IT was a gala night at the National Opera House, and the theatre was crammed from floor to roof, for Melba was sustaining a new part, and all London had gathered to listen. It was rarely indeed that so fashionable an audience assembled in February. The boxes were ablaze with diamonds. On the grand tier, however, there was one box which was not filled with gaily garbed women and which attracted attention by the fact that its sole occupants were a girl and two men.