SUPPOSED CAUSES OF THE AURORA.
[Sidenote: Supposed causes of the Aurora. Sulphurous vapours. Magnetic
effluvia.]
At first the Aurora was described to be sulphurous vapours issuing from
the earth; and Musschenbroek pointed out that certain chemical mixtures
sent forth a phosphorescent vapour, in some respects resembling the
Aurora. Dr. Halley originally proposed a similar theory, but ultimately
concluded that the Aurora might be occasioned by the circulation of the
magnetic effluvia of the earth from one pole to another.
[Sidenote: Zodiacal light.]
M. de Mairan, in 1721, in a treatise, ascribed the Aurora to the impulse
of the zodiacal light upon the atmosphere of the earth.
[Sidenote: Luminous particles of our atmosphere.]
Euler combated this theory, and ascribed the Aurora to the luminous
particles of our atmosphere driven beyond its limits by the light of the
sun, and sometimes ascending to the height of several thousand miles.
[Sidenote: Electric fluid _in vacuo_ resembles Aurora.]
Mr. Hawksbee very early showed that the electric fluid assumes, _in
vacuo_ or in highly rarefied atmosphere, an appearance resembling the
Aurora. Mr. Canton contrived an imitation of the Aurora by means of
electricity transmitted through the Torricellian vacuum in a long glass
tube, and showed that such a tube would continue to display strong
flashes of light for 24 hours and longer without fresh excitation.
[Sidenote: Experiment with electrical machine and exhausted receiver.]
In the ‘Edinburgh Encyclopædia,’ date 1830, is mentioned an experiment
in which an electrical machine and air-pump are so disposed that strong
sparks pass from the machine to the receiver of the air-pump.
[Sidenote: Dr. Franklin’s theory.]
As the exhaustion proceeds the electricity forces itself through the
receiver in a visible stream, at first of a deep purple colour; “but as
the exhaustion advances it changes to blue, and at length to an intense
white, _with which the whole receiver becomes completely filled_.” [It
will be noticed that this experiment bears a close resemblance to Prof.
Ångström’s exhausted flask referred to later in treating of the spectrum
of the Aurora.]
Dr. Franklin gave a different form to the electric theory of the Aurora,
supposing that the electricity which is concerned in the phenomenon
passes into the Polar regions from the immense quantities of vapour
raised into the atmosphere between the tropics (Exper. and Observ. 1769,
p. 43).
[Sidenote: Mr. Kirwan’s theory.]
Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora
Borealis and Australis was occasioned by the combustion of inflammable
air kindled by electricity.
[Sidenote: Mons. Monge’s.]
Mons. Monge proposed the theory that the Auroræ were merely clouds
illuminated by the sun’s light falling upon them after numerous
reflections from other clouds placed at different distances in the
heavens (Leçons de Physique par Prejoulz, 1805, p. 237).
[Sidenote: Mons. Libes’.]
Mons. Libes propounded a theory that the electric fluid, passing through
a mixture of azote and oxygen, produced nitric acid, nitrous acid or
nitrous gas, and that these substances, acted upon by the solar rays,
would exhibit those red and volatile vapours which form the Aurora
Borealis (Traité de Physique, ou Dictionnaire de Physique, par Libes;
Rozier’s Journal, June 1790, February 1791, and vol. xxxviii. p. 191).
[Sidenote: Mr. Dalton’s.]
Mr. Dalton considered the Aurora a magnetic phenomenon whose beams were
governed by the magnetism of the earth. He observed that the luminous
arches were always perpendicular to the magnetic meridian (Dalton’s
Meteorological Observations and Essays, 1793, pp. 54, 153).
[Sidenote: Abbé Bertholon’s.]
The Abbé Bertholon ascribed the Aurora Borealis to a phosphorico-electric
light (Encyc. Méthod. art. Auroræ).
[Sidenote: Dr. Thompson concluded the arches to be an optical deception.]
Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the
observations of Mr. Cavendish and Mr. Dalton, concluded there was no
doubt that the arched appearance of the Aurora was merely an optical
deception, and that in reality it consisted of a great number of straight
cylinders parallel to each other and to the dipping-needle at the place
where they were seen.
[Sidenote: Artificial Auroræ produced in exhausted tubes.]
With many of us (at least it was so in my own case) our first viewed
Auroræ have been artificial ones, devised by electricians and having
their locus at the Royal Polytechnic in Regent Street or in some
scientific lecture-room. The effects in these cases are produced in tubes
nearly exhausted by means of an air-pump, and then illuminated by some
form of electric or galvanic current.
[Sidenote: Tubes described.]
In one instance the tube is usually of the form shown on Plate X. fig.
9, supported on a base with a brass ball electrode at the lower end, and
a pointed wire at the upper. In another case the tube is of the form
shown on same Plate, fig. 8. After exhaustion it is permanently closed,
the current passing through it by means of the platinum-wire electrodes
introduced into each end of the tube. The first form of tube is usually
excited by a frictional plate machine; the second by a galvanic current
from a Grove or bichromate battery, which, by the aid of a Ruhmkorff
coil, has had its character changed from quantity to intensity. In each
instance, upon connexion with the source supply of the electric current,
a very similar effect is produced.
[Sidenote: Effects described.]
Brilliant streams of rose-coloured light pass between the electrodes,
sometimes as a single luminous misty band, sometimes in divided vibrating
sprays or streams, and sometimes in a flaky column of striæ.
All this, before the spectroscope took its part in the investigation, we
were content to accept as a very fair and probable explanation of the
Aurora accompanied by a mimic representation of the phenomenon.
These appearances may, of course, be produced at will in tubes having
electrodes; but it is, moreover, possible to produce them, though with
less effect, in certain other forms of tube having no such direct
communication with the external electric machine.
One electrode only may be connected with the coil or electrical machine.
The appearance is then a faint representation of what happens when the
current entirely passes (but see experiments with a single wire detailed
in Part III.).
[Sidenote: Tube without electrodes.]
In the case of an exhausted tube having no electrodes, the wires from
the coil may be made into a little helix and placed at each end of the
tube, and the induced currents within will show themselves in flashes and
streams of light, varying in colour and tint according to the gaseous or
other contents of the tube.
[Sidenote: Tube excited by friction.]
In some cases the ordinary forms of galvanic or electrical machine for
supplying the current of electricity may be dispensed with. A long
straight tube exhausted and closed at each end, and without electrodes,
Plate X. fig. 6, being slightly warmed and then excited by friction with
the dry hand or a piece of flannel, silk handkerchief, or the like, is
soon filled with the most brilliant flashes of light playing in the
interior, and when once thoroughly charged needs but little further
excitation to keep up the effect.
[Sidenote: Geissler’s mercury tube.]
Geissler has introduced a form of tube in which electricity in its form
of flashes and glow of light is produced by the friction of mercury. The
outer tube is strong, and contains within it a smaller tube of uranium
glass with balls blown upon it (Plate X. fig. 7). The tubes are exhausted
and a small quantity of mercury is introduced which has access to both
surfaces of the inner tube, as well as to the inner surface of the outer
tube. Upon the tube being reversed end for end or shaken, the mercury
runs up or down the tube and causes a very considerable display of
whitish light.
The before-described tubes are also referred to, and their spectra
described, in the section “On the comparison of some tube and other
Spectra with the Aurora” (Part II.).
The aura or brush from the electrical machine has been considered as
resembling the Aurora, while the hissing and crackling accompanying it
has been supposed to corroborate the reports of similar noises having
been heard during an auroral display.
[Sidenote: Prof. Lemström’s instrument to demonstrate the nature of
Auroræ.]
Prof. Lemström, of the University of Helsingfors, has devised an
instrument for the purpose of demonstrating that Auroræ are produced by
electrical currents passing through the atmosphere. An illustration of
this instrument (for which I am indebted to the Editor of ‘Nature’) is
introduced (fig. 1).
The instrument was exhibited at the recent Scientific Loan Collection at
South Kensington, and a full description of it, together with an essay by
Prof. Lemström, “On the Theory of the Polar Light,” will be found in the
third edition of the Official Catalogue, p. 386. no. 1751. The apparatus
is intended to show that an electric current passing from an insulated
body does not produce light in air of normal pressure; but as it rises to
the rarefied air in the Geissler tubes a phenomenon very like the real
Polar Light is produced.
[Illustration: Fig. 1.]
A is an electrical machine, the negative pole being connected with a
copper sphere and the positive with the earth.
_s s´_ is of ebonite as well as R R _d_, so that B is quite insulated as
the earth is in space. B is surrounded by the atmosphere. _a´ a´ a´ a´
a´ a´_ are a series of Geissler tubes with copper ends above and below.
All the upper ends are connected with a wire which goes to the earth;
consequently a current runs in the direction of the arrows through the
air, and the Geissler tubes become luminous when the electrical machine
is set into operation.
The Geissler tubes represent the upper part of the atmosphere which
becomes luminous when the Aurora Borealis is observed in the northern
hemisphere. The phenomena produced by the Lemström apparatus are
considered consistent with the theory advocated by Swedish observers that
electrical currents emanating from the earth and penetrating into the
upper regions produce Auroræ in both hemispheres. The experiment differs
from the apparatus of M. de la Rive, who placed his current _in vacuo_,
and did not show the property of ordinary atmospheric air, in allowing to
pass unobserved, at the pressure of 760 millims., a stream of electricity
which illuminates a rarefied atmosphere.
[Sidenote: M. de la Rive’s apparatus described.]
De la Rive’s apparatus was also exhibited at the same time, and will be
found described at p. 385 of the Catalogue, No. 1749. A large sphere of
wood represented the earth, and iron cylinders the two extremities of the
terrestrial magnetic axis. These penetrated into two globes filled with
rarefied air, simulating the higher regions of the Polar atmosphere. The
electric discharge turned around a point situate in the prolongation of
the axis, in a different direction at either pole, when the two cylinders
were charged by means of a horseshoe electro-magnet, in accordance with
observations on the rotation of the rays of the Aurora.
[Sidenote: De la Rive’s magnet in an electric egg.]
De la Rive placed an electro-magnet in an electric egg. As soon as the
magnet was set in action the discharge which had before filled the egg
was concentrated into a defined band of light, which rotated steadily
round the magnet.
[Sidenote: Gassiot’s experiment with 400 Grove cells and exhausted
receiver between poles of magnet.]
Gassiot describes an experiment with his great Grove’s battery of 400
cells, in which an exhausted receiver was placed between the poles of the
large electro-magnet of the Royal Institution.
“On now exciting the magnet with a battery of 10 cells, effulgent strata
were drawn out from the positive pole, and passed along the under or
upper surface of the receiver according to the direction of the current.
