Although the nucleus of a typ- 
ical human cell is relatively 
large (about 7 micrometers in 
diameter), most organelles 
vary from a width of only 1 
micrometer to structures so fine 
that they must be measured in 
nanometers (which are 1,000 
times smaller than micro- 
meters), or even in angstrom 
units (10 times smaller than 
nanometers). To see such tiny 
particles under a microscope, 
scientists must bypass light 
altogether and use a different 
sort of “illumination,” one 
with a shorter wavelength. 
The invention of the electron 
microscope in the 1930’s filled 
the bill. In this kind of micro- 
scope, electrons are speeded 
up in a vacuum until their 
wavelength is extremely 
short — only one hundred- 
thousandth that of white fight. 
Beams of these fast-moving 
electrons are focused on a cell 
sample and are absorbed or 
scattered by the cell’s parts so 
as to form an image on an 
electron-sensitive photographic 
plate. 
If pushed to the limit, electron 
microscopes can make it 
possible to view objects as 
small as the diameter of an 
atom. Most electron micro- 
scopes used to study biological 
material can “see” down to 
about 10 angstroms — an 
incredible feat, for although 
this does not make atoms 
visible, it does allow researchers 
to distinguish individual 
molecules of biological 
importance. In effect, it can 
magnify objects up to 1 
million times. Nevertheless, 
all electron microscopes suffer 
from a serious drawback. 
Since no living specimen can 
9 
This is the actual size 
of a typical microscope 
built by van Leeuwenhoek. 
He peered through the 
tiny lens opening on one 
side of a metal plate 
(left) to see the specimen 
mounted on the point of 
a pin on the other side 
(right). The specimen 
could be moved into focus 
by a system of screws. 
