One of the first usable microscopes ever made was this simple instrument by Antoni van Leeuwenhoek (1632-1723). Optics consisted of a simple bi-convex lens with a 1/4" focus mounted between two plates with a small diameter opening for viewing.  Leeuwenhoek make his lenses by pulling about a glass rod in a hot flame until he produced a thread-like thinness.  Next he broke the thread and put the end back into the edge of the flame until a tiny ball formed.  This ball became the lens.  He obtained different size lenses and, consequently, different magnifications by varying the sizes of the glass spheres.  The subject to be viewed could be affixed on the pointed shaft on the reverse and positioned with the screws.  The microscope was held very close to the eye for viewing. The microscope was quite small and difficult to use.  The plates measured about 1 7/8" in length.   Magnification ranges were in the neighborhood of 50-275x.  Using his simple microscope, Leeuwenhoek was able to discover the existence of single-celled organisms he called, "animalcules", which we know as microorganisms. Only nine original Leeuwenhoek microscopes are known to exist.  They were typically made of silver or copper.  Photos above show a reproduction instrument based on original diagrams.