SCIENCE MATTERS - Microscopes unlock invisible world


The Royal Society of London's first major publication was in 1665. It didn't anticipate this work would be a harbinger for a great leap in understanding of life. Within 15 years this august scientific society would find it necessary to abandon its ideas about the spontaneous generation of lower life forms.

The book was Robert Hooke's Micrographia. Its drawings and descriptions of objects as seen through a microscope generated great excitement. There were illustrations of a flea, a fly's eye and the point of a needle. For the first time in a biological context, Hooke used the word "cell" to describe the structure of cork.

Microscopes weren't new. The classic configuration of a microscope, two lenses mounted at either end of a tube, dates to about 1590. Galileo is known to have constructed an instrument in 1609 that was later referred to, for the first time, as a microscope.

By the early 1670s, a Dutchman, Anton van Leeuwenhoek, had developed methods for polishing lenses with sufficient curvature to yield magnifications of about 300. This was a huge improvement over earlier lenses. He began sending accounts of his observations to the Royal Society.

Initially, his observations were well received. Eventually, the Society would have to come to grips with the implications of Leeuwenhoek's work. It had been thought that lower forms of life were spontaneously generated from "corruption" of natural material. Fleas were thought to be produced from sand or dust, and flour mites from rotten wheat.

Leeuwenhoek's descriptions of microscopic organisms in droplets of water were met with skepticism. After further investigation, Leeuwenhoek was vindicated by the Society and extended a membership in 1680. Fundamental ideas about life had been transformed.

Leeuwenhoek is now generally recognized as the father of microbiology.

A human eye is unable to see objects smaller than about a tenth of a millimeter (4 thousandths of an inch). The best of optical, or light, microscopes have limitations to their magnification.

Typically, magnification is offered as a measure of the utility of a microscope. Under the best conditions, magnifications of about 1,200X are achievable. However, for an image of a very small object to be useful, it must have both magnification and good resolution.

To see detailed structure, greater resolution is essential. This is the ability to distinguish two objects that are very close to one another. Images become blurred when the size of an object approaches about half the wavelength of the light with which it is being viewed.

Light microscopes are not useful for resolving structures less than about .200 microns (about 8 millionths of an inch).

A red blood cell is about 7 microns across. It is easy to see from this that it is a real challenge to see the structural details of cells.

This limitation can be overcome if an object is probed with shorter wavelengths. This is accomplished using subatomic particles.

In 1924, physicist Louis de Broglie introduced his theory for the wave properties of electrons.

In the strange world of quantum mechanics, all physical (massive) objects in motion have wavelike properties. The wavelengths of very large objects like balls, airplanes and planets are so short as to be of no real consequence. The wavelengths of subatomic particles such as electrons are significantly longer, but are still about 100,000 times shorter than those of visible light.

Using these properties, the first electron microscope was built in 1931 by two Germans, Max Knoll and Ernst Ruska. It has subsequently been used to image a wide range of materials. With the most current technology, resolution of about two angstroms (about 8 billionths of an inch) can be achieved. This is approximately the distance between atoms in a solid. This resolution is 1,000 times greater than an optical microscope and about 500,000 times than that of the human eye.

For biological samples, researchers are able to visualize the large molecules central to life. For this exquisite resolution there is a price. Electron beams must be accelerated in very high vacuums. Organism cannot survive under such severe conditions. Consequently, observations of the dynamics of living cells cannot be seen.

There is the added risk the electron beam can damage biological materials. Finally, sample thicknesses must be very thin to allow penetration of the electron beam. This means preparation of samples can be difficult and time consuming.

In the transmission electron microscopes (TEM) described above, a focused electron beam passes through the object being studied. Where a lens is used to focus light in an optical microscope, metal apertures and magnets focus electron beams.

Scanning electron microscopes (SEM) use an electron beam to scan over the surface of an object. Several kinds of signals are generated as the beam interacts with atoms on the object's surface. Those signals are used to generate an image. Resolution can be as good as 10 nanometers (0.4 billionths of an inch).

The utility of these instruments seems limitless. One can view the finest structures within cells, the details of microfossils, and the architecture of semiconductors manufactured for microelectronics.

Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at


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