One of two profile gauges with concave profile which was used by the 3rd Earl of Rosse to measure the 72 inch mirrors used on the largest Rosse telescope. Profile gauge with concave profile
Photograph film positive, 16" x 20", portion of the Moon, photographed with the 100-inch Hooker Telescope of the Mount Wilson Observatory, 1919 September 15, region near Copernicus; including Western end of Apennines, with Eratosthenes, size of image 15" x 19 3/16" Photograph film positive