1876.] 
Sidereal Astronomy. 
55 
first based their arguments on the universality of gravitation ; 
and, moreover, to-day we find it described in all treatises of 
astronomy as being the most interesting of the orbital 
systems. But on reuniting the hundreds of micrometrical 
measurements taken during this long interval of time, I 
have arrived at the conclusion that the two components of 
61 Cygni do not turn round each other at all , but that they 
are carried together in space by a rapid proper movement, 
which, whilst it is common to both, removes them slowly 
from each other. These two suns are truly associated, but 
do not gravitate round each other, and their relative 
displacement is performed in a right line. This proper 
movement is one of the most rapid that we know ; it is 
515" in a century, a velocity which represents a minimum of 
many millions of leagues a day. These two suns should be 
one behind the other in perspective, and advance or 
retire from us in such a way that their line of junction 
would appear more in face, and their angular distance appear 
to augment. This system is not the only one of its kind; 
I have found many others like it. Thus there are stars 
associated by a proper common movement which do not 
gravitate round each other. 
Amongst the couples of which the components remain 
not only fixed with regard to each other, but altogether 
immovable the one to the other at the same point of the 
celestial sphere, many can form physical couples, and 
many optical couples. Their distance in the depths of 
infinity, the slowness of their absolute or relative move- 
ment, will only permit us to recognise their nature after 
centuries of continued observation. Absolute rest does not 
exist in the immense universe. 
Many very distinct causes thus operate on the double 
stars to give this real or apparent movement : the rotation 
of the components of a binary, tertiary, or multiple 
system, round their common centre of gravity ; the gravita- 
tion of two or more stars carried together in space under' 
the influence of unknown sidereal attractions ; the differing 
proper movements of two stars much separated one behind 
the other in the line of the visual ray ; the secular transla- 
tion of our solar system in space, which reaCts by giving to 
less distant stars an apparent displacement in a contrary 
direction. 
In giving a general review of the double stars, I have 
found 614 systems having certain orbital movement, and for 
each of these I have traced the figure of the observed move- 
ment. Thirty couples have travelled over a part of their orbit 
