Microorganisms, as their name implies, cannot be seen with the naked eye. Although they were observed as early as 1674 by Anton Van Leeuwenhoek using a simple, single-lens microscope, it was not until the development of the modern compound microscope that the real diversity of microorganisms became apparent.
There are two basic categories of microscopes: light microscopes and electron microscopes. Light, or optical, microscopes require light waves to provide the illumination while electron microscopes use electrons to provide the illumination. Light microscopes are used for most general laboratory work, while electron microscopes are used to view extremely small objects such as sub-cellular components or viruses. In each basic category of microscope, there are a variety of sub-types. Light microscopes may be bright field, dark field, phase contrast, or fluorescence, while electron microscopes can be either transmission or scanning. The most commonly used laboratory microscope is the bright-field microscope, so this lab will be concerned exclusively with bright-field microscopes.
PARTS OF A MICROSCOPE:
- Base Supports the microscope
- Light Source Illuminates the object to be studied
- Iris Diaphragm Controls the light intensity on the object
- Condenser Concentrates light on the object
- Stage Platform which supports the slide containing the object to be studied
- Arm Carrying handle
- Coarse Adjustment Knob Large outer knob which brings the image into rough focus
- Fine Adjustment Knob Small inner knob which brings the image into clear focus
- Low Power Objective Magnifies the object ten times (10×)
- High Power Objective Magnifies the object 43 times (43×)
- Ocular Lens Magnifies the image produced by the objective lens ten times (10×)
- Revolving Nose piece holds the objective lenses and allows you to change directly from one objective to another without having to refocus Basically, the microscope consists of a support system, a light system, a lens system, and a focusing system. Each of these systems works together to produce a magnified image of the specimen.
The support system consists of the base, arm, and stage. The base and arm are structural elements which hold the other parts of the microscope in place while the stage holds the slide. Depending on the microscope, the slide can be positioned under two spring clips and moved by the fingers, or it can be held in place by a mechanical stage and moved by means of two control knobs.
- Light System The light system passes light through the specimen using the light source, the condenser, and the iris diaphragm. In a bright-field microscope, an incandescent bulb is usually used as the source of illumination. Light from the light source then passes through the condenser which focuses the light on the specimen.
- An iris diaphragm is used to control the intensity, or brightness, of light which passes through the specimen, thus allowing the operator to adjust the intensity and achieve an optimum viewing contrast.
The lens system forms the actual image which you will see when you look through a microscope. A typical compound microscope has two lenses - an objective lens near the specimen and an ocular lens at the top - each of which magnifies the image of the specimen by a certain amount. The ocular lens on most microscopes magnifies 10x. In contrast, the typical microscope has at least three objective lenses mounted on a revolving nose piece to allow for different magnifications. There is a limit to the amount of useful magnification one can achieve with a light microscope. The highest magnification which can be achieved without producing a poorly resolved image is known as the resolving power of the lens. The resolving power is the shortest distance between two closely adjacent points which can be seen and is based on the wavelength of light used for illumination and on the nature of the lens.
The final system at work in the microscope is the focusing system. So far, we have learned how all of the components of the microscope are held together by the support system, how the light system sends light through the specimen, and how the lens system uses that light to magnify the specimen's image and transmit it to our eyes. The focusing system adjusts the distance between the slide and the objective lens so that the image comes into focus. The focusing system consists of two knobs - the coarse adjustment knob and the fine adjustment knob. When focusing, the operator first turns the coarse adjustment knob (which is the larger focus knob) in order to move the stage a large distance and bring the image into the focal plane of the objective lens. At this stage, the image will be visible but fuzzy. Then the operator turns the smaller knob, known as the fine focus knob, to fine tune the focus and to make the image sharply focused.