The stereo, stereoscopic, operation, or dissecting microscope is an optical microscope variant designed for low magnification observation of a sample, typically using light reflected from the surface of an object rather than transmitted through it. The instrument uses two separate optical paths with two objectives and eyepieces to provide slightly different viewing angles to the left and right eyes. This arrangement produces a three-dimensional visualization for detailed examination of solid samples with complex surface topography. The typical range of magnifications and uses of stereomicroscopy overlap macrophotography. The stereo microscope is often used to study the surfaces of solid specimens or to carry out close work such as dissection, microsurgery, watch-making, circuit board manufacture or inspection, and examination of fracture surfaces as in fractography and forensic engineering. They are thus widely used in manufacturing industry for manufacture, inspection and quality control. Stereo microscopes are essential tools in entomology.
History Although a binocular microscope with two separate optical paths had been designed and built in 1671 by Chérubin d'Orléans, the first practical stereomicroscope was designed in 1892 by American zoologist Horatio Saltonstall Greenough and became commercially available in 1896, produced by Zeiss AG in Jena, Germany. Greenough's invention built on the prism arrangement designed by Ignazio Porro to properly show relief and texture.
Greenough grew up in the elite of Boston, Massachusetts, the son of the famous sculptor Horatio Greenough Sr. Without the pressures of having to make a living, he instead pursued a career in science and relocated to France. At the marine observatory at Concarneau on the Bretton coast, led by the former director of the Muséum national d'histoire naturelle, Georges Pouchet, he was influenced by the new scientific ideals of the day, namely experimentation. While dissection of dead and prepared specimens had been the main concern for zoologists, anatomists and morphologists, during Greenough's stay at Concarneau interest was revived in experimenting on live and developing organisms. This way scientists could study embryonic development in action rather than as a series of petrified, two-dimensional specimens. In order to yield images that would do justice to the three-dimensionality and relative size of developing invertebrate marine embryos, a new microscope was needed. While there had been attempts to build stereomicroscopes before, by for example Chérubin d’Orleans and Pieter Harting, none had been optically sophisticated. Furthermore, up until the 1880s no scientist needed a microscope with such low resolution. Greenough took action and, influenced by his Concarneau colleague Laurent Chabry’s attempts to construct intricate mechanisms to turn and manipulate the live embryo, conceived of his own instrument. Building on the recent discovery of binocularity as the cause of depth perception by Charles Wheatstone, Greenough designed his instrument with the phenomenon of stereopsis in mind.
Optical design The Greenough design uses two independent objective lenses focused on a single object. Most modern stereo microscopes use a binocular design with a single common main objective; the light path is parallel for each eye within the microscope body, which facilitates changing magnifications in discrete or continuous steps.
The stereo microscope should not be confused with a compound microscope equipped with double eyepieces and a binoviewer. In such microscopes, first popularized by a design credited to Francis Herbert Wenham around 1860, the magnified image is split after the objective lens using a prism and both eyes see the same image. The binoviewer was refined using roof prisms by Jentzsch and Siedentopf in the 1910s; for these designs, the two eyepieces serve to provide greater viewing comfort. However, the image in those microscopes is no different from that obtained with a single monocular eyepiece.
Working distance Great working distance and depth of field are important qualities for this type of microscope. Both qualities are inversely correlated with resolution: the higher the resolution (i.e. the greater the distance at which two adjacent points can be distinguished as separate), the smaller the depth of field and working distance. The large working distance at low magnification is useful in examining large solid objects such as fracture surfaces, especially using fibre-optic illumination as discussed below. Such samples can also be manipulated easily so as to determine the points of interest. Optimal working distances range from approximately 150 to 400 mm (5.9 to 15.7 in).
Magnification Some stereo microscopes can deliver a useful magnification up to 100×, which corresponds to the combination of a 10× objective and 10× eyepiece in a normal compound microscope; however, this is around one tenth the maximum useful resolution of a compound microscope, which range up to 500–1000×. The practical upper limit of total magnification for stereo microscopes is around 60×. In the Greenough design, the upper limit results from the need to maintain physical separation between the two objective lenses.
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