VELOCITY OF LIGHT. This quantity through the efforts of many experimenters has been determined with a high degree of precision and plays an important part in both theoretical and practical optics and astronomy. After vain attempts at direct measurements made by Gali leo, Roemer in 1075 was able to calculate the velocity of light from observations of the eclipse of .Jupiter's satellites, making the value about 192,000 miles per second. Little further was done, hot•ever, until the nineteenth century, when Fizeau, using a revolving toothed wheel to in terrupt a beam of light, made a direct measure ment of light which had traveled a distance of 8033 meters. This method was also employed by Corm], who had improved apparatus constructed and used a distance of 23 kilometers. A method involving the use of a rapidly rotating mirror was employed by Foucault in 1862, and was noteworthy in that it permitted a short distance, such as could he found in a laboratory, to be used as the pathway of the brain. By this
method, which was improved by Miehelson and Newman), it was possible to measure the in liquids. The values obtained by various ex perimenters are given in the following table: Kilometers.
Foucault, 1862 2118,0181 Comm (1), 1574 298,500 •• (2), 1878 9110,1(0 Loran, (112m.usand b1• 1,181Ing 29'.1,990 Young and Vorbps, 1550-81 '101,382 Michelson 2)10.910 " (2) 290,893 liewpoinb (au1neted results) 240,5M '• results) 209,810 A detailed description of the methods Will be found in the article Linter in the paragraph on relocily of bight. to whiell referenee should be made. Consult: Preston. Theory of Light (Lon• don and New York. 1895) ; Comm. in Reports of International Congress of Physics (Paris. 1900) ; and iklichelson. "The Velocity of Light," vol. ix., Decennial Publications University of Chicago (Chicago, 1902).