“On making the circuit of the magnet and breaking it immediately, the
luminous strata rushed from the positive pole and then retreated, cloud
following cloud with a deliberate motion, and appearing as if swallowed
up by the positive electrode.” Mr. Marsh considered this bore a very
considerable resemblance to the conduct of the auroral arches, which
almost invariably drift slowly southward.
[Sidenote: Mr. Marsh considered the Aurora an electric discharge between
the magnetic poles of the earth.]
He considered it probable that the Aurora was essentially an electric
discharge between the magnetic poles of the earth, leaving the immediate
vicinity of the north magnetic pole in the form of clouds of electrified
matter, which floated southward, bright streams of electricity suddenly
shooting forth in magnetic curves corresponding to the points from
which they originated, and then bending southward and downward until
they reached corresponding points in the southern magnetic hemisphere,
and forming pathways by which the electric currents passed to their
destination; and, further, that the magnetism of the earth caused these
currents and electrified matter composing the arch to revolve round the
magnetic pole of the earth, giving them their observed motion from east
to west or from west to east.
[Sidenote: Varley’s observation on a glow-discharge _in vacuo_. Spark
surrounded by an aura which could be separated.]
Varley showed that when a glow-discharge in a vacuum tube is brought
within the field of a powerful magnet, the magnetic curves are
illuminated beyond the electrodes between which the discharge is taking
place, as well as in the path of the current, and also thought that this
illumination was caused by moving particles of matter, as it deflected a
balanced plate of talc on which it was caused to infringe. It has also
been shown that in electrical discharges in air at ordinary pressure,
while the spark itself was unaffected by the magnet, it was surrounded by
a luminous cloud or aura which was driven into the magnetic curve, and
which might also be separated from the spark by blowing upon it.
Most of the foregoing interesting results and experiments will be found
repeated and verified in Part III.
_Prof. Lemström’s Theory._
[Sidenote: Prof. Lemström’s theory.]
Prof. Lemström thinks that terrestrial magnetism plays only a
comparatively secondary part in the phenomena of the Polar Light, this
part consisting essentially in a direct action upon the rays.
That the experiments of M. de la Rive do not all furnish the proof that
the rays of the light are really united under this influence.
[Sidenote: Character of the Polar Light.]
That the Polar Light considered as an electrical discharge gives the
following results:—
(1) An electric current arising from the discharge itself, which takes
place slowly.
(2) Rays of light consisting of an infinite number of sparks, each spark
giving rise to two induction currents going in opposite directions.
(3) A galvanic current going in an opposite direction to that of the
discharge, and having its origin in the electromotive force discovered
by M. Edlund in the electric spark. That these currents require a closed
circuit; but this is not necessary in the case of the Aurora, as the
earth and rarefied air of the upper regions are immense reservoirs of
electricity producing the same effect as if the circuit were closed. That
permanent moisture in the air, a good conductor of electricity, is the
cause of a slow and continuous discharge assuming the form of an Aurora,
instead of suddenly producing lightning as in equatorial regions and mean
latitudes.
[Sidenote: Polar Light due to electric discharges only.]
He sums up, that the electric discharges which take place in the Polar
regions between the positive electricity of the atmosphere and the
negative electricity of the earth are the essential and unique cause
of the formation of the Polar Light, light the existence of which is
independent of terrestrial magnetism, which contributes only to give to
the Polar Light a certain direction, and in some cases to give it motion.
This Prof. Lemström maintains contrary to those who believe they see
in terrestrial magnetism, or rather in the induction currents, what is
capable of developing the origin of the Polar Light.
_Theories of MM. Becquerel and De la Rive._
[Sidenote: Theories of MM. Becquerel and De la Rive.]
M. Becquerel’s theory is that solar spots are cavities by which hydrogen
and other substances escape from the sun’s protosphere. That the hydrogen
takes with it positive electricity which spreads into planetary space,
even to the earth’s atmosphere and the earth itself, always diminishing
in intensity because of the bad conducting-power of the successive layers
of air and of the earth’s crust. That would then only be negative, as
being less positive than the air. The diffusion of electricity through
planetary space would be limited by the diffusion of matter, since it
cannot spread in a vacuum. That gaseous matter extends further than the
limits usually assigned to the earth’s atmosphere, is proved by the
observation of Auroræ at heights of 100 and 200 kilometres, where some
gaseous matter must exist. M. de la Rive agrees with M. Becquerel as to
the electric origin of the Aurora, but considers the earth is charged
with negative electricity and is the source of the positive atmospheric
electricity, the atmosphere becoming charged by the aqueous vapour rising
in tropical seas. The action of the sun he considers is an indirect one,
varying with the state of the sun’s surface, as shown by coincidences in
the periods of Aurora and sun-spots.
_M. Planté’s Electric Experiments._
[Sidenote: M. Planté’s experiments. Effects produced resembling Auroræ.]
M. Planté has performed some experiments with a very considerable series
of secondary batteries. By inserting the positive electrode after the
negative in a vessel of salt water, luminous and other effects were
observed which were considered to have a strong resemblance to those of
Auroræ.
M. Planté advocates the theory that the imperfect vacuum of the upper
regions, acting like a large conductor, plays the part of the negative
electrode in his experiments, while the positive electricity flows
towards the planetary spaces, and not towards the ground, through the
mists and ice-clouds which float above the Poles.
[Sidenote: M. Planté’s experiments producing a corona, an arc, or a
sinuous line.]
In an article in ‘Nature,’ March 14, 1878, a further account is given
of M. Planté’s experiments, under the head of “Polar Auroræ;” and it is
stated that, in these experiments, the electric current, in presence
of aqueous vapour, yielded a series of results altogether analogous to
the various phases of Polar Auroræ. If the positive electrode of the
secondary battery was brought into contact with the sides of a vessel
of salt water, there was observed, according to the distance of the
film (electrode?) from the liquid, either a corona formed of luminous
particles arranged in a circle round the electrode (fig. 2, p. 90), an
arc bordered with a fringe of brilliant rays (fig. 3), or a sinuous line
which rapidly folded and refolded on itself (fig. 4). This undulatory
movement, in particular, formed a complete analogy with what had been
compared in Auroræ to the undulations of a serpent, or to those of
drapery agitated by the wind. The rustling noise accompanying the
experiment was analogous to that sometimes said to accompany Auroræ, and
was caused by the luminous electric discharge penetrating the moisture.
As in Auroræ, magnetic perturbations were produced by bringing a needle
near the circuit, the deviation increasing with the development of the
arch.
The Auroræ were produced by positive electricity, the negative electrode
producing nothing similar.
Illustrations of these miniature Auroræ are given in ‘Nature,’ and
reproduced on p. 90. No mention of any spectroscopic observations is made.
[Sidenote: Mr. Holden’s views.]
In a communication to the Metropolitan Scientific Association
(‘Observatory,’ March 1, 1879, p. 389), Mr. A. P. Holden, after
supporting the theory of a connexion between the waxing and waning of the
solar corona and sun-spots, adopts Mr. F. Pratt’s hypothesis “that the
Aurora is simply light filmy cirrus cloud, first deposited at the base
of a vast upper body of highly rarefied vapour, and illuminated by the
free electricity escaping in the condensation through the very rarefied
medium above, towards the north or south. The Aurora would, according to
this theory, have its origin in a vast electrical storm, resulting from
a violent condensation of vapour which causes a flow of electricity from
the pole to restore equilibrium.” The Aurora would thus, in Mr. Holden’s
opinion, “depend on storm phenomena of an intense character; and the
frequency of Auroræ at the sun-spot maxima would indicate the connexion
of the latter with the weather.”
[Illustration: Fig. 2. The corona.
Fig. 3. The arc and rays.
Fig. 4. The sinuous line.]
PART II.
THE SPECTRUM OF THE AURORA.
CHAPTER X.
SPECTROSCOPE ADAPTED FOR THE AURORA.
[Sidenote: Must be of moderate dispersion, with ready mode of measuring
line-positions.]
Any form of spectroscope of moderate dispersion will suffice for
observations of the spectrum of the Aurora; but, for sake of convenience,
a hand or direct-vision spectroscope is to be preferred, and it is
desirable also to have some quick and ready mode of measuring the
position of the lines while the Aurora lasts.
[Sidenote: Mr. Browning’s instrument described.]
Mr. John Browning arranged for me a form of instrument which I have
found very convenient for observations by hand of the Aurora-lines, and
also, when fixed on a stand, for tube and chemical investigations. A
representation of this instrument is given on Plate X. fig. 1. A brass
tube carries a large compound (5) direct-vision prism (shown dark in the
drawing). An arrangement is made so that a second prism can at will be
slipped into the tube (shown in outline in the drawing). With one prism
and a fine slit the D lines are widely separated, and the field of view
extends at one glance from near C to near G. When the second prism is
inserted and used in combination, the nickel line can be seen between the
two D lines, and the instrument may be used for solar work. A photograph
of the sun’s spectrum, taken with one prism only, shows a great number
of the dark solar lines and many of the bright ones, ascribed by Prof.
Draper to oxygen and nitrogen.
[Illustration: Plate X.]
[Sidenote: Diaphragm micrometer described. Mode of use of the micrometer.]
The collimator and observing telescope are respectively 6 inches in
length, and carry achromatic lenses of one inch aperture. The telescope
traverses the field so that the extremities of the spectrum may be
observed. The dispersion of the instrument was ascertained by a set
of observations of the principal solar and some metallic lines, made
with an excellent filar micrometer. For the Auroral observations, Dr.
Vogel has described an instrument (see Appendix E) in which the usual
spider’s-web wires are replaced by a needle-point, as being easily seen
upon a faint spectrum. Illuminated wires may also be used; but I was led
ultimately to employ, in preference, a diaphragm micrometer which the
spectrum itself illuminates, as being adapted for speedy, yet fairly
accurate, observations. It was made in this manner:—A card was first of
all prepared (Plate X. fig. 2), and within a circle described on this,
a scale was drawn of moderately wide white spaces, with black divisions
between, short and long, so as to read off easily by eye. The upper half
of the circle was then entirely filled in with black; and from the card
as thus prepared a reduced negative photograph was made. In this the
spaces and lower half of the circle were opaque, and the upper half of
the circle and the lines between the spaces were transparent (Plate X.
fig. 4). This photograph was about the size of a shilling (fig. 3, same
Plate). It was mounted carefully in Canada balsam, with a thin glass
cover, and then placed in the focus of the eyepiece. In use, the spectrum
is brought upon the scale so that the upper half shows above the scale
without any interruption at all; while the lower half illuminates the
scale and renders the divisions visible, showing the spectrum-lines
falling either upon them or the spaces between. The photographed scale
was next enlarged to a considerable size and printed upon faintly ruled
paper; and the enlargement was so arranged as to comprise five of the
faint ruled lines between each division of the scale. Each of these faint
lines in turn represented a certain portion of the spectrum as read off
with the filar micrometer; so that the scales as constructed with the
filar micrometer and with the photographed micrometer corresponded for
all parts of the spectrum included in the field of the eyepiece.
