S M. Arago Ofi Double Stars. 



that is to say, when it has arrived at the meridian, the immove- 

 able thread is horizontal. 



The instrument, then, which is employed, gives for the mo- 

 ment in question, and for the moment of the passing the meri- 

 dian, the angle which a horizontal line proceeding from the 

 greater star, forms with the straight line that joins this same 

 star to the smaller. This is what is called the angle of 'po- 

 sition. 



According to this method, the observations of different astro- 

 nomers, of different days, of different years, become objects of 

 comparison amongst themselves. The table of the successive 

 values of the angle of position, teaches at a glance whether the 

 lesser star revolves round the greater from west to east, or from 

 east to west ; if the movement is uniform or otherwise ; and what 

 are the points of the greatest and the least rapidity. 



Another instrument composed of two threads, the one fixed, 

 the other moveable in parallelism with the former, an apparatus 

 which bears the name of a micrometer .^ enables us to ascertain if 

 the apparent distance of the stars be constant or variable, and, 

 when there is a variation, to what extent this variation is con- 

 fined. 



This is all that observation supplies. But these data are am- 

 ply sufficient to enable us to determine, by the assistance of cal- 

 culation, the form of the curve described by each star, also to 

 know whether this curve is circular or elliptical ; and in the lat- 

 ter case what is the extent of the eccentricity. 



Fou>r values of the angle of position, and of the apparent mi- 

 crometrical distances corresponding to known epochs, are, in ge- 

 neral, necessary to determine the form and the position of the 

 curve which the lesser star describes around the larger. 



When it happens that the plane which contains the curve 

 passes through the earth, the movement of the satellite star 

 seems to take place along a straight line ; and then there are not 

 successive angles of position to be measured ; all is reduced to 

 micrometric observations of distances, and jive of these observa- 

 tions are necessary to arrive at the results which the^i^r supply 

 in the preceding hypothesis. 



Finally, if the observer is not supplied with a micrometer, and 

 can thus observe only angular displacements, six angles of posi- 



