i60 An Attempt to clafsifj/ certain Imn'inous Vhatnomena 



Imninous appearances observed about the sun and moon, 

 and occasiontd bv the refraction ot" their light through an 

 intervening cloud of |)ecu1iar structure, which are in com- 

 mon called halos, coronae, burs, &c. But as these phae- 

 noniena varv considerably, and as I know of no existing 

 nonienclature calculated to express, with precision, their 

 several appearances ; and as the above terms are very fre- 

 quently confustd with one another, I shall take the liberty 

 to offer one, which, however imperfect, may serve till a 

 better shall be found. I shall endeavcur to classify them 

 (for want of a better criterion) according to the various 

 shapes or figures which thev present. It must be remem- 

 bered, that their various figures are the result of the parti- 

 cular construction of the cloud which refracts their light: 

 a correct attention, therefore, to these appearances may lead 

 to a more perfect knowledge of the structure of the refract- 

 ing medium. 



Halo*. Def. Circulus vel arimdus hicidus aream inclu' 

 dens, in citjus centra sol, ant luna apparet. 



Ohser. By a halo I uhdersiand an extensive luminous 

 rinC) including a circular area, in the centre of which the 

 sun or moon appears, whose light, refracted through the 

 intervening cloud, gives rise to the phsenomenon. Halones 

 are called Lunar or solar, according as they appear round the 

 moon orsun. They aregenerally pretty correct circles ; I once, 

 however, saw a halo of a somewhat oval figure. Halones 

 are sometimes coloured with the tints of the rainbow. 



Halo duplex. Def. Duo anmdi, in quorum centra 

 communi sol aut luna videatur. 



Ohser. A double halo is not a very common occurrence, 

 I have observed that simple halones are generally about 45 

 degrees in diameter: in case of double halo, it might be 

 worth while to take the diameters of each of the concentric 

 circles. 



Halo triplex. Def. Tres aiumliy in quorum centro 

 communi sol aut luna apparet. 



Olser. Triple halones are extremely rare occurrences. 



Halo discoidks. Def. Annulus aream reliqua nuhis 

 parte lucidiorem contineus, in cujus centro luna aut 

 sol visus est. 



Ohser. A discoid halu may be said to he a halo consti- 



* The word hali cr ha!r,s is evidently derived friim the Greek «x«v or £>..«;, 

 signifying an area. The Latin writers appear to liave spoken indifferently 

 of halones, halyses, coi-onx, circuli, &c. witliout sufSciently distinguishing 

 between" the curona and the halg — in other woidl, between the himiicuus dhh 



and the luwiinuris ring, _ - 



luting 



