(6) The Plücker air-tube (Plate XVII. fig. 2) was placed between the

poles of the magnet, and the negative wire only was connected with the

straight electrode. A pale violet glow was seen round this electrode, and

another, but rather fainter, glow of a similar description at the ring

electrode, the intermediate space being filled with a salmon-coloured

light. This violet glow was condensed into an arc by the action of the

magnet. Reversing the current, the violet glow still remained at each

electrode, and that between the poles of the magnet was still influenced

into an arc.

[Sidenote: Geissler tube substituted, with similar results.]

(7) An air Geissler tube was substituted for the Plücker tube, with very

much the same result. Whichever wire was attached, a violet glow appeared

at the connected electrode, and a fainter one of the same character at

the other; and the magnet influenced both. The connecting salmon-coloured

glow was faint.

[Sidenote: Globular receiver treated with phosphoric anhydride.]

(8_a_) The globular receiver had some phosphoric anhydride shaken into

it; and it was then exhausted. The cobweb streamers and violet glow each

appeared according to which wire was connected. There was no marked

difference between the receiver with the anhydride and without; except

that in the former case the streamers and glow were reduced in extent and

strength, and were comparatively faint.

[Sidenote: Discharge in water-vapour described.]

(_b_) The anhydride having been washed out, first with plain and

afterwards with distilled water, some drops of the latter were allowed

to remain in the receiver. On exhaustion a vapour-cloud was formed, and

the discharge passed (both terminals being connected with the coil)

through this. The rosy stream of light was formed as usual, but was

more flickering and unsteady. As the exhaustion was lessened, the rosy

stream disappeared, and the cobweb streams began to fill the receiver.

These were, however, not so bright and sharp as in a dry receiver, but

were faint and broad; while some diffused and nebulous streams of light,

running (slightly bent) from pole to pole, and from ¼ to ⅜ of an inch

broad, were intermixed with them. When one wire only was connected, the

glow and streamers from the electrode were very faint.

[Sidenote: Large bell-receiver and plate described. Receiver exhausted

and stream of light formed.]

(9) A large bell-shaped receiver, 11 × 8 inches, was next used. It was

open at the bottom, which was ground as usual; and had a small opening

at the top, also carrying a ground edge. A solid brass plate was

prepared, ground only round the edge (in order to take the receiver),

and in the centre of this brass plate were inserted two disks of soft

iron, corresponding in position and size with the poles of the Ladd

electro-magnet (see Plate XVIII. fig. 11). When this plate was placed on

the magnet, the poles and disks were in contact; and the disks became

N. and S. poles within the receiver. A small brass plate carrying a tap

and exhaust-tube, and a binding-screw for attaching an electrode within,

closed the receiver at top. The receiver and plate being placed on the

magnet-poles, the former was exhausted until the discharge became a rosy

slightly-diffused stream of light; with a small unilluminated space

between it and the negative pole, where the usual violet glow appeared

round the wire.

This stream of light was used for the experiments after detailed. In some

cases the conical armatures were placed within the receiver, in others

the disks alone were used as the magnetic poles.

[Sidenote: Effect on same when magnet excited.]

(_a_) With the apparatus arranged as shown on Plate XVIII. fig. 10, and

the magnet excited, a violet glow appeared round the end of the wire

which was negative. A small unilluminated space then intervening, the

stream ran in a curve between the wire and the armature, which latter was

positive. The stream was not steady and had a tendency to rotate; but as

this was better observed with the disks only, it is described further on.

[Sidenote: Experiments with the conical armature removed. Vibrating

stream.]

(_b_) The conical armature within the receiver was removed, and the

stream allowed to connect with the centre of the pump-plate. When the

magnet was excited, the stream was violently projected at right angles

to the poles, with a vibrating movement to either side according to the

direction of the current. When the wire was positive the movement was

towards the left, with a slight inclination towards the N. pole. When

the wire was negative the movement was to the right, but in a rather

strong curve towards the N. pole. The vibrating motion was very distinct,

and gave the appearance of six or seven streams running off at regular

intervals (see Plate XVIII. fig. 12).

[Sidenote: Rotating wire over S. pole.]

(_c_) The wire was next placed over the centre of the disk forming the

S. pole. With the wire negative and the pole positive, rotation of the

stream was decidedly, but not very strongly, from right to left from the

centre of the plate (as the hands of a watch). With the wire positive

and the pole negative, rotation was strongly left to right, with a

disposition to spiral twist in the stream (see Plate XVIII. fig. 13).

[Sidenote: Same over N. pole.]

(_d_) The wire was placed over the disk forming the N. pole. With the

wire negative and the pole positive, rotation of the stream was left to

right. With the wire positive and the pole negative, rotation was right

to left (see Plate XVIII. fig. 14).

[Sidenote: Stream thrown across the receiver above the magnet-poles.]

(_e_) The stream was thrown across the receiver from the lateral

binding-screws above, and at right angles to, the disks, and afterwards

in the opposite direction, _i. e._ along them. In neither case was there

any marked change when the magnet was excited.

(_f_) The conical armatures were placed with the pointed ends upon the

disks in the receiver, and the stream thrown above and along them. It

diverged—one part running straight across between the electrodes, whilst

another stream and some cobwebs ran from each electrode to its nearest

pole. The streams and cobwebs flickered a good deal. There was no marked

change when the magnet was excited.

_Some of Baron Reichenbach’s Magnetic Researches tested._

[Sidenote: Baron Reichenbach’s researches.]

In 1846 Dr. W. Gregory published an abstract of Baron Reichenbach’s

‘Researches on Magnetism and on certain allied subjects, including a

supposed new Imponderable.’

[Sidenote: Auroræ considered to be magnetic lights. Flames seen by

“sensitive” persons.]

From a paragraph in this work, it would seem that the Baron considered

his observations as tending to an explanation of the Aurora Borealis;

and, since it was generally admitted that these phenomena occur within

our atmosphere, that there appeared a great probability of Auroræ being

visible magnetic lights. The Baron, in the original work, fully describes

the Aurora Borealis; and concludes it must be similar in its nature

to the flames of light seen streaming from the magnet-poles by Mdlle.

Reichel and other sensitive patients of the Baron’s. It is unfortunate

that these flames were only seen by certain “sensitive” persons. The

drawings given of them, too, show no analogy to the magnetic curves.

[Sidenote: Magnet tested for such flames.]

Having the opportunity of a powerful magnet in that used during our

tube-experiments, we made an attempt to detect the Baron’s magnetic

flames, on or around the poles of our magnet, in a perfectly dark room.

Arrangements were made to silently connect and disconnect the battery

with the magnet, without the knowledge of any one except the operator.

The experiment proved a complete failure; no flames or discharges of

light of any kind were to be seen. The observers were five in number, two

gentlemen and three ladies, but not one of the party proved “sensitive.”

[Sidenote: Mr. Brooks’s experiments on action of the magnet on a

sensitive photographic plate.]

Some experiments made by Mr. W. Brooks, and detailed in a paper read by

him before the South London Photographic Society, seem to corroborate

(to a certain extent) the statements made by the Baron in regard to the

influence of the magnet on a sensitive photographic plate.

Remembering, however, how it has been demonstrated that light may be

“bottled up” as an actinic source for a considerable period of time, it

seems a question whether the images obtained were not due to some such

source rather than to any magnetic aura.

SUMMARY OF THE FOREGOING EXPERIMENTS AND THEIR RESULTS.

[Sidenote: Summary of the experiments.]

Chapter XIV. Action of magnet on glow and spectrum of Geissler gas

vacuum-tubes demonstrated.

Chapter XV. Action of magnet on glass capillary tube negatived. Faraday’s

experiment with heavy-glass bar repeated.

Chapter XVI. Action of magnet on glow in wide air-tube demonstrated. Note

on stratification. In Plücker tube, action of magnet on negative pole

(arc formed) and positive pole (Gassiot’s rings produced) demonstrated.

