PHYSICS. 361 



The weight of the cubic iiicli of {ether would be only 2,467,5oo,ooo,L,ooo,ooo,ooo 

 ouuce. (Phil. Mag., September, 1S8G, V, xxii, 255.) 



J. W. Gibbs has giveu a resume of the results obtained by Newcomb 

 (see Nature, May, June, 1886, xxxiv, 29, 170) aud by Michelson in their 

 experiments on the velocity of light. As the final result of his Wash- 

 ington experiments Newcomb gives 299,800 i 30 kilometers per second 

 as the velocity of light m vacuo. Michelson's Cleveland experiments 

 give 299,853 rt GO; a result substantially identical. Combining the first 

 of these values with Nyren's value of the aberration constant (20.492"), 

 the value 149.60 is obtained as the sun's distance in millions of kilome- 

 ters. Although both the above experimentors paid especial attention to 

 the question whether there was any dilierence between the velocity of 

 r. d and of blue light, not the least indication of any difference was ob- 

 served. A difterence of one-thousandth in these velocities would have 

 given a well-marked color to the return image of the slit in Newcomb's 

 experiments. But no such ettect could be detected. Michelson covered 

 one half the slit with a red glass; the two halves of the return image 

 were exactly in line. Gibbs also gives the values which haVe been most 

 recently obtained of the ratio between the electro- magnetic and electro- 

 static units, which, according to Maxwell's electro-magnetic theory of 

 light, represents the velocity of this agent. These values, as corrected 

 for the true value of the ohm, are as follows: Ayrton and Perry, 1878, 

 296.1; Hockin, 1879, 296.9; Shida, 1880, 295.6; Exner, 1882, 291.7 (!); 

 J. J. Thomson, 1883, 296.3; Klemencic, 1884, 301.88 (?). They should 

 be compared with the velocity of light in air, expressed in millions of 

 meters per second— 299.778 according to Newcomb. Setting aside Ex- 

 uer's and Klemencic's values, the other four are closely accordant, their 

 mean being nearly identical with that of J. J. Thomson, which appears 

 to be by far the most worthy of confidence, aud differing by only one 

 per cent, from the velocity of light. Michelson's experiments on the ve- 

 locity of light in carbon disulphide afford, the reviewer thinks, an inter- 

 esting illustration of the difference between the velocity of waves and 

 the velocity of groups of waves — a subject to which Rayleigh has called 

 attention in an appendix to the second volume of his " Theory of Sound." 

 The quotient of the velocity in vacuo divided by the wave velocity in 

 carbon disulphide, calculated from Verdet, is for the line D 1.624 and 

 for E 1.637 ; while the group velocity, when used as a divisor, gives 

 1.722 and 1.767, respectively. Michelson's experimental result was 

 1.76 ± .02, agreeing well with the latter. (Am. J. Sci., January, 1886, 

 III, XXXI, 62.) Subsequently Shuster has argued in support of Eay- 

 leigh's conclusion that the velocity measured by the revolving-mirror 

 method of Foucault is really neither the wave velocity V nor the group 



Y2 



velocity U, but is — . But he shows that if only one revolving mirror 



be used, the experiment can not be performed in the way mentioned. 

 Hence the Foucault method really measures neither V nor TJ, nor yet 



