Why does this occur out of focus?

In focus, the dispersion of reflected beams is very small and they form such a thin line that it is imperceptible. When the objective is raised away from the preparation, the dispersion is increased in size and the area covered by the reflected beams grows bigger, displacing the line towards the centre of the medium with the higher refractive index. If they are separated too much, the image becomes out of focus, but additionally, as the rays spread over a greater area, the increase in light intensity produced stops being perceptible.

As the objective gets nearer to the preparation, the reverse occurs.

When the refractive index of the mineral is lower than that of the medium, logically the luminous line is displaced away from the mineral.

Remember this simple rule: as the objective is defocussed by drawing it away from the preparation, the Becke line will always move towards the medium with the higher index.

In the figure, the mineral has a higher index than the immersion medium (the two lines indicate when the objective is focussed).

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