I'm uncertain which article to use in the following examples:

- calotype, invented by an/the Englishman William Henry Fox Talbot 
- paper, written by a/the Scottish professor John Dudgeon 
- a studio founded by an/the American George R. West 
- In 1859, a/the Swiss photographer Pierre Joseph Rossier (c.1829-1897) took the first photo....

Seeing as *Scottish* and *Swiss* work as adjectives, but *Englishman* is a noun, is there a preference for using one article over another?