Light is convergent because auxiliary condensor produces a cone of light which is focused on the sample, it passes through the sample and is collected by the objective lens.

1. Light which travels along the optic axis is not split into two rays, n_epsilon' = n_omega, and exits the mineral to form the melatope. No retardation "between" rays.
2. Light following paths 2 & 4 experience moderate retardation
   n_epsilon' < n_omega ~ 550 nm
3. Light following paths 3 & 5 experience moderate retardation
   n_epsilon' << n_omega ~ 1100 nm because light makes a larger angle with optic axis and must take a longer path through the sample.

Optic axis is vertical and optical properties vary symmetrically about the optic axis, rings of equal retardation are produced around the melatope = isochromes.

Number of isochromes depends on retardation and the thickness of the sample.

Simplified, ignored the splitting of light into its two component rays, each of which refract differently.