A number of people have read all or part of the manuscript at various stages of its development. They are too numerous to be listed, but I want to thank them all for their help and criticism. Byron Wien, in particular, has gone beyond the call of duty in reading and commenting on, the manuscript at three different stages of development. Special thanks are due to Antonio Foglia, who generated the graphics that illustrate the real-time experiment. Larry Chiarello supplied the figures.
