What Is a Compound Light Microscope?

Light, Contrast, Ocular and Objective Lenses & Total Magnification

Sep 11, 2008
Tami Port

Microscope Mechanical Stage and Objective Lenses - Tami Port

Learn about the features of a compound light microscope, the importance of contrast and how to determine total magnification of ocular and objective lenses.

Many students new to using a microscope struggle and become frustrated when trying to find objects, particularly at higher magnifications. This is the first in a series of articles clearly describing the mechanics and operation of a compound light microscope.

What Is a Compound Light Microscope?

The ‘Light’ Part of a Microscope

A light microscope uses a beam of visible light to illuminate and contrast the object being viewed through the scope. The light source is typically an incandescent bulb which is turned on by a toggle switch at the side of the base. The beam of light shines up from a lamp in the microscope's base, through a hole or ‘window’ in the stage (area of the scope where the specimen sits), ultimately up through the specimen.

Contrast results when the area surrounding the object is bright, from the light, and the object being viewed is darker in comparison. It is important to be able to control the level of contrast when using a light microscope, since too much light can make it difficult to see a somewhat transparent or light-colored object (this is called ‘burn out’ in scope speak). Illumination can usually be controlled two different ways:

By adjusting the iris diaphragm, just under the stage: This lever controls the amount of light that enters the condenser which will allow for appropriate contrast. In general, close the diaphragm on low power, (allowing for reduced amounts of light to reach the stage) and open the diaphragm to let more light through at the higher power objectives.

By adjusting a dial on the side of the base: This dial turns the light source up and down, making it brighter or dimmer.

The ‘Compound’ Part of a Microscope

The magnification of a compound microscope is the result of two lenses:

the ocular lens or lenses
the objective lenses

The ocular lens is the one closest to the eye, and usually magnifies objects ten times (10X) their actual size. The objectives are a collection of lenses located on the rotary nosepiece. There are usually three or four objective lenses, each allowing for different degrees of magnification:

Scanning power objective: The shortest objective. This lens usually has a red stripe around it, and magnifies objects four times actual size (4X).
Low power objective: The next shortest objective. This lens usually has a yellow stripe around it, and magnifies objects ten times their actual size (10X).
High-dry objective: This is usually either the longest, or second longest objective. This lens typically has a blue stripe around it, and magnifies objects forty times their actual size (40 X).
Oil immersion objective: If a compound scope has this lens, it is the longest, and typically has a black or both a black and white stripe around it. The oil immersion lens magnifies objects one hundred times (100X), and must be used with a drop of oil placed directly on the specimen and fills the space between the lens and specimen.

The total magnification is determined by multiplying the power of the objective by the power of the ocular. (For example, the total magnification at scanning power is 4X times 10X = 40X). Using these two sources of magnification is what makes a microscope compound.

To learn more about the history of microscopy see the article Anthony Van Leeuwenhoek to learn about the man who made great contributions to what has become the modern microscope of today.