Colour vision deficiencies IX : proceedings of the ninth symposium of the International Research Group on Colour Vision Deficiencies, held at St. John's College, Annapolis, Maryland, U.S.A., 1-3 July 1987 / ed. by B. Drum and G. VerriestColour vision deficiencies X : proceedings of the tenth Symposium of the International Research Group on Colour Vision Deficiencies, held in Cagliari, Italy, 25-28 June 1989 / ed. by B. Drum, J.D. Moreland and A. SerraColour vision deficiencies XI : proceedings of the eleventh symposium of the International Research Group on Colour Vision Deficiencies, held in Sydney, Australia, 21-23 June 1991, including a joint IRGCVD-AIC meeting on mechanisms of colour vision, 24 June 1991 / ed. by B. DrumColour vision deficiencies XII : proceedings of the twelfth symposium of the International Research Group on Colour Vision Deficiencies, held in Tübingen, Germany, July 18-22, 1993 / ed. by B. Drum ; with associate ed.: A.J. Adams ... [et al.]