Download The Compound Microscope

Chapter 7
THE MICROSCOPE
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 3
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 4
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 5
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 6
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-byside magnified view of bullets.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 7
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 8
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7- 9
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7-10
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7-11
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7-12
Figure 7–4 Optics of the compound microscope. Courtesy Leica
Microsystems, Buffalo, N.Y., www.leica-microsystems.com.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7-13
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.
Criminalistics, 10e
Richard Saferstein
© 2011, 2007, 2004, 2001, 1998, 1995 Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
7-14