tubeless "aerial telescopes" in his 1684 book Astroscopia Compendiaria, and their invention has been attributed to him and his brother Constantijn, although similar designs were also used by Adrien Auzout; the idea is even sometimes attributed to Christopher Wren. The Huygenses contrived some ingenious arrangements for aiming these "aerial telescopes" at an object visible in the night sky. The telescope could be aimed at bright objects such as planets by looking for their image cast on a white pasteboard ring or oiled translucent paper screen and then centering them in the eyepiece. Fainter objects could be found by looking for the reflection of a lamp held in the observer's hand being bounced back by the objective and then centering that reflection on the object. Other contrivances for the same purpose are described by Philippe de la Hire and by Nicolaas Hartsoeker. The objectives for aerial telescopes sometimes had very long focal lengths. Christiaan Huygens states that in 1686 he and his brother made objectives of 8 inch (200 mm) and 8.5 inch (220 mm) diameter and 170 and 210 ft (52 and 64 m) focal length, respectively. Constantijn Huygens, Jr. presented a 7.5 inch (190 mm) diameter 123 ft (37.5 m) focal length objective to the Royal Society of London in 1690. Adrien Auzout and others made telescopes of from 300 to 600 ft (90 to 180 m) focal length, and Auzout proposed a huge aerial telescope 1,000 ft in length that he would use "to observe animals on the Moon". Applications Astronomer Giovanni Domenico Cassini had the wooden Marly Tower, originally built as part of the Machine de Marly to lift water for the reservoirs and fountains at the Gardens of Versailles, moved to the grounds of the Paris Observatory. On this tower he mounted long tubed telescopes and the objectives of aerial telescopes made for him by the Italian optician Giuseppe Campani. In 1684 he used one of his aerial telescopes to find Dione and Tethys, two satellites of Saturn. James Bradley, on December 27, 1722, measured the diameter of Venus with an aerial telescope whose objective had a focal length of 212 ft (65 m). Francesco Bianchini tried to map the surface of that same planet and deduce its rotational period in Rome in 1726 using a 2.6" (66 mm) 100 foot focal length aerial telescope. Obsolescence The extreme difficulty of using these very long focal length