HALOS. 161 



paleness of the sun at sunset, by means of which, sailors are ac- 

 customed to presage a storm. 



The colored rings or halos which are often seen surrounding 

 the sun and moon are evidently occasioned by very thin vapor 

 diffused through the atmosphere. They are supposed chiefly to 

 encircle the moon, but scarcely a day passes without light misty 

 clouds, when at least portions of halos may be seen near the sun, 

 and in order to perceive them, it is only necessary to remove 

 the glare of light which makes the delicate colors appear white. 

 Thus, if we examine the reflection from a smooth surface of wa- 

 ter, we will perceive that the sun gilds the fleecy clouds with seg- 

 ments of beautifully colored rings. This effect is more distinctly 

 seen, if the rays from a hazy or a mottled sky, be received upon a 

 sheet of white paper held before a small hole in the window shut- 

 ter in a dark room. But even when the sun shines from an azure 

 firmament, circles of the richest tints may be produced by experi- 

 ment, thus, holding a hot poker below, and a little before the small 

 hole in the shutter, above mentioned, throw, a few drops of wa- 

 ter upon it, and the sun will be painted upon the paper like the 

 glowing radiations of the passion flower. 



Halos are produced by what is termed the diffraction of light, 

 i. e. the rays of light in passing near the edges of a body appear 

 to be bent from their rectilineal course. This diffraction maybe 

 easily observed by viewing objects through a minute hole, it will 

 be found that the edges of straight bodies will be curved if viewed 

 near the edge of the hole, and a line of bright white light, will 

 appear tinged with orange on the side nearest the edge of the 

 hole, and with blue upon the other. Halos are much more com- 

 mon in the northern latitudes than in warmer climates, a fact 

 which is owing doubtless, to frozen particles of water floating in 

 the air, though Humboldt remarks that lunar halos are much 

 rarer in the northern than the .southern countries of Europe, and 

 seen more especially when the sky is clear and weather settled. 

 He observes that in the torrid zone they appear almost every night, 

 and often in the spdke of a few minutes disappear several times. 

 Between the latitude of 15 N. and the equator, he has seen 

 small halos around the planet Venus. The next figure exhibits 



