z 
4 
\, 
, 
and the Absolute Size of the Fixed Stars. 301 
A proper motion of the large amount of 3-58” is participated in 
by both the stars, a fact which pretty clearly proves a physical con- 
nection between them; for while they are now very nearly in the 
position they were in 100 years ago, when observed by the Abbé 
Lacaille, they would have separated by this time upwards of five mi- 
nutes, if one only was pursuing this anomalous path amongst the 
rest of the stars. 
The first person to remark on this physical connection was Pro- 
fessor Henderson, who, in the concluding paragraph of his memoir 
on the parallax, says, 
“The two stars appear to be approaching each other. The 
earliest observations of @ Centauri made with a telescope which I 
have found, are those of Richer at Cayenne in 1673, but neither he 
nor Halley, who observed it at St Helena in 1677, mentions it as 
being double. Their telescopes were of course anachromatic, and 
probably not of much power. Feullée appears to have been the first 
person who observed the star to be double, as he mentions in the 
Journal of his Voyage in South America in July 1709. La Conda- 
mine next observed the star during the scientific expedition to Peru 
for measuring an are of the meridian.” But neither of them made 
any observations of real service in determining the nature of the 
physical connection of the two stars. ‘* From Lacaille’s observations 
in 1751-2, the distance of the two stars appears to have been then 
22°5”. Maskelyne, who observed them at St Helena in 1761, says 
(Philosophical Transactions, 1764, p. 383): The bright star in the 
foot of the Centaur, marked a in the catalogues, when viewed through 
a telescope, becomes divided into two stars, one of which is about the 
second and the other the fourth magnitude. They were both ob- 
served by the Abbé De Lacaille. I found their distance by the 
divided object-glass micrometer, fitted to the reflecting telescope, to 
be 15” or 16”. I have not found any observations,’’ continues Pro- 
fessor Henderson, “ of the distance of the two stars made between 
1761, and the institution of the Paramatta Observatory : there, in 
the end of 1825 or the beginning of 1826, the distance was observed 
to be 23” (Memoirs of Astronomical Society, Vol. iii., p. 265), 
since which time it has been decreasing at the rate of . more than 
half a second per annum. The angle of position scarcely appears 
to have changed since Lacaille’s ee, whence it may be interred, 
that the relative orbit is seen projected into a straight line or very 
excentric ellipse ; that an apparent maximum of distance was attained 
in the end of the last or the beginning of the present century ; and 
that about twenty years hence the stars will probably be seen very 
near each other, or in apparent contact, but the data are at present 
insufficient to give even an approximation to the major axis of the 
orbit and time of revolution.” 
The next authority on the subject is Sir John Herschel, who spe- 
cially applied himself to the subject of the Southern double stars 