[Sidenote: Advantage of the method.]
One of the photographed enlargements being laid on the table under the
spectroscope, the observed lines were marked off with ease and accuracy
upon it; and as the photograph was an exact copy of the scale, any want
of exactitude in the divisions was of no moment.
One great advantage of this method was, that all the lines seen could
be recorded at one time and with all in view, and without the risk of
slight shift in the instrument, which frequently happens when lines are
read off seriatim.
I found this plan most effective for the rapid and correct recording of
a faint and evanescent spectrum, and it gave close results when compared
with traversing-micrometer measured spectra. The records, too, admitted
of subsequent examination at leisure.
[Sidenote: Double-slit plate arrangement.]
Mr. Browning subsequently constructed for me a double-slit plate (lately
in the Scientific Loan Collection at South Kensington) for the same
instrument (Plate X. fig. 5). The lower half of the plate is fixed. The
upper half traverses the lower by the aid of a micrometer-screw. The slit
is widened or closed at pleasure by loosening the small screws by which
the jaw-plates are attached. A scale is engraved on the fixed lower half
of the plate for an approximate measurement, while the division of the
micrometer-screw-head completes it.
In use, one half of the spectrum slides along the other, and a bright
line in the upper spectrum is selected as an index. The distances between
the lines of the lower half of the spectrum are read off by means of the
bright line above. This form of micrometer was suggested by Mr. Procter
(in ‘Nature’) many years ago as a substitute for a more complicated
apparatus by Zöllner. Other instruments on a similar principle have been
lately introduced, but for Aurora purposes I prefer a fixed scale.
[Sidenote: Photographed spectrum suggested.]
In ‘Photographed Spectra’ I have pointed out that we shall probably
obtain no spectrum of the Aurora to be absolutely depended upon for
comparison with other spectra until we succeed in a photographed one.
From experiments made with a special prism of the Rutherfurd form,
constructed for me by Mr. Browning (with which many gas-spectra have
been already photographed), I see no reason, should an unusually bright
Aurora favour us with a visit, why its spectrum may not be recorded in a
permanent form, and with lines sufficiently well marked to be compared
with other spectra. Rapid dry plates would be especially useful for such
a purpose, and some Auroræ, if wanting in brilliancy, would doubtless
compensate by their period of endurance.
[Sidenote: Mr. Hilger’s half-prism spectroscope.]
Mr. Adam Hilger has also made for me one of his “half-prism”
spectroscopes, in which considerable dispersion is obtained with but very
little loss of light. This instrument has a simple and rapid micrometer
arrangement, with a bright line as an index. I have (for want of Auroræ)
had no opportunity of trying it, but I doubt not it is well adapted for
such a purpose.
_Spectrum of the Aurora described._
[Sidenote: Lines or bands and continuous spectrum.]
The spectrum of the Aurora consists of a set of lines or bands upon a
dark ground at each extremity of the spectrum, but with more or less of
faint continuous spectrum towards the centre. The extreme range of the
spectrum, as observed up to the present time, is from “_a_” (between C
and D) in the red to “_h_” (hydrogen) in the violet.
[Sidenote: Lines nine in number.]
The lines have been classified and arranged by Lemström and others as
nine in number, but I believe not more than seven have ever been seen
simultaneously.
The author of the article “Aurora Polaris,” in the ‘Encyclopædia
Britannica,’ classes the lines as nine, and gives a table with the
following results (to these I have added Herr Vogel’s lines, for the
purpose of identification and comparison):—
[Sidenote: Table from Encyc. Brit.]
+------+-------------+------+--------+-------+
|No. of| Number of | Mean |Probable|Vogel’s|
|line. |observations.| W.L. | error. |lines. |
+------+-------------+------+--------+-------+
| 1. | 5 | 6303 | ± 8·1 | 6297 |
| 2. | 10 | 5569 | ± 2·9 | 5569 |
| 3. | 4 | 5342 | ±16 | 5390 |
| 4. | 6 | 5214 | ± 5·4 | 5233 |
| 5. | 4 | 5161 | ± 9·7 | 5189 |
| 6. | 6 | 4984 | ±11 | 5004 |
| 7. | 4 | 4823 | ± 9·3 | |
| 8. | 8 | 4667 | ± 9·8 | 4663 |
| 9. | 8 | 4299 | ± 9·3 | |
+------+-------------+------+--------+-------+
The probable errors are large, and it is a question whether any thing is
gained by thus endeavouring to average the lines.
[Sidenote: Ångström’s line. Zöllner’s line in the red. Other Lines of the
spectrum.]
The principal and brightest line, in the yellow-green, is generally
called “Ångström’s,” and his (probably the first) measurement of its
position at 5567 adopted. This was in the winter of 1867-68, and he saw
in addition, by widening the slit, traces of three very feeble bands
situated near to F. Zöllner is credited with the first observation of
the line in the red. These two lines are generally described as with
similar characteristics, and in about the same respective positions, by
all observers, and have never been remarked to spread into bands. The
other lines in the spectrum are difficult to position, owing to the many
discordant observations of them. They seem also variable in intensity
as well as in number (sometimes even in the same Aurora), and are not
unfrequently observed to have their places supplied by bands.
[Sidenote: Second German expedition observations. Austro-Hungarian.]
The spectroscope was used in the second German expedition, but only the
one brightest line seen—Dr. Börgen stating he had never seen a trace of
the weak lines in the blue and red, which were observed so distinctly
with the same spectroscope on 25th October, 1870, after the return of the
expedition. Lieutenant Weyprecht used a small spectroscope during the
Austro-Hungarian Expedition, and saw only the well-known yellow-green
line.
[Sidenote: Swedish expedition, 1868. Lemström’s observations.]
In the Swedish Expedition, 1868, Lemström mentions that in the
Aurora spectrum there are nine lines (he does not say he saw them
simultaneously), which he considers to agree with lines belonging to the
air-gases. He also thinks the Aurora could be referred to three distinct
types, depending on the character of the discharge.
[Sidenote: Spectrum or Aurora seen at Tronsa.]
At Tronsa an Aurora was seen October 21st, 1868, which commenced in the
north and became very brilliant. The spectroscope showed:—
1. A yellow line at 74·9.
2. A very clear line in the blue at 65·90.
3. Two lines of hair’s breadth, with very pronounced (horizontal?) striæ
on the side of the yellow, one at 125 and the other about 105.
[I presume the striæ were really vertical, and that the explanation
intended to convey that these lines shaded off towards the yellow. From
a comparison of the figures they must have been in the red, and are the
only instance recorded of two auroral lines in that region. They are
subsequently spoken of as “shaded rays.”—J. R. C.]
[Sidenote: MM. Wijkander and Parent’s observations.]
M. Auguste Wijkander and Lieut. Parent, of the Swedish Expedition in
1872-73, under Professor Nordenskiöld, used a direct-vision spectroscope,
with a micrometer-screw movement of the prisms, the reading being
afterwards reduced to wave-lengths upon Ångström’s line-values.
The following Table gives the results, with Dr. Vogel’s lines added for
the sake of comparison:—
+------+------------------------+-----------------------+-------+------+
| |Observations, Wijkander.| Observations, Parent. | | |
| |-------+-----+----------+-------+-----+---------|Mean of| |
|Lines.| | | Probable | | |Probable | both. |Vogel.|
| |Number.|W.L. | error. |Number.|W.L. | error. | | |
+------+-------+-----+----------+-------+-----+---------+-------+------+
| .. | .. | .. | .. | .. | .. | .. | .. | 6297 |
| .. | .. | .. | .. | .. | .. | .. | .. | 5569 |
| (1) | 5 | 5359| ±3 | .. | .. | .. | 5359 | 5390 |
| (2) | 6 | 5289| ±5 | 3 | 5280| ± 1 | 5286 | .. |
| (3) | 6 | 5239| ±4 | 2 | 5207| ±11 | 5231 | 5233 |
| .. | .. | .. | .. | .. | .. | .. | .. | 5189 |
| (4) | 5 | 4996| ±9 | .. | .. | .. | 4996 | 5004 |
| (5) | 1 | 4871| .. | 1 | 4873| .. | 4872 | .. |
| (6) | 8 | 4692| ±2 | 10 | 4708| ± 5 | 4701 | 4663 |
| (7) | 1 | 4366| .. | .. | .. | .. | 4366 | .. |
| (8) | 4 | 4280| ±3 | 3 | 4286| ±16 | 4284 | .. |
+------+-------+-----+----------+-------+-----+---------+-------+------+
The brightest line in all Auroræ, 5567, was intentionally not included in
the Tables. The red line was not seen. Nos. 5 and 7 were only seen once,
and not in the same Aurora.
[Sidenote: Spectrum of Aurora of October 24th, 1870.]
The Aurora of October 24th, 1870, came at a time when spectroscopes of a
direct-vision form were being introduced, and a number of observations
were communicated at the time to ‘Nature.’
[Sidenote: T. F.’s observations. W. B. Gibbs’s observation. Elger’s
observation.]
A correspondent, T. F., writing from Torquay, saw, with a direct-vision
spectroscope, one strong red line near C, one strong pale yellow line
near D, one paler near F, and a still paler one beyond, with a faint
continuous spectrum from about D to beyond F. The C line was very
conspicuous and the brightest of the whole. It was intermediate in
position and colour to the red lines of the lithium and calcium spectra.
Plainly there were two spectra superposed, for while the red portions of
the Aurora showed the four lines with a faint continuous spectrum, the
greenish portions showed only one line near D on a faint ground. W. B.
Gibbs saw, in London, only two bright lines, one a greenish grey, situate
about the middle of the spectrum, and the other a red line very much like
C (hydrogen). Thomas G. Elger, at Bedford, on the 24th and 25th, saw:—(1)
a broad and well-defined red band near C; (2) a bright white band near D
(same as Ångström’s W.L. 5567), on 25th visible in every part of the sky;
(3) a faint and rather nebulous line, roughly estimated to be near F; (4)
a very faint line about halfway between 2 and 3. The red band was absent
from the spectrum of the white rays of the Aurora, but the other lines
were seen.
[Sidenote: J. R. Capron’s observation.]
With a small Browning direct-vision spectroscope on the 24th, I found
no continuous spectrum, but two bright lines, one in the green (like
that from the nebulæ, but more intense, and considerably flickering),
the other in the red (like the lithium line, but rather duskier: Plate
V. fig. 6). The latter was only well seen when the display was at its
height; it could, however, be faintly traced wherever the rose tint of
the Aurora extended. The line in the green was well seen in all parts of
the sky, but was specially bright in the Auroral patches of white light.
[Sidenote: Mr. Browning’s observation. Alvan Clarke’s, jun.,
observations.]
