Photograph of the observatory building for the reversible transit circle at the Cape Observatory, South Africa. Made by T Cooke & Sons Ltd, the instrument was used in conjunction with an accurate timepiece called a regulator clock. The transit circle was used to measure the position of stars as they cros the north-south meridian and to check the running and accuracy of clocks. In this view the sighting telescope is being turned around so that the instrument's axes are reversed with respect to their pivots. This procedure improves the accuracy of this type of meridian measuring instrument.
© Science Museum / Science & Society Picture Library