Measurement of the refractive indices of mineral crystals
The principle is very simple: Mix ethanol and bromonaphthalene (e.g.) to produce a series of media with a known refractive index, then find out the medium in which the mineral crystal just disappears visually. Unfortunately, this method is very inaccurate.
Using the "Line of BECKE" is much nore accurate. If you look at a grain of sand mounted with water you can see a bright line surrounding this grain. It is called "the Line of BECKE". If you lower the eyetube this line moves into the the surrounding water, the medium with the lower refractive index (LLL-rule: lowering leads to lower index). In case of Bromonaphthalene this line will move inwards instead. Varying the mounting medium, you can fix the refractive index quite accurately, because, if there is no difference, there is still a weak line which is displaying many colours. This is the consequence of dispersion: In case of short-wave light (blue and green) the refractive index is always higher than in case of long-wave light (yellow and red), the difference depending on the media. Let us assume that the indices are identical in case of green light, and that the diffraction of the sand is higher than that of the mounting medium. Then the blue line will be inside the grain whereas the red line will be outside. The method explained here is very useful, but it is useless in case of very high refractive indices, as no proper mounting media are available then.