A USER'S GUIDE TO THE COMPOUND MICROSCOPE

The compound microscope is still one the basic tools of the biologist and you will use it extensively during this course. Therefore, it is important for you to use the compound microscope correctly. The instructions below will provide you with a working knowledge of how to clean, align, and focus the optical elements of the compound microscope.

How to clean the exposed optical elements of the microscope

Dirt and grease accumulate on the three exposed optical surfaces of the microscope, the ocular lenses, the objective lenses and the glass plate covering the light source. To clean these surfaces we need to use a solvent or set of solvents that will remove both polar and nonpolar residues. A good solvent to start with is distilled water. A distilled water rinse can be followed with an organic solvent, such as chloroform or xylene. However, you have to be careful using organic solvents on microscope lenses because they dissolve the glues that are used to cement together the 4 to 10 elements which form the objective lenses. To avoid this problem in our laboratory we will use a commercial lens cleaner - "The Green Stuff," which is a dilute solution of acetic acid, isopropyl alcohol, and acetone (Some microscope companies recommend using products such as GlassPlus or Windex as a solvent!).

Lens paper can be used to clean the large optical surfaces of the ocular lenses and the glass plate covering the light source. The small optical surfaces of the objective lenses are best cleaned using cotton swabs (Q-tips). In addition to being small, some of the objective lenses, such as the 40x objective, have surfaces that are recessed (concaved) and very difficult to clean with a piece of lens paper. To clean a lens with a cotton swab, first add 1 or 2 drops of solvent to the swab and then gently rub the swab over the surface of the lens. Do not grind the swab into the lens. Use a clean swab for each optical surface, so that you do not transfer "gunk" from one lens to the next.

Adjusting the interpupilary distance and focusing the binocular lenses for your eyes (Or how to prevent that pounding headache).

1. Focus on a specimen using the 10x objective.

2. Move the two oculars apart until they are in a position that is conformable for your eyes (i.e. you should see only one image of the specimen).

3. Look at the scale positioned between the oculars. The reading on that scale is your interpupilary distance.

4. There are similar scales on the sides of both oculars. Turn the right ocular until the reading on its scale matches your interpupilary distance.

5. Using only your right eye, focus the microscope so that the image of the specimen appears sharp to you.

6. Now, using only your left eye, turn the left ocular until the image of the specimen is in focus.

The binocular optics of the microscope are focused for your eyes and your interpupilary distance. If you change the interpupilary distance of the oculars, the oculars will no longer be in focus for your eyes!

Aligning the optical elements of the microscope

1. Focusing the condenser: Close the field diaphragm which is the iris diaphragm located on top of the light source. Raise or lower the condenser to focus the image of the field diaphragm and the plane of the specimen. It may be necessary to refocus the microscope on the specimen as you focus the condenser. It is important to have the images of both the specimen and the field diaphragm in focus.

2. Centering the condenser: There are two set screws on opposite sides of the condenser which can be used to adjust the position of the condenser, so that the image of the field diaphragm appears in the center of your image. This is not an adjustment which has to be made frequently on our microscopes but you should be aware that this adjustment can be made.

3. Open the field diaphragm to just fill the field of view.

4. Adjust the iris diaphragm on the condenser (the condenser diaphragm) until you have obtained the best image. The more you close or stop down the condenser diaphragm the more you will increase the contrast of the image. Sometimes more contrast is good and sometimes it is not!

Steps 1-4 must be repeated each time you change objectives if you want to obtain the optimal image with each objective lens.

Illumination

Remember there are two controls for the light source. One is the on/off switch. The other control is a rheostat that controls the light intensity (#1 being the lowest, 10 being highest). The lower the number the more yellow the light. Due to its longer wavelength, yellow light decreases the resolving power of the microscope compared to blue light which is produced at higher light intensities. However, it is also important for you to set a light intensity that is conformable for your eyes. That is, the resolving power of your eyes decreases dramatically as you burn out your retinas!