The Structure and Function of a Light Microscope

We may never know who made the first microscope. It could have been a spectacle maker, or a telescope maker who discovered that a telescope reversed magnified objects. However, two of the greatest pioneers in microscopy were Robert Hooke who used both a compound microscope and a single lens microscope around 1665 in England. In a compound microscope the magnification is the result of the eyepiece magnification multiplied by the objective magnification. The other microscopist was Antonie van Leeuwenhoek, a Dutch fabric merchant who created single lens microscopes that were able to magnify approximately 400X. Antonie discovered bacteria and protozoa around 1674 and he is known as the Father of Microbiology. Robert Hooke is known for having discovered cells in cork and gave “cells” their name. To learn more about the interesting history of the microscope see Brian J. Ford (1973) “The Revealing Lens - Mankind and the Microscope”.

An optical microscope until recently could resolve objects separated by about 0.2 microns where a micron = 0.001 mm. Super resolution optical microscopes were developed in the early 1990s and can visualize molecules down to about ~250 nm (nm = 0.000001 mm). Bacteria (prokaryotes cells without a nucleus) are about 1 micron and larger, whereas eukaryotic cells (cells with a nucleus) are about 6 microns in size or larger. The microscope is significant because it allowed scientists to identify contagion (bacteria) that could infect and harm humankind. Light?microscopes are routinely used to examine blood samples and the structure of plant and animal cells. Below is an image of a large bacterium found in ponds magnified 630X.

The purpose of this article is to describe the structure and function of a modern light microscope and its’ components. For those new to microscopes they might appear complicated. However, only a few hours of instruction will reveal how to use one. An entry level microscope costs about $150. A super resolution microscope can cost hundreds of thousands. There are three common types of light microscope used in biology that magnify objects: stereo microscopes, compound microscopes and inverted microscopes.

First you need to be able to identify the major parts of a microscope and then learn what their function is. The parts of a polarizing light microscope are shown in the labelled diagram Fig. 3.

A polarizing microscope includes the use of two polarizing filters and an accessory filter (wave plate or retardation filter). Polarizing filters can be added to any light microscope allowing some specimens to be viewed in color (crystals, hair, and plants) without staining them. A polarizing microscope can also be used to see subjects by white light called bright field microscopy. Learning how to use a light microscope is easier when someone shows you, even children can learn how to use one.

Cells and tissues viewed by a light microscope often appear translucent because of their high water content. One solution is to stain tissues with various dyes. Unfortunately this requires thin sections or isolated cells that are fixed and stained before being viewed. Processing involves treating the cells with a fixative (e.g. alcohol or formalin), embedding the tissues in paraffin and cutting thin sections which are mounted on microscope slides. Sometimes thin sections can be made simply with just a razor blade.

Scientists have developed optical techniques that can add color and contrast to cells. These techniques include dark-field illumination, phase contrast, fluorescence, polarized light, differential interference contrast and others. When crystal-like subjects made up of more than one refractive index are observed with polarized light they display different colors which can reveal information about the molecular properties of the substance. The crystals may also form beautiful abstract patterns.

Click to read the full article for more information, including different parts of optical microscopes, different types of condensers, differences between finite Mechanical tube length microscopes with infinity microscopes ... and more

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