Page 23 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

3. Objective lenses: Initial magnification of your specimen occurs here. Most

brightfield light microscopes have 3 or 4 objective lenses seated into the resolving

nose piece base; 4X, Low (10x), High (40x), and Oil Immersion (100x)

4. Coarse focusing knob: larger of the two knobs, the coarse adjustment knob moves

the stage up or down to bring the specimen into focus. It is very sensitive, even

small partial rotation of this knob can bring about a big change in the vertical

movement of the stage. ONLY use coarse focusing at the beginning with the 4X,

10X low powered objectives in place. If you use it with the higher powered

objectives, it can damage the objective if you crash the lens through your glass

specimen slide.

5. Fine focusing knob: smaller of the two knobs, fine adjustment knob brings the

specimen into sharp focus under low power and is used for all focusing when using

high power lenses such as the 100x oil immersion lens.

6. Stage & Mechanical stage: The horizontal surface where you place the

slide specimen is called the stage. The slide is held in place by spring loaded clips

and moved around the stage by turning the geared knobs on the mechanical stage.
The mechanical stage has two perpendicular scales that can be used to record the

position of an object on a slide, useful to quickly relocate an object.

7. Illuminator: contains the light source, a lamp made either of an incandescent

tungsten-halogen bulb or an LED. There is normally a switch to turn on/off or

a rheostat located on the side that you can use to adjust the brightness of the light.

General Microbiology & Immunology (PM 401) Practical note Page 12