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 »
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 »
Since the retinal nerves radiate from the cup of the optic nerve (the seat of the blind spot) the loss of retinal function tends to occur in a way that is diagnostic. For example: the group of nerve fibres that tend to be affected first are those that function in arches around the centre of vision. The next group to go are those providing vision in the nasal field (the lateral part of the retina). But it is all too possible for a patient to be unaware of these losses of vision, and because the condition is symptom-free in its early stages and only gradually progressive, diagnosis is often difficult. Read the rest of this entry »
Until the advent of contact lenses or intraocular lens implants spectacles were the only means of achieving a finely focused image on the retina after a cataract operation. Although in many cases this is most satisfactory, there are limitations to such correction. At first some patients find that wearing thick cataract glasses causes a number of problems which arise from magnification of the image, distortion in the peripheral part of vision, and some limitation of the field of clear vision.
These all result from the spectacle lens itself rather than from the operation. Read the rest of this entry »
Squint, or strabismus, is a general term used to describe any one of several conditions in which the two eyes are not properly co-ordinated. One eye focuses on an object, but the other eye fails to align itself to the same fixation. Most commonly the errant eye is aligned inwards (cross-eye or esotropia) or outwards (wall-eye or exotropia); less commonly it looks upwards (hypertropia) or downwards (hypotropia). Other words to describe these conditions are ‘cast’ and ‘turn’. The phrase ‘evil eye‘ has also doubtless been used in association with squint. Where a squint remains uncorrected the person who has it is often forced to adopt an unusual, even grotesque, head- posture, adding to a general impression of unsightliness. But because the squint sufferer has the potential for normal stereoscopic vision the treatment of squint is an important ophthalmic activity. Read the rest of this entry »
Basically, good and bad characteristics, healthy and unhealthy trends, are divided into dominant and recessive. Those that are dominant are handed down and show in the offspring; those that are recessive are handed down but do not show. Further, dominance and recession are often linked to the sex of the individual. Thus we talk about traits that are ‘dominant in females’, ‘dominant in males’, ‘recessive in females’, and ‘recessive in males’.
Suppose, for example, that both parents had a recessive trait `A’. If they had nine children the chances are that only three of them would be marked by ‘A’. But if they had only two children, the chances of either of their offspring showing trait ‘A’ are small indeed, whether the trait is for weak legs or musical genius. But if both parents had dominants of a certain trait, this would almost certainly come out in the offspring. Read the rest of this entry »
