i66 



NA TURE 



[Dec. 27, 1877 



an inclination that the star, carriei on by the ^ daily 

 movement, will cross the field of the instrument. For 

 this purpose the interior circles fixed on the axis of the 

 telescope carry a rough scale which may be seen by 

 means of a pointer telescope fixed on the east wall. A 

 clamp which clasps the edge of this circle serves to fix 

 the instrument. The observer then places himself on the 

 observing chair in the position indicated on Fig. 2. The 

 star soon appears, enters the field of vie*v on the west 



and procteds towards the east side. With the star 

 the observer sees in the field of view a network of 

 spider threads stretched vertically and traversed by a 

 horizontal thread. Listening to the beats of the clock, 

 he notes the second and the fraction of a second at 

 which the star passes under each of the vertical 

 threads ; the mean of these times is the precise moment 

 of the passage across the middle thiead. At this 

 same moment he slightly displaces the telescope by a 



Fig. I. — Keveisiug Apparatus. 



movement given to the clamp and brings the star under 

 the horizontal thread. The direction of a line deter- 

 mined by the crossing of this thread and the middle 

 vertical one and the optical centre of the object- 

 glass is that along which the star is seen at the 

 moment of its passage across the middle thread. 

 To fix this direction it is necessary to connect it 

 with two points of an absolute fixity. For this pur- 

 pose the telescope is provided with a circle of a metre 



in diameter, the limb of which is very finely and very 

 exactly divided ; this turns with the telescope in front of 

 six microscopes permanently fixed to the east pillar. M. 

 Eichens has adopted for these microscopes the arrange- 

 ment devised by Sir George Airy for the meridian circle 

 of Greenwich. The tube of each of these is formed by 

 the side of a hole pierced in the block of marble which 

 forms the upper part of the pillar ; the positions of these 

 microscopes is then permanently fixed to that of the wall, 