Mr. Browning also saw the red line, but found comparison difficult. On
the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston,
used a chemical spectroscope of the ordinary form, with one prism and a
photographed scale illuminated with a lamp. Four Auroral lines were seen
at points of his scale numbered 61, 68, 80, and 98. These were reduced to
wave-lengths by Professor Pickering, with the following results:—
+-----+---------+--------+-------+-------------------+--------+
|Line.| Reading | Wave- |Assumed| Comments. |Probable|
| |on scale.|lengths.| line. | | error. |
|-----+---------+--------+-------+-------------------+--------|
| 1. | 61 | 5690 | 5570 |Common Aurora-line.| -20 |
| 2. | 68 | 5320 | 5316 | Corona line. | + 1 |
| 3. | 80 | 4850 | 4860 | F, hydrogen. | - 3 |
| 4. | 98 | 4350 | 4340 | G, hydrogen. | + 6 |
+-----+---------+--------+-------+-------------------+--------+
[61 is evidently wrong, and was probably a mistake for 63.]
[Sidenote: G. F. Barker’s observations.]
George F. Barker, observing at New Haven (U.S.A.), saw, on November 9th,
a crimson and white Aurora, which he examined with a single glass-prism
spectroscope, by Duboscq, of Paris. The line positions were obtained by
an illuminated millimetre scale. In the white Aurora were four lines (the
red one being absent); in the red Aurora five. The wave-lengths of the
Aurora-lines were run out as follows:—
(1.) Between C and D, 6230 (Zöllner’s 6270).
(2.) ” D and E, 5620 (Ångström’s 5570).
(3.) ” E and _b_, 5170 (Winlock’s 5200).
(4.) ” _b_ and F, 5020.
(5.) ” F and G, 4820 (Alvan Clarke’s, jun., 4850).
[Sidenote: Spectrum of Aurora of Feb. 4, 1872. Prof. Piazzi Smyth’s
observations.]
Mr. Procter’s Aurora-lines will be found noticed in connexion with the
spectrum of oxygen; and Lord Lindsay’s lines, with a comparison scale
drawing, are separately described further on in this Chapter. The Aurora
of February 4th, 1872, had many observers; some of whom communicated
at the time spectroscopic notes. Professor Piazzi Smyth minutely
describes the display as seen in Edinburgh, and saw “Ångström’s green
Aurora-line perpetually over citron acetylene[13] at W.L. 5579, and the
red Aurora-line between lithium _a_ and sodium _a_, but nearer to the
latter, say at W.L. 6370.” Extremely faint greenish and bluish lines also
appeared at W.L. 5300, 5100, and 4900 nearly.
[Sidenote: Rev. T. W. Webb’s observations.]
The Rev. T. W. Webb, with a very fine slit, saw the green Auroral line
even in the light reflected from white paper. With a wider slit he saw a
crimson band in the brighter patches of that hue, and beyond an extent of
greenish or bluish light, which he suspected to be composed of contiguous
bands.
[Sidenote: R. J. Friswell’s observations.]
R. J. Friswell, coming up the Channel at 9.40, with a Hoffman’s
direct-vision spectroscope (the observing telescope removed), saw the
green line, a crimson line near C, and faint traces of structure in the
blue and violet.
[Sidenote: The Rev. S. J. Perry’s observations.]
The Rev. S. J. Perry observed at Stonyhurst four lines, and, on examining
one of the curved streamers, found the red line even more strongly marked
than the green. A magnetic storm was observed to be at its height from 4
to 9 P.M. of the same day.
[Sidenote: J. R. Capron’s observations.]
With a Browning 7-prism direct-vision spectroscope I saw the green line
in all parts of the Aurora, attended with a peculiar flickering movement.
I did not see the other lines.
[Sidenote: His catalogue of lines up to Nov. 9, 1872.]
In a letter to ‘Nature,’ dated November 9th, 1872, I catalogued the lines
observed up to that date as follows:—
1. A line in the red between C and D. W.L., Ångström, 6279.
2. A line (the principal one of the Aurora) in the yellow-green, between
D and E. W.L., Ångström, 5567.
3. A line in the green, near E (corona line?). W.L., Alvan Clarke, jun.,
and Backhouse, 5320.
4. A faint line in the green, at or near _b_. W.L., Barker, 5170.
5. A faint line or band in the green, between _b_ and F. W.L., Barker,
5020 (chromospheric?).
6. A line in the green-blue, at or near F. W.L., Alvan Clarke, jun., 4850.
7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350.
8. The continuous spectrum from about D to beyond F.
[Sidenote: Dr. H.C. Vogel’s observations of Auroral lines. Spectrum
described.]
Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel,
and since of the Astrophysical Observatory, Potsdam, made several
observations of the Auroral lines, October 25th, 1870. Besides the bright
line between D and E, he found several other fainter lines stretching
towards the blue end of the spectrum on a dimly-lighted ground. February
11th, 1871, he observed the same set of lines, and an average of six
readings gave 5572 as the W.L. of the Ångström line. February 12th gave
5576 as Dr. Vogel’s reading, and 5569 as Dr. Lohse’s. April 9th gave
5569, and April 14th 5569. The Aurora of April 9th, 1871, was exceedingly
brilliant, so that micrometer measurements of the lines were taken. The
spectrum consisted of one line in the red, five in the green, and a
somewhat indistinct broad line or band in the blue. The lines are thus
described:—
[Sidenote: Table of lines.]
Table of Dr. Vogel’s lines. Aurora, April 9th, 1871.
+-----+--------+------------------------------------------------+
|W.L. |Probable| Remarks. |
| | error. | |
+-----+--------+------------------------------------------------+
|6297 | 14 |Very bright stripe. } |
| | | } |
|5569 | 2 |Brightest line of the spectrum, } |
| | | became noticeably fainter at } On a faintly |
| | | appearance of the red line. } lighted |
| | | } ground. |
|5390 | .. |Extremely faint line; } |
| | | unreliable observation. } |
| | | } |
|5233 | 4 |Moderately bright. } |
| | | |
|5189 | 9 |This line was very bright when the red line |
| | | appeared at the same time; otherwise equal |
| | | in brilliancy with the preceding one. |
| | | |
|5004 | 3 |Very bright line. |
| | | |
|4694}| |{Broad band of light, somewhat less brilliant |
|4663}| 3 |{ in the middle; very faint in those parts of |
|4629}| |{ the Aurora in which the red line appeared. |
+-----+--------+------------------------------------------------+
A translation of Dr. Vogel’s interesting paper will be found printed _in
extenso_ in Appendix E, and his lithographed drawings of the spectrum in
the green and red portions of the Aurora respectively on Plate VI. figs.
2 and 3. The observations of April 9th by Dr. Vogel are probably, up to
the present time, the most exact of any one Aurora, and I have therefore
in most cases used them for comparison.
[Sidenote: Mr. Backhouse’s catalogue of lines.]
Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue
of lines, gave the following as the latest determinations from his own
observations:—
No. 1. Wave-length 6060
2. ” 5660
3. ” 5165
4. ” 5015
6. ” 4625
7. ” 4305
(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr.
Backhouse never saw a line at 5320 again. He found the continuous
spectrum to reach from No. 2 to No. 7, being brightest from a little
beyond No. 2 to No. 6. This part of the spectrum did not give him so much
the idea of a true “continuous spectrum” as of a series of bright bands
too close to be distinguished.
[Sidenote: Subsequent full catalogue of Auroral lines.]
I have subsequently, in another section of this Chapter, added a full
catalogue of the Auroral lines, prepared by myself from the foregoing and
other sources and observations; and I also append to it a Plate [Plate
XII.], in which these lines are positioned and the wave-lengths and names
of observers are given. The numbers of the lines on the Plate correspond
with those in the catalogue. The solar spectrum and the spectrum of the
blue base of a candle-flame are added for purposes of comparison. [The
telluric bands in the solar spectrum are shown more distinctly than they
actually appear, and do not profess to give details.]
_Flickering of the Green Line._
[Sidenote: Flickering of the green line. Herschel’s observation. J. R.
Capron’s observation.]
A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable
circumstance connected with the appearance of the single line observed on
this occasion was the flickering and frequent changes with which it rose
and fell in brightness; apparently even more rapidly than the swiftly
travelling waves, or pulsations of light, that repeatedly passed over the
streamers, near the northern horizon, towards which the spectroscope was
directed.” In the spectrum of the Aurora of 20th October, 1870, I saw
and noted the green line as “considerably flickering;” and in the Aurora
of 4th February, 1872, I again saw and noted “the peculiar flickering”
I had remarked in 1870. I have not seen the peculiarity noted by other
observers.
_Mr. Backhouse’s graphical Spectra of four Auroræ._
[Sidenote: Mr. Backhouse’s graphical spectra of Auroræ.]
Mr. Backhouse has been good enough to supply me with some details of
four several Auroræ seen by him at Sunderland, accompanied by drawings,
showing in a graphical way the spectrum of each display as seen with a
spectroscope with rather a wide slit and as drawn by eye. I have reduced
the four drawings to the same scale, and in this way they are extremely
interesting for comparison (Plate V. fig. 4). The line on the left in
each spectrum is Ångström’s bright Auroral line, and is supposed to be
considerably prolonged. The height of the lines denotes intensity.
[Sidenote: April 18, 1873.]
April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which Mr.
Backhouse had not perceived before. No. 5 had not been visible lately,
and Mr. Backhouse thought it must belong to Auroræ of a different type
from those which had appeared latterly.
[Sidenote: Feb. 4, 1874.]
February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw
seven lines, all that he had ever seen. (The red line, not shown in the
diagram, makes the seventh.)
The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr.
Backhouse had only once before seen No. 4, and it became quite invisible
between 7.45 and 7.55, though the other lines were as bright as before
and the red line had appeared.
[Sidenote: Oct. 3, 1874.]
October 3rd, 1874. This spectrum was examined, and diagram made between
10 and 10.25 P.M. Five lines only are indicated.
It is mainly distinguished from the two preceding spectra by the
brightness of the continuous spectrum on which the lines 2, 3, and 4 lie,
and by the weakness of No. 5.
[Sidenote: Oct. 4, 1874.]
October 4th, 1874. Taken between 11.10 and 11.20 P.M.; distinguished,
like the last, by a considerable amount of continuous spectrum and by a
faint line (No. 3), not seen in the last spectrum, while No. 3 in the
last is missing in this spectrum.
[Sidenote: Mr. Backhouse’s remarks as to comparative frequency of some of
the Auroral lines.]
