The isochromes form in the interference figure because they reflect varying degrees of retardation of the light.
With the auxiliary condensor in place a strongly convergent cone of light is focussed into the mineral grain. Only light which follows one of the optic axes emerges from the grain with zero retardation.
This point is the MELATOPE (M), and there are two in biaxial minerals.
Light travelling along any other path through the mineral will exhibit varying degrees of retardation depending on the length of the path through the mineral and birefringence for that path.
The amount of retardation increases outward, ideally concentrically as seen in uniaxial minerals, from the melatope.
Near the melatope, the light experiences lower birefringence, travels a shorter distance through the mineral and therefore has a lower retardation.
Retardation increases slowly from melatopes towards the Actue Bisectrix (Bxa) (indicatrix axes).
The increased birefringence is partially compensated for by a shorter path.The result is that the isochromes are stretched out towards Bxa, defining a tear drop shape or figure 8 pattern. The number of isochromes is dependant on:
high birefringence & thick crystals = numerous isochromes
FORMATION OF ISOCHROMES OVERHEAD