680 
Proceedings of the Royal Irish Academy. 
The star was measured by Struve. 
1827-28 : 337°-8 : l"-24, 
so that the distance seems nearly constant. 
14. a Geminorum (Castor) = % 1110. The change of position angle 
since 1719 has been about 125°. Sadler thinks that "a good orbit 
for Castor has yet to be computed." Some recent measures are 
as follows : — 
1882-82 
234°-35 
5" 
•69, Doberck. 
1882-88 
234 
•29 
5 
•559, Perrotin. 
1883-03 
232 
•28 
5 
•44, Doberck. 
1884-17 
232 
•15 
5 
•840, „ 
1885-110 
232 
•93 
5 
•78, Tarrant (^.iV., 2866.) 
1885-15 
232 
•2 
5 
•74, Engelmann. 
1885-99 
231 
•15 
•36, Doberck. 
1886-00 
: 232 
•93 
: 5 
•83, ,, 
1886-33 
: 232 
-2 
. 5 
•75, Engelmann. 
1888-28 
: 230 
•9 
: 5 
•85, Schiaparelli. 
1889-09 
. 229 
•6 
: 5 
•68, Leavenworth. 
A long list of measures by Dr. Doberck from 1875 till 1886 will be 
found in the " Transactions of the Eoyal Irish Academy," vol, xxix., 
part XIII. (1890). 
Using Johnson's parallax of 0-1984", I find the sum of the mass of 
the components of Castor only 0-052398 of the mass of the sun, a result 
which would imply that the component stars are gaseous bodies! 
Johnson's parallax is, however, of doubtful value. 
15. ^ Cancri (AB) = Struve, 1196. Seeliger's orbit (1888) is pro- 
bably the best. Nearly two revolutions have now been completed 
since its discovery by Sir W. Herschel in 1781. All three stars form 
probably a physically connected system, but the motion of C round AB 
is slow. Professor Seeliger has recently investigated the motion of 
this system, and has come to the conclusion that to make the obser- 
vations agree with theory it is necessary to assume that the third star 
C is in reality a very close double, of which the components revolve 
round their centre of gravity in about 17-6 years, and both round the 
centre of gravity of the components of the close pair. This supposed 
duplicity of C has not yet been detected with the telescope. Burnham 
in 1889, using a power of 1500, failed to see any other component. 
The following are some recent measures : — 
