I invite you to perform an experiment. Take a small object, let's say a dead ant. You will also need the use of a five-year- old boy. Give the ant to the child in a place where the light is good, and ask the boy to look at the ant and tell you what he sees.

If he has normal eyesight, you will notice that he holds the ant ridiculously close to his face. You find it hard to believe that he can bring it into focus at all. Yet he is able to see great detail, more than you can observe at any distance or in any light. There are two reasons why this is possible. First, in a child the lens of the eye is extremely flexible, so the eye muscles can deform the lens to a shape that permits focus at very close distances. Second, the sensitivity of the eye's retina decreases with age. By age seventy-five, the loss is ninety percent or more.

Before the discovery that lenses can magnify an object's apparent size, a description at the level of detail visible to a sharp-eyed child was the best that humans could hope for. Magnifying glasses were in use in Roman times, maybe even in the Minoan civilization, 2,500 years ago. However, the discovery of the microscope, usually involving more than one lens, did not come until the end of the sixteenth century. For the first time ever, people had a chance to examine very small objects, with, as Galileo said in 1610, "a fly looking as big as a hen."

The most famous name in microscopy in the seventeenth century was the Dutchman, Anthony van Leeuwenhoek. He might be even more famous if he had a name easier to pronounce. His biographer, Clifford Dobell, remarks that in attempting it most people "invent and emit noises unintelligible to any Hollander," and he recommends that English speakers, if they must say anything, say "Laywenhook."

Leeuwenhoek was an amateur microscope maker of incredible skill (his day job was as a merchant draper). His microscopes had tiny single lenses with very short focus, so strictly speaking they were no more than magnifying glasses; but with them and cunning lighting methods he was able to see the living organisms in a drop of pond water, and even study large bacteria. Curiously, he seemed to make a new microscope for every new set of observations, and he had little urge to sell the better specimens.

Others lacked Leeuwenhoek's vast skill and patience, but fortunately during the eighteenth and nineteenth centuries the design and construction of high quality compound (multi-lens) microscopes continually advanced. Smaller objects came into view. The anthrax bacillus, much in the news today, is only a few millionths of a meter long and was first seen in 1850. Viruses are even smaller. Humans caught their first sight of the cowpox virus, one quarter of a millionth of a meter across, in 1887.

At this point, a new variable entered as a limiting factor. Light (or any radiation) consists of waves. Any object we study causes distortion in the light pattern, rather like waves breaking around a rock. If the rock is too small, we will be able to make out no details of its structure. In the case of visible light, it is impossible to see any detail in an object smaller than about half a wavelength. That lower limit of size is about one five-millionth of a meter.

This might seem like the end of the road for microscopes, although some properties of smaller objects could be studied by employing much shorter wavelength radiation, such as X-rays. However, during the 1920s a completely new possibility emerged. In 1924, Louis de Broglie had shown that particles exhibit wave-like properties, and he showed how to calculate their characteristic wavelengths. (For this work he received the 1929 Nobel Prize in physics). The wavelength associated with an electron is tiny - less than a billionth of a centimeter. The wavelength of visible light is a million times as large. How about the idea of using a beam of electrons, rather than light, to illuminate a target, thus making an "electron microscope"?

In 1928, Leo Szilard suggested this to Denis Gabor, who was afraid that anything exposed to such an electron beam would be totally destroyed by it. Others like Max Knoll and Ernst Ruska, more optimistic, pressed on, learning how to converge and diverge electron beams using magnetic fields rather than lenses, and accepting the fact that observation of a specimen might well destroy it. The first picture using an electron microscope was of a plant tissue section, and it was made in 1934. As with optical microscopes, following that pioneering picture the level of detail gradually improved, until objects only a fraction of a nanometer across could be seen. To put this in perspective, it is about the diameter of a carbon atom. However, further improvement of the electron microscope seemed difficult.

Once again, we had reached an apparent limit. And once again, in 1980, a new approach came to the rescue. Suppose that we make a sharp needle, so sharp that the tip is only an atom or two across. If such a needle point is brought close to the surface of a target, electron clouds surrounding atoms on the tip of the needle will overlap the cloud surrounding an atom on the surface, permitting a current to flow between the two even though the tip does not actually touch the surface. A current like this is called a "tunneling current" and is a well-known result of quantum theory. By moving a needle tip across the surface ("scanning" it), an atom-by-atom image can be built up. Not surprisingly, the practical problems of moving the needle point precisely across the target are substantial. However, this instrument, known as a "scanning tunneling microscope" or a "scanning probe microscope," was perfected by Heinrich Roehrer and Gerd Binnig in the early 1980s. Imaging of single atoms became routine. In 1986, they shared the Nobel Prize in physics with Ernst Ruska, who had fifty years earlier been a pioneer in the development of the original electron microscope.

We have come a long way, from ants to bacteria to viruses to molecules to atoms. Is this, finally, the real end of the road for microscopy?

It may seem there is nowhere else to go, except that at the center of the atom we find the nucleus, one ten-thousandth the size. And hidden within the nucleus lie the quarks, invisible forever according to basic nuclear theory. And theories, even basic ones, come and go.

Don't be surprised if school Websites/holograms/brain downloads, fifty years from now, offer great 3-D images of quarks

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