Two very different observers
The first recorded sightings of cells and tiny living things in the 1600s came mostly from two men who had little in common. Robert Hooke was a salaried experimenter at the Royal Society in London, skilled at design and drawing. Antony van Leeuwenhoek was a cloth merchant and town official in Delft, in the Dutch Republic, who taught himself lens making and had no university training. Both used the microscope, but with different instruments and different goals.
Hooke and Micrographia
In 1665 the Royal Society published Hooke’s Micrographia, a large illustrated book of observations. Hooke used a compound microscope, lit by an oil lamp with a glass globe of water to concentrate the light. He looked at everyday things and drew them in detail: the point of a needle, the edge of a razor, a snowflake, a louse, the eye of a fly, and a flea.
The flea engraving is probably the best known. The book was widely read, and its detailed plates showed that familiar objects, up close, had intricate structure. It was also one of the first popular science books in English.
Cork and the word cell
Hooke sliced thin pieces of cork and saw that they were made of many small, box-like compartments. He compared them to the small rooms, or cells, that monks lived in, and the name stuck. It is worth being careful here. What he saw were the empty walls of dead plant tissue. He did not know that cells were living units, and the idea that all living things are made of cells developed much later, in the 1800s.
He also looked at other plant material and noted similar porous structures. His text is cautious and descriptive, with little theory.
Leeuwenhoek and the animalcules
Leeuwenhoek’s work began to reach the Royal Society in the early 1670s, in letters written in Dutch. He used small single-lens instruments of his own making, which he kept for his own use and rarely shared. Over decades he wrote many letters describing what he saw.
Among the things he reported:
- Tiny living creatures in rainwater, pond water and other water samples, which he called animalcules. This is generally dated to the mid-1670s.
- Organisms in scrapings from his own teeth, in a letter usually dated to the early 1680s, which are widely read as some of the earliest descriptions of bacteria.
- Red blood cells and the flow of blood in thin tissues.
- Spermatozoa, which he described in animals and humans.
- The fine structure of wood, hair and insect parts.
He measured sizes by comparing them with grains of sand or the width of a hair, and the figures he gave were often in the right range for what we now know. His descriptions of the different shapes, rods, spheres and spirals, can be matched with bacteria that microbiologists know today, though identification from letters alone is naturally uncertain.
Doubt, then confirmation
Not everyone believed him. A world of living creatures in a drop of water was a strange claim, and few people could reproduce his results, since his lenses were special and his methods undisclosed. The Royal Society asked Hooke to try to repeat the observations. After some difficulty, in the 1670s Hooke succeeded in seeing the animalcules, and Leeuwenhoek’s reports were accepted. He was later elected a fellow of the Society.
Others in the same period
Hooke and Leeuwenhoek were not alone. Marcello Malpighi in Italy used a microscope to see the capillaries in a frog’s lung, supporting the idea that blood circulates in a closed system. Jan Swammerdam, in the Netherlands, studied insect anatomy. Nehemiah Grew, in England, wrote about plant structure. Together these works made the case that the microscope could answer real questions in biology.
What they did not know
It is easy to read modern ideas into these early reports, so some limits are worth stating.
- Neither Hooke nor Leeuwenhoek had any idea that the organisms they saw could cause disease. That link was not firmly made for roughly two centuries, and it eventually underpinned public measures such as water treatment and milk pasteurization.
- The cell was an anatomical curiosity, not yet a basic unit of life.
- Many seventeenth-century thinkers believed that small creatures could arise from decaying matter. That belief, often called spontaneous generation, was not fully rejected until later experiments.
The observations were therefore a beginning. They revealed that the unseen world was crowded and varied, but making sense of it took far better instruments and many more people. The long road from these first glimpses to routine medical use is told in later accounts of germ theory and the microscope in hospitals.
Why these observations lasted
Two features made the work durable. Hooke’s drawings were accurate enough that later readers could check them, and Leeuwenhoek wrote down sizes and shapes plainly, so his letters could be compared with later findings. Both men also had an eye for ordinary materials, such as cork, water and blood, that anyone could obtain. That is partly why the field of microscopy grew so quickly after them: the subjects were everywhere.