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Lesson 1 - Lab 1 - The Use of the Compound Light Microscope

Student Performance Objectives
1. Focus on a slide of stained blood cells under both low and high-dry power.
2. Identify from a diagram, the parts of a compound light microscope.
3. Explain the concepts of field of view with different power objectives, working distance,
    and depth-of-field.

Lesson Outline
A. Compound Light Microscope basic parts and their functions
    1. Proper way to carry a microscope: always use two hands - one hand gripping the arm and the other supporting the microscope from its bottom surface. Note that if turned upside down, the ocular lenses can fall out so never swing a microscope.
    2. Ocular and objective lenses: the lenses that your eye directly looks through are the ocular lenses of a microscope - there are two on a binocular microscope and only one on a monocular scope. The lens closest to the specimen on the microscope's stage is the objective lens. The objective lens is usually attached to a revolving turret that allows one to move and "click" the chosen objective lens into place. The different objective lenses have different magnifying powers - usually 4x, 10x, 43x, and 100x. The ocular lens has a fixed power of usually 10x. The total magnification when looking at a specimen is the multiplication of the power of the ocular lens and the objective lens. E.g., if the 43x objective lens is swiveled into place, then the total magnification is 430X, 10x times 43x. The 100x objective is seldom used in the anatomy laboratory as it requires the placement of a drop of oil on the slide and for the objective to be immersed in the oil during viewing of the specimen. The oil immersion, 100x objective is used extensively in the microbiology laboratory.
    3. Light source, condenser, diaphragm, and mechanical stage. An electric light at the base of the microscope provides the source of illumination. A condenser focuses the light onto the stage where the opening is and where the slide with the specimen is located. Between the condenser and the stage is the diaphragm that controls the amount of light reaching the stage from the condenser. The diaphragm is opened and closed manually by means of a control lever, just under the stage and attached to the diaphragm. The stage has manual control knobs that allow precise movement of the stage as a whole. This allows for precise viewing of any region of the specimen on the slide being observed.
    4. Working distance and the parfocal concept.
         a. Working distance - The distance between the bottom of the objective lens and the slide on the stage is called the working distance. With more and more powerful objectives swiveled into place, the working distance becomes less and less. Under the "high,dry" power - 43ox, there is only a tiny distance between the objective and the slide. For this reason, when observing a specimen at 430x, you should only use the fine adjustment focusing knob. The coarse adjustment focusing knob may be used at 40x and 100x.
         b. Parfocal - the microscopes in the anatomy and physiology laboratory are parfocal. This means that once you have focused on an object under low power, the object will be in focus, or very close to it, when you swivel the next, more powerful objective lens into place. You do not need to raise the microscope's tube up with the coarse adjustment, change objectives, and then refocus
. This concept does not hold for the oil immersion objective and it is best not to use the 100x objective unless your laboratory professor gives you specific instructions on the use of the oil immersion lens.
    5. Correct posture when using the microscope. Your back should be straight and you should use both eyes when looking through the binocular microscopes provided to you in the A&P laboratory. (You should not have a curved back and be squinting through one eye). To do this you may need to adjust the height of your laboratory seats and also adjust the distance between the two ocular lenses on the microscope by compressing them medially or moving them laterally.

Your laboratory instructor will guide you through the remaining exercises utilizing prepared slides:        
B. Letter e slide for basic orientation to microscopic visual fields.
C. Silk thread slides for basic depth-of-field concept at varying powers.
D. Observation of prepared slides: blood smear: erythrocyte and leukocyte observation.

E. Optional Internet assignment: report of the following different microscope types:
    1. Phase contrast microscope- concept and website: 
2. Electron microscope (scanning and transmission) - concept and website:

Biomedical Terminology:

depth of field
electron microscope
field of view
light microscope
mechanical stage
objective lens
ocular lens
phase contrast microscope

working distance