115 IMPROVED REIXECTING CIRCLE. 



Double ver- trouble verniers to the index, carrying the telescope and horizon 

 niers. glass; these read upon opposite sides of the circle, and if a 



difference is observed between these readings, by taking the 

 mean of them the errour arising from any eccentricity the index 

 Centringofthe may have, will be corrected. And the third consists in fixing the 

 in ex g abs. j^jg^ glass upon an axis, accurately fitted into the centre of the 

 circle. By this means it is assured,that the index glass, in turning 

 round, shall always be exactly perpendicular to the plane of 

 the circle. In the old method, when the index-bar was merely 

 fitted on a pin txed in the centre of the circle, it was impossible 

 to make the circle so perfectly flat, or keep the index so 

 Ixplanation of accurately in contact with it, as by having an axis. To explain 

 the plate. these improvements more perfectly, the reader is referred to 

 plate III, which contains a perspective view of the instrument j 

 A, is the circle with six arms j B, is the index carrying the 

 telescope C, and the horizon-glass D, with the two clusters of 

 dark glasses E and F. At the opposite ends of this index are tha 

 two verniers a and b ; the former has the clamp screw and slow 

 movement attached to it ; consisting of a screw c, which fixes 

 the index to the circle ; and d the tangent screw, which will 

 move the index a small quantity when turned, to adjust it 

 accurately. G is the index mirror screwed upon the index H, 

 which has also a vernier, and a clamp and tangent screw e e^ 

 similar to the other, I is the handle by which the instrument is 

 held when in use j it is fitted to a socket K, which is screwed to 

 the centre of the circle, and is unscrewed from the circle when 

 packed away. The handle is fitted to a springing socket, so as 

 to turn round upon the socket K, that it may be turned to any- 

 side of the circle for the convenience of holding it j it may be 

 fastened by a small milled nut, seen in the figure, which bindj 

 the ends of the spring socket together. * L is a magnifying 

 glass for the purpose of reading the divisions of the verniers j 

 it is fitted upon a pin screwed into the indexes, and may be 

 applied to either. The figure 2 in the corner of the plate is a 

 section, showing the construction of the central part of the 

 circle, where M is a section of the thickness of the circle, witii 

 a hole through the centre, and a recess turned out in the lower 

 side to receive a centre piece N, which is fixed in with three 

 small screws ; a hole is turned in the centre of this piece, and 

 an axis O is fitted into it with the utmost accuracy ; this axis 



has 