Effects of magnet upon glow and spectrum of tin-chloride vacuum-tubes

demonstrated.

Chapter XVII. Effect of magnet upon after-glow in a bulbed phosphorescent

tube demonstrated. Effect of magnet upon glow in small phosphorescent

(powder) tubes examined. Marquis of Salisbury’s experiments (lighting-up

with one wire only) tested, and confirmatory results arrived at.

Chapter XVIII. Action of magnet on aura of electric spark demonstrated.

Chapter XIX. Effects of magnet on discharges _in vacuo_ in larger vessels

demonstrated. Ångström’s flask experiment tested; same results not

obtained unless one wire only was connected. Experiments demonstrating

the action of a magnet on an electric stream, viz. vibration between, and

rotation round, poles. Baron Reichenbach’s magnetic flames tested without

result.

CHAPTER XX.

SOME CONCLUDING REMARKS.

It is usual, in concluding a work on a special subject, to sum up its

contents, and to examine the general results arrived at. This, however,

it is not easy to do in the present case. The contents of our volume

comprise a short history of the Aurora, its qualities and spectrum; and

a statement has been given of the several conclusions at which various

observers have arrived as to its character and causes. In the present

state of our knowledge of the subject, to add an opinion to these might

seem to savour of presumption; and the questions involved may perhaps

be better treated as still _sub judice_, and as requiring further and

fuller evidence before arriving at a verdict. The following observations

must therefore be taken rather as further notes and memoranda, than as

conclusions. Apart from the spectroscopic questions involved, the oldest

and most received theory of the Aurora—that of its being some form of

electric discharge in the more rarefied regions of the atmosphere,—seems

to hold its own: and if, as is probable, some form of phosphorescence is

involved in the discharge, M. Lecoq de Boisbaudran’s observations on the

brightening of the red line under the influence of cold, and the falling

of the yellow-green line within a band of phosphoretted hydrogen, come

into play; and a connexion, though slight and imperfect, may be in this

respect traced between the discharge and its spectrum. The experiments

detailed in Part II. seem to have an important bearing, as showing the

very marked effect of the magnet on the rarefied glow, as well as on the

spark in air at ordinary pressure. The well-defined arc formed by the

aura of the spark, the flickering jets which replace the even edge of

the arc when partially withdrawn from the magnetic influence, and the

streamers formed when the aura is blown away from the spark (Plate XVII.

figs. 6, 7, and 8), are certainly highly suggestive of frequent forms

of Auroral discharge; and, but for trial and failure, might lead one to

expect results from a comparison of the line air-spectrum with that of

the Aurora. The experiments with a wire attached to one electrode only,

show how the glow may be affected and varied in colour and character when

the discharge is interrupted and incomplete. Differences in electric

tension may also considerably vary the character of the discharge.

The influence of the magnet in exciting and brightening the glow and

spectrum of one gas, while it depresses and extinguishes the glow and

spectrum of another gas in the same tube, suggests an explanation of

the observed variation in intensity, and difference in number, of the

Aurora-lines. Intensity of lines depending on temperature, and this

again on resistance, and it appearing that resistance is influenced by

the magnetic action, the same effects of brightening or depressing of

the spectrum are probably produced in the Aurora, as in the vacuum-tubes

placed between the magnet-poles.

In the Marquis of Salisbury’s observations, paraffin-vapour gave C

and H lines when connected with both poles of the battery, but C

lines only when connected with one pole; and in that case the lines

were equally sharp on both sides. These observations (repeated in our

experiments) may afford an explanation why the hydrogen-lines are not

seen in the Aurora-spectrum; although there can be hardly any doubt

that the phenomenon usually takes place in air more or less moist.

Professor Ångström’s researches on the violet-pole glow are not entirely

corroborated by our experiments; and it seems doubtful whether his

results in the exhausted flask were not obtained from the negative pole

only. One great difficulty in the comparison of the Aurora-spectrum

with the violet pole of air-tubes and some other spectra (including

oxygen), arises from the presence in the latter of broad bands; and it

is difficult to understand how these bands can be aptly compared with

the definite, though faint, lines observed by Dr. Vogel and others in

the Aurora-spectrum. It must, too, be borne in mind that the conditions

under which we may consider the Aurora to obtain, are such as can be

only very imperfectly imitated in the laboratory. Auroræ also no doubt

differ in density and thickness of layer; and Kirchhoff’s observation

must be remembered:—“That if thickness of a film of vapour be increased,

the lines are increased in intensity, the bright lines more slowly than

the fainter; and it may happen that the spectrum appears to be totally

changed when the mass of the vapour is altered.” Were it possible to test

with the spectroscope a cloud or film of gaseous vapour corresponding

in some degree in density and thickness with an Auroral discharge, we

might perhaps get nearer the truth. Mr. Procter also remarks (as we

proved in our magnet experiments):—“That frequently very small traces

appropriate to themselves the whole of electrical discharges at low

pressures, and completely mask the spectra of any other gases present.”

The oxygen-spectrum, with its possible variation by the conversion of

that gas into the allotropic condition termed ozone, seemed at first

to afford a prospect of close relation to the Aurora-spectrum; which,

however, disappeared on closer examination. If nitrogen could be modified

in some such way as oxygen is converted into ozone, it might perhaps

afford another opportunity for investigation; but we have no evidence

at present of such a change. The spectrum of nitrogen is usually found

singularly distinct and persistent; and, except as varied from band to

line by intensity of the discharge, not liable to alteration[16].

Colours of lines are functions of wave-length, subject, however, to the

observation that in a weak spectrum the colours lose their intensity. The

red line in the Aurora has sometimes been found brighter than the green.

It has been suggested that the red and green may be independent spectra;

but the variations of tint observed in the capillary of hydrogen and

other tubes according to resistance of the current, demonstrate that the

varying colours of the Aurora may be connected with the lighting-up of

particular parts of the spectrum, and do not necessarily indicate that

different gases and spectra are excited.

Absorption may also play an important part in the nature of the

Aurora-spectrum (Zöllner’s theory that the lines are really spaces

between absorption bands). Most gases will give a continuous spectrum

under certain circumstances, even at a low pressure.

The question of cosmic dust is inviting, but the facts collated hardly

warrant at present its probable connexion with the Aurora.

If Auroræ were composed of incandescent glowing meteors, it would be

reasonable to expect to find in the spectrum the lines of iron, a metal

constituting so prominently the composition of meteorites. No connexion

between the iron and the Aurora-spectra is, however, proved; though it

may be suspected. The iron-spectrum, as remarked elsewhere, contains so

many lines that some may, as a mere accidental circumstance, closely

agree with the Aurora-lines.

The iron-lines are, it may be remarked, as a rule, sharper and finer than

the Auroral lines, though it is possible that these characteristics might

vary if the spectrum were obtained in a rarefied medium. Tubes with iron

terminals are said to evolve a compound gas of H and Fe. I have not had

an opportunity to verify this.

It may be added that the comparative faintness of the more refrangible

lines of the Aurora-spectrum suggests a feeble resistance to the exciting

current, and a low temperature inconsistent with a meteoric theory; and

this is not contradicted by the brightness of the red and green lines,

if these are due to a phosphorescent origin. Expansion of a line is

recognized to be dependent on pressure, and consequently the breadth

of the green or red lines might indicate the height of the Aurora;

while their brightness or otherwise might also give some idea as to its

density. No observations in this direction have, as far as I am aware,

been recorded.

