466 michelson's recent researches on light. 



direction and velocity of the movement of the solar system through 

 space. For finding the direction he used the method of Folic. For 

 calculating the velocity he combined the observations on three groups 

 of stars, the brightest belonging probably to the solar nebula. The 

 resulting velocity was only about 10,000,000 miles a year. Homann, 

 working on the spectroscopic observations at Greenwich, had obtained 

 a velocity of 527,000,000 of miles. As late as 1887, Fizeau studied the 

 nature of the phenomena when light was rellected from a mirror mov- 

 ing with a great velocity, and inferred that aberration was the same in 

 this case as when the light was taken directly from a star. 



The solar parallax, calculated from Coidu's last experiment on the velocity 

 of light aid Delambre's equation of light (493". 2 being the time for pass- 

 ing over the radius of the earth's orbit) =8". 878 



From Struve's observed aberration 8". 797 



From Bradley's observed aberration 8". 881 



From Foucaiilt's velocity with Struve's aberration 8".860 



From Le Verrier's latitudes of Venus by transits 8". 853 



From meridian observations of Venus during 106 years 8". 859 



From occultations of ^^ Aquarius in 1672 8". 866 



Glasenapp calculated the time taken by the light in travelling the 

 mean distance of the earth's orbit as equal to 500^85 rt 1.02. This 

 time combined with Michelson's velocity of light makes the solar paral- 

 lax 8". 76. Struve's constant of aberration with Michelson's velocity 

 gives a jiarallax of 8".81. From Gill's mean of the nine best modern 

 determinations of aberration (=20".496) the parallax comes out equal 

 to 8". 78. If we regard the solar parallax as known, the eclipses give 

 nearly the same velocity as aberration, though the former is a group- 

 velocity and the latter a wave velocity. Gill's parallax from observations 

 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 5 experiment however showed but little differ- 

 ence. Lord Rayleigh found formulip for the relation between these two 

 kinds of velocity, which involved the wave-length and the idex of re- 

 fraction, and J. Willard Gibbs has compared them, and other formulae- 

 proposed by Schuster and Gouy, with the experimental velocities 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 



