Glass has long made it possible for people to see the unseeable. The earliest glass “lenses” were actually magnifiers placed directly on objects, helping their users to inspect or read documents that strained their eyesight. True lenses that improved vision appeared first in spectacles around 1285, and more than 300 years later in telescopes and microscopes. In the intervening period, many inventors claimed to have made devices for extending vision. Some were earnest, others charlatans; in each case, their ideas were more imaginative than real. Ironically, no matter how fantastic their ideas were, the actual inventions were more startling, and the resulting discoveries more significant, than anything they ever imagined.

Microscopic discoveries made by scientists and artists in the early 1600s created, and then fed, a widespread hunger to learn more about nature. They increased the popularity of microscopes and drove improvements in scientific glass that made it possible to see ever tinier worlds otherwise invisible to the human eye. These advances culminated in the 19th century with the advent of modern scientific glassmaking and the subsequent realization of the nearly perfect optical microscope.

The first telescopes and microscopes shared much in common. They were made of the same materials, mostly wood and paper tubes, often covered with leather and tooled with gold leaf. They used variations of spectacle lenses, with those variations requiring precise grinding for producing a suitable image. And both revealed worlds previously inaccessible.

There was one very important difference. A person could view a distant (terrestrial) object through a telescope, and then move closer to that object and confirm what had been seen. By contrast, there was no independent way to access microscopic objects. A viewer could rightly be uncertain if details seen with a microscope were real or illusions; more importantly, such a viewer would often be puzzled as to what they might be. This previously invisible world was completely new, without any comparables in the visible world.

It is still unknown when and where the first microscope appeared, or who made it. The earliest report is connected with Dutch inventor Cornelius Drebbel in London around 1620, but by then, early forms of microscopes had been around for a decade. The word “microscopio” first appeared in print in 1625 on the title page of a book by Francesco Stelluti and Frederico Cesi on the microscopic investigation of honey bees; not coincidentally, Cesi had hosted the banquet in Galileo’s honor a decade earlier at which Cesi had publicly unveiled the word “telescope”. Stelluti’s Persio tradotto in verso sciolto (1630), a translation of ancient poetry, featured the first images of organisms viewed through a microscope, including a bee and a grain weevil.

Some early microscopes were made or improved by making analogies with telescopes, such as by adding a lens. Another method, the most widely used in the 17th century, was to make a single lens from a tiny glass ball. Antoni van Leeuwenhoek, the most famous microscopist of the period, initially used his single-lens microscopes to inspect the quality of cloth he sold, but later used them to become the first person to see microorganisms, such as bacteria and sperm, as well as blood flowing through small blood vessels. He crafted over 500 optical lenses, although fewer than a dozen survive. It was difficult and time-consuming to put a specimen at the narrow focal point of such a small lens, so Van Leeuwenhoek and other early microscopists often used a different microscope for each specimen.

Robert Hooke’s Micrographia (1665) includes the first published illustration of a compound (multiple-lens) microscope, and features spectacular drawings of the flea and the louse. (View the full text)

Soon after, Francesco Redi demonstrated that “life comes from life” rather than spontaneously generating, as had been the prevailing thought since the time of Aristotle.

Because glass is the most important part of any microscope, most makers fiercely guarded their secrets for making, grinding, and polishing glass. Antonio Neri was a rare exception. In his L’arte Vetraria (1612, Art of Glass), he summarized and disseminated the knowledge of glassmaking. Going one step further, Christian Gottlieb Hertel made public what had long been the secret art of grinding lenses and building optical instruments.

But no glassmaker then could fix the vexing issue of chromatic aberration, the introduction of color distortion into images produced by telescope or microscope lenses. In his Opticks (1704), Isaac Newton even claimed (incorrectly) that it is impossible to make achromatic lens systems that would have no such problems, delaying the appearance of achromatic microscopes by over a century.

In the following decades, instrument makers and sellers in London and Paris created an extensive system of wholesale and retail markets, with instruments sold and resold multiple times. Using trade cards and broadsheets, makers and sellers tried to distinguish their wares from each other, in part by impressing their potential buyers with an astonishing assortment of products.

The Industrial Revolution (c. 1760 to 1840), a period of new manufacturing systems and large scale social changes, also saw the rise of science popularization. Benjamin Martin, the most successful popularizer, lectured in cities and towns around England, enabling self-education with the aid of his books and scientific instruments, with telescopes and microscopes particularly popular. Lesser-known figures might post a handbill and draw audiences from the newly created middle class, another Industrial Revolution product. Surviving price lists provide fascinating insights into the realities of the science marketplace. Some advertised instruments cost the 2015 equivalent of $1000 - $17,000, clearly targeting not just the ordinary person in the street.

Between 1700 and 1850, microscopes improved through incremental mechanical, material, and optical innovations. These included tripods that increased the stability of the device, improved and more precise focusing mechanisms, metal rather than wood and paper components, improved relationships between the various lenses, and variations for positioning and lighting specimens. Although some artificial lighting was available, microscopes continued to use illumination primarily from the sun.

Motivated by the need to investigate plant and animal cells with greater resolution and clarity, Joseph Jackson Lister is often credited with the invention of the achromatic microscope (1826), which enabled his work with Thomas Hodgkin in identifying the cancer of white blood cells (Hodgkin lymphoma). Motivated by the discoveries made by his father and others, including Louis Pasteur, Joseph Lister developed the principles of antiseptic surgery, spraying carbolic acid on surgical instruments, incisions, and dressings. The effectiveness of Lister’s spray inspired an American chemist, Joseph Lawrence, to develop a surgical antiseptic that would eventually morph into a more general germicide and mouthwash: Listerine.

The mid-19^th-century microscope commanded a presence when displayed. Its elegant metal form and sleek lines conveyed precision, and represented an ever-increasing knowledge of the previously invisible worlds within our own bodies. Around 1850, binocular microscopes – microscopes with eyepieces for both eyes – appeared on the market. These instruments enabled users to see specimens with 3-D depth or, by manipulating polarized light, features of cells and tissues not visible in other ways.

The difference between early- and late -19^th-century microscope illustrations is quite stunning. Instead of viewing relatively crude sketches, one could now clearly see details of muscles, cells, and microorganisms. By the 1860s, varying strengths of dyes were used to highlight certain features. This differential staining marked a new phase in microscope studies, but it did not take off until the widespread development of synthetic dyes (and better glass) made it possible to use this strategy to resolve ever finer details.

Even so, the most important innovations came from understanding and improving the optical properties of glass. In the 1870s, the trio of Carl Zeiss, Ernst Abbe, and Otto Schott systematically researched and developed optical glass with very specific properties. This led to a greatly improved understanding microscope optics, as well as a new high-quality apochromatic microscope objective that was even better than the long desired achromatic lens. These accomplishments were incorporated into the Zeiss corporation logo, which features two lenses each made from different glass compositions. This collaboration also led to the development and classification of a vast array of specialty glasses, and to the origins of modern scientific glass.

The development of precisely engineered glass signaled the culmination of optical microscopes for over a century. Ernst Abbe had showed how the wavelength of light defined the limits of the details we could see with optical microscopes, leading eventually to the invention of very different, non-optical microscope technologies: electron microscopes and scanning probe microscopes.

But the optical microscope is not gone. Since 2000, two new ingenious, Nobel-Prize-winning approaches have made it possible to see the unimaginably small nano-worlds using optical microscopes. This revolutionary optical microscope is leading to discoveries that will continue to spark our imagination for decades to come.

Published on January 5, 2016