The Parsons Telescope is a historic astronomical instrument located in Ireland. Built in the mid-19th century by the third Earl of Rosse, William Parsons, it was the largest telescope in the world at the time. The telescope features a large reflecting mirror, measuring 1.83 meters (72 inches) in diameter, which allowed astronomers to observe celestial objects in greater detail than ever before. This telescope played a significant role in the discovery of various astronomical phenomena, including the spiral structure of galaxies. Its design and construction influenced future telescope development, making it a key milestone in the history of astronomy and a notable achievement in the field of optics.