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  BACKGROUND

You have probably used bright field illumination during their introduction to the microscope. However, bright field illumination, which uses direct white light from the  condenser of the microscope to produce a dark image against a white field (background), can obscure the details of some specimens, especially those that are translucent or nearly translucent. To get a better idea of how this works, try looking for dust motes suspended in the air in a well-lit room.

Now recall how easily they are seen within an oblique shaft of light passing through a dark room such as that cast by a projector. This is because in a dark room you only see the light reflected or refracted by the dust motes, making them shine against the dark background. Dark field illumination works on the same principle. Direct light from the condenser of the microscope is blocked, and only the oblique light reflected or refracted by the specimen is seen. This results in a bright image against a black or dark field, a tremendous advantage when viewing translucent or nearly translucent specimens.

To create dark field illumination, an opaque circle called a patchstop is placed in the  condenser of the microscope. The patchstop prevents direct light from reaching the objective lens of the microscope. The only light that does reach the lens is reflected or refracted by the specimen.

How to make a patchstop for dark field illumination
1. Place your microscope in an appropriate work area.
2. Select the low-power objective.
3. Remove the eyepiece and place it somewhere clean and safe.
4. Open the diaphragm as far as possible while looking through the opening left by the missing eyepiece. Now slowly close the diaphragm until it just impinges on the visible circle of light.
5. Lean over or squat down so that you can see the diaphragm from below. The diameter of its opening is slightly smaller than the patchstop you will make.
6. Use the hole punch to make several circles of black construction paper. Compare one to the size of the diaphragm's opening. If the circle is more than 10% larger than the opening, trim it so that it is about 10% larger than the opening. If the circle is smaller than the opening, cut out a larger circle.
7. Cut out a square piece of transparency film measuring 5 cm on each side.
8. Glue the circle of construction paper about 2 cm from one corner of the piece of transparency film.
9. Label the film with the magnification of the objective. The patchstop is ready for use.
10. Select the medium-power objective and repeat steps 4–9 to create a patchstop for it.
11. Select the high-power objective and repeat steps 4–9 to create a patchstop for it only if it is not an oil-immersion type.
12. Replace the microscope's eyepiece.

How to view a slide using dark field illumination
 
1. Make a wet mount slide of diatomaceous earth and view it using the low-power objective.
2. Select the patchstop for the low-power objective. Slide it between the filter holder and the rest of the condenser, starting at the corner with the black circle. Note: If your microscope does not have a filter holder, hold the patchstop beneath the condenser.
3. Remove the eyepiece and set it somewhere clean and safe.
4. Open the diaphragm completely and look through the opening left by the missing eyepiece. Adjust the patchstop until the light is completely blocked. Note: If your microscope does not have a condenser, you must secure the patchstop in place with tape once you have it properly adjusted.
5. Replace the eyepiece.
6. View the slide and compare the image to the original.
7. Switch to the medium-power objective. How does it affect the image?
8. Repeat steps 1–6 with the same slide but using the medium and high-power objectives and patchstops. Can you explain what you have observed?

Try It, You'll Like It!!!!      

                                                  Tungston Filament

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