As the general result of spectrum work on the Aurora up to the present

time, we seem to have quite failed in finding any spectrum which, as to

position, intensity, and general character of lines, well coincides with

that of the Aurora. Indeed, we may say we do not find any spectrum so

nearly allied to portions even of the Aurora-spectrum, as to lead us to

conclude that we have discovered the true nature of one spectrum of the

Aurora (supposing it to comprise, as some consider, two or more). The

whole subject may be characterized as still a scientific mystery—which,

however, we may hope some future observers, armed with spectroscopes

of large aperture and low dispersion, but with sufficient means of

measurement of line positions, and possibly aided by photography, may

help to solve. The singular absence of Auroræ has, for some time past,

given no opportunity in that direction. May some of my readers be more

fortunate in obtaining opportunities of viewing the glorious sky-fires,

and assist to unravel so interesting a paradox!

APPENDICES.

APPENDIX A.

REFERENCES TO SOME WORKS AND ESSAYS ON THE AURORA.

(Most of these are cited in the ‘Edinburgh Encyclopædia’ and the

‘Encyclopædia Britannica.’)

Musschenbroek, Instit. Phys. c. 41.

‘Trai. Phys. et Hist. de l’Aurore Boréale,’ par M. de Mairan. Paris, 1754.

Beccaria, ‘Dell’Elettricismo Artif. e Nat.’ p. 221.

Smith’s ‘Optics,’ p. 69.

D’Alembert’s ‘Opuscules Mathématiques,’ vol. vi. p. 334.

‘Philosophical Transactions’ as under:—

Vol. Pages

1716 406

1717 584, 586

1719 1099, 1101, 1104, 1107

1720 21

1721 180, 186

1723 300

1724 175

1726 128, 132, 150

1727 245, 301

1728 453

1729 137

1730 279

1731 53-55

1734 243, 291

1736 241

1740 368

1741 744, 839, 840, 843

1750 319, 345, 346, 499

1751 39, 126

1752 169

1753 85

1762 474, 479

1764 326, 332

1767 108

1769 86, 307

1770 532

1774 128

1781 228

1790 32, 47, 101

‘Miscell. Berolinens.’ 1710, vol. i. p. 131.

‘Comment. Petrop.’ tom. i. p. 351, tom. iv. p. 121.

‘Acta Petrop.’ 1780, vol. iv. p. 1.

‘Mem. Acad. Paris,’ 1747, pp. 363, 423; 1731; 1751.

‘Mem. Acad. Berl.’ 1710, vol. i. p. 131; 1747, p. 117.

Schwed. ‘Abhandlungen,’ 1752, p. 169; 1753, p. 85; 1764, pp. 200, 251.

Bergman, ‘Opusc.’ vol. v. p. 272.

‘Americ. Trans.’ vol. i. p. 404.

‘Mém. de Mathémat. et Phys.’ tom. viii. p. 180.

Rozier, vol. xiii. p. 409; vol. xv. p. 128; vol. xxxiii. p. 153.

Franklin’s Works, vol. ii.

Weidler, ‘De Aurora Boreale.’ 4to.

Nocetus, ‘De Iride et Aurora Boreale, cum Notis Boscovisch.’ Rome, 1747.

Chiminello, ‘Mem. Soc. Ital.’ vol. vii. p. 153.

Gilbert’s ‘Journal,’ vol. xv. p. 206; and (particularly) Dr. T. Young’s

‘Nat. Phil.’ vol. i. pp. 687, 716, and vol. ii. p. 488.

Wiedeburg, ‘Ueber die Nordlichter.’ Jena, 1771.

Hüpsch, ‘Untersuchung des Nordlichts.’ Cologne, 1778.

Van Swinden, ‘Recueil de Mémoires.’ Hague, 1784.

Wilke, ‘Von den neuesten Erklärungen des Nordlichts,’ Schwed. Mus.

Wismar, 1783.

Dalton’s ‘Meteor. Observ.’ 1793, pp. 54, 153.

Loomis, ‘Sill. Journal,’ 2nd series, xxxii. p. 324; xxxiv. p. 34. The

same, 3rd series, v. p. 245; B. V. Marsh, 3rd series, xxxi. p. 311.

Oettingen and Vogel, Pogg. Ann. cxlvi. pp. 284, 569.

Galle and Sirks, _ibid._ cxlvi. p. 133; cxlix. p. 112.

Silbermann, ‘Comptes Rendus,’ lxviii. pp. 1049, 1120, 1140, 1164.

Prof. Fritz, “Geog. Distrib.,” Petermann’s Mitth., Oct. 1874.

Zehfuss, ‘Physikalische Theorie.’ Adelman, Frankfort.

‘Nature,’ iii. pp. 6, 7, 28, 104, 126, 346, 348, 510; iv. pp. 209, 213,

345, 497, 505; x. 211 (Ångström).

‘Edinburgh Astronomical Observations,’ vol. xiv. 1870-1877.

‘English Mechanic,’ No. 461 (January 23, 1874), pp. 445-447; and No. 462,

pp. 475, 476.

APPENDIX B.

EXTRACTS FROM THE MANUAL AND INSTRUCTIONS FOR THE (ENGLISH) ARCTIC

EXPEDITION, 1875.

_Note on Auroral Observations. By Prof. STOKES, Sec. R.S._

The frequency of the Aurora in Arctic regions affords peculiar facilities

for the study of the general features of the phenomenon, as in case the

observer thinks he has perceived any law he will probably soon, and

repeatedly, have opportunities of confronting it with observation. The

following points are worthy of attention:—

_Streamers._—It is well known that, at least as a rule, the streamers

are parallel to the dipping-needle, as is inferred from the observation

that they form arcs of great circles passing through the magnetic zenith.

It has been stated, however, that they have sometimes been seen curved.

Should any thing of this kind be noticed, the observer ought to note the

circumstances most carefully. He should notice particularly whether it

is one and the same streamer that is curved, or whether the curvature is

apparent only, and arises from the circumstance that a number of short,

straight streamers start from bases so arranged that the luminosity as a

whole presents the form of a curved band.

Have the streamers any lateral motion? and if so, is it from right

to left or left to right, or sometimes one and sometimes the other,

according to the quarter of the heavens in which the streamer is seen, or

other circumstances? Again, if there be lateral motion, is it that the

individual streamers move sideways, or that fresh streamers arise to one

side of the former, or partly the one and partly the other? Do streamers,

or does some portion of a system of streamers, appear to have any uniform

relation to clouds, as if they sprang from them? Can stars be seen

immediately under the base of streamers? Do streamers appear to have any

definite relation to mountains? Are they ever seen between the observer

and a mountain, so as to appear to be projected on it? This or any other

indication of a low origin ought to be most carefully described.

When streamers form a corona, the character of it should be described.

_Auroral Arches._—Are arches always perpendicular to the magnetic

meridian? If incomplete, do they grow laterally? and if so, in what

manner, and towards which side? Do they always move from north (magnetic)

to south? and if so, is it by a southerly motion of the individual

streamers, or by new streamers springing up to the south of the old ones?

What (by estimation, or by reference to known stars) may be the breadth

of the arch in different positions in its progress? Do arches appear

to be nothing but congeries of streamers, or to have an independent

existence? What relations, if any, have they to clouds? and if related,

to what kind of clouds are they related?

_Pulsations._—Do pulsations travel in any invariable direction? What

time do they take to get from one part of the heavens to another? Are

they running sheets of continuous light, or fixed patches which become

luminous, or more luminous, in rapid succession? and if patches, do these

appear to be foreshortened streamers? Are the same patches luminous in

successive pulsations?

_Sounds_ (?).—As some have suspected the Aurora to be accompanied by

sound, the observer’s attention should be directed to this question when

an Aurora is seen during a calm. If sound be suspected, the observer

should endeavour, by changing his position, brushing off spicules of

ice from the neighbourhood of the ears, his whiskers, &c., to ascertain

whether it can be referred to the action of such wind as there is on

some part of his dress or person. If it should clearly appear that it

is not referable to the wind, then the circumstance of its occurrence,

its character, its relation (if any) to bursts of light, should be most

carefully noted.

