Drawing, Aerial telescope by Robert Hooke Illustration of Christian Huygens's aerial telescope after the engraving in his 'Astroscopia compendiaria' (The Hague, 1684). The illustration accompanies the text of Robert Hooke's discussion of Huygens's telescope read to the Royal Society on 25 June 1684. The illustration follows the printed version closely and shows a 50-foot mast, an object lens with a 70-foot focal length, an eyepiece attached to the object lens with a strand of silk, a trestle for the observer to lean against and a dark-lantern. Subject: Astronomy 25 June 1684