Engraving
Inscribed IH.W., attributed to Johannes Wierix, 1572. 

Transcription of Inscription [Lampsonius]:

DE GUILIELMO CAIO, BREDANO, PICTORE. 
Quas hominum facies, ut eos te cernere credas
     Expressit Caii pingere docta manus,
(Si tamen excipias unum, me iudice, Morum)
     Culpari Belgae nullius arte timent.

Translation of Inscription [Lampsonius]:

About Willem Key of Breda, painter. 
What faces of people the hand of Key, learned in painting, expressed, so that you could believe you were looking at them!¹ – if however, you except one, Mor,² in my opinion the Belgians do not fear to be found wanting because of anyone’s skill. 

1. In the Latin, “eos” can only refer to “homines”, not “facies”.  English cannot do this, so one would have to translate “looking at the people themselves”, vel sim. to avoid ambiguity.
2. The renowned portraitist Anthonis Mor (c.1517/20 – c.1576), who was a friend of Lampsonius.