A major obsession for Johnson was recording the coins of Carausius and Allectus and a 100-page manuscript to be called ‘The Decennium of Carausius and Allectus, Emperors of Britain’ survives with the Spalding Gentlemen's Society. Incorporated into it is correspondence with leading antiquaries of the day such as Stukeley and Kennedy, who were also avid collectors of these coins. Johnson’s detailed preparatory notes survive, as for example one in which he highlights the virtus (courage) of Carausius: ‘A frequent compliment to this great and very brave Prince, well deserving it from the Britains whom he deliver’d and preserv’d by his courage and conduct from the insolent tyranny of Diocletian and avaritious cruelty of Maximian’ (p.96). There are sketches by Johnson of some twenty coins of Carausius and Allectus scattered throughout the Decennium manuscript, a few coins of particular interest appearing more than once. Johnson was fastidious in his record-keeping for next to many coin sketches, in minute lettering, is a reference to the collection from which the coin came. A name that appears more than once in this context is that of Robert Cotton. [Goes on to identify two coins illustrated by Johnson from the Cotton collection which are now in the British Museum]' (Barker 2013)
