It is a pleasure to thank Wes Colley, Frederic Courbin, Emilio
Falco, Henk Hoekstra, Neal Jackson, Tomislav Kundic, Geraint
Lewis, and Andrzej Udalski for permission to use their figures. I
would also like to thank Matthias Bartelmann, Emilio Falco,
Jean-Paul Kneib, Bohdan Paczynski, Sjur Refsdal, Robert Schmidt,
Liliya Williams, and David Woods for their careful reading of
(parts of) the manuscript at various stages and their useful
comments. Of particular help were the comments of Jürgen Ehlers
and an unknown referee which improved the paper considerably.