Compound microscopes use a combination of lenses -- the eyepiece lens and the objective lenses -- to achieve magnification of the object being viewed. The objective lens system usually consists of three or four lenses of different focal lengths from low to high power.
The High Power Objective
With a series of three lenses, the first one is usually about 4x (4 times) magnification, the next is 10x, and the highest, or "high power objective", is generally 40x.
How It Works
The magnification achievable is a multiple of the eyepiece and the objective lens. If the eyepiece is 10x and the low power objective is 4x, the total magnification will be 40x.
How It Is Used
When examining a specimen slide it is usual to start with the lowest magnification to orient the view. The next objective is then rotated into use and the object examined again. If more detail is needed, the high power objective will be used.
The act of rotating the next objective into use gives a coarse focus. A fine adjustment knob can then be used to bring the magnification into sharp focus.
A Higher Power
Some microscopes have a fourth objective (100x) that is oil-based for greater clarity at 1,000x magnification. It is used by placing a drop of oil on the specimen slide and lowering the objective until it is just in contact with the oil, giving less distortion than viewing through air at that magnification.