Sight is a double act performed by the eye and the brain, with the eye converting light into modified electrochemical impulses while the brain converts those impulses into pictures. The double act is extremely complicated and far from understood, but in this chapter I will try to deal with the main principles of the amazing mechanism of vision.

Vision at the retina

Human beings have two different seeing skills: the dominant one is centralvision, but there is another called side (or peripheral) vision. As explained in the previous chapter, we also have two different classes of photoreceptors, the rods and cones. In this section we will find out a little more about sight and the retina.

Central versus side vision

Central vision is the dominant form of vision in humans and it is concentrated around the macula, the spot in the retina where there is a high concentration of cones (see Plate 4). As the receptor involved in colour vision and seeing things sharply, the cone needs a lot of light in order to work properly. Thus central vision is daylight vision, and the light-focusing tissues of the eye (the cornea and lens) make sure the central region and the macular region in particular have more than their fair share of the light that is available.

Side vision is the province of most of the retina, in fact all the retina outside a narrow ring around the macula. Although there are some cones, this part of the retina is the domain of the rod. The rod cannot recognize colour and rod-dominated vision is associated with rather fuzzy images, but the rod has the great advantage of being able to work in low levels of light. Side vision is the vision of the night, or rather twilight for humans, because we do need some light in order to see.

To demonstrate the difference between central and side vision, take two coins and look at them on a table in front of you in reasonable lighting conditions. Shut one eye and concentrate with the other on the two coins. This is pure central vision in operation. Lower your head if you need to, to see all the fine detail on the coins.