lines that are closer together than 
0.27 micrometers will be seen as a 
single line, and any object with a 
diameter smaller than 0.27 microme- 
ters will be invisible — or, at best, 
show up as a blur. 
Although the nucleus of a typical 
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 
micrometers), or even in angstrom 
units (10 times smaller than nanome- 
ters). 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 micro- 
scope in the 1 930's filled the bill. In 
this kind of microscope, electrons are 
accelerated in a vacuum until their 
wavelength is extremely short — only 
one hundred-thousandth that of white 
light. Beams of these high-speed 
electrons are focused on a cell 
sample and are absorbed or scat- 
tered by the cell's parts so as to form 
This is the actual size of a typical 
microscope built by van Leeuwen- 
hoek. 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. 
10 
