Anton van Leeuwenhoek 
(1632-1723) was an early 
niicroscopist who ground 
his own lenses as a hobby 
and was the first to 
observe such living cells 
as sperm and pond ivater 
microorganisms . 
he had been examining. This 
description of what was later 
called the cell’s nucleus was 
the first suggestion that ani- 
mal cells had an internal 
structure. Throughout the 
18th and 19th centuries, 
improvements in microscopes 
and techniques for selectively 
staining cell parts enabled cell 
biologists to distinguish other 
particles within the cell. 
However, researchers could 
not study these minute flecks 
in detail because they met an 
insurmountable obstacle: the 
wavelength of light. 
A light microscope — even one 
with perfect lenses and perfect 
illumination — simply cannot 
be used to distinguish objects 
that are smaller than half the 
wavelength of light. White 
light has an average wave- 
length of 0.55 micrometers, 
half of which is 0.275 micro- 
meters. (One micrometer is a 
thousandth of a millimeter, 
and there are about 25,000 
micrometers to an inch. 
Micrometers are also called 
microns.) Any two lines that 
are closer together than 0.275 
micrometers will be seen as a 
single line, and any object 
with a diameter smaller than 
0.275 micrometers will be 
invisible — or, at best, show 
up as a blur. 
