The reader who wishes to acquaint himself further with the different forms of halo and the methods of observing them will find a comprehensive article on the subject (devoid of mathematical discussions) in the “Monthly Weather Review” (Washington, D. C.) for July, 1914. [Illustration: SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF THE 22° HALO When the sun is high they unite to form the “circumscribed halo.” (Altitude of sun shown in the center of each figure.)] The ice crystals that produce halos consist of hexagonal plates or columns, occasionally including complications of structure, such as pyramidal bases, combinations of plates and columns, etc. These have the well-known effect of prisms in refracting and dispersing light that passes through them. It is evident that there are many possible paths for the light rays through the sides and bases of such crystals, resulting in different deflections and corresponding differences in the forms and positions of the halos produced. The attitudes assumed by the crystals as they slowly sink through the air, and the oscillations they undergo, are further points to be considered in working out the theory of each form of halo by the application of the laws of optics. Nearly all the known forms have been fully explained. A few species of halo--notably the parhelic circle (called _paraselenic circle_ when formed by the moon)--are due to simple reflection from the faces of the ice crystals, and not to refraction. The last group of optical phenomena that we shall consider consists of those due to the process called _diffraction_, which occurs when light is bent around objects in its path, instead of passing through them, as in refraction. The process involves separation of the prismatic colors. The diffraction phenomena of the atmosphere are produced by the water drops of clouds and fog, or sometimes by fine dust. Everybody is familiar with the nocturnal spectacle which Tennyson describes as ... the tender amber round Which the moon about her spreadeth, Moving thro’ a fleecy night. This diffuse reddish or rainbow-tinted circle is called a _corona_. It occurs about the sun as well as the moon (though not easy to see on account of the glaring brightness of the luminary), and also about street lamps and other terrestrial lights when viewed through a misty atmosphere. Unlike the halos, it has no definite angular size. It is usually only a few degrees in radius. Small coronas are produced by large water drops and large coronas by small drops, while the largest of all coronas, known as _Bishop’s ring_, is due to exceedingly fine dust in the atmosphere, and has been seen after great volcanic eruptions.