The Telescope and the Microscope: New Eyes on the World
How refracting and reflecting telescopes and compound microscopes, invented in the early seventeenth century, opened the heavens and the microworld to observation.
The Telescope and the Microscope: New Eyes on the World
Two instruments invented in the early seventeenth century did more than any single theory to convert the Scientific Revolution from a textual debate into a visual one. The refracting telescope, first appearing in the Netherlands in 1608 and improved by Galileo within a year, opened the heavens to direct observation. The compound microscope, developed over the following decades, opened a previously invisible biological world. Together, they made the empirical case for the new science in a way that no book of philosophy could match. This section examines both instruments. It is part of the Major Discoveries. It links upward to the Galileo for the telescopic story, and to the Isaac Newton for the reflecting telescope. It links downward to two articles: Who Invented the Telescope?, which traces the Dutch origins of the refracting telescope and its rapid spread, and What Did Early Microscopes Reveal?, which describes Hooke’s Micrographia and Leeuwenhoek’s “animalcules.” For the broader technological and industrial consequences of the new science, see Inventions of the Scientific Revolution.
The Refracting Telescope
A refracting telescope, in its simplest form, is a tube containing two lenses. The objective lens, at the front (the end pointed at the object), gathers light and forms a real image of the distant object inside the tube. The eyepiece lens, at the back, magnifies that image for the observer. The two-lens arrangement is called a Keplerian telescope if the eyepiece is a converging lens, and a Galileian telescope if it is a diverging lens. The Dutch telescopes of 1608 were Galileian; Galileo’s improvements were also Galileian. Kepler’s design, proposed in 1611, was sharper and became the standard for astronomical telescopes. The first telescopes magnified only a few times — three diameters was typical of the earliest Dutch instruments. Galileo’s best telescopes reached about thirty diameters, which was enough to resolve the moons of Jupiter, the phases of Venus, and the major craters of the Moon. The limit was set by the quality of the lenses: a small piece of bad glass, or a poorly figured surface, would smear the image into a coloured blur. The chromatic aberration of a simple lens — the tendency of different colours to focus at different distances — was the principal optical defect and was not corrected until the invention of the achromatic doublet in the mid-eighteenth century. The growth of the refracting telescope, and the discoveries it enabled, are described in detail in Who Invented the Telescope?. Galileo’s telescopic observations are described in the Galileo and in Galileo’s Telescope Discoveries.
The Reflecting Telescope
The refracting telescope has an inherent limit set by the absorption of light in the glass and by the chromatic aberration of a single lens. The first attempt to overcome these limits was the reflecting telescope, in which a curved mirror at the back of the tube gathers the light and forms the primary image, and a smaller secondary mirror or lens sends the image to the eyepiece. Newton built the first successful reflecting telescope in 1668. His design used a small flat mirror tilted at forty-five degrees to the optical axis to send the image out through a hole in the side of the tube. The design, now called the Newtonian telescope, became one of the standard configurations of reflecting telescopes and is still used in amateur astronomy today. Newton’s telescope was tiny — its mirror was only about three centimetres in diameter — but it produced sharp images and demonstrated the principle that a mirror could substitute for an objective lens. Larger reflecting telescopes were built in the eighteenth century by John Hadley and others, and the great reflectors of the nineteenth and twentieth centuries — William Herschel’s telescopes, the Leviathan of Parsonstown, the Mount Wilson and Palmirar reflectors, the Hubble Space Telescope — were all descendants of Newton’s design. The construction of larger and larger telescopes is one of the principal stories of post-Newtonian observational astronomy.
The Compound Microscope
The compound microscope is, in optical terms, the same instrument as the refracting telescope, but used the other way around: it looks at a small, close object through a short-focal-length objective lens that produces a magnified real image, which is then further magnified by an eyepiece. The earliest compound microscopes date from the late 1590s or early 1600s, and there is evidence that Zacharias Janssen and his father Hans in Middelburg may have built a compound microscope as early as the 1590s. Whether the device they built was a true compound microscope or a simple magnifier is debated; the first unambiguous reports of compound microscopes producing useful magnifications come from the 1620s. The first great work of microscopy was Robert Hooke’s Micrographia of 1665. Hooke, who was also Curator of Experiments at the newly founded Royal Society, used a compound microscope of his own design to examine a wide range of small objects. The illustrations of Micrographia — the cork cells from which the word “cell” entered biology, the compound eye of a fly, the surface of a needle, the structure of a gnat, the surface of a leaf — are among the most striking images in this history. They are described in What Did Early Microscopes Reveal?.
Antonie van Leeuwenhoek and the Single-Lens Microscope
The greatest microscope-maker of the seventeenth century was not Hooke but the Dutch draper Antonie van Leeuwenhoek of Delft. Leeuwenhoek ground his own single-lens microscopes from small glass spheres, achieving magnifications of more than two hundred diameters and resolutions that no compound microscope of his day could match. Between 1674 and his death in 1723, he sent more than three hundred letters to the Royal Society describing what he saw: bacteria, protozoa, spermatozoa, the structure of muscle, the circulation of blood in capillaries, the growth of small animals. Leeuwenhoek’s discoveries are described in What Did Early Microscopes Reveal?. They opened a previously invisible biological world and, in time, made possible the rejection of spontaneous generation, the growth of cell theory, and the eventual discovery of the microbial causes of disease.
