470 THE POPULAR SCIENCE MONTHLY. 



distinction from the objective rings, AH, A J, AN. In the same 

 way, the particles situated on the cone PAS will contribute to the 

 formation of a bright subjective ring of radial angle RAP. The 

 center R will appear to be surrounded by dark and bright rings. 



Now we must introduce the supposition of many-colored or poly- 

 chromatic light — the white light of many wave-lengths that comes 

 from the sun. Such light, passing a fine thread, forms a series of pris- 

 matic bands on a screen ; passing a single particle, it forms a series of 

 concentric prismatic rings with the blue inside ; for the first blue ring 

 will fall a little inside of the first yellow, and the first yellow inside of 

 the first red — and so on with the others, until at a distance from the 

 center the outer rings overlap irregularly. The subjective rings seen 

 when white light passes through a transparent medium containing 

 many one-sized particles will, for the same reason, appear many-colored, 

 with the blue inside and the red outside ; the central area will be white, 

 with a reddish margin. 



Finally, the actual case is reached when the suspended particles are 

 of dijQFerent sizes. The colors of the central area now overlap so ir- 

 regularly that they unite to form a whitish or silvery disk ; but the 

 outer red margin of the central area formed by the smallest particles 

 is still uncounterbalanced. The silvery disk will be reddish about the 

 circumference ; and the colors thus deduced by theory are so closely 

 like those observed in Bishop's ring around the sun that it may be 

 safely considered a diffraction corona. The outer rings are too faint 

 to be seen in daytime. 



Colored coronal rings may be seen around a light when looking at 

 it through a glass strewed lightly over with spores of lycopodium ; 

 they are so nearly of the same size that a number of concentric rings 

 appear. Kiessling describes some interesting experiments with thin 

 artificial clouds of condensed vapors, through which the sun is seen 

 surrounded with coronal rings. The moon is often surrounded with 

 similar I'ings of small diameter, formed by diffraction, probably on 

 small floating particles of ice, even when the sky seems clear. These 

 are easily distinguished from halos. The latter are of definite and 

 much larger diameter, and, when seen around the moon, are gen- 

 erally whitish ; if formed around the sun, they are visibly colored with 

 the red inside ; and they are due to refraction and reflection on mi- 

 nute ice-crystals. 



All this is safe enough ; it is the origin of the diffracting particles 

 and the long endurance of their effect that give trouble. Indeed, the 

 experimental and mathematical knowledge of optics, based on the un- 

 dulatory theory of light, has advanced so far that the physicist is now 

 better able to suggest processes by which effects may be produced 

 than the meteorologist is able to apply them. The physicist can safely 

 say that a sufiicient supply of extremely fine liquid or solid dust scat- 

 tered through the atmosphere would produce just such a solar corona 



I 



