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The Microscope The

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									The Microscope

         Chapter 7
       Introduction
   A microscope is an optical instrument that uses a lens or a
    combination of lenses to magnify and resolve the fine
    details of an object.
   The earliest methods for examining physical evidence
    relied solely on the microscope.
   The magnified image seen by looking through a lens is
    known as a virtual image, whereas an image viewed
    directly is known as a real image.
   The object to be magnified is placed under the lower lens,
    called the objective and viewed through the upper lens,
    called the eyepiece.
   Various types of microscopes are used to analyze forensic
    specimens.
       The Compound Microscope
   In the basic compound microscope, the object to
    be magnified is placed under the lower lens
    (objective lens) and the magnified image is viewed
    through the upper lens (eyepiece lens).
   The magnification of the image can be calculated
    by multiplying the magnifying power of the
    objective lens times the magnifying power of the
    eyepiece lens.
   The microscope is composed of a mechanical
    system which supports the microscope, and an
    optical system which illuminates the object under
    investigation and passes light through a series of
    lens to form an image of the specimen.
The principle of the compound microscope. The passage of light through
two lenses forms the virtual image of the object seen by the eye.
       The Compound Microscope
   The Mechanical System
      Base: the support.

      Arm: the C-shaped upright structure.

      Stage: the plate on which the specimens are placed.

      Body Tube: the hollow tube on which the objectives and

       eyepiece lenses are mounted.
      Coarse Adjustment: the knob used to focus the

       microscope lenses by moving the body tube.
      Fine Adjustment: the knob also used to focus the lenses

       by moving the body tube, but by a much smaller
       magnitude.
        The Compound Microscope
   The Optical System
       Illuminator: artificial light, usually supplied by a
        light bulb, to illuminate the specimen.
            Transmitted Illumination: when the light is directed
             up through the specimen from the base.
            Vertical or Reflected Illumination: when the light
             comes from above and reflects off the specimen.
       Condenser: lens system under the microscope
        stage that focuses light onto the specimen.
         The Compound Microscope
   The Optical System
      Objective Lens: the lens closest to the specimen;
       usually several objectives are mounted on a
       revolving nosepiece.
            Parafocal: when the microscope is focused with one
             objective in place, another objective can be rotated
             into place and the specimen remains very nearly in
             correct focus.
       Eyepiece or Ocular Lens: the lens closest to the
        eye.
            Monocular: a microscope having only one eyepiece
            Binocular: a microscope having two eyepieces.
       The Comparison Microscope
   The comparison microscope consists of two
    independent objective lenses joined together by an
    optical bridge to a common eyepiece lens.
   When a viewer looks through the eyepiece lens of
    the comparison microscope, the objects under
    investigation are observed side-by-side in a circular
    field that is equally divided into two parts.
   Modern firearms examination began with the
    introduction of the comparison microscope, with its
    ability to give the firearms examiner a side-by-side
    magnified view of bullets.
       The Stereoscopic Microscope
   The stereoscopic microscope is actually two
    monocular compound microscopes properly spaced
    and aligned to present a three-dimensional image
    of a specimen to the viewer, who looks through
    both eyepiece lenses.
   It is particularly useful for evidence not requiring
    very high magnification (10x–125x).
   Its large working distance makes it quite applicable
    for the microscopic examination of big, bulky items.
 Schematic
diagram of a
stereoscopic
microscope. This
microscope is
actually two
separate
monocular
microscopes,
each with its own
set of lenses
except for the
lowest objective
lens, which is
common to both
microscopes.
        Polarizing Microscopy
   Light that is confined to a single plane of vibration is
    said to be plane-polarized.
   The examination of the interaction of plane-polarized
    light with matter is made possible with the polarizing
    microscope.
   Polarizing microscopy has found wide applications for
    the study of birefringent materials; materials that split a
    beam of light in two, each with its own refractive index
    value.
   The determination of these refractive index data
    provides information that helps to identify minerals
    present in a soil sample or the identity of a man-made
    fiber.
Polarization of light.
       The Microspectrophotometer
   The microspectrophotometer is a
    spectrophotometer coupled with a light
    microscope.
   The examiner studying a specimen under a
    microscope can simultaneously obtain the visible
    absorption spectrum or IR spectrum of the material
    being observed.
   This instrument is especially useful in the
    examination of trace evidence, paint, fiber, and ink
    evidence.
       The Scanning Electron
       Microscope
   Finally, the scanning electron microscope (SEM)
    bombards a specimen with a beam of electrons
    instead of light to produce a highly magnified image
    from 100x to 100,0000x.
   Its depth of focus is some 300 times better than
    optical systems at similar magnification.
   The bombardment of the specimen’s surface with
    electrons normally produces X-ray emissions that
    can be used to characterize elements present in the
    material under investigation.
A schematic diagram of a scanning electron microscope displaying the
image of a gunshot residue particle. Simultaneously, an X-ray analyzer
detects and displays X-ray emissions from the elements lead (Pb),
antimony (Sb), and barium (Ba) present in the particle.

								
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