Someone brought me a mid-Victorian rapid symmetrical doublet yesterday... evidently attempted a repair using cheap UV windshield resin instead of standard Canada balsam or optical grade Norland... Predictably, the cure shrinkage produced severe radial starburst fractures along the crown interface... It took forty-eight hours in xylene just to soften the perimeter... The discoloration is permanent, of course... People really should read the basic literature before touching 19th-century flint with hardware store adhesives...