COMPOUND MAGNIFYING SYSTEMS: THE MICROSCOPE
The fundamental design of the microscope has not changed much since its original invention; improvements to nearly every component, however, have made even the most inexpensive microscopes suitable for basic applications. In this section, refer to Figure 4.6 for a diagram of the important parts of a microscope. Starting at the top, the eyepiece or ocular is the lens that the observer looks into when viewing an object microscopically. A microscope may be monocular, having one eyepiece, or binocular, having two eyepieces; most microscopes found in laboratories today are binocular. Many microscopes today are trinocular; they have an eyepiece that accommodates a video or digital camera. Typically, the eyepiece(s) will have a magnification of 10× and may be focusable; this allows the viewer to adjust the eyepieces if one eye is stronger than the other. The area seen when looking through the eyepieces is called the field of view and will change if the specimen is moved or the magnification is changed.
FIGURE 4.6 The various parts of the microscope.
FIGURE 4.7 Courtesy Olympus USA. The objective lens, so called because it is closest to the object or specimen being viewed. The objective is the most important part of the microscope and comes in many types and magnifications. The information on the lens is very specific: “10×” is the magnification, “0.25na” is the numerical aperture, “170 mm” is the tube length (some objectives are now infinity-corrected and are labeled “∞”), and “0.17 mm” is the recommended thickness of cover slip to use.
The next lens in the microscope is called the objective lens (or just the objective) because it is closest to the object or specimen being studied. The objective is the most important part of the microscope. Objectives come in many types (see “In More Detail: Lens Corrections”) and magnifications (typically, 4×, 10×, 15×, 20× and 25×; higher magnifications are possible). Each objective will have information about it engraved into its body in a specific format, as shown in Figure 4.7. Although the information may vary by manufacturer, objectives will usually have the magnification, the numerical aperture (NA), the tube length, and the thickness of coverslip that should be used with the objective. The numerical aperture is an angular measure of the lens’s light-gathering ability and, ultimately, its resolving quality, as shown in Figure 4.8 (see “In More Detail: Why Resolution Is More Important Than Magnification”). The tube length is the distance from the lowest part of the objective to the upper edge of the eyepiece; this has been standardized at 160 mm in modern micro scopes. Because the tube length determines where the in-focus image will appear, objectives must be designed and constructed for a specific tube length (however, read about “infinity-corrected” lenses in “In More Detail: Lens Corrections”). Coverslips, the thin glass plates that are placed on top of mounted specimens, protect the specimen and the objective from damage. They come in a range of thicknesses measured in millimeters (0.17 mm, for example). All of this information is important to the microscopist’s proper use of a particular objective.
The microscope stage is the platform where the specimen sits during viewing. The stage can be moved up or down to focus the specimen image, meaning that portion of the specimen in the field of view is sitting in the same horizontal plane; typically, stages are equipped with a coarse and fine focus. Stages may be mechanical (that is, having knobs for control of movement), rotating (able to spin in 360° but not move back and forth), or both The condenser is used to obtain a bright, even field of view and improve image resolution. Condensers are lenses below the stage that focus or condense the light onto the specimen field of view. Condensers also have their own condenser diaphragm control to eliminate excess light and adjust for contrast in the image. The condenser diaphragm is different from the field diaphragm, a control that allows more or less light into the lens system of the microscope. The illumination of the microscope is critical to a quality image and is more complicated than merely turning on a light bulb. Two main types of illumination are used in microscopy, critical and Köhler. Critical illumination concentrates the light on the specimen with the condenser lens; this produces an intense lighting that high lights edges but may be uneven. Köhler illumination, named after August Köhler in 1893, sets the light rays parallel throughout the lens system, allowing them to evenly illuminate the specimen. Köhler illumination is considered the standard setup for microscopic illumination (Davidson and Abromowitz, 2005).
FIGURE 4.8 The numerical aperture is an angular measure of the lens’ light-gathering ability. It is an indication of the lens’ resolving power.
