Eyepiece

• Most compound microscopes, the most common kind used in labs and science classes, get their name because the total magnification is computed using the product of two lenses---the ocular and the objective. A typical ocular, or eyepiece, magnifies the subject by a power of 10, called 10X magnification. A binocular microscope has two eye pieces, one for each eye, and the individual focus and distance between them can be adjusted separately. This will enable people who are nearsighted or farsighted in one eye to use the microscope without glasses, but users with astigmatism still need to wear glasses with binocular microscopes.
  
  

Objectives and Adjustments

• The objectives, or main magnifying lenses of the microscope, can be damaged if they are not used properly. The part of the microscope that holds the rotating objective lenses is called the nosepiece. The lowest-power objectives can be either 4X (scanning) or 10X (low power). Use these lenses first to center and focus the item on the slide.
  
  Coarse adjustment (the larger focusing knob closest to the body of the microscope) may be used to get the item in focus. Most compound microscopes are parfocal, meaning that an item that's in focus under one objective should be nearly in focus on another. DO NOT use the coarse adjustment with any objective with a magnification greater than 10X, or the objective and slide could collide, damaging the lens.
  
  The next higher powers are called high-dry (40X) and oil immersion (100X). The oil immersion objective gives a total magnification of 1000X (100X times the 10X of the eyepiece). A drop of oil must be placed on top of the slide before the oil immersion lens is moved into place. The oil helps reduce blurriness caused by the image passing through the air. High-dry is so named because it is a high-power lens but does not require oil as the "wet" immersion lens does.
  
  

Light

• Light passing through the slide can be controlled in three different ways. A rheostat controls the amount of electricity used by the light bulb. A condenser focuses the light onto the stage. The iris diaphragm regulates the amount of light passing through the slide, and is the most common light adjustment used. Sometimes an object is easier to see using more or less light. Remember to never stare directly into the light of a microscope, or eye damage could result.
  
  

Stage

• The stage holds the slide for viewing using clamps, and it can move on an X or Y axis (forward and backward, and side to side) using the x-y adjustment knobs, or up and down using the coarse and fine adjustment knobs. This is why the coarse adjustment can never be used with the large oil immersion objective; it moves the stage too much and the lens or slide can be scratched or broken.
  
  

Moving and Storage

• When a microscope is not in use, it should always be stored with the lowest-power objective in place. Clean the oil immersion lens with special lens paper before storage, until the paper comes off clean with no oil on it. Failure to do so will turn the lens cloudy over time. A microscope should always be carried with one hand under the base and the other hand gripping the arm, or the part that supports the stage and nosepiece. Clear a space for the microscope and decide where you want it to go before setting it down; sliding it across a table could cause damage.