For hundreds of years scientists have observed the structure of once-living cells through microscopes, whether they be simple lenses (such as those used by Robert Hooke) or compound microscopes (like those found in most school laboratories) or large electron microscopes (such as those found in most hospitals today). All of these devices have one major fault: the inability to observe living cells. The phase-contrast microscope was invented to make observation of living cells possible.

When an object is placed under a normal microscope and a light is placed below it the object will block a good deal of the light and create shadows. A viewer looking through the eyepiece sees the dark (shadowed) areas compared to the light (non-shadowed) areas. This comparison between light and dark is called contrast.

Because living cells are largely transparent and allow most of the light to pass through unaffected, they...