The human eye also belongs to a relatively small group that is equipped to identify different colours. Most insects, fish, birds and many animals can only distinguish different shades and textures. The basis of colour sense is the mixture of three transparent colours — red, green and blue. When they are combined in the correct intensities they make up white. People with a well-developed colour sense can recognize many hues of the same colour, perhaps even as many as a hundred. But colour sense is highly variable, and it is estimated that one male in eight is `colour-blind‘; that is to say, Read the rest of this entry »
Typically, something moves at the periphery of our field of vision. The eyes shift to bring whatever it is into detailed vision by projecting its image on to the centres of the retinae. The retinae then provide the data that the brain uses to decide whether the moving object is threatening, edible, sexy, inconsequential, or, if you are playing cricket, catchable.
By contracting the pupil the iris assists vision in three ways. First, it stops light from the sides of the cornea forming blurred images on the retina. Secondly, it prevents too much light from entering the eye. Thirdly, it enables depth of focus. Read the rest of this entry »
The colour of your eyes is inherited. The colour and pattern of the iris are as individuated as fingerprints. It is coated on its back with brown-black pigment. This prevents light penetrating to the back of the eye except through its centre, or pupil. This hole can change its size, dilating or contracting as the level of light requires. In bright light, or when we wish to scrutinize a near object, it contracts. In dull light, or when we wish to relax our eyes and stare in the distance, it dilates. It also becomes bigger if we are frightened or excited, and it dilates in death. Read the rest of this entry »
It would seem appropriate, therefore, to regard the whole visual system at birth as a more or less ready-to-use computer that has not yet been programmed. The actual programming takes place soon after birth; but just when this happens is not the same with all living beings. Once the programme has been supplied it remains built-in for life; but to function well it requires frequent use, especially in the early years. Read the rest of this entry »
Of all our faculties, sight has consistently been considered the most miraculous, the most beneficial. In a moving passage from his correspondence, Charles Darwin refers to a time when ‘the thought of the eye made me cold all over’. And with good reason: for when, in 1859, he first published Origin of Species, by far the commonest objection to his revolutionary theory of evolution by natural selection was that a process so dependent upon chance and accident could not possibly account for such an intricate Read the rest of this entry »
