M. Arago 07i Double Stars. 7 



telescope. The one must pass across the centre of the circular 

 space, which is denominated the field of vision, and must he fix- 

 edy immovably fixed, to the tube. The other thread must turn 

 round the same centre, so as to coincide, when we wish it, with 

 the stationary thread, or to make with it, to the right or to the 

 left, upwards or downwards, every imaginable angle. These 

 angles are to be measured upon a graduatedcircle, placed either 

 interiorly or on the outside of the tube. 



In making an observation, the more brilliant star is first 

 placed, as accurately as possible, at the point of the intersection 

 of the two threads. We then turn the moveable thread till we 

 get it accurately over the centre of the second star. Thus, in 

 reading off the degree at which it has stopt, we know the angle 

 which the direction of the fixed thread forms with the visual 

 line drawn from the centre of the greater star to the centre of 

 the less. 



According to this method of preparing the telescope, and 

 owing to the particular direction which is given to the immove- 

 able thread, whatever the hour is at which the observation of the 

 angle is taken, the same number is invariably found.* More- 

 over, if the telescope is turned towards the star at the moment 

 when it has arrived at its greatest elevation in its nightly course, 



• This result must needs astonish those who have remarked the apparent 

 changes which the constellations undergo betwixt their rising and their set- 

 ting. Nevertheless, we cannot think for a moment of doubting it. It is eveu 

 the peculiar characteristic property of those instruments, known in our ob- 

 servatories under the name of Parallactic instruments. In these, the move- 

 ment of the telescope is effected around a cylinder parallel to the axis up- 

 on which the starry sphere appears to execute its revolution. Well, then, 

 turn the telescope, thus mounted^ towards any constellation at the moment of 

 its rising. Let us suppose that, at this moment, one of the threads which it 

 incloses is parallel to the line which would join two of the stars contained in 

 the field of vision. This parallelism will exist at any hour whatever that 

 the observation may be repeated. It will be so at its rising, — it will be so at 

 its setting ;— at the moment of its passing the meridian, and at all interme- 

 diate epochs. Without doubt, betwixt the time of rising and that of setting, 

 the line connecting the two stars will take, in relation to the horizon, posi- 

 tions having very different inclinations ; but it will be also identically so with 

 the thread with which we compare it, since the starry sphere and the teles- 

 cope of the parallactic instrument move round one and the same axis. 



We trust that these details will suffice to exhibit the possibility of the 

 species of observations which are alluded to in the text. 



