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Chapter 7
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
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
7-3
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
lenses 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
7-4
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.
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
The compound microscope
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
7-8
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 threedimensional 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
Polarizing Microscopy
• Light that is confined to a single plane of
vibration is said to be plane- polarized.
• The examination of the interaction of planepolarized 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
7-12
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
7-13
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
7-14