micitelson's recent researches on lioiit. 459 



ing' through it at all. Hoiico the refraction is the siune whether the 

 ])ri.sm be at rest or in motion through space." Maxwell is more guarded 

 in his own statement of the case. He says: "We can not conclude 

 certainly that the jether moves with the earth, for Stolces has shown 

 from Fresnel's liypothesis that the relative velocities of the aether in the 

 l)rism and that outside are inversely as the scjnare of the index of re- 

 fraction, and the deviation in this casi* would not be sensibly altered, 

 the velocity of the earth being only one ten-thousandth of the velccity 

 of light." 



In 1879, IMaxwell wrote to Prof. D. P. Todd, then at the Nautical 

 Almanac Oftice in Washington, askinghim if he had observed an appar- 

 ent retardation of the eclipses of Jupiter's satellites depending on the 

 geocentric position of the planet. Sueh observations, he thought, would 

 furnish the only method he knew of linding the direction and velocity 

 of the sun's motion through the surrounding medium. In terrestrial 

 methods of measuring the velocity of light, it returns on its ])ath, and 

 the velocity of the earth in relation to the a'thei' would alter the whole 

 time of passage by a quantity dei)ending on the scjuare of the ratio of 

 the velocities of the earth and light, and this is quite too small to be 

 observed. 



In 1839, Babinet made a very delicate experiment on the relation of 

 the luminiferous ;^'ther to the motion of the earth. He found that when 

 two i)ieces of glass of equal thickness were ]daced across two beams of 

 light which interfered so as to produce fringes, one of them moving in 

 the direction of the earth's motion and the other contrary to it, the 

 fringes were not displaced. The experiment was made three times by 

 Babinet, with new apparatus each time. He concludes that here is a 

 new condition to be fulfilled by all theories in regard to the propaga- 

 tion of light in refracting media. According to all the theories admit- 

 ted or proposed, the displacement of the fringes should have been equal 

 to many lengths of a fringe — that is, many millimeters — whileby obser- 

 vation it was nothing. Stokes has calculated the result according to 

 Fi'esnel's theory, or his own modifi(;ation of it, and found that the retar- 

 dation expressed in time was the same as if the earth were at rest. 

 Fizeau has pointed out a compensation in the effect of Babinet's exper- 

 in)ent. lie says : " When two rays have a certain difference of march, 

 this difference is altered by the reflection from the turning mirror." By 

 calculating the two efiects in Babinet's experiment, Fizeau finds that 

 they have sensibly equal values, and of opposite sign. 



In 18G0, Angstriim communicated to the Royal Society of Upsala a 

 method of determining the motion of the solar system by observations 

 OH the bands of interference i)rodu(;ed f)y a glass grating. In ISO.), he 

 published the results which he had obtained. After allowing for Bab- 

 inet's correction on account of the motion of the grating, Angstrom 

 finds that a difference in the direction of the observing telescope with 

 reference to the earth's motion might produce a displacement of the 



