November 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Optical magnification is only useful to
the point where resolution is limited by the wavelength of light representing the object under observation.
Astronomer William Dawes first provided a means of quantification based on an ability to visually resolve
closely spaced stars. Known as the Dawes Limit, a value of 4.56/D arc seconds was empirically determined
(D is aperture of instrument in inches). A theoretical upper limit to magnification of any optical system
with perfect optics is around 2,000. The electron microscope removed that resolving limit by shooting
a stream of electrons with radii much less than the wavelength of visible light, and measuring its reflection.
Images are necessarily in 'false color' because we cannot perceive the real wavelength/color of the
surface revealed by the electron beam.
The Amazing Electron Microscope
By Morris M. Rubin
Long after optical microscopes have reached their limit, the electron microscope goes on magnifying
... up to 200,000 times.
Since the time of Anton von Leeuwenhouk - the first great microscope designer - scientists have relied
on the microscope as one of their key tools. Year after year, as techniques for manufacturing optical
glass were improved, new and better microscopes enabled scientists to see increasingly minute objects.
Then, about 1890, it seemed that light microscopes had come to the end of the line. Beyond about 2000
times magnification, even the finest, most perfectly designed microscope showed only a fuzzy blob.
Blocking the development of more powerful microscopes was a basic characteristic of light itself.
Similar to sound, light travels in waves of measurable length. In the middle of the visible light spectrum,
for example, the waves have a length of about 6/250,000 of an inch. In order for light waves to differentiate
between two points of an object, they must be separated by about one-third the length of a light wave,
or about 2/250,000 of an inch. Objects smaller than about half a wavelength cannot be magnified clearly
by a light microscope, no matter how perfect its lenses.
Scientists reasoned that since the basic difficulty was caused by the relatively long wavelengths
of "ordinary" light, if it were possible to employ some type of light which had shorter wavelengths,
greater useful magnification could be achieved. This possibility was explored, and by utilizing ultraviolet
light (which has a wavelength only one-third that of visible light), microscopes were designed which
could magnify up to 5000 times, over double the limit of visible-light microscopes.
At this point, the light microscope reached the zenith of its design capacities. If scientists wanted
more magnification, they had to find a new way of getting it.
Electrons to the Rescue.
The theory of the electron microscope was suggested in the 1920's. Experiments showed that electrons
acquired a measurable characteristic wavelength when they were speeded up by subjecting them to high-voltage
fields. The higher the voltage, the greater was the velocity of the electrons, and the shorter became
the apparent wavelength. Further, it had been proven that electrons could be bent or refracted by magnetic
fields in much the same way light is bent and refracted by optical lenses.
Comparison of electron microscope and ordinary light microscope. The basic principles
are the same, but the electron microscope uses coils of wire to deflect and focus an electron beam magnetically
rather than glass lenses to bend and refract light.
Therefore, it seemed logical that light, the limiting factor in the process of magnification, could
be replaced by a stream of electrons which would have a much shorter wavelength and so would allow greater
magnification. With this important concept under their hats, scientists began work on the design of
an electron microscope.
By the late 1930's, experimental microscopes were in operation in Europe, Canada, and the United
States. And then, in 1940, RCA marketed the first commercial American electron microscopes. These initial
instruments, though crude by present-day standards, were fantastically superior to the best light microscopes
Two of the most widely used electron microscopes are shown here. At left is the RCA
EMU-3, capable of 200,000X magnification. At right is the Norelco EM100B, which magnifies to 90,000
Whereas even an ultraviolet microscope was limited to a magnification of 5000 times, these early
electron microscopes were capable of enlargements of 100,000 times. Today's models magnify more than
200,000 times - enough to see an object one-millionth the diameter of a human hair - and by enlarging
the image still further by photographic means, useful magnifications of over one million diameters are
Co-inventor of the first RCA electron microscope, Dr. James Hillier, is shown at
left with RCA's Model EMB, marketed in 1940.
