What Did Early Microscopes Reveal?
Robert Hooke's Micrographia, Antonie van Leeuwenhoek's animalcules, and the seventeenth-century discovery of a hidden biological world beneath the limits of naked-eye vision.
What Did Early Microscopes Reveal?
The seventeenth century opened a second visible world, just as astonishing as the heavens revealed by Galileo’s telescope. Through the compound microscope, and later through Antonie van Leeuwenhoek’s single-lens instruments, observers saw for the first time that everyday objects — a drop of pond water, a slice of cork, a hair, a flea, a drop of blood — contained structures and inhabitants that no naked eye had ever seen. The discovery of this hidden world is one of the great empirical achievements of the Scientific Revolution, and it took longer than the telescopic discoveries to be fully integrated into biological theory.
This page is an article in the Telescope and Microscope, under the Major Discoveries. It assumes familiarity with the broad context, which is sketched in the section overview. For the invention of the microscope, see Who Invented the Telescope? (which discusses the related optical history). For the optical theory that underlies microscope design, see Newton and Optics. For the broader technological and industrial consequences of these instruments, see Inventions of the Scientific Revolution.
Robert Hooke and Micrographia
Robert Hooke, the Curator of Experiments at the Royal Society of London, published in 1665 a book that is, by general consent, the most beautiful work of microscopy in the seventeenth century. Micrographia, or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries Thereupon combined careful observations, detailed illustrations, and speculative theoretical discussion. The illustrations, drawn by Hooke himself and engraved by the best London craftsmen, were the first time the microscopic world had been shown to a wide public in a form that the printing press could reproduce.
The most famous illustrations in Micrographia include the compound eye of a fly (showing the individual lenses, or ommatidia, of which it is composed), the surface of a needle (showing the ragged, irregular edge left by the grinding process), the structure of cork (showing the boxy compartments that Hooke called “cells,” from the Latin cella), the surface of a leaf with its stomata, the head of a gnat, and the leg of a flea, drawn at a magnification at which the joints and bristles are clearly visible.
Micrographia is a landmark in the history of biology for several reasons. It introduced the word cell into biological vocabulary, although Hooke did not understand that the cells of cork were once living — he saw only the cell walls, since the contents had died. It established the principle that small structures could be systematically described and illustrated, even when their function was not yet understood. And it made the case, by example, that the new instruments were producing a new kind of evidence that deserved to be taken as seriously as the testimony of the ancient authorities.
Hooke’s Compound Microscope
The microscope Hooke used for Micrographia was a compound instrument of his own design, with an objective lens, a field lens, and an eyepiece. It could magnify by factors ranging from about twenty to several hundred, depending on the lens combination. The illumination was supplied by a lamp, with a water-filled glass sphere used as a condenser to concentrate the light on the specimen.
Hooke described his microscope design in detail at the front of Micrographia, and his design became the prototype for the compound microscopes used in England and on the Continent for the next century. The instrument was difficult to use — focusing required a careful screw mechanism, the field of view was narrow, and the spherical and chromatic aberrations of the simple lenses were severe — but it produced images that were, for the time, remarkably detailed. Hooke also developed a method of mounting specimens on a small pin that could be rotated and tilted, allowing the observer to view the specimen from different angles.
The Disadvantages of Compound Microscopes
Despite Hooke’s success, compound microscopes of the seventeenth century had a fundamental optical limitation. The simple lenses available at the time suffered from severe spherical aberration (a lens focuses light at slightly different distances depending on where on the lens the light passes through) and chromatic aberration (a lens focuses different colours at different distances). The combination of these two effects severely limited the useful magnification of compound microscopes. Above a few hundred diameters, the images became indistinct and were surrounded by coloured halos.
The achromatic doublet, in which a convex lens of crown glass is paired with a concave lens of flint glass to cancel out most of the chromatic aberration, was invented in the 1730s by Chester Moore Hall and reinvented by John Dollond in 1758. Achromatic objectives transformed microscopy in the late eighteenth and nineteenth centuries, allowing useful magnifications of a thousand diameters or more. Until that improvement, however, seventeenth-century observers looking for the highest possible resolution were forced to use single-lens microscopes, in which the aberrations of a single piece of glass are easier to control.
Antonie van Leeuwenhoek
The greatest microscope-observer of the seventeenth century was Antonie van Leeuwenhoek, a Dutch draper from Delft with no formal scientific training. Leeuwenhoek ground his own single-lens microscopes — small glass spheres mounted between two brass plates, with a specimen pin and focusing screws — and he achieved 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, Leeuwenhoek sent more than three hundred letters to the Royal Society describing what he saw. The first observation of bacteria, the first observation of spermatozoa, the first observation of protozoa, and the first observation of the capillary circulation of the blood are all due to Leeuwenhoek. He was the first human being to see a living cell, in the modern sense of the term — a discrete unit of living matter — although the full significance of the cell as a universal building block of life would not be understood for another century and a half.
Leeuwenhoek’s letters to the Royal Society, translated from the Dutch and published in the Philosophical Transactions, are remarkable documents. They include detailed descriptions of bacteria (“animalcules”) in pepper water, in scrapings from teeth, and in pond water; of the structure of muscle fibres, plant tissues, and the eye of a fly; of the growth of small animals such as the weevil from egg to adult; and of the capillary circulation of blood in the tail of a tadpole. The Royal Society sent several visiting delegations to Delft to verify his observations, and his claims were eventually accepted, although some of his contemporaries remained sceptical.
The Discovery of the Cell
The word cell entered biology through Hooke’s Micrographia, but Hooke’s cells were the empty cell walls of dead plant tissue. Leeuwenhoek observed the contents of living cells, but he did not use the term cell. The cellular theory of life — the idea that all living things are composed of cells, that the cell is the basic unit of life, and that all cells arise from pre-existing cells — was developed in the nineteenth century by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.
Schleiden, a German botanist, proposed in 1838 that all plants are composed of cells. Schwann, a German physiologist, extended the idea to animals in 1839. Virchow, in 1855, completed the theory with the famous omnis cellula e cellula — every cell from a cell. The cellular theory is one of the foundational ideas of modern biology, and the seventeenth-century observations of Hooke and Leeuwenhoek were its necessary precursors. Without the microscope, the cell could not have been seen; without the cell, the unity of plant and animal life could not have been understood.
The Gradual Shift in Understanding of Life
The seventeenth-century microscopic discoveries did not, on their own, transform biology. The great anatomical discoveries of the century — William Harvey’s circulation of the blood (1628), the lymphatic system, the role of the lungs, the structure of the heart — were made without significant use of the microscope. The full integration of microscopy into biology had to wait until the growth of achromatic lenses in the late eighteenth century, better staining techniques in the nineteenth, and electron microscopy in the twentieth.
What the seventeenth century did establish was that a hidden biological world existed, that it could be observed directly with the right instruments, and that observation was a legitimate way of answering questions about life. The tradition of careful, illustrated, written descriptions of microscopic specimens — established by Hooke, refined by Leeuwenhoek, and continued by later observers such Jan Swammerdam, Marcello Malpighi, and Nehemiah Grew — was the foundation on which the nineteenth century built the cellular theory of life and the twentieth century built molecular biology.