Remove pulp from four large oranges, by cutting fruit in halves crosswise and using a spoon. Sprinkle with two tablespoons powdered sugar, and add two tablespoons finely chopped mint, and one tablespoon each lemon juice and Sherry wine. Chill thoroughly, serve in glasses, and garnish each with a sprig of mint. Should the oranges be very juicy, pour off a portion of the juice before turning the mixture into glasses.