400 PROCEEDINGS OF THE AMERICAN ACADEMY 



vations of Mars (8". 78) agrees with Michelson's velocity of light and 

 the mean constant of aberration. 



In 1877-78, Lord Rayleigh, in his profound treatise on the Theory 

 of Sound, discussed the distinction between wave-velocity and group- 

 velocity. In 1881, he recognized the same difference in the case of 

 luminous waves. In the experiments of Young and Forbes, the wave- 

 velocity might be nearly three per cent less than the group-velocity. 

 With toothed wheels and the revolving mirror, group-velocity was the 

 subject of observation. Aberration gave wave-velocity ; Jupiter's sat- 

 ellites, group-velocity ; experiment, however, showed but little differ- 

 ence. Lord Rayleigh found formulae for the relation between these 

 two kinds of velocity, which involved the wave-length and the index 

 of refraction, and J. Willard Gibbs has compared them, and other for- 

 mulae proposed by Schuster and Gouy, with the experimental veloci- 

 ties of light. Michelson's experiment on the index of refraction of 

 carbon disulphide agrees with the assumption that he was dealing with 

 the group-velocity. 



Although there is not a complete accordance between the results of 

 different methods of investigation, astronomers and physicists will be 

 slow to abandon the theory of undulations, and take up again the cor- 

 puscular theory of light. The latter theory has received fatal blows 

 from which it cannot recover. The undulatory theory, which started 

 with Huyghens more than two hundred years ago, and was elaborated 

 by Fresnel sixty years ago has survived many crises in its history, 

 and is supported by a wonderful array of experiments. Some of the 

 experiments of Mr. Michelson may require a modification in Fresnel's 

 interpretation. Stokes and Challis have worked for many years upon 

 it, and established it on mathematical principles differing from Fres- 

 nel's and from each other. Ketteler in his TJieoretische Optlh, pub- 

 lished in 1885, builds upon the Sellmeier hypothesis, that ponderable 

 particles are excited by the ethereal vibrations and then react upon 

 them. There remains Maxwell's electromagnetic theory of light, 

 which has been elaborated by Glazebrook and Fitzgerald, and is sup- 

 ported, to say the least of it, by remarkable numerical coincidences. 



Discrepancies between theory and experiment are always to be 

 welcomed, as they contain the germs of future discoveries. We have 

 learned in astronomy not to be alarmed by them. More than once 

 the law of gravitation has been put again on trial, resulting in a new 

 discovery or in improved mathematical analysis. We may not expect 

 in light such a brilliant discovery as that of the planet Neptune. The 

 luminiferous ether is a mysterious substance, enough of a fluid for the 



