telescopes led astronomers to develop alternative designs. One was the reflecting telescope. In 1721 John Hadley showed a Newtonian reflecting telescope to the British Royal Society with a 6-inch diameter mirror. The instrument was examined by Society members James Pound and James Bradley who compared its performance to the 7.5 inch (190 mm) diameter aerial telescope built by Constantijn Huygens, Jr. that the Society had in their collection. In the comparison they noted that the Hadley reflector "will bear such a charge as to make it magnify the object as many times as the latter with its due charge", and that it represented objects as distinct, though not altogether so clear and bright as the Huygens aerial telescope. The need for very long focal length refracting telescope objectives was finally eliminated with the invention of the achromatic lens in the middle of the 18th century. Replicas In May 2014 a working replica of an aerial Huygens telescope was unveiled at the Old Leiden Observatory in Leiden. It was commissioned by Hans de Rijk, a Dutch science promoter. It was unveiled during the first annual 'Kaiser Lente Lezingen' (Kaiser Spring Lectures), which is a local astronomy lecture event. Unlike the original telescopes this one only has a 4-meter focal length, making it much easier to operate as compared to the original. The telescope is so far the only known fully working replica in the world. It can be seen on open days at the Old Observatory and on special request during tours. See also History of the telescope Infinite-axis telescope List of telescope types List of largest optical telescopes of the 18th century Notes References External links Huygens and His Great Refractor Institute and Museum of the History of Science - Galileo's telescope - Chromatic aberration Animation explaining the need for very long focal length lenses due to Chromatic aberration. ESA - Proceedings of the International Conference Titan, from discovery to Encounter. Extensive detail on Cassini, Huygens and Optics, and Campani's lenses. Telescope types