These questions are prepared merely to lead the observer to direct

his attention to various features of the phenomenon. Answers are not

demanded, except in such cases as definite answers can be given; and

the observer should keep his attention alive to observe and regard any

other features which may appear to be of interest. It is desirable that

drawings should be made of remarkable displays.

Observations with Sir William Thomson’s electrometer would be very

interesting in connexion with the Aurora, especially a comparison of the

readings before, during, and after a passage of the Aurora across the

zenith.

_Spectroscopic Observations. By Prof. G. G. STOKES, Sec. R.S._

_Spectrum of the Aurora._

The spectrum of the Aurora contains a well-known conspicuous bright line

in the yellowish green, which has been accurately observed. There are

also other bright lines of greater refrangibility, the determination of

the positions of which is more difficult on account of their faintness,

and there are also one or more lines in the red, in red auroras.

Advantage should be taken of an unusually bright display to determine the

positions of the fainter lines. That of the brightest lines, though well

known, should be measured at the same time to control the observations.

The character of the lines (_i. e._ whether they are strictly lines,

showing images of the apparent breadth of the slit, or narrow bands,

sharply defined or shaded-off) should also be stated.

Sometimes a faint gleam of light is seen at night in the sky, the

origin of which (supposed from the presence of clouds) is doubtful. A

spectroscope of the roughest description may in such cases be usefully

employed to determine whether the light is auroral or not, as in the

former case the auroral origin is detected by the chief bright line.

The observer may thus be led to be on the look-out for a display which

otherwise might have been missed.

It has been said, however, that the auroral light does not in all cases

exhibit bright lines, but sometimes, at least in the eastern and western

arch of the Aurora, shows a continuous spectrum. This statement should be

confronted with observation, special care being taken that the auroral

light be not confounded with light which, though seen in the same

direction, is of a different origin, such, for example, as light from a

bank of haze illuminated by the moon.

Sir Edward Sabine once observed an auroral arch to one side (say north)

of the ship, which was in darkness. Presently the arch could no longer

be seen, but there was a general diffuse light, so that a man at the

mast-head could be seen. Later still, the ship was again in darkness, and

an auroral arch was seen to the south.

Should any thing of the kind be observed, the whole of the circumstances

ought to be carefully noted, and the spectroscope applied to the diffuse

light.

_Polarization of Light. By W. SPOTTISWOODE, M.A., LL.D., Treas. R.S._

It has been suggested that the Aurora, inasmuch as it presents a

structural character, may afford traces of polarization. Having reference

to the fact that the striæ of the electric discharge in vacuum-tubes

present no such feature, the probability of the suggestion may be

doubted. But it will still be worth while to put the question to an

experimental test.

If traces of polarization be detected, it must not at once be concluded

that the light of the Aurora is polarized; for the Aurora may be seen on

the background of a sky illuminated by the moon, or by the sun, if not

too far below the horizon, and the light from either of these sources is,

in general, more or less polarized; therefore, if the light of the Aurora

is suspected to be polarized, the polariscope should be directed to an

adjacent portion of clear sky, free from Aurora, but illuminated by the

moon or sun as nearly as possible similar, and similarly situated to the

former portion; and the observer must then judge whether the polarization

first observed be merely due to the illumination of the sky.

The presence of polarization is to be determined:—

(1) With a Nicol’s prism, by observing the light through it by turning

the prism round on its axis, and by examining whether the light appears

brightest in some positions and least bright in others. If such be the

case, the positions will be found to be at right angles to one another.

The direction of “the plane of polarization” will be determined by

that of the Nicol at either of these critical positions. The plane of

polarization of the light transmitted by a Nicol, is parallel to the

longer diagonal of the face; and, accordingly, the plane of polarization,

or partial polarization, of the observed light is parallel to the longer

diameter of the Nicol when the transmitted light is at its greatest

intensity, or to the shorter when it is at its least.

(2) The observation with a double-image prism is similar to that with a

Nicol. This instrument, as its name implies, gives the images which would

be seen through the Nicol in two rectangular positions, both at once,

so that they can be directly compared; and when in observing polarized

light the instrument is turned so that one image is at a maximum, the

other is simultaneously at a minimum. Both these methods of observation,

(1) and (2), are especially suitable for faint light; because in such a

case the eye is better able to appreciate differences of intensity than

differences of colour.

(3) The observation with a biquartz differs from (1) only by holding a

biquartz (a right-handed and a left-handed quartz cemented side by side)

at a convenient distance beyond the Nicol, and by observing whether

colour is or is not produced. If the Nicol be so turned that the two

parts of the biquartz give the same colour (choose the neutral tint,

_teint de passage_, rather than the yellow), we can detect a change in

the position of the plane of polarization by a change in colour, one

half verging towards red, the other towards blue. This observation is

obviously applicable to a change in the plane, either at different parts

of the phenomenon at the same time, or at the same parts at different

times.

(4) We may use a Savart’s polariscope, which shows a series of coloured

bands in the field of view. For two positions at right angles to one

another corresponding to the two critical positions of a Nicol, these

bands are most strongly developed; for two positions midway between the

former the bands vanish. In the instruments here furnished, the plane of

polarization of the observed light will be parallel to the bands when the

central one is light, perpendicular to them when the central band is dark.

_Instructions in the use of the Spectroscopes supplied to the Arctic

Expedition._

_By J. NORMAN LOCKYER, F.R.S._

_Spectroscopic Work._

Scales prepared on Mr. Capron’s plan, together with forms for recording

positions, also accompany the instrument.

_A._ In using these, carefully insert the principal solar lines in their

places on the forms, as taken from a fine slit, and keep copies of this

scale for use. If the slit opens _only on one_ side, note on scale in

which direction the lines widen out, whether towards red or violet. Also

fill up some of these forms with gas and other spectra, as taken at

leisure _with the same instrument_ and scale.

When observing, close the slit (after first wide opening it) as much as

light will permit, and then with pen or pencil record the lines as seen

upon the micrometer-scale on the corresponding part of the form, and note

_at once relative intensities_ with Greek letters, α, β, &c. (or numbers).

Reduce at leisure line-places on scale to wave-lengths, and note as to

each line the _probable limits of instrumental error_. _B._

In case the auroral spectrum is so faint that the needle-point or

micrometer-scale is invisible, half of the field of view may be covered

with tinfoil, with a perfectly straight smooth edge running along the

diameter of the field, in perfect focus, and parallel to the lines of the

spectra. The reading-screw being set to 10, the bending-screw should then

be adjusted so that the green line of the Aurora is just eclipsed behind

the blackened edge of the tinfoil. A similar eclipse of other lines will

give their positions.

In this instrument the reference-prism is brought into action by turning

the slipping piece to which is fixed the two terminals. Care should be

taken that the prism itself is adjusted before commencing observations,

as it may be shaken out of position on the voyage. The tubes provided

for the reference-spectra may be either fastened to the terminals or

arranged in some other manner. The air-spectrum may also be used as a

reference-spectrum. To get this, two wires should be screwed into the

insulators, their ends being at such a distance apart and in such a

position that the spectrum is well seen.

_General Observations regarding the Spectrum of the Aurora[17]._

_C._ Note appearance, colour, &c. of _arc_, _streamers_, _corona_, and

_patches_ of light.

Get compass positions of principal features, and _note any change of

magnetic intensity_. If corona forms, take its position and apparent

height.

Look out for _phosphorescence_ of Aurora and adjacent clouds. Listen

for reported sounds. Note any peculiarity of cloud scenery, prior to or

pending the Aurora.

Sketch principal features of the display, and indicate on this sketch the

parts spectroscopically examined.

Examine line in _red_ specially in reference to its assumed connexion

with _telluric_ lines (little _a_ group), and note _as to its brightening

in sympathy with any of the other lines_.

