3: Microscope

Last updated

Mar 12, 2025
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- 2: Basic Lab Equipment
- 4: Medical Terminology

Kari Christenson, Western Technical College

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Learning Objective

Course Competency 5: Use basic lab equipment.

Introduction

An important piece of equipment used in several departments in a clinical laboratory is the microscope. In the hematology department, microscopes are used to examine stained blood smears. Stained microorganisms are examined in the microbiology department. In urinalysis, the microscope is used to view urine sediment. The microscope is a delicate and expensive piece of equipment, so care must be taken to use it properly.

Clinical microscopes are light microscopes. The microscope uses bright light to illuminate through the specimen and provides an inverted image at high magnification and resolution. There are two lenses that magnify the image of the specimen – the objective lens on the nosepiece and the ocular lens (or eyepiece). To determine the total magnification of the specimen, you must multiply the objective lens magnification with the ocular lens magnification.

When viewing unstained cells, the use of phase-contrast is helpful. A compound brightfield microscope can be fitted with a special objective and phase condenser. In phase contrast microscopy, the background looks grey, and the specimen is bright. In the lab, urine sediment is visualized using phase contrast.

A third type of microscope, called an epi-fluorescent microscope, can also be used in clinical labs. These microscopes use ultraviolet light to view the specimen. The objects need to be stained with a fluorescent dye in order to visualize them.

Microscope Parts

This is a complex image depicting a microscope with the parts labeled. — Figure 1. Labeled parts of a microscope.

Oculars

Clinical microscopes are binocular, which means they have two eyepieces. They are designed for the user to utilize both eyes when viewing the specimen. This helps to prevent eyestrain. The eyepieces, or oculars, are attached to a tube that is connected to the arm of the microscope. There is a lens in each ocular. Typically, the magnification of the ocular lens is 10x.

Objective Lenses

On the underside of the microscopes arm, is a revolving nosepiece. This part of the microscope holds the objective lenses. The three most common objective lenses used in clinical labs are the low power, high power, and oil immersion objectives. The low power objective is typically 10x magnification. The high-power objective is 40x and the oil immersion objective is 100x. Each objective is marked with a colored band and the magnification. As the magnification of the objective increases, so does the overall length of the tube. Recall, to determine the total magnification, you must multiply the magnification of the ocular lens by the magnification of the objective lens.

Table $\PageIndex{1}$ : Magnification

Objective Magnification	Ocular Magnification	Total Magnification
Low power 10x	10x	100x
High power 40x	10x	400x
Oil immersion 100x	10x	1000x

The oil immersion objective is the only objective that requires the use of oil. Oil is placed between the slide and the objective. This prevents air from refracting the light out of focus on this very magnified lens. The high power (40x) objective is long enough to touch any oil that is left on a slide, but this objective is not designed to be in contact with oil. The oil immersion objective is the only one that should ever have oil on it.

Stage

The stage is located under the revolving nosepiece with the objective lenses. The slide with the specimen is placed on the stage. It is held in place with the stage clip. The slide is moved with the stage control knobs.

Iris Diaphragm

Lever that opens and closes to control the amount of light reaching the specimen.

Condenser

The condenser focuses the available light through the specimen and into the objective lens. It enhances specimen contrast. The condenser can be raised up to the specimen when more light is needed and can be lowered away from the specimen when less light is needed.

Light Source

The slide is illuminated by the light source in the base of the microscope.

Light Intensity Knob

The light intensity knob, also called a rheostat, controls how bright the light bulb is. It’s like a dimmer switch in your house!

Coarse and Fine Focus Adjustment Knobs

The course and fine focus adjustment knobs are typically on the side of the microscope base. The course adjustment is used only with the low power objective. The fine focus is used to sharpen the focus of the image after the object is found with the course adjustment. The fine focus knob is used for the high power and oil immersion objectives. Use of the course adjustment knob with these objectives causes too much movement and may break the slide.

