Looking at samples under a microscope that are unstained typically shows nothing. Without a stain to show the sample, most specimens would look invisible and transparent. However, one technique that has been used in microscopes for a very long time is called differential interference contrast (DIC) microscopy and is dependent on a very simple principle. There have been many other methods to make it more quantitative but it is first important to understand how this works.
In one line, the method translates the changes in phase (invisible to the human eye) to amplitude which is visible to the human eye. The method is especially sensitive to the rate of change across the length. Thus long flat surfaces such as the flat surface of the cytoplasm do not generate as much contrast as would sharp gradients that are presented by smaller vesicles.
In the microscope, two identical light beams are generated that are polarized. These two beams are then separated by a small distance and transmitted through the sample. A small phase lag is then introduced into one of the sample light beams. These two beams of light then interfere with one another in the image plane that is seen by the observer.
The images from the DIC microscope appear almost three dimensional and generate contrast where none was seen before. See more images at the link below with a comparison of phase contrast with DIC.