Introduction

The history of spectacles is steeped in antiquity. The earliest evidence dates from 500 B.C. in Egypt, where artifacts have been found in tombs. In China, the writing of Confucious refers to the use of crystals to restore vision to a poor cobbler.

The word "spectacle" has been attributed, circa 1317 A.D., to Salcino de Armato, a Dominican friar and a friend of Roger Bacon, the famous father of science. Subsequent references to glasses and spectacles appeared in Chaucer's "The Wifes Tales" in 1386. The only significant advance afterwards in the field of spectacles occurred when Benjamin Franklin in 1784 designed the first bifocal lens.

Countless names may be mentioned but through the ages from the day the original glasses were worn, thousands of years ago to the present, the concept has remained the same—a concave lens for short sight, a convex lens for long sight and a cylindrical lens for astigmatism, all in the form of two pieces of polished glass supported by a frame resting on the nose.

What are spectacles?

Spectacles are optical appliances consisting of lenses fitted in a frame or a mount with sides extending outwards to the ear.

What is the difference between a frame and a mount?

A mount is essentially a frame with no rim at the bottom so that the lenses are supported by holes or slots.

What are the parts of spectacles?

A central "bridge" with "pads" which rest on the nose; the lenses which are supported by the "rims" which extend to form the "lug", an extension on each side fitted with a "joint" to which is connected a "side" or "temple" which passes over the ears.

What is a lens?

Basically a solid material, glass or plastic or even synthetic resin combinations. Its only requirement is that it be transparent and be capable of bending or refracting the light.

How does one know one has a good "lens" fitted in one's spectacles?

(a) Have them made by a "reputed" dealer. He has his reputation to maintain.

(b) Have them checked by your doctor.

(c) Look for lens flaws, striations, pits in the glass, the edge finish and the way it is mounted in the spectacles.

Unfortunately there are many reject lenses that find their way through unscrupulous operators to "bargain houses".

How does one know if a spectacle lens is plus, minus or cylindrical (astigmatic)?

A convex lens is thicker in the centre than at the edge. Looking through it seems to magnify the letters. It is used to correct long sight and is denoted by a plus (+) sign. Holding up the lens near the eye and moving it will show distant objects moving in a direction opposite to the movement of the lens.

A concave lens is thinner in the middle than at the edges. Therefore objects look smaller when you look through the lens. It is used for short sight or myopia. It is always denoted by a minus sign (-). Holding up the lens near the eye and moving it will show that distant objects will move in the same direction as the movement of the lens.

Astigmatism requires a tonic or cylindrical lens. To know if a lens is cylindrical look at a straight edge or line, say the edge of a door and rotate the lens. If the line tilts as you rotate the lens, it is a cylindrical lens. Cylindrical lenses may be plus or minus and have their angle denoted by an axis setting.

How do spectacles work?

Spectacles are purely a device to converge or diverge the rays of light. Much has been made of this simple contraption. It has been praised to the skies by one factions, while the other "without glasses" body of opinion has placed all the ills of the world at its door.

The simple facts are that spectacles refract the light rays to a sufficient degree to permit the eye to take over and give adequate vision.

How is the power or number of a spectacle lens expressed?

The power or number is expressed in terms of dioptres. It is the most convenient system for ophthalmic purposes. A lens of 1 dioptre focuses parallel rays of light to its focal point, 1 metre away. A lens of focal length of half a metre, will be twice as strong as that with 1 metre and its power is 2 dioptres. Thus a 4 dioptre lens has a focal length of 25 cms and so forth.

What is the optical or "dioptre" power of the human eye?

The human eye has a power of roughly + 60.00 dioptres. Considering that average spectacles are hardly ever more than ± 5.0, it shows what little difference they make to the actual full power of the eye.

What are the normal problems with vision due to a spectacle lens?

Since a spectacle lens is a far from perfect optical system, there are inadequacies of light fucusing, differences in clarity and object size (termed collectively as optical aberration). These aberrations have been reduced in specialised optical units like a microscope, binocular, telescope and good cameras by a combination of two types of optical lens, usually of different density material, with specialised anti-reflection coatings. This is not however possible in spectacles, since it would increase the size and the weight of the lens and give a poor cosmetic appearance.

The common optical aberrations are:

(1) Spherical aberration

Looking through the centre of a lens gives much higher clarity of vision than from the sides. This is because the light from the sides does not have an identical plane of focus.

(2) Chromatic aberration

White light passing through a prism tends to split up into various colours. The edges of lenses, especially in myopic patients, tend to act like prisms, breaking up light. The result, especially in daylight, is that objects have fine coloured rings along their edges.

(3) Multiple reflections

Reflections occur in spectacle lenses both from the back and sides. Reflections are normal to lenses but some people find them extremely bothersome. They an be however reduced by anti-reflection coatings.

(4) Image distortions

These are extremely obvious in large spectacle lenses, typical of the present-day fashion. The horizontal and vertical lines tend to be distorted, thus affecting perspective, the lines bulging outwards in a plus lens and inwards in a minus lens.

(5) Comma

(Literally like a comma used in punctuation). Instead of a fixed point of focus, the edges of a spectacle lens blur out in a comma like fashion.

Most of these aberrations are inherent to spectacle lenses. However by a proper selection of curves and coating these problems can be reduced to the level where they would be acceptable.

Dustin Flores

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