Microscopes have played a fundamental role in the development of biology as an experimental science. It was Robert Hooke who, when using a compound microscope in 1655, noticed that thin slices of cork were made up of identical and small selfâ contained units, which he called â cells.â The generalization of this observation and its acceptance, though, had to wait until the late 1830s, when German microscopists Matthias Schleiden and Thcodor Schwannâ working independentlyâ introduced the â cell theoryâ of complex organisms. By the second half of the 19th century Magnus Retzius, Santiago Ramón y Cajal and Camillo Golgi were busy completing the microscopic anatomical description of the cell. A highâ resolution instrument that can operate in liquids is making complex biological structures accessible to study in conditions close to those that exist in living organisms. © 1995, American Institute of Physics. All rights reserved.