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Wednesday, 13 November 2013

Metallurgical Microscope

Metallurgical Microscope
Points : Metallurgical Microscope, Definition Definition The metallurgical microscope is similar in optical principles to any other microscope, but differs from some of them in the methods by which the specimen is illuminated. Most biological specimens can be prepared as thin, transparent slices mounted between sheets of thin glass, so that illumination can be arranged simply, by having a source of light behind the specimen. Metals, however are opaque substances, and since they must be illuminated by frontal lighting, it follows that the source of light must be inside the microscope tube itself.
This is usually accomplished, as in by means. of a small plain-glass, reflector, R, placed inside the tube. With this system of illumination much of the light is lost both by transmission when it first strikes the plate and, by reflection, when the ‘returning ray form the specimen strikes the inclined plate again. Nevertheless, a small 6-volt bulb is usually sufficient as a source of illumination. The width of thee beam is controlled by the iris diaphragm, D. Generally speaking, this should be partly closed so that the beam of light is just sufficient to cover the back component of the objective lens. An excess of light, reflected form the sides of the microscope tube, will cause light-scatter and, consequently. ‘glare’ in the held of view. The optical system of the microscope consists of two lenses, the objective, 0, an the eyepiece, F. the former is the more important and expensive of the two lenses, since it has to resolve the tine detail of the object being examined. Good- quality objectives are corrected for chromatic and spherical aberrations, and hence, like camera lenses, are of compound construction. The magnification given by the objective depends upon its focal length-the shorter the focal length. The higher the magnification. In addition to magnification. resolving power is also important. This is defined as the ability of a lens to show clearly separated two lines which are very close together. In this way resolution can be expressed as a number of lines per mm. Thus resolving power depends upon the quality and it is useless to increase the size of the image. either by extending the tube length of the microscope or iv using a higher-power eyepiece. beyond a point where there is a falling off emf resolution. A parallel example in photography is. where a small 90 mm x 60 mm snapshot, on enlarging, fails to show any more detail than it did in its original size and in fact shows blurred outlines as a result.

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