Dipleidoscope

A dipleidoscope is an instrument used to determine true noon; its name comes from the Greek for double image viewer.

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).

Bloxam (a barrister), he found he had also been working on his own design using reflections, which Dent felt was superior.

[1] 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.

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.