All the microscopes in the lab are parfocal. That means that if the slide is in focus under one objective, it will stay largely in focus if the objective is changed. In practical terms, this means you should usually only need to use the coarse focus knob once per slide. You get the slide in focus under the lowest-power objective (where focusing is easiest), then, from that point onward, only make minor adjustments with the fine focus knob even if you change objectives.
When you first get a new slide, you can usually determine the location of the specimen by looking at the slide while it is still in your hand. The specimen is usually a patch of colour somewhere near the center of the coverslip. After you clip your slide securely onto the stage with the stage clips, use the stage control knobs to move the patch of colour until it is directly over the hole in the center of the stage where the light comes through. Now when you look through the eyepieces using the lowest objective (always start with the lowest objective) you should be able to find the specimen and get it quickly in focus.
Occasionally the eyepiece or objective lenses will have specks of dirt or dust on them, making it difficult to focus on the specimen. To clean lenses, always use lens paper supplied by the lab instructor or a pure cotton swab. Do not use any other type of cloth or paper as they might scratch the lens. To remove dirt with lens paper, first roll up the lens paper and try to dry brush away the dirt in a spiraling motion that circles from the center of the lens out. If that doesn’t work, moisten the lens paper or pure cotton swab with water (do not apply the water to the lens directly) and clean in a spiraling motion from the center of the lens out.
Below is a checklist for initially setting up a microscope. Every time you get a new slide, you should use this checklist.
Lab 4 Exercises 4.3
Get a slide with the letter “e” on it. Then following the checklist below, checking off each item as you complete it. For this slide, the letter “e” is the specimen we are interested in.
Checklist for setting up a microscope and beginning with a new slide.
|1. Plug in the microscope & turn on light source.|
|2. Pick up microscope by carrying arm, position it so it is accessible to your seat, with open side of the stage facing you|
|3. Rotate the objectives so that the lowest power objective (smallest in size) clicks into place.|
|4. Look at the slide with your naked eye and find the location of the specimen.|
|5. Clip the slide into place with the stage clips. The cover slip on the slide must face up. Find the stage controls and make sure that, when they are turned, the slide moves smoothly left and right or up and down, depending on the knob.|
|6. Use the stage controls to move the slide so that the light source is shining directly on to the specimen to be magnified.|
|7. Find the coarse and fine focus knobs. Watching the stage and objective, use the coarse focus knob to bring the low power objective as close to the slide as it will go.|
|8. Put your eye to the eyepiece (or eyepieces, if the microscope is binocular) and rotate the coarse focus knob in the lowering direction until some aspect of the specimen comes into focus.|
|9. Move your hand to the fine focus knob and get the specimen into perfect focus for your eyes. Do NOT touch the coarse focus knob again.|
|10. Use the stage control knobs to move you specimen to close to the exact center of your field of view.|
|11. Move to the next highest power objective (do not skip the individual objectives) and use only the fine focus to get your image into perfect focus for your eyes.|
|12. If you need further magnification, move to the next highest power objective and use only the fine focus to get your image into perfect focus for your eyes.|
|13. Do not use the 100x objective (if you have one) in this course. It must be used with immersion oil and we won’t have students doing that.|