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(optical microscope) An optical instrument that contains one or more lenses and is used in the laboratory to enlarge objects that are too small to be examined in detail by the naked eye. Its maximum resolution (the capacity to observe fine detail clearly), is about 0.3 μm (i.e. it can distinguish points only 0.3 μm apart) as compared to about 80 μm for the average human eye (see resolving power). Light is passed through a condenser, which converges the light rays onto the specimen, and then through one or more lenses. The earliest instruments were simple microscopes with a single lens (i.e. basically magnifying glasses). Such instruments are little used today as they have to be positioned close to the eye and have a limited field of vision. Also the illumination and mounting of the specimen is very tricky. The compound microscope has two lenses, one at either end of the body tube. The objective lens is at the base of the tube close to the specimen and the eyepiece or ocular at the other end close to the eye. The image is focused either by moving the body tube or by moving the microscope stage to which the specimen is secured. Most modern compound microscopes have several objective lenses of different magnifications, fitted to a revolving nose-piece. Some microscopes have a built-in light source while others are fitted with a mirror to reflect light from a lamp. The adjustment of an iris diaphragm reduces glare by limiting the part of the specimen that is illuminated. The modern compound microscope is used extensively for observing fine detail of microorganisms and thin sections through tissues and organs. Sections are often stained to increase contrast. The stereoscopic binocular microscope is used for work requiring lower magnifications, such as dissection, or for viewing detail of comparatively large specimens. It has two eyepieces that give a three-dimensional image, and the specimen is usually placed on a contrasting background.
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