Transcript Document

THE MICROSCOPE
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-1
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-2
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-3
Figure 7–2 The principle of the compound microscope. The passage of light
through two lenses forms the virtual image of the object seen by the eye.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-4
The Compound Microscope
• The Mechanical System
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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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-5
The Compound Microscope
• The Optical System
– Illuminator: artificial light, usually supplied
by a lightbulb, 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-6
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-7
Figure 7–4 Optics of the compound microscope. Courtesy Leica
Microsystems, Buffalo, N.Y., www.leica-microsystems.com
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-8
The compound microscope
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-9
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-10
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-11
Figure 7–8 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-12
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-13
Figure 7–9 Polarization of light.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-14
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-15
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-16
Figure 7–16 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. Courtesy Aerospace
Corp., El Segundo, Calif.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-17