I should like to recordmywarmthanks to PhillipAllcock, Russell Allcock, Steven Balbus, Roger Blandford, Stephen Blundell,Stephen Justham, Tom Lancaster, LathaMenon, John Miller, and Paul Tod for many helpful comments on drafts of this book, to Stephen Blundell for preparing the diagrams, and to Steven Lee for assistance with the optical observations.

KMB

Oxford,

April 2015