Some objects are far too small to be seen with the human eye. However, by using a microscope many can be viewed in great detail. There are many types of microscopes that range from low–level magnification (i.e., hand-held magnification lens) to very high-power magnification (i.e., an electron microscope). In the middle of that range lies the light microscope, or for our purposes, the compound light microscope, which uses multiple lenses.
Often times, you may have to make several unit conversions before arriving at the appropriate units for that measurement. For example, if you knew it took you 3450 seconds to drive from your house to the post office, determining the number of minutes it would take involves converting units.
The compound light microscope (Figure 1) has two sets of lenses:
the ocular lenses (close to your eyes)
the objective lenses (close to the “object” on the stage).
Along with a light source, these lenses work together to magnify the object being viewed. In the case of the compound light microscope, the total magnification is equal to the magnification power of the ocular lens multiplied by the magnification power of the objective lens. For example, if the ocular lens magnifies 10X (this means 10 times) and the objective lens magnifies 10X, the total magnification is 100X.
Parts of a Compound Light Microscope
Base: The flat support of the microscope.
Light: Illuminates the object being viewed. This can be either in the form of a light source or a mirror that reflects ambient light onto the image. In the latter case it is important to be working in an environment with adequate ambient light.
Stage:Supports the slide or other material to be viewed.
Diaphragm: Controls the amount of light allowed on the object.
Stage Clips: Secure the slide in place.
Revolving Nosepiece: Rotates the objective lenses of different magnifications and allows one of
them to be positioned over the slide.
Arm: Connects the lower base and the upper head of the microscope (also used to carry the microscope).
Head: Supports both the ocular lens and the revolving nosepiece.
Ocular Lens (eyepiece): The lenses on the microscope typically have a magnification of 10X. If your microscope has a pointer, which is used to indicate a specific area of the specimen, it is attached here.
Figure 1: Compound microscope.
Types of Microscopes
Monocular Microscope: Has a single ocular eyepiece. Binocular Microscope: Has two ocular eyepieces.
How to Use a Microscope
Always carry a microscope with one hand securely around the arm and the other underneath the base for support.
Place the microscope on a table, plug it in, and turn on the light source (or adjust the mirror as necessary). It is a good practice to ensure that the light intensity switch is always in the “Off” position, or at the lowest level, prior to plugging in the microscope. The light bulb should also be turned off before the microscope is powered off. Note: When cleaning a microscope, do not use paper towels or cloths as this will scratch the lens. To preserve the microscope, use only lens paper that will not scratch the optics.
To prevent damage to the lens or slides, always start and end with the scanning power objective lens (the shortest one) above the light source.
Place your slide on the stage and secure it with the stage clips. It is helpful to visually orient the slide so the object to be viewed is directly in the middle of the opening in the stage
where the light is directed up toward the slide.
While viewing the microscope from the side, turn the course adjustment knob to bring the stage all the way up to the scanning power objective lens. While looking through the lens,
use the course adjustment knob to slowly lower the stage until the specimen comes into focus.
Note: When using a binocular microscope, adjust the distance between the two oculars
until only one object is seen. Record this distance and set your microscope to this distance every time you use it. If someone else uses the microscope, the lenses may be re
-adjusted for their eyes.
To adjust the light, open or close the diaphragm located over the light source. When properly illuminated, the specimen should not be gray or exceptionally bright.
With the object is in general focus, rotate the revolving nosepiece to the low-power lens (the next longest). After focusing with the course adjustment knob, switch to the fine adjustment
knob to obtain more precise and greater detail. It may also be necessary to adjust the light, because more light reduces contrast (sharpness).
To become familiar with the mechanical stage knobs around the base of the microscope (if present), turn one slowly to the right, noting that the image will be moving toward the left. This image inversion is caused by the lenses.
If you need higher magnification, slowly rotate the high-power lens into place (the next longest lens). This will bring the tip of the lenses very close to the slide. Do not use the
course adjustment knob with the high-power lens in place as this could damage the lens.
Make sure the objective lens does not touch the slide.
Whenever you use the high-power lens, only use the fine adjustment knob. If the object was well focused while viewing with the low-power lens, very little adjustment should be necessary.
If you cannot bring the object into focus, return to the low-power lens, focus the object, and then return to the high-power lens.
When finished, move the revolving nosepiece to the scanning objective lens position before
How to Prepare a Wet Mount Slide
To make a wet mount for a specimen that is not already in liquid, take a clean slide and place the specimen in the center.
Add a drop of water. Note: For cells that are transparent, it may be necessary to add a small drop of stain as opposed to water.
Carefully add a coverslip by placing one side or end down and slowly lowering the other end. Note: If the coverslip is added too quickly, large air bubbles may become trapped which can
cause difficulty viewing the slide. If this happens, gently remove the coverslip, add another drop of water and try again.
Remove excess liquid on the bottom of the slide or around the edges before it is placed on the microscope to avoid damage to the lens. Just touch a tissue to the edge of the coverslip
to draw away the water (this is an example of capillary action).
If the specimen is already in liquid, place a drop in the middle of the slide and add the coverslip
as you did in Step 3.
