Eye Care Blog

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, they are deficient in the red to green areas. Blue blindness is very rare, as is total colour-blindness. The loss of colour sense after birth can be a sign of abnormality in either the eye itself or the nervous system, and its detection can be of importance. But instances of such loss are rare, and most colour defects are hereditary. Interestingly, women very rarely inherit colour-blindness. (While the evolutionary advantages of better colour sense among the females of the species are by no means clear, it does perhaps help explain, along with many cultural and social factors, why the visual arts have been and are dominated by males. It may be valid to say that five women, looking at the same full-of-colour scene, will all give the same interpretation and therefore there is no discussion. Conversely, a ’striking and original sense of colour‘ is a phrase common both among artists and art critics. Great and original art may not always be as contrived as the pundits make out.)

The photo-sensitive chemicals in the rods and cones become rapidly exhausted after particles of light have entered the cell. Time is required for the chemicals to re-form. Vitamin A, carried to the retinae from the blood circulation (via the choroid), is vital to the continuing manufacture of these chemicals. There are continuous minute periods when the cells are not functional, but the process is randomized so that not all the cells are `recharging’ at the same moment. Sometimes, however, in sudden very bright light, all the cells are exhausted more or less simultaneously, and there is a noticeable period of recovery. If there is any vitamin deficiency or metabolic malfunction, this period will be yet slower. The cells also have a mechanism for blocking out impulses to avoid complete exhaustion. This time-period between impulses being fired into the brain works out to our advantage. For instance, the cinema presents to the eye different pictures with split-second intervals between them, but the brain does not register the absence of pictures, only their continuity. By this method we appreciate the smooth movement of objects.

Perhaps the most interesting aspect of the whole optical system is the brain’s ability to fuse the two images, one from each eye, into a single ‘picture’ that has the sensation of depth. This happens, or is allowed to happen, because the eyes are set apart, usually by between 50 and 70 mm, but nevertheless can focus on the same spot or object. The horizontal displacement of the eyes means that each eye will see a slightly different object. Only by making allowance for this discrepancy can the brain achieve a fusion. It seems that it does this by operating a size value which it uses as a common reference to both images. The perception of depth is thus caused by the small differentials in relation to the standard exhibited by either one or both pictures. Using one eye on its own, depth can be judged only by memory or knowledge of the shadows formed, and, to a limited extent, by intensity of colour. Foreground tends to have high colour values, and background lower intensities (haze effect). Artists use perspective shadow and changes in colour intensity to mimic depth. Perversion of these techniques, whether or not deliberate, can result in new or original art forms. ‘Realism’ in art often heightens detail and colour intensity to such a degree that every part of the picture appears in focus (which is rather unrealistic compared to natural vision). Complete absence of colour differentiation can cause the viewer confusion, especially if the artist still retains other clues to depth such as perspective. Some forms of abstract and semi-abstract art even separate or `desynthesize’ perspective and colour and use several dimensions or views of one object on a single canvas. (This is particularly the technique of the Cubist painters such as Cézanne, Braque and early Picasso.) Another example of artistic distortion is the work of the Spanish Renaissance painter El Greco, whose elongated figures suggest he may have suffered some kind of astigmatism. The perception we have of objects is stored in our brains as memory pictures, and many people find it difficult to subjugate these so as to accept modern art forms. Their rejection of what they may find in museums of modern art is as often biologically conservative as it is culturally reactionary, and the animosity caused may not be as irrational as the artists themselves would like us to believe.

Biological conservativeness, however, has its own sad limitations. In old age the first tissues to degenerate are those composed of the most highly specialized cells, which are also, generally, the cells that require most energy to function. Live long enough and you will experience a sort of evolutionary build-down. The blood vessels bringing energy-laden molecules to the surfaces of the body begin to block up and contract. In particular the nutrition of the optical nerve system begins to falter. The optic nerve itself, that pipeline of nervous fibres, becomes atrophic in old age. (There are also some diseases that mimic old age in young people, with the same result.) Vision gradually becomes dimmer. It is not uncommon to operate for cataract on very old people, only to discover in the convalescent period that the optic nerve has suffered irreparable deterioration. But even if this does not happen, as age creeps upon us the ability to focus the eyes dwindles. The crystalline lens hardens, and therefore cannot change its shape to form a lens of higher power. The muscles that might do such a job have themselves weakened.

To sum up: at an early stage in life we develop an organ of light reception able to focus at will from distance to very near and see objects in great detail with fine discrimination of colour.

The adult is aware of a field of vision nearly 180 degrees in every direction using both eyes. Only the nose and eyebrows tend to cause obstruction. But then, as middle age approaches (35-45), the ability to change the focus from distance to near begins receding. Thus a young person who could focus on something 10 cm away finds in middle age that he or she can no longer focus on anything closer than 20 cm. By the age of fifty this distance has grown to perhaps 50 cm, and at seventy it is 100 cm. There are exceptions, which will be discussed, but in general everyone must learn to make accommodations as time goes by.

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