Hermetic was a term much bandied about by late 19th and early 20th century occultists. They claimed that there was a magical tradition going back to Hermes Trismegistos (thrice greatest Hermes), the Greek name for the Egyptian god Thoth, which had been carefully preserved in secret to keep it from profane use and persecution by the church. The idea was that magical rites performed in the physical world symbolised the the real magic which was occurring within the magician's soul. There was a very complicated system of symbolisms involved, which are still used in things like lucky colours or stones for different astrological signs. So what your author is trying to say, I think, is that the physical descriptions in Hardy's novels are symbolic of the characters' mental and spiritual states.

Bingley

Bingley