Meaning of “dark-glass way”

Question

Excerpted from poetryfoundation.org:

And so, in its dark-glass way, “If—” reflects modern uncertainty after all. It’s a masterpiece of timing, of structure, of rhetoric (the genre that Yeats pointedly contrasted with poetry). But the more you read it, the more you hear a countersong beneath the assurance. In that long series of perfectly balanced clauses, you hear a mounting fear that the child won’t succeed. The sentence keeps building; the number of required conditions keeps growing. Maturity starts to seem like a very big “if.”

What is dark-glass way?

Answer 1

Dark-glass way is a reference to the phrase through a glass darkly who's meaning is that we can not see things clearly or perfectly.

In your text, the meaning is explained by

...reflects modern uncertainty...

The original phrase was used in Paul's letter to the Corinthians.

The glass is a mirror, as in Alice's Through the Looking Glass. When a mirror becomes dark as with the mercury in antique mirrors, the reflected image becomes obscured.