These depictions of the first fifteen Roman Emperors were first published by Jaffe in 1971, loc. cit. Jaffe noted that they relate to a series painted by Rubens in the mid-1620's that he hypothesized may have been inspired by his meeting in the suite of Cardinal Francesco Barberini the antiquarians Girolamo Aleandro, Giovanni Doni and Cassiano del Pozzo. That series was, however, in his opinion anticipated by a much earlier, lost group painted by Rubens in circa 1599 and recorded only through the present group and another in the Staatsgalerie, Stuttgart (the latter depicting Julius Caesar, Augustus, Caligula, Nero, Otho, Vitellius, Nerva, Trajan, Antoninus Pius, Marcus Aurelius and Commodus).
Jaffe noted the evident artistic debt displayed in the series by Rubens to his master Otto van Veen (a debt that was even described by Rubens' nephew, Philip, in his biography of his uncle); indeed until 1600 the former was still working as the latter's assistant. He did not however, suggest that the present group - rather than being after the Rubens' own prototypes but influenced by Van Veen - were in fact directly by the latter and his workshop. This present attribution was first proposed by Professor Hans Vlieghe, who noted in the group the classicist characteristics of Van Veen's workshop and, in some of the pictures (notably those of Augustus, Nero, Galba, Vitellius, Vespasian and Hadrian) the hand of Van Veen himself.
That the group should be the work of Van Veen, and that they should subsequently have had an influence on Rubens' series of the mid-1620s, is not necessarily surprising. Van Veen, one of the most influential artists in Antwerp at the end of the 16th century, was himself a noted proponent of humanist learning who was greatly to influence his illustrious pupil in this field. Furthermore, the concept of a series of depictions of Roman Emperors was not unheard of: at least three such (two by David Teniers I and one, known from engravings, by Frans Floris) are mentioned in early records. Perhaps the most famous example was that later commissioned between 1618 and 1625 by several leading Netherlandish artists, including Rubens, Janssens, Goltzius, Honthorst and Terbrugghen for the Stadtholder Frederick Hendrick (Berlin, Museum Grünewald).
What remains uncertain is the full extent of the present series. The Stuttgart pictures suggest that the group originally began with Julius Caesar and ran at least to the Emperor Commodus. The latter would represent an obvious terminus, being the last of the Antonine dynasty and the immediate precursor to the internecine civil wars of the third century. The only other traditional stopping point is the death of Domitian and the consequent end of the Flavian dynasty: the last of Suetonius' twelve Caesars. Supposing the full group therefore to have run from Caesar to Commodus, the present set lacks three: Julius Caesar, Marcus Aurelius and Commodus (assuming in all likelihood that Lucius Verus, Marcus Aurelius' co-emperor from 161-169 was never included); it is, however, chronologically complete within itself and, given that Caesar was not himself an Emperor, represents all of the first fifteen Roman Emperors.