The human eye
Simple design, extraordinary abilities
Our eyes are our most important sensory organs, capable of performing extraordinary feats. Just a few highly sensitive "components" complement each other to create a remarkable visual instrument.
The optical part of the eye can be compared to a camera. The functions of the camera lens are performed in the eye by the cornea, the lens and the intervening aqueous humour. The focal length adjustment needed to bring objects at different distances into sharp focus is possible thanks to the eye lens's capacity to vary the curvature of its surfaces. This accommodative capacity decreases with age; the lens tissue loses its elasticity and hardens.
Iris and retina
In front of the lens are the iris and pupil. Much like a camera aperture, the iris alters its diameter according to the illuminance on the retina, which, in turn, can be compared to a light-sensitive film. At the same time, the iris regulates the depth of field.
On the inner wall of the eye is the retina, the "projection screen", with around 130 million visual cells. Close to the optical axis of the eye, is a small depression, the fovea, where the visual cells for day and colour vision are concentrated. This is where the maximum visual acuity is achieved.
Legend: a = cornea, b = lens, c = pupil, d = iris, e = suspensory ligaments/ciliary muscles, f = vitreous humour, g = sclera, h = retina, i = blind spot, j = fovea, k = optic nerve
Visual cells: cones and rods
Visual cells come in two types: cones and rods. Each plays a different role in the visual process, depending on the level of brightness (luminance). The 120 million rods are highly sensitive to brightness but relatively insensitive to colour. So the darker it gets, the more active they are.
The seven million or so cones are responsible for vision at higher levels of luminance – e.g. in daylight or in bright artificial lighting – and for colour vision. There are three types of cone, each with a different spectral sensitivity (red, green and blue). Together, they create an appearance of colour.
Unlike a camera, which generates a static image on light-sensitive film, our eyes supply new data to the brain several times a second – even if we stare at the same object for a long time. The brain processes this stream of data to create an image.
