Microscopes help you see things that are small. This makes them a key tool for studying structure and function in all kinds of organisms. You'll need to get comfortable with two kinds of microscopes for Bio 6A: dissecting and compound scopes.
Dissecting microscopes, also called stereo microscopes, have some key characteristics that make them useful for looking at large specimens and doing dissections:
- Low magnification. For large specimens, you don't need much magnification to be able to see the important details. Many dissecting microscopes have a zoom feature, allowing you to change the magnification.
- Large field of view. In other words, you can see a relatively large area of your sample; field of view is described in more detail below.
- Large working distance: you'll have plenty of room between your specimen and the microscope's lens, so you can reach in with forceps or other tools to move things around.
With the dissecting scopes, you can use either reflected light or transmitted light. Reflected light is the way you normally see things: light comes from a light source, bounces off something, and you see it. If you use the upper light source on the dissecting scope, you're using reflected light. Transmitted light is light that passes through your specimen, instead of bouncing off. This only works with specimens that are somewhat transparent. If you're using the light source that comes from below your specimen, you're using transmitted light. When using the dissecting scope, try both kinds of lighting to see which works best. Usually it's one or the other; if you have light from above and from below, your image will be washed out and you'll see less. Properly controlling the light will make invisible things become visible.
Overall, dissecting microscopes are simple to understand and use. Compound
microscopes are a little more complex.
Compound scopes provide higher magnification than dissecting scopes. The high magnification makes compound scopes powerful tools, but it also creates some diffuculties. Important characteristics of compound scopes:
- High magnification.
- Small field of view: you can only see a small part of your sample.
- Must use transmitted light (light that shines through your sample); therefore...
- Samples must be thin & transparent, and mounted on glass microscope slides.
In this lab, you'll mostly use the compound scope with commercially prepared microscope slides. These slides generally hold ultra-thin slices of specimens. The specimens are so thin that they are more or less transparent, so the slides are stained to allow you to see specific structures. In general, the colors you see on commercially prepared slides are the colors of the stains, not the colors of the living organism. This is a good thing, because the people who make the slides can choose particular stains to highlight particular features on the sample.
See How to use the compound microscope for more detail.
Magnification means how much larger an object appears in the microscope as compared to without the microscope. If you look at a leaf without a scope, you're seeing it at 1x magnification -- in other words, life size. If you put the leaf under the scope, you might see it at 100x, or one hundred times life size. The total magnification of a compound microscope depends on two sets of lenses:
Ocular (also called eyepiece). This is the lens you look into; it typically has a magnification of 10x.
Objective. This is the lens that's closest to the object you're looking at. Compound scopes usually have several objectives, such as 4x, 10x, 40x, and 100x.
The total magnification is the ocular magnification times the objective magnification. With a 10x ocular and 40x objective, the total magnification is 400x.
Field of view is the diameter of the area you can see when you look through the microscope. When you use higher magnification, the field of view becomes smaller. It is possible to measure the field of view by placing a tiny ruler, called a stage micrometer, on the scope instead of a normal microscope slide.
It's useful to know what your field of view is so you can estimate the size of things you're looking at. If the field of view at a given magnification was 200 microns, and you were looking at a cell that was half as long as the field of view, the cell would be about 100 microns.
Scale bars. Most photomicrographs (pictures taken with a microscope) should have a scale bar -- a small line on the picture, with text indicating the length of the line.
For example, the picture at right (a mushroom cross-section) shows a scale
bar labeled 200 microns. Of course, this scale bar appears much bigger than
200 microns on your computer screen, because it's magnified. How much is
it magnified? That depends not only on the magnification of the microscope,
but also on your computer screen. You could also print this picture larger
or smaller. However you enlarge the picture, you'll always be able to see
that the reddish-brown spores are much smaller than 200 microns. That's
why scale bars are better than writing something like "100x" on
Depth of field describes how much of your image is in focus. A typical small digital camera has vary large depth of field: everything will be in focus, from close to the camera to far away. With a compound microscope, the depth of field may be only a few microns. You may find that you can't get your whole sample in focus at once when you're using the higher-magnification objectives.
This page updated September 17, 2011