From τὰ φυσικά (ta physika) to physics – XXXIV

Possibly the biggest ever upheaval in the history of optics was the emergence of the telescope at the end of the first decade of the seventeenth century, closely followed by that of the microscope. Although mirrors in various forms–flat, convex, concave–had been known and their properties intensely investigated and described by optical investigators since antiquity, the lens only emerged in the Middle Ages. 

There are claims that the roughly ground eight-century BCE rock crystal found in 1850 during excavations of the Assyrian palace in Nimrud, and now erroneously called the Nimrud Lens, was a lens used as a magnifying or burning glass or even in one very wild speculation as a telescope eyepiece.

The object has been very carefully, scientifically scrutinised and it has been shown that if it was used as a lens then it would have been a very crappy one. There are other mostly hearsay references to the possible use of lenses in antiquity:

The oldest certain reference to the use of lenses is from Aristophanes’ play The Clouds (424 BCE) mentioning a burning-glass:

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

 Pliny the Elder (1st century) noted the use of glass vases filled with water to concentrate sunlight heat intensely enough to ignite clothing, as well as convex lenses that were used to cauterize wounds Pliny also has the earliest known reference to the use of a corrective-lens when he mentions that Nero was said to watch the gladitorial games using an emerald (presumably concave to correct for near-sightedness, though the reference is vague). Both Pliny and Seneca the  (3 BC–65 AD) described the magnifying effect of a glass globe filled with water. (Wikipedia)

There is, however, no systematic description of lenses before the ninth century with the emergence of reading stones (lapides ad legendum). These are limpid rock crystal, amethyst, cairngorm or smoky quartz, and garnet plano-convex and aspherical ground stones, which when laid on a written manuscript function as a magnifier.

The lens as we know it only emerged with the invention of eye glasses, which according to two different sources took place in Venice around 1280-1295. The Swiss historian of optics, Rolf Willach^[1], has a hypothesis to explain the origin of eyeglasses that is generally accepted by other historians. He thinks that the monks, who ground and polished the precious and semi-precious stones with which reliquaries were ornamented, noticed that looking through the stones helped them to overcome their presbyopia, physiological insufficiency of optical accommodation associated with the aging of the eye, and started to produce ground quartz lenses purposefully for just this purpose. 

Going into the fourteenth century lenses for hypermetropia quickly followed and by the beginning of the fifteenth century, lenses for myopia were also freely available throughout Europe. Spectacles rapidly became big business the level of money that could be made being clearly indicated by the Republic of Venice, which made both the best glass and the best spectacle lenses, introducing the death sentence for any glassmaker or lens grinder, who revealed the secrets of their trade to outsiders. 

However, it should be noted that the invention of eyeglasses was made by artisans, who made a serendipitous discovery, which they then exploited. This took place without the involvement of scholars or optical theory in any way whatsoever. Although, the production of eyeglasses became big business in the succeeding centuries it continued to be an entirely artisanal activity, which involved no abstract optical theory. The lens grinders produced the necessary convex and concave lenses in increasing steps of focal length, although the term itself held no meaning for them offering the produced spectrum of lenses to their customers, who selected the ones that fit their visual deficiency by trial and error, in the same way was  people select the mass produced reading glasses sold today in supermarkets. Medieval spectacle hawkers graded their ware by age in ten year intervals. 

From the beginning of spectacle production at the end of the thirteenth century down to the late seventeenth century, nobody produced an optical theory of lenses and how they functioned to correct errors in visual perception until Francesco Maurolico (1494–1574) wrote his Photismi de lumine et umbra and Diaphana. Photismi de lumine et umbra, on light and shadow as the title says, was written in 1521, the first part of  Diaphana in 1523 and the second and third parts in 1552.

Unfortunately, this text was first published posthumously in 1611, so the credit for the first publication of a text on the function of lenses and their corrective use in visual perception, goes to Johannes Kepler (1571–1630) and his d Vitellionem Paralipomena, Quibus Astronomiae Pars Optica Traditur (Supplement to Witelo, in Which Is Expounded the Optical Part of Astronomy) published in 1604, which gave the first correct explanation of visual perception and having done so explained how spectacles function. 

