OPTICS. 
311 
the effects produced by them on the rays of 
light ; but in different states of the air, in- 
stead of drops of rain, vapour falls to the 
earth in different forms of sleet, snow, and 
Bail. In the two latter states there cannot be 
a refraction of the rays of light ; but in the 
former state, when a drop is partly in a con- 
gealed and partly in a fluid form, the rays of 
light will be diiFerently affected, both from 
the form of the drop and its various refract- 
ing powers. Hence we may expect a variety 
of curious appearances in the heavens ; and 
to these drops, in different states, we may at- 
tribute the formation of halos, parhelia, and 
many other phenomena, detailed in the Phi- 
losophical Transactions, or in tire histories of 
every country. 
The halo, or corona, is a luminous circle 
surrounding the sun, the moon, a planet, or 
a fixed star. It is sometimes quite white, 
and sometimes coloured like the rainbow. 
Those which have been observed round the 
moon or stars are but of a very small diame- 
ter ; those round the sun are of different 
magnitudes, and sometimes immensely great. 
When coloured,, the colours are fainter than 
those of the rainbow, and appear in a dif- 
ferent order, according to their size.. In those 
which sir Isaac Newton observed in 1692, 
the order of the colours, from the inside next 
the sun, was in the innermost blue, white, 
red ; in the middle purple, blue, green,, yel- 
low, pale red ; in the outermost pale blue, 
and p^le red. Huygens observed one red 
next the sun, and pale blue at the extre- 
mity. Mr. YVeidler has given an account of 
one yellow on the inside, and white on the 
outside. In France one was observed, in 
which the order of the colours was w hite, red, 
blue, green, and a bright red on the out- 
side. 
Artificial coronas may be made in cold 
weather, by placing a lighted candle in the 
midst of a cloud of steam ; or if a glass win- 
dow is breathed upon, and the flame of a 
candle placed at some distance from the win- 
dow, while the operator is also at the distance 
of some feet from another ’part of the win- 
dow, the flame will be surrounded with a co- 
loured halo.. 
When M. Bouguer was on the top of mount 
Pichinea, in the Cordilleras, he and some 
gentlemen who accompanied him, observed 
a most remarkable phenomenon. When the 
sun was just rising behind them, and a white 
cloud was about thirty paces from them, 
each of them observed his own shadow (and 
no other) projected upon it. All the parts 
of the shadow were distinct ; and the head 
was adorned with a kind of glory, consisting 
of three or four concentric crowns, of a very 
lively colour, each exhibiting all the varieties 
of the primary rainbow, and having the cir- 
cle red on the outside. 
Similar to this appearance was one which 
occurred to Dr. M’Fait, in Scotland. This 
gentleman observed a rainbow round his sha- 
dow in a mist, when he was situated on an 
eminence above it. In this situation the 
whole country appeared to be immersed in a 
vast deluge, and nothing but the tops of hills 
appeared here and there above the tlood ; at 
another time he observed a double range of 
colours round his shadow. 
The parhelia, or mock suns, are the most 
splendid appearances of this kind. We find 
these appearances frequently adverted to by 
the antients, who generally considered them 
as formidable omens. Four mock suns were 
seen at once by Scheiner at Rome, and by 
Muschenbroeck at Utrecht ; and seven were 
observed by Hevelius at Sedan, in 1661. 
The parhelia generally appear about the size 
of the true sun, not quite so bright, though 
they are said sometimes to rival their parent 
luminary in splendour. Wlien there are a 
number of them they are not equal to each 
other . in brightness. Externally they are 
tinged with colours like the rainbow. They 
are not always round, and have sometimes a 
long (iery tail opposite the sun, but paler to- 
wards the extremity. Dr. Haller observed 
one with tails extending both ways. Mr. 
Weidler saw a parhelion with one tail point- 
; ing up and another downward, a little crook- 
ed ; the limb which was farthest from the 
sun being of a purple colour, theother tinged 
with the colours of the rainbow. 
Coronas generally accompany parhelia : 
some coloured, and others white. There is 
also, in general,, a very large white circle, pa- 
rallel to the horizon, which passes through all 
the parhelia ; and, if it was entire, would go 
through the centre of the sun : sometimes 
there are arches of smaller circles concentric 
to this, and touching the coloured circles 
which surround the sun ; they are also tinged 
with colours, and contain other parhelia. 
One of the most remarkable appearances 
of this kind was that which was observed at 
Rome by Scheiner, as intimated above ; and 
this may serve as a sufficient instance of the 
parhelion. 