Mr. Backhouse, as to both these last spectra, remarks that the lines were
very variable in intensity, and sometimes some were visible and sometimes
others. They varied also in relative brightness in different parts of
the sky at the same time. Mr. Backhouse, in a communication to ‘Nature,’
referring to a statement of Mr. Procter’s, that the bands of the Auroral
spectrum are seldom visible, except the bright line at 5570, says that
he always found two bands, “doubtless Winlock’s 4640 and 4310,” to be
invariably visible when the Aurora was bright enough to show them. Of
thirty-four Auroræ examined by Mr. Backhouse, fourteen showed the lines
4640 and 4310, and three others at least one of these, while eight showed
the red line. (Ångström only once saw this line.) In five Auroræ, all
more or less red, he saw a faint band, the wave-length of which he placed
at 5000 or 5100. He never saw the line 5320 (also Winlock’s coronal
line), unless it were once, probably from want of instrumental power.
With regard to these observations, I may say that with a Browning’s
miniature spectroscope I saw only two lines (the red and the green) in
the grand display of the 24th October, 1870; and with an instrument of
larger aperture the green line only on the 4th February, 1872; while I
saw the green line and three others towards the violet with the same
instrument during the Aurora of 4th February, 1874. (See description of
this Aurora, _antè_ p. 21, and drawing of spectrum, Plate VI. fig. 1 _a_.)
_Lord Lindsay’s Aurora-Spectrum, 21st October, 1870._
[Sidenote: Lord Lindsay’s Aurora of 21st Oct., 1870.]
Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on the
night of the 21st October, 1870. It commenced about 9.30, reached its
maximum about 11, and faded away suddenly about 11.30 P.M.
[Sidenote: Spectrum described.]
A spectrum obtained in the north-west gave five bright lines with a
Browning’s direct-vision spectroscope—two strong, one medium, two very
faint. A tallow candle was used to obtain a comparison spectrum of sodium
and carburetted hydrogen.
A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1 is
a sharp well-formed line visible with a narrow slit.
No. 2, a line very slightly more refrangible than F. The side towards D
is sharp and well defined, while on the other side it is nebulous.
No. 3, slightly less refrangible than G, is a broad ill-defined band,
seen only with a wide slit.
No. 4, a line near E, woolly at the edges, but rather sharp in the
centre. This, says Lord Lindsay, should be at or near the position of the
line 1474 of the solar corona.
No. 5, a faint band, coincident with _b_, extending equally on both sides
of it.
The lines are numbered in order of intensity. It is questionable,
from observations with instruments carrying a scale, whether the
line-positions are exact; but the description of their characters is
valuable.
[Sidenote: Candle-spectrum.]
As a candle blue-base spectrum is at times a ready and handy mode of
reference in Auroral observations (as was found in this instance), I
have, on Plate XI. fig. 5, given a representation of it as seen with my
Auroral spectroscope. Dr. Watts’s corresponding carbon-spectrum is added
on the lower margin. The numbers on the upper margin refer to my scale.
[Illustration: Plate XI.]
_Spectrum of the Aurora Australis._
[Sidenote: Captain Maclear’s spectra of Aurora Australis.]
Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874
(_antè_, p. 27), with the spectroscope, found three prominent lines in
the yellow-green, green, and blue or purple, but not the red line. In
the Aurora of March 3rd, 1874 (p. 27), he could trace four lines, three
bright and one rather faint. They must have been exceedingly bright to
show so plainly in full-moon light.
[Sidenote: Instrument used, and mode of registering lines.]
The spectroscope used was a Grubb single-prism with long collimator. A
needle-point in the eyepiece marked the position of the lines; and a
corresponding needle-point, carried on a frame by a screw movement in
concord with the point of the eyepiece, scratched the lines on a plate of
blackened glass. Two plates were taken. On the first were scratched the
auroral lines and the solar lines as seen in the moonlight; on the second
plate were scratched the auroral lines, the Solar lines from the moon,
and the carbon lines in a spirit-lamp.
[Sidenote: Copies of the two spectra obtained. Discrepancy in the
spectra. Remarks on the spectra.]
The next morning the solar lines were verified in sunlight. I subjoin
(Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the
auroral lines being marked A, the solar lines by the usual designating
letters, and the carbon by _Car_. To these spectra I have added for
comparison Dr. Vogel’s spectrum of the Aurora Borealis. Captain Maclear
could not account for the different positions of the auroral lines in the
two plates; for the prism, as far as he was aware, was not moved during
the observations. As the solar lines are indicated in the same place in
both spectra, the case would seem one of actual change of position of the
auroral lines during observation. A comparison of the two spectra gives
the impression that the lower one is the same as the upper, except that
the dispersion is greater, the lines remaining relatively in position.
One does not, however, see how the dispersion could have so varied in a
single-prism instrument, and the position of the solar lines is adverse
to such an explanation.
There is a suspicion that Auroræ are not always identical in position
of some of the lines; but the line in the green (considerably out of
place in the lower Australis spectrum) has always, within small limits,
the same position. It will be noticed how much further the Australis
spectrum runs into the violet than Vogel’s Borealis, the latter having no
lines much beyond F.
The faint line (No. 2) mentioned by Captain Maclear possibly corresponds
with Dr. Vogel’s band. The absence of the four lines of the Aurora
Borealis in the green part of the spectrum of the Australis is peculiar;
and in this respect, too, the two Australis spectra agree.
[Sidenote: Comparison of the lines.]
The nearest approaches to Captain Maclear’s lines (of the upper spectrum)
which I can find are:—
Line Corresponding line.
1. 5567, Ångström.
2. (The faint line.) Vogel’s band, 4694-4629.
3. I find no approximately corresponding line.
4. 4350, Alvan Clarke.
But the comparisons are not by any means close. Further observations of
the Australis spectrum are very desirable.
_Prof. Piazzi Smyth’s Aurora-Spectra._
[Sidenote: Prof. Piazzi Smyth’s chemical and auroral spectra.]
Prof. Piazzi Smyth, in volume xiv. of the ‘Edinburgh Astronomical
Observations,’ 1870-77, has compared simultaneously the Aurora-spectrum
with the sets of bright lines seen in the blue base of flame—the lines of
potassium, lithium, sodium, thallium, and indium being also introduced
for comparison. The spectra are drawn as seen under small dispersion,
and will prove most useful in cases where an Aurora is not bright enough
to admit of the lines being measured by micrometer, and the eye and
comparison spectrum are obliged to be resorted to.
[Illustration: Plate XII.]
_Author’s Catalogue of the Auroral Lines._
(See Plate XII.)
1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zöllner. Seen
only in red Auroræ; stands out on a dark ground, without other lines
near it. Character of line sharp and well defined; varies in colour from
dusky red to bright crimson. Intensity, Herschel, 0 to 4 or 8. According
to same, position coincident with atmospheric absorption-group “_a_” in
solar spectrum (between C and D). I confirm this position according to
my scale of solar lines, and a drawing of the coincidence (in which,
and in Plate XII., the absorption-lines are drawn too dark) is given on
Plate XIII. fig. 2. Herschel says this line coincides with a red band in
the negative glow-discharge, but its identity is doubtful. Its isolation
and want of adjacent lines seem to separate it from the air-spectrum and
gas-spectra in general. At the appearance of this line, 5569 (No. 2)
becomes noticeably fainter. When this line is bright, 5189 (No. 5) is
bright also (Vogel).
If we propose to assign to this line, as well as to 5569, a
phosphorescent origin, it would be strongly confirmatory of such a theory
(in connexion with the phosphoretted-hydrogen spectrum) to find it
brighten at low temperatures.
_Note._—Sir John Franklin says, in his ‘Polar Expeditions,’ that a low
state of temperature is favourable for the production of brilliant
coruscations. It was seldom witnessed that the Auroræ were much agitated,
or that the prismatic tints were very apparent, when the temperature was
above zero.
2. Line in the yellow-green. Brightest of all lines in the
Aurora-spectrum. W.L. 5567, Ångström; 5569, Vogel. Intensity 25,
Herschel. To me more pale green than yellow, sometimes flickering
and changing in brightness (Herschel and Capron). Seen in all Auroræ
usually sharp and bright, but Procter has once recorded it nebulous. Its
character as to width, sharpness, and intensity, if carefully observed,
might indicate height and structure of Aurora. Becomes noticeably
fainter at appearance of red line (Vogel). Found by me to correspond in
position with a faint atmospheric absorption-band (see Plate XIII. fig.
2). According to Ångström and Herschel, arising from a phosphorescent
and fluorescent light, emitted when air is subjected to the action of
electrical discharge.
3. Line in green near last. W.L. 5390. An extremely faint and unreliable
observation (Vogel). Seen only by him, unless Alvan Clarke’s 5320
(coronal?) be the same.
4. Line in green-blue. W.L. 5233, moderately bright (Vogel); 5200,
Winlock. Intensity, 2 or 0? to 6, Herschel. Coincides with line in the
negative glow according to same. Frequently observed.
5. Line in green-blue. W.L. 5189. This line is very bright when the red
line appears at the same time; otherwise equal in brilliancy with No.
3 (Vogel); Winlock, 5200. Not so frequently observed as No. 3. Barker
gives a band extending from 5330 to 5200. Intensity of 5189, 0 to 8,
Herschel, who considers it coincident with a constant strong line in the
spark-discharge.
6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker
(coronal?). Intensity 2 or 0? to 8, Herschel. Coincides with line of
nitrogen in the nebulæ according to same. Barker gives a band extending
from 5050 to 4990.
7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. W.L.
4850, Alvan Clarke; 4820, Backhouse and Barker. Intensity, Herschel, of
4820-4870, 0 to 4? Herschel suspects this and No. 4 to be seen only in
Auroral streamers of low elevation. Barker gives a band extending from
4930 to 4850.
8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the
middle; very faint in those parts of the Aurora in which the red line
appears (Vogel). Intensity 3-6 (Herschel). A double band, consisting of
two lines, the first rather more frequently noted than the second in
Auroral spectra, agrees well in position with the principal band in the
negative glow-spectrum (same). Barker gives a band extending from 4740 to
4670; Backhouse and Winlock give a line at 4640, situate within the same.
9. There seems a good deal of confusion about a fairly bright line
(intensity 0-6, Herschel) seen in most Auroræ (not, however, by Vogel,
April 9th, 1871), and situate somewhere near G in the solar spectrum.
Alvan Clarke places it at 4350, on the less refrangible side of G;
Backhouse and Barker at or very near to G; while Lemström and others
position it on the more refrangible side of G. Accurate observations, for
which a quartz spectroscope might be useful, are much wanted. Herschel
makes this line, at 4285, correspond with a strong band in the violet in
the negative glow-spectrum.
Herschel also refers to an apparently additional line near the
hydrogen-line, or between G and H₁, in the solar spectrum, as mentioned
once by Lemström at Helsingfors. I am not aware of any other observation
of this line, which must be considerably beyond that at or near G, and
would probably be difficult to detect, except in instruments specially
adapted for examination of the violet end of the spectrum.
_Theories in relation to the Aurora and its Spectrum._
[Sidenote: Lemström’s.]
