Year: 1871

From the Boston Transcript.


Red auroras, such as that which was
visible here on the evening of Nov. 8,
have a peculiar interest to the man of
science as well as to the ordinary spectator.
This is due to the fact that their colOrs diff
fer in certain respects from any other red
which we are in the habit Of seeing. For
example, a gentleman who is partially color
blind, and distinguishes ordinary red tints
with difficulty, if at all, found the red light
of the recent aurora plainly visible. But a
more convincing proof of the singular char
acter of the aulroral red is furnished by the
spectroscope.


Most persons are now aware that the pur-
pose of spectroscopic observations is to as
certain the comparative refrangibility of the
light furnished by luminous objects, and to
learn by that means something with respect
TO their chemical constitution. If the light
observed is all of one kind it will be re-
fraeted to one place in the field of the spec
troscope, and we shall see a single bright
line, having the color appropriate TO light
of that degree of refrangibility. If the light
is compound, the spectroscope will sepa-
rate it into its elements, or will show,
by continuous band of prisnatic colors,
that the luminous vibrations are prevented
from fully developing themselves. This is
the case with all light proceeding from solid
and LIQUID bodies, and even with that emitted
by gases subjected TO sufficient pressure.
But in general, an incandescent gas gives
out light which appears in the spectroscope
in the form of bright lines, separated from
each other by dark intervals. The auroral
light which is presumed to be due to the
discharge of electricity through gases imper-
fectly known to US, existing high above the
ordinary atmosphere of the earth, appears
accordingly when examined by the spectre
scope, mainly as a set of bright lines, the
most conspicuous of which is green, as is
natural, since the eye perceives q greenish
tint in all our usual auroras. Still, in some
remarkable cases, arches of auroral light
have been observed which give a continu-
Gus spectrum. Several phenomena of this
kind have been recorded within the past few
years at Harvard College Observatory.
They are apparently to be accounted for by
assuming the light to proceed from solid or
liquid particles floating in gaseous medi-
um, or from q gas under pressure, and
hence probably nearer the surface of the
earth than that which is the source of ordi-
nary auroral light.


Red light, as is known to all who have
amused themselves by experiments with a
prism, is less refrangible than green light.
Hence we should expect that when the light
of red aurora is examined by the spectre
scope, lines would be seen beyond the usual
green line toward what is called the red
end of the spectrum. Some observers have
indeed reported such lines as visible, but it
is clear that they are not always to be seen
when the red aurora appears. Nearly all
the auroras which have shown light during
the past two years have been carefully exc
amined with spectoscopes of various kinds at
Harvard College Observatory, with the sin
glllar result that while lines indicating light
more refrangible than that Of the usual
green line are noticed whenever the quanti-
ty of usll. is sufficient TO make them yisi-
bic, no line indicating light Of less refrangi-
blity has ever been observed. In the recent
aurora, for example, lines were seen in the
violet, the blue and green (besides the Or
binary green line) and these blue and green
lines were seen most plainly in the light
from the reddest portion of the aurora.
Between these lines there was q certain
amount of light, which curiously enough,
still seemed red to the eye Of the observer
at the spectroscope.


The conclusion seems necessary that
what we call red light in aurora is, in fact,
often, if not always, made up of rays Of the
more refrangible kinds. Some experiments
tried by combining flames tinted with va
rious chemical substances confirm this sup
position, and have resulted in the produc-
ton of fair imitation of auroral red with
out the use of any originally red fame.
As mere speculation, it is, perhaps, also
allowable to refer to recent conjectures as
to the possible existence of what has been
called another octave of colors, in which
light beyond the violet end of the spectrum
produced by vibrations of double the rapidi-
ty of those which yield ordinary red light,
may be supposed to appear to the eye in
unison with that. 1t is conceivable that
both forms of red may be visible to ordi-
pary ewes under special conditions, and
that red in the upper octave may impress
an eve for which the pitch of ordinary red
is too low, as we may say if we adopt the
analogy of musical sounds.


Another fact ofinterest, and confirming
better established theory, was also noticed
on Nov 8, at Harvard College ObServ-
alory. 1t is now generally admitted that
electrical disturbances upon the earth are
dependent to some extent upon the changes
constantly going on in the gaseous envel-
ope surrounding the sun. Since 1868 it has
been possible for observers to examine the
protuberances, as they are called, upon the
body of the sun, which had previously been
seen only at the time of total eclipse.
This is effected by means ofthe spectroscope
used in peculiar manner. On the fore
noon of Nov. 9, unusually extensive
ard sudden changes in the form CT these
protuberances were observed, indicating a
degree of cOmmotion in the solar atmos-
phere which might be expected to produce
some striking terrestrial phenomena of the
electrical kind. The aurora of the evening,
we can hardly doubt, was part of one great
series of events originating at the sun, and
possibly extending to the remotes planet.