1. Crawford, A., Bull. Br. Psychol. Soc., 17, 2A–3A (1964).
2. Allport, D. A., Br. J. Psychol., 59, 395–406 (1968).
3. Mollon, J., thesis, Univ. Oxford.
4. Crawford, B. H., Proc. R. Soc., B 128, 552 (1940).
5. Morgan, M. J., and Thompson, P., Perception (in the press).