Examine line in yellow-green (Ångström’s) as to _brightness_, _width_,

and _sharpness_ (_or nebulosity_) at the edges. Notice as to a peculiar

_flickering_ in this line sometimes seen; note also whether this line is

_brighter_ (or the reverse) _with a fall of temperature_. Note _ozone_

papers at the time of Aurora.

Note whether the Auroræ can by their spectra be classed into distinct

types or forms, and examine for _different spectra_ as under:—

α. The auroral _glow_, pure and simple.

β. The white arc.

γ. The streamers and corona.

δ. Any phosphorescent or other patches of light, or light cloud

in or near the Auroræ. _D._

The information collected together in the ‘Manual’ should be carefully

consulted, and the line of observations suggested by Ångström’s later

work followed out. To do this, not only record the positions of any

features you may observe in the spectrum, but endeavour to determine,

if any, and if so which, of the features vary together. Compare, for

instance, the two spectra of nitrogen in the Geissler tube supplied, by

observing first the narrow and then the wider parts of the tube. It will

be seen that the difference in colour and spectrum results simply from an

addition to the spectrum in the shape of a series of channelled spaces

in the more refrangible end in the case of the spectrum of the narrow

portion.

Try to determine whether the difference between red and green Auroras may

arise from such a cause as this, and which class has the simpler spectrum.

See whether indications of great auroral activity are associated with the

widening or increased brilliancy of any of the auroral lines.

Remember that if auroral displays are due to gaseous particles thrown

into vibration of electric disturbance, increased electric tension

may either (1) dissociate those particles and thus give rise to a new

spectrum, the one previously observed becoming dimmer; or (2) throw the

particles into more intense vibration without dissociation, and thus give

rise to new lines, those previously observed becoming brighter.

Careful records of auroral phenomena from both ships may enable the

height of some, observed from both, to be determined. It will be very

important that those the heights of which are determined by such means

should be carefully observed by the spectroscope, in order to observe

whether certain characteristics of the spectrum can be associated with

the height of the Aurora.

APPENDIX C.

EXTRACTS FROM PARLIAMENTARY BLUE BOOK, CONTAINING THE “RESULTS DERIVED

FROM THE ARCTIC EXPEDITION 1875-76.” (Eyre and Spottiswoode, 1878.)

_Auroras observed 1875-1876, at Floebery Beach and Discovery Bay._

_By Lieutenant A. C. PARR, R.N._

Though the auroral glow was often present, and served in some degree to

lighten the darkness of the sky during the long winter, when the moon was

absent, the actual appearances of the Aurora itself were few, and the

nimbus worthy of any particular remark extremely small. Those which were

stationary assumed the form of low arches, with streamers flashing up to

them from the horizon, and usually to the eastward. But the more common

form was for an arch to appear low down in some part of the sky where the

glow was brightest; at first it was very faint and narrow, but as it rose

gradually in the heavens it would increase both in size and intensity,

till on arriving near the zenith, with its ends extending nearly to the

horizon, it would be about the breadth of three or four rainbows, and its

colour that of white fleecy clouds lit up by the rays of the full moon.

On reaching this point, however, its course was nearly run; for after

appearing to remain stationary, as little white gaps would suddenly rend

the arch asunder, the portions thus detached seemed to roll together and

concentrate all their brightness in the smaller space, and then gradually

fade away and become extinct. Sometimes a very pale green would show

itself in the more luminous patches, and once or twice there was a slight

suspicion of red; but never was the whole sky illuminated by streams

running in all directions, and forming coronæ, while these colours varied

every moment.

When instead of the arch rising up from the horizon a streamer appeared,

its origin was in the north. From the northern horizon it would stretch

out towards the zenith, passing nearly overhead, and reaching to within

a few degrees of the land to the south. In appearance they would be the

same as the arches, but sometimes a second would grow out of the first,

and on one occasion three were visible at the same time. They had lateral

motion either from east to west, or west to east, but there was no

flashing to brighten them, and they gradually faded away.

The time at which Auroras usually occurred was between 9 P.M. and

midnight, the last display being on February 19th, commencing at 11

P.M. It was a beautifully clear night, without mist or haze of any

description, and small stars visible close down to the horizon. At

the above-named hour two arches made their appearance, and remained

stationary; the lower one was the brighter, being of a pale green colour,

its centre bearing E.S.E. (true), and having an altitude of about 5°,

with a breadth of about twice that of a rainbow. The second arch was

concentric with the first, and about 7° above it, but rather broader and

fainter. These arches maintained their altitude, the upper one at about

the same intensity, but that of the lower one varied considerably. It

would gradually lighten up, then send flashes to the upper one, then

break up and fade away; before, however, it had quite disappeared,

flashes would come up to it from the horizon which seemed to endue it

with new life, for the arch would be reformed, brighten up, and the same

performance would be again repeated. This occurred three or four times in

the course of three quarters of an hour; but the flashes from the horizon

never extended beyond the lower arch, and those from the lower never went

beyond the upper. During this display the citron-line was obtained very

clearly with the spectroscope, but no other lines were visible.

On six or seven occasions Auroras were visible at the same time on board

both the ‘Alert’ and ‘Discovery;’ but the absence of characteristic

features makes it impossible to determine whether they were the same

display, or merely two distinct ones which happened to occur at the same

time. But as by far the larger number of those recorded in the one ship

were not visible at the other, it was certainly only under exceptional

conditions that they could be simultaneously observed at both stations,

if, indeed, they ever were. Auroras seemed to appear indifferently both

when there was wind and when it was calm, with either a high or low

barometer, and seemed quite unconnected with the temperature, although on

an occasion the thermometer was observed to fall 3° during the display,

and to rise 2° almost immediately afterwards. But it was never seen

illuminating the edges of clouds, as we saw it on the passage home, nor

playing about the outline of the land, and never was there the slightest

suspicion of sound being produced by it.

The opportunities for observing the spectrum of the Aurora in this

position have been most unsatisfactory, as the displays were small in

number and deficient in brilliancy.

The form they generally assumed was to rise like an arch from a portion

of the horizon where there was a luminous glow, at first very faint, but

gradually increasing in brilliancy till near the zenith, where it would

remain stationary for a short time and then break up and disappear.

Sometimes they would rise up as streamers, but only occasionally was more

than one visible at a time, and they lasted for such a short time, that

even if they had been bright it would have been very difficult to make

satisfactory observations.

Very few showed any signs of colour, and those only the slightest tinge.

Nearly all that were observed gave the citron-line with the small pocket

spectroscope with more or less distinctness, though no signs of any other

lines were ever seen; but on only two occasions was it bright enough to

get the line with Nury’s spectroscope, and then only for such a short

time that a satisfactory measure could not be obtained.

* * * * *

Then follows a descriptive list of the Auroræ seen, from which I have

selected three of the finest, viz. January 2nd, February 14th, and

February 19th, 1876.

January 2nd, 1876. Lieut. Parr. _Floeberg Beach._—9 P.M. Streams of

Aurora. Stars shining brightly.

Register. _Discovery Bay._—9 P.M. Observed an Aurora like a pale band of

light in the form of an arch whose centre was on the true meridian and

15° from the zenith. It shortly afterwards broke up into feathered edges,

their direction being a little to the eastward of the zenith. The arch

grew fainter, and shifted to the eastward of the meridian four points;

the left extremity of the arch faded away, and the right assumed the

shape of the folds of a curtain doubled over. The weather was clear and

calm. The display lasted upwards of 30 minutes.

A spectroscope, one of Browning’s 8-in. direct-vision, was directed

towards the Aurora, but the light was not sufficient to give any spectrum.

The temperature was -39°. Barometer 29·56 inches. No wind. Clouds stratus

2. Eight meteors were observed during the time the Aurora was visible.

February 14th. Register. _Discovery Bay._—At 2 A.M. a faint Aurora

passing across the heavens from S.E. to S.W. was observed, like an arch

of a pale colour. It lasted only a short time, and was very indistinct.

