Sidereal Astronomy. 
1876.] 
53 
this case there is only an optical group, and the two stars 
are not physically associated. 
To obtain the data necessary to determine the nature of a 
double star, it must be observed with care during a great 
number of years, and the relative position of the two com- 
ponents measured as exactly as possible, to ascertain their 
state and their position. When we possess these precise 
observations made at intervals of ten, twenty, or fifty years, 
and concordant intermediate observations, we find either 
that the two components have remained fixed, stationary, 
invariable in position one to the other, or else that their 
relative position has changed, and that they are respectively 
displaced. In this last case it is necessary to examine 
carefully the form of the movement. We take the most 
brilliant of the two components as a point of comparison — 
we suppose it to be immovable, and we place the second 
star at the different points observed during the series of 
years of observation. If these successive positions follow a 
curve round the principal star, this curve is studied, the 
missing part is obtained by calculation, and we find that the 
little star turns round the large one, following an ellipse 
more or less elongated. This is the apparent orbit as seen 
from the earth. Often instead of forming a curve, the 
displacement observed forms a right line. In studying this 
rectilinear movement, it is found at times that it also 
resembles an ellipse very much elongated, of which the 
plane passes through our visual ray, and instead of turning, 
the secondary star appears to oscillate in a right line from 
one side to the other of its primary. A wheel which is seen 
turning in front shows us a circular movement ; seen 
obliquely, it offers an elliptical movement ; seen all together 
from the side, it shows a rectilinear movement, and if it 
were transparent we should see a mark made on its circum- 
ference simply ascend and descend above and below the 
mean position. The examination of the rectilinear move- 
ment shows sometimes, on the contrary, that it is owing to 
a proper movement of one of the two stars in space, and 
that the second far distant star rests immovably in the 
depths of the heavens. There are even many cases where 
a star passes before two or three others much farther off 
and immovable. Sometimes these stars pass so exactly 
one before the other that they eclipse themselves. 
When they are of different colours, this eclipse of one 
star by another, so unexpected and so new to science, 
offers new phases to which no other celestial phe- 
nomenon can be compared. There are thus in this 
