[ London ]: E. J. Dent circa 1860. We are pleased to offer a E.J. Dent dipleiodoscope. Primarily made from brass (with lead weighting in the base) and glass. 83.6 mm wide x 52 x mm deep x 59.6 mm high. It is marked "E. J. DENT PATENTEE 431" on the smaller triangular panel, and "P" on the base - but otherwise unmarked as shown in the first photograph. This example does not have the protective brass cover which noted the instrument name as the "Dent Meridian instrument". One corner dented and other wear as expected (and appreciated) on an instrument of this age. No original lacquer remaining - this instrument was used regularly not left on the shelf.
One of (at least) two forms that Dent made, the other with a fancy scrolled base and brass cover plate. Dent made these instruments starting circa 1843. After his death in 1853, his adopted son continued by some accounts into the 1870s. Clifton's Directory of British Scientific Instrument Makers 1550-1851 shows Edward John Dent operating in London from 1826 until his death in 1853. We've seen serial numbers as high as the 900 range on these but expect production was heaviest towards the end of the period, hence our 1860 estimate. A nice scrolled example sold at the spectacular Time Museum auction and sold in the $1200 range. Very Good. 
"A dipleidoscope is an instrument used to determine true noon, its name comes from the Greek for double image viewer. It consists of a small telescope and a prism that creates a double image of the sun. When the two images overlap, it is local true noon. The instrument is capable of determining true noon to within ten seconds. The dipleidoscope was invented by Giovanni Battista Amici in the first half of the 19th century.
Edward John Dent, a chronometer and clockmaker in London, was working in the 1830s on a simple contrivance that would allow the public to set clocks correctly based on the transit of the sun (more complex and expensive transit telescopes had been developed by Ole Rømer in 1690). By 1840 he felt he had come to a suitable design using shadows, however when he communicated his ideas to Mr J.M. Bloxam (a barrister), he found he had also been working on his own design using reflections, which Dent felt was superior. The two formed a partnership and worked together on the device, and after a further 2 years work they finalised the design and patented it (GB Patent 9793 of 1843), with Mr Dent manufacturing and selling it as Dent's Dipleidoscope. The instrument could use the moon as well as the sun and when correctly calibrated and aligned the accuracy was said to be less than a second. Dent exhibited the device at the Great Exhibition of 1851. After Edward Dent died in 1853, his son Frederick William Dent took over manufacture.
The significance of this device relates in part to the development of the railways, when an absolute knowledge of the time became more important, whereas previously it was often sufficient that an entire rural community would use the parish clock, and this would periodically be set by 'the announcement of the guard of the mail coach' or similar. The instrument came with a detailed instruction booklet, which had a substantial section on correcting local time to Greenwich Mean time (as used by the railways). " (wikipedia)