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 »
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 »
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 »
For the first two weeks or so of the immediate postoperative period vigorous activity is discouraged. Stooping, lifting heavy objects, sneezing or coughing, and any very violent physical effort should be avoided. Any activity that makes the patient feel that his collar is too tight or that his neck is swelling should be stopped because movements which cause congestion in the head also provoke an increase in pressure within the eye. Until the wound is properly healed this could lead to its leaking, to haemorrhage, to poor healing, and, in the worst event, to loss of the eye. After two weeks, however, the surgical wound should be well healed. At the routine outpatient visit which should be made at this time, the state of healing will be assessed by the surgeon, who will tell the patient what physical activities may be undertaken. Read the rest of this entry »
The lids consist of movable tissue. Inside they are coated by a smooth, delicate tissue-membrane which, if traced up and down, forms a fold coming back on itself and then enveloping the eye, but stopping at the edge of the cornea. This fine membrane is called the conjunctiva. Thus the inside of the lid and the surface of the eye are in direct contact, more so when the lids are closed and the cornea itself is covered. Sometimes this membrane becomes subject to a mild infection, of a non-specific type, and the result is conjunctivitis. This is not usually a serious condition, and is easily treated either with drops or by bathing the eye in a prepared solution. The symptoms are irritability, discharge, redness, particularly along the top edge of the lower lid, and occasionally swelling. Most people will experience conjunctivitis at least once in their lives. Some have more than one attack each year. People who wear contact lenses are more prone, since the daily routine of placing lenses on the eye offers a greater opportunity for an infective organism or irritant chemical to reach the eye. Read the rest of this entry »
These then are the common errors of vision which may cause eye-strain. They are short- and long-sightedness, near-vision deficiency and astigmatism. Most commonly astigmatism is present with one of the other three conditions. They may all be measured by both objective and subjective methods. Objective measurement means the use of instruments, while subjective measurement requires you to state an opinion. In ordinary practice the practitioner will use both methods, and thereby establish a pattern which enables him to prescribe the right optical correction. If you repeatedly give a wrong answer to a subjective inquiry the experienced occulist will proceed to further objective (optometrical) tests. Read the rest of this entry »
The symptoms of eye-strain are diverse and confusing. Inability to use the eyes for a specific task leads to frustration in most intelligent people, regardless of whether such inability is temporary or permanent. There are states where the individual finds that, by squeezing the eyes, or by holding objects at abnormal distances, better vision is possible, but this only leads to fatigue and congestion of the delicate musculature. Read the rest of this entry »
Strabismic disorders in the adult may also be associated with certain diseases and drugs affecting the nervous system. In the child, however, the chief causes are congenital. Treatment of squint can be generalized as follows. The first step must always be to establish why the eyes are not working together, and this necessitates a full ophthalmic examination. Special (orthoptic) tests to measure the degree of weakness in the affected eye and to clarify whether the defect is sensory or motor, using electronic equipment, may be required, especially if the patient is a child. 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 »
