<CHAP NUM="7" ID="CH.00.007">chapter 7
<FM><TTL>The Microscope</TTL>
<KTSET><TTL>Key Terms</TTL>
<KT>binocular</KT>
<KT>condenser</KT>
<KT>depth of focus</KT>
<KT>eyepiece lens</KT>
<KT>field of view</KT>
<KT>microspectrophotometer</KT>
<KT>monocular</KT>
<KT>objective lens</KT>
<KT>parfocal</KT>
<KT>plane-polarized light</KT>
<KT>polarizer</KT>
<KT>real image</KT>
<KT>transmitted illumination</KT>
<KT>vertical or reflected illumination</KT>
<KT>virtual image</KT></KTSET>
<OBJSET><TTL>Learning Objectives</TTL>
<P>After studying this chapter you should be able to:
<OBJ><P><INST>< </INST>List and understand the parts of the compound microscope</P></OBJ>
<OBJ><P><INST>< </INST>Define magnification, field of view, working distance, and depth of focus</P></OBJ>
<OBJ><P><INST>< </INST>Contrast the comparison and compound microscopes</P></OBJ>
<OBJ><P><INST>< </INST>Understand the theory and utility of the stereoscopic microscope</P></OBJ>
<OBJ><P><INST>< </INST>Appreciate how a polarizing microscope is designed to characterize polarized light</P></OBJ>
<OBJ><P><INST>< </INST>Appreciate how a microspectrophotometer can be used to examine trace physical evidence</P></OBJ>
<OBJ><P><INST>< </INST>Compare and contrast the image formation mechanism of a light microscope to that of a scanning electron microscope</P></OBJ>
<OBJ><P><INST>< </INST>Outline some forensic applications of the scanning electron microscope</P></OBJ></P></OBJSET></FM>
<CASE NUM="1" TY="CS"><TTL>The Lindbergh Baby Case</TTL>
<P>On the evening of March 1, 1932, a kidnapper crept up his homemade ladder and stole the baby of Charles and Anne Lindbergh directly from the second-floor nursery of their house in Hopewell, New Jersey. The only evidence of his coming was a ransom note, the ladder, a chisel, and the tragic absence of the infant. A couple of months later, though the $50,000 ransom had been paid, the baby turned up dead in the woods a mile away. There was no additional sign of the killer. Fortunately, when finally studied by wood technologist Arthur Koehler, the abandoned ladder yielded some important investigative clues (see case study on page 198).</P>
<P>By studying the types of wood used and the cutter marks on the wood, Koehler ascertained where the materials might have come from and what specific equipment was used to create them. Koehler traced the wood from a South Carolina mill to a lumberyard in the Bronx, New York. Unfortunately the trail went cold, as the lumberyard did not keep sales records of purchases. The break in the case came in 1934, when Bruno Richard Hauptmann paid for gasoline with a bill that matched a serial number on the ransom money. Koehler showed that microscopic markings on the wood were made by a tool in Hauptmann’s possession. Ultimately, handwriting analysis of the ransom note clearly showed it to be written by Hauptmann.</P></CASE>
<BM><P>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 in crime laboratories relied almost solely on the microscope to study the structure and composition of matter. Even the advent of modern analytical instrumentation and techniques has done little to diminish the usefulness of the microscope for forensic analysis. If anything, the development of the powerful scanning electron microscope promises to add a new dimension to forensic science heretofore unattainable within the limits of the ordinary light microscope.</P>
<P>The earliest and simplest microscope was the single lens commonly referred to as a <ITAL>magnifying glass</ITAL>. The handheld magnifying glass makes things appear larger than they are because of the way light rays are refracted, or bent, in passing from the air into the glass and back into the air. The magnified image is observed by looking through the lens, as shown in <LINK LINKEND="FG.07.001">Figure <FIGIND NUM="1" ID="FG.07.001"/>7–1</LINK>. Such an image is known as a <KT>virtual image</KT><SIDEIND NUM="1" ID="MN2.07.001"/>; it can be seen only by looking through a lens and cannot be viewed directly. This is distinguished from a <KT>real image</KT><SIDEIND NUM="2" ID="MN2.07.002"/>, which can be seen directly, like the image that is projected onto a motion picture screen.</P>
<P>The ordinary magnifying glass can achieve a magnification of about 5 to 10 times. Higher magnifying power is obtainable only with a <ITAL>compound microscope,</ITAL> constructed of two lenses mounted at each end of a hollow tube. The object to be magnified is placed under the lower lens, called the <KT>objective lens</KT><SIDEIND NUM="3" ID="MN2.07.003"/>, and the magnified image is viewed through the upper lens, known as the <KT>eyepiece lens</KT><SIDEIND NUM="4" ID="MN2.07.004"/>. As shown in <LINK LINKEND="FG.07.002">Figure <FIGIND NUM="2" ID="FG.07.002"/>7–2</LINK>, the objective lens forms a real, inverted, magnified image of the object. The eyepiece, acting just like a simple magnifying glass, further magnifies this image into a virtual image, which is what is seen by the eye. The combined magnifying power of both lenses can produce an image magnified up to 1,500 times.</P>
<P>The optical principles of the compound microscope are incorporated into the basic design of different types of light microscopes. The microscopes most applicable for examining forensic specimens are as follows:
<NL><ITEM><P><INST>1. </INST>The compound microscope</P></ITEM>
<ITEM><P><INST>2. </INST>The comparison microscope</P></ITEM>
<ITEM><P><INST>3. </INST>The stereoscopic microscope</P></ITEM>
<ITEM><P><INST>4. </INST>The polarizing microscope</P></ITEM>
<ITEM><P><INST>5. </INST>The microspectrophotometer</P></ITEM></NL></P>
