being used. For example, the high-powered objective of short focal length (4 mm) will need the stage raised so the objective is very close to the specimen, while the low-powered objective of a longer focal length (16 mm) will need the stage lowered so the objective is farther from the specimen. The coarse control knob is used initially to bring the specimen’s image into approximate focus. Once this is accomplished, the fine control knob sharpens the image. Coarse Control Knob The coarse control knob is the larger and inner knob. Rotating the coarse control knob allows the image to appear in approximate focus. Fine Control Knob The fine control knob is the smaller and outer knob. Rotating this control knob renders the image clear and well-defined. FOCUSING THE MICROSCOPE The process of focusing consists of adjusting the relationship between the optical system of the microscope and the object to be examined so that a clear image of the object is obtained. The distance between the upper surface of the glass slide on the microscope stage and the faces of the objective lens varies depending upon which of the three objectives is in the focusing position. It is a good practice to obtain a focus with the low-power objective first, then change to the higher objective required to avoid accidentally damaging the objective lens, the specimen, or both. Most modern microscopes are equipped with parfocal objectives (meaning that if one objective is in focus, the others will be in approximate focus when the nosepiece is revolved). With the low-power objective in focusing position, observe the following steps in focusing. 1. Seat yourself behind the microscope, then lower your head to one side of the microscope until your eyes are approximately at the level of the stage. 2. Using the coarse adjustment knob, lower the body tube until the face of the objective is within 1/4 inch of the object. Most microscopes are constructed in such a way that the low-power (green) objective cannot be lowered and make contact with the object on the stage. 3. While you are looking through the ocular, you should use the coarse adjustment knob to elevate the body tube until the image becomes visible. Then use the fine adjustment knob to obtain a clear and distinct image. Do not move the focusing knob while changing lenses. 4. If the high-power objective (yellow) is to be used next, bring it into position by revolving the nosepiece (a distinct “click” indicates it is in proper alignment with the body tube). Use the fine adjustment knob only to bring the object into exact focus. 5. If specimen is too dark, you can increase lighting by opening the iris diaphragm of the condenser. 6. The oil-immersion objective (red) is used for detailed study of stained blood and bacterial smears. Remember that the distance between objective lens and object is very short, and great care must be employed so the specimen is not damaged. After focusing with the high-power objective and scanning for well-defined cells, raise the objective, place a small drop of immersion oil, free of bubbles, on the slide, centering the drop in the circle of light coming through the condenser. Next, revolve the nosepiece to bring the oil-immersion objective into place, and, by means of the coarse adjustment knob, slowly lower the body tube until the lens just makes contact with the drop of oil on the slide. The instant of contact is indicated by a flash of light illuminating the oil. The final step in focusing is done with the fine adjustment knob. It is with this lens in particular that lighting is important. The final focus, clear and well-defined, will be obtained only when proper light adjustment is made. CARE OF THE MICROSCOPE The microscope is an expensive and delicate instrument that should be given proper care. Moving or transporting microscopes should be accomplished by grasping the arm of the scope in one hand and supporting the weight of the scope with the other hand. Avoid sudden jolts and jars. Keep the microscope clean at all times; when not in use, microscopes should be enclosed in a dustproof cover or stored in their case. Remove dust with a camel hair brush. Lenses may be wiped carefully with lens 7-9