Are you talking about the final orb located just above the (locked)entrance to the Antedeluvian chamber? If so, you need to jump on a series of platforms that de/materialize as you wear/take off your ESP cap. The first platform is located past the bridge made of planks. Have fun getting to that orb, I certainly did.
You can also assign a hotkey for the helmet to un/equip it with just one button press. Did I just save you from having an immense bout of nausea? YES I DID! Or maybe not.
If it's some where within the academy's 2nd floor, wear the ESP helmet and look for a lever in the same room. pull to materialize 2 platforms.