Lemström (1):—That the Polar light is caused by an electric current
passing from the upper rarefied layers of the air to the earth, producing
light-phenomena that do not arise in the denser layers of the air. (2)
That there are nine rays (lines or bands) in the Aurora-spectrum, which
in all probability agree with lines which belong to the gases of the air.
(3) That the Aurora-spectrum can be referred to three distinct types,
which depend on the character of the discharge.
[Sidenote: Vogel’s.]
Dr. Vogel:—(1) That the Auroræ are electric discharges in rarefied-air
strata of very considerable thickness. (2) That the Aurora-spectrum is a
modification of the air-spectrum, involving the question of alteration of
the spectrum by conditions of temperature and pressure.
[Sidenote: Ångström’s.]
Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final
cause in electrical discharges in the upper strata of the atmosphere, and
that these, whether disruptional or continuous, take place sometimes on
the outer boundary of the atmosphere, and sometimes near the surface of
the earth.
(2) That the Aurora has two different spectra.
(3) That the green line is due to fluorescence or phosphorescence, and
that there is no need to resort to Dr. Vogel’s variability of gas-spectra
according to circumstances of pressure and temperature.
(4) That an agreement exists between the lines of the Aurora (except the
red and green before mentioned) and the lines or bands of the violet
light which proceed from the negative pole in dry air.
[Sidenote: Zöllner’s remark as to temperature of Aurora and character of
spectrum.]
Zöllner has pointed out that the temperature of the incandescent gas of
the Aurora must be exceedingly low, comparatively, and concludes that the
spectrum does not correspond with any known spectrum of the atmospheric
gases—only because, though a spectrum of our atmosphere, it is one of
another order, and one which we cannot produce artificially.
CHAPTER XI.
THE COMPARISON OF SOME TUBE AND OTHER SPECTRA WITH THE SPECTRUM OF THE
AURORA.
[In part from an Article in the ‘Philosophical Magazine’ for April 1875.]
[Sidenote: Testing Ångström’s Aurora theory. Battery and spectroscope
described. Vogel’s spectrum selected for comparison.]
In order to test Professor Ångström’s theory of the Aurora, referred to
in the last Chapter, in an experimental way, I examined, in the winter of
1874, some tube and other spectra, not only for line-positions, but also
for general resemblance to an Aurora-spectrum. It did not seem desirable
to use powerful currents. A ½-inch-spark coil, worked by a quart
bichromate-cell, was found sufficient to illuminate the tubes steadily.
The spectroscope used was one made for me by Mr. Browning specifically
for Auroral purposes, and of the direct-vision form, being the same
instrument as is described _antè_, p. 91, and figured in Plate X. fig. 1.
The micrometer was the diaphragm one, also before described and figured
on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel’s spectrum for
comparison, it being, so far as I am aware, the most accurately mapped,
with regard to wave-length, at one observation, of any Auroral spectrum.
It seemed an unsafe plan to attempt to obtain an average Aurora by
comparison of different observations made at various times by different
observers with all sorts of instruments—the difficulty, too, being
increased by the suspicion that the spectrum itself at times varies in
number and position as well as intensity of its lines.
[Sidenote: Central part only of spectrum mapped.]
In most cases the central part of the spectrum only (corresponding to
the central lines of the Aurora) was mapped, the red line in the Aurora
not being found to correspond with any prominent line in the gas-spectra
examined, and the Auroral line near solar G being so indefinitely fixed
as to render comparison almost valueless. (See Plate XIII. fig. 1.)
Dr. Vogel’s spectrum does not comprise the line near G; but I have added
this (in an approximate place only) in order to complete the set of
lines. For drawing of Dr. Vogel’s spectrum, with its scales attached, see
Plate XIII.
[Illustration: Plate XIII.]
_Hydrogen-tube._
[Sidenote: Hydrogen-tube. Colour of glow varied with intensity of
current.]
This tube was one of Geissler’s and of rather small calibre. On
illumination the wide ends were easily lighted with a silver-grey
glow, having a considerable amount of stratification. The capillary
part glowed brilliantly with silver-white, bright green, and crimson
light, according to the intensity of the current. With the commutator
slowly working, white running into green and bright green were the main
features of the thread of light; on the current passing more rapidly,
the capillary thread became of an intense crimson, at the same time
apparently increasing in diameter (an effect probably due to irradiation).
[Sidenote: Spectrum described.]
The spectrum was very brilliant, consisting of the three bright lines
usually distinguished as Hα, Hβ, and Hγ, and a number of shaded bands
and fainter lines between these, with a bright continuous spectrum as a
background to the whole.
[Sidenote: Lines α, β, and γ varied in intensity with colour as seen by
eye. Fainter lines or bands described.]
The lines Hα, Hβ, and Hγ were found to vary in intensity with the
current, and in accordance with the colour of the light as seen by the
eye—a fact, as I think, not without bearing on the question of the
Aurora, the varying tints of which are so well known. The fainter lines
or bands were mostly stripes of pretty equal intensity throughout, and
all about the width of the Hβ line. I did not trace any marked degrading
on either side of the lines, though the edges were not uniformly so sharp
as Hα and Hβ. Some of the lines were found coincident in position with
lines of the air-spectrum.
[Sidenote: Purity of subsidiary lines questioned.]
It is a question whether these subsidiary lines are hydrogen, or are due
to some tube impurity. A photograph I have taken of this tube-spectrum
shows 17 lines in the part of the spectrum between F and H₂, some of
which are repeated in the hydrocarbon-tube spectra.
[Sidenote: Coincidence of lines with Aurora-spectrum.]
No principal line, and one subsidiary line only, actually coincide with
the Aurora-spectrum, this last being that to which Dr. Vogel assigns an
identical wave-length, viz. 5189. Other of the subsidiary lines, however,
fall somewhat near the Aurora-lines 5569, 5390, 5233, and 5004, two faint
lines also falling within the band 4694 to 4629.
[Sidenote: Comparison of the lines.]
The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when
compared, as under:—
{4694}
Aurora 5569 5390 5233 5189 5004 { to } band.
{4629}
Hydrogen 5555 5422 ... 5189 5008 4632
I remarked that a line (5596) described by Dr. Vogel as “very bright” in
his H spectrum does not appear in my tube, though in most other respects
our H spectra agree.
[Sidenote: Effect of distance on the spectrum.]
I thought this tube afforded a good opportunity for testing the effect
of distance upon the spectrum. The slit was made rather fine. At 6
inches distance from it the line α in the blue-green (F solar) was very
bright. The lines marked β, γ, δ, ε, and ζ also survived, but were faint.
At 12 inches from the slit α and γ were alone seen, and at 24 inches
α stood by itself upon a dark ground. I also noticed that the red and
yellow parts of the spectrum first lost their light on the tube being
withdrawn from the slit; and this appeared to account for β disappearing
while γ survived. For drawing of the hydrogen-tube spectrum see Plate
XIV. spectrum 1.
The question of effect of distance upon the spectroscopic appearance of
a glowing light, as tested for this and other tubes, seems an important
one. It may possibly account for the generally faint aspect of the lines
in the more refrangible part of the Auroral spectrum.
[Illustration: Plate XIV.]
_Carbon- and Oxygen-tubes._
[Sidenote: Carbon- and oxygen-tubes. Tubes described. Carbon-tubes lighted
up. Spectra of the carbon-tubes described.]
The following tube-observations were taken together, because my friend
Mr. Henry R. Procter (to whom I am indebted for many profitable hints and
suggestions in Auroral work) was disposed to regard the spectra found
in the carbon-tubes and in those marked “O” as identical, suggesting
that pure O, with the ordinary non-intensified discharge, gives only a
continuous spectrum; and that the O tubes are in fact generally lighted
up by a carbon-spectrum, as the result of impurity from accidental
causes. The tubes examined for the purpose of comparison were as
follows:—A coal-gas tube, a tube marked “C.A.,” three O tubes (two of,
I believe, London make, and the third from Geissler), and an OH₂ tube,
also from Geissler. The carbon-tubes were both brilliantly and steadily
lighted by the current. The C.A. tube glowed with a peculiar silvery-grey
green light in the capillary part, and with a grey glow, considerably
stratified, in the bulbs. The coal-gas tube illumination was whiter and
still more brilliant than the C.A., and with even finer stratification in
the bulbs. The spectra of both tubes were conspicuous for the same three
well-known principal bright lines or bands in the yellow, green, and blue
(with one fainter in the violet), all shading off towards the violet, and
in both cases with fainter intervening bands or lines. These last bands
or lines only partially coincided when the two tubes were compared.
The spectra in both cases were rich and glowing, with a certain amount of
continuous spectrum between the lines; and the three principal bands or
lines showed well and distinctly their respective place-colours.
[Sidenote: Tubes tested for distance.]
_Tubes tested for distance._—In the case of the C.A. tube, at 18 inches
from the slit the continuous spectrum and fainter lines disappeared,
while the four principal lines still shone out, that in the green being
the strongest. At 24 inches the same lines were still visible, though
somewhat faintly.
In the case of the coal-gas tube, at 24 inches the whole spectrum was
quite brilliant, the four principal lines being very bright and even
preserving their distinctive colours. The H line, near the line or band
in the blue, was also plainly seen.
[Sidenote: O tubes lighted up.]
The O tubes, when treated by the same current as the carbon-tubes, were
found to be all three identical in general features. The discharge
lighted up each of the tubes feebly and somewhat intermittently. Grey
in the bulbs and a faint but decidedly pinkish white in the capillary
part were the distinguishing light colours; while nothing could be more
marked than the difference in brilliancy between these and the preceding
carbon-tubes.
[Sidenote: OH₂ tubes lighted up. O tubes spectra described.]
The OH₂ tube presented very much the same character; but the discharge
occasionally varied from a pinkish white to a yellow colour, somewhat
like that which artists call brown-pink, and reminding one of the “golden
rays” in certain Auroræ. These O spectra presented, in common with the
carbon-tubes, three principal bright lines or bands in the yellow, green,
and blue, with a fainter one in the violet, all shading off towards
the violet. The bands, however, showed but very little trace of local
colour, and the whole spectrum had a faint and washed-out look, very
different from the carbon-spectra. (I certainly, by a little management,
subsequently succeeded in getting the same look to the C.A. spectrum; but
it was only by removing the tube to some distance from the slit, and thus
depriving the spectrum of very much of its brightness.) The hydrogen line
(solar F) was bright, more so than any of the O lines.
The intensity of the three principal lines seemed to me to run in the
following order:—
Yellow. Green. Blue.
Coal-gas β α γ
Oxygen γ α β
Between the lines γ and α in the Geissler O tube I found a rather bright
line, which I shall have occasion to refer to hereafter.
At 12 inches distance from the slit the O spectrum lost nearly all
its light; the H line, and the three lines γ, α, and β, alone faintly
remaining, α being decidedly the brightest. At 24 inches no spectrum at
all was to be seen.
