In 1665, Robert Hooke observed the first cells by putting two sets of lenses together to get an increase in magnification. That was the beginning of microscope development and discovery. Now they are used to view blood samples, tissue samples and body fluids. Medical professionals use them to diagnose infection and foodborne illness. Botanists use them for plant identification and lab technicians use them to differentiate between cell lines and even to diagnose cancer.

Many different organisms can be viewed with different types of microscopes. Bacteria, which cause many infections, range in size from .1m to 5m or more. These can be viewed with a common light microscope. Fungi are larger than bacteria, and their single-cell stage, called yeast, can be viewed with a light microscope. Parasites are organisms that derive nutrients from a living host. Many parasitic eggs can be detected using a light microscope. A virus is the smallest known organism and cannot live without a host. They are generally less than 1m in size. Viruses can only be seen with an electron microscope.

What are the Different Types of Microscope

Compound Microscopes have two sets of lenses (objectives and oculars) that use visible light as the source of illumination.

Darkfield Microscopes have a device to scatter light from the illuminator so that the specimen appears white against a black background.

Electron Microscopes use a flow of electrons, instead of light, to produce an image. They enhance images of viruses, ribosomes, proteins, lipids and even small molecules.

Fluorescence Microscopes use an ultraviolet light source to illuminate specimens that will fluoresce. Usually a fluorescent dye or antibody has been added to the specimen that is being viewed.

Phase Contrast Microscopes allow examination of structures inside the cells by using a special condenser. They take advantage of different refractive indexes and allows for examination of live organisms, because it is unnecessary to stain the cells to give good differentiation of parts. The final image is a combination of light and dark to produce the image.

Microscope Terminology

Achromatic Description of the viewing area of a standard microscope. With an achromatic microscope, the edge of the viewing area is curved. This curve changes the perception of the specimen at the edge of the circular viewing area and makes them seem rounder and longer. This is similar to objects perceived in the edges of a magnifying glass.

Plan Achromatic Description of the viewing area of a microscope in which a more expensive lens system compensates for the distortion at the edges of the viewing area; a true representation of the full viewing area can be observed.

Pole Microscope Microscope where the focus adjustment knob moves the entire top of the unit on a pole and not on an arm. This type is a little less sturdy than the traditional arm-style microscope.

Refractive Index The relative velocity through which light passes through a substance.

Resolution or Resolving Power The ability to distinguish fine detail with a magnifying instrument. Also known as the ability to distinguish between two points a specified distance apart.

Total Magnification Magnification of a specimen, determined by multiplying the ocular lens magnification by the objective lens magnification.

Microscope Parts

Arm Area on the back of the microscope that supports the oculars and objectives. Part of the handle used to lift and carry it.

Base The bottom of the scope. Usually houses the light source, if one is present. The extended, rear portion of the base also functions as the handle and is used to lift and carry the microscope.

Course Focusing Knob Used to adjust the position of the objective lenses. This should be adjusted while continuing to make sure the objective is NOT going to hit the slide. It should be stopped once the object is seen through the ocular. Any further adjustment of the knob can break either the slide or the lens in the objective, which is very expensive to replace.

Fine Focusing Knob Used to bring a specimen into perfect focus once the specimen has been sighted with the course focusing knob. Adjust very slowly to prevent contact between the objective and the specimen.

Illuminator The light source of a microscope.

Objective Lens or Numerical Aperture This part of a compound microscope is the lens closest to the specimen.

Ocular Lens In a compound light microscope, this is the lens closest to the viewer.

Oil Immersion Lens This is a 100x objective lens. The lens is small in order to achieve high magnification and high resolution. Because of its small size, it is important to get as much light through the lens as possible. By immersing the lens in oil, the refraction (bending) of light is eliminated, because glass and oil have approximately the same refractive index. In this manner, the light is maximized and gives the clearest image. If an oil immersion lens is used without oil, the image becomes fuzzy and has poor resolution.

Commonly Asked Questions About Microscopes

Q. Why do I have to use immersion oil with my 100x objective?

A. In order to get an intense magnification, the lens has to be cut extremely small. Because of this, it is important that as much light go through the lens as possible, so the viewer can see the object in question. The oil changes the refraction of the light, so the light goes through the lens instead of bending and being scattered in all directions.

Q. Which is better: a plan achromatic microscope or an achromatic microscope?

A. Both types of microscopes are excellent for viewing specimens in the center of the viewing area. But if you must count bacteria or distinguish the shape of a bacterium in any position in the field of view, a plan achromatic microscope is better suited. These microscopes have a lens system that compensates for the distortion at the edges of the viewing area, which makes specimens appear extremely clear.

Q. How do I know the total magnification of what I am viewing?

A. The first thing to determine is the magnification of the ocular lens; that is the lens closest to the viewer. The magnification of this lens will be etched into the metal. The standard magnification of an ocular lens is 10x. Then you must find the magnification of the objective lens. That is the lens closest to the specimen. That number will also be etched into the objective. These two magnifications are then multiplied together to get the total magnification. For example, if you had a standard 10x ocular and were using a 40x objective, your total magnification would be 400x.

Sources for More Information

Microscope Primer
Glossary of Terms