Temperature -47°. Barometer 30·44 inches. No wind or clouds.

Lieut. Aldrich. _Floeberg Beach._—2 A.M. A faint Aurora towards the S.W.

Weather calm. Cumulus-stratus clouds 3. Temperature -46°. 8 P.M. Faint

flashes of Aurora in the E. and S.W.

Lieut. Aldrich and Lieut. Parr. _Floeberg Beach._—11.50 P.M. A moderately

bright arch of Aurora extended from due N. to about S.S.W., where it

terminated close down to the horizon in a crook turned to the eastward.

In a few moments a streamer flashed from the end of the crook parallel

to the first and right across the heavens, its edges being quite sharp

and parallel to each other. A third streamer shot up a minute afterwards,

but did not extend more than 80° upwards. The streamers were visible for

a very short time, the first remaining longest. The second-named arch

gradually faded away till within a few degrees of the S.S.W. horizon, and

(still being a continuation of the crook) bent round to the eastward,

and towards the horizon, going on to what was left of the stump of the

third arc. A lateral motion to the eastward now began, the whole body

gradually turning round until it disappeared about due south. Stars were

visible through it at its brightest, but not very distinctly. This is

the most intense and variegated Aurora we have experienced, but scarcely

any colours were to be seen. Temperature -51°. Barometer 30·43 inches,

stationary. Calm weather. Clouds cumulus 1. Preceded and followed by calm

weather.

Meteorological Register. _Discovery Bay._—9.15 P.M. An Aurora was

observed to the southward, spreading out like a fan in separate ways.

It was faint. A few cirro-stratus clouds were visible, apparently

between the observer and the Aurora. It lasted about 40 minutes, and

then gradually faded away. Temperature -47°. Barometer 30·51 inches,

stationary. No wind. Clouds cirro-stratus 4.

February 19th. Meteorological Report. _Discovery Bay._—9.45 P.M. An

Aurora like a fluted arch, with rays flashing towards the Pole, was

observed spanning the hills from the south to the east. The direction of

the lines of light from all parts of the arch was towards the zenith.

Above the arch a pale band of colour appeared, like a secondary arch

above the other. It appeared very much as if it was caused by the

reflected light of the Aurora. The Aurora was bright for a few seconds,

and then gradually died away. It lasted altogether about 30 minutes.

The centre of the arch bore S.E., having an altitude of about 30°. The

secondary arch was about 15° above the former. Both arches were of a pale

light colour, the upper one very faint. Temperature -34°. Barometer

29·87 inches, rising rapidly. Weather calm. Misty. No clouds.

Lieut. Parr. _Floeberg Beach._—An Aurora appeared shortly after 11

P.M., consisting of bright arch, whose centre bore about E.S.E., and

had an altitude of about 5°, with a second broader and fainter arch

about 7° above the first. These arches maintained their altitudes, the

upper one at about the same intensity, but that of the lower one varied

considerably. It would gradually brighten up, then send streamers up to

the second, then break up into light patches, and gradually fade away.

This happened three or four times during the 40 minutes that the display

lasted. At times streamers would come up from the horizon to the lower

arch, for it was a splendidly clear night, and seemed to brighten it up,

but none of them extended beyond it. Neither did the streamers from the

lower arch extend beyond the upper one. It was slightly green in colour

when brightest, and the citron-line was well defined, but no others were

visible. Temperature -46°. Barometer 29·95 inches, steady. Weather calm.

Cumulus clouds 4. Misty.

TABLE of DATES when AURORAS were observed by the ARCTIC EXPEDITION,

1875-76.

----------------+--------------------------+-----------------------

| H.M.S. ‘Alert,’ | H.M.S. ‘Discovery,’

Date. | Floeberg Beach. | Discovery Bay.

----------------+--------------------------+-----------------------

1875, October 25| 11.45 P.M. Faint. | Cloudy.

” ” 26| 10 P.M. Very faint. | 10 P.M.

” ” 30| Sky obscured. Faint. | Ditto.

” November 1| Ditto. Faint, but well |

| marked. | Ditto.

” ” 2| 9 to 10 P.M. Arches and |

| streamers. | A few clouds.

” ” 21| Ditto. Bright | 9 to 10 P.M.

| streamer. | and 10 to 11 P.M.

” ” 22| 2 P.M. and 8 P.M. Slight,|

| red. | Clear sky.

” ” 25| 9.30 A.M. Character not |

| recorded. | Ditto.

” ” 26| 10 A.M. Stream of light.}| A few clouds.

” ” 26| Cloudy to 10 P.M., }|

| bright afterwards. }| 10 P.M.

” ” 27| Midnight. Slight. | 11.40 P.M.

” ” 28| 1 A.M. Bright streak. | Clear sky.

” ” 29| Cloudy, brighter at 11 |

| A.M. Faint glow. | 9.30 A.M.

” ” 30| A few clouds. Very faint.| 4.30 A.M.

” ” 30| 5 P.M., 8 P.M., and 10 |

| P.M. Flashes. | 5 P.M.

” December 2| Evening. Streamers. | Clear sky.

” ” 3| 1 A.M. Flashes. | Ditto.

” ” 3| Bright sky. Faint Aurora.| 2.30 P.M.

” ” 16| 10 P.M. Slight; showed |

| citron-line. | 11 P.M.

” ” 19| 3 P.M. to 5 P.M., faint; |

| and 9 to 10 P.M., | Very clear sky.

| moderately bright arc. |

” ” 22| 10 P.M. Slight. | Ditto.

” ” 23| 6 P.M. Ditto. | Ditto.

” ” 24| Misty, a few stars |

| visible. Arch. | 9 A.M.

” ” 26| Very bright sky. Faint. | 6 P.M.

” ” 29| Ditto. Very faint. | 6.15 P.M.

” ” 31| 4 P.M. Same. | Sky obscured.

1876, January 1| 5 P.M. and 11 P.M. |

| Slight. | A few clouds.

” ” 2| 9 P.M. Described and |

| figured. | 9 P.M.

” ” 17| Very bright sky. Very |

| faint streamers. | 9.25 A.M.

” ” 18| 9.45 P.M. and 10.5 P.M. |

| Character not recorded. | 10.15 P.M.

” ” 19| Very bright sky. Faint. | 9.45 P.M.

” ” 20| 2 A.M. Slight. | 2.30 A.M.

” ” 23| 7.55 A.M. and 2 P.M. |

| Slight. | 8.45 P.M.

” ” 24| Bright sky. Slight flash.| 2 A.M.

” ” 24| 5 P.M. and 11.15 P.M. |

| Faint Aurora. | Very clear sky.

” ” 27| 2 A.M. to 3.45 A.M. |

| Faint. | 1 A.M. to 4 A.M.

” ” 27| Very bright sky. Faint |

| double arch. | 8.30 P.M.

” ” 28| 6 P.M. and 7 to 9 P.M. |

| Faint flashes. | 7.20 P.M.

” ” 30| 8 P.M. Streak. | 7.50 to 9 P.M.

” ” 31| 8.30 A.M. and 7.30 |

| P.M. Very faint. | 8.25 A.M., 5.30 P.M.

” February 3| 10 P.M. Slight flash. | Very clear sky.

” ” 11| Sky obscured. Very faint.| 11 P.M.

” ” 13| 11 P.M. Flashes. | Clear sky.

” ” 14| 2 A.M., 9.15 to 10 P.M. |

| Described and figured. | 2 A.M. and 11.50 P.M.

” ” 19| 9.45 P.M. | 11 P.M.

” ” 20| 2 A.M. Very faint. | 2.30 A.M.

” ” 22| 2 A.M. Character not |

| recorded. | Very clear sky.

” ” 24| Bright sky. Very faint. | Midnight.

” ” 26| 10 P.M. and 11 P.M. |

| Faint flashes. | Sky obscured.

