomical valuation by about three per cent., it was certain 

 that either the experiment was in error, or the received aberration 

 was too small, or the reputed distance of the sun was too large. An 

 error of three per cent, in the experiment or in the aberration was 

 inadmissible. But it was conceivable that the distance of the sun 

 should be at fault, even to this extent. The popular announcement 

 that Foucault had picked a flaw in the astronomer's work was not 

 correct. Astronomers had always known what those who pinned 



