Microscope is a piece of laboratory optical equipment that is used to magnify small things that are too small for the details to be seen by the naked eye. The microscope is the microbiologist’s most basic tool, and every student of microbiology needs some background knowledge on parts of a microscope and how microscopes work.
The microscope was developed in the 16th century. Antony van Leeuwenhoek made the first modern microscope. He is also known as the father of microscopy. Microscopy is the technical term in which the microscope in used for investigation.
Do you know?
Antoni van Leeuwenhoek is the first person to see bacteria.
There are different types of microscopes based on their working mechanism and functions but the microscopes can be broadly classified into;
- Light (optical) microscope and
- Electron microscope
The Light Microscope
Light microscopes are used to examine cells at relatively low magnifications. Magnifications of about 2000X are the upper limit for light microscopes. The highest resolution of a light microscope is about 0.2 μm. The use of blue light to illuminate a specimen gives the highest resolution. It is because the blue light is of a shorter wavelength than white or red light. For this reason, many light microscopes come fitted with a blue filter over the condenser lens to improve resolution.
The common light microscope used in the laboratory is called a compound microscope. It is because it contains two types of lenses; ocular and objective. The ocular lens is the lens close to the eye and the objective lens is the lens close to the object. These lenses work together to magnify the image of an object.
Parts of Compound Microscope
There are twelve parts in a compound microscope. They are as follows:
1. Illuminator (Light Source)
A mirror or electric bulb is provided as the source of light rays. The function of the mirror is to provide reflected light from a lamp or sunlight. Most microscopes today have built-in lamps that provide necessary illumination.
You can turn on and off the light source using a switch and adjust the light intensity by turning the light adjustment knob. This knob is calibrated with a scale of 1 to 10; 1 is low intensity, and 10 is high intensity.
2. Diaphragm (Irish)
Many microscopes have a rotating disk under the stage known as the diaphragm or iris. The diaphragm has different-sized holes that control the amount of light passing through it. Based on the transparency of the specimen, adjustment of the diaphragm setting to achieve a needed degree of contrast is possible.
Beneath the stage is a group of lenses that comprise the condenser. The condenser accepts parallel light rays produced by an illuminator and condenses them into a strong beam. It causes light rays from the light source to converge on the microscopic slide. The clarity of the image increases with the higher magnification of the condenser.
Abbe condenser is present in the more sophisticated microscopes with a higher magnification of 1000X. The condenser focus knob helps in the up-down movement of the condenser and aids in controlling the focus of light on the specimen.
It is the hole present in the microscopic stage. Through the aperture, the transmitted light reaches the stage from the source.
The stage is a flat platform positioned about halfway up the arm. It is the part that holds the slides. The stage can also be moved up or down with a sub-stage adjustment knob. The stage holds the microscope slides using either simple or mechanical stage clips. A mechanical stage allows the operator to move a slide around smoothly during viewing by the use of stage control knobs.
6. Objective lens
These are primary lenses that magnify the specimens. Four objective lenses are present in the compound light microscope. The shortest lens has the lowest power. Similarly, the longest one is the lens with the greatest power. The higher power objective lenses are retractable, i.e., when they hit a slide, the end of the lens will push in, thereby protecting the lens and the slide.
- (4X): It is a scanning objective lens. It also provides the lowest magnification power of all objective lenses.
- (10X): It is a low-power lens. Lower magnifications locate specimen samples in certain areas on a microscope slide.
- (40 X): It is a high-power lens. 40X objective lens is used for examination of wet preparations e.g, hanging drop, and ova and cyst examination in the stool.
- (100 X): It is the oil-immersion lens. The lenses on which oil is used are called oil-immersion lenses. Visualization of bacteria generally requires immersion oil with 100X objective (i.e. total magnification of 1000X). Magnification of 1000X is sufficient for the visualization of fungi, most parasites, and bacteria but is not enough for observing viruses that require magnification of 100,000X or more. Electron microscope provides such magnification.
Most ocular lens magnifies the image ten times. So the total magnification of a microscope is calculated by multiplying the power of the objective lens by the power of the eyepiece (10x). For example, if you are observing an object by a scanning objective lens (4x), you are observing a 40 times magnified image (10x eyepiece lens multiplied by 4x scanning objective lens).
7. Body Tube
It transmits the image from the objective lens to the ocular lens.
8. Ocular Lens (eye-piece)
It is located at the top of the microscope, and the ocular lens or eyepiece lens is used to look through the specimen. It also magnifies the image formed by the objective lens, usually ten times (10x) or 15 times (15x). Usually, a microscope has an eyepiece of 10x magnification power. Advanced microscopes have eyepieces for both eyes and are called binocular microscopes.
The eyepiece tube, also known as the eyepiece holder, holds the eyepiece lens together. They are flexible in the binocular microscope that rotates for maximum visualization. They are not flexible in the monocular microscopes.
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9. Revolving Nose Piece
The revolving nosepiece holds several objective lenses of varying magnification. It is movable, and the user can rotate it to achieve desired magnification levels. Ideally, a microscope should be parfocal, i.e. the image should remain focused when objectives are changed.
10. Coarse and Fine Adjustment Knob
Coarse Adjustment Knob
The coarse adjustment knob located in the arm of a microscope moves the stage up and down to bring the specimen into focus. The coarse adjustment helps to get the first focus. The gearing mechanism of the adjustment produces a large vertical movement of the stage with only a partial revolution of the knob. Because of this, the coarse adjustment should only be used with low power (4x and 10x objectives) and never with high power lenses (40x and 100x).
Fine Adjustment Knob
A fine adjustment knob is generally present inside the coarse adjustment knob. It helps in bringing the specimen into sharp focus under lower power. It also helps for overall focusing when using a high-power lens.
The arm of the microscope supports the tube and connects it with the base. The arm as well as the base help to carry the microscope. In the case of high-quality microscopes, an articulated arm with more than one joint is present.
The base is the bottom of a microscope. It helps to support the microscope. A microscopic illuminator is also present in it.
References and further readings
- Madigan, M. T., Martinko, J. M., Stahl, D. A., & Clark, D. P. (2011). BROCK Biology of Microorganisms (13thedition). Benjamin Cumming.
- Prescott, L. M. (2002). Microbiology (5th edition). The McGraw-Hill Companies.
- Abramowitz, M., & Davidson, M. Eyepieces (Oculars). Evident. Retrieved 6 June 2022, from https://www.olympus-lifescience.com/en/microscope-resource/primer/anatomy/oculars/.