Haidinger's brush
Haidinger's brush is an optical phenomenon first described by Austrian physicist Wilhelm Karl von Haidinger in 1846. Many people are able to perceive polarization of light. It may be seen as a yellowish horizontal bar or bow-tie shape (with "fuzzy" ends, hence the name "brush") visible in the center of the visual field against the blue sky viewed while facing away from the sun, or on any bright background when looking through polarized sunglasses. The direction of light polarization is perpendicular to the yellow bar (i.e. vertical if the bar is horizontal). Fainter bluish or purplish areas may be visible between the yellow brushes (see illustration). Haidinger's brush may also be seen by looking at a white area on many flat panel computer screens, in which case it is often diagonal.Some insects and fish are sensitive to polarized light.
Haidinger's brush for vertically polarized light
Haidinger's brush is usually attributed to the dichroism of the pigment of the macula. The brush's size is consistent with the size of the macula. The macula's dichroism is thought to arise from some of its pigment molecules being arranged circularly. The small proportion of circularly arranged molecules accounts for the faintness of the phenomenon. As well, the outer layer of the cones is said to be birefringent; this is thought to contribute to the phenomenon. The cornea has also a slight birefringence.