Who invented bifocals and why is the fish Anableps so remarkable?

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Long time ago before the invention of bifocal glasses, Anableps – one little freshwater fish, has already used such bifocals “single-lens optics.” These fishes up to 30 inches like minnows and may be found in waters of northern South America and southern Mexico. The amazing thing is that at first glance it seems that Anableps has four eyes (one pair looks down, the other – up).

But this is only an optical illusion. In fact, they have two big round eyes and each is divided by horizontal strips of leather. These fishes swim near the surface and therefore the upper half of their eyes are like periscopes, watching the sky and the bottom half is under the water, watching what’s going on there. The fish needs such eyes in order to during the search for food in the water, she could watch the surface: whether the hungry waterfowl on the top not come close.

However, to see under the water, the fish need a thicker lens surface. How does the fish come out of the situation? Bifocals come to help her. Each fish-eye is equipped with a whole oval lens that is thicker at the bottom than at the top. Therefore, under the water this fish sees through the thickened part of the lens and the sky is observed through a thin upper half.
The image clarity dependents much on the purity of the lens. But how do the fishes clean them? When the lens dry out the fishes just dips their heads and comes up already with brilliant bifocal eyepieces. Their wonderful eyes, of course, reflect the wisdom of nature, which endowed the fish with such a complex device.

The history holds back about whether Benjamin Franklin knew about these amazing fishes and the properties of their vision, but, nevertheless, more than two hundred years ago, the U.S. president took two pairs of glasses, one for long-sightedness and the other for short-sightedness, and cut the lenses of these glasses in half. Then put them in a frame: on the top the half lenses for farsightedness and on the bottom for myopia. Thus the first bifocals appeared.
who invented bifocalsWith all the merits of such a decision, the people who use such glasses sometimes mark the increased fatigue of view, the feeling of dizziness and disorientation. Today, thanks to the achievements of modern technologies, the scientists were able to develop a unique prototype of such glasses in which for instant adjust of the focus lens is used the liquid crystals. Glasses are switched between the “long distance” and “reading” with one button, thus preventing the eye strain that occur with ordinary wearing of bifocals.

How do the bifocals work?

These glasses consist of two parts: the lower part is designed for a short distance vision and the upper part, a large area, for distance vision. The shape and the size of a segment for near vision may be different. Most often, the segments have horseshoe width of 2.5-3 cm. There are also rounded segments of the same size. One can find fully segmental bifocal lens, which segment occupies the entire lower half of the lens.
Bifocal lenses are widely distributed for the correction of presbyopia (age-related farsightedness). In most cases, the users quickly get used to these glasses, although a sharp transition between the zones for distance and near vision when looking from distant objects on nearby, and vice versa, due to the difference in optical power segments, sometimes causes some discomfort. It should be noted that the difference in strength of the lens of the upper and lower zones should not be too large, maximum 2-3 diopters. Many people do not who invented bifocals_2like the fact that bifocal shows the age, since the boundary segment is clearly visible and we all know that the need for glasses to correct presbyopia occurs most often after age 40. In addition, the segments for proximity increase the lower eyelids and the area under the eyes.
Bifocals are used not only for reading and working at a distance of the arm’s length. You can order bifocal glasses for specific purposes, such as golf. The upper segment is designed for distance vision when the lower adjusted to the growth of the player for a good sight during the hitting of the ball. Or you can place a segment for near vision on the top, for example, users who need in the force of their profession to look at monitors or devices located above the head.

Bifocals can simultaneously correct nearsightedness and farsightedness.

With the years it is sometimes difficult our eyes to focus on close objects and there is a disease called presbyopia or age farsightedness. Unfortunately, many people had already have myopia and to correct both disorders with one glasses is very problematic. Short-sighted and long-sighted people have to use a couple of different glasses or special bifocals in which the lens is composed of two parts. However, in the latter case, one has to focus on one of the images by tilting the head or moving the glasses.
Glasses allow clearly see the surrounding objects practically in all range of the distances – from 33 cm to infinity. Glasses lenses have a complex microrelief of 25 round 2-mm structures, which form two rings with the depth of one and width of a few hundred micrometers. The rings shift the phase of the oscillations of light waves passing through the lens glasses and cause simultaneously amplifying and destructive interference. Thanks to this the light is focused into a solid center channel and not a single point, as in ordinary glasses. In bifocal glasses Zalewski one has no need to “look for” point of focus, it is enough to wear them and the axial channel will cover the entire retina.
During the tests on human volunteers was demonstrated the possibility of using these glasses simultaneously adjusting both visual defects. There has been little reduction in the brightness of the image, but the human eye copies very well with low-light conditions, moreover, many people constantly wear more dark sunglasses and do not experience any problems.


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