The Cascade of Telescopic Discoveries
The telescopic discoveries of 1609–1612 were the most dramatic single set of empirical findings in the Scientific Revolution. The Moon, observed by Galileo, was no longer a perfect celestial sphere: it had mountains and craters, and its surface resembled that of the Earth. The Milky Way, observed by Galileo, resolved into countless individual stars too faint to be seen with the naked eye. Jupiter, observed by Galileo, had four moons that changed position from night to night, demonstrating that there was at least one other centre of motion in the universe. Venus, observed by Galileo, exhibited a full set of phases, demonstrating that it orbited the Sun. The Sun, observed independently by Galileo, Christoph Scheiner, and others, had spots that moved across its disc, demonstrating that it rotated on its axis and was not the unblemished emblem of divinity that Aristotelian cosmology had assumed. Saturn, observed by Galileo and (with better instruments) by Christiaan Huygens and Giovanni Domenico Cassini, had a ring system that the early telescopes could not resolve, so it appeared at first as a “triple” planet and then, as the rings turned edge-on and disappeared, as a single disc. Huygens announced the true nature of the rings in 1659. Cassini later discovered the great division in the rings and four of Saturn’s moons. These discoveries are described in Galileo’s Telescope Discoveries. Their cumulative effect on the new cosmology is described in the Heliocentrism and the New Astronomy section. For the practical instruments and their improvement, see Who Invented the Telescope?.
The Cascade of Microscopic Discoveries
The microscopic discoveries of the seventeenth century were equally consequential, although they took longer to integrate into the scientific mainstream. Hooke’s Micrographia introduced the word “cell” to describe the boxy compartments he saw in cork, although Hooke himself did not understand their biological significance. Leeuwenhoek’s letters to the Royal Society between 1674 and 1723 described a previously invisible world of “animalcules” — bacteria, protozoa, spermatozoa — and of structures within larger organisms, including the striations of muscle, the structure of plant tissues, and the capillary circulation of the blood that William Harvey had postulated in 1628. These observations are described in detail in What Did Early Microscopes Reveal?. The technical challenges of the early microscopes — poor optics, distortion, the difficulty of illuminating the specimen — are addressed, and the role of these instruments in the eventual overthrow of spontaneous generation and the growth of cell theory is sketched.
From Observation to Theory
A common theme of the section is the way in which new instruments generated new theory. Galileo’s observations of Jupiter’s moons gave visual evidence for the existence of centres of motion other than the Earth. Hooke’s observations of plant cells gave biology a new fundamental unit. Leeuwenhoek’s observations of microorganisms opened the long path to germ theory. The instruments did not merely extend the senses; they suggested theoretical possibilities that had not been conceivable without them. This is a recurring pattern in this history, and one that the Scientific Revolution illustrates especially clearly. The new science was not only a new theory; it was a new way of producing theoretical evidence. The telescope and the microscope, together with the air pump, the thermometer, the barometer, the pendulum clock, and the vacuum pump, made the laboratory into a place where phenomena could be reliably produced, controlled, and observed. The empiricism of the Scientific Revolution was, in this sense, an instrumented empiricism.
Optics as a Branch of Physics
The construction of the telescope and the microscope was inseparable from the growth of optics as a mathematical science. Kepler, in his Dioptrice of 1611, gave the first clear account of the principles of the refracting telescope, including the design with a converging eyepiece that bears his name. René Descartes, in the Dioptrice appended to his Discours de la méthode (1637), extended the theory to the lens and the eye. Willebrord Snell in 1621 and Descartes in 1637 independently formulated the law of refraction (Snell’s law), which states that the sines of the angles of incidence and refraction are in a constant ratio for a given pair of media. Newton, in his optical work, took a different approach. In a series of experiments described in the Opticks (1704), he showed that white light is a mixture of light of all colours, that each colour is refracted by a glass lens by a slightly different amount, and that this chromatic dispersion is the reason simple refracting telescopes produce coloured images. The work led Newton to the conclusion — wrong, as it turned out, since achromatic doublets were eventually made — that refracting telescopes could never be perfected, and to the construction of the reflecting telescope described earlier in this section. For the full optical story, see Newton and Optics. The wave theory of light, proposed by Christiaan Huygens in his Traité de la lumière (1690) and confirmed experimentally by Thomas Young in 1801 and Augustin-Jean Fresnel in the 1810s, eventually replaced Newton’s corpuscular theory. But the question of whether light is a wave or a particle was not finally settled until the twentieth century, when quantum mechanics showed that light behaves as both, depending on the experiment. The seventeenth-century optics of the telescope and the microscope were the working context in which these questions were first sharpened.
The Limits of Resolution
The microscopes and telescopes of the seventeenth century had inherent limits of resolution, set by the wave nature of light. The smallest object that a microscope can resolve is roughly the wavelength of the light used to illuminate it, about half a micrometre for visible light. The most distant object that a telescope can resolve as a point source is set, in principle, only by the diameter of the objective lens or mirror and the wavelength of the light collected. These limits were not understood in the seventeenth century. Newton, in the Opticks, speculated about a “natural limit” beyond which observation would fail, but the diffraction theory that explains the limit was developed in the nineteenth century by Fresnel and others. The fact that the limit exists at all — that the wave nature of light imposes a finite resolution on any optical instrument — is a profound feature of the physical world, and it was the seventeenth-century work on the telescope and the microscope that made its practical significance clear. The diffraction limit of optical telescopes was eventually the motivation for radio astronomy, infrared astronomy, X-ray astronomy, and the great space-based observatories of the late twentieth and early twenty-first centuries. The Hubble Space Telescope, launched in 1990, and the James Webb Space Telescope, launched in 2021, are direct descendants of the small telescopes built in Middelburg and Padua in 1608 and 1609. The history of the telescope is, in this sense, a single continuous story running from Lipperhey’s three-diameter spyglass to the most sophisticated instruments of contemporary astronomy.