Eye Care Blog

Typically, something moves at the periphery of our field of vision. The eyes shift to bring whatever it is into detailed vision by projecting its image on to the centres of the retinae. The retinae then provide the data that the brain uses to decide whether the moving object is threatening, edible, sexy, inconsequential, or, if you are playing cricket, catchable.

By contracting the pupil the iris assists vision in three ways. First, it stops light from the sides of the cornea forming blurred images on the retina. Secondly, it prevents too much light from entering the eye. Thirdly, it enables depth of focus. As the pupil becomes smaller the need for optical accuracy in the system regresses. In fact, if the pupil shrinks to less than one millimetre in diameter the individual could see clearly without any of the `lenses’. A short-sighted person who looks through a pinhole made in a black card will be able to see things markedly better, even though he or she is not wearing glasses or contact lenses.

The sensory part of the retina is thinner than tissue paper and is almost transparent, and is backed by a layer of pigment. Both are closely applied to a third layer of blood vessels, which ‘feed’ the whole retinal system. This is called the choroid. The retina coats the back half of the eye and consists of light-sensitive cells. It is a little similar to the hemisphere of a radar scanner.

The light-sensitive, or sensory, cells are of two types, called rods and cones. Those cells shaped like rods are arranged in groups, several sharing one nerve. The cone-shaped cells, on the other hand, have individual pathways to the brain: that is, they have one nerve each. In all there are hundreds of thousands of nerves which carry electrical messages from the eye to the brain. They are bunched together at the back of the eye and taken deep inside the brain in a sort of transatlantic cable, which is called the optic nerve. Mostly the flow of information is one-way, but there are signals that travel in the opposite direction, from the brain to the retinae.

The rods and cones may be described as small chemical factories. They make molecules of pigment that are sensitive to light (rods) and sensitive to colour (cones). The rods can be sensitive to very low levels of light, and they are the receiving cells for night vision. The cones, which are chiefly situated at the centre of the retina, are equally sensitive to light, but they also distinguish colour. Because of the nerve structure the cones give a ‘one-to-one’ account of themselves to the brain and therefore provide the ‘detailed’ focus. The rods, on the other hand, because there may be several sharing one nerve fibre, give much coarser information. They tell us where we are, both by day and night, and help to keep us orientated to our environment. The cones, on the other hand, enable us to do our work. Both kinds of cell, however, function on the same broad principles. Light and colour coming into the eye trigger minute electrical discharges which are conveyed along the nerves, or relays of nerve fibre, until they reach the visual cortex at the back of the head. These tiny impulses disturb the brain’s normal electrical rhythms, and the ‘picture’ is created. (It is now possible to record the eye’s electrical discharges, which is of some diagnostic value. The electrical changes in the visual cortex of the brain can also be collected from the scalp and be used to measure vision accurately, although this method is still chiefly of research value. The functions of the retinae can be interpreted crudely, and sometimes in detail, by their electrical behaviour, which can be measured from the front of the eye or adjacent tissues.)

The ability to see clearly, therefore, is determined not only by how well the image is in focus, how good are the lenses and how clear the fluids of the eye, but also by how active are the cones and how well their impulses travel through the brain. When the messages reach the back of the brain the image is reformed in a new electrical energy pattern, and other parts of the brain interpret this as a picture. For the developing baby and child, many patterns will be seen for the first time, and memory of them stored until reinforced and eventually identified when the image is next seen again. The acquisition of visual sense is, in this respect, similar to spoken language. Everything depends upon repetition. If we do not remember the original turmoil of birth then that may well be because it only happens once.

The rods, far more numerous than the cones, work at many different levels of light intensity. Thus, when the light becomes very dim, they still manage to transmit signals back to the brain. But because they are arranged in groups they are less useful in forming pictures. They are, however, vital for visual orientation. They tell us where objects are in space, what is our immediate and distant environment. Without this system, which with two eyes can roughly locate any object in space, we would not be able to move where we want with any confidence. Not only do the rods direct the eye muscles via the brain, but, through the same route, they also co-ordinate all the limbs.

The presence of both cones and rods make the primate’s eye exceptionally sophisticated. Elsewhere in the animal kingdom the power of vision is not so well developed. Many insects (the bee is the most familiar example) have a polygonal or faceted cornea and each polygon forms a separate optical system (like a tube or tunnel) to the retinal cell at the back of the eye. Such an eye is designed to work all round the creature without movements. Hence the difficulty of swatting a fly. Whichever way you come at it, it can see you. But the actual quality of vision is nowhere near as good as ours. At best a fly’s or a bee’s acuity can only be like that of a newspaper photograph, hundreds of small dots, while we can see in superchrome. Some birds have an optical system that can change the position of the crystal lens, and some birds have a supplementary lens in a third eyelid that can be flicked across the eye when necessary in order to find small insects. But these creatures do not have binocular vision: the pictures delivered by each eye are not synthesized.

It is really memorable to have Drop Ship Eyeglasses Lens, Skin Care Products, Other Vision Cares Drop Shipping, Natural Skin Care Products, Eye Care Massager Vision Care Accessories, Buy SONY Dome Surveillance Camera, Maintenance & Care Drop Shipping Wholesale, Spy Cameras, Vision Care Drop Shipping, Mini size SONY CCD Security, Drop Ship Surveillance Cameras, Eye Treatments Wholesale Price, Drop Ship Mini DV, Anti Aging Skin Care and Wholesale Nursery Night Lights drop shipping wholesale, a powerful online wholesale list.

Possibly related posts: (automatically generated)
The Optics of the Eye: from Birth to Old Age part 4

• The Optics of the Eye: from Birth to Old Age part 3
• The Optics of the Eye: from Birth to Old Age part 2
• The Optics of the Eye: from Birth to Old Age part 1
• Without Eyeglasses, Defective Vision and Mind part 2
• The Retina, the Crystal Lens of the Eye, Relax your Eyes from long time hard work
• Eye Sight, Short- and Long-Sightedness continued
• Eye Vision care, why the two Eyes do not view together, Squint (Strabismus) and Treatment continue...
• The Optics of the Eye: from Birth to Old Age part 5
• The Appearance of Contact Lenses part 1
• The Appearance of Contact Lenses part 2