A lens is a transparent refracting medium, bounded by one or two surfaces, one or both of which are usually curved. Early lenses were made of glass but, more recently, plastic materials are being used. Such a device allows a beam of light rays to converge or diverge on passing through it.

Traditionally the history of advances in lenses has focussed on 16th to 18th Century Europe, particularly in Italy and the Netherlands, but there have been some anomalous archeological discoveries that contradict this simplistic view.

From Burning Glasses to Microscopes

Lenses were no doubt being made soon after the art of glass-making was discovered, before about 2000 BC.

The earliest lenses, which were magnifying lenses of a sort, were known to the Greeks and Romans. These were glass bowls filled with water which were used as burning glasses ¹; but their effect in showing up details was well recognised, as can be seen from a reference in Seneca.(The Roman philosopher, Lucius Annaeus Senecus, 3 BC-65 AD). The great comic dramatist, Aristophanes, in 424 BC, mentions a similar burning glass in Act II of Comedy of the Clouds:

STREPSIADES:

Have you ever seen a beautiful, transparent stone at the druggists', with which you may kindle fire?

SOCRATES: You mean a crystal lens.

STREPSIADES: That's right. Well, now if I placed myself with this stone in the sun and a long way off from the clerk, while he was writing out the conviction, I could make all the wax, upon which the words were written, melt.

Furthermore, Pliny relates that such bowls were used by physicians for burning. The glass bowl was obviously used as a condensing lens, though it was no doubt a marvel to the Romans that cold water should be able to cause a burn! So, however dimly, the biconvex lens was already known in classical times.

True glass lenses were not known in classical times; they were probably first manufactured at the end of the 13th century in Europe, and Roger Bacon's name is associated with this. Bacon discussed the use of segments of spheres and showed that letters and small objects on which they are placed appear to be magnified. He commented,

"For this reason such an instrument is useful to old persons and to those with weak sight, for they can see any letter, however small, if magnified enough."

This observation of the magnifying properties of segments of spheres was not original to Bacon. His real contribution was the clear recognition of their use for old people and those with weak sight.
Although it has been claimed that Bacon actually invented spectacles this seems to be a myth. For more information on this, click on the link.

The lens surface of spectacles can be either spherical or cylindrical.

A spherical surface has a uniform curvature across its surface. The equations governing the optical properties of lenses are thought to have been devised by the French philosopher and mathematician René Descartes in the 17th Century. But he could not find anyone skillful enough to make one.

In the 17th Century, the best manufacturers of lenses in the world were in Holland. Foremost amongst these was Anton van Leeuwenhoek (1632-1723) who made his own simple microscopes using only one lens. He became expert at making tiny lenses with strong magnifying power. Objects were mounted on the point of a pin and brought into focus of the lens by adjusting screws to the pin. Leeuwenhoek later added a second lens to produce simple 'compound microscopes' so that he has become known as the 'father of microscopy'. He was the first person able to see and describe cells and micro-organisms.

Later on, in Britain, Robert Hooke (1605-1703) improved Leeuwenhoek's design by adding a third lens behind the original two. Click here for more information on the history of optical microscopes.

The most common types of glasses used in optics are crown glasses, which are composites of silicon dioxide (silica), sodium oxide (soda) and calcium oxide (lime) and flint glasses. Prior to the mid-18th Century only crown glass was available. However, by 1758, flint glass, which contains lead oxide and which is denser and disperses light into a spectrum more strongly, started to become available. This enabled the spectacle-maker, John Dolland (1706-1761) to manufacture small 'achromatic' telescopes. Although flint glass had long been in use for making bottles, it had been difficult to manufacture in a quality suitable for lenses. It was only the efforts of a Swiss glass-maker, Pierre Guinand (1748-1824) that allowed a significant improvement to occur. Guinand devised a process for stirring the glass whilst it was molten, and so removed the bubbles and unevenness in the mixture. This was so successful that he applied his method to the manufacture of all optical glass, and during the 19th Century he set up a works at Benediktbeuren, in Bavaria, together with a German manufacturer whose manager was Joseph von Fraunhofer (1787-1826). Here, excellent optical glass was made and Fraunhofer produced, not only optical lenses, but also the facility for mounting them. His telescopes were of such high quality that both Friedrich Bessell (1784-1846)and Friedrich Struve (1793-1864) used them for measuring stellar distances.

Why the name lens? Some of the earliest lens makers were Italian. It is claimed that because Italian lenses were bi-convex, they resembled the lentils they used to make soup - so 'lens' came from the Latin for lentil (Lens culinaris).

Ancient Lenses?

In 1998, Archaeologists discovered clear quartz discs at a http://news.bbc.co.uk/1/hi/sci/tech/702478.stmViking settlement in Sweden, dating from 700-1000 AD. These were at first thought to be jewellery, but optics specialists led by Olaf Schmidt of the University of Applied Science in Aalen, Germany, have discovered that they are, in fact, sophisticated lenses. Their shapes closely match an ellipse. The optical properties of some of them are comparable to modern lenses. Hence Vikings were making lenses hundreds of years earlier than first thought. Schmidt believes the lenses were made on a simple lathe and used for focusing sunlight to cauterise wounds and light fires.

It is interesting to note that the book 'Ancient Inventions' by Peter James and Nick Thorpe, published by Ballantine Books in 1994 contains the following information:

"Two lenses of optical quality are on display at the Heraklion Museum of Ancient Cretan Civilization. As many as fifty were reported as having been found [by Heinrich Schliemann] in the excavations of Troy, though only a handful have been properly published.

Some lenses from these sites have impressive magnifying powers. One lens, probably of the fifth century BC, found in Crete, can magnify with perfect clarity up to seven times. If it is held farther away from the object viewed, it will actually magnify up to twenty times, though with considerable distortion". For an intriguing viewpoint regarding the availability of ancient lenses read The Crystal Sun (paperback) Robert Temple. Arrow (London), 2001.

The processes used in lens manufacture have not changed essentially since the Middle Ages, except for the utilization of pitch as a polishing medium, introduced by Isaac Newton.

Recent Developments

The recent development of plastics and of special processes for moulding them has led to their increasing use for the manufacture of lenses. Plastic lenses are cheaper, lighter, and less fragile than glass ones. However, they suffer from the aesthetic disadvantage that they are thicker than glass lenses of equivalent power.

Bibliography