What is a Compound Microscope?

The compound microscope get its name from its use of compound magnification. This refers to two separate lenses which each magnify the image to create even higher levels of magnification. It starts with the objective lens which is the lens closest to the object being viewed. These lenses typically range in magnification from 4X to 100X. The magnified image created by the objective lens is further magnified by an ocular lens – also called an eyepiece. Eyepieces typically range in magnification from 5X to 30X with the most common being 10X. When these two lenses work together they can produce magnifications well over 1000X.

Because compound microscopes are capable of producing such high levels of magnification, they are used by laboratories, schools, manufacturers, agricultural professionals, and so many others. These microscopes are widely seen in science classrooms for use when studying cells. However, they are also used by geologists and mineralogists for viewing rock specimens, and by manufacturers for viewing materials including metals, plastics, and fabrics. The wide variety of applications has led to specialized types of compound microscopes including metallurgical, polarizing, and fluorescence models.

Upright vs Inverted

Compound microscopes have two common forms: upright and inverted. By far the more common form is upright. The upright configuration places the objective lenses above the specimen, which also means that the specimen stage would be below the objective lenses. Any light source which would project light through the specimen would be located below the stage. The inverted configuration places the objective lenses below the specimen, and therefore the stage and any transmitted light source would be above the objectives lenses. The purpose of the inverted form is to allow for larger or heavier objects which would not fit on the stage of an upright microscope, or would otherwise make focusing impossible.

Light Source

One common characteristic of all compound microscopes is that they require a light source to function, especially at the highest magnifications. The original compound microscopes relied on candlelight and mirrors to provide the needed illumination. Modern microscopes are often equipped with built-in illumination systems utilizing incandescent, fluorescent, or LED light sources. Technology in lighting continues to improve and provide energy-efficient options with brightness and color-accuracy.

Transmitted vs Reflected Light

The most prevalent type of illumination in compound microscopes is transmitted – also called diascopic. This refers to the light passing through the object being viewed. In order to work, transmitted light would only be used with transparent or translucent specimens. Specimens are typically very thin so that light can pass through them. For upright microscopes, the transmitted light source would be located below the specimen stage, while inverted microscopes would place the transmitted light source above the specimen stage.

Reflected illumination – also called episcopic illumination -- is commonly used to observe surface features of a specimen. Instead of passing through the specimen, the light reflects off of it. This is useful for observing opaque objects such as rock sections, metals, and other materials. Some entry-level microscopes may have a light source mounted behind the objective lenses, which would project light onto the specimen from an angle. Specialized microscopes such as metallurgical models, actually project light through the objective lenses. These microscopes use a complex internal system to direct light through the objective lenses and eventually into the viewing head.