CHAPTER I. 



Historical Introduction. 



1. The history of the radial velocity determinations of the stars is — as far as the 

 last century is concerned — very short. The results deduced from the material, so labo- 

 riously brought together, have also been very uncertain. The most interesting problem, 

 to determine the motion of the sun relative to the stars, from observations in the line 

 of sight, led to very varying values concerning the magnitude of the velocity as 

 well as its direction. In fact, as late as 1900 these fundamental quantities were 

 known from radial velocity stars with very small accuracy. 



Undoubtedly there were many difficulties to overcome especially regarding the 

 small sensibility of the instruments, but another and perhaps greater difficulty was 

 to be found in the circumstance that the measurements of the displacement of the 

 spectral lines were made visually. Evidently in this way it was only possible to 

 measure the spectra of the brightest stars. 



In 1875 and the following years systematic radial velocity determinations were 

 made at the Greenwich Observatory especially by Mr. Maundee. These results, pub- 

 lished in the Monthly Notices \ were subjected to discussion by different persons and 

 many attempts were made to determine the velocity of the sun in space. 



In 1883 KövESLiGETHT 2 makes, perhaps, the first determination of the solar velocity. 

 The number of stars, however, being too small to give a good value of the direction 

 of the apex, he assumes this latter to be known. Adopting the values of the coordi- 

 nates of the apex derived from proper motion observations a = 261 u . o § = + 35°. i 

 he got the mean value of the velocities of the stars surrounding this apex to be 63.8 km 

 per sec. The mean value of the velocities of the stars surrounding the parallactic 

 equator was found to be 7.5 km per sec. and he considered this evidence as proof 

 of the accuracy of the solution. 



Some years later, in 1885 Homann 3 makes a more complete solution, based 

 upon different sets of spectroscopic observations, namely the measurements previously 



1 Monthly Notices Tome XXXII and following. 

 - Aetron. Nachr. 114. 327. 1886. 



3 In Astron. Nachr. 114. 25. 1886 the author gives a short summary of his results. 



