Photograph of one the telescopes at the United States Naval Observatory in Washington when it occupied its original site at Foggy Bottom. This view shows an equatorially mounted refracting telescope of 26 inch aperture made by Alvan Clarke and Sons from Cambridge, Massachusetts in the United States. The telescope installed at the observatory in 1873, but was later remounted, when the observatory moved to a better site in 1893. The instrument became famous in 1877, when the astronomer Asaph Hall (1829-1907) used this telescope to discover the two moons of Mars, Phobos and Deimos.
© Science Museum / Science & Society Picture Library