[Sidenote: Comparison of spectra of coal-gas and O tubes.]
I carefully compared together the three principal lines of the two
spectra of coal-gas and O by means of:—
1st, the photographed micrometer before described;
2nd, a comparison-prism on the slit plate;
3rd, a piece of very fine brass foil cut as a pointer and fixed in the
focus of a positive eyepiece.
The lines or bands in both tubes were found to be slightly nebulous
towards the less-refrangible end (where they were measured), and the O
tube was not bright under a moderately high power (positive eyepiece).
Subject to these remarks, the three principal lines in both tubes were
found to correspond in position within the limits of my instrument. The
spectra did not, however, I am bound to say, _look_ alike.
[Sidenote: Dr. Vogel’s O spectrum reduced and compared.]
Puzzled by these observations, it then occurred to me to reduce Dr.
Vogel’s spectrum of O, given in his memoir, to the same scale with my
own. This I did independently, and I then compared the result with my own
spectrum as mapped out. From the comparison, I judge that if my O tubes,
one and all, showed a carbon-spectrum, the learned Doctor’s tube must
have been subject to a similar infirmity, as the tubes all agreed in main
features.
There is, however, one point to which I desire to draw attention, which
is this, that common to both the Doctor’s and my own Geissler spectrum I
found the before-mentioned rather bright line between γ and α. This line
I found no equivalent for in either of the carbon-tubes. For spectra of
coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4.
[Sidenote: Tube- and flame-spectra of carbon do not correspond.]
In comparing the spectra, it should be remembered that the tube- and
flame-spectra of carbon do not correspond. Compare, for instance, the
spectrum of coal-gas or CO₂ in tube, and the well-known lines or bands
in the blue base of a candle-flame. The sharper edge of the yellow
line or band of the carbon-tubes will be found about midway between
the two bright yellow candle-lines or bands. The first of the very
beautiful group of lines or bands in the green in the candle-flame
falls considerably behind the sharper edge of the green line or band
in the tube, while the third bright band in the tube, alone of the
three, corresponds with a very faint band in the candle-flame. A line
or band in the violet in the tube-spectrum finds no equivalent in the
candle-spectrum. For comparison of the carbon-tube and flame spectra (the
principal lines of the tube being alone shown), see Plate XVI. spectra 6
& 7.
[Sidenote: Prof. Piazzi Smyth’s measurements of the components of the
citron-band in a coal-gas flame.]
_Note._—Prof. Piazzi Smyth has been good enough, at my instance, to
measure the components of the citron band of the carbo-hydrogen spectrum
(near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged
with common air.
The spectroscope used had prisms giving 22° of dispersion between A and
H, and the observing telescope magnified 10 times. The following is a
table of the results communicated to me by the Professor:—
Reading of
Intensity. Micrometer.
Reference line, lithium β 4 16·55
” sodium, α1 10 18·45
” ” α2 10 18·51
Citron band. Carbo-hydrogen.
Line 1, exquisitely clear 6 21·28
2, ” 5 21·88
3, ” 3 22·44
4, faint but clear 2 22·95
5, faint 1 23·38
6, faint and hazy 1 23·70
7, doubtful ? 23·92
Reference line, thallium α 10 25·08
[Sidenote: Comparison of Dr. Vogel’s O lines and Dr. Watts’s
carbon-lines.]
From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal
carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube
as under:—
Yellow. Green. Blue.
Dr. Vogel’s oxygen-lines 5603 5189 4829
Dr. Watts’s carbon-tube bands or lines 5602 5195 4834
Now these wave-length differences are so small that they raise a
presumption of the possibility of the spectra being identical. On the
other hand, assuming the spectra are not identical, the comparison tells
the other way, viz. that the differences are so minute as to escape
detection in instruments of moderate dispersion. With my own instrument
I found the O spectrum too faint to increase the dispersive power with
advantage. Considering the extremely different character of the two
discharges, the identity of all the O tubes, and the presence of the line
found between γ and α in the O spectrum, I think the two spectra are
independent, though I admit there is room for doubt.
[Sidenote: O and CO₂ spectra photographed.]
_Note._—Since this examination I have photographed both spectra side
by side (see ‘Photographed Spectra,’ Plate XXXI., text, pp. 69, 70).
The pictures include, of course, only the blue and violet parts of
the spectrum; but they are widely different in aspect, and show that,
photographically at least, in this part of the spectrum there is a
complete want of identity. Subsequent investigations, however, by
Schuster and others (detailed later in this Chapter), go to establish
that the principal lines shown in mine and Dr. Vogel’s tubes were due to
(probably hydrocarbon) impurity. The exception is the single line common
to mine and Dr. Vogel’s tubes, but absent from the coal-gas spectrum.
This line proves to be oxygen. Compare oxygen-tube spectra (Plate XIV.
spectra 3 and 4) with Schuster’s oxygen-tube spectrum (Plate XVIII. fig.
15). The line in question is found identical in the three tubes.
The tube OH₂ was found to give the principal lines of the O and H spectra
combined on a faint continuous spectrum.
_Geissler Mercury-tube_ (Plate X. fig. 7) _and Barometer Mercurial
vacuum_.
[Sidenote: Mercury- and barometer-tubes examined. Mercury-tube described.
Barometer-tube.]
I next examined two vacuum-tubes of an entirely different character. The
one was a tube from Geissler of stout glass, some fifteen inches long,
without electrodes, and an inch across. Within this tube was a second
of uranium glass, with bulbs blown in it. In contact with both tubes a
quantity of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this
tube with the hand brilliant flashes of blue-white light, like summer
lightning, flashed out. These were discernible (though faintly) even in
daylight. The fine terminal wires of the coil being wrapped round each
end of this tube, when the current passed, a bright and white induced
discharge, with a considerable amount of stratification, was seen in the
tube. The other tube was that of a mercurial siphon-barometer. This being
placed in a stand, one terminal wire was placed in the mercury in the
short leg of the siphon, while the other terminal was made into a little
coil and placed on the upper closed extremity of the barometer-tube. On
passing the current, the entire short space above the mercury was filled
with a grey-white light, not stratified, but showing a conspicuous bright
ring just above the level of the mercury.
[Sidenote: Spectrum of both these tubes described.]
Both these tubes, when examined with the spectroscope, showed four bright
rather uniform bands (the central one being the brightest), which I
assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6).
The Geissler tube was probably filled designedly with coal-gas. In the
case of the barometer-tube the spectrum must be assumed to be the result
of some carbon impurity.
No lines of mercury could be detected in either case.
An effort was made to examine the light of the Geissler mercury-tube as
excited by motion only, but the spectrum could not be kept in the field;
the four lines were, however, seen to flash out as the light passed
before the slit.
[Illustration: Plate XV.]
_Air-tubes._
[Sidenote: Air-tube illuminated.]
The first tube I examined was an ordinary Geissler tube charged with
rarefied air. The bulbs, on passing the discharge, were filled with the
well-known rose-tinged light like to the Aurora-streams. This in the
capillary part was condensed into a brighter and whiter thread, while the
platinum wire of the negative pole was surrounded by its characteristic
mauve or violet glow.
[Sidenote: Spectrum described.]
The spectrum, even with a weak current, was quite bright, and consisted
mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ,
and some of the intermediate lines of the H tube.
The double line α was undoubtedly the brightest in the spectrum when
taken in the capillary part of the tube. After this followed β, and then
γ(H), δ, and ε. I was, however, uncertain as to the relative brightness
of the last three, and marked their intensities with hesitation. I tested
them several times independently with differing results, and suspected
them of variability with the current.
The rest of the lines were very much of the same intensity. (For drawing
of spectrum of air-tube in capillary part see Plate XV. spectrum 1.)
_Violet [negative] Pole, same tube._
[Sidenote: Violet (negative) pole: spectrum described.]
I next turned my attention to the violet or negative-pole glow; and here
a remarkable change took place in the spectrum, not only in the position
of the principal bands or lines, but in their relative intensity (see
Plate XV. spectrum 2).
The double line α in the capillary part was replaced in the violet glow
by a shaded band of second intensity β, the sharp edge of which was
extended towards the red, and formed (except for some faint indications)
the limit of the spectrum in that direction. The somewhat faint line
next α in the capillary tube had its faint representative in the violet
pole; but the next two lines (capillary) were represented by the bright
band γ in the violet pole lying in a position between them. Next γ in
the violet pole came three faint lines, representing β, γ, and δ in the
capillary spectrum; and then the bright band α, which was the brightest
of the violet-pole group, and represented a medium-intensity band in the
capillary spectrum. After this was a faint band near α, representing
two rather bright ones in the capillary spectrum, this last being
succeeded by other bands in the violet. α, β, and γ in the violet pole
were examined carefully for relative brightness, and were, I believe,
correctly marked.
_Red [positive] Pole._
[Sidenote: Red (positive) pole: spectrum described.]
The red [positive] pole was next examined, but presented no peculiar
features. It appeared as a fainter representation of the capillary
air-spectrum, with some few lines or bands absent, and (as will be seen
after) was also a fair representation of a diffused air-spectrum (see
Plate XV. spectrum 3).
Examined for comparative intensity, at 24 inches from the slit, the whole
capillary air-spectrum showed faintly. The marked lines in the centre of
the spectrum generally retained their prominence; but after α I judged
ε next in brightness. On examining the violet pole at 12 inches from
the slit, the whole spectrum was faint and the bands α and β were alone
distinctly seen.
_Aurora (air)-tube._ (Plate XV. spectrum 4.)
[Sidenote: Aurora-tube: discharge described. Spectrum described.]
Next to the Geissler air-tube I examined an “aurora”-tube, about 15
inches long and 1¼ inch across, with platinum terminals, and of the same
diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red
colour, and the long flickering stream from pole to pole certainly
much reminded one optically of an auroral streamer. Spectroscopically
examined, the discharge presented a faint banded air-spectrum similar to
that of the positive pole (see Plate XV. spectrum 4); but the relative
intensity of the lines was somewhat altered, while a very bright line
in the green (seen also in the tube next described) was characteristic
of the spectrum, and in this respect distinguished it from the ordinary
air-spectrum.
_Phosphorescent tube._
[Sidenote: Phosphorescent tube described. Discharge described. Spectrum
described.]
Following this last tube I examined one purchased as “phosphorescent.”
It was rather short (6½ inches), of equal calibre, and about the size of
the bulb of a Geissler tube. It was filled with a white powder (probably
one of the Becquerel compounds). On passing the current between the
electrodes, a bright rose-coloured stream appeared; and wherever this
was in contact with the powder, the tube glowed with a brilliant green
light. On stopping the current, the tube still continued to shine, but
with a fainter green glow, which gave only a continuous spectrum. When
examined in full glow, the tube-spectrum was also in the main continuous
and of a green tinge; but upon it were bright lines in the blue and
violet portions of the spectrum, while in the red, yellow, and green a
faint but distinct air-spectrum was seen; and with this was also found
the same bright line in the green which distinguished the “aurora”-tube.
