The invention of the compound microscope is generally attributed to Hans and Zacharias Janssen, a father-son team of spectacle makers. By placing two lenses in a tube, they discovered that the object at the end was significantly more magnified than by a single magnifying glass. This invention opened up an entirely new 'invisible' world to human inquiry. It was later used by Robert Hooke to discover cells and by Antonie van Leeuwenhoek to observe bacteria and protozoa (which he called 'animalcules'). The microscope revolutionized biology and medicine, allowing scientists to understand the cellular nature of life and the germ theory of disease. It remains one of the most essential tools in scientific research, from forensic pathology to the development of new vaccines and materials at the nanotechnology level.