On the whole, A. Merritt did not consider himself to be a writer of what is now known as "fantasy fiction". Most of his novels were intentionally based on scientific principles--albeit, he stretched many of those principles to the limit. Some of his fiction, like the classic novel, The Ship of Ishtar, was and is unabashedly "fantastic". However, Merritt was an educated, worldly man who prided himself on keeping up with the various advances in the sciences. Check out this letter he sent to The Argosy in 1932:

"Now and then I read a letter in your ‘Argonotes’ expressing doubt as to the scientific accuracy of this or that in my stories. Now and then I read letters from people who quite simply and frankly say they don’t like them. With the latter, I haven’t the least quarrel. If one doesn’t like something, I can’t, for the life of me, see why they shouldn’t say so. As the old rhyme goes—’Some like their pudding hot, Some like it cold, Some like it in the pot, Nine days old.’ The Lord knows everybody is entitled to pick his own pudding—prohibitionists to the contrary. I pick mine.

But I am a bit sensitive concerning criticisms of my scientific accuracy. (...)

There was some criticism of ‘The Snake Mother’ [the second half of what would later become the novel, The Face in the Abyss]. Some even called it a ‘fairy tale.’ That was rather funny, because, for example, if all the novelists and playwrights who have rewritten Cinderella could be laid head to foot they would reach to the moon and back. And every so-called ‘realistic’ story can be paralleled in plot by Grimm and Hans Andersen. However, there is not a single scientific statement in ‘The Snake Mother’ that cannot be substantiated. If any one, even at this late date, desires to ask any question about it I will be glad to answer him. Or her."

As one can clearly see, ol' Abe didn't take kindly to accusations that he was just making things up as he went along and writing, in effect, "fairy tales". Like Edgar Rice Burroughs, Merritt wrote most of his novels within a scientifically-based framework. That, my friends, is the basic definition of "science fiction". 

Any other crap, such as requiring that each and every element of the story rigidly conform to the most-agreed-upon principles--at that particular moment--by the scientific community, that crap is the insidious game invented by literary nerds to shut down the good times being had by normal readers of SF adventure fiction. That game--it is literally called "The Game" by its geekish adherents--is also known as "Hard Science Fiction" and the influence of this fallacy has tainted SF for well over fifty years. I have a blog post in the works addressing this idiocy.

Back to Merritt and "The Snake Mother"... The blog, Skulls in the Stars, is the website of a man generally known to his readers as "Doc Skyskull". As the title of his blog would suggest, he is a fan of Robert E. Howard. He is also a fan of A. Merritt and Skyskull has done great reviews of every Merritt novel on his blog.

However, Doc Skyskull is also, as he tells you on his blog, "a professor of physics, specializing in optical science, at UNC Charlotte." That's right, he's a physicist. Who is also an A. Merritt fan. Skyskull decided to use his powers for good and wrote an article on the science of invisibility--as it stood in 2011--for Scientific American. In it, he namechecks A. Merritt and quotes from The Face in the Abyss. Read it here:

 Invisibility: After several years of research, it's just gotten weirder

The money-quote is this:

"This is such a simple and elegant idea, it is quite surprising that it wasn't tried long ago. In fact, a weird fiction novel from 1923, A. Merritt's The Face in the Abyss, uses exactly the water in the stream image to explain invisibility:

'Conceive of something that neither absorbs light nor throws it back. In such case the light rays stream over that something as water in a swift brook streams over a submerged boulder. The boulder is not visible. Nor would be the thing over which the light rays streamed.' "

Oh yeah. Doc Skyskull--whose street name is Greg Gbur--got A. Merritt into the frikkin' Scientific American in 2011. You know ol' Abe gave a thumbs-up to that from the hereafter.

What Merritt was referring to were the "Messengers"--also called "flying serpents"--who served the Snake Mother of Yu-Atlanchi. They were able to cloak themselves with a form of invisibility. That is just one of the many cool things to be found in The Face in the Abyss. There are also dino-riders, lizard-men and spider-people. Oh, and arena-fights, bloody rebellions and hot chicks.

You can read Doc Skyskull's excellent review of The Face in the Abyss here.