The Dorset County Express and Agricultural Gazette.
Dorchester, Dorset, England, 1865-05-30 00:00:00

MIRAGE, RAINBOW, AND ATMOSPHERIC

SPECTRA.—No. L
To the Editors.

"A singular atmospheric phenomenon (says the
Times' Parisian Correspondent of date 4th October,
1862) was observed near Yvetot, (Seine-Inferieure),
three evenings since, (let) about 6 o'clock,by several
pereons who were walking along the Havre-road.
The sky was covered with dark clouds. The gusts
of wind, with occasional flashes of lightning, in the
west, seemed to portend a storm. All at once, a
strange appearance presented itself in the S.W.
horizon. The clouds seemed to form an inclined
plane, sloping down to the shores of a lake, the blue
water@ of which were rippled by a gentle breeze.
The lake seemed to be surrounded by trees of un-
equal height, and the whole background of the
picture presented a vast amphitheatre. This mirage
remained visible till lost in the darkness of night."
The number and variety of these phenomena, to
which indeed might be added others within the
ordinary range and rules of vision, such a. doable
and sometimes triple rainbows, parahelia or mock
sues, and moreover the Aurora Borealia, and most
probably the Zodiacal light, all show that they are
dependent partly on the laws of reflection,and partly
on thoae of refraction, in the transmission of those
rays or bodies of light which compose entire images.
Indeed we have no difficulty in resolving the trans-
mission of visible images through the immense dis-
tances for which it will be shown the most of the
foregoing appearances have travelled or been trans-
mitted from the real vortex of their existence, into
a aeries or succession of telescopic phenomena, in
which the action of something in nature, equivalent
to the artificial reflector, and the lens, is clearly de-
monstrated in the cane of those images which appear
inverted to the eye--their appearing inverted or not
being of course dependant on the correction of one
lens or another, and on the point of perception being
within on beyond the focal distance at the moment
when the image is beheld.
It may be objected that if these effects really
emanate from cloud and sky, or, in other words,
, atmospheric combinations, why are there places
more remarkable than others for exhibiting them?
This, however, is a matter palpably involving the
conditions for perceiving the spectra, and forms part
of the optical arrangement by which they are really
produced, the same as looking from Salisbury Crags,
near Edinburgh, towards the rock on which the
Nelson monument is reared upon the Calton-hill, it
is not difficult to trace a colonial figure of Nelson
himself in the natural contour of the rock; or as the
hills of Dunsappie nd Arthur Seat, rising above
Salisbury Crags, viewed from the Edinburgh '
meadows, clearly resemble a lion couchant. Thou-
sands of such natural reeemblances, requiring no
grad stretch of fancy for their identification, no
doubt exist, and are known to the curious in all ro-
mantic parts of the country ; as, for example, the
figure of George 111. on horseback, seen on the head-
land which forms the eastern point of Weymouth
harbour. We do not refer to the white equestrian
figure cut into the chalk by stripping oil the turf,
but to a dark outline visible in the face of the cliffs,
which has no doubt suggested the other, and is an
imitation of the five colossal figures, on the hill faces
above the celebrated White Horse Valley, all of
which are artificially cleared down to the chalk.
Then there is a field which we were wont to pass
several times a week lying on thitalopes that descend
from the north to the meadow grounds behind Dor-
chester, and here in passing about five in the after-
noon, when the sun is getting low in the west, the
level beams when refracted by the falling drops of an
evening shower, seldom fail in producing reduplica-
tions of the rainbow image, generally forming ?,hree
complete arches, of which the centre image is the
real, and the right and left, which are respectively
higher and lower, the complementary expressions.
The inversion of the order in which the due series of
prismatic colours severally occurs in the fainter
images, proves also their repetition by means of re-
flection against the showery medium from the more
vivid central image. The fact is that in this hollow
under Poundbury ridge the lie of the ground consti-
tutes the spectacular portion of the sky into a vast
cup, or concave mirror, in whale misty background
occur the foregoing phenomena. The same may be,
and often are, seen elsewhere; here they seldom
but are peculiar to the spot; in the same
way as the Cape hns its peculiar phantom
ship, "The Flying Dutchman," the Brokken its
spectral " giant," the Straits of Messina their
Fats Morgana, the Arabian desert its mirage, and
the mist-clad heaths and moors of Ayrshire and
ether desolate parts of Scotland its movements
of armies in the air. In the case of the rainbow.,
Iwo take the oolours of the central arch in the usual

