Halo. 
Sir Isaac 
Newton’s 
explanation 
of halos. 
616 
supposed to have been at first globules, formed of the 
softest and finest particles of snow. As soon asa glo- 
bule is formed by a collection of these particles, many” 
more particles will adhere to the bottom of it, but not 
to its sides, on account of thecurrent of ascending va- 
pours. The globules will thus have an oblong cylin- 
dri¢al figure; and when the warmth of the sun or of 
the air shall have melted the outsides of these cylin- 
‘ders, a smaller cylinder of snow will remain in the 
middle of each of them, surrounded with water ; and 
after a certain part is melted, the cylinders within will 
become round and perfect, and will remain in this state 
for some time. If this coat of water should be frozen, 
Huygens supposes that it may possibly remain suffi- 
ciently transparent and polished to transmit, refract, 
and reflect the rays of the sun iri aregularmanner. By 
the aid of these assumptions, Huygens has ingeniously 
explained, in a very minute manner, almost all the 
principal phenomena of halos which had been seen at 
the time when he wrote. It is extremely improbable, 
however, that such hailstones do exist, and still more 
improbable that they should have such properties as 
to produce constantly the diameter of 47°. _ 
Sir Isaac Newton ascribes the halo of 223 degrees by 
refraction from floating hail or snow, and he accounts 
for the small coloured corone by his doctrine of fits 
of easy reflexion and transmission. ‘“ As light re- 
flected by a lens,” says he, ‘ quicksilvered on the back- 
side, makes the rings of colours above described, so it 
ought to make the like rings of colours in passing 
through a drop of water. At the first reflection of the 
rays within the drop, some colours ought to be trans< 
mitted, as in the case of a lens, and others to be reflect- 
ed back to the eye. For instance, if the diameter of a 
small drop or globule of water be about the 500th part 
ofan inch, so that a red-making ray, in passing through 
the middle of this globule, has 250 fits of easy trans- 
mission within the globule, and that all the red-making 
rays which are at a certain distance from this middle 
ray round about it have 249 fits within the globule, 
and all the like rays at a certain further distance round 
about it have 248 fits, and all those at a certain far- 
ther distance 247 fits, and so on; these concentric 
circles of rays, after their transmission, falling on a 
white paper, will make concentric rings of red upon 
the paper, supposing the light, which passes through 
one single globule, strong enough to be sensible ; and, 
in like manner, the rays of other colours. Suppose 
now that, in a fair day, the sun shines through a thin 
cloud of such globules of water or hail, and that the 
globules are all of the same bigness, and the sun seen 
through this cloud shall appear encompassed with the 
like concentric rings of colours, and the diameter of 
the first ring of red shall be 74°, that of the second 
104°, that of the third 12° 33’. And according as the 
globules of water are bigger or less, the rings shall be 
less or bigger. This is the theory, and experience an- 
swers it. For, in June 1692, I saw, by reflection in a 
vessel of stagnating water, three halos, crowns, or rin 
of colours about the sun, like three little rain-bows 
concentric to his body ; the colours of the first or inner- 
most crown were blue next the sun, red without, and 
white in the middle between the blue and red. Those at 
the second crown were purple and blue within, and pale 
red without, and green in the middle; and those of 
the third were pale blue within, and pale red without. 
These crowns inclosed one another immediately, so that 
their colours proceeded in this continual order from the 
sun outward ; blue, white, red ; purple, blue, n; 
pale yellow, and red ; pale blue, pale red. The diame- 
ter of the second crown measured from the middle of the 
HALO. 
yellow and red on one side ofthe sun, to the middle of 
the same colour on the other side, was 94°, or thereabouts. 
The diameters of the first and third I had not time to 
measure ; but that of the first seemed to be about five 
or six degrees, and that of the third about 12°. The 
like crowns appear sometimes about the moon ; for, in 
the beginning of the year 1664, Fe’ 19th, at 
night, I saw two such crowns about her. 
ter of the first or innermost was about 8°, and that of 
the second about 53°. Next about the moon was a cir 
cle of white, and next about that the inner crown, 
which was of a bluish green within next the white, and 
of a yellow and red without, and next about these colours 
were blue and green on the inside of the outward 
crown, and red on the outside of it. At the same time, 
there appeared a halo about 22° 35’ distant from the 
centre of the moon. It’ was elliptical, and its long dia~ 
meter was ndicular to the nieces verging below 
farthest from the moon. I am told, that the moon has 
sometimes three or more concentric crowns of colours 
encompassing one another next about her body. The 
more equal the globules of water or ice are to one 
another, the more crowns of colours will appear, and 
the colours will be the more lively. The halo at the dist- 
ance of 223° from the moon is of another sort. By its 
being oval and remoter from the moon below than 
above, I conclude, that it was made by refraction in 
some sort of hail or snow floating in the air in an hori- 
zontal , the refracting being about 58° or 
60°.” See Newton’s Optics, ii. Part iv. Obs. 18. 
ilateral triangular prism, He con 
jectures, that the hard flakes of snow which fall during 
a paabbenty Si ee at fi of stars, are 
composed of little nts like equi ‘al prisms, 
—- ~_— — are like erm leaves, Re maple 
easily seen by microscope. U, examining the 
filaments which com the pa ovis he found them 
cut into three equal facets, and they exhibited rainbows 
when placed in the sun. Mariotte then supposes, that 
before the hoar frost is formed, some of these separate 
prisms float among thethin vapours in the air, beforethey 
unite into the compound figures. ‘ These little stars,” 
says he, “ are very thin and er oa and the little fi- 
laments which compose them are still more so, and may 
often be supported along time in the air by the winds. 
Hence, when the air is moderately filled with them, so 
as not to be much darkened, many of them, whether se- 
parate or united, will turn in every direction as the air 
impels them, and will be disposed to transmit to the 
eye _ ae awe ond tpestalaaniae like ya —_ 
whichwould be Wi requi isms of 
M. Mariotte then waitabentsie angles ; Secsiiey deduct 
ing 16! for the sun’s semidiameter, and 30’ for the de- 
viation of the red)rays, he finds 22° 50’ tobe the semi- 
diameter of the halo produced by equiangular prisms. | 
In attempting to account for parhelia, Mariotte ob- 
serves, ‘ that they are usually at the same altitude as 
the sun. Among the prisms of snow, there are often 
many heavier at one end than at the other, and conse- 
quently situated in a vertical direction. These cause a 
bright parhelion with a tail, which cannot be above 70° 
long. I have read an account of a halo seen in May, 
soon after sun-rise, with parhelia in its circumference, . 
which, after two or three aK: were more than a de- 
gree distant from it. This appearance arises from the 
coincidence of the sun’s rays with the transverse sec- 
tion of the prism, when they are nearly horizontal, and 
from their obliquity, when the sun is elevated, causing 
a greater deviation, and throwing the parhelia outwards, 