[Five out of six of the lines in the blue and violet will be also found
in Schuster’s oxygen-tube, violet pole (Plate XVIII. fig. 15). The
air-spectrum probably arose from impurity.]
[Illustration: Plate XVI.]
_Spark in Air._
[Sidenote: Spark in air: spectrum described.]
I next took a ½-inch spark in air between platinum terminals (see Plate
XV. spectrum 6). The principal lines in this spectrum were the line α
(by far the brightest), corresponding to γ in the violet pole; next was
β, a line in the yellow, not appearing in the tube-spectrum, and then
other lines of less intensity. In the “aurora” and “phosphorescent” tubes
was found, as before mentioned, a line in the green prominent for its
brightness, and, indeed, in the “aurora”-tube the only one which survived
when it was moved away from the slit. This line also appeared in the
spark-spectrum, but there only of an average brightness. I examined it
carefully for position in the respective tubes; and on comparing them by
means of a pointer in the eyepiece, found it coincident with the ridge or
centre of the wedge-like bright-green broad band which is so conspicuous
in the air-tube spectrum.
I think this edge-like centre has actually a line coincident with the
line I refer to; but if so, its intensity little exceeds that of the band
itself.
_Spark over Water._
[Sidenote: Spark over water: spectrum described.]
To complete the set of air-experiments, I examined the same spark taken
from the surface of a small meniscus of water, placed in a glass cup upon
the lower platinum wire. In this case the air-spectrum was plainly, but
not brightly, seen at the violet end of the spectrum—the red, yellow,
green, and blue being filled with a continuous spectrum, through which
some of the air-lines faintly showed (see Plate XV. spectrum 7).
_Phosphoretted-Hydrogen Flame._
[Sidenote: Phosphoretted-hydrogen flame.]
This was obtained from a hydrogen-bottle fitted with glass tubing, two or
three minute pieces of phosphorus being placed with the zinc. The flame
was of a bright yellow colour, with a cone of vivid green light in its
centre.
[Sidenote: Spectrum described.]
The spectrum was found to consist mainly of three bright bands in the
yellow, green, and green-blue respectively (see Plate XVI. spectrum 3).
[Sidenote: Mons. Lecoq de Boisbaudran’s remarks on the spectrum
increasing in brilliancy when the flame is cooled.]
The central band was very striking in its emerald-green colour, while
all the bands were remarkable as being very broad in proportion to the
slit (which, however, was not fine). The yellow band had a rich glow of
colour. My spectrum was mapped out at ordinary temperature, and I found
the bands sufficiently bright; but Mons. Lecoq de Boisbaudran, in his
‘Spectres Lumineux’ (texte, p. 188), has described how the brilliancy
of these bands is increased when the flame is artificially cooled
(_refroidie_).
The idea of cooling the flame was due to M. Salet, who effected it either
by a jet of water or by an air-blast.
The less refrangible bands seem the most susceptible to increase of
brilliancy.
Mons. Boisbaudran also makes the important remark that the relative
intensities of the bands are in such case altered, adding:—“La
plus importante de ces modifications consiste en un renforcement
très-considérable de la bande rouge δ 97·03 (W.L. 5994) qui devient
vive de presque invisible qu’elle était en l’absence du refroidissement
artificiel de la flamme.”
Full details of the changes are given by M. de Boisbaudran.
The bearing of these observations as connected with the variable
character of the red line in the Aurora-spectrum seems to me in the
highest degree noteworthy.
_Iron-Spectrum._
[Sidenote: Iron-spectrum.]
A comparison of this spectrum suggested itself, partly from the suspected
relations between the Aurora and solar corona, and partly from a
consideration of the views expressed by M. Gronemann and others in favour
of the Aurora having its origin in the fall of an incandescent meteoric
powder.
[Sidenote: How obtained. Spectrum described. Mons. Lecoq de Boisbaudran’s
spectra also given.]
The spectrum was obtained from a spark taken over a solution of
perchloride of iron in a small glass cup, and was remarkable for its
brightness in and about the green region. The lines varied considerably
in intensity, and with a fine slit the principal ones were sharp,
distinct, and clear. A group of three lines (α) stood out boldly in the
green as the most marked, and next to these a group of three others more
towards the violet end of the spectrum (see Plate XVI. spectrum 4). By
the side of my phosphoretted-hydrogen and iron spectra I have placed the
principal lines of Mons. Lecoq de Boisbaudran’s same spectra (reduced
to my scale), and with figures of wave-lengths for comparison with the
Aurora-spectrum (see Plate XVI. spectra 1 and 2).
[Sidenote: Comparison of iron- and Aurora-spectrum.]
A difficulty in comparing the iron-spectrum with that of the Aurora
arises from the large number of fine lines found in the former spectrum.
In a photograph (taken with the same prism as before described) of a
small piece of meteoric iron fused in an electric arc by the aid of 40
Grove cells, about 154 lines are easily counted in the blue and violet
parts of the spectrum. Double this number at least would be seen with a
spectroscope of moderate dispersion in the region comprising the entire
set of auroral lines.
_Spectrum of Mercury._
[Sidenote: Mercury-spectrum. How obtained.]
This spectrum is given as useful for comparison with the bright and
principal Aurora-line. It is easy to obtain with a small coil, the metal
being used as one electrode. The yellow lines are distinct and steady;
but the green, which is very bright, is apt to flicker as the spark moves
on the surface of the metal (see Plate XVI. spectrum 5).
_The following Table was compiled for the purpose of comparing the
foregoing results with the Aurora-spectrum._
[Sidenote: Table of coincidences.]
TABLE showing comparative position of Aurora-lines with the principal
lines in the examined spectra. C. means coincident within the limits of
my instrument and scale, N. near, and VN. very near.
-------------+--------+------+------+------+------+------+--------+------
{| | | | | | | 4694 |
Aurora-lines{| 6297 | 5569 | 5390 | 5233 | 5180 | 5004 |to 4629 | 4350?
{| β. | α. | ζ. | δ. | δ. | γ. | ε. | ε.
-------------+--------+------+------+------+------+------+--------+------
| | | | | C., | | Band |
Hydrogen-tube| | N. | | N. | same | |includes|
| | | | | W.L. | |2 lines.|
-------------+ +------+------+------+------+------+--------+
| | | | | | | Band |
Coal-gas | | | | N. | VN. | |includes|
tube | | | | | | |1 line. |
-------------+ +------+------+------+------+------+--------+
Oxygen-tube | | | | | VN. | | |
-------------+ +------+------+------+------+------+--------+
| | Band | | | | Band |
Air, | | includes | | N. | |includes|
capillary | | | | | |2 lines.|
-------------+ +------+------+------+------+------+--------+
| | | | |Band | | Band |
Air, violet- | * | | | | in- | C. |includes| **
pole | | | | |cludes| |1 line. |
-------------+ +------+------+------+------+------+--------+
Air, red-pole| | See Air, capillary. |
-------------+ +-------------------------------------------+
Aurora-tube} | | See Air, capillary; and note bright line. |
and phos- } | | |
phorescent} | | |
tube } | | |
-------------+ +------+------+------+------+------+--------+
| | | | | | | Band |
Air, spark | | N. | | | N. | C. |includes|
| | | | | | |2 lines.|
-------------+ +------+------+------+------+------+--------+
Air, spark | | Continuous spectrum and faint | Band |
over water | | air-lines. |includes|
| | |2 lines.|
-------------+ +------+------+------+------+------+--------+
Phosphoretted| | N. | Faint| Band | | | |
hydrogen | | | band.| in- | | | |
| | | |cludes| | | |
-------------+ +------+------+------+------+------+--------+
Iron | | VN. | N. | VN. | VN. | | |
-------------+--------+------+------+------+------+------+--------+------
* No results in the examined spectra; but see Plate XIII. fig. 2.
** Too uncertain in position for comparison (see Plate XIII. fig. 1).
Tested by coincidence, or close proximity of lines to those of the
Aurora, we arrange the spectra in the following order:—(1) iron, (2)
air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6)
carbon and oxygen.
The air-tube spectrum might perhaps stand higher in the scale but for its
broad bands, which make comparison doubtful. Lines of oxygen possibly
escape detection in the Aurora from the faint character of its spectrum.
The phosphorus and iron spectra are especially interesting in connexion
with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the
Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546).
[Sidenote: Additional Table of compared spectra.]
As an addendum to the foregoing, on Plate IX. fig. 1 will be found a
Table I have prepared, in which a type Aurora and also Vogel’s and
Barker’s Auroræ are compared with eight other spectra, viz.:—
S. Solar spectrum.
N. Nitrogen (air): Watts.
O. Oxygen (air): Watts.
C.H. Carburetted-hydrogen vacuum-tube: Watts.
C.I. Carburetted-hydrogen flame: Watts.
C.C. Blue base of candle-flame: Capron.
O.P. Oxygen vacuum-tube: Procter.
I. Iron: Watts.
The divisions and vertical lines will guide the eye in making comparison
of the spectra.
CHAPTER XII.
SOME NOTES ON PROFESSOR ÅNGSTRÖM’S THEORY OF THE AURORA-SPECTRUM.
[The substance of these appeared in the ‘Philosophical
Magazine’ for April 1875, in conjunction with the “Comparison
of the Tube and other Spectra” (Chapter XI.), but they are now,
for the sake of convenience, made a separate article.]
[Sidenote: Professor Ångström’s propositions.]
In a contribution by the late Professor Ångström to a solution of the
problem of the Aurora-spectrum (an abstract of which appeared in ‘Nature’
of July 16, 1874), the Professor is stated, amongst other things, to have
laid down certain propositions in substance as follows:—
[Sidenote: That the Aurora has two spectra.]
1st. That the Aurora has two different spectra—the one comprising the one
bright line in the yellow-green only, and the other the remaining fainter
lines.
[Sidenote: That bright line does not coincide with HC₂.]
2ndly. That the bright line falls within a group of hydrocarbon lines,
but does not actually coincide with any prominent line of such group, and
that Dr. Vogel’s finding this line to coincide with a not well-marked
band in the air-spectrum must be regarded as a case of accidental
coincidence.
[Sidenote: That moisture is _nil_ in Aurora region.]
3rdly. That moisture in the region of the Aurora must be regarded as
_nil_, and that oxygen and hydrogen must alone there act as conductors of
electricity.
[Sidenote: Ångström’s flask-experiment described.]
Professor Ångström then details the examination of an exhausted dry
air-flask filled with a discharge analogous to the glow of the negative
pole of a vacuum air-tube.
[Sidenote: Flask-spectrum compared with Aurora-spectrum.]