384 PROCEEDINGS OF THE AMERICAN ACADEMY 



of resolving it. Babinet thought that the experiment admitted of ten 

 times greater accuracy. With three times only it might correct 

 Struve's value of aberration. 



In 1873, Cornu, another French physicist, repeated the experi- 

 ments of Fizeau with a toothed wheel, the work extending over three 

 years. The observer was stationed at the Ecole Polytechnique. 

 The reflecting mirror and collimating telescope were placed on Mont 

 Valerian, at a distance of about 33,816 feet. Three different wheels 

 were tried, having 104, 116, and 140 teeth respectively, and rotating 

 between 700 and 800 times a second, the velocity being registered by 

 electricity. Cornu used at times all the eclipses from the first to the 

 seventh order. Calcium and petroleum liglit were tried, as well as 

 sunlight. A chronograph with three pens recorded automatically 

 seconds, the rotations of the toothed wheel, and the time of the eclipse. 

 More than a thousand experiments were made, 600 of which were 

 reduced. The velocity of light as published by Cornu in 1873 was 

 185,425.6 miles per second. The probable error was one per cent. 

 In 1874, Cornu gave the result of a new set of experiments made 

 by him in conjunction with Fizeau, over a distance of more than 

 14 miles between the Observatory and Montlhery. The experiments 

 were repeated more than five hundred times, mostly at night with 

 the lime light. The light was sent through a 12 inch telescope and 

 returned through a 7 inch telescope. The toothed wheel which pro- 

 duced the eclipse was capable of rotating 1,600 times a second. From 

 these experiments the velocity of light was placed at 186,618 miles. 

 The probable error did not exceed 187 miles. The time was re- 

 corded accurately within a thousandth of a second. 



I come now to that which most interests us to-night, viz. the part 

 taken in this country for the measurement of these great velocities. 

 About 1854, Dr. Bache, chief of the U. S. Coast Survey, appropri- 

 ated $1,000 for the construction of apparatus to be used in repeating 

 Wheatstone's experiment on the velocity of electricity. But those 

 who were expected to take part in the investigation were called to 

 other duties, and the money was never drawn. 



In 1867, Professor Newcomb recommended a repetition of Foucault's 

 experiment, in the interest of astronomy, to confirm or correct the re- 

 ceived value of the solar parallax. In August, 1879, Mr. Albert A. 

 Michelson, then a Master in the United States Navy, presented a 

 paper to the meeting of the American Association for the Advance- 

 ment of Science, on the measurement of the velocity of light. This 

 paper attracted great attention. Mr. Michelson adopted Foucault's 



