460 michelson'.s recent researches on light. 



fringes amouuting to 49". 8. Selecting tbe line D in the fourth spec- 

 trum, he thought that the influence of the earth's annual motion was 

 verified, but that of the motion of the solar system Mas less decided. 

 The observations were more consistent with the assumption that the 

 solar system moved with a velocity Cijual to one-third of that in its 

 orbit, than with an equal velocity, or none at all. In 1862-'G3, Babinet 

 presented to the Academy of Paris a paper on the influence of the mo- 

 tion of the earth on the plienomena produced by gratings, which depend 

 not on reflection, refraction, or diflVaction, but on interference. His 

 principal object was a study of the motion of the solar system. He 

 calculated the eflects to be expected, but published no observations. 

 In 1867, Van der Willigen measured the length of waves of light by 

 means of a grating. When a slit was used, no eftect was produced by 

 the motion of the earth, the slit iiartaking of that motion. With a star, 

 a movement of the earth in the direction of the light had an eftect. 

 This is the theoretical result, and agrees with Babinet's experiment, 

 but is not applicable to solar light when reflected by a mirror. That 

 behaves as light from a terrestrial source. In 1873, he rejects the 

 proposition that the refraction of light is modified by the motion of its 

 source or of the prism. In 1874, he seems to doubt the reality of the 

 eftect produced on diftraction. 



In 18G7, Klinkerfnes used a transit instrument having a focal length 

 of 18 inches. In the tube was a column of w^ater 8 inches long, and a 

 prism. He observed transits of the sun and of certain stars whose 

 north polar distance was equal to the sun's, and which passed the 

 meridian at midnight. The difterence of riglit ascension is aftected by 

 double the coefficient of aberration. He computed that the column of 

 M'ater and the prism would increase the aberration by 8". The amount 

 observed was 7".l. In 1868-'69, Hoek of Amsterdam discussed the in- 

 fluence of the earth's motion on aberration. Delambre had calculated 

 from the eclipses of Jupiler's satellites that light must take 493^2 in 

 coming from the sun. Hence the aberration must be 20".255. Struve's 

 observed aberration made the time 497^8. Hoek decided in favor of 

 Struve; but he thought that it was desirable that a new set of obser- 

 vations should be made on the eclipses. Klinkerfues espoused the side 

 of Delambre. Hoek said that, if the eartli's motion was taken into 

 account, according to Fresnel's fraction, different results would be 

 harmonized. In 1868, he made experiments on a divided beam of light, 

 the two parts going in opposite directions through tubes filled with 

 water. TJiere was no interference attributable to the eftect of the 

 earth's motion. As to any influence to be expected from the motion of 

 the solar system, he thinks that motion must be insignificant compared 

 with the initial motion of the comets, and with the cometary orbits, 

 which are parabolas with few hyperbolas. 



In 1872, and on several jirevious occasions, one of the grand prizes 

 of the Academy of Paris was oftered for an investigation of the eftect 



