GREENWICH OBSERVATORY. 
271 
covered minor planets — he made a revolutionary change of 
double character, first, by designing an instrument which, while 
carrying a large object-glass, should in itself combine the func- 
tions of both a Transit and a Mural Circle, and second, by caus- 
ing such an instrument to be constructed of a solidity hitherto 
undreamt of. The engineer became instrument-maker in place 
of the optician, and cast and wrought iron-work of the 
former supplanted the tender and perishable brass of the latter. 
Not that the optician was ignored, but his work was confined 
to the delicate parts, the glasses, micrometers, and divided 
circles ; and he and the engineer worked upon this instrument, 
as in all later additions to Greenwich, in judicious concert 
under one head. 
The Transit Circle consists essentially of a telescope, 12 feet 
long with an object-glass of 8 inches diameter, turning between 
two massive stone piers. The telescope tube and the pivots are 
of cast iron strengthened by internal braces. In the focus is 
the series of vertical cobweb wires, the central one marking the 
meridian, for observations in Right Ascension. Upon the 
western side of the axis (right hand in the cut) is a circle 6 feet 
in diameter, divided on silver to five minutes of arc, and read by 
six long micrometers whose eye-ends are on the other side of 
the pier, which is pierced for the purpose. The micrometers 
read to *01 of a minute. The circle reads Zenith Distances, 
which are afterwards converted into North Polar Distances. 
One horizontal wire in the telescope, whose position is registered 
by a micrometer screw on the eye-piece, is used to bisect the 
star or object under observation, and the reading of its micro- 
meter is combined with the circle reading. Upon the eastern 
axis (left hand in the cut) is another circle ; this is for clamping 
only. A trough of mercury (for obtaining the horizontal point 
of the circle by observation of the same star directly and by re- 
flexion) is carried on a circular tramway on the eastern pier by 
means of parallel bars which, with the counter-weights of the 
trough, appear in the picture : the trough does not appear. 
North and south of the instrument are two fixed inverted tele- 
scopes (the end of one appears in the cut) ; these are for giving 
a line of collimation : each has a wire at its focus which can be 
viewed by the other through an aperture in the central cube of 
the great telescope, or by raising the great telescope from its 
bearings, for which there is due provision ; the wires are ad- 
justed to coincidence, and then by observing each with the 
great telescope the collimation error of the latter is obtained. 
The instrument is never reversed. The error of level of the 
axis is found by measuring the amount of non-coincidence of 
the meridional cobweb with its own image reflected from a 
trough of mercury placed perpendicularly under the telescope. 
