How to Use a Compound Microscope to Study Cells
What is a Compound Microscope?
- Also referred to as the 'light microscope', the compound microscope is the most common type of microscope, found in almost every school or microbiology lab.
- A compound microscope is fitted with two or more convex lenses which enable a high magnification and thus a detailed image of cells, tissues and micro-organisms.
- In the microscope there is an objective lens which provides the primary magnification as well as an eyepiece lens which further magnifies the image.
- The image that a compound microscope produces is two dimensional as opposed to a stereo microscope or dissecting microscope which produces a three dimensional image.
- Ordinarily, compound microscopes have four objective lenses that can be rotated into different positions in order to view specimens at different magnifications (x4, x10, x40 and x100). Compound microscopes have a 'resolving power' of 200nm meaning that if the specimens that you are viewing are closer than 200nm apart then they will be seen as just one object. This limitation is due to the size of the wavelength of light.
- Sectioning: specimens are covered in wax and then cut into thin sections. Sectioning prevents damage to the structure of the specimen whilst cutting it, especially when cutting soft tissue.
- Staining: Although you can view some specimens under the microscope without any preparation, a lot of biological matter is transparent which makes it extremely hard to see in any detail, even under a microscope. In order to see these you must use coloured stains that contain chemicals that bind to the chemicals that are found on the specimen you are trying to view - this allows them to be seen under the microscope. Some stains bind to specific parts of the cell, for instance a stain called acetic orcein stains the DNA of a cell a dark red colour.
Using The Microscope
Now you have prepared your specimen and put it onto the glass microscope slide, adjust the microscope so that it is on the lowest magnification (by rotating the objective lenses) and then put your glass slide onto the viewing tray. Look into the eye piece and turn the larger focus knob until you begin to see the image of your specimen more clearly. To refine the image, turn the smaller fine focus knob until the image is sharper. If you wish to study the specimen in more detail, then rotate the objective lenses to a higher magnification, you may need to adjust the focus knobs again because the image may become blurry when you change the magnification.
Compound microscopes can be fitted with an eyepiece graticule, this is transparent with a small ruler drawn onto it. The ruler can be seen on top of the specimen when you view it through the microscope and therefore allows you to measure the specimen.
What You Should See In a Human Cell
Depending on the strength of the microscope, you should see different common structures of an animal cell in different levels of clarity.
At the very least, you should be able to identify where the cell starts and ends (by its circular membrane) and see its nucleus.
You may also see ribosomes (small round globules), the golgi apparatus and the rough endoplasmic reticulum (easy to identify by their many folds).
Here are two useful equations you may need to know:
Magnification = size of the image ÷ actual size of the object
Actual object size = size of the image ÷ magnification
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