michelson's recent researches on light. 45.'^ 



aud IMontllu'ry. The experiments were reix'atcMl more than iive lumdred 

 times, mostly at night with the limelight. The light was sent through 

 a 12 iucb telescope and returned thiough ji 7-incii telescope. Tlie 

 toothed wheel which produced the eclipse was capaltle of rotating six- 

 teen hundred times a second. From these experiments the velocity of 

 light was placed at 180, ({18 miles. The probable error did not exceed 

 187 miles. The time was recorded ac(;urately 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. Jiache, chief of the U. S. Coast Survey, appropriated 

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

 stone'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 I^ewcomb 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 Advancement of 

 Science, on the measurement of the velocity of light. This paper at- 

 tracted great attention. Mr. Michelson adopted Foucauli's method with 

 important niodilicatious. In Foucault's experiment the detlection of 

 the light produced by the revolving mirror was too small for the most 

 accurate measurement. Mr. Michelson phiced the revolving mirror 500 

 feet from the slit (which was ten times the distance in Foucault's experi- 

 ment) and obtained a deflection twenty times as great, although themir 

 ror made only one hundred and twenty -eight turns in a second. With 

 apparatus comparatively crude, he obtained for the velocity of light 

 180,500, with a probable error of 300 miles. This preliminary exi)eri- 

 ment, made in the laboratory of the Naval Academy in May, 1878, in- 

 dicated the directions in which improvements must be made in order 

 to insure greater accuracy. The distance from the slit to the revolving- 

 mirror must be increased, the mirror must revolve at least two hun- 

 dred and fifty times a second, and the lens for economizing the light 

 must have a large surface and a, focal length of about 150 feet. With 

 the aid of $2,000 from a private source Mr. Michelson was able to carry 

 out his ideas on a liberal scale. 



His new experiments were made in the summer of 1871). The revolv- 

 ing mirror, made by Alvan Clark & Sons, was moved by a turbine wheel. 

 Its rapidity of revolution was measured by optical comparison with an 

 electric fork which made about one hundred and twenty-eight vibrations 

 a second, the precise value being accurately measured by reference to 

 one of Konig's standard forks. The velocity generally given to the 

 mirror was about two hundred aud (iftysix turns a second. The dis- 

 tance between tlie revolving aud the fixed mirror was 1,980.26 feet. 