<P>After describing these five microscopes, we will talk about a completely different approach to microscopy, the scanning electron microscope (SEM). This instrument focuses a beam of electrons, instead of visible light, onto the specimen to produce a magnified image. The principle and design of this microscope permit magnifying powers as high as 100,000 times.</P>
<H1><SCAP></SCAP>The Compound Microscope</H1>
<P>The parts of the compound microscope are illustrated in <LINK LINKEND="FG.07.003a">Figure <FIGIND NUM="3" ID="FG.07.003a"/>7–3(a)</LINK>. Basically, this microscope consists of a mechanical system, which supports the microscope, and an optical system. The optical system illuminates the object under investigation and passes the light through a series of lenses to form an image of the specimen on the retina of the eye. The optical path of light through a compound microscope is shown in <LINK LINKEND="FG.07.003b">Figure <FIGIND NUM="3" ID="FG.07.003b"/>7–3(b)</LINK>.</P>
<P>The mechanical system is composed of six parts:
<UL><ITEM><TTL>Base (1).<INST> </INST></TTL><P>The support on which the instrument rests.</P></ITEM>
<ITEM><TTL>Arm (2).<INST> </INST></TTL><P>A C-shaped upright structure, hinged to the base, that supports the microscope and acts as a handle for carrying.</P></ITEM>
<ITEM><TTL>Stage (3).<INST> </INST></TTL><P>The horizontal plate on which the specimens are placed for study. The specimens are normally mounted on glass slides that are held firmly in place on the stage by spring clips.</P></ITEM>
<ITEM><TTL>Body tube (4).<INST> </INST></TTL><P>A cylindrical hollow tube on which the objective and eyepiece lenses are mounted at opposite ends. This tube merely serves as a corridor through which light passes from one lens to another.</P></ITEM>
<ITEM><TTL>Coarse adjustment (5).<INST> </INST></TTL><P>This knob focuses the microscope lenses on the specimen by raising and lowering the body tube.</P></ITEM>
<ITEM><TTL>Fine adjustment (6).<INST> </INST></TTL><P>The movements effected by this knob are similar to those of the coarse adjustment but are of a much smaller magnitude.</P></ITEM></UL></P>
<P>The optical system is made up of four parts:
<UL><ITEM><TTL>Illuminator (7).<INST> </INST></TTL><P>Most modern microscopes use artificial light supplied by a lightbulb to illuminate the specimen being examined. If the specimen is transparent, the light is directed up toward and through the specimen stage from an illuminator built into the base of the microscope. This is known as <KT>transmitted illumination</KT><SIDEIND NUM="5" ID="MN2.07.005"/>. When the object is opaque—that is, not transparent—the light source must be placed above the specimen so that it can reflect off the specimen’s surface and into the lens system of the microscope. This type of illumination is known as <KT>vertical</KT> or <KT>reflected illumination</KT><SIDEIND NUM="6" ID="MN2.07.006"/>.</P></ITEM>
<ITEM><TTL>Condenser (8).<INST> </INST></TTL><P>The <KT>condenser</KT><SIDEIND NUM="7" ID="MN2.07.007"/> collects light rays from the base illuminator and concentrates them on the specimen. The simplest condenser is known as the <ITAL>Abbé condenser</ITAL>. It consists of two lenses held together in a metal mount. The condenser also includes an iris diaphragm that can be opened or closed to control the amount of light passing into the condenser.</P></ITEM>
<ITEM><TTL>Objective lens (9).<INST> </INST></TTL><P>This is the lens positioned closest to the specimen. To facilitate changing from one objective lens to another, several objectives are mounted on a revolving nosepiece or turret located above the specimen. Most microscopes are <KT>parfocal</KT><SIDEIND NUM="8" ID="MN2.07.008"/>, meaning that when the microscope is focused with one objective in position, the other objective can be rotated into place by revolving the nosepiece while the specimen remains very nearly in correct focus.</P></ITEM>
<ITEM><TTL>Eyepiece or ocular lens (10).<INST> </INST></TTL><P>This is the lens closest to the eye. A microscope with only one eyepiece is <KT>monocular</KT><SIDEIND NUM="9" ID="MN2.07.009"/>; one constructed with two eyepieces (one for each eye) is <KT>binocular</KT><SIDEIND NUM="10" ID="MN2.07.010"/>.</P></ITEM></UL></P>
<P>Each microscope lens is inscribed with a number signifying its magnifying power. The image viewed by the microscopist will have a total magnification equal to the product of the magnifying power of the objective and eyepiece lenses. For example, an eyepiece lens with a magnification of 10 times (10´) used in combination with an objective lens of 10 times has a total magnification power of 100 times (100´). Most forensic work requires a 10´ eyepiece in combination with either a 4´, 10´, 20´, or 45´ objective. The respective magnifications will be 40´, 100´, 200´, and 450´.</P>
<P>In addition, each objective lens is inscribed with its numerical aperture (N.A.). The ability of an objective lens to resolve details into separate images instead of one blurred image is directly proportional to the numerical aperture value of the objective lens. For example, an objective lens of N.A. 1.30 can separate details at half the distance of a lens with an N.A. of 0.65. The maximum useful magnification of a compound microscope is approximately 1,000 times the N.A. of the objective being used. This magnification is sufficient to permit the eye to see all the detail that can be resolved. Any effort to increase the total magnification beyond this figure will yield no additional detail and is referred to as ...
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