Information most probably from the circle around the clairvoyant Rudolf Steiner: 

 "It was Christian Rosenkreuz who, appearing in a particular human form for Rembrandt, taught Rembrandt the secrets about light and darkness. Rembrandt twice gave a portrait of the man who brought him the knowledge of light and darkness. One is the warrior, which is hanging in Glasgow, and the other is the so-called Polish Rider, which is New York." 

 http://wn.rsarchive.org/RelAuthors/LievegoedBernhard/To21st_c02.html