ON LIGHT. 465 



At each corner four mirrors were placed, by reflection from which the 

 length of path traversed by the light was increased to ten times its 

 former value. The width of the fringes of interference, which were 

 the subject of observation, measured from forty to sixty divisions of 

 the observing micrometer. The light came from an Argand burner sent 

 through a lens. To prevent jars from stopping and starting, the float 

 was kept constantly in slow circulation, revolving once in six minutes. 

 Sixteen equidistant marks were made on the stationary frame-work 

 within which the float moved. Observations were taken on the fringes 

 whenever any one of these marks came in the range of the micrometer. 

 The observations were made near noon and at (J P. m. The noon and 

 evening observations were plotted on separate curves. One division 

 of the micrometer measured one fiftieth of a wavelength. Mr. Michel- 

 son was confident that there was no displacement of the fringes exceed- 

 ing one-hundredth of a wave length. It should have been from twenty 

 to forty times greater than this. Mv. Michelson concludes that this 

 result is in opposition to Fresnel's tlieory of aberration. 



As late as 187li, Le Verrier thought that a new measurement of the 

 velocity of light by Fizeau very important in the interest of astronomy ; 

 and in 1871, Cornu wrote that the parallax of the sun, and hence the 

 size of the earth's orbit, were not yet known with tlie desirable precis- 

 ion. In 1875, Villarceau made a communication to the Paris Academy 

 on the theory of aberration. He says that the parallax of the sun by 

 astronomical measurement is 8". 86. Foucault's velocity of light com- 

 bined with Strure's aberration makes the sun's parallax 8".86. Cornu\s 

 velocity of light gives the same result only when it is combined with 

 Bradley's aberration, which differs from that of Struve by 0".20. Vil- 

 larceau thinks that there is an uncertainty about the value of aberra- 

 tion on account of the motion of the solar system. In 1883, M. O. Struve 

 discussed seven series of observations made by his father, Nyren, and 

 others, with various instruments and b^^ different methods, at the Ob- 

 servatory of Pulkowa. He was certain that the mean result for the 

 value of aberration was 20". 192, with a [)robable error of less than ,^,- 

 of a second. This aberration, co ubined with the velocity of light as 

 deduced from the experiments of Cornu aud Michelson, made the paral- 

 lax of the sun 8". 781; differing from the most exact results of the geo- 

 metric method by only a few liundredtLs of a second. Villarceau i)ro- 

 posed to get the solar motion by aberration ; selecting two places on 

 the earth in latitude 3iP 10' north and south, and after the exam])le ol 

 Struve, observing the zenith distances of stars near the zenitii. The 



tangents of these latitudes are J: .^ so that they contain the best sta- 

 tions for obtaiDing the constant of aberration, and the three components 

 of the motion of translation of the solar system. In 1887, Ubaghs, a 

 Belgian astronomer, published his results on the determination of thf* 

 H. Mis. 224. 3(1 



