362 RECORD OF SCIENCE FOR 1886. 



y2 y2 



— , but ^^ ; tbe calculated value of which is 1.758, practically iden- 

 tical with 1.7C, the value obtained by Michelson. If a second rotating 

 mirror be used, Eayleigh's value — remains true. He concludes, then, 



that while the aberration method measures Y, and the eclipse of Jupi- 

 ter's satellites and Fizeau's method measure U, Foucault's revolving- 



y2 ■ Y2 

 mirror method measures either ^r or ^^^^y- (Nature, March, 188G, 



XXXIII, 439.) In reply to this note Gibbs calls attention to the fnct 

 that it follows from Rayleigh's formulas that while the individual wave 

 rotates, in the revolving-mirror experiment, the wave-normal of the 

 group remains unchanged; or, in other words, that if we fix our atten- 

 tion on a point moving with the group, and therefore with the velocity 

 U, the successive wave planes, as they pass through that point, have 

 all the same orientation. "To get a picture of the phenomenon," he 

 says, " we may imagine that we are able to see a few inches of the top 

 of a moving carriage-wheel. The individual spokes rotate, while the 

 groups maintain a vertical direction." This consideration, he thinks, 

 greatly simplifies the theory of Foucault's experiment, and makes it 

 evident that the results of all such experiments depend upon the value 

 of U and not upon that of V. (Nature, April, 1886, xxxiii, 582.) 



Gony has repeated, by means of Foucault's method, experiments to 

 determine the relative velocity of red and blue rays in carbon disul- 

 phide. Several concordant series of obervatious show the deviation to 

 be greater for blue than for red light, the difference being about 5", or 

 one twentieth of the deviation for the red ray. This result he thinks is 

 in accord with theory. (C. R,, July, 1886, cm, 211.) 



Michelson and Morley have repeated Fizeau's experiment on the in- 

 fluence of the motion of the medium upon the velocity of light. He 

 had announced the remarkable result that the increment of velocity 

 which the light experienced was not equal to the velocity of the medium, 

 but was a fraction of this velocity, which depended on the index of 

 refraction of the medium. The formula of Fresnel is equivalent to the 

 statement that the tether within a moving body remains stationary, 

 with the exception of the portions which are condensed around the par- 

 ticles. If this condensed atmosphere be insisted on, every particle, to- 

 gether with its atmosphere, may be regarded as a single body, and then 

 the statement is simply that the aether is entirely unaffected by the 

 motion of the matter which it permeates. The authors used essentially 

 the same form of apparatus as Fizeau. Light from a given source falls 

 on a half-silvered surface where it divides, one-half being reflected 

 tiirough one of two tubes 28""" in diameter and 6 meters long and back 

 through the second, the other half passing through the second tube 

 directly and being reflected back through the first. The tubes being- 

 filled with distilled water, the light from an electric lamp was directed 



