(2315) 
The relations (1) show a remarkable analogy to those existing’ 
between the values of the rotation a and «' about a and a’ and 
the rotation round the central axis, the latter evidently amounting 
to @, 
We have namely : 
a =o iin: (ca'), 
a' = @ sin (ca). 
Comparing these relations to (1), we see that “OAA' = / (ca). 
Now the plane (4a’) being the polar plane of A, it follows that 
the rotation of this plane, when A describes the line O'A, is equal 
to the rotation of OA: in other words, the rotation of a is equal 
to that of OA. 
If now we imagine a line passing through O parallel to a, we 
immediately see that when 4 describes the line O'A, the plane (Oa) 
assumes a motion originating from a double rotation with equal 
components about the plane O44’ and the plane normal to it through 9. 
If finally we make the thus generated system of planes turn 
about the plane through O and the central axis c, we obtain the 
complete image of the reduction. 
The results arrived at here entirely agree with those of Dr. W. A. 
WytHorr in his dissertation: „De Biquaternion als bewerking in de 
ruimte van vier afmetingen.”’ 
Astronomy. — “On J. C. Kaptryn’s criticism of Atry’s method 
to determine the Apex of the solar motion,’ By J. Sruin S.J. 
(Communicated by Prof. H. G. van DE SANDE BAKHUYZEN). 
At the meeting of the Section of Sciences of Jan. 27th 1900, 
Prof. J. C. Kapreyn has given some critical remarks on the methods 
followed till now to determine the co-ordinates of the Apex of the 
solar motion. In his paper the writer would point out: first, that 
neither Arry's nor ARGELANDER’s method is based on the known 
hypothesis on the proper motions: “the peculiar proper motions of 
the fixed stars have no preference for any particular direction.” 
Secondly he has tried to develop a method satisfying this condition. 
(Proceedings Vol. II, pag. 353). 
It seems to me that this charge against Arry’s method is unjust ; 
and IJ hold that this method, even when the equations of condition 
