Optics of solar halos. 18th-century diagrams on optics, showing the various types of halos that can be produced by sunlight in the Earth's atmosphere. The light interacts (refracts and reflects) with ice crystals. The most common type of halo is the circular 22-degree halo. These diagrams were published in 'A Compleat System of Opticks' (1738) by English mathematician Robert Smith (1689-1768).