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Microscopy in Forensic Science - Chapter 8 The Microscope

Introduction to the Microscope

  • 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.
  • Virtual image: The magnified image seen by looking through a lens.
  • Real image: An image viewed directly.
  • The object to be magnified is placed under the lower (objective) lens, and viewed through the upper lens (eyepiece).
  • Various types of microscopes are used to analyze forensic specimens.

Basic Concepts: Images and Light

  • The passage of light through two lenses forms the virtual image of the object seen by the eye. This is illustrated in Figure 8–2: The Principle of the Compound Microscope.
  • Objective lens creates a real, enlarged image which is viewed through the eyepiece to form a virtual image for the eye.

The Compound Microscope (Overview)

  • 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).
  • Magnification is calculated by multiplying the magnifying power of the objective lens times the magnifying power of the eyepiece lens:
  • M = M{ ext{objective}} \times M{ ext{eyepiece}}

The Compound Microscope: Structure and Systems

  • The microscope has two main systems:
    • Mechanical system: supports the microscope (base), arm, stage, etc.
    • Optical system: illuminates the object and passes light through lenses to form an image.

The Compound Microscope (Mechanical System)

  • Base: the support.
  • Arm: the C-shaped upright structure.
  • Stage: the plate on which specimens are placed.

The Compound Microscope (Optical System)

  • Body Tube: hollow tube on which the objectives and eyepiece lenses are mounted.
  • Coarse Adjustment: knob used to focus the lenses by moving the body tube.
  • Fine Adjustment: knob used to focus with smaller movements for precision.
  • Illuminator (Optical System): artificial light (usually a light bulb) to illuminate the specimen.
  • Transmitted Illumination: light directed up through the specimen from the base.
  • Vertical/Reflected Illumination: light from above reflects off the specimen.

The Condenser and Illumination

  • Condenser: lens system under the microscope stage that focuses light onto the specimen.
  • Objective Lens: the lens closest to the specimen; several objectives are mounted on a revolving nosepiece.
  • Eyepiece (Ocular Lens): the lens closest to the eye; can be monocular (one eyepiece) or binocular (two eyepieces).
  • 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.

The Comparison Microscope (Overview)

  • The comparison microscope consists of two independent objective lenses joined together by an optical bridge to a common eyepiece lens.
  • When looking through the eyepiece, 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, enabling side-by-side magnified views of bullets.
  • Figure 8–5 shows: The Comparison Microscope—Two Independent Objective Lenses Joined Together by an Optical Bridge.

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.
  • Particularly useful for evidence not requiring very high magnification (10×–125×).
  • Large working distance makes it applicable for examining big, bulky items.
  • Figure 8–8 schematically diagrams a stereoscopic microscope (two separate monocular microscopes sharing a common objective).

Polarizing Microscopy

  • Polarizing microscopy uses plane-polarized light: light confined to a single plane of vibration.
  • The polarizing microscope enables the study of birefringent materials—materials that split a beam of light into two rays, each with its own refractive index.
  • Determining refractive index data helps identify minerals in soil samples or the identity of man-made fibers.

The Microspectrophotometer

  • The microspectrophotometer is a spectrophotometer coupled with a light microscope.
  • It allows the examiner to obtain the visible absorption spectrum or IR spectrum of the material being observed while viewing it under the microscope.
  • Particularly useful for trace evidence, paint, fiber, and ink evidence.

The Scanning Electron Microscope (SEM)

  • The SEM bombards a specimen with a beam of electrons instead of light to produce a highly magnified image from 100x to 1000000x.
  • Its depth of focus is roughly 300 times better than optical systems at similar magnification.
  • Bombarding the surface with electrons normally produces X-ray emissions that can be used to characterize elements present in the material under investigation.
  • A schematic diagram (not shown here) of an SEM display shows how an image is formed and how X-ray analysis can be attached to element detection.
  • Example: Gunshot residue particle analysis using SEM/EDS to identify elements such as lead (Pb), antimony (Sb), and barium (Ba).
  • A schematic diagram caption mentions: A Scanning Electron Microscope displaying the image of a gunshot residue particle and X-ray emissions for Pb, Sb, and Ba.

Primer Residue on Hands

  • The firing of a weapon propels residues toward the target, but also blows gunpowder and primer residues back toward the shooter.
  • Traces of these residues are often deposited on the shooter’s firing hand, and detecting these residues can provide information about whether an individual has recently fired a weapon.
  • With SEM, examiners measure the amount of barium (Ba) and antimony (Sb) on relevant portions of the suspect’s hands (e.g., the thumb web, back of the hand, palm).
  • SEM can also characterize the morphology of particles containing Ba and Sb to help determine whether a person fired, handled a weapon, or was near a discharged firearm.

Forensic Palynology

  • Forensic palynology involves the collection and examination of pollen and spores connected with crime scenes, illegal activities, or terrorism.
  • The microscope is the principal tool used in forensic palynology.
  • The information gained from pollen/spore analysis has several uses:
    • Link a suspect or object to the crime scene or the victim.
    • Prove or disprove a suspect’s alibi.
    • Include or exclude suspects.
    • Track the previous whereabouts of some item or suspect.
    • Indicate the geographical origin of some items.

Connections, Implications, and Practical Notes

  • The choice of microscopy technique depends on the evidence type, required magnification, depth of field, and whether 3D structure or elemental composition is needed.
  • Polarizing and birefringence analysis helps identify minerals and fibers that would be indistinguishable under normal light.
  • SEM with EDS (X-ray analysis) provides elemental composition, enabling identification of materials in trace evidence (e.g., GSR particles containing Pb, Sb, Ba).
  • The stereoscopic microscope is valuable for handling and inspecting large, non-miniature items before high-magnification analysis.
  • The microspectrophotometer integrates chemical/spectral information with visual context, enabling correlation between color and composition in evidence like paints and inks.
  • Palynology offers probabilistic, not absolute, connections. pollen and spores can migrate, be ubiquitous, or contaminate samples; interpretations should consider geography, timing, and environmental context.
  • Practical implications include the need for proper sample collection, chain of custody, contamination control, and cautious interpretation of evidence in light of limitations and probabilistic reasoning.

Quick Reference Terms

  • Virtual image: The magnified image seen by looking through a lens.
  • Real image: An image that can be viewed directly.
  • Plane-polarized light: Light confined to a single plane of vibration.
  • Birefringence: A property of materials that split a light beam into two rays with different refractive indices.
  • Refractive index: A measure of how much a material bends light.
  • Parafocal: Focusing is maintained when switching objectives.
  • Objection/eyepiece: The lenses through which the observer views the specimen.
  • Transmitted illumination: Light that passes upward through the specimen.
  • Vertical/Reflected illumination: Light that shines on the specimen from above.
  • Condenser: Focuses light onto the specimen.
  • SEM: Scanning Electron Microscope, uses electrons and detects X-rays for elemental analysis.
  • EDS: Energy-dispersive X-ray spectroscopy, often paired with SEM for elemental identification.
  • Forensic palynology: Study of pollen and spores to connect evidence to places/teams, or to establish alibis and origins.

Notes on Figures and Terminology

  • Figure 8–2: The Principle of the Compound Microscope – light passes through two lenses to form a virtual image.
  • Figure 8–4: Optics of the Compound Microscope – depicts components in the optical path.
  • Figure 8–5: The Comparison Microscope—Two Independent Objective Lenses Joined Together by an Optical Bridge.
  • Figure 8–8: Schematic Diagram of a Stereoscopic Microscope.