The (usually rose-colored) glasses which one views the past. Viewing through retrospectacles causes one to overlook past problems or disasters and remember only the good aspects. Thus, retrospectacles are often the device which allows one to intentionally repeat the same ill-advised course of action.
Adam: "I know she cheated on me the last 5 times we got together, but we had some really good times too. Maybe this time will be different, I think I'll give her a call"
Bob: "Adam you idiot, put down the phone and take off your retrospectacles"