Lenses found in October 1983 by Sakellarakis in the Idaean Cave of central Crete are of unusually fine optical quality. The discovery prompted an investigation of similar finds, and speculation on the likely use of lenses in antiquity and on the methods of their manufacture.
In Figure 1 is shown a plano-convex lens, 8 mm. in diameter and 4 mm. thick, which has a focal length of 12 mm., thereby giving a nominal magnification of 20X. The useful magnification is limited by distortions and is a subjective evaluation; this lens has at least a 7X useful magnification. The high quality of the polish and the perfection of the shape are evident. It is made of rock crystal (single crystal quartz), as is revealed by its birefringence when rotated between crossed polaroids. When viewed under a 7X loupe, very shallow, slight, circumferential polishing marks can be seen on the convex side.
A similar lens was found at the same time and place (fig. 2). It is 15 mm. in diameter, 6 mm. thick, with 25 mm. focal length, which gives a nominal magnification of 10X; but its useful magnification is limited to about 2.5X. The edge of this lens has light tooling marks around the circumference inclined at 30 degrees from the axis of the lens. These marks are consistent with shaping the periphery with a cutting stone harder than quartz and using a round template. The two lenses were found in a disturbed stratum in the cave, but they are presumed to be Archaic Greek in keeping with the majority of objects found with them. From the studies by Boardman of artifacts from earlier excavations of the cave, it could be inferred that the cave was not used as a shrine after the sixth century B.C.2 Recent excavations, however, show that the shrine was in use much later than the Archaic period.3
These lenses should be considered in context with the much older ones found by Sir Arthur Evans in the Palace of Knossos and in the nearby Mavro Spelio Cemetery which date from 1400 B.C. The Bronze Age lenses received some attention in 1928 soon after the time of their excavation,4 but subsequently have received little mention. There are now 23 ancient lenses on display in the Archaeological Museum at Herakleion and many more are in storage there. They are also made of rock crystal and are of optical quality, with generated plano-convex surfaces. However, many have one surface lightly etched, presumably from the action of the chemicals in the soil during their long burial. A typical example is shown in Figure 3. Its diameter is 14 mm., with a thickness of 4 mm., and the focal length is 22 mm., which gives a nominal magnification of 11X. When the concave surface is wetted to minimize the scatter of the light from the etched surface, it magnifies objects without appreciable distortion.
Two plano-convex rock crystal lenses were found in central Anatolia at Gordion by Gustav and Alfred K6rte in 1901 but cannot be dated.5 These lenses also magnify with little distortion. Recently four similar lenses were found on Cyprus but have not yet been studied in detail.6 Also a number of rock crystal objects were found at the Temple of Artemis at Ephesos by Hogarth;7 their precision led him to identify them as "lathe-turned." These objects are plano-concave and therefore could not have served as burning glasses or as magnifiers. The optical quality of one of these objects is demonstrated in a photograph by B. Freyer-Schauenberg, in which it is shown reducing print about 20% without distortion.8 There has been speculation over the possible use of these objects, but recently Brein has presented a convincing demonstration that they were used as ear ornaments by the Archaic Greeks.9 ...
There are several probable uses for lenses in antiquity. One use, as a burning glass to kindle fires, is mentioned by Aristophanes in his comedy, The Clouds.25 The lenses found by Schliemann that had a central hole were probably burning glasses. A very convenient way to carry such a useful and valuable object would be by a cord through a central hole, a less expensive and safer method than a metal frame with an attached ring. A central hole would only slightly reduce the effectiveness of a burning glass; however, it would reduce the usefulness of a magnifying glass. Manufacturing a burning glass was well worth the effort; consider the inconvenience of preserving hot coals or the effort of igniting a fire by a bow drill or by striking flint and pyrites.
The wide use in antiquity of finely engraved seal stones to seal storage jars, inventory lists, and messages in the Middle East is apparent from the multitude of seal impressions and seal stones displayed in museums, and usually only a small fraction of the collection is on display. The collection at the Archaeological Museum at Herakleion from the Minoan Palace of Knossos is an impressive testimony to the extensive use of seals. At Ur clay seal impressions were so numerous that Sir Leonard Woolley defined excavation strata in terms of their relative presence. The high cost of making seal stones is revealed in Assyrian texts of 1800 B.C. studied by Larsen; the cost of manufacture was approximately equal to that of an ox and slightly less than that of a slave girl.26 Although many seals have high ornamental value, their function of ensuring the security of stores and of authenticating information must have been very important in order to warrant such expense.
Gorelick and Gwinnett believe that the fine details of these seal stones could have been carved by young men, whose eyes could focus on objects held at close distance, or by older men with myopia, and that the use of magnifying glasses would not have been necessary.27 Of course, a magnifying glass would have been helpful. The official in the royal storeroom probably was neither young nor myopic, but he would have had to compare seal impressions with authentic exemplars to detect forgeries. As this required very close observation, a magnifying glass with a magnification between 3X and 7X would have been indispensable. The ownership of magnifying glasses may even have been restricted in order to reduce the quality of forged seals. The details of the imperfections of impressions from somewhat crudely carved seal stones might have been more revealing of authenticity than the perfection of an impression from a smoothly finished seal.