Transit instrument at Dunsink Observatory, near Dublin, Ireland. A transit instrument is used to measure the positions of the stars as they transit a pre-defined point. Founded in the 1780s, Dunsink Observatory is the oldest scientific organisation in Ireland, and the directors of the observatory were the Astronomers Royal of Ireland until that post ended in 1921. This photograph is from the late-nineteenth to mid-20th century. By the late-20th century, increasing light pollution had meant that the observatory was no longer used for active research, and was instead used mainly for public education programmes.