Page:Scientific Papers of Josiah Willard Gibbs - Volume 2.djvu/263

 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 experiments 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 ± 60. All these experiments were made with the revolving mirror, but the arrangements of the two experimenters were in other respects radically different. The first of these values of the velocity of light with Nyrén'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 theoiy of light makes the velocity of light in air equal to the ratio of the electromagnetic and