I've seen this happen to other players; In the quest "Reveal the Unseen" you're supposed to use cooling and heating spells to adjust the focusing crystal, but on the PC version neither spell as any affect on the crystal. I've seen reports of this glitch on a few gaming websites, when I searched for a solution when I couldn't get the spells to work properly.

The bug is inconsistent, as at least one other PC player(visible on Youtube clip..) was able to successfully complete the quest, without cheating and skipping that portion of the quest..