UNM Biology Undergraduate Labs

Microscopes

Useful Reading

Campbell, Biology 6th Ed - pgs 109-111

Campbell, Biology 7th Ed - pgs 95-97

Definitions

Microscope – device used to view objects which are too small to see or explore with your eye alone.

Compound – when a scope has a minimum to two magnifying lenses.

Binocular – two eyepieces.

Monocular – one eyepiece.

Stereoscopic (dissecting) microscope – a microscope with relatively low magnification that is good for viewing large, thick objects.

Transmitted light – light passing through an object, used by many types of microscopes.

Reflected or incident light – light directed down on an object, used by dissecting scopes.

Magnification – the ratio of the image size to the actual size; apparent enlargement of an object.

Resolution - the ability of a microscope to distinguish between two very close points.

Electron microscope – microscope that uses electrons as a source of illumination.


Parts of the compound microscope

A compound microscope is composed of many important, functional components. There are at least two lenses – the ocular lens and objective lens. The ocular lens is in the eyepiece; it is usually 10x power. The objective lenses are near the stage on the revolving nosepiece. There are usually multiple objective lenses that are used to vary the magnification; on many microscopes the objectives are 4x, 10x and 40x magnification. The condenser and iris diaphragm are important devices used to adjust how light hits the sample, thus influencing the visualization of the sample. For more information on microscope parts, click here.

 

Using a compound microscope

To use a microscope, you must know how to focus on the sample, adjust the condenser and diaphragm to optimize visualization of the sample. For binocular scopes, you must also set the appropriate interpupillary distance and adjust the diopter ring to focus both eyes on the sample.

The image you see through the microscope is inverted, which should be kept in mind when moving the stage and sample. The field of view and depth of field decrease as the magnification increases; by knowing the diameter (mm) of the field of view for each objective lens, the size of objects can be measured.

  

 

Stereoscopic (dissecting) microscopes

A stereoscopic or dissecting microscope is a relatively low magnification (often 2x-30x) microscope that is good for viewing large objects. As its name implies, it is better than a compound microscope for dissecting many materials (e.g. small animals, plants, organs) because it produces a 3-dimensional image.

Although its magnification is generally less than that of a compound microscope, it has a couple of advantages over compound scopes. First, it has a large depth of field, so you can observe thick objects with most parts in focus at the same time. Second, you can illuminate your sample using incident light as well as transmitted light, so that thick objects which will not transmit much light can be illuminated adequately to view.

Dissecting microscopes have an objective lens which often allows a continuous range of magnification (from 2-30x), controlled by a magnification knob. There is a focus knob, and a source of transmitted light (adjustable); many scopes also contain a source of incident light. Another difference from a compound microscope is that the stage is much farther from the objective lens, allowing large objects to be placed on the stage.

       

For more on dissecting scopes, click here.

 

Electron Microscopes

 An electron microscope visualizes objects with electrons as the source of illumination, allowing much greater magnification and resolution than a compound microscope. Transmission electron microscopes transmit electrons through a thin slice of sample. Scanning electron microscopes use incident electrons on whole samples to get a 3-dimensional image.

Similar to compound microscopes, electron microscopes have an energy source (electrons instead of light), an objective lens, and a condenser lens. Images are viewed through an eyepiece, or projected onto a screen or photographic plate.

The following images show: a golgi apparatus at unknown magnification,

the head region of a deer tick magnified 500x,

Spirogyra (Protista-green algae) magnified 500x,

and a diatom magnified 15,000x.

 These images show a mosquito at increasing magnification.

For great images from electron microscopes, click on the following links:

For a page with magnification of fly, mosquito and ant body parts, click here.

For interesting websites on how electron microscopes work, click on the following links:


Review Questions

-What does it mean for a microscope to be “compound”?
-What is the total magnification if the ocular lens is 10x and the objective lens is 40x?
-Why is it important to use the diopter ring on a microscope?
-If you’re looking through a compound microscope and the following problems arise, how can you fix them?

1. The image appears very grainy?

2. You can only see a portion of the object, and you’d like to see more of it in one field of view?

3. The image is very dark?

4. You see dual images of the sample, right next to each other?

-If you are observing a sample through a compound microscope and you want to move the image to the left, which way should you move the stage?
-If you wanted to observe a whole insect at ~400x magnification, would you use a compound microscope or an electron microscope? If you wanted to observe a very thin object at the same magnification, what would be an advantage of using an electron microscope for this sample?
-What structures are common between compound and electron microscopes?
-What type of microscope would you use if you wanted to observe how a worm ingests a piece of food?