Reg Barker was the leader of the Percussion Section at the orchestra of the Royal Opera House. It was part of his duties as leader to study the score before a new work was performed, working out which of his section members would play which instrument, for instance, would the side drum have enough time to get to the cymbals, etc.
On call for a performance one evening shortly before rehearsals for the new opera started, he decided to use the interval to go down to the library and start on the job. He was only half way through the task when the interval bell sounded, calling the musicians back to the pit.
Reg had a glass eye; when he heard the bell, he took it out, shuffled the papers into a pile, placed his eye on top of it and said: "here, keep an eye on it for me, and carry on, if you would. I've got to get back to work".