objective lens varies according to which of the three objectives is in focusing position. Thus, the intervening distance with the low-power objective (10X) is the greatest (16 mm), that for the oilimmersion lens (100X) is the smallest (1.8 mm), and that for the high-power objective (45X) is intermediate (4 mm). As a result, the focusing operation must be conducted with skill to avoid damage to the objective lens, the specimen, or both. It is good practice to obtain a focus with the low-power objective first, then change to the higher objective required. Most modern microscopes are equipped with parfocal objectives, which means 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. Seated behind the microscope, 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 manner that the low-power (green) objective cannot be lowered to make contact with the object on the stage.
3. While looking through the ocular, 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. Of course, light adjustment must be made; open the iris diaphragm of the condenser to accommodate more light.
5. 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. After focusing with the highpower 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.
The microscope is an expensive and delicate instrument that should be given proper care.
Moving or transporting the microscope should be done 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.
Make sure the microscope is kept clean at all times; when not in use, enclose it in a dustproof cover or store in its case. Remove dust with a camel hair brush. Lenses may be wiped carefully with lens tissue.
When the oil immersion lens is not being used, remove the oil with lens tissue. Use oil solvents, such as xylol, on lenses only when required to remove dried oil and only in the minimal amount necessary. Never use alcohol or similar solvent to clean lenses.
The complete blood count consists of:
The red blood cell count is made to determine the number of red cells in one cubic millimeter