The Becke line test is a technique in optical mineralogy that helps determine the relative refractive index of two materials.
It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to move.
If a different mounting medium is used, its refractive index should be recorded on the slide, to avoid loss of the information.
Media used for impregnating a specimen before sectioning (either for mechanical strength, or to pick out porosity with a contrasting colour) are also usually chosen with the same 1.55 refractive index.
If a specimen is mounted without a cover slip - for microprobe analysis, backscattered electron microscopy, reflected light microscopy ... - then an immersion oil can be chosen with whatever refractive index is desired for the study.