OF ARTS AND SCIENCES. 399 



combined with the velocity of light as deduced from the experiments 

 of Coruu and Michelson, made the parallax of the sun 8". 784 ; dif- 

 fering from the most exact results of the geometric method by only a 

 few hundredths of a second. Villarceau proposed to get the solar 

 motion by aberration ; selecting two places on the earth in latitude 

 35° 16' north and south, and, after the example of Struve, observing 

 the zenith distances of stars near the zenith. The tangents of these 



latitudes are ± — ~; so that they contain the best stations for obtaining 



V- 



the constant of aberration, and the three components of the motion of 

 translation of the solar system. In 1887, Ubaghs, a Belgian astron- 

 omer, published his results on the determination of the direction and 

 velocity of the movement of the solar system through space. For 

 finding the direction, he used the method of Folic. For calculating 

 the velocity, he combined the observations on three groups of stars, 

 the brightest belonging probably to the solar nebula. The resulting 

 velocity was only about ten million miles a year. Horaann, working 

 on the spectroscopic observations at Greenwich, had obtained a ve- 

 locity of 527 millions of miles. As late as 1887, Fizeau studied the 

 nature of the phenomena when light was reflected from a mirror 

 moving with a great velocity, and inferred that aberration was the 

 same in this case as when the light was taken directly from a star. 



The solar parallax, calculated from Cornu's last experiment on the 

 velocity of light and Delambre's equation of light (493".2 being 

 the time for passing over the radius of the earth's orbit) = 8".878 

 From Struve's observed aberration, 8".797 



From Bradley's observed aberration, 8".881 



From Foucault's velocity with Struve's aberration, 8".860 



From Leverrier's latitudes of Venus by transits, 8". 853 



From meridian observations of Venus during 106 years, 8".859 



From occultations of x Aquarius in 1672, 8".866 



Glasenapp calculated the time taken by the light in travelling the 

 mean distance of the earth's orbit as equal to 500". 85 ± 1.02. This 

 time combined with Michelson's velocity of light makes the solar par- 

 allax 8".76. Struve's constant of aberration with Michelson's velocity 

 gives a parallax of 8".81. From Gill's mean of the nine best modern 

 determinations of aberration (= 20".496) the parallax comes out equal 

 to 8".78. If we regard the solar parallax as known, the eclipses give 

 nearly the same velocity as aberration, though the former is a group- 

 velocity and the latter a wave-velocity. Gill's parallax from obser- 



