I have an Omega D-II with variable condenser. In the FIXED condenser portion of the assembly, I have two HUGE (6/12 lens) lens. On top of it, there is a piece of flat round glass with a slight tint. This is the piece that if you take the fixed assembly off, it will be sitting at the top of the top lens (that is facing concave side down) and in contact with this lens. What is this flat glass for?? This flat glass does not show up on an instruction manual I found for D-2 with VC, but I am aware, Omega has produced similar models for years so there could be some changes to its design. Thank you.