When we say someone has black eyes, blue, or brown eyes, we refer to the color of a part of the eye-the iris. Mythology goes that Iris was the ancient Greek goddess who personified the rainbow. The word iris means "eye of heaven". Iris, it was believed, would carry messages from "the eye of Heaven" to earth traveling on the arc of the rainbow.
The iris is actually a small circular muscle, which controls the amount of light that enters the eyes. In the center of the iris is a small opening called the pupil. The iris controls the size of the pupil so that more or less light enters the eye. When you are in a bright environment, the iris contracts making the pupil smaller, to allow less light through. When it is dark, the, the iris relaxes, and the pupil expands to allow more light to reach the back of the eye. The aperture of a camera is designed, basically copying the pupil and the iris. When a photograph is taken with bright light, the aperture is reduced so that less light from the object enters the camera and falls on the film, if the setting is darker, the aperture is opened wider to allow more light from the object to fall on the camera lens so that a clear image of the object is possible. The same works for the iris and the pupil.
The iris contains different types of the pigment melanin, which determine its color. A combination of the pigments eumelanin and pheomelanin gives the eye its black, brown or blue color. In the UK, this unique property of the iris is being tried as new identification system. This is because scientists believe that the iris is of a person is unique to him or her. Much more unique than even a fingerprint, they say. Called iris recognition technology, the image of a person's iris is stored on a computer. When a person's identity has to be checked, for example in an airport, a picture of the eye is taken and matched with that on the computer.
Another interesting information about the iris, is that some scientists believe that iris is an indicator if disease in any part of the body. This is called the science of iridology. However, many medical doctors do not believe in this science as a foolproof method of disease diagnosis.