These are quaternion images created by Henrik Engstrom  with a
program he wrote himself. Some of the later images, (the HOLO
series) were later rendered with the POV-RAY package, I think.
His program can be found on this site in the
[pub.fractals.programs.ibmpc] directory in the QJULIA.ZIP file.
His email address is "d0henke@dtek.chalmers.se"