eequesce, red, orange, yellow, green, blue, purple,
violet ; those on the side of red, in the faint arch
aeareet ne will read reversely, blue, yellow, red,
with the complementary colours formed by the
Junction of these ; and those in the further arch will
read the same—the truth being that on the side
nearest the original red prismatic ray of the central
*rob, red is repeated in the reflection or arch on this
aide; and on the side nearest the primitive blue,
blue is reflected in an opposite direction, so that in
point of fact all the colours read atrai,ght-forwardly
read alike. This proves, we think, that a double
reflection takes place in the concavity of the sky.
My daughter Informs me that only a few day. ago
the observed no fewer than five distinct rainbow
area, the outer bows having been again reflected.
Take the cue of halos, as expounded by an great
in optical authority as Sir David Brewster ; he
11 notices that Sir Isaac Newton in Jane, 1692,
' observed by reflection in a vessel of standing water,
three rings of colour round the ran like three little
rainbows. The colours of the first or innermost were
blue next the sun, red without, and white in the
middle. The colours of the second ring were purple
and blue within, pale red without, and green in the
middle. The colours of the third ring were pale
blue within, and pale red without. This is stated
more distinctly thus-
-Ist ring blue, white, red;
2nd ring purple, blue, green, pale yellow, red;
3rd ring pale blue, pale red.
The readings of these reflected colours are in fact
one and the same, the extended enumeration in the
second ring comprising only the colours comple-
mentary to the three primitive colours in the seoond;
and though two only are enumerated, more faintly
pronounced in the third, the central ring being made
up in the usual way of the blended colours of the
other two.

Taking then these familiar indications of the
mirroring power of the atmosphere, and its constant
tendency to reflect and multiply, as well as refract
Images of objects, or appearances formed within its
bosom, we may perhaps come to understand how and
In what manner images may in the more rarified
States of the atmosphere come to be reflected from
stage to stage, always varying their axis, however,
at every stage, so as to become visible at the most
immense and yet incalculable distances, being alto-
gether deflected from the straight line. The exami•
nation of certain of those phenomena may lead or
help us to a solution of the origin of some of those
interesting and magical phenomena that chequer the
world at times with creations seemingly super-
natural.
The ease of a vessel "looming" in the distant
base of the ocean, or appearing perhaps above the
horizon, and sometimes with its inverted image
immediately over it in the air, although in reality,
not yet physically, "in sight," is a paradox which
can only be explained by the state of the light in
which the single or doable object is made prema-
turely to appear, and by the relative position
of the observer to the image and the
light. Thus, if the light acting from
beyond the visible horizon should be strong
enough to project the image so as to bring it in view,
or to throw it up against any of the reflecting media
with which ire have seen that the atmosphere is
abundantly tarnished; then it will depend on
another condition, and that is, the position of the
spectator, whether and to what extent the objects
otherwise too distant for observation may come into
view. The story of the spectre ship, or Flying
Dutehman, of the Cape of O'ood Hope, has generally
been regarded as a fanciful creation of the brain of
Cameos, the Portuguese poet, who introduces the in-
(Admit with startling et, as the artist, David
Scott, has painted it on an enormous canvass, now in
the Trinity House at Leith, in depicting the voyage
of Yasoo di Gains round the Cape of Storms. But
the F/ying Dutchinan is a real, or at all events a
visible object. Captain W. F. W. Owen has, in our
own times, testified Szi the truth of this in his
"Account isf the -.Towage of the Leven and
Baresoouta."
W. WALLACE FYFE.
To Ur eentinued.

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