Note:  Do not rely on this information. It is very old.

Haidingers Brushes

Haidinger's Brushes, in physical Optics, are appearances that present themselves when plane-polarised light is received into the eye under certain conditions. Thus, if the eye be rapidly directed from one image to another of an object viewed through a rhomb of Iceland spar, which is a transparent crystal that gives two images of any small object viewed through it, a pale yellow patch will appear, bounded by curved arcs on either side, and with a violet patch contained in each set of curves. The brushes seem to be due to the fact that the eye is itself capable of polarising light, especially in the neighbourhood of the yellow spot, the coloured appearances corresponding to those that are seen when plane-polarised light is viewed through an external polarising medium.