On the Origin of the Eye’s Sensitivity Curves

Abstract

The spectral brightness sensitivity function of the ordinary human eye may be considered in terms of the availability of natural sources of illumination. The human eye possesses both a cone system of vision and a rod system of vision. Here it is speculated that their peak brightness sensitivities arose in phylogenetic development, with the cone system being relatively recent in appearance. The older rod system became adapted to the role of low-illumination, or ‘twilight’ vision. The comparative physiology of vision offers support to these conjectures, based on differences in illumination for terrestrial animals and aquatic animals.