Dipleidoscope

A dipleidoscope is an instrument for determining true noon, using a small telescope and a prism to create a double image of the sun; local true noon occurs when the two images overlap, with accuracy to within ten seconds. The device was invented by Giovanni Battista Amici in the first half of the 19th century. Its significance relates in part to the development of the railways, when absolute knowledge of time became more important than relying on parish clocks set by mail coach announcements.