(MIC) Compound Microscope Lesson

Compound Microscope

The compound microscope uses a series of lenses in order to magnify an image so that the subtle characteristics of that object are more clearly seen. Historically, the development of the compound microscope has been attributed to several people. Zaccharias Janssen and his son Hans were said to have created a tube with lenses on each end that could be used to magnify objects in the 1590s, but the focus of the device could not be adjusted or changed. Though it could not be focused, the instrument developed by the Janssens drove the invention of the modern compound microscope as scientists sought a way to focus on various magnified areas and adjust the image accordingly. Perhaps the most famous and accepted history of the modern compound microscope is that of Galileo Galilei who is said to have developed a compound microscope with adjustable focus in 1609. It was given the name of "microscope" in 1625. The term microscope comes from the Greek words micron and skopein which mean "to look at small things".

The compound microscope works by gathering light, redirecting it through a condenser lens and into the path of the specimen. The condenser lens focuses or condenses the light onto the specimen and is needed for higher magnification because it increases the illumination of the light and the resolution. The image of the specimen is then directed to the back portion of the microscope, called the focal plane, by the objective lens. The image from the focal plane is then received by the ocular lens and the image is redirected to the eye. Once the image reaches the eye, it is actually viewed in reverse of its orientation on the slide; essentially the image is upside down and backwards from the orientation on the stage. A compound microscope can generally magnify a specimen in a range of about 40X to 400X, but could be magnified up to 1000X in some compound microscopes.

While the compound microscope is very useful, it is also limited by the constructs of resolution and magnification. Resolution is the shortest distance between two separate points in a microscope's field of view that can still be distinguished as distinct entities. It directly relates to the clarity of the image when viewed. If the image lacks resolution, it will appear "fuzzy" and individual components or characteristics of the image may be obscured. Magnification is the act or process of enlarging the physical appearance or image of something. Magnification does not affect the clarity of the image; it merely makes the image appear larger to the eye so that more qualities or characteristics of the image can be observed. The total magnification of a microscopic image can be calculated by multiplying the magnification of the objective lens by the magnification of the ocular lens.

For example, what would be the total magnification on a microscope with a 10X ocular lens and a 40X objective lens?

A simple but powerful tool in the Forensic Scientist's arsenal, the compound microscope is an essential part of crime labs for very small or dense pieces of evidence. Compound microscopes are most useful when high magnification is needed, but are limited by the size of the object to be viewed. The item must be small enough to fit on slides on the stage while still fitting under the objective lenses. Most compound microscopes used in crime labs have high quality cameras integrated so that microscopic images can be documented photographically at the touch of a button or click of a mouse! The cameras are often programmed so that each photo document is stamped automatically with a date, time, magnification and other important information. In addition, photo documentation of microscopic evidence is stored in multiple places on computers, back-up files and in the case file to prevent loss of important evidence documentation.

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