Page 26 - MNUmicrobiology practical 2025

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

a) Oil immersion lenses like this one are used to improve resolution.

(b) Because immersion oil and glass have very similar refractive indices, there is a
minimal amount of refraction before the light reaches the lens. Without immersion oil,

light scatters as it passes through the air above the slide, degrading the resolution of
the image.

How to use a Microscope

FOCUSING:

1. Use one of the pre-made, Gram-stained, bacterial slides.

2. Make sure the condenser is all the way up and the iris diaphragm is open all the way,

letting the maximum amount of light contact your slide.

3. Always start at 4X, stage lowered, focus with coarse focus knob first.

4. Once in focus move to 10X and focus using the fine focus knob.

5. Once in focus move to 40X and focus using the fine focus knob.

General Microbiology & Immunology (PM 401) Practical note Page 15