LUMINOUS PHENOMENA. 
68 
It must be remembered, that their various figures are the result 
of the particular construction of the cloud which refracts their i 
light : a correct attention, therefore, to these appearances, may 
lead to a more perfect knowledge of the structure of the re- 
fracting medium. 
Hilo. Halo.* Def. Circulusvel Annulus lucidus aream includens, in 
cujus centra Sol aut Luna apparel. 
Ol'ser, By a halo I undeistand an extensive luminous ring, ; 
including a circular area, in the centre of which the sun or 
moon appears ; whose light, passing-th rough the interverjing j 
cloud, gives rise to the phenomenon. Halones are called Lunar i 
or 5o/ar, according as they appear round the moon or sun. i 
Those about the moon are the most common. They are gene- 
rally pretty correct circles : I once, however, saw a halo of a 
somewhat oval figure, Halones are sometimes coloured with 
the tints of the rainbowf. 
I 
Double halo. Halo duplex. Duo Annuli, in quorum centra communi Sol l| 
aut Luna videatur. i| 
Olser. A double halo Is not a very common occurrence, 
I have observed, that simple halones are generally about 45“ I. 
in diameter : in case of double halo, it might be worth while j. 
to take the diameters of each of the concentric circles. 
TripleLalo. Halo XEIPLEX. Tres Annuli, in quorum centra communi Sol il 
aut Luna apparent. 
Olser. Triple halones are extremely rare occurrences. 
iscoid halo. Halo discoides. Annulus aream reliqvd nulis parte lucidiorem n 
■ continens, in cujus centra Luna aut Sol visits est. 
Olser, A discoid halo may be said to be a halo constituting ;i 
• The word hah, or halos, is evidently derived from the Greek Sxwv ■, 
or aXwf, signifying an wea. The Latin vvi iters appear to have spoken 
indifferently of halones, halyses, eoronae, eircnli, &c. without suf- | 
ficiently distinguishing between the corona and the halo — in other | 
words, between the luminous disk and the luminous ring. 
* The coloured halo is generally seen in a denser cloud. 
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