36 TRANSACTIONS OF THE [Nov. 4, 
is true that the period covered by Rutherfurd’s observations is 
only about three-and-a-half years, or about one fifty-fourth part 
of the whole period of revolution of the star, and this circum- 
stance is unfavorable to the success of the suggested research. 
But, on the other hand, the component changed its position 
during the period of Rutherfurd’s observations by about seven- 
tenths of a second of arc. So that if we adopt Struve’s ratio of 
the masses, which is one to three, the principal star must have 
changed its position with respect to the centre of gravity of the 
system by nearly two-tenths of a second. So large a quantity 
as this could hardly fail to be determined by the Rutherfurd 
measures, in view of the very large number of comparison stars 
available. It is, of course, obvious that we should not attempt 
to determine any of the other elements of the orbit from these 
measures, but employ in the computations one of the orbits that 
have been deduced from the measures of the system as a double 
star. The most recent orbit of this kind is that of Dr. See 
(Astr. Jour., Vol. XV.,p. 54), and this would be the best one to 
adopt for our present purpose. 
The method to be pursued in this inv estigation appears readily 
from the following considerations. It is well known that boti 
components of the system revolve about their common centre 
of gravity, and that this centre of gravity, therefore, isa fixed 
point in the system. It always lies on the line joining the two 
components, and divides this line in the inverse ratio of their 
masses. Consequently, in the apparent orbit, which is the pro- 
jection of the real orbit, and which is what we actually see on 
the sky, there is also a fixed point, which is the projection of 
the centre of gravity. This fixed point always lies on the line 
joining the apparent positions of the two components on the 
sky, and it divides that line in the same inverse ratio as before. 
It is this fixed point, and not the principal star, which should 
retain a constant distance from all the other surrounding stars 
on the plate, after the effects of proper motion, parallax, ete., 
have been removed. 
From what has been said, it is clear that if we compute from 
Dr. See’s elements an ephemeri is of the minor component referred 
to the principal star, and if we put: 
po’, =the distance and position angle of the minor component, 
as ‘evel by the ephemeris. 
M, M’=the masses of the principal star and companion re- 
spectively ; then if we compute m by the equation : 
MM’ 
M+ M’ 
m= — 
