Compound Microscope Basics
by Laura Templeton on May 27, 2020
What is a compound Microscope?
Compound microscope is a general term and covers a wide variety of microscopes; they can be anything from a basic student scope you would see in a classroom all the way up to highly expensive and specialized microscopes used in research labs. However, regardless of their price, all compound microscopes utilize a set of two lenses (one found in the eyepieces and one found in the objective lenses) to produce a highly magnified image far beyond what our unaided eyes can see. These sets of lenses are collectively known as a compound lens system.
(White Blood Cells Captured Under a Compound Microscope)
(Left: The Z4 Stereoscope Right: The Revelation III Compound Microscope)
Stereoscopes vs. Compound Microscopes
Compound microscopes differ from stereoscopes, another common type of microscope, in several ways. Compound microscopes are designed to view specimens that are transparent -- they have been stained and affixed to a slide. Stereoscopes are able to view non-transparent objects at much lower magnifications than compound microscopes. The lower magnification of a stereoscope is not a shortcoming but a design decision. Stereoscopes are used to view larger, three dimensional objects. If you wanted to view a slide of red or white blood cells, you would use a compound microscope. However, if you wanted to look at an apple or a circuit board, you would use a stereoscope. These two types of microscopes serve very different purposes, so it’s important to know which you will need to fit your application.
A compound microscope needs some form of illumination. As light shines from the base of the microscope onto the object, the lens nearest the object—called the objective lens—produces an enlarged image of the object for you to view through the eyepieces. Modern compound microscopes almost exclusively use LED illumination due to its low energy consumption and long life. When purchasing a new compound microscope, we highly suggest you purchase a microscope with LED illumination over an older halogen scope. If you’re interested in picking up starter microscope that includes LED illumination, we recommend you look at our Levenhuk 50L.
(Image of a tumor under a compound microscope)
Most microscopes also include a diaphragm that allows you to further control the amount of illumination that hits the specimen. The diaphragm is located directly below the stage and above the light source. Usually diaphragms are of two types: a disc diaphragm (a disc with a set of predrilled holes that gives you fixed levels of adjustment) or a better choice, the iris diaphragm (which, like a camera lens, offers unlimited adjustment). Depending upon your specimen, more or less light may be required for the best viewing.
Objective Lenses and Magnification
If you look back at the microscope diagram at the top of the page, you’ll notice that there are multiple objectives attached to the middle of the microscope. Each of these objectives offers a different level of magnification (commonly called “power” in microscopy). The ability to adjust magnification is a key component of a compound microscope. If your goal is to view only a specific area or part of a specimen, a low power may not be enough for you. By turning the nose piece, the piece on the microscope that the objectives are attached to, you can adjust the magnification of your image. The most common objective magnifications are 4x, 10x, 40x, and 100x.
The microscope eyepieces that you look into have their own level of magnification as well, usually 10x. By multiplying the eyepiece magnification by the objective magnification you can determine the total level of magnification. For example, a 10x eyepiece in conjunction with a 40X objective, will give you a magnification factor of 400. The object will be magnified 400 times larger than you can view it with the naked eye.
As a fun fact, a compound microscope can go no higher than 2000x level of magnification. Beyond that limit you could indeed magnify it, but neither your eyes nor your brain would be able to recognize the image. Incredibly small specimens, such as viruses, molecules and atoms, are beyond the capabilities of a compound microscope and can be viewed only with an electron microscope. Before your rush out and buy an electron microscope just know that the prices for them range from hundreds of thousands to even millions of dollars, so you might want to curb your enthusiasm a little and start with a compound microscope!
TIP: No matter how high of a magnification power you want to use, it’s best to start by bringing the specimen into focus with the lowest objective power and work your way through each objective until you achieve the desired image. It can be time consuming (and frustrating!) to use just the 40x or 100x objective to attempt to target the exact area you want to view.
