AN 18TH CENTURY IDEA MATERIALIZES

There is another scientific development this month, not strictly speaking related to energy, but nevertheless important enough that I think readers ought to know about it. It is a new kind of micros-copy.

A microscope works by a combination of magnifying lenses, and a magnifying lens works in a way that you learned in high school by tracing the ray from the top of an arrow to the focus and another straight ray through the center of the lens. If you have forgotten or were never taught it, don't worry, because that is not what I am after now.

What I am after is the concentration of rays, coming from a dis-tant point source on the left to a focus by a convex lens. The lens concentrates the rays to the focus not only geometrically in the sense that the rays all pass through the focus; there is something else. Light is a wave motion with positive crests, nodes and nega-tive crests all traveling at the same speed through air or vacuum, and also traveling at the same, but slower speed through glass. Now follow one of the positive crests, which I have indicated by the dotted lines perpendicular to the rays on the left.

shaped lens from the left and converging to a point on the right of of the lens.

Ray 1 has the longest way to go to the focus, but it does not get slowed down anywhere. Ray 2 has a little shorter path, and is slowed down only on a small segment in the glass. Ray 4 has the shortest possible path, a straight line, but is slowed down most, since it travels through the fattest part of the lens. From that alone it does not follow, but it is true anyway (as you can read up in any textbook on optics) that the focus is not just a point where the rays converge geometrically, but they arrive there all with the same phase: at any one moment, all the rays bring to the focus a positive crest, or they all bring in a node (null), or they all bring in a nega-tive crest, etc. Let's call this the principle of equal phases at the focus for further reference.

Through a microscope one can see only objects that are much larger than a wavelength of light (roughly from 350 nanometers for violet to 700 nm for red). When the object is comparable or smaller than a wavelength of light, it will not reflect the light into the microscope, but bend (diffract) it, resulting in an indefinite and unrecognizable blotch.

Are there many objects smaller than 0.000000001 meters or 0.00000000025 inches? Plenty. Molecules, atoms, viruses, the intel-ligent part of Sen. Dole's brain, and many others.

The most common solution to this problem is an electron microscope. Electrons have waves associated with them, and streams of them can be manipulated by magnetic fields in exactly the same way as a lens manipulates light. In fact, such arrange-ments are quite understandably called electronic lenses, and they again not only concentrate the electrons into a focus, but they do so with the same phase for every electron. The quantum-mechani-cal waves associated with an electron depend on the mass of the electron, which is pretty well unchangeable, and on its velocity, the higher the velocity, the smaller the wavelength.

This is fine if you are studying mineralogy, but if you have a live specimen (such as a cell) to investigate, the electrons will kill it, and the higher you raise the electron velocity, the more distinctly you will see the smashed corpse of what you wanted to investigate.

Not only electrons, but all elementary particles have quantum-mechanical (or "de Broglie") waves associated with them. Ideally, you could produce short wavelengths with low velocities and heavy particles (such as atoms or molecules) and keep your sample un-scathed. But these particles are neutral and their path cannot be changed by a magnetic field, i.e., by an electronic lens.

But there is a third type of lens, invented by the greatest of all opticians, and one of the greatest geniuses of all time, Augustine Jean Fresnel (the s is silent), who made an incredible amount of basic discoveries and correct predictions in his short life of 39 years (1788-1827), ten of which he spent on road construction be-cause the French bureaucracy (he was a government engineer) would not grant his wish to be transferred to a lighthouse.

Fresnel (much of whose work was devoted to defending the wave theory against the then fashionable corpuscular theory of light) realized that to get all the rays from a source into a focus, he did not need a lens. Imagine a plane close to a point source of light. One point on that plane will be hit by a ray at right angles. Let's choose a moment when a negative crest hits the plane at that point, at the center of the figure as indicated. Now if you move away from that point along the plane, the distance from the point source will increase, so that eventually you will find a ring where the distance has changed by half a wavelength, and you will be in the zone of positive crests for the same moment, and so on. Now if you paint all the positive-crest zones black and leave the other zones transparent, you guarantee that always only one phase of the wave can get through the arrangement, called a Fresnel lens or a zone plate.

And it works exactly like a lens. For a few dollars you can buy such a magnifying lens in novelty shops and mail-order houses. They have many hundreds of concentric circles per inch put on a wallet-sized plastic plate photographically. I carry one myself in my wallet for reading fine print.

Fine-grain photographic reduction is a byproduct of semicon-ductor technology. But Fresnel was not a man to be stopped by the lack of a technology that was still some 250 years away. He proved his point by making a zone plate reflect light at a small angle, so that the zones became elliptical and much bigger.

Now back to microscopy. Using a Fresnel lens rather than an electronic lens, and a jet of neutral helium atoms rather than electrons, the same results as with an electron microscope can be achieved (same small wavelength), but without the destructiveness of high-velocity electron beams. (A helium atom has roughly 3,400 times the mass of an electron.)

The principle was recently demonstrated at a Southern German university.