The Human eye

The human eye is perhaps our most important sense organ. We depend upon sight more than any other sense to supply us with important information about the world we live in.

In many ways, the eye operates like a camera. Light rays enter through a small area of transparent material (called the cornea). The amount of light allowed in, is controlled by the widening or narrowing of the pupil, which acts like a camera aperture. (The pupil is the black hole in the middle of the iris or coloured part of the eye.) The light rays are then brought into focus by the lens behind the pupil and projected onto a membrane at the back of the eyeball. This membrane acts like the film inside the camera and is called the retina. As in a camera, the image projected is inverted (upside-down).

However, the similarity between a camera and the eye ends there. When we take a photo, we are just recording images onto film. Our eyes do much more than this. They allow us to make sense of what we see. This is because the eyes are connected to the brain. The images that are formed on the darkened black wall of the retina are converted into electrical impulses. These impulses are sent to the brain along the optic nerve, and are then converted into messages that we understand.

There are two particularly important things the brain does. Firstly, it puts together the separate information it receives from each eye so that we do not see double. Secondly, it reads the upside-down images on our retina and turns them the right way up. The brain learns to do both these things automatically when we are very young, but small babies actually do see double and upside-

down for a short while.

The brain also sorts out the information it gets from the two different types of light-sensitive nerve cells on the retina-the rods and the cones. The rods are highly sensitive to light intensity but do not understand colour. The cones see colour and fine details, but only work in bright light. This is why when daylight begins to fade, our sight is less clear and colours disappear or become a green-grey or blue.