Care and Use of the Microscope
Laboratory

Objectives

When you finish this laboratory you should be able to:

1. Define these terms and use them in an appropriate context:
a. diopter value

b. FOV

c. interpupillary distance

 d. parfocal

e. resolution

2. Carry a microscope safely to your desk.

3. Name the parts of the microscope and their functions.

4. Discuss the advantages and disadvantages of using compound microscopes and dissecting scopes.

5. Calculate interpupillary and diopter values.

6. Prepare dry and wet mounts for use with the compound microscope.

7. Correctly focus the microscope on tissue and be able to calculate cell size.
8. Clean the microscope and correctly prepare it for storage.

Introduction

Many organisms are too small to be observed with the naked eye. These organisms are multicellular as well as single celled. Representatives of all five phyla, plants, animals, fungi, protists, and bacteria are microscopic in size. Microbes are less than 1 millimeter in size and usually measured in micrometers (1 um=1/1000 mm.)

Use of the Dissecting Scope

Today in lab, we will begin by using the dissecting scope which magnifies objects as much as a magnifying glass. The field of view for this device is large and the depth of field that is in focus at one time is also large. Light can be directed from above the object or below. Magnification is controlled by a zoom knob.

*Carry the dissecting microscope by its arm with one hand supporting the base. After placing it on the table, name the parts and associated functions.

Parts of the dissecting scope (Fig. la)(back Page)

Base with illuminator.

Light switch: Turns on the illuminator.

Illuminator control knob: Two positions for one switch may alternately select lighting from above or below the stage or both lighting sources may be used simultaneously.

 Oculars: Move them together or apart until you feel comfortable looking into the microscope. A single circle of light should be observed.

Zoom knob: Controls magnification of the specimen. Rotate it to increase or decrease

Cleaning the Lenses

Nothing should touch the microscope lenses except the special lens paper and appropriate solvent.

The lenses may be cleaned with clean lens paper. Lens cleaner must be used to clean grease or oil on the lenses. Handle solvents carefully and wash your hands after use.

Calculating Interpupillary Distance and Diopter Values.

Before viewing a slide one should calculate the interpupillary distance using the following steps:

(1) Focus on a specimen using the low power objective.

(2) Sit back from the microscope and slowly move the oculars apart.

(3) While attempting to look through the oculars, notice that you can see two images. Carefuly move the oculars together until the split image is viewed as one.

(4) Repeat the procedure.

(5) Locate the scale on the microscope between the oculars. Record the number in your lab book.

The diopter values must also be calculated:

(1) After the interpupillary distance has been determined, locate the diopter adjustment rings on the ocular lenses.

(2) Turn the diopter rings so that the mark on each ring aligns with the midpoint of the microscope scale on the ocular.

(3) Close the left eye, and use the fine focus to find the clearest possible image.

(4) Open the left eye, and close the right eye.

(5) Do not change the fine focus, but instead, turn the diopter ring on the left lens from one extreme to the other to find the best viewing place.

(6) Repeat the procedure, and record the diopter values in your lab book.

(Note: Your interpupillary and diopter values will apply to other microscopes in the lab.)

Viewing a Slide

Prepare a dry mount by placing a small piece of newsprint on a glass slide and covering it with a cover slip. Place the slide on the stage, using the stage clips to hold the slide securely. Adjust the light source to provide 75% of the total light. Always begin slide F viewing with the lowest power objective available. Click the scanning objective in place and turn the coarse adjustment knob until the letters come into focus. Then use the fine adjustment knob to refine the image. Adjust the amount of light coming through the newspaper print using the light rheostat and the iris diaphragm lever.

As you examine the print, is the letter reversed?

While you are looking at the stage move the adjustment knobs to observe whether the objective is moving closer or further from the stage.

Turn the mechanical stage drive knobs, one at a time. Note the direction that they move the stage when turned clockwise and counterclockwise.

Focus on the letter, change objectives by moving to the next highest objective. The microscope is parfocal, therefore it should not be necessary to adjust the view with the coarse adjustment knob. Use only the fine adjustment knob from this point. Note that the field of view has gotten smaller while the magnification increased. The depth of field also is decreased. Only a thin layer of tissue is in focus at higher magnifications. By focusing up and down you can view different parts of a single cell or different cell layers.

The specimen must be centered over the hole in the stage through which light passes.

Oil Immersion

The resolution of the image is increased by filling the space between the slide and the lens with oil. Oil has a much higher index of refraction than air and contributes to the magnification of the image.

1. Locate the object and center it with scanning lens.

2. Move to low, then to high power, refocus while using the fine adjustment kob only.

3. Swing the high power lens away from the slide. Place a drop of immersion oil on the center of the coverslip and move the oil immersion lens into position. Refocus using the fine adjustment knob only.

