Portable transit instrument, with case and components

Portable transit instrument of 2½-inch aperture and 24-inch focal length, with striding level and lamp, by Troughton and Simms, with wooden box and cast iron stand.

Dated to the mid nineteenth century, this portable transit instrument was made by the London instrument makers Troughton & Simms. In conjunction with a very accurate clock, it was used to find the celestial longitude of stars known as 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 across the east--west cross-wires in the telescope eyepiece. To make the observations easier, the small lamp on the right was used to illuminate the eyepiece. A small mirror in the middle of the tube directed the light downwards towards the eyepiece cross-wires.