Gradually, standards were developed in microscope design, and these were applied by many manufacturers of optical instruments in the early days of industrial mass production. Advances in microscope design made groundbreaking discoveries in the fields of botany, histology, cytology, bacteriology and medicine possible, such as the medical advances by Robert Koch* and Rudolf Virchow**. Suitable fixing, embedding and sectioning methods (the microtome), specialized stains and preservatives were developed at the same time.
* Robert Koch: primary founder of medical bacteriology, discoverer of the tuberculosis bacillus and Nobel Prize winner for medicine and physiology in 1905.
** Rudolf Virchow: founder of the disciplines of cellular pathology (study of the cellular basis of disease), comparative pathology (diseases common to humans and animals), anthropology (the study of humanity) and ethnology (cultural studies). Quote: "Every cell originates from another cell (Omnis cellula e cellula)".
Figure right: Standard microscope from around 1883 by Ernst Leitz, Wetzlar, with the typical features of the time: a horseshoe stand and blackened brass parts with a cellulose lacquer finish. At the time, all microscope manufacturers had at least one horseshoe stand in their product lines. Brass replaced the cardboard, wood and ivory of very early microscopes. Recently, a wide range of plastics have also been used.
In a genuine stereo microscope, each eye must be able to observe the object through a dedicated microscope. Parallel to the development of telescopes and microscopes, work was already being done to design instruments for both eyes in the 17th century. Inspired by the description of a binocular microscope by the Capuchin monk Antonius Maria de Rheita from 1645, his fellow monk Chérubin d’Orléans applied the familiar principle of the binocular telescope to the design of a microscope that would permit tiny objects to be viewed with both eyes at the same time in the year 1677. His goal was not a three-dimensional image as such; he believed that image quality could be improved by viewing objects with both eyes at the same time. The principle of stereoscopic vision was not known at that time – it was first described by the English physicist Charles Wheatstone in the year 1832.
In 1853, John Leonhard Riddel, a chemistry professor and postmaster in New Orleans, presented a binocular microscope with a single objective and a prism system. The prisms were arranged so that the right eye only received light from the right half of the objective and vice versa. The image was three-dimensional, but confusing because the relief appeared reversed (pseudoscopic).
Greenough and Cycloptic® principles
Binocular microscopes of the day featured a simple lens system and the same design as traditional compound microscopes. They only attained low magnifications and did not permit significant working distances. These dissecting microscopes, as they were then known, were used primarily in biology for dissection purposes; there were no technical applications for them at the time.
Around 1890, the American biologist and zoologist Horatio S. Greenough introduced a design principle which is still used today by all major manufacturers of optical instruments. Stereo microscopes based on the "Greenough principle" deliver genuine stereoscopic images of a very high quality.
In 1957 the American Optical Company introduced the modern stereo microscope design with a shared main objective and named it Cycloptic®. Its modern aluminum housing contained two parallel beam paths and the main objective, as well as a five-step magnification changer. This stereo microscope type, which was based on the telescope or CMO (Common Main Objective) principle, was adopted in addition to the Greenough type by all manufacturers and used for modular, high-performance instruments. Two years later, another American company, Bausch & Lomb, presented its StereoZoom® Greenough design with a groundbreaking innovation: a stepless magnification changer (zoom).
Although the basic stereo microscope has been around for a very long time, it has recently assumed an even more important role. Microscopes are involved in the manufacture or development of nearly all products for everyday use, medical technology or other high-tech applications. For instance, the demanded quality and performance specifications of today’s innumerable mobile phones can only be met by using a stereo microscope. The same applies to watches, irrespective of whether they are luxury or economy models. Stereo microscopes are also used in medical technology products, e.g. artificial hearts, defibrillators or stents for holding hollow organs open.
However, the use of stereo microscopes is not confined to the manufacture of goods. There is an equal variety of applications in research, development and forensics, whether for gaining new information on qualities and processes in biology or materials technology, or for convicting criminals with the evidence of microscopically small fibers.