4. When you are finished, swing the lens away. Do not put any other lens in position.

5. Remove the slide and wipe most of the oil off with paper, then with lens cleaner.

6. Clean the lens also with lens cleaner and then dry lens paper.

Computing Magnification

Magnification is important in examination of cells and tissues. It is not the only concern in observation. Objects must not only appear larger, but also clearer. Resolution or resolving power measures how clearly details can be seen. It is the smallest distance between two points which can be separately distinguished as different points.

Total magnification using the light compound microscope can be calculated by multiplying the magnification of the ocular lens by the magnification of the specific objective.

Table 1.2

Measuring Objects Using the Microscope

Remember: as you increase the magnification you are decreasing the FOV (Field of View). If you know the FOV and the magnification when viewing an object you can calculate the size of the cell or organelle that you are observing. There is a mathematical relationship between the magnification and the diameter of the FOV.

X,   =  D₂

  Where X, = Magnification with a specific objective.

X₂       D,                   X2= Magnification using a different objective.

                                   D, = Diameter of FOV at magnification 1.

                                  D₂ = Diameter of FOV at magnification 2.

We can therefore calculate any of these values if the other three are available. Such as: ₂=P.X₁

1t₂

Record the diameters of the FOV for all objectives that you are using today in Table 1.2. Measuring cells.

Today you will observe cell types presented by your instructor. They may be prepared slides or wet mounts that you have prepared with materials provided for you. Calculate cell size by estimating the number of cells that would fit across the FOV and then divide the diameter of the FOV by the number of cells.

cell size = diameter of the FOV number of cells

As we estimate the size of the cell that we are viewing we must take into consideration the objective that we are using to view the object with and the diameter of the field of view (FOV.) Using the scanning objective, the diameter of the FOV is 4.5 mm. The diameter of the FOV for low power is 1.8 mm, the diameter of the FOV for high power is.45 mm and the diameter of the FOV for oil-immersion is-.18 mm.

For example: if you count 5.5 cells that would fit across the diameter of the field of view using the scanning objective, you would divide 4.5 mm by 5.5 cells giving a cell length of .818181.... (Using the rules in your appendix for significant digits you know that we cannot have more than two significant digits in the answer.) Thus we will round off our answer to .82 mm. To convert this to um we use the conversion factor that relates mm to um.

.82 mm X 1000 um = 820 um­lmm

Use the best objective for viewing the cell. It is not always best to use the highest power objective. (However you are likely to estimate more accurately and you don't have to count as many cells.)

Example: 12 cells fit across the diameter of the FOV using low power. Calculate cell size.

cell size =,diameter of FQV

number of cells across the diameter of the FOV

cell size = 1.8 mm

12.0 cells = .15 mm/cell

Using low power, calculate the cell size in the following diagram:

cell size = diameter of FOV

number of cells across diameter of FOV

Calculate cell size in the following diagram of a single organism viewed under high power. (Remember that you must estimate how many cells would fit across the diameter f the FOV.)

cell size = diameter of the FOV

number of cells across diameter of the FOV

Preparation of a wet mount.

Wet mounts are useful in observing living tissues or cells for characteristics such as movement, color or behavior. Follow these steps:

(1) Obtain a slide, coverslip and perhaps a teasing needle.

(2) Place a drop of the substance on your slide.

 (3.) Touch the coverslip to one margin of the drop and slowly lower it. The slide should not have bubbles if you perform this carefully, but they do give you something to focus on!

Obtain a thin section of onion membrane by snapping a layer of onion in half and transferring the transparent thin film to a slide. Apply one or two drops of I₂KI, Lugol's Solution, to stain it.

Place the edge of the coverslip slowly on the slide over the stained material at an angle rather than dropping it on the slide from a parallel position. One should observe the following:

HAND-IN Sheet                        Name_____________________________

Use the following chart to record your calculation of the sizes of  5 different cells.

Table 1.4                                                                                                                                          Drawing of Cells

(ie: :# cells(4)/ dia. FOV 1.6 =.4mm x1000=400 um)

Electron Microscopes

Electron Microscopes are not generally available to introductory biology students but you may have the opportunity to observe the use of one. Various types of electron microscopes allow viewers to increase the magnification without losing the resolution of the picture. The view does not become fuzzy with increased magnification. The use of these has increased greatly for study of cell structure and identification. Most larger hospitals have electron microscopists to examine tissue samples on site.

Electron microscopes use a beam of electrons as an illumination source. The wavelength of the electron is a hundred thousand times smaller than the wavelength of visible light.

Transmission electron microscopes, scanning electron microscopes and probe electron microscopes are all available for more sophisticated study of cell structure and structure of macromolecules.

Student Questions
1. Discuss the difference between magnification and resolution.

2. Discuss the relationship between: magnification, diameter of FOV and relative brightness for all objectives.

3. Discuss how you would calculate cell size of a single celled organism.

4. Compare and contrast the dissecting and light microscopes.