In the 300 years since Van Leeuwenhoek died, some of the details around his life and his work have provided material for discussion or dispute. As archives and libraries are being scanned and technology improves, information is becoming more readily available. This review therefore aims to take a new look at some of those discussions, and Van Leeuwenhoek’s possible experimental methods. Digital photography has made it possible to show exactly what can be seen through his simple microscopes, and how he could have obtained his results by, for example, modifying his microscopes and lighting. Equally, the completion of the series known as the Collected Letters, begun in 1931 with volume 1 published in 1939 and to be completed in 2023, allows researchers to see complete letters in English and modern Dutch. Theories about experimental methods can be tested and the results recorded photographically. Additionally, new, non-destructive techniques such as neutron tomography have improved the evaluation of the authenticity of surviving microscopes.

When Antoni van Leeuwenhoek began his work with microscopes in the late 17th century, western medicine was mostly based on the work of a Roman doctor called Galen (129-199 ad), theological interpretation, superstition, and folk remedies. During modern discussions of Van Leeuwenhoek's work, a common question from listeners is why it took so long for the link between Van Leeuwenhoek's discoveries and infectious disease to be accepted. Published literature, examples of which are discussed here, shows that many researchers, doctors, and others reported the link, even during Van Leeuwenhoek's lifetime. However, it was frequently not taken seriously by the most influential people. The scientific establishment included a faction of the Royal Society of London who called themselves the 'Mechanical Philosophers'. They ridiculed those reporting animalcule-linked infection, dismissing them as 'Contagionists'. The medical establishment also included many influential people with a lot to lose if they changed their established approaches, and many quack doctors. Most religious ministers were strongly orthodox, some even claiming that helping the sick angered God. A major problem, of course, was that technology and biological understanding also lagged far behind. Despite the fact that the use of vaccination was under active discussion in the Royal Society at the time of Van Leeuwenhoek's death and quarantine was in regular use, a possible microbial connection was apparently not considered. It was not until late in the 19th century, that Robert Koch (1843-1910) isolated Bacillus anthracis, proved that it caused anthrax, and was believed. This paper follows a lecture given during the online Microbe Forum in June 2021, and illustrates the difficulties of establishing the true link between Van Leeuwenhoek's animalcules and infectious disease in humans, animals, and plants.

A single-lensed microscope made by the Van Musschenbroek workshops between 1690 and 1750 has been examined experimentally, and the method of use established. The results were compared with those obtained with facsimile van Leeuwenhoek microscopes. As might be expected, each microscope had its stronger and weaker points.

It is often claimed that Antoni van Leeuwenhoek was secretive about his methods. However, closer examination of his letters suggests that this reputation was not always deserved. Some letters were not published in the Royal Society Proceedings, and others were edited. This paper describes the repetition of his experiments with the eyes of bees and dragonflies using his own account of his experiments, despite the fact that only one of the four letters was published in the Proceedings.

Possible techniques for lighting opaque samples while using Van Leeuwenhoek microscopes have been tested, and the results are presented in relation to published material. The design of the microscope causes the sample to be in shadow with any form of top lighting. It is therefore suggested that Van Leeuwenhoek's hinted 'particular method of observing' might refer to a different style of microscope as shown in the frontispiece of the sale catalogue for his microscopes, and available at that time for purchase from sellers of optical equipment.