This type of reflecting circle was introduced by Edward Troughton in London
in 1796. Here the telescope, mirror, and filters are on one side of the circle,
while the silver scale is on the other. This scale is graduated to 20 minutes,
and read by three verniers (one has a tangent screw) to 20 seconds. The circle
is supported on a heavy brass stand with a counterweight. Troughton & Simms
were still offering instruments of this sort in the 1850s–at a cost of £23.
This example was probably made in the late 1830s. It belonged to Worcester
Polytechnic Institute.
Ref: Abraham Rees, The Cyclopaedia; or, Universal Dictionary of Arts,
Sciences, and Literature (London, 1819), vol. 8, art. "Circle,"
and Plate III of "Astronomical Instruments."
"Directions for observing with Troughton’s Reflecting Circle,"
quoted in F. W. Simms, A Treatise on the Principal Mathematical Instruments
(Baltimore, 1836), pp. 51-54.
"Catalogue of Instruments Made By Troughton & Simms," appended
to William Simms, The Achromatic Telescope (London, 1852).
