XVI. 



REVIEWS OF NEWCOMB AND MICHELSON'S "VELOCITY 

 OF LIGHT IN AIK AND REFRACTING MEDIA" AND 

 OF KETTELER'S " THEORETISCHE OPTIK." 



[American Journal of Science, ser. 3, vol. xxxi. pp. 62-67, Jan. 1886.] 



Velocity of Light in Air and Refracting Media. 



Astrononomical Papers prepared for the use of the American Ephemeris and 

 Nautical Almanac, vol. II. parts 3 and 4,* Washington, 1885. 



PEOFESSOK NEWCOMB obtains as the final result of his experiments 

 at Washington 299,860 30 kilometers per second for the velocity of 

 light in vacuo. Professor MICHELSON'S entirely independent experi- 

 ments at Cleveland give substantially the same result (299,853 60) 

 His former experiments at the Naval Academy, after correction of 

 two small errors which he now reports, give 299,910 + 50. All these 

 experiments were made with the revolving mirror, but the arrange- 

 ments of the two experimenters were in other respects radically 

 different. The first of these values of the velocity of light with 

 Nyren's value of the constant of aberration (20"'492) gives 149*60 for 

 the distance of the sun in millions of kilometers. On acount of the 

 recent announcement by Messrs. Young and Forbes of a difference 

 of about two per cent, in the velocities of red and blue light, 

 especial attention was paid to this point by both experimenters, 

 without finding the least indication of any difference. In Professor 

 Newcomb's experiments, a difference of only one thousandth in these 

 velocities would have produced a well-marked iridescence on the 

 edges of the return image of the slit formed by reflection from the 

 revolving mirror. No trace of such iridescence could ever be seen. 

 Professor Michelson made an experiment, in which a red glass covered 

 one-half the slit. The two halves of the image the upper white, 

 the lower red were exactly in line. 



Since Maxwell's electromagnetic theory of light makes the velocity 

 of light in air equal to the ratio of the electromagnetic and 



*[Part 3, "Measures of the Velocity of Light," S. Newcomb; part 4, "Supple- 

 mentary Measures of the Velocities of white and colored Light in air, water, 

 and carbon disulphide," A. A. Michelson.] 



