A Gregorian telescope is a type of reflecting telescope which uses mirrors to gather and focus light. The Gregorian system was named after James Gregory, a Scottish mathematician who described it in 1663. Gregory was not able to make a working version at this time, however, and it was not until the 18th century that they began to be made commercially. In May of 1779, Nathanael Greene, then Quartermaster General, was advised by Colonel Pettit that the reflecting telescope that Robert Erskine ordered had arrived, and that it would be sent on to him.