Microscope Quick Guide

Basic Rules for Effective Use of A Compound Light Microscope

1) Always carry a microscope with TWO hands!

2) To focus on a prepared slide:

a)Always start with the LOW POWER (10x) objective. On some microscopes there may be a SCANNINGobjective (4x) but for most ‘scopes the low power objective is the lowest power objective.

**You should also begin with the light on the lowest/narrowest setting, and increase as needed while viewing. Too much light saturates the field of view, which means decreased contrast. This can eliminate the details of your specimen from view.

b)While watching from the SIDE (not through the eyepiece), use the COARSE adjustment (big knob) to move the 10x objective and the slide towards each other. Then, while looking into the EYEPIECE, use the coarse adjustment to slowly move the 10x objective lens and the slide apart until the specimen comes into focus. (This method prevents you from scratching the glass lens of the microscope, which is very delicate & expensive to replace!)

c)When you are ready to view the specimen under HIGH POWER, first make sure the part you want to see in high power is clearly focused and in the CENTER of your field of view in low power. THEN, gently turn the 40x objective into place, WITHOUT CHANGING THE FOCUS.

You must center your specimen before changing magnification because the higher the magnification of the objective lens, the smaller the field of view. You could lose sight of your specimen if you switch magnification without centering first.

d)In high power, use the FINE adjustment (the smaller knob) ONLY to focus on your specimen. This knob moves the lens less distance per rotation and will give you the fine control needed to focus at high magnification.

e)When you switch to high power, you most likely will need to INCREASE the amount of light coming through the specimen by adjusting the DIAPHRAGM. This is because higher magnification lenses require more light. Does your specimen look darker when you switch to high power?

Troubleshooting in High Power:

**If you cannot find your specimen when you switch to high power, go back to LOW POWER, re-focus with the coarse knob and make sure the part you want to see is centered in your field of view. Then, go back to high power to try again.

3) Lastly, a MICROSCOPE DRAWING should always include:

• the Field Of View (the circle) with your drawing done TO SCALE within the FOV

• TOTAL Magnification (e.g. 100x)

• the name or description of what the specimen is (e.g. cork cells or newspaper “e”)

Resolution And Magnification

Magnificationis how much an image is enlarged under a microscope. An image being seen at a magnification of 100X means that its linear dimensions are 100 times those of the object giving rise to the image.

The compound microscope has two separate lens systems, the objective and the ocular. The objective, nearest the specimen, magnifies the specimen a certain amount, and the ocular further magnifies this image. Thus, the image as seen by the eye has a magnification equal to the product of the magnifications of the two systems.

Resolutionis the amount of detail that can be seen. It refers to the ability of the microscope to distinguish separate and distinct objects that lie in close proximity. Resolution is usually expressed in terms of the minimum distance observable between two objects. The smaller the distance that can be seen between two objects, the better the resolution.

You can enlarge a photograph indefinitely using more powerful lenses, but the image will blur together and be unreadable. Therefore, increasing the magnification will not improve the resolution.

Resolution in some cases is even more important than magnification. A greatly enlarged blur is still a blur. There is no upward limit to the magnification of a microscope, but there is a limit to useful resolution. One can have increase in magnification without an increase in visible detail. The basic limit in seeing objects is resolving power, not magnification.

Attaining the maximum resolution depends primarily on the design of the objective lens. An objective lens capable of utilizing a large angular cone of light coming from the specimen will have a higher resolving power than a lens limited to a smaller cone of light. Of course, the resolvable detail must be magnified a sufficient amount in order to be seen—hence the relationship between magnification and resolution.

In a compound microscope, the wavelength of the light waves that illuminate the specimen limits the resolution. The wavelength of visible light ranges from about 400 to 700 nanometers. The best compound microscopes cannot resolve parts of a specimen that are closer together than about 200 nanometers.

These pictures have the same magnification (the cells are the enlarged the same amount), but the left picture has better resolution than the right picture, which appears more blurry.

What is Resolution?
Magnificationis how much an image is enlarged under a microscope.
Resolutionis the amount of detail you can see in an image. You can enlarge a photograph indefinitely using more powerful lenses, but the image will blur together and be unreadable. Therefore, increasing the magnification will not improve the resolution. This is also known as the resolving power.
Compound Microscope Resolution:
In a compound microscope, the wavelength of the light waves that illuminate the specimen limits the resolution. The wavelength of visible light ranges from about 400 to 700 nanometers. The best compound microscopes cannot resolve parts of a specimen that are closer together than about 200 nanometers.
Dissection Microscope Resolution:
Just like in a compound microscope, the wavelength of light limits resolution. This microscope does not use light to see through the specimen, but uses light to aid in viewing the specimen under magnification. The resolution of the dissecting or stereoscope is about 120 nanometers.
Scanning Electron Microscope Resolution:
In a SEM, an electron beam scans rapidly over the surface of the sample specimen and yields an image of the topography of the surface. The resolution of a SEM is about 10 nanometers (nm). The resolution is limited by the width of the exciting electron beam and the interaction volume of electrons in a solid.
Transmission Electron Microscope Resolution:
In a TEM, a monochromatic beam of electrons is accelerated through a potential of 40 to 100 kilovolts (kV) and passed through a strong magnetic field that acts as a lens. The resolution of a TEM is about 0.2 nanometers (nm). This is the typical separation between two atoms in a solid.