Microscope

A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. The science of investigating small objects using such an instrument is called microscopy, and the term microscopic means minute or very small, not easily visible with the unaided eye. In other words, requiring a microscope to examine.

Compound optical microscope

The diagrams below show compound microscopes. In its simplest form—as used by Robert Hooke, for example—the compound microscope would have a single glass lens of short focal length for the objective, and another single glass lens for the eyepiece or ocular. Modern microscopes of this kind are usually more complex, with multiple lens components in both objective and eyepiece assemblies. These multi-component lenses are designed to reduce aberrations, particularly chromatic aberration and spherical aberration. In modern microscopes the mirror is replaced by a lamp unit providing stable, controllable illumination.

Related Topics:
Robert Hooke - Focal length - Eyepiece - Aberrations - Chromatic aberration - Spherical aberration

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• ocular lens or eye-piece
• objective turret, or nosepiece
• objective lenses
• coarse adjustment knob
• fine adjustment knob
• object holder or stage
• mirror
• diaphragm and condenser

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Compound optical microscopes can magnify an image up to 1000× and are used to study thin specimens as they have a very limited depth of field. Typically they are used to examine a smear, a squash preparation, or a thinly sectioned slice of some material. With a few exceptions, they utilize light passing through the sample from below and special techniques are usually necessary to increase the contrast in the image to useful levels (see contrast methods). Typically, on a standard compound optical microscope, there are three objective lenses: a scanning lens (4×), low power lens (10×), and high power lens (40×). Advanced microscopes often have a fourth objective lens, called an oil immersion lens. To use this lens, a drop of oil is placed on top of the cover slip, and the lens moved into place where it is immersed in the oil. An oil immersion lens usually has a power of 100×. The actual power or magnification is the product of the powers of the ocular (eyepiece), usually about 10×, and the objective lens being used.

Related Topics:
Depth of field - Contrast - Image - Contrast methods - Magnification - Eyepiece

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To study the thin structure of metals (see metallography) and minerals, another type of microscope is used, where the light is reflected from the examined surface.  The light is fed through the same objective using a semi-transparent mirror.

Related Topics:
Metal - Metallography - Mineral

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Stereo microscope

The stereo, binocular or dissecting microscope is designed differently from the diagrams above, and serves a different purpose. It uses two eyepieces (or sometimes two complete microscopes) to provide slightly different viewing angles to the left and right eyes. In this way it produces a three-dimensional (3-D) visualisation of the sample being examined.

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The stereo microscope is often used to study the surfaces of solid specimens or to carry out close work such as sorting, dissection, microsurgery, watch-making, small circuit board manufacture or inspection, and the like. Great working distance and depth of field here are important qualities for this type of microscope. Both qualities are inversely correlated with resolution: the higher the resolution (i.e., magnification), the smaller the depth of field and working distance. A stereo microscope has a useful magnification up to 100×. The resolution is maximally in the order of an average 10× objective in a compound microscope, and often much lower.

Related Topics:
Microsurgery - Magnification

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Special designs

Other types of optical microscope include:

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• the inverted microscope for studying samples from below; useful for cell cultures in liquid;
• the student microscope designed for low cost, durability, and ease of use; and
• the research microscope which is an expensive tool with many enhancements.