Haidinger's brush, more commonly known as Haidinger's brushes is an image produced by the eye, an entoptic phenomenon, first described by Austrian physicist Wilhelm Karl von Haidinger in 1844. Haidinger saw it when he looked through various minerals that polarized light.
Simulated appearance of a computer screen viewed through a polarizer, showing typical size and intensity of Haidinger's brush
Entoptic phenomena are visual effects whose source is within the human eye itself.
Purkinje tree depiction