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Written by John G. Reynolds
Friday, 11 March 2011

How do bifocal contact lenses work? Specifically, how is the reading part of the lens held in the correct position?

* There are at least three kinds of bifocal contact lens: "translating", concentric and aspheric. The question relates mainly to the translating type, because these are the only ones that have to be positioned correctly all the time. A translating bifocal lens is in two halves, with the part that corrects for short-sightedness at the top and that for long-sightedness at the bottom, pretty much the same design as for bifocal glasses.

Such lenses are relatively small, covering only the centre part of the eye, and they are made of a more rigid material than the more usual soft lenses.

In addition, a small segment of the lens is cut away at the bottom leaving a flat edge. The flat edge ensures that the lens stays correctly aligned when you blink Its small size and rigidity allow the eyelid to hold the lens centrally while the eyeball translates - slithering up and down between far and near vision.

In concentric lenses, the centre is made up of concentric bands of material that alternate between correction for short sight and long sight. This design exploits the adaptability of the human brain, which can keep a mixture of both near and distant objects in focus at the same time.

The wearers "train" their brains to "see" either close-up objects or distant ones as required, because in effect the eye is registering both at the same time. Such lenses move with the eyeball and it doesn't matter if they rotate on the surface of the eye because they are symmetrical.

Finally, aspheric lenses are similar to the progressive lenses in glasses: correction for short-sight is in the centre of the lens, and it progressively changes to correction for long sight towards the outside. Once again the brain has to sort out conflicting information from the two parts of the lens.

People who wear progressive lenses will remember the first time they tried them, when head movements produced a wildly oscillating, completely disorientating visual field. Somehow the brain sorts this out and, after a few hours, everything returns to normal and perfect vision ensues.

Terence Hollingworth Blagnac, France

* I wear toric contact lenses, which correct for my astigmatism as well as my short-sightedness. Because these lenses are not symmetrical they have a heavier portion at the bottom designed to "drag" the lens into the correct position after insertion. The lens itself also has a small mark at the six o'clock position so I can insert it the correct way up when I start wearing them. This mark is invisible to the wearer once the lens is on the eye.

Rhian Chapman Luton, Bedfordshire, UK

This week's questions

Pooling resources

When I open a new jar of marmalade the contents are a nice, semi-solid, homogenous mass with a smooth surface, however old the jar is. Yet when I make a spoonful-sized hole in the flat surface to remove some marmalade, the next time I open the jar a couple of days later, the hole has started to fill with a syrupy liquid. What is it about breaking the surface of the marmalade that sets this process in motion? It continues until the jar is empty.

Kenneth Crowther Derby, UK

Malt teaser

In my kitchen cupboard I have olives in brine and pickled onions in vinegar. There is no doubt that when I go to remove an olive or onion with my fingers (I know I should use a fork) the vinegar feels warmer than the brine. I even filled a jar with water and left it overnight in the cupboard to check. It felt colder than the vinegar. And friends have verified my finding. Surely the vinegar should be at cupboard temperature? Has anybody any idea what's going on?

Bill Brinkley Exeter, Devon, UK

Ducking the issue I have never seen a duck stand as erect as the one shown in the centre of the attached photo (below) which I took at Rowsley, Derbyshire. Does anyone know if there is an explanation for this posture or is it just an unexpectedly tall duck?

Vince Sellars Sheffield, South Yorkshire, UK

Last Updated ( Friday, 11 March 2011 )