THE SUN'S MOTION IN SPACE. 189 



spectral type. 1 In all there were 2,357 stars, of which 1,189 



belonged to the first, 1,106 to the second, and 62 to the third 



spectral type ; these types were considered because any general 



theory of distance as related to magnitude is obviously imperfect 



unless the character of the emitted light is regarded. The general 



result of Kapteyn's researches, in which Ludwig Struve's position 



of the apex of solar motion was accepted for the purpose of the 



reductions, is given hereinafter, see Kapteyn 1898. These values 



were 



R.A. = 276°, D. = +34° 



for the epoch 1865. 



(96) Kobold, 1893.— Kobold opened his 1893 treatise 2 with a 

 short discussion on the essential nature of the methods previously 

 adopted for determining the sun's path in space. Stating that 

 these may be divided into two species, viz., those that avoid all 

 hypotheses in computing the direction of motion, and those that 

 are deduced on some definite hypothesis, Argelander's and 

 Bessel's methods belonging to the one, and Airy's and Schonfeld's 

 to the other. Argelander so determined the direction that the 

 sum of the squares of the differences of direction between the 

 observed and the computed parallactic motion should be a mini- 

 mum. Bessel computed the poles of the observed proper motions, 

 and determined the direction of the solar motion as the pole of a 

 great circle so situated, that the pole of the proper motions should 

 approximate as near to its pole as possible. Airy determined the 

 direction and magnitude of the motion together, and was forced 

 to adopt an assumption as to relative distances of the stars, so as 

 to suitably combine his data. Finally Schonfeld introduced by 

 way of explanation of the differences between the deduced paral- 

 lactic motion and the observed proper motion, the notion of 

 rotation parallel to the plane of the Milky-Way. As already 



1 Over de verdeeling van de sterren in de ruinate. Verslagen der Afd. 

 Natuurk. kon. Akad. v. Wetenschappen, 28 Jan. 1893, pp. 125 - 140. See 

 also The Observatory, Yol. xvi., p. 275, 1893. 



2 Ueber die Bestimmung der eigenen Bewegung des Sonnensystems — 

 Astr. Nach. Bd. cxxxn., pp. 305 - 326, 1893. 



