Somehow I did the Afland part of this quest that relates to the Main Quest (actually I am completely done with the Main Quest) but when I go back to finish this quest, I have the atunement sphere but my blank lexicon is gone. I don't know if I sold or trashed it. Can anyone tell me where to get another? Or was I supposed to take it with me after grabbing the ES and I just left it (aararrggh!)?

I think you were supposed to have grabbed it off of the pedestal after you unlocked the vessel holding the Elder Scroll...

You must have left it back there, as it's a quest item and cannot be dropped or sold.

The Lexicon is transcribed at the same time as the Elder Scroll is revealed, so you need to pick it back up from the stand and bring it to Septimus. This one puzzled me the first time I did it, but the stand was the only thing I could still activate so I figured I was supposed to retrieve the cube. Turned out I was right.

Aaaargh--was afraid of that, as this is the longest "dungeon" in the game. Thanks for the help, both.

Especially as you can't go back in the back door......

Don't despair, I did the same thing and had to go back for this part of the quest too.

i did the same... wasn't bad exploring blackreach though

Yep, I did just plain leave it sitting there. Sheesh!

I did the same thing. Just go in the way you came out (through the back entrance.) and it takes a couple of minutes.

This, I had to run through the Dwemer Ruins again and find the dammed Lexicon.