The duplicity theory of vision concerns the comparisons (both differences and similarities) and interaction between the cone and rod systems in the visual pathways, with the assumption that the cone system is active during daylight vision and the rod system functions in low light (night time). Research on this aspect of vision dates back to the 17th century and the work of Newton, and is still ongoing today. This book describes the origin and development of this fundamental theory within vision research whilst also examining the Young-Helmholtz trichromatic colour theory, and the opponent...