The strong vertical contrasts in air temperature that occur in the polar regions produce many remarkable examples of mirage. The pictures and descriptions of those observed a century ago along the coast of Greenland by Captain William Scoresby, Jr., have become classical. A recent episode connected with mirage was the expedition sent north in 1913 to explore “Crocker Land,” which Peary believed he had sighted from an elevated point in Grant Land in 1906, and which for a time figured on all maps of the Arctic. The later explorers found no land at the place indicated, but they observed the same mirage that Peary had mistaken for distant hills and mountains. Currents of air of different densities produce, through their varying effects on atmospheric refraction, the twinkling or scintillation of the stars, as well as of distant terrestrial lights. Twinkling is much more violent near the horizon than near the zenith, and more pronounced on some nights than others. The shimmering of the air over heated surfaces faces and the “boiling” of celestial objects as seen in the telescope are analogous phenomena. [Illustration: INFERIOR MIRAGE (_From American Museum Journal. Drawing by Chester A. Reeds._) Out-of-doors when a layer of warm rarefied air arises from contact with heated ground or warm water, occupying a position below the colder, more dense normal air, two images of a distant object may be seen--one inverted beneath the other. This is “_inferior mirage_” and is explanatory of the appearance of trees and their reflections, which haunts the desert traveler with the hope of water.] [Illustration: SUPERIOR MIRAGE (_From American Museum Journal. Drawing by Chester A. Reeds._) When a zone of warm rarefied air is sandwiched between normal air above and colder air below, a “_superior mirage_” of distant objects may be seen. Three images are produced, one above the other, the middle one inverted.] In the refraction phenomena that we have thus far considered the air is the medium in which the light rays are bent and distorted. In the production of the _rainbow_, light undergoes refraction, dispersion (separation of the spectral colors) and reflection by passing through drops of water in the atmosphere; especially falling raindrops.