Saying the walls have no refractive power isn't too bad. The refractive index of earthly-air is very close to one.
That is, it has almost no refractive power. The authors probably didn't want people to just imagine glass.
I like the cover posted here more than the one on the front page; it references the story better.
Newton Applefig, I mentioned a "wheat-field" effect as another possible way a trench might hide:http://www.thewallpapers.org/photo/21922/2-Wheat-Field.jpg
Quite right, a low refractive index would be nearly invisible for a normal wall in an earth-type air with fairly low humidity that had fairly even temperature ( the gas has an even index of refraction ). Of course, Venus is foggy, loaded with cyanogens, apparently, and one would suppose has a large amount of air circulation to produce such constant rain. Also, the maze is not 1 pane of transparent aluminum... it's a giant round structure with nested walls 20' high that... one assumes... the foggy air is being blown against and bouncing off and swirling around inside etc.
But, I'll throw that bone. If it is a force field, it might even allow gases through freely while repelling anything thicker than a liquid ( leaving aside water fog is generally made of small droplets rather than gases ).
My point was none of that. It was that based on how the authors threw the terms around, I suspect they didn't understand the principles involved. I'm not definitive on it, but it's fishy. It's possible they did understand the physics, and were just a little sloppy with the phrasing. That's not a fatal for me.
But especially... the mud. The mud is a real place where I bog down. Ha ha....
Really though, an "edge on" illusion is possible. A nanometer thread cannot be seen by human eyes, it's microscopic. Microscopic walls would not be visible, and very very thin ones would be likewise harder and harder to see in evenly colored mud, even close up, the closer they got to invisibly small.
But Lovecraft never mentions it. That's a pretty big one not to mention, that they were so small. Ah, the irony.
Such a person-deflecting wall made so thin would also be hard and "sharp" on it's edges, unless they were rounded in some way. I don't think that's fatal flaw. I joked "molding" but actually I can "see" a solution to that. If their "doors" were actually formed by slow curving turns with no real "edge" it could work. After all, the maze appears to be based on a sea-shell design, a giant circle with "spiraling" corridors. I suppose it's not impossible.
But Lovecraft never mentions that either. He has an almost painfully long narrative describing turn after turn, maybe to up the word count for sale to publisher, but he leaves out the most critical bits for verisimilitude?
It's a lot to ask of the reader.
so I have to conclude either:
A) it's an oversight of logic
B) it's an oversight of storytelling.
Either way, iI have to conclude "what a lot of gaps we have to fill in for Lovecraft". We are working pretty hard here on his behalf.