This novel was originally serialized in Argosy All-Story Weekly in 1921, and it was reprinted frequently after that, both in other pulp magazines and in books. I read the Ace paperback from the mid-'60s, with a cover sporting the claim, "The most famous fantastic novel of all time." I'm not sure that it's been reprinted since then. It seems largely forgotten except as the target of one of Damon Knight's critical demolition jobs in the '50s.
It's not hard to see why. It is a ramshackle book, with the first eighteen chapters written by Hall, then a few chapters by Flint, then another long section by Hall to conclude. A large cast of characters is introduced sporadically over the course of the long story, and many of them seem irrelevant or are forgotten. As Knight gleefully points out, Hall writes as though English were a foreign language to him, and his ideas are often sophomoric. It's also old-fashioned in its concerns, but I think it's worth asking what the original readership found so wonderful about the book, which is something Knight doesn't bother to do.
Two things struck me about what
The Blind Spot was up to. The first is that, like A. Merritt in
The Moon Pool, the authors seem to be trying to rationalize the occult. As one character puts it, "In other words, Dr. Holcomb has certainly proved the occult by material means." Unfortunately, this rationalization is largely done by assertion, and the assertions frequently seem idiotic. Here's more of the statement I quoted from above:
'In other words, Dr. Holcomb has certainly proved the occult by material means. He has done it with a vengeance. In so doing he has left us in doubt as to ourselves; and unless he discovers the missing factor within the next few hours we are going to be in the anomalous position of knowing plenty about the next world, but nothing about ourselves.'
Since these characters largely disappear from the story at this point, it's hard to see if they ever regain certitude about themselves. Perhaps their loss of self-knowledge is what causes them to disappear! As to whether Dr. Holcomb discovers the missing factor, that's also hard to say. He gives an exposition on his discoveries at the end, explaining something like a parallel world system, but undermines his own theories with comments such as, "I throw out the idea mainly as a suggestion. It is not necessarily the true explanation."
Perhaps this is admirable agnosticism and a scientific call for further testing. What is being suggested, if I can follow the bafflegab, is that what we have heretofore perceived as occult or spiritual is in fact a blurry, limited perception of a parallel universe that exists inside the interstices of the very atoms that compose us. Or something like that. At least that's the explanation for all the mysterious goings-on in San Francisco in the early parts of the novel, although Hall never gets around to telling us whether it explains all gods and monsters and ghosts. Whatever the case, it's quite possible that early science fiction readers found this grounding of the spiritual in the material to be very exciting. From my perspective, the big problem is that Hall seems just as ignorant of the occult as he is of science.
The other thing that struck me is that the parallel world we are shown in the latter part of the novel is, unlike the other worlds of Merritt or Burroughs, not derived from lost world literature. Perhaps it derives more from utopian concepts, although it is not presented as a utopia. Highly advanced, and yet specifically archaic in other ways. The society is relatively complex and conflicted. There is a millennial strain in it. There is interesting technology, and unexplained phenomena. It almost feels like something out of an Arthur C. Clarke novel. From my still limited reading in the era, this may have been a fresh approach to world-building -- a breakthrough -- although Flint was doing something similar in his Dr. Kinney stories, starting in 1919.