392 PROCEEDINGS OF THE AMERICAN ACADEMY 



regard to the propagation of light in refracting media. According to 

 all the theories admitted or proposed, the displacement of the fringes 

 should have heen equal to many lengths of a fringe, that is, many 

 millimeters, while by observation it was nothing. Stokes has calcu- 

 lated the result according to Fresnel's theory, or his own modification 

 of it, and found that the retardation 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 experiment. He says : " When two rays 

 have a certain difference of march, this difference is altered by the 

 reflexion from the turning mirror." By calculating the two effects 

 in Babinet's experiment, Fizeau finds that they have sensibly equal 

 values, and of opposite sign. 



In 1860, Angstrom communicated to the Royal Society of Upsala 

 a method of determining the motion of the solar system by obser- 

 vations on the bands of interference produced by a glass grating. In 

 18G3 he published the results which he had obtained. After allow- 

 ing for Babinet'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 dis- 

 placement of the fringes amounting to 49".8. Selecting the line D 

 in the fourth spectrum, he thought that the influence of the earth's 

 annual motion was verified, but that of the motion of the solar system 

 was less decided. The observations were more consistent with the 

 assumption that the solar system -moved with a velocity equal to one 

 third of that in its orbit, than with an equal velocity, or none at all. 

 In 1862-63, Babinet presented to the Academy of Paris a paper on 

 the influence of the motion of the earth on the phenomena produced 

 by gratings, which depend not on reflection, refraction, or diffraction, 

 but on interference. His principal object was a study of the motion 

 of the solar system. He calculated the effects 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 effect was produced by the motion of the earth, the slit par- 

 taking of that motion. With a star, a movement of the earth in the 

 direction of the light had an effect. 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 effect produced on diffraction. 



In 1867, Klinkerfues used a transit instrument having a focal 



