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 »
The spectacle lens that corrects short sight or myopia is a negative power: light rays from distant objects are bent in such a way that they become more, not less, divergent. The degree of divergence will depend on the power of the lens. When the degree of (artificial) divergence is equal to the degree of short sight, the eye will form a clear image on the retina. A usual degree of short sight is up to —8.00 dioptres. But in fact it is how healthy the back of the eye remains, and how thin the outer coats of the eye become, that determine the future of the shortsighted person. Read the rest of this entry »
When light rays from a distant object pass through the cornea only the central rays are likely to form an image on the central and most sensitive part of the retina. Only the central part of the cornea (an inner diameter of between 3 and 5 mm) is sufficiently curved to bend the light-rays regularly. The light entering the more peripheral parts of the cornea only stimulate the more peripheral parts of the retina. These are bent irregularly and do not form a clear central retinal image. This ‘peripheral vision‘ is most useful for locating objects in space, and, by a reflex nerve stimulation, regulating the size of the pupil. Read the rest of this entry »
Secondly, such rays (peripheral vision) may be scattered by such scarring and cause unusual sensitivity to bright light. The cuts may also damage the very sensitive deeper layers of the cornea creating problems in later life, although the operation is too new to know whether and to what extent this is the case. The effects of cuts cannot easily be measured, while the ability of the tissue to heal totally may prejudice even the short-term benefits. The presence of a small degree of short sight may seem a great inconvenience to a young person, and he or she is often willing to take a long-term risk for the sake of an immediate improvement. Read the rest of this entry »
There have been many research programmes involving humans and primates aimed at finding answers to the sort of question suggested in the preceding paragraph, but generally each attempt has only uncovered a further set of factors making any final solution, any final map of the ‘genes versus environment‘ problem, less and not more likely. Perhaps in some future era when education returns to a system that doesn’t require books, but where knowledge is computer-stored and audio- retrievable, then short-sight, where it occurs, will be more easily explicable. Meanwhile we should teach our children not to hold their heads too close to the printed page, not to read for too long periods, to use good light and to develop their distance vision in outdoor pursuits. Perhaps, too, we should encourage them to develop their thought-processes, rather than seek to fill their heads with useless data as though they were merely memory banks. Read the rest of this entry »
It is known that because of inheritance not everyone will have eyes of the same size and optical power, that some eyes will fail to achieve ‘normal’ functional standards. Thus at least 15 per cent of us will have developed some degree of short-sightedness by the age of twenty-five, owing to the eye not growing in balance with the rest of the optical system seen as a whole. But this statistic applies only to post-industrial populations. The incidence of short-sightedness is much less among preindustrial peoples, and so it cannot be hereditary factors alone that are at work. The correlation is between short-sightedness and socio-industrial development, not necessarily between short-sightedness and race. There are as many short-sighted Japanese as there are short-sighted Europeans. It could be argued that where pre-industrial conditions still exist the mechanics of natural selection have ‘weeded out’ congenital short sight, but it seems much more likely that a tendency-towardsshort-sight-given-certain-conditions is inherited, and inheritable, among all peoples. Read the rest of this entry »
In modern times we have learned to manufacture glass of a much higher and controllable refractive capacity. Correct refraction is as important as the shape of the lens for the efficacy of an optical aid.
The grinding and polishing of regular curvatures on to transparent media to produce lenses that minify objects has thus been going on for several hundred years but the scientific principles underlying the measurement and accurate reproduction of thin spectacle lenses belong to modern times. Read the rest of this entry »
There are further classifications according to lens design, finish, colour, etc., which make the whole subject very confusing. If you want what is best the answer must be, ‘Whatever your practitioner advises as the best optical correction according to your eye sensitivity and required duration of wear.’ The classifications given are in fact properties of the lens, and a combination of them gives a particular kind of lens. Some types are commercially available by trade names on an `off-the-shelf’ basis, others have to be manufactured by specification. They are all made of plastics, however, though some absorb water while others do not. Most absorb and transmit gas, a necessary function to maintain the chemical equilibrium of the eye-system. Read the rest of this entry »
