Portable transit instrument, 1840-1860.
© Science Museum / Science & Society Picture Library
This portable transit instrument with a cast iron stand was made by the London instrument makers Troughton & Simms. It was used, in conjunction with a very accurate clock, to find the celestial longitude of stars known as the 'Right Ascension'. In operation, the instrument was oriented so that the telescope was aligned with the north-south meridian. Star positions were then measured by noting the time when the star moved acros the east-west cros-wires in the telescope eyepiece. To make observations easier, the small lamp on the right was used to illuminate the eyepiece and a small mirror in the middle of the tube directed the light downwards towards the eyepiece cros-wires.