64 Royal Astronomical Society. 



have no tangent-screw, the bisection being in all cases effected by 

 the micrometer in the field of view of the telescope. I propose that 

 the divisions be illuminated by a single lamp in the prolongation of 

 the axis, without reflectors ; and that the microscopes be in a conical 

 surface, passing through one pier, the eye-ends being in a circle of 

 two feet diameter ; and that the divisions be cut upon a limb of metal 

 which is so bevelled on the circle that the light of the lamp will be 

 reflected up the microscopes. Several microscopes to be permanently 

 mounted, in positions proper for ascertaining with the utmost ex- 

 actness the errors of division. Microscopes to be mounted for ascer- 

 taining the laws of movement of points on the ends of the pivots. 

 The instrument never to be reversed ; but an apparatus to be pro- 

 vided for raising it so far that a collimating telescope firmly fixed on 

 a solid pier on the north side, and one on the south side, can be ad- 

 justed on each other ; then when the instrument is dropped into its 

 usual place, the error of coUimation and the flexure will be deter- 

 mined without reversion, by observation of the two collimators. No 

 spirit-level or equivalent instrument to be used, but the error of level 

 to be determined by observation of the image of the wires by reflexion 

 in a trough of mercury. A parallel- motion apparatus to be used for 

 carrying the trough, and a peculiar arrangement for facilitating the 

 process of cleaning the mercury. In regard to the material, I pro- 

 pose that the whole be made of cast-iron, the axis being in two parts 

 (which enables the founder to make the pivots of hard chilled iron, 

 while the rest is of soft iron), each end of the telescope being in one 

 part, and each of the two circles being cast in one piece. An instru- 

 ment thus constructed would probably be more accurate for right 

 ascensions than the present transit, in so far as the frequent obser- 

 vation of the well-mounted collimators would add to the knowledge 

 of its azimuthal error ; and perhaps more accurate for zenith distances 

 than Troughton's circle, in so far as the circle is in a state of less 

 strain, while its construction possesses greater firmness. But the 

 reasons for recommending it, as is known to the Visitors, are the 

 power of carrying a larger object-glass, and the enabling one observer 

 to complete the observation of the two elements." 



The observations in polar distance were made with Jones's Cape 

 circle, until Troughton's circle was erected in another apartment, 

 where it is and will be used until the circle-room is rebuilt and fur- 

 nished. The transit will not be disturbed till the new instrument 

 is at work. 



The zenith tube has been taken down. It was proposed by some 

 of the Visitors that it should be erected in another and less objection- 

 able position than that which it formerly occupied, and a new site 

 was pointed out by the Astronomer Royal. Mr. Airy, however, 

 greatly prefers a different construction, if a continuous series of ob- 

 servations of y Draconis be required. The principle of this con- 

 struction, which is singularly simple, is thus described by Mr. Airy : — 



" Let the micrometer be placed close to the object-glass, the frame 

 of the micrometer being firmly connected with the object-glass cell, 

 and a reflecting eye-piece being used with no material tube passing 



