Plate holder at the Lick Observatory, Mount Hamilton in California, USA. This is a compound slate rest, used to carry the negative plate of the 36 inch refractor telescope (also known as the Great Lick Refractor). The telescope was built at the same time as the Lick Observatory, between 1880 and 1888. 36 inches refers to the diameter of the refracting lens, which was, at that time, the largest in the world. The observatory was built at an altitude of 1280 metres and was the world's first permanent mountaintop observatory. Its location provided excellent viewing conditions for years until light pollution from the nearby city of San Jose began to interfere with results.