Mural Circle by Thomas Jones. 1831. Diameter: 56"; 3-inch aperture telescope of 63" focal length. The telescope was a duplicate of that of the Jones transit instrument. This instrument was described by Robinson (see Mem. Roy. astr. Soc., 9, 1835) where he mentions the peculiar shape of the pier: "built of Armagh marble, joggled together, and cemented with mortar of marble lime and Loughneagh sand, which in a few years attains the hardness of compact limestone. The bracing circle is continuous and clasps each radius rather than being screwed to it. The axis is 36" long with steel pivots. The motion-wheel, a uniform plate of copper, is 55.5" extreme diameter. The reading microscopes are of unusual size, being 24.5" from the micrometers to the object glasses which are triple achromatics by Tulley of 0.75" aperture and 7" focus".
Robinson composed a paper on the constant of refraction from observations with this instrument (see Trans. Roy. Irish Acad., 19, 177, 1843).
In 1848, Robinson formulated the plan of converting the mural circle into a transit circle, by adding to it a second axis supported on a pier, and substituting a telescope of larger aperture (7") and about the same focal length as the original telescope. The new telescope was made by Thomas Grubb of Dublin and was attached to the circle, but without a second pier. Two small collimators were mounted in the same room on iron pillars, north and south of the circle. These improvements were completed in 1862 and a new series of observations of stars selected from those observed by Lalande at the close of the 18th century was then commenced.
Last Revised: 2009 November 5th