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 »
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 »
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 »
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 »
Symptoms
All the symptoms of cataract are visual, the usual complaint being of a general mistiness of sight, more particularly for distance vision, as reading is often unaffected in the earlier stages. One eye is frequently worse than its fellow, but sooner or later both eyes alter. At this stage dazzle may be a cause of great distress. In normal lighting conditions vision may not be seriously disturbed, but in bright sunlight it becomes obscured by dazzle in the same way that a dirty windscreen becomes almost opaque in the headlights of oncoming cars. In this situation patients may be greatly helped by wearing a brimmed hat or tennis shade. Tinted glasses are of limited assistance because it is the direction of the light as much as its brilliance that causes the trouble. Read the rest of this entry »
The first step in the operation itself is the securing of the eyelids. This is usually done with a small spring speculum, which gently holds the lids open without in any way pressing on the eye.
The removal of the cataract can be achieved in a number of ways, the aim being to clear all the opacity from the pupil of the eye. Basically, there are two methods either of which may be used: Read the rest of this entry »
At one time almost all intraocular surgery was performed under a local anaesthetic. For two reasons local anaesthesia is used much less commonly today. First, the whole science of general anaesthesia has greatly advanced and the likelihood of coughing has been considerably reduced. Second, methods of repairing the cataract wound are now better than they used to be. Coughing and sneezing after the operation consequently constitute less of a threat than formerly. Read the rest of this entry »
There is a popular misconception that cataract surgery cannot be undertaken until the cataract has reached a particular ripeness. This was the case many years ago, when the only method of removing the cataract was by the extracapsular method, which involved opening the capsular bag and washing away its contents. If the cataract was very ‘immature’, it was technically rather difficult to do this. Read the rest of this entry »
