312 TRANSLATIONS AND SELECTED ARTICLES. 
by reason of the large apparent diameter of this body, it has been pos- 
sible to observe separately different points of his contour. If the sun. 
were removed from us to the distance where his apparent diameter 
would be inappreciable, as that of the stars is, the method would be- 
come inapplicable. The colored rays, proceeding from different points 
of his contour, would then be found: closely mingled, and. we have 
already said that their mixture would produce white, It appears then. 
that we must give up the application to stars not possessing sensible 
dimensions of the process which has led us so well to our goal in the 
case of the sun. There are however certain of these stars which lend 
themselves to this: method of investigation. I allude to variable stars. 
Astronomers have remarked stars whose brightness changes con-. 
siderably. There are some of them which pass in a very small 
number of hours from the second to the fourth magnitude. There are 
others in which the change of brilliancy is much more decided. Such 
stars, very visible at certain epochs, disappear afterwards totally, to 
. appear anew after intervals, longer or shorter, and subject to some 
slight irregularities. Two explanations of these curious phenomena 
present themselves to the mind. One of which consists in supposing 
that the star is not equally luminous at all points of its surface, and 
that it has a motion of rotation on its own axis. Consequently, it 
appears brilliant when its luminous face is turned towards the earth, 
and sombre when its dark face comes into that position. On the 
second hypothesis, a satellite, opaque and not self-luminous, revolving: 
about the star, would periodically eclipse it. 
In reasoning on one or other of these two suppositions, the light 
which is sent. to us some time before the disappearance or the re- 
appearance of the star, has not issued from all the points of its contour, 
and there can no longer be occasion for the complete neutralisation of 
the tints we just now spoke of. If a variable star, examined with the 
polariscope, remains perfectly white in all its phases, we may be sure 
that its light proceeds from a substance like our clouds or burning 
gases. Now, such is the result of the small number of observations 
that we have yet been able to make, and which it will be of much 
utility to complete.* This same mode of investigation requires more 
eare, but succeeds equally well when it is applied to stars which 
*T am not aware that these experiments have been successfully prosecuted, but the method 
of prismatic examination of Kirchoff and Bunsen, alluded to in a previous note, has been 
applied with success to various stars, and has resulted in similar conclusions to those 
drawn in the case of the sun.—(Zvrans.) 
