Brian, it sounds like the rear group on someone's Xenar or Tessar are starting to separate. Canadian balsam is the optically clear cement holding the two rear elements together as a group.
If it is slight, which would be only towards one edge, then the lens may be ok when stopped down.
- Andrew in Austin, Texas -
35mm Gear Bessa R, Leica II, - IIIg, - M2
Just for fun 35mm Gear Oly 35RC, 35RD and a XA
Modern Medium Format Fuji GW 690III
Vintage MF Folders a Bessa II, ZI Mess Ikonta - 524/2, an Agfa Super Isolette & Record III
Digital a D300 with some primes
"Who spilled the Dektol on the bathroom carpet?"