We gratefully acknowledge the hospitality and the substantial allotment of observing time at the 48-in. Schmidt telescope of Hale Observatories at Mount Palomar which made this project possible. We thank the technical staff of ESO and in particular Drs K. Banse;
P. Grosbol and J. Melnick for assistance at the Optronic plate measuring machine and the related data reducu00ACtion facilities. We are also grateful to Drs D. King and K. Hartley of the Royal Greenwich Observatory for help with the image-processing facilities at the RGO STARLINK node. Finally we thank Drs Richard Ellis;
John Skakeshaft and Tom Shanks for very valuable discussions and for critical readings of the manuscript. JVW acknowledges support from the Royal Society via a Jaffe Donation Research Fellowship held at the Cavendish Laboratory;
Cambridge when the project commenced;
CRB acknowledges support from the SERC;
the Cavendish Laboratory and Trinity College;
Cambridge. This work was supported in part by NATO via Research Grant No. 1785;