It consists of a cylinder and a plane
1. The cylinder surface is parallel in the direction parallel to the axis.
2. The cylinder surface is circular in the direction perpendicular to the axis and has the greatest width.
3. These two directions become the two principal meridional directions of the cylinder.

Light rays passing through the axial meridian (vertical direction in the figure) do not change the convergence and divergence; Light passing through the refraction meridian (horizontal direction in the figure) will show a change in convergence and divergence.