173 



an aspect of Doppler's principle noticed by Fizeau 1 in 1848. It 

 was not till April 1868 however, that definite estimations of 

 movements to or from our system were made : they were then 

 communicated to the Royal Society of England by Huggins. 2 It 

 is evident that the motion in the line of sight affords a perfectly 

 independent method of computing the solar motion. Eighteen 

 months after Huggins had reported his results, Zollner 3 devised 

 his ingenious reversion-spectroscope, which by doubling the line 

 displacements increased the possibility of their accurate measure- 

 ment. 



(68) Vittarceau, 1875. — In 1875 Villarceau contributed a 

 second note, in continuation of the subject referred to in his note 

 of 1872. 4 No further remark is here necessary. 



(69) Maxwell-Hall, 1876.— -By 1876 not only had some con- 

 siderable advance been made in the determination of velocities 

 in the line of sight, a similar progress had also been made in the 

 estimation of the parallax, and therefore in the distance of stars. 

 In September of that year, Maxwell-Hall published his first memoir 5 

 commenced in 1869, on the sidereal system, in which the sun, 

 and some of the nearer so-called ' fixed ' stars were regarded as 

 bound together in a great dynamical system, assumed to be subject 

 to the ordinary laws of gravity. Hall supposed the stellar orbits 

 to be circular, and employing the same axes as Biot and Airy, 

 used heliocentric polar coordinates in the developments of his 

 equations. In adopting the direction of solar motion for the 

 purpose of examining the evidence of the existence of a dynamical 

 stellar system, Hall remarks that the mean of the results from 



1 Paper read before Soc. Philomathique, Paris, 23 Dec. 1848. See 

 Annal. de Chim. et de Phys., t. xix., pp. 211-221, 1870. 



2 Further observations etc., with an attempt to determine whether stars 

 are moving to or from the earth etc. — Phil. Trans., Vol. clviii., pp. 529 



-564,(1868). 



3 Leipzig, Ber. math, phys., Bd. xxiii., pp. 300-306, 1871. 



4 Becherches sur la theorie de Faberration, et considerations sur 

 Finfluence du mouvement absolu du systeme solaire, dans le phenomene 

 de Faberration. — Comptes rendus, t. lxxxi., pp. 163 - 171, 1875. See also 

 Conn, des Temps. Additions 1878. 



5 Mem. Eoy. Astr. Soc, Vol. xliii., pp. 157 - 197, Sept. 1876. 



