5.1: Microscope

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To be able to distinguish two objects (i.e. to resolve them), they must be separated by some minimum distance, i.e., the limit of resolution. If they are any closer they will appear as one object. The best human eyes have a limit of resolution of about 0.1 mm.

To observe objects that are too small to be seen (or resolved) with the naked eye, we must use a microscope. The simple purpose of the microscope is to observe things that are too small to be seen clearly. All microscopes are designed to both magnify and to increase resolution. The limit of resolution of the light microscope is usually 0.2 \(\mu\)m. This means that many objects of biological importance, from cells and some sub-cellular structures to small tissues and organs are visible.

The compound light microscope is used for magnifying objects, typically 100\(\times\) (or “100 times larger than actual”) to 1000\(\times\). There are several additional features of this microscope. First, there are two adjustment knobs: coarse and fine. The coarse adjustment knob is only used when using the lowest power objective lens. When the higher power objective lenses used, focus using only the fine adjustment knob.

There are several important safety rules which everybody must remember:

Good microscope is a really expensive equipment, and it is not easy to mend or replace it.
Never use the coarse adjustment knob with high (more than \(30\times\)) magnification lenses!
Never ever leave microscope on high lenses! If you finish your work or go out of microscope for a while, switch to smaller lenses (e.g. \(5\times\))
In our labs, we do not use the highest lenses available (\(100\times\)). Do not switch to it. Our highest is \(40\times\). Also, remember that with \(40\times\) you must always use the slides with cover glasses.
Do not move microscope, move yourself! It is made heavy for the reason.
Always use manipulator, do not move slide with you fingers.

A magnifying glass is a simple microscope because it consists of only one lens; our microscope is called compound because it consists of two or more lenses. The two lenses on the microscope are called the ocular lens (the eyepiece, or lens closest to your eye) and the objective lens (the lens that is variable and can be changed). The ocular lens typically magnifies objects \(10\times\), whereas the objective lenses (used one at a time and mounted on a nosepiece) vary from \(4\times\) to \(100\times\).

As two or more lenses are used together, their effects are multiplicative; this means that a lens that magnifies objects 10 times used with a lens that magnifies objects 4 times will together magnify an object 40 times (i.e., \(4 \times 10 = 40\)).

Total magnification refers to this magnification produced together by the ocular lens and the objective lens in use. When discussing magnification, the microscope user should be clear about whether “objective lens magnification” or “total magnification” is being used!