"the feeling of the medium in which, that June evening out-of-doors, I had had my first sight of Quint"
A photograph purporting to be of a man with the spirit of his deceased first wife, partially materialized
Public DomainA photograph purporting to be of a man with the spirit of his deceased first wife, partially materialized - Credit: William Hope

This is evocative of late 19th century beliefs concerning the manifestation of spirits. 

Spiritualism — belief in a world populated by spirits of the dead, able to communicate with and enter into the world of the living — was a major phenomenon in the Victorian era. Mediums who claimed to be able to mediate between the two realms attracted great fascination and séances became hugely popular.

Both the medium and the spirit were thought to exude ectoplasm, a kind of energy field which enabled the dead to interact with the living. Descriptions vary wildly, with some reporting a vapour, others a tissue and others a force which could be felt but not seen. The term was first coined by Charles Richet in 1894 and so would have been known to Henry James, if not his governess.