Final answer:
Antonie van Leeuwenhoek was the first to examine a drop of pond water under the microscope and observe the microscopic "animalcules" that darted back and forth. He used a simple and powerful microscope to observe single-celled organisms, which we now know as bacteria and protists.
Step-by-step explanation:
Antonie van Leeuwenhoek was the first to examine a drop of pond water under the microscope and observe the microscopic "animalcules" that darted back and forth. In 1675, using a simple but powerful microscope, Leeuwenhoek was able to observe single-celled organisms, which he described as "animalcules" or "wee little beasties," swimming in a drop of rainwater. From his drawings of these little organisms, we now know he was looking at bacteria and protists.