A compound light microscope is a microscope that uses both light and multiple magnifying lenses in order to provide a better view of specimens. The light of a compound microscope is located below the stage that the specimen you are looking at is put on and is reflected through the specimen and up into the eyepiece of the microscope.
The eyepiece of the microscope has the magnification of 10X and is used in conjunction with three other objectives (in your typical light microscope). These objective lenses which are closest to the specimen are able to project into the microscope's body tube and then into the ocular lens for people to view the magnified image. These other objectives are generally, 4X, 10X and 40X.
The 4X objective when combined with the 10X of the eyepiece will magnify the object 40 times (10x4). This 4X objective is usually considered to be the scanning lens. Use of this objective, which is the shortest of the three objectives along with the eyepiece objective, is usually used just to scan the object you wish to look at. Use of the coarse adjustment knob to bring the object you are looking at into view is acceptable with this lens since the objective is far enough from the stage to not hit the slide/specimen you are looking at.
The second most powerful objective and the second shortest is the 10X objective. Combined with the 10X objective of the eyepiece, this magnifies that object you're looking at by 100 times. This objective should be adjusted with the fine adjustment knob because this objective is closer to your slide and because only a small amount of adjustment should bring the slide into a clear view.
The third most powerful objective and one of the longest is the 40X. This objective when combined with the eyepiece magnifies the object 400 times. This is considered the high power lens and should only be adjusted with the fine adjustment knob. If the coarse adjustment is ever used for this, not only can your slide be broken, but you may also damage your lens.
Some compound microscopes (such as those found in clinical laboratories) have a fourth objective called, oil immersion. The oil immersion lens is 100X and therefore magnifies the specimen 1000 times. To use this objective, a drop of oil must be placed on the slide between the specimen and the objective. The fine adjustment must be used carefully so that great care is taken not to damage the lens.