----------------+--------------------------+-----------------------

I have added to the above Table the character of the Aurora in each

instance as taken from the fuller descriptions given.—J. R. C.

_Auroras and Magnetic Disturbances._

The appearances of Auroras and the synchronous movements of the

declinometer-magnet were subjects of special observation during the stay

of the ‘Alert’ and ‘Discovery’ at their winter-quarters. The Table on

page 187 gives the dates and hours when Auroras were visible. On all

occasions they were observed to be faint, with none of those brilliant

manifestations which are described by our own officers as seen at Point

Barrow, and by the Austro-Hungarian Expedition at Franz-Josef Land, where

the magnetical instruments were so sensibly disturbed.

These phenomena were not observed either in the ‘Alert’ or the

‘Discovery,’ especially no connexion between magnetical disturbances and

the appearances of Auroras could be traced.

This is quite in accordance with the remarks of previous observers within

the region comprehended between the meridians of 60° and 90° west, and

north of the parallel of 73° north. For example:—

In the Phil. Trans. 1826, Part IV. p. 76, Capt. Parry and Lieut. Foster

remark, in the discussion of their magnetical observations at Port

Bowen:—“As far, however, as our own observations extended, we have

reason to believe that on no occasion were the needles in the slightest

degree affected by Aurora, meteors, or any other perceptible atmospheric

phenomenon.”

Again, in the Smithsonian Contributions, vol. x., 1858, Mr. A. Schott,

in his discussion of Dr. Kane’s observations at Van Rensselaer Harbour,

in 1854, remarks—“In conformity with the supposed periodicity of this

phenomenon as recognized by Professor Olmstead, no brilliant and complete

Auroras have been seen; with an exception of very few, they may all be

placed in his fourth class, to which the most simple forms of appearances

have been referred.” The following statement is given in the same page

as a footnote:—“The processes have no apparent connexion with the

magnetic dip, and in _no_ case did the needle of our unifilar indicate

disturbance.”

* * * * *

The following description of the Aurora observed on 21st November, 1875,

is given by Commander Markham and Lieut. Giffard, in their abstract of

observations at Floeberg Beach:—

“Between 10 and 11 P.M. bright broad streamers of the Aurora appeared

10° or 15° above the north horizon, stretching through the zenith, and

terminating in an irregular curve about 25° above the south horizon,

bearing S.S.W. During the Aurora’s greatest brilliancy the magnet was

observed during five minutes to be undisturbed.”

[_Note._—I applied for a loan of the lithographic stones

to enable me to give copies of the three diagrams of

Auroræ referred to in the Arctic “Results;” but the Lords

Commissioners of H.M. Treasury refused this, except on the

terms of my paying one third of the original cost of production

of such diagrams. I did not think it worth while to accept

these conditions. Only one of the drawings has any special

interest; and this is a “curtain” Aurora, similar to that

figured on Plate II. of this work.—J. R. C.]

APPENDIX D.

THE AURORA AND OZONE.

[Sidenote: Aurora and ozone. Dr. Allnatt’s notes and conclusions deduced

therefrom.]

While Part I. was in the press, Dr. Allnatt, formerly of Frant, and for

many years the well-known meteorological contributor to ‘The Times’

newspaper, kindly placed at my disposal his large series of notes. Upon

an examination of these we came to the following conclusions:—

1. That Auroral periods are also periods of comparative abundance of

ozone.

2. That instances are by no means wanting in which an abnormal

development of ozone appears to be coincident with the manifestation of

an Aurora.

[Sidenote: Year 1870 remarkable for sun-spots, auroræ, and ozone.]

In reference to the first point, it is found, as the result of an

examination of Dr. Allnatt’s notes, that particular years and months are

notable at once for Auroræ and for ozone in abundance. 1870 was one of

these years, and was specially recorded by Dr. Allnatt, in his ‘Summary

for the Year,’ as remarkable for sun-spots, Auroræ, and ozone.

[Sidenote: Particulars of some of the monthly records.]

The month of February in that year was marked by intense cold and

brilliant Auroræ. Atmospheric electricity was feeble, but ozone was,

throughout the month, well developed; and there was no tangible period of

antozone.

In the month of April of the same year, eight days consecutively (19th to

26th) were marked for ozone 10, the maximum of Dr. Allnatt’s scale.

In May of the same year there were magnificent Auroræ, and atmospheric

electricity was intense. Ozone was scanty; but this was accounted for by

the wind being generally E.N.E., ozone being mostly developed with a W.

or S.W. wind, and a moist state of the atmosphere.

In August 1870 the unusually large number of 22 days were recorded for a

maximum of ozone.

September 1870 was hardly less remarkable, with 19 days of maximum. It

was recorded that there were splendid Auroræ during this month, and the

solar spots were very large.

October 1870 had 20 days of maximum ozone, and November had several

fine Auroræ and maxima of ozone noted. In fact, nearly every month in

that year was referred to by Dr. Allnatt for displays of Aurora (of

both Arctic and Antarctic forms) and for a development of ozone very

considerably above the average.

[Sidenote: Year 1871.]

The year 1871 had more or less of the same character. In the month

of October of that year, fine Auroræ were prevalent, and ozone was

registered as at its maximum during 22 days.

There seems reason to conclude that if a systematic comparison of annual

or other periods of Aurora and ozone development were made, it would

result in disclosing a connexion (probably an intimate one) between the

two phenomena.

[Sidenote: Instances showing a connexion between a specific Aurora and an

ozone maximum.]

With reference to the second point, the following (among other) instances

may be quoted, for the purpose of showing a connexion between a specific

Aurora and an ozone maximum.

The Aurora of 24th September, 1870, was splendid and universal, being

seen in Europe, Asia, Africa, America, and Australia. Ozone reached, on

the morning of the 24th, 8 of the scale (the scale running from 1 to 10),

and, on the morning of the 25th, 10, the maximum.

In October 1870 there were grand displays on the 14th, 20th, 22nd, 24th,

and 25th, and ozone was correspondingly abundant, as is seen by the

following Table:—

+-------------------+----------+------+------------------------+

| Date. | Aurora. |Ozone.| |

+-------------------+----------+------+------------------------+

|1870, October 14th.|Aurora. | 8 |The display of the 24th |

| ” ” 20th.|Aurora. | 10 | was accompanied by |

| ” ” 21st.|None seen.| 5 | the formation of a |

| ” ” 22nd.|Aurora. | 10 | corona, and that of |

| ” ” 23rd.|None seen.| 8 | the 25th was splendidly|

| ” ” 24th.|Aurora. | 10 | seen in Edinburgh. |

| ” ” 25th.|Aurora. | 8 | |

+-------------------+----------+------+------------------------+

The foregoing figures somewhat point to the conclusion that ozone

quantity rises and falls coincidently with the Aurora displays.

The following seems, however, a case still more strongly in point.

+-------------------+-------+---------------------+------+

| Date. | Wind. | Aurora. |Ozone.|

+-------------------+-------+---------------------+------+

|1871, January 25th.| E.S.E.| None seen. | 0 |

| ” ” 26th.| N.N.W.| None seen. | 2 |

| ” ” 27th.| E.S.E.| Aurora at night in | |

| | | N. and S. horizons.| 10 |

| ” ” 28th.| E. | None seen. | 8 |

| ” ” 29th.| S.E. | None seen. | 2 |

+-------------------+-------+---------------------+------+

It is curious, in examining the above Table, to note how the ozone rose,

notwithstanding an east wind, from 0 on the 25th, and 2 on the 26th, to

10 on the 27th, when the Aurora appeared, and 8 on the 28th, when it

might have lingered; and how it again descended to 2 on the 29th.

