I have a six element Beseler Color Pro 90mm f4.5 enlarging lens that was suffering from severe lens separation problems. I took it apart and put the doublet in the oven at 325 deg. and separated the rear cell elements. Using acetone, I removed all the balsam and, not having any UV cement on hand, re-assembled the doublet in its metal cell. I have to note here that the cemented junction is composed of two flat surfaces, not concave-convex, which surprised me. The two flat surfaces mate quite nicely and, playing around, put the assembled, but not cemented, lens in the enlarger and made beautiful prints. What is the downside to leaving as is and not cementing the doublet back together. I probably will order UV cement from Summers after I talk to them but I am curious about leaving the two flat surfaces alone for the time being. If the lens performs well without cement, why were the two flat surfaces cemented? The doublet has a slight concave on the inner element facing the aperture blades and the rear lens surface is convex, as are most lenses. Again, the joined surfaces are flat. Optical experts, what say ye? -F.