438 THE POPULAR SCIENCE MONTHLY 



Many proverbs foretelling rain and bad weather are based on the 

 appearance of solar and lunar haloes and coronas, and as these form 

 only when there is much moisture in the air and some condensation the 

 proverbs of this class are well founded. 



Coronas are the small colored rings of light that encircle any bright 

 object when seen through a mist, though the term commonly is used 

 to designate only the colored rings around the sun and moon. They are 

 due to diffraction (the bending of light at the boundary of an object 

 into its geometric shadow) caused by water globules, and have one or 

 another angular diameter depending on the size of the droplets that 

 produce them, in the sense that the larger the droplets the smaller the 

 corona. Hence a decreasing corona implies growing drops and the 

 probability of an early rain. 



Haloes, on the other hand, are the rings of large diameter, usually 

 colorless or nearly so, due to reflection and refraction by ice spicules, 

 and are often seen in the high cirrus clouds that have been caught up 

 from the tops of storms and carried forward by the swiftly moving air 

 currents that always prevail at such elevations. It is this usual position 

 of haloes relative to storm centers, that is, in front of them, that makes 

 them the good indicators they are of approaching bad weather. 



Typical of such proverbs is that of the Zuni Indians, who say : 



' ' When the sun is in his house it will rain soon. ' ' 



Several others refer to the apparent diameter of the circle. Thus we 



have: 



' ' Far burr, near rain. ' ' 



' ' The bigger the ring, the nearer the wet. ' ' 



"When the wheel is far the storm is n'ar; 

 When the wheel is n'ar the storm is far." 



These latter can not refer to the corona, which actually does change 

 in angular size, because in that case just the reverse is true ; the bigger 

 the ring the farther off the storm. Clearly then they apply only to the 

 halo, and as the apparent size of an object of constant angular diameter 

 depends upon its seeming distance away, it follows that the supposed 

 changes referred to are optical illusions, due to erroneous impressions 

 of distance. 



A good illustration of this kind of illusion is furnished by the moon 

 as seen by different people, or as seen by the same person at different 

 elevations above the horizon. When high in the heavens, where it ap- 

 pears to be comparatively near, it looks smaller than it does when close 

 to the horizon where it seems to be farther away; and yet careful meas- 

 urements show but little change in its angular diameter, and that little 

 just the reverse of appearances. 



Hence, when the actual distance to a halo is less than it seems to be, 



