There’s two Illuminati with the last name Jung identified in Hermann Schüttler’s Die Mitglieder des Illuminatenordens 1776-1787/93 (Munich: Ars Una 1991): Franz Wilhelm Jung (1757-1833) and Johann Sigmund Jung (1745-1824).
The latter, it turns out, was probably the uncle to the famed Swiss psychoanalyst’s grandfather, Carl Gustav Jung (1794-1864).
Relatively late in the research for the book Perfectibilists, I became aware of an short account from Henry Crabb Robinson, in his Diary, Reminiscences, and Correspondence, where he had gone to Gotha in 1804 and met with Adam Weishaupt. I hadn’t included this exchange in my own book (as it wasn’t readily accessible to me at the time). However, recently, Google books has scanned in a full-view copy of the diary. Here, then, is Henry Crabb Robinson’s encounter with Adam Weishaupt (in Diary, Reminiscences, and Correspondence, Vol. I, Boston: Fields, Osgood, & Co. 1869, pp. 124-6):
[...] my object in referring to this visit to Gotha is to say something of a man whose name belongs to the history of the last century, though it was raised to undue importance by the malignant exaggerations of party spirit.