19th century neurobiology as means of "lifting the veil" of consensus reality as mediated by flawed senses, and terrible repercussions of both this hidden knowledge and what may cross back through such lifted veils. An influence on Lovecraft, apparently (obviously), and as such it could be improved somewhat by, rather than just telling us that things are cripplingly horrifying, actually giving us some more of the specifics. Of course, the merits of the unknown, suggestion, etc -- so it still rather works. Excellent, evocative descriptions of landscape, too, Machen's native Wales, fully imbued with mystery and arcane history (apparently the psychogeographers have gotten ahold of this). Incidentally, Walerian Borowczyk seems to have borrowed bits of this when adapting Stevenson, who Machen also admired and borrowed from.