Most often an inability to focus the specimen is due to a dirty lens. Most often it is dirty due to the presence of OIL (see below) which will make it impossible to focus (remains fuzzy/cloudy).
Take a piece of paper towel and clean off the prepared slide. This will make it easier to focus on the specimen and not on a piece of dirt.
2. Place the slide onto the stage and rotate the nosepiece until the
10x lense clicks in place.
Adjust the occular lenses until one big field is seen.
3. Using the course adjustment knob move the objective as close to the slide as possible.
4. While viewing the specimen, adjust the focus using the coarse adjustment (big knob) until the specimen is in focus. It may be necessary to use the fine adjustment knob (small knob) to bring the specimen into focus. Using the stage adjustment knobs, move the specimen to the center of the field.
5. To go to the next magnification, typically 40x, rotate the nosepiece until the next objective is clicked into position. As most lenses in quality microscopes are parfocal and parcentral, the specimen should only need minor adjustment with the FINE FOCUS knob. Other than the 10x lens, use only the fine focus adjustment knob for all other focussing. If the coarse adjustment knob is used with any other lens, it is possible to smash the slide with the objective, braking the slide or breaking the objective or breaking both. If you have used prepared slides that looked cracked, this is how they were cracked, not by dropping them.
6. Oil Immersion Lens:
The 100x lens is typically an oil immersion lens. This means that, in order for it to work properly, it should be connected to the specimen using a drop of special oil. ONLY the 100x oil immersion lens is constructed to be used with oil. If the lens is used without oil, the specimen will not focus sharply. If any other lens is used with the oil, that lens will not focus properly and may be PERMANENTLY DAMAGED.
To use this lense: After the specimen is in focus at 40x, unclick the 40x objective and stop. Put a drop of oil on the specimen and then click the 100x objective into place. The 100x objective should be touching the oil. Now using the fine focus only, bring the image into focus.