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You're thinking of 'slow glass', from Bob Shaw's short story 'Light of Other Days': http://www.tor.com/blogs/2010/10/slow-glass-seen-from-all-around-bob-shaws-o

• swamp40 13y ago
  Analog, 1966 - Well, I guess he didn't get the idea from me, then. Melancholy synopsis from Wiki: While on vacation, they visit a slow glass merchant and notice the man's wife and child through the window, playing inside the house. When they enter, however, the house is deserted; only then do they realize that the window is made of slow glass, giving the lonely man his final glimpses of his long-dead wife and child. It's funny how stories from your youth can permeate into your memories.