Mechanical Stage The mechanical (movable) stage holds the specimen in place and is the means by which the specimen may be moved about on the stage. Base The base is the structure on which the microscope rests. ILLUMINATION SYSTEM Ideal illumination of a specimen viewed under the microscope requires even light distribution. The objectives must also be entirely filled with light from the condenser. To fulfill these requirements, the illumination system of the compound microscope consists of three parts: an internal light source, a condenser, and an iris diaphragm. See figure 7-5. Internal Light Source The internal light source is built into the base of the microscope. It provides a precise and steady source of light into the microscope. Condenser The condenser is composed of a compact lens system and is located between the light source and stage. The condenser concentrates and focuses light from the light source directly through the specimen. Iris Diaphragm An iris diaphragm located on the condenser controls the diameter of the light source’s beam. To improve resolution, the operator should adjust the opening of the iris diaphragm to approximately the same size as the face of the objective lens. In addition to the diaphragm on the condenser, an iris diaphragm may be located on the internal light source. This iris diaphragm controls the amount of light sent to the condenser from the internal light source. MAGNIFICATION SYSTEM The magnification system of the compound microscope contains at least two lens systems. The two lens systems are mounted on either end of a tube called the body tube. The lens nearest the object is called the objective lens, and the lens nearest the eye is the ocular lens or eye piece. See figure 7-5. Objective Lenses On a compound microscope, there is usually a set of three objective lenses (or “objectives”). This set of objectives is the component most responsible for the magnification and resolution of detail in a specimen. Each objective lens has a different focus distance and magnification power. A set of objectives normally consists of a low-power lens (approximate focus 16 mm, magnification 10X), a high-power lens (approximate f o c u s 4 m m , m a g n i f i c a t i o n 4 5 X ) , a n d a n oil-immersion lens (approximate focus 1.8 mm, magnification 100X). Objective lenses are color coded for easy recognition: 16 mm-10X (green), 4 mm-45X (yellow), and 1.8 mm-100X (red). Revolving Nosepiece The revolving nosepiece contains openings into which objective lenses are fitted, and revolves objectives into desired position. Body Tube The body tube is a tube that permits light to travel from the objective to the ocular lens. Ocular Lenses Ocular lenses, or eyepieces, are located on top of the body tube and usually have a magnification power of 10X. To calculate the total magnification of a specimen, you multiply the magnification power of the objective by the magnification power of the ocular lens. Examples of total magnifications are provided in table 7-1. FOCUSING SYSTEM Focusing is accomplished by moving the stage up or down with the coarse and fine control knob (fig. 7-5). Whether the stage needs to be raised or lowered depends on the focal length of the objective 7-8 Objective Lens Color Code 10X Ocular Total Magnification 16 mm-10X 4 mm-45X 1.8 mm-100X Green Yellow Red 10X 10X 10X 100X 450X 1000X Table 7-1.—Examples of Total Magnifications