My 3rd great grandfather Robert Sillars/Sellars/Sellers was baptised on 15 Oct 1830 at Dundonald, Ayr, Scotland and spent much of his adult life from about 1854 to about 1870 (when he died is unknown) in North and South America.
There is an intriguing record at FindMyPast for a Robert Sillers listed in the Index to Register of Passport Applications 1851-1903 that has an image showing one dated 2 Oct 1861 with number 49990.
The image has "Crown Copyright Images reproduced by courtesy of The National Archives, London, England" along the bottom so I assume the index is held there, but are the Passport Applications themselves still held anywhere so that I might be able to find some more details about this one?