Acknowledgements

Dr John Gill is Chief Scientist at Royal National Institute of the Blind, RNIB Scientific Research Unit, 105 Judd Street, London WC1H 9NE, UK. (Tel: +44 20 7391 2244; Fax: +44 20 7391 2318; Email: john.gill@rnib.org.uk).

The author is grateful for the help he has received from Colin Baptie, Julian Harding, Vicky Morland, Richard Tischler and Leo Van Hove.

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