Final answer:
In the 1600s, Antony van Leeuwenhoek developed a large collection of single-lens microscopes. He observed the movements of single-celled organisms and sperm, and is credited as being the first person to observe microorganisms.
Step-by-step explanation:
In the 1600s, Antony van Leeuwenhoek developed a large collection of single-lens microscopes. Van Leeuwenhoek, a Dutch shopkeeper, had great skill in crafting lenses and was able to observe the movements of single-celled organisms and sperm, which he collectively termed 'animalcules.' He is credited as being the first person to observe microorganisms and made significant contributions to the development of the microscope.