Of mansions never quite disclosed And never quite concealed (5-6)
Our human eyes can't quite make out whatever's out there in the cosmos, but we can sometimes see hints of it. This idea of partial or incomplete knowledge is essential to the poem.
The Apparatus of the Dark To ignorance revealed. (7-8)
It seems we will always remain at least partially "ignorant," since all that's revealed to us is the "Apparatus," or the framework, of whatever higher power is out there. We can see it reflected in things like natural phenomena, but when it comes to actually seeing or knowing what's going on beyond our world, our understanding is strictly limited.