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 »
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 »
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 »
The immediate appeal of contact lenses is social. The desire of spectacle wearers to be ‘normal’, to appear in public without a facial contraption, is very real. In the USA, Europe and Japan most young short-sighted people have, by the age of twenty-five, at least tried contact lenses if their vision requires the daily or continual use of spectacles. Some patients will even persist through considerable discomfort in order to achieve ‘normalization’. And as more and more people adopt contact lenses, it becomes less and less normal to wear glasses. It is as well therefore that, apart from its cosmetic advantages, the contact lens also has some optical advantages. Read the rest of this entry »
In order to maintain its transparency the lens, a living structure, requires nourishment and metabolic activity. Any agency which disturbs the normal metabolism of the lens will cause a greater or lesser opacification, which is by definition a cataract. Nourishment is provided by the aqueous humour in which the lens lies, the necessary substances passing through the outer capsular membrane to reach the cells within. There are no blood vessels in the lens. Most of it consists of a form of protein, rather like egg-white, which does not occur elsewhere in the human body. Lens protein in different animals is exactly the same. This curiosity, which is called organ specificity rather than species specificity, means that if someone becomes allergic to any animal’s lens, he will also become allergic to his own. Read the rest of this entry »
The eye, we have seen, is a ball with a stalk behind that conveys data to the brain. The inside of the eye has been described as consisting of the light-sensitive retinal film in the back half, and a lens called the cornea and a pupil (entry hole) in the front half. The iris or coloured part has a black pigment behind it so that the whole back part of the eye globe is in darkness. Between the pupil and the retina, suspended by fine fibres or ligaments, is the inner lens of the eye, which is made of transparent layers of cells. Read the rest of this entry »
