ABSTRACT
Staining a specimen is not always convenient or even possible, especially in the case of living cells. It is clear that all the structures inside the living cell differ in some way from their surroundings. Why, then, are we often unable to see them in the microscope? The answer is that they differ from their surroundings in refractive index but not in the amount they absorb light. To make these structures visible, we need some way to convert these differences in refractive index into differences in amplitude (color or intensity) in the image.
