212 C. Abbe on the Repsold Portable Circle. 
constant or systematic errors. By reason of the ease with which 
it is put in position, and the brightness of the stars observed, as 
well as by the accuracy of its divided circles, level and micro- 
scopes, there is no time lost nor money expended in building 
stations, nor in waiting for nightfall, nor in tedious repetition of 
observations. Some of the principles embodied in the Repsold 
construction of this instrument may be found introduced into 
other instruments previously constructed for Struve by these 
artists, by Ertel, by Brauer, &c.; but the first of those on the 
rfected pattern now adopted was constructed in 1851, from 
designs furnished+ by His Excellency, Otto Struve, the present 
Director of the Central Observatory. This one belonged to the 
topographical staff of the Imperial Military Academy, and was 
destined to be used in the surveys of the Caucasus; it could 
therefore only be once used in the new revision of the latitudes 
of the thirteen principal points of the Russian-Scandinavian are; 
the results for that one station, Kilpi Miki, 1852, afforded,, how- 
ever, a very satisfactory proof of the quality of the instrument. 
The considerable number of these instruments already made by 
the Messrs. Repsold, (and especially the fine one ordered for the 
hydrographical staff, and into which were introduced a number of 
minor improvements suggested by Mr. Brauer), have by their 
continual use and their successive improvement, led to the belief 
that the vertical circle has as yet but begun its course of useful- 
ness, and will, with further improvements, eventually be entirely 
depended on for doing the work that it is so admirably fitted to 
accomplish,—and farther, that the principles carried out in 18° 
construction, i.e., compactness, high magnifying powers, revel 
sibility, &c., have received authoritative confirmation as to their 
correctness. 
The first of the memoirs of Colonel Smyssloff above quoted 
gives a detailed description of the instruments used and the 
work done in a chronometrie expedition carried out by himself 
1 circle used and of the plan of the expedition. 
The aperture of the telescope used was 14 inches with a focal 
length of 20 inches—these dimensions, especially the aperture, 
have in later instruments been somewhat increased. The con- 
ical tube holding the objective is 94 inches long, being screwed 
to the cube containing the prism of total reflection whose center 
is 5 inches distant from the two Ys in which the pivots of the 
axis rest, and 94 inches distant from that end of the steel axis 
that carries the two parallel wires midway, between which the 
bserved star is to be brought. The opposite end of the axis 
being perforated admits light for the illumination of the field. 
+ See Are du Meridien, Introd., p. xxxviii. 