It is very important to learn how to handle and use a microscope properly. Review the following rules and tips for using and handling your microscope.

General Procedure

Always use one hand around the microscope arm and one hand under the microscope base.
Carry it in a vertical position without swinging, tipping, dropping or bumping the microscope.
Place the microscope gently on the lab bench. Never slide it across the table.
Clean all lenses with lens cleaner and lens paper. Never use paper towels or Kimwipes.
Carefully plug in the microscope.
Turn on the microscope and rotate the nosepiece ring (turret) to snap the 10x objective lens in place. Do not use the objective lens to rotate!
Turn the light control (rheostat) halfway to adjust the amount of light.
On the side of the microscope are two knobs, one on top of the other. The larger of the two knobs is the coarse focus adjustment knob. Turn the knob so that the stage goes down as far as it can.
Obtain a slide.
Place the slide on the stage (specimen side up) and secure it with the stage clip.
Use the coarse focus knob to move the stage as high as it can go.
Use stage adjustment knobs to center the specimen so that the light from the light source can pass through it.
Looking through the ocular lenses, adjust them to fit your eye span (interpupillary distance). Only one image should be seen and both eyes need to be open. Then lower the stage with the coarse focus adjustment knob until the specimen comes into view.
Use the fine focus adjustment knob to make the image as clear as you can.
At this point there are different adjustments you can make to improve the quality of the image:
1. The rheostat on the side of the microscope controls the intensity of the light. If it is too bright or dim at any time, use this knob to adjust the light.
2. The condenser will also adjust the light intensity. The condenser gathers and focuses the light to illuminate the specimen. Move the condenser with the condenser adjustment knob so that it approaches the stage. It should almost be touching the bottom of the slide.
3. The iris diaphragm adjusts the aperture of the opening and controls the amount of light that exits the condenser (or illuminates the specimen). You can open and close this aperture, for most purposes it should be fully open, but sometimes partially closing it will increase contrast in the image.
Once the specimen is in focus, turn the nosepiece to the 40x objective lens and snap it into place. Use the fine focus to make the image clear. (NEVER use the coarse focus at this or any higher magnification or you risk snapping the slide or worse, snapping the lens!!!) If needed, make any light adjustments with the rheostat, condenser, or diaphragm.
To use the oil immersion lens, rotate the nosepiece BETWEEN the 40x and the 100x lenses so that the wand or dropper containing the oil can reach the slide. Place a generous drop of oil on the slide and snap the 100x objective lens into place. The lens will slide into the drop of oil.
Use the fine focus to make the image clear. If needed, make light adjustments with the rheostat, condenser, or diaphragm.
NEVER return to the 40X objective lens after there is oil on the slide. If you are having trouble focusing using the oil immersion lens, you must go back and use the 10x lens to re-center (turn the nosepiece so that the 40x objective lens is NOT dragged through the oil on the slide). Then go directly back to the 100x lens. If this doesn’t work, the slide must be wiped clean and you should start over.
When finished with the slide, lower the stage and remove the slide. (Do not lower the stage if you are going to view a different slide). Clean the oil off the slide.
Clean the oil immersion objective with lens paper to remove all oil.
Clean the oculars and objectives with the clean lens paper and lens cleaner.
Turn off the microscope light and unplug the microscope.
Position the nosepiece in the lowest position with the course adjustment.
Center the stage so it doesn't stick out on the sides.
Cover the microscope and return to storage.

General Rules/Procedural Reminders

Carry the microscope with two hands - one under the base and the other holding the arm.
Never turn the nose piece by the objective lens.
Use only lens paper to clean microscope lenses (oculars and objectives). Other materials with scratch.
Only use immersion oil with the oil-immersion objective.
Avoid jarring or bumping the microscope.
Use the course adjustment only with the low power objective.

References

Content on this page was adapted from:

1.5: Microscopy by Orange County Biotechnology Education Collaborative; Public Domain

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