In this chapter we examine some aspects of the sensory receptor cells in our most complex sense organ, the eye. The optical apparatus of the eye focuses an image of the visual field on the retina. The retina contains, in humans, about 100 million photoreceptor cells, which are connected in a rather complicated fashion to about a million fibres in the optic nerve. When light falls upon the photoreceptor cells they are excited, and their excitation eventually leads to the production of action potentials in the optic nerve fibres.

The light sensitivity of the eye is caused primarily by the existence in the receptor cells of a visual pigment, whose function is to absorb light and, in so doing, to change in some way so as to start the chain of events leading to excitation of the optic nerve fibres. As a reflection of this photochemical change in the pigment, we find that the pigment molecules are bleached by illumination, and have to be regenerated before they regain their photosensitivity.

The range of sensitivity of the eye is enormous: the intensity of the brightest light which we can see is about 1010 times that of the dimmest. There are a number of mechanisms which enable this wide range to be perceived, which constitute the phenomena of visual adaptation. Dark adaptation is the increase in sensitivity which occurs when we pass from brightly lit to dim surroundings, and light adaptation is the reverse of this.