This celebrated phenomenon is represent- 
ed in Plate 111. tig. 17, in which A is the 
place of the observer, B his zenith, C the true 
sun, and AB a plane passing through the ob- 
server’s eye, the true sun, and the zenith. 
About, the sun C there appeared two con- 
centric rings, .-not complete, but diversified 
with colours. The lesser of them, DEF, was 
fuller, and more perfect ; and though it was 
open from I) to !■', yet those ends were per- 
petually endeavouring to unite, and some- 
times they did so. The outer of these rings 
was much fainter, so as scarcely to be dis- 
cernible. It had, however, a variety of co- 
lours, but was very inconstant. The third 
circle, KI.MN, was very large, and entirely 
white, passing through the middle of the 
sun, and every where parallel to the horizon. 
At first this circle was entire ; but towards 
the end of the phenomenon it was weak and 
ragged, so as hardly to be perceived from M 
towards N. 
In the intersection of this circle and the 
outward iris GKl, there broke out two par- 
helia, or mock suns, N and K, not quite per- 
fect, K being rather weak, but N shone 
brighter and stronger. The brightness of 
the middle of them was something like that 
of the sun ; but towards the edges they were 
tinged with colours like those of the rain- 
bow, and they were uneven and ragged. The 
parhelion N was a little wavering ; and sent 
out a spiked tail NP, of a colour somewhat 
fiery, the length of which was continually 
changing. 
The parhelia at L and M, in the horizontal 
ring, were not so bright as the former, but 
were rounder, and white, like the circle in 
which they were placed. The parhelion N 
disappeared before K; and while M grew 
fainter, K grew brighter, and vanished the 
last of all. 
It is to be observed farther, that the order 
of the colours in the circles DEF, GKN, was 
the same as in the common halos, namely, 
red next the sun ; and the diameter of the 
inner circle was also about 45°, which is the 
usual size of a halo.. 
Parhelia have been seen for one, two, 
three, and four hours together ; and in North 
America they are said to continue some days, 
and to be visible from sun-rise to sun-set. 
When they disappear it sometimes rains, or 
snow falls in the form of oblong spicuke. 
Mr. Wales says, that at Churchill, in Hud- 
son’s-bay, the rising of the sun is always pre- 
ceded by two long streams of red light. These 
rise as the sun rises ; and, as they grow 
longer, begin to bend towards each other, till 
they meet directly over the sun, forming 
there a kind of parhelion, or mock sun. 
These two streams of light, he says, seem 
to have their source in two other parhelia, 
which rise with the true sun ; and in the win- 
ter season, when the sun never rises above 
the haze or fog which he says is constantly 
found near the horizon, all these accompany 
him the whole day, and set with him in the 
same manner as they rise. Once or twice he 
saw a fourth parhelion under the true sun ; 
but this, he adds, is not common. 
The cause of these is apparently the reflec* 
tion of the sun’s light and image from the 
thick and frozen clouds in the northern at- 
mosphere, accompanied also with some de- 
gree of refraction. To enter upon a mathe- 
matical analysis of these phenomena would 
be only tedious, and very foreign to our pur- 
pose. From what has been said upon this 
subject it is evident, that all the phenomena 
of colours depend upon two properties of 
light, the refrangibility and reflexibility of its 
rays.. 
Of the inflection of light . — The direction, 
of the rays of light is changed, as we have 
seen, in their approach, to certain, bodies, by 
reflection and to fraction ; and consequently 
we must admit that there is some power in 
these bodies by which such effects are uni- 
versally produced. If reflection was pro- 
duced simply by lire impinging of particles 
of light on hard or elastic bodies, or if they 
were in themselves elastic, the same effects 
would follow as in the impulse of other clastic 
bodies ; but the angle ot incidence could not 
be equal to the angle of reflection, unless the 
particles of light were perfectly elastic, or the 
bodies on which they impinged were per- 
fectly elastic. Now we know that the bodies 
on which these particles impinge are not per- 
fectly elastic ; and also that if the particles of 
light were perfectly elastic, the diffusion of 
light from the reflecting bodies would be 
very different from its present appearance: 
for as no body can be perfectly polished, the 
particles of light, which are so inconceivably 
small, would be reflected back by the inequa- 
lities on the surface in every direction ; con- 
sequently we are led to this conclusion, that 
the reflecting bodies have a power which 
acts at some little distance from their sur- 
faces. 
If this reasoning is allowed to be just, it 
necessarily follows, that if a ray of light, 'in- 
stead of impinging on a body, should pass so 
