n The method for determining refractive index of a transparent particle by noting the direction in which the Becke line moves. The halo (Becke lines) will always move to the higher refractive index medium as the focus is raised. The halo crosses the boundary into the lower refractive index medium when the microscope is focused down. The particle must be illuminated with a narrow cone of axial light obtained by closing the aperture diaphragm of the condenser to a small aperture. (Nesse WD (2003) Introduction to optical mineralogy. Oxford University Press, New York).