The Microscope Stage
We’ve learned how to get a closer view of our specimen, but we haven’t discussed how you bring a specimen into focus. This is where the microscope’s stage comes in. The stage is the part of a compound microscope that holds the specimen to be viewed.
Most compound microscope will come equipped with a stage control knob that allows you to adjust the x (horizontal) and y (vertical) position of the stage, allowing you focus the specimen. More expensive microscope utilize what is known as a mechanical stage. A mechanical stage is a more precise stage that also allows you to control the z (depth) position of the specimen. Without a mechanical stage, you must manually adjust the specimen’s position with your hands. While this will work fine at lower magnifications, when you get into high levels of magnifications, moving the stage merely an inch can result in movement that feels like a mile! Mechanical stages are more costly, but if you’re going to be performing tasks that require high magnifications, we’d strongly recommend you purchase a microscope that is equipped with one as they provide a much finer level of control.
If you're looking for a low-cost microscope with a mechanical stage, take a look at our Student Advanced Pro Microscope.
Focusing a Specimen
Once you have secured your specimen, set your microscope to the lowest powered objective (hopefully you remembered the above tip!) and, try to align the specimen as near to the center of where the objective will come down. This will make it easier to locate when viewing through the eyepiece. Before looking through the eyepiece, turn the coarse focus knob, which will lower the objective, until the objective is almost in contact with the slide.
It’s important that you use the coarse focus knob (generally found on the side of the microscope) for this step and not the stage’s vertical adjust. If you are looking through the eyepiece and you move the objective downward, you run a very high risk of contacting the slide with the objective and damaging one or both of them. Slides are cheap—objectives aren’t. By starting in the lowered position, your focus will bring the objective up, away from any potential damage. As you turn the coarse focus knob, your specimen will come into focus. Some microscopes will come with a fine focus knob (usually located on the coarse focus knob), this allows you to make even more minute adjustments to your specimen.
With your specimen now in focus on the lowest power, you’ll want to apply all the steps we’ve discussed above and further adjust the specimen until you receive the desired result. Do remember that focusing a specimen under a microscope is a skill unto itself. You may initially struggle to find the desired result you want or make a small move and totally throw everything off. If this happens, go back to your 4x objective and refocus the image and try again. Most importantly of all, don’t get discouraged! With time and practice you’ll greatly improve your skills.
TIP: You should now begin to learn an important skill that will significantly increase your enjoyment of the instrument. You must learn to view through the eyepiece(s) with both eyes open! Whether you have a monocular microscope (one eyepiece,) or a binocular microscope (two eyepieces,) start from the beginning to use both eyes. This might seem strange or even unnatural at first, especially if you only have one eyepiece, but try this experiment. Close one eye and squint as long as you can. Not very comfortable, is it?
(Image of a Pap Smear)
Now, try this. Open both eyes, but place your hand over one eye. Isn’t that more comfortable? You could probably do this indefinitely. If you were spending hours each day looking through a microscope, which would you prefer? Train yourself to always keep both eyes open. It may seem difficult, but your eye will automatically shut out the image from the eye not used for viewing through the monocular microscope, and with the binocular microscope, both eyes will focus on the image. Like riding a bicycle it might feel unnatural at first, but once you learn it, you’ll never forget it.
Final ThoughtsWe hope you enjoyed this brief guide on the parts and applications of a compound microscope. If you’re interested in jumping into microscopy and purchasing your own scope, we can make a few recommendations:
For teachers or beginner hobbyist the Levenhuk 50L is an excellent place to start.
If you're looking to spend a little more money, we'd recommend the LW Scientific Student Advanced Pro Microscope. This microscope includes a mechanical stage making fine adjustments much easier.
Finally, if you're looking for professional grade microscope, the LW Scientific Revelation III is being used in labs all around the world every day and offers fantastic performance for a low price!
And of course, if you have any questions do not hesitate to contact us! We’ll be more than happy to answer any questions you have and help you find the microscope that best suits your needs!