Rolf Willach devoted many years trying to find the reason why it took three hundred years from the invention of spectacles to the invention of the simple telescope, which basically just consists of two spectacle lenses in a tube. He showed that the initial problem lay with the quality of hand ground lenses. For a particular focal length, a spherical lens requires a particular curvature. With hand ground lenses the curvature is usually fairly accurate at the middle of the lens but becomes increasingly inaccurate towards the edges. This means that rays passing through the middle of the lens focus correctly creating a relatively sharp image, whilst those passing through the periphery don’t, causing in total, an often, serious distortion of the image. With spectacles worn close to the eye, only the rays passing through the middle of the lens are perceived by the eye producing a sharp clear image. Moving the lenses away from the eye, as is the case with the telescope, allows the peripheral rays to be perceived, which causes the image to seriously degenerate.

Willach showed this to be the case by analysing many, many early spectacle lenses. He hypothesised that the problem was solved by adding an aperture stop or diaphragm to eyepiece thus cutting out the unfocused peripheral rays. We don’t know if this was the solution used by Hans Lipperhey (c. 1570–1619), the first person to present publicly a viable, simple, two lens telescope, because he didn’t say and we don’t have any of his instruments. However, we do know that all the early instruments of Galileo (1564–1642) did have an aperture diaphragm. 

Once again, as with spectacles, we have an optical instrument, the telescope, which was invented by an artisan, Lipperhey was a spectacle maker, probably by trial and error and not by the application of abstract optical theory. This situation would prove problematic when it came to the various spectacular astronomical discoveries made with this wonderful new scientific instrument. 

The evidence on which empirical science was based prior to 1600 was collected by the five senses, sight, hearing, touch, smell and taste. When one reads accounts of the things that early physicians and alchemists tasted, I surprised that not more of them poisoned themselves. Visual evidence acquired by sight was considered reliable if it was confirmed by witnesses, preferably clerical ones. If measuring devices were used, a complete description of the device and its use should be included in any report. The telescope was the very first instrument that increased the scope of one of the senses, namely sight. This being the case how could one be sure that the images gathered through the telescope were truthful representations of reality? Because there was no reliable theory explaining how a telescope functioned, one couldn’t. There were even some claims, as Galileo published his astronomical observations, the first to do so, that the things that he saw were not in the heavens but in his telescope! 

What was desperately need was a reliable theoretical explanation of just how a telescope functioned. At that time only two people appeared to have done the ground work necessary to provide that explanation, Francesco Maurolico and Johannes Kepler. Maurolico’s work on the basic function of lenses didn’t appear in print till a year after Galileo published his discoveries and by then he had been dead for more than thirty years. This just left Kepler, who in a tour de force delivered in less that a year the required detailed, scientific explanation of just how Galileo’s telescope functioned in his Dioptrice, already completed in September of 1610, just months after Galileo had published his Sidereus Nuncius, and published in 1611.

In the long preface, Kepler comments on Galileo’s recent discoveries made with the telescope and their importance in supporting the theories of Copernicus. From the starting point of the information on lenses and the eye that he had already published in his Pars Optica, Kepler now laid down all of the basic theory of lenses both individually and in combination in a telescope. He described in detail real, virtual, upright and inverted images, as well as the concept of magnification. Having laid down the basics he went on to give a detailed explanation of just how the Dutch or Galilean telescope, with its convex objective and concave eyepiece, functioned. He then goes on to describe the optics for a, at this point in time non-existent, telescope with convex lenses for both objective and eyepiece. He showed that although this telescope produced an inverted image, as opposed to the upright image of the Dutch telescope, it was capable of much higher magnification. This became known as the Keplerian or astronomical telescope. It would not be long before telescope makers added a third inverter lens to their Keplerians turning the image upright. With this publication, Kepler totally moved the discipline of optics from the theory of vision to the behaviour and properties of light. It would have a major influence on the development of the discipline in the following century.

Before he published his Sidereus Nuncius, Galileo was a virtually unknown, middle aged professor of mathematics, the lowest of the low in the academic hierarchy, at a North Italian university. Now, a year later when Kepler’s Dioptrice was published, he had become the most feted astronomer in Europe and, although others were also making telescopic astronomical discoveries, he was Signore Telescope with a capital S. Instead of greeting the publication of Dioptrice, as a confirmation that the images produced by his telescopes were reliable empirical evidence, he dismissed the book, in his usual arrogant style, as unreadable. He also dismissed the Keplerian telescope as inferior to “his” telescopes, meaning that because of his influence the acceptance of the superior instrument was delayed. 

Despite Galileo’s rejections, Dioptrice opened up a new era in geometrical optics and in time the Keplerian or astronomical telescope, later with multi-lens eyepieces would go on to dominate the discipline.

^[1]  Rolf Willach, Long Route to the Invention of the Telescope: A Life of Influence and Exile: Transactions, American Philosophical Society (Vol. 98, Part 5) (Transactions of the American Philosophical Society), 2008