54 Sidereal Astronomy . [January, 
world of double stars an immense variety of movements, to 
which may still be added the effects produced by the 
sidereal translation of the star system across the changing 
perspective of the starlit space. It is thus found that there 
are amongst the double stars couples possessing definite 
orbital movements, optical couples, due only to the chance 
of perspective and to our arbitrary position in space, 
and couples which have remained fixed since their dis- 
covery. Do these last represent true systems of stars 
associated by a physical bond, or are they only perspective 
groups? Both species can be found among them. When 
the components of a double star — the whole having pre- 
served the same relative position with regard to each other 
— are brought together into space by a mutual proper 
movement, it is thereby evident that they are mutually 
associated. But do they necessarily turn one round the 
other, in an extremely slow movement, which may not reach 
5° in a century (for this variation would be noticed), and the 
duration of which may exceed an average of seven thousand 
years, and may extend, perhaps, to ten, twenty, fifty, or a 
hundred thousand years ? This is probable, at least in the 
great majority of cases. But although the laws of universal 
gravitation appear to oppose all other hypotheses, and 
although astronomers have until lately declared that it 
must be so, we think that there are cases in which two or 
more stars might be brought near together by a proper 
movement in space, without thereby turning round a 
common centre of gravity situated within their group. 
This extraordinary faeft is proved by the analysis that I have 
made (among others) of all the observations and of all the 
measurements taken of 61 Cygni. This beautiful double 
star is one of the most celebrated in the entire heavens, 
because it is the first of which the terrestrial homunculus 
had the audacity and the glory of determining the distance. 
Its parallax being 0*51", its distance is 403,600 times the 
distance from here to the sun, that is to say, 14,933,200 
millions of leagues. At the rate of 77,000 leagues a second, 
light would have to fly for six years and five months to cross 
the abyss which separates us from it. Well ! this star is 
double, and it has been observed as such since 1753, that 
is to say for 123 years. The illustrious Bessel believed he 
was able to calculate its orbit, and had estimated its period 
at 400 years. Since then this period has been raised to 450, 
520, and even to 600 years, and its mass has been supposed 
to be calculated. By a singular chance, it was precisely on 
the supposed orbit of this famous star that astronomers at 
