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Three Classes of Microscopes

Scientists often refer to three main types of microscopes: dissection, compound and electron microscopes. Electron microscopes are often subdivided into two categories: the scanning electron microscope (SEM) and transmission electron microscope (TEM).

When discussing microscopes, it is important to understand a few terms. Magnification refers to the size ratio between the what is seen by the individual using the microscope and the object being examined. The amount of the object being examined that is seen is called the field of view. The clarity and amount of detail that can be seen is referred to as resolution. Parfocal refers to a microscopes ability to remained focused despite changing magnifications.

With the terms understood, we can discuss each type more in depth.

Dissection Microscope
- Dissection microscopes use a light source to illuminate the object being examined and produce a three-dimensional image. It its generally used to look at larger sections of small objects and has a lower magnification range as a result. Individual cells cannot be seen with this microscope. The three-dimensional images produced have a very high resolution.
Compound Microscope
- The compound microscope is the most commonly used of the three types. Like the dissection microscope, the compound microscope is light-illuminated. However, this microscope produces a two-dimensional image. Although the resolution for compound microscopes is low, the magnification power is very high, often magnifying objects up to 100 times their normal size. Individual cells can be viewed with this type of microscope.
  
  Most compound microscopes contain the same components. Individuals look at the object through a lens called the ocular. The lens itself magnifies the object being examined to 10 times its actual size. The barrel/body of the microscope extends from the ocular down to a nosepiece that contains other lenses used for various magnification levels. The plate which the object being examined rests on can also be adjusted to increase or decrease the field of view.
Electron Microscopes
- Both the scanning and transmission electron microscopes use electron illumination. Scanning electron microscopes produce three-dimensional images. In order to produce the images, the object being observed is first coated in gold so that the electrons bounce off of it and produce an image of the object's surface. A SEM produces a high magnified, high resolution image that is in black and white.
  
  Transmission electron microscopes produce two-dimensional images. In order to produce the images, thin slices of objects are obtained, and electrons are passed through the object. High resolution, high magnified images are produced with TEMs.

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