The Lightning is a yellow Fork From Tables in the sky (1-2)
This image seems simple and even a bit on the silly side, but it actually expresses a <em>lot</em> – primarily, a kind of scope and vastness that's difficult to process.
By inadvertent fingers dropt (3)
This is a mystifying and brief moment in which we have to wonder whose enormous fingers are capable of using lightning as a fork. Also, what are the implications of their "inadvertent," seemingly clumsy nature?
The awful Cutlery Of mansions never quite disclosed And never quite concealed (4-6)
The lightning is a pretty scary thing – and if it's merely the awe-inspiring "Cutlery" of these mysterious "mansions" in the sky, then what else is up there? We'll never know ... the unknowable nature of the heavens makes them all the more fascinating – and a little scary.
The Apparatus of the Dark (7)
This evocative phrase expresses the total incomprehensibility of whatever higher power is lurking out there. The speaker's description of it as "the Dark" demonstrates its utter obscurity.