'I want a coin of that emperor [Caligula] with his head on it in large brass in my collection; and if you see our friend, and he has not disposed of it, should be, with my service to him, much obliged to him for it towards compleating my series. I have too much other business to hunt after coins for that purpose; but when a student, having several good parcels from relations and friends, have an ample collection, and applied them to the use of exhibiting chronologically at our Society’s meetings to the company, with some little discourse on them from Cassivelaun and his contemporary Julius Caesar, in the way of British history, bringing in the Romans only as they fill up space of time; and more fully when, like Julius, Claudius, Nero, Vespasian, Hadrian, Antoninus, Severus, Caracalla, Geta, &c. they had personally or by great prefects very considerable dealings here. These have well helped on a pinch to support and enliven our chat; and last month I got to about anno Domini 253, where the Upper Empire ends, and which is good work; and shall next on like occasion, when the company of correspondents at any time fails to furnish, begin with those of the Lower Empire, scil. Valerian and his son Gallienus, in whose unhappy reign the empire was distracted, and XXX tyrants usurped in one or other of its provinces; from some of which there is now and then something to be learned. Indeed, there is a middle state, both as to government and workmanship, reckoned from the end of the Antonines to Valerian. There was no triumphal appellation of the Roman emperors were more fond (and some vainly proud) of then BRITANNICUS. But I think none of them but Claudius, Hadrian, Antoninus Pius, and Severus, could be justly said to assume it; though perhaps Albinus and Geta, with Carausius, and some few of the Constantine family after him might merit it. On coins of Geta, neither Spanheim, Paterol, Occo, nor any other medalist, rightly accounts for both L SPET and P SEPT being prefixed to Geta, which they make the same man, son of Severus. On coins with the former inscription he has a beard; these with the other represent him as a youth.'
(Nichols 1781-1790, pp. 96-9; Burnett 2020b, p. 1662)