Final answer:
Antonie van Leeuwenhoek was the first person to observe "animalcules" under the microscope. He observed single-celled organisms using his own microscopes with powerful lenses that he developed.
Step-by-step explanation:
The first person to observe "animalcules" under the microscope was Antonie van Leeuwenhoek. Born in 1632, van Leeuwenhoek was a Dutch shopkeeper who had great skill in crafting lenses. He observed single-celled organisms, which he collectively termed "animalcules," using his own microscopes with powerful lenses that he developed.