Royal Astronomical Society. 479 



cipal axis telescope, and partly by the equate-real motion, either of 

 the whole apparatus, or by the rotatory movement of the principal 

 axis, by means of the declination telescope. This motion, I may 

 mention by the way, of the principal or polar axis may be made at 

 pleasure, independently of a large frame- work attached to the in- 

 strument, which is moved by the clock-work, There is an hour- 

 circle in the plane of the meridian, fixed to this outer frame- work, 

 and another circle fixed to the lower extremity of the polar axis, 

 which may be clarfijped or freed from that which belongs to the frame- 

 work. The speculum, at the other extremity of the axis, is so con- 

 trived that it moves along with the frame-work. 



" It will therefore be understood, that if the upper speculum be 

 so directed towards a star that the rays reflected from it pass down 

 the polar axis telescope, they will be received and reflected, first, 

 from the central speculum, and secondly, from the speculum in the 

 declination telescope, to the eye, in whatever position the declina- 

 tion telescope may be. Now, if the hour-circles be clamped, so as 

 to form one, and the frame-work be put into gear with the clock- 

 work, the whole will move round at the rate observed by the heavens, 

 and, consequently, the image of the star reflected from the upper 

 speculum will continue in the centre of the field of the declination 

 telescope, for any required length of time, and in every possible po- 

 sition of that telescope. 



" Suppose, now, that the relative position of the equatoreal circle, 

 fixed to the frame, and that carried by the polar axis, be carefully 

 ascertained by reading off their graduated circumferences, by micro- 

 scopes or otherwise, and that then the circle carried by the polar 

 axis be undamped, that axis will be left free to revolve and to carry 

 with it the declination circle, and Likewise the declination telescope, 

 but without interfering with what may be called the celestial move- 

 ment of the frame, or that of the upper speculum, which, by going 

 along with, continues to reflect the rays from the star to which it 

 was originally directed ; and, consequently, to preserve the image of 

 that star constantly in the centre of the field of the declination tele- 

 scope. This declination telescope is now directed to any other given 

 star whatsoever, the image of which, viewed directly, is to be brought 

 into coincidence with that seen by reflection from the upper specu- 

 lum. If now the equatoreal circles be clamped, and a second set 

 of readings be made, it is obvious that the difference between the 

 two will be the difference, in arc, of the right ascensions of the 

 stars. 



" When the observation commences, the declination telescope is 

 directed to the standard star, as well as the upper speculum, so that 

 the images, seen direct and by reflection, are made to coincide in 

 the centre of the field of view of the declination telescope. The 

 graduations of the declination circle are then read off, to be com- 

 pared with those when the second observation is made, or that of 

 the star whose place is to be determined. The difference of these 

 readings will give the difference of the declinations of the two stars, 

 in the same manner that the difference of the readings of the two 



