Light rays passing through an object of high refractive index will be retarded in comparison with light rays passing through a surrounding medium with a lower refractive index. The retardation or phase change for a given light ray is a function of the thickness and the index of refraction of the material through which it passes. Thus, in a given unstained specimen, transparent regions of different refractive indices retard the light rays passing through them to differing degrees. Such phase variations in the light focused on the image plane of the light microscope are not visible to the observer. The phase contrast microscope is an optical system that converts such phase variations into visible variations in light intensity or contrast. The phase microscope therefore allows cytologists to observe the behavior of living, dividing cells. See Chronology, 1935, Zernicke.
Subjects: Genetics and Genomics.