Sidereal System called the Milky Way. 89 
all placed along a plane* ring of enormous dimensions, and all 
moving in it in periods which only myriads of centuries can 
measure; following them in their long and slow courses, ima- 
gine them to approach promiscuously but alternately the outer 
and inner edge of the ring, and you will have an idea of the 
sidereal system in which we are placed, such as I have conceived 
it, and such as I have wished to show it you in this discourse. 
Addenda.—\. The distinguished Mr. Henderson, Director 
of the Edinburgh Observatory, communicated to the Royal 
Astronomical Society of London, at the beginning of the 
present year, the result of the observations which he and 
Lieut. Meadows have made at the Cape of Good Hope, on 
the double star «! a! of the constellation of the Centaur. This 
result shows that that star, which has a very sensible motion 
of its own, has an annual parallax of about one second, that is, 
three times as great as that which M. Bessel found for the 
double star 61 Cygni, so that the former star must be three 
times as near to us as the latter. The double star a!’ a of 
the Centaur is projected on the edge of the Milky Way on 
the side of the constellation of the Cross, that is, in that part 
of the ring in which we have said our solar system is at pre- 
sent situated; and according to what we have shown it is 
clear that it is on this very side that we should have a greater 
chance of meeting with stars nearer to us. In general, the 
fact that stars which are more visible and which have a more 
sensible motion of their own, such as Bootes, Sirius, Pro- 
cyon, are found in that hemisphere of the heavens in which 
the segment of the ring of the Milky Nebula which is nearest 
to us is situated, is a circumstance that tends to confirm the 
constitution of the sidereal system which we have attempted 
to explain in the foregoing discourse. 
* Although we have always spoken of a plane ring, yet to satisfy all 
appearances we must suppose it of a slightly conical form, i. e. we must 
suppose it to have the shape of a portion of the surface of a very obtuse 
right cone intercepted between two planes parallel tothe base. For if 
the ring were plane, the Milky Way would appear narrower in the neigh- 
bourhood of the constellation Cassiopeia, yiz. in the part opposite that in 
which we are placed, than on our side, viz. in the neighbourhood of the 
constellation of the Cross; whereas, in fact, in the former, though it sheds 
a weaker light, it has the appearance of greater width. This can be explained 
by assuming the ring of the above-mentioned shape. To an observer si- 
tuated near the interior edge of the ring on theside of the constellation of the 
Cross, the Milky Way on this side would appear of a thickness correspond- 
ing to the angle contained between two visual rays, which, penetrating to a 
certain distance between the stars in the ring, embrace its thickness, while 
on the opposite side, besides the thickness, a portion of the internal sur- 
face would be visible in a very oblique direction, and this would give to 
the Milky Way the appearance of a greater width. 
