The Microscope

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The Microscope

• Chapter 7

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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.

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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.

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The principle of the compound microscope. The
passage of light through two lenses forms the
virtual image of the object seen by the eye.
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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.

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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.

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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.

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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.

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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 (10x125x).
• Its large working distance makes it quite
  applicable for the microscopic examination of
  big, bulky items.

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 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.
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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.

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Polarization of light.
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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.

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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 specimens surface with
  electrons normally produces X-ray emissions that
  can be used to characterize elements present in
  the material under investigation.

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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.