There are some interference methods that have been devised to measure indexes of refraction. Early refractometers, such as the Rayleigh refractometer take advantage of interference for accurate measurements. Interference methods will measure the mechanical displacement between the two substances compared. It can be a somewhat indirect approach. The range of the interference is limited, a path difference should not exceed a few dozen wavelengths.
The rayleigh refractometer works by having light pass through a vertical slit, and then collimated through a lens which passes it through two vertical slits to line it up parallel with two test tubes that the index of refractions will be compared. Typically one of the tubes will be filled with something such as air so that it has a known index. After the light is refracted in the test tubes, It wil pass through a piece of glass and then a diffraction image will be produced in the focal plane of a lens L2. The fringes produced are very fine. To improve the accuracy of reading these fine lines, a cylindrical lens is directed towards it with a 2mm diameter. This will magnify the diffraction image making it clearer to see the spacing between the lines. The glass that the refracted light passes through can then be maneuvered at specific angles to slow down the light until both the light rays are in phase. Then comparing the difference in angle of the two glasses will give a sense of the difference of index of refraction of the two. As long as one of the indexes of known and the difference between the two is known, then it is just a simple calculation to find out the unknown index of refraction.
Here is an example of the refractometer from the top view:
Looking at it from the side:
You can see how the two glasses "K" and "L" can be rotated in order to put the two rays in phase. Whenever they are altered the thickness of the glass will change for each beam.
Refractometers are very important in industry. In the next page read on about the basics of modern ones.