Michael Terry, Once a Week, Series 1, Volume VII… (1862)
“ It is doubtful whether anyone besides the Emperor himself can have known whether he took its upper part to represent the Latin letter P, or the Greek one for R. The great comparative prominence of the said upper part on early monuments, joined to Constantine’s ignorance of Greek, inclines us to the former opinion, and perhaps Eusebius as an enthusiastic Oriental gave rise to the latter. There is some evidence that the Roman Emperor Probus brought the monogram, or something like it, from Egypt in the third century. ”
