296 Royal Astronomical Society. 



or 9 inches below the top of the great tube, and consequently 5 

 inches below the point of free suspension of the wire, a micrometer- 

 microscope was fixed externally to the tube, on the same side on 

 which the wire tube is placed, for reading the position of the wire. 

 Very close to the bottom of the tube, but on the side opposite to 

 that on which the wire tube is placed, another micrometer-micro- 

 scope was placed for reading the position of the lower part of the 

 wire ; the wire tube here was perforated to permit vision of the 

 wire. This microscope was almost completely buried in the great 

 tube (its micrometer and eyepiece alone projecting from it) ; it was 

 not, however, firmly screwed down ; it was thrust in by hand into 

 a hole (where it was held by friction only) in a sliding-plate within 

 the great tube, which sliding-plate was moved by external screws. 

 It is evident that, if these microscopes were firm, and if the wire 

 retained in all positions of the instrument its peculiarities of curva- 

 ture, &c. in constant relation to the instrument, while its general 

 direction of dependence had respect to nothing but the direction of 

 gravity, the readings of the micrometer-microscope would give most 

 accurate information upon the position of the great tube with 

 respect to the vertical (the observation of the sides of the plumb- 

 wire being exceedingly delicate). Below the lower plate of the 

 great tube was the eyepiece of the telescope, a 4-glass diagonal eye- 

 piece, its eye-end projecting horizontally below the eyepiece of the 

 lower micrometer- microscope, and nearly to the same distance. 

 With this was viewed the wire (carried by a micrometer, to which 

 was given for distinction the name of grand micrometer), by which 

 the bisection of the star was effected. The lower plate of the tele- 

 scope was supported by four arched pieces of brass, attached to the 

 lower plate at its circumference, and united about 6 inches below 

 its centre ; and at the place of union was the pin which turned in 

 the foot-plate. The intervals between the brass arches left room 

 for the eyepiece, the micrometer, and the plumb-line cup. The 

 foot-plates had adjusting screws, which were, however, very rarely 

 used. The uj)per guides of the great tube were about 5 feet below 

 the top ; they were carried by an enormous iron tube, which inclosed 

 the great tube to within about 4 feet of the bottom, and was there 

 supported by large iron arches rising from four large iron pillars, 

 which at some distance surrounded the pier of the foot-plates, being 

 placed at the angles of a square, and leaving a clear space of 4^ feet 

 between each pillar and that opposite to it. 



When the present Astronomer Royal took charge of the Royal 

 Observatory, he found the grand micrometer of this instrument in 

 the following state : — The wire-plate of the micrometer carried also 

 the long heavy eyepiece projecting sideways more than 6 inches ; 

 the long micrometer-screw extended nearly from side to side of the 

 lower plate of the tube, was supported at its extremities by projec- 

 tions from that lower plate, and was tapped through the case of the 

 diagonal mirror, thus acting at a distance of about 1 inch from the 

 wire-plate. The supports of the ends of the screw, upon whose 

 firmness the correctness of the action of the screw must entirely 



