If you haven't been watching ``Lost Empires'' on PBS' ``Masterpiece Theater,'' you've been losing out on a good thing. It's not too late to catch up. Sunday's episode, at 9 p.m. on Ch. 8, is just No. 3 in this stylish seven-parter. ``Lost Empires'' is the British adaptation of J.B. Priestley's book about that most English of all English entertainments, the music halls. Over here, the closest thing was vaudeville, and ``Lost Empires'' would have been called ``Lost Palaces.'' 

 The music hall is a world within the world, and it fairly springs to life through the eyes of an innocent lad who leaves his small-town home to go to work for his uncle. His uncle is a top-of-the-bill magician. 

Colin Firth plays young Richard with quiet effect, and John Castle is mesmerizing as his uncle, the magician. As usual, all of the acting support is top drawer. ``Lost Empires'' is a fascinating backstage foray, and a fine dramatic evocation of a long-gone entertainment era... 

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