OF ARTS AND SCIENCES. 393 



lentrth of eighteen inches. In the tube was a column of water eight 

 inches long, and a prism. He observed transits of the sun and of 

 certain stars wliose north polar distance was equal to the sun's, and 

 which passed the meridian at midnight. The difference of right as- 

 cension is affected by double the coefficient of aberration. He com- 

 puted tliat the column of water and the prism would increase the 

 aberration by 8". The amount observed was 7".l. In 1868-69, 

 Hoek of Amsterdam discussed the influence of the earth's motion on 

 abei-ration. Dehimbre liad calculated from the eclipses of Jupiter'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 observations should be 

 made on the eclipses. Klinkerfues espoused the side of Delambre. 

 Hoek said that, if the earth'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. There was no 

 interference attributable to the effect 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 mo- 

 tion of the comets, and with the cometary orbits, which are parabolas 

 with few hyperbolas. 



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

 of the Academy of Paris was offered for an investigation of the effect 

 produced by the motion of the luminary or of the observer. This 

 prize, consisting of a gold medal or three thousand francs, was awarded 

 in 1874 to Mascart. He maintained that in Arago's experiment the 

 change in refraction produced by the fraction of the earth's motion 

 was compensated by the displacement of the observing telescope. 

 Mascart repeated Babinet's experiment with gratings, where the effects 

 of the motion of the telescope and of the grating would be addi- 

 tive, and found the sum small compared with Babinet's calculation. 

 He thinks that the change in the length of the wave caused by the 

 motion is compensated by the displacement of the measuring apparatus. 

 He concludes that reflection, diffraction, double refraction, and circular 

 polarization are powerless to show the motion of the earth, either with 

 solar light or that from a terrestrial source. 



In 1871, Airy used a vertical telescope, and measured the merid- 

 ional zenith distance of y Draconis, the star by which Bradley dis- 

 covered aberration. It is about 100" north of the zenith. The tube 



