Abstract
The rise of microscopy in the seventeenth century allowed scientists to discover a new world of microorganisms and achieve great physiological advances. One of the first microscopes of the epoch was Antonie van Leeuwenhoek's microscope, a deceptively simple device that contains a single ball lens housed in a metal plate allowing the observation of samples at up to ×250 magnification. Such magnification was much greater than that achieved by rudimentary compound microscopes of the era, allowing for the discovery of microscopic, single-celled life, an achievement that marked the study of biology up to the nineteenth century. Since Leeuwenhoek's design uses a single ball lens, it is possible to fabricate variations for educational activities in physics and biology university and high school classrooms. A fundamental problem, however, with home-built microscopes is that it is difficult to work with glass. We developed a simple protocol to make ball lenses of glass and gelatin with high magnification that can be done in a university/high school classroom, and we designed an optimized support for focusing and taking photographs with a smartphone. The protocol details a simple, easily accessible, low-cost, and effective tool for the observation of microscopic samples, possible to perform anywhere without the need for a laboratory or complex tools. Our protocol has been implemented in classrooms in Chile to a favorable reception.