A bishop's ring is a diffuse brown or bluish halo observed around the sun. It is typically observed after large volcanic eruptions. Most observations agree that the inner rim of the ring is whitish or bluish white and that its outside is reddish, brownish or purple. The area enclosed by the ring is significantly brighter than its surroundings. From the sequence of colors with the red on the outside one can conclude that the phenomenon is caused by diffraction because halos always have their red part on their inside.