The scientifi c study of the eye is believed to have originated with the Greek physician Herophilus, who lived from about 335 to 280 BCE. Indeed, from his work came the words that we use today to describe the various parts of the eye, including the words retina and cornea. In Herophilus’ day, scientists believed that we could see because beams of light came out of our eyes and fi xed on objects. In the centuries since, doctors and anatomists have discovered that vision relies on just the opposite effect. Human eyes are actually light collectors. Light rays travel from objects around us and stimulate the light-sensitive cells in our eyes.

This book takes a look at these amazing organs and how they function to allow us to see the world. Anyone staring into another person’s eye would notice that its exterior is mostly white. This part of the human eye, the sclera, is made up of fi brous tissue and provides a tough protective coating around the whole eyeball. The most noticeable part of any eye is the coloured iris and the dark pupil that it surrounds. The iris, which works much like the aperture of a camera, expands in darkness to let more light into the pupil and contracts in bright light to keep the light-sensitive cells from being overwhelmed. The colour of the iris comes from melanin, a substance that protects the eye from absorbing strong light. In the centre of the iris is the pupil, which allows light and other visual information into the interior of the eye. The iris and pupil are protected by a transparent, domelike cover called the cornea.