PHYSICA]. ASTRONOMY. 163 
in the transit instrument, as also the same errors and corrections. To the 
latter is to be added the verification by the observed altitude or polar 
distance of the star. The best meridian circles of an earlier date are those 
of Ramsden (particularly the one at Palermo) and of Troughton (at Leipzig). 
Perhaps the most perfect of more modern construction are those made by 
Reichenbach, and especially by the brothers Repsold in Hamburg. | 
In the meridian circle erected at the Hamburg observatory in 1836, its 
constructors, A. and G. Repsold, sought to solve the problem of avoiding 
every error arising from flexion, by the greatest equality and counter- 
balancing of the individual parts. For this reason the instrument is 
symmetrical in all its parts, the axis, BB (pl. 15, fig. 11), being bored 
within as well as without; two circles, F’, F, of equal weight, with the 
accompanying microscope carriers, burden equally the axis, BB, and require 
equally heavy counterpoises, L, L, on both sides. To avoid a possible 
alteration of the axis, the attachment is very near the telescope, and a cor- 
responding counterpoise on the other side restores the equilibrium of the 
whole. The circles are of cast brass, 3 feet 2 inches (French) in diameter, 
graduated on silver for every 2 minutes. These circles have four verniers, 
by which the angles can be read off to single seconds by lenses, R, R, 
fastened to the microscope carrier, FF. The massive centre, B (pl. 15, 
jig. 12), of this microscope carrier, itself composed of hollow tubes for the 
purpose of measuring absolute heights, is fitted to the axis in such a manner 
as to move freely without great friction in the boxes. The telescope 
(jig. 11), CE, with a Fraunhofer object glass, C. of 5 feet focus, consists of 
two equally heavy conical tubes, CB, BE, of hammered brass, which, 
firmly united to the axis BB, admit of no bending. The illumination of the 
cross lines is effected through the hollow axis by a mirror in the tube, a 
lamp being placed in one of the tubes running out in the prolongation of 
the axis. The obscuration, as also the regulation of the illumination, is 
quickly effected by a wedge of colored glass worked by arack. The beds 
of the axis, entirely independent of the other parts of the instrument, are 
screwed fast to blocks of brass in the pillars, A, A’, behind which are the 
brass plates which support the posts, M, K, for the counterpoises L, L. 
88. The meridian circle at the central observatory of Pulkowa (St. 
Petersburg), also erected by the brothers Repsold, is very similar to the 
preceding, though on a larger scale. Sig. 3, pl. 15, represents it in per- 
spective. The two pillars, A and A’, are of grey granite, 72 feet high, and 
18 inches broad each way at the upper end. The telescope, CB, has a 
focal length of 83} inches, 54 inch aperture of objective, and possesses mag- 
nifying powers of 170, 238, and 245. Thewire plate in the focus at C, 
consists of two horizontal and nine vertical wires. Eiach of the two circles, 
BKEK and BK’E’, has a diameter of 48 inches, and is divided on silver 
to two minutes. For the counterbalancing, the counterpoises, G and 
G’, are attached to special metal posts, 1, H and I’, MH’. The whole instru- 
ment can be raised at F and I’. The level N rests on the cross piece M. 
Fig. 4 exhibits the microscope carrier on a larger scale. This consists of 
the hoop EE; the four microscopes themselves are at K, K, K, K; LL and 
163 
