The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1870-01-10

TIE HALO.


The pralllnnt AtmosphericnI Phenomenon Of
Iast Evening, and How it was Produced.
Last evening the moon was surrounded by halo
OF more than usual bliliancy and distinctness. At
O'Clock the moon was about 40 degrees above the
horizon and the brilliant circle by which It was sur
rounded had radius apparently OF about twenty
degrese, which actual measurement would have
shown TO be little less than twenty-five degrees
The inner edge of the halo was clearly defined, but
on the outer edge the bright light was gradually
merged in darkness. Within the circle the sky was
of a very dark color much darker than II was for q
distance OF several degrees without the halo; and
lust within It, at the extreme eastern end gnttereu
the planet Venus even more beautiful than usual
because displaying her beauty on such an ample ana
unbroken background. According to the popular
Impressions prevailing, the appearance OF such phe-
nomena Is regarded as a forerunner, in winter time
OF snow, theory to which science lends assent The
halo is formed, In summer time as wel as In winter
by the refracton OF the light OF the sun or moon as
tho case may be, through crystals OF ice noating In
the atmosphere in summer, when the phenomenon
is frequently observed, the condensed vapor OF the
atmosphere at moderate elevation is frozen, even
An the very hottest weather; while In winter the ice
CRYSTALS assume the shape OF nakes OF SNOW NO
matter In what position any particular ice or snow
crystal may be placed the effect is the same, and the
raw of light from tie sun or moon is refracted AT
such an angle to make it coincide with all other
Tefractcd rays in forming the halo with radius OF
twentytwo degrees. The space within the circle Is
darker than the space without 1t, because from no
mari of this space can ray OF light refracted by Ice
nr,sms reach the eye OF the observer. Elaborate
mathematical calculations sh ow that, IN the case OF
a halo around the sum- when the different col0rs OF
the prism usually apparent the inner edge being
soered red and the outer pale blue, the deviation
of the red rays Of the suns light is 81 degrees ~
minutes and that OF the violet rays ~ degrees ~
minutes The average OF eighty-three measure
ents Of the radius OF the red edge Of the halo Is 2
degrees &c minutes which is almost identical with
the result OF the theoretical computation The ordt-
nary halo OF % degrees radius Is formed by tue re-
fraction of the rays through the simplest form OF ice
crystals which are right prisms terminated by hexs-
pona bases perpendicular to the sides. The alter
nate faces OF such prsms are inclined TO each other
at an angle OF 60 degrees But the angle formed by
either base and OF the sides or faces s one OF 30
degrees and where the rays are refracted through
tbe lower portions of the ice crystals, second halo
,s produced having radius OF about 46 degrees,
and exhibiting phenomena similar to the inner halo
already described in this circle, as in the former,
actual observation comedies almost exactly with
theoretical computation, the deviation OF the red
Eays through a prism OF Ice having an angle OF 80
degrees being found to be 45 degrees minutes, and
mat of the blue rays &c degrees 80 mnuJcs, while
The average OF the best observations gives radius OF
Al degrees 4 minutes to the brightest portion OF the
outer halo. A third halo has occasionally been
seen around the sun, having radius OF about so de-
prees but only three observations OF this rare phe
nemenon are on record and its exact dimensions
are not well determined can IL be accounted for
satisfactorily on scientific principles, as in the case
Of the first and second haloes. 1t is quite an easy
experiment 10 produce artificial haloes. The eXpe-
yimcnter may crystalize some salt like alum upon a
glass plate, and looking through this plate at the
san or candle he will observe three distinct circles
of light possessing the same characteristics as the
natural halo. few drops of saturated or strong
solution Of alum, spread on q plate Of glass will
coon it with layer of minute crystals, having
the shape OF regular octahedron, with truncated
angles OF angles formed by planes oblique to some
OF the faces. The different faces are inclined to
each other angles Of TO- 90, and 10 degrees and
the three haloes observed are formed by the refrac
ton OF rays OF light through corresponding pairs OF
faces inclined to each other at the different angles
seyerally named.