James Boswell's uncle John was a member of a sect known as the Glasites.
Prince of Hesse
Boswell met a Prince of Hesse at the Hague on December 24, 1763. The Prince was in the army at the time, and Boswell met him at a parade at which he was on duty.
The editors of Boswell in Holland 1763-64 were not certain which Prince of Hesse Boswell met, but possibly candidates seems to be the later Elector of Hesse-Kassel Wilhelm I (1743-1821) or one of the Landgrafs of Hesse-Kassel, Karl (1744-1836) or Friedrich (1747-1837). All three were sons of Landgrave von Hesse-Kassel Friedrich II (1720-1785). One would think that the most likely candidate is the eldest, Wilhelm, as it seems to have been traditional only to refer to the hereditary Prince of Hesse as Prince, whereas his brothers were Landgrafs. Wilhelm married Princess Wilhelmine Caroline of Denmark in Copenhagen on September 1, 1764. However, when Boswell visited Hessen in 1764 he mentioned the hereditary prince specifically without referring in any way to them having met before. It is possible, of course, that he is referring to an entirely different branch of the royal family of Hessen, and that the title of Prince was used somewhat more liberally at the time.