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A Guide to the Compound Microscope
A Whole New World
Ever since its invention in the 16th century, the microscope has allowed its users to glimpse another world. Even a drop of pond water is found to be teeming with unimaginable life forms. Indeed, there are marvels hidden in our surroundings that the naked eye is unable to see.
The compound microscopes of today still rely on the same principles employed in the 16th century. Very thin samples, mounted on microscopic slides, are placed in the path of a beam of light. These samples are magnified first by the objective lens, and then by the ocular lens. The result is amagnified image that is projected onto the retina of the eye.
With the compound microscope becoming more affordable nowadays, anyone can pick up microscopy as a hobby. As such, I will introduce everything you need to know about the compound microscope. Are you ready to explore a whole new world?
About 1595, Zachiarias Janssen, with the help of his father, probably invented the first compound microscope. In 1665, Robert Hooke published Micrographia, a book showcasing specimens taken under the compound microscope that have never been seen before.
Anatomy of a Microscope
There are many kinds of compound microscopes. However, all of them consist of the same basic parts, albeit with slight variations.
The base provides support for the microscope.
The illuminator is located at the base.
The stage is located above the base, and contains an opening for light from the illuminator to pass through.
The microscopic slide is a thin piece of glass used to contain the sample.
Stage clips are used to hold the microscopic slide in place on the stage.
The condenser's purpose is to gather diffuse light from the illuminator and concentrate it onto the sample.
An iris diaphragm controls the amount of light entering the condenser.
Two major control knobs are found on most compound microscopes. The largest knob is for adjusting the coarse focus. Close to it is a smaller knob for adjusting the fine focus. These two knobs control the vertical movement of the objective lenses.
Compound microscopes have two to five objective lenses, mounted on a revolving nosepiece. An objective lens provides the initial magnification of a specimen. Most microscopes contain objective lenses with low, middle and high magnifications.
Good microscopes also have objective lenses that are parfocal with one another. As such, once you focus with the lowest magnification and switch to higher magnifications, the sample will remain in decent focus.
The user views the sample through the ocular lens, which further magnifies the image. Most microscopes nowadays are binocular. Diopter rings on the ocular tubes allow for possible eyesight inconsistencies for one or both eyes.
Buying a Compound Microscope
Things to Note
With so many different brands of compound microscope out there, finding a suitable one can be confusing for the novice. As such, here are some points to note when considering your first purchase.
First and foremost, a compound microscope is typically used to be view samples that are 'truly microscopic', such as bacteria, water organisms and blood cells. If you're looking to view 'bigger' samples like plants, then a stereo microscope is a better choice.
A binocular microscope is recommended as it puts less strain on the eyes compared to one that is monocular. Furthermore, most binocular microscopes come with a mechanical stage, allowing for more precise movements of the sample.
Even if you're purchasing a compound microscope for your child, avoid those that are made from low quality materials such as plastic. They are likely to break easily, and do little to facilitate your child's learning.
Of course, price is an important factor when choosing a compound microscope. Fortunately, reputable brands such as OMAX and AmScope regularly sell good quality microscopes at affordable prices. Currently both brands are offering a 90% discount on Amazon!
The objective lenses should be DIN (Deutsch Industrie Norm) compatible. Since DIN is the international standard, in the event that you break or lose a lens, you can easily obtain any replacement.
A good microscope should have a condenser with an iris diaphragm to optimize brightness, contrast and evenness of illumination, so as to achieve satisfactory results. An Abbe condenser is considered ideal for a compound microscope.
So You Got Everything You Need
Before anything else, you need to calibrate it to your eyesight for an optimal viewing experience (assuming that you have a binocular microscope).
- Adjust the interpupillary distance by moving the ocular tubes together or apart so that you only see one image.
- Rotate the diopter ring on the left ocular tube until it matches your interpupillary distance (in numerical value).
- Close your right eye and focus on the sample using the coarse and fine adjustments.
- Close your left eye and make the necessary adjustments to the diopter ring on the right ocular tube until you see a clear image.
Tips and tricks
When working with a compound microscope, you should always start with the objective lens with the lowest magnification.
The exact magnification is listed on the side of the objective lens mounted on the nose piece.
At the highest magnification, only the fine focus knob should be used in order to prevent destroying the slide by pushing the objective lens through it.
Finding the sample can be frustrating at first because the series of lenses reverses your movement. However, practice makes perfect.
Usually the power of the ocular lens is multiplied by the power of the objective lens to obtain the degree of magnification. However, there is a point where no more information can be gained by increasing magnification. Any further increase is called empty magnification because what you're doing is magnifying the blur inherent in the sample.
The tiny opening of the objective lens should be completely filled with light of the proper intensity passing through the sample. Note that as less light reaches the sample, the image has more contrast but is not as sharply defined.
The sample must be thin enough to easily allow light to pass through. It must have contrasting elements of dark and light. For this reason, staining is often used to accentuate the features.
Care and Maintenance
The following precautions should be observed when handling and caring for a microscope. First, the microscope should always be lifted with one hand on the arm, and the other hand on the base.
Fingers should be kept off slides, mirror surfaces and ocular lenses. If smudges accidentally occur, lens paper can be used to remove them. If an objective lens becomes dirty, it may be cleaned by using a cotton swab dipped in benzene or ethanol.
The microscope should be covered after use, ideally with something that is dust repellent.