26 Descriptioti of a Halo or Coro?ia. 



tion of the halo. The outer circles, however, one only of which 

 appeared to be perfect, were composed of pure white light, and 

 had for their centres the circumference, or a point near it, of the 

 inner ring. Consequently, their circumferences, if all the circles 

 had been perfect, would necessarily have passed through the ap- 

 parent situation of the sun. I mentioned, however, that one only 

 of these rings was perfect, the others were concentric arcs of cir- 

 cles which crossed one another, as seen in the accompanying 

 diagram. 



In the centre of the inner circle and bounded by it, a bluish 

 mass of dense vapor was perceptible, which gave to the whole 

 an embossed appearance, and added much to the beauty and 

 brilliancy of the scene. Around and within the exterior circles 

 there were also perceptible masses of vapor, though obviously 

 much less dense than the mass which was nearer the sun. With 

 the exception of these masses of vapor, and a large cumulus 

 which lay to the south of us, and here and there a few scattered 

 cirri, the sky was cloudless and the atmosphere calm and serene. 

 The mercury in the thermometer stood at 86°. The weather 

 continued thus for thirty six hours, when we had a smart fall of 

 rain, and a descent of the mercury in the thermometer to 36°, at 

 which point or near this, it has remained until about three days 

 since, when it rose to 66°. 



Coronas and parhelia have frequently been observed and ac- 

 curately and glowingly described, by many scientific gentlemen, 

 and various and conflicting opinions have been entertained re- 

 specting their causes, some attributing them to the peculiar state 

 of the air consequent upon intense cold, while others, probably 

 more correctly, attribute them to the refraction and reflection of 

 the rays of light through masses of vapor which are formed in 

 such aggregations as are not heavy enough to fall in the form of 

 drops. Descartes remarks, that halos never appear when it rains. 

 Coronas have frequently been observed around the moon, and 

 even around Sirius and Jupiter, but, as far as my information ex- 

 tends, they have been but seldom variegated, even when they 

 have encircled the sun. 



I know not to what cause this phenomenon can be attributed, 

 unless it be to the refraction and reflection of the sun's rays 

 through the masses of vapor. Doubtless the first circle was thus 

 formed, and if we suppose the rays of light from the circumfer- 



