Dressing Room 10 on the 40th Street side of the Metropolitan Opera House is the most unprepossessing chamber. All but airless, it has a decor which is garishly drab and furniture which is barely serviceable.... Yet over the years, this dismal room with its many mirrors has been the silent witness to scenes of hope, triumph, and despair. Here the great singers of the world, the soon-to-be great, and the not-quite great wait before their debuts, trembling in every nerve, straining toward the moment when they can sweep on stage to the triumph that sometimes comes, and more often does not. Through Room 10 this year have paraded that trio of great European divas: The Italian Renata Tebaldi, Victoria de los Angeles of Spain, and Birgit Nilsson, who was born and trained in Sweden. Joining them, for the first time at the Metropolitan, came two Americans of equal rank, Eileen Farrell and Leontyne Price.