S6 M. Arago (yti Double Stars. 



reserve in page 22, concerning the relative change in position of 

 two objects at different distances. 



Tliis change, we then said, depends'entirely on one of the two 

 objects, — on that which is the nearest ; whensoever the other is 

 at such a distance, compared with the space over which the ob- 

 server can move, that these variations of angular elevations are 

 insensible. Then the second object becomes the most exact of 

 marks with which we may compare the first, to recognize and to 

 measure its changes of elevation. Then, too, there is no need 

 either of a large mathematical instrument, which shall be un- 

 changeable during its transport from the first to the second sta- 

 tion, or of a level or plummet ; — then to know if there have 

 been a change of position, a simple coup (Toeil is sufficient, when 

 the objects are nearly in contact ; and then, besides, the extent 

 of this change is measured with the help of a little instrument 

 known under the name of a micrometer, and which, enclosed in 

 the telescope, is simply composed of two threads, the one fixed 

 and the other moveable, with the help of a screw. 



If it be supposed, as is natural, that the difference of bril- 

 liancy between the two stars depends, in general, on the diffe- 

 rence of their distances from the earth; — that stars of the 

 seventh, eighth, and ninth magnitude, are much more dis- 

 tant than stars of the first, second, and third, we shall find in 

 the heavens many binary combinations which will satisfy the re- 

 quired conditions. Every amateur who is supplied with a power- 

 ful glass, may henceforward work for the determination of the 

 distance of the stars, with as much prospect of success as those 

 who are astronomers by profession. 



This is in fact the method of observation of which we have for- 

 merly spoken. It is now evident, then, that no one should apply 

 himself to the observation of stars which almost seem to touch. 

 In the groups of this sort, the difference of the lustre of the two 

 stars avails so little as it regards the difference of the distance, 

 that, in describing its orbit, the lesser star, without ceasing to be 

 the lesser, interposes itself betwixt us and the greater. As to 

 the binary combinations, where a star of the first, second, or 

 third magnitude is remarked at the angular distance of three, 

 four, or five minutes^ from a star of the sixth, seventh, or eighth 

 magnitude, there are, without doubt, a considerable number 



