A compound light microscope is a scientific tool typically used to look at very small living things, such as stained plant and animal cells, and even minute bacteria. This type of scope has a built in light source and two sets of lenses.
* The ‘Light’ Part of a Compound Light Microscope *
A light microscope uses a beam of visible light to illuminate the specimen and create contrast between the object being viewed and the background. The light source is typically an incandescent bulb which is turned on by a toggle switch. 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 allows for appropriate contrast. In general, it is best to have the diaphragm opening smaller on low power (allowing for reduced amounts of light to reach the stage) and larger at higher power objectives, to let more light through.
By adjusting a dial on the side of the base: This dial turns the light source power up and down, making it brighter or dimmer.
* The ‘Compound’ Part of a Compound Light Microscope *
The magnification of a compound microscope is the result of two lenses, the ocular lens and 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 nose piece. There are usually three or four objective lenses, each allowing for different degrees of magnification:
1. Scanning power objective: The shortest objective. This lens usually has a red stripe around it, and magnifies objects four times actual size (4X).
2. 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).
3. 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 (40X).
4. 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 that fills the space between the lens and specimen.
The total magnification of a compound microscope 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.