The discoverer of microbes had to have a magnifying glass to be able to enlarge objects that are not visible to the naked eye. However, it was neither an optician, nor a technician or biologist, but a cloth merchant who was the first person to see microbes: Antoni van Leeuwenhoek from Delft used small magnifying glasses, as was common in his day, to assess the structure of woven fibres and thus the quality of a fabric. Using a process he developed himself, he succeeded in producing tiny glass lenses of exceptionally high quality.
Leeuwenhoek's "single-lens microscopes" enabled magnifications of 250 to 300 times. This was not only sufficient to visualise sperm or protozoa such as euglena and to precisely describe red blood cells. In cloudy pond water and in a suspension of ground pepper, he discovered much smaller, sometimes mobile objects in 1675, which he called "animalcules". He also found large numbers of such bacteria in dental plaque. He even recognised different shapes, which we know today as rods, cocci and spirillae. Leeuwenhoek reported to the Royal Society of London that the number of these little animals in a person's mouth must be greater than the number of subjects in a kingdom. He was not yet able to measure the absolute size of his microbes. However, through size comparisons and special calculations, he came to the conclusion that more than 110 million of them would be just the size of a grain of sand.
He never revealed how he made his glass lenses. They were probably not created by grinding alone, but from tiny spheres of molten glass. He attached such a sphere with a diameter of one millimetre between two drilled metal plates and held it close to his eye to view an object. Leeuwenhoek thus achieved a much greater magnification than the English naturalist Robert Hooke. Ten years earlier, Hooke had already built microscopes consisting of two glass lenses and used them to observe various biological objects, including moulds. The structures he recognised led him to introduce the term "cell" into biology - the name for the basic building block of all living things. However, his apparatus only provided a maximum magnification of 50 times. It was not until more than 200 years later that microscopes with a resolution similar to Leeuwenhoek's devices became available again.
The reports of the existence of a previously invisible microbial world were initially rejected by many scientists at the Royal Society, of which Robert Hooke was a member. The amateur researcher Leeuwenhoek was not believed. He therefore had to have what he saw witnessed by trustworthy personalities - including ministers, doctors and lawyers - and send corresponding statements to London. The people of his time knew about telescopes that could be used to discover new celestial bodies. But nobody wanted to believe that looking through a glass lens could visualise a microcosm of unknown life forms.
Incidentally, single-lens microscopes are now widely used again as an attachment for the camera of smartphones. You can buy such a small microscope relatively cheaply or, with a little skill, make one yourself following the instructions. The observed objects can then be easily photographed and recorded on video.
© Text Joachim Czichos / VAAM, czichos[at]czience.de, Nutzung gemäß CC 4.0
Abbildung: Antoni van Leeuwenhoek developed the first microscope: the object on the needle was viewed through a lens and focussed with two screws, use according to CC 4.0