Electrons Replace Light. Similar in principle to light microscopes, the electron
microscope uses a series of lenses to magnify the specimen in a step-by-step process. But while a light
microscope uses glass lenses to bend light rays, the electron microscope's "lenses" are coils of wire
- similar to the deflection coils of a television set - which bend and deflect a stream of electrons.
Electrons emitted by the electron gun pass through the condenser lens which concentrates the beam
of electrons on the specimen. Since the specimen has been sliced so thin as to be partially transparent,
the electrons pass through it in varying numbers depending on the density of the specimen at anyone
point. Thus a pattern of varying electron densities is produced.
Although this pattern is invisible to the eye, it could be shown by placing a fluorescent screen
below the specimen. In practice, however, the electrons pass through the objective lens, which provides
the first step of magnification. Just before they reach the projector lens, a "spread-out" representation
of the density pattern is formed, the center area of which is then further magnified by the projector
A 1959 Russian unit exhibited at the recent Soviet Exhibition in New York.
The enlarged specimen can be viewed directly on a fluorescent screen (which looks and works like
a television screen), or the image can be photographed by special cameras - usually built right into
the electron microscope. Enlargement of the resulting photographs allows further magnification of the
Price Tags. In addition to its optical system, an electron microscope must have
an ultra-stable high-voltage supply and a highly efficient vacuum system. This complexity accounts for
the big price tags on today's electron microscopes - from $12,000 to $40,000, depending on the magnification
desired, the make, etc.
Norelco (Philips of Holland) and RCA are the largest producers of these units. Also active in the
field are manufacturers in Germany and Japan. And the Russians have gotten into the act, too, by producing
an electron microscope which seems to be an adaptation of a 1940 RCA model.
Limitations. Useful though the electron microscope may be, it still has its limitations. Since the
high-voltage electrons are fatal to living organisms, the electron microscope cannot be used to view
live bacteria, viruses, etc. Also, the beam of electrons can't penetrate more than 1/25,000-of-an-inch,
so the electron microscope cannot be used to view objects that are any thicker - the wing of a fly,
The solution to the latter problem has been the development of special devices that can carve off
slices of the object to be viewed which are thin enough to allow the passage of electrons. It's easy
to see how such a "slicer" works with softer materials, but how do we go about slicing off a layer of
steel 1/25,000-of-an-inch thick? The answer to this question is surprisingly simple. A "replica" of
the steel's surface is made on a soft material - such as wax. The replica is easy to slice and, when
it is mounted on an exceedingly thin transparent membrane, it takes the place of the original object
in the microscope.
Importance. About one thousand electron microscopes are now in use in laboratories across the nation.
While they are invaluable tools in research aimed at finding the causes of diseases, especially cancer,
they are also useful in solving a wide variety of industrial problems. For example, the wearing qualities
of rubber tires can be judged by careful examination of an electron microscope photograph, thus eliminating
the need for long and tedious road tests.
But it is in the study of cells that the electron microscope finds its most exciting application.
Cells are grown, nourished, and reproduced through a process of protein synthesis. With the aid of electron
microscopes, scientists have been able to see these processes - which are truly the "secrets of life"
- for the first time.
Man is an insatiably curious creature. One of his most efficient means of satisfying his hunger for
knowledge and understanding is the electron microscope.
Can You Identify These Pictures?
All were taken with the aid of an electron microscope
(answers are at bottom of page)
(a) metal surface
(b) human skin
(d) wool rug
(a) wing of a fly
(b) moon craters
(d) cellulose fiber
(a) lead carbonate
(b) golf balls
(d) cotton balls
(a) an ant's toe
(b) polio virus
(c) a sloppy
(d) limesoap grease
(a) Sahara Desert
(b) silk fiber
(d) Monday morning at Coney Island
1. (a); total magnification 160,000X; courtesy of Philips Electronics, Inc.
2. (c); total
magnification 425,000X; courtesy of College de France and RCA
3. (c); total magnification 112,000X;
courtesy of Dr. C. E. Hall, M.I.T.
4, (d); total magnification 68,000X; courtesy of Esso Research &
5. (c); total magnification 14,680X; courtesy of Dow Chemical Co, and RCA
Posted May 23, 2014