The case of the Aurora of 6th of October, 1869, when a broad belt of

Aurora was in the north, is also an illustrative one, as will be seen by

the following data:—

+------------------+------+------+-------+

| Date. |Wind. |Ozone.|Aurora.|

+------------------+------+------+-------+

|1869, October 5th.|S.S.W.| 1 | — |

| ” ” 6th.|S.S.E.| 5 |Aurora.|

| ” ” 7th.|S.S.W.| 10 | — |

| ” ” 8th.| S. | 10 | — |

| ” ” 9th.| S.E. | 5 | — |

+------------------+------+------+-------+

The Aurora of the night of the 6th was here represented by the

ozone-paper of the morning of the 7th with a maximum of 10, which lasted

till the 8th.

[It should be borne in mind, in examining these Tables, that the Aurora

is of the night of the given date, while the ozone-papers are taken and

recorded in the morning of the date quoted.]

[Sidenote: Other instances.]

We will now take instances where the ozone has not reached its maximum;

but even in these cases a certain amount of rise and fall of the ozone

development towards and from the Aurora is traceable.

+-------------------+--------+------+-------------------------+

| Date. | Wind. |Ozone.| Aurora. |

+-------------------+--------+------+-------------------------+

|1871, April 8th| S.S.E. | 5 |Aurora on 9th, but |

| ” ” 9th| S.S.E. | 8 | wind E. and unfavourable|

| ” ” 10th| S.E. | 5 | to ozone. |

+-------------------+--------+------+-------------------------+

|1871, November 9th| N. | 5 |Aurora on all three |

| ” ” 10th| N.W. | 8 | nights. |

| ” ” 11th| N. | 5 | |

+-------------------+--------+------+-------------------------+

|1872, February 3rd| S.W. | 4 |Aurora on night of the |

| ” ” 4th| S.S.W. | 5 | 4th represented by |

| ” ” 5th| S.W. | 8 | ozone-paper of morning |

| ” ” 6th| S.W. | 5 | of the 5th. |

+-------------------+--------+------+-------------------------+

Other cases are, we are bound to say, found, in which ozone was either

not remarkable for quantity, or positively fell during the Aurora, as,

for instance, this:—

+-----------------+------+------+------------------------------+

| Date. | Wind.|Ozone.| Aurora. |

+-----------------+------+------+------------------------------+

|1874, March 16th |W.N.W.| 6 |Aurora on the 18th represented|

| ” ” 17th | S.W. | 6 | by test-paper of |

| ” ” 18th | W. | 5 | the 19th with only two |

| ” ” 19th |S.S.W.| 2 | degrees of discoloration. |

+-----------------+------+------+------------------------------+

It is, however, possible that such instances may be accounted for, either

by some reaction on the test-papers after they have been coloured, or

by some accidental antagonistic circumstance affecting the tests. The

following is a case well illustrating this:—

+------------------+------+------+---------------------------------+

| Date. | Wind.|Ozone.| Aurora. |

+------------------+------+------+---------------------------------+

|1874, January 31st|N.N.W.| 6 |There was an Aurora on |

| ” February 1st| N.W. | 8 | the night of the 2nd represented|

| ” ” 2nd| N.W. | 2 | by the ozone-paper |

| ” ” 3rd|N.N.W.| 4 | (4 only) on the |

| ” ” 4th|E.N.E.| 8 | morning of the 3rd. |

+------------------+------+------+---------------------------------+

This instance would seem strongly opposed to the theory of a connexion

between Aurora and ozone but for the fact that on the 2nd, when the

Aurora was seen at night, and on other days in the same month, Dr.

Allnatt has recorded a strong wave of antozone to have swept over the

whole of England, and blanched the ozone-papers, however deep their

coloration might have previously been. Indeed, it is easy to understand

that some antozonic influence may, at times, disturb the evidence of the

test-papers, even in so elevated and apparently pure an atmosphere as

that of Frant.

It may not be considered that the foregoing instances are enough to

establish a case of ozone=Aurora; but there seems, at least, sufficient

to base a requisition for further inquiry upon.

It would, too, be interesting to investigate whether Auroræ and ozone

development are respectively localized. Mr. Ingall’s fine Aurora, seen

at Champion Hill, S.E., July 18th, 1874 (_antè_, pp. 22 and 23), was not

observed at Frant, and the ozonoscopes there were described as blanched

by antozone.

APPENDIX E.

INQUIRIES INTO THE SPECTRUM OF THE AURORA.

BY H. C. VOGEL.[18]

The frequent appearance of the Aurora in the past winter, as well as

this spring, has given me opportunity to institute exact inquiries into

the spectrum of the Aurora. It is known that the nature of Auroræ is as

yet but little explored. It has been considered necessary to abandon

the former view—that they are discharges of the electricity collected

at the poles—because it has been hitherto found impossible to bring the

chief lines of the Aurora-spectrum into coincidence with the spectra of

the atmospheric gases. Theoretical considerations, based on the great

alterations to which the spectrum of the same gas is subject under

varying conditions of temperature and density, have very recently led

Zöllner to the opinion that probably the spectrum of the _Aurora does not

coincide with any known spectrum_ of the atmospheric gases, only because

it is a spectrum of another form of our atmosphere hitherto incapable of

artificial demonstration[19].

The following article will show how far I have succeeded, in conjunction

with Dr. Lohse, in supporting this view by exact observations of the

Aurora-spectrum itself, as well as by comparison with the spectra of the

gases constituting the air.

The star-spectrum apparatus belonging to the 11-inch equatorial of the

Bothkamp Observatory was used for these observations. It consists of

a set of prisms _à vision directe_, five prisms with refracting angle

90°, slit, collimator, and observing telescope. The lowest eyepiece

(magnifying four times) of this telescope was employed. The telescope is

capable of being moved in such a way, by the aid of a micrometer-screw,

that different portions of the spectrum can be brought into the centre

of the field of vision. As fractions of the rotation of this screw are

marked, the distances of the spectral lines can be readily found.

Repeated measurements of 100 lines of the solar spectrum have enabled

me, upon the basis of Ångström’s Atlas (‘Spectre normal de Soleil’), to

express the indications of the screw directly in wave-lengths.

In place of the cross wires originally introduced into the focus of the

observing telescope, I have inserted a tiny polished steel cone, the very

fine point of which reaches to the centre of the field of vision. The

axis of this cone stands perpendicular to the length of the spectrum,

therefore parallel with the spectral lines, and the setting of the point

of the cone on the latter is accomplished with great sharpness. If the

spectrum is very faint, or consists only of bright lines, the cone is

lighted by a small lamp. For this purpose, opposite to the point of the

cone, there is an opening in the telescope, through which, regulated by

a blind, light can be thrown on the point. As the latter is polished, a

fine line of light thus appears, which extends to the centre of the field

of vision, and the brilliancy of which can be altered by withdrawing

the lamp to a greater distance or lowering the blind, so that even the

faintest lines of a spectrum can be brought with facility and certainty

into coincidence with this line of light.

The head of the micrometer-screw is divided into 100 parts, and each

part, in the neighbourhood of the Fraunhofer line F, answers to about

·00016 wave-length. The probable error of position on one of the

well-marked lines in the sun’s spectrum amounts to about 0·008 of a turn

of the screw with the lowest eyepiece of the telescope. I have subjected

the screw itself to a thorough examination with reference to such range,

as well as to periodical inequalities in the single worms of the screw,

but could discover no error exceeding 0·01 of a turn of the screw. I

have to mention, further, that after each observation in the position in

which the instrument was used, readings followed on the sodium-lines,

or on some of the hydrogen-lines, in order to eliminate errors which

might arise in the unavoidable disturbance of any particular part of the

spectral apparatus.

1. _Observations of the Aurora._

1870, Oct. 25th.—A very bright Aurora. In the brightest parts, besides a

very bright line between D and E, several other fainter lines were to be

discerned, situated further towards the blue end of the spectrum. They

appeared on a dimly-lighted ground, and stretched out over the Fraunhofer

lines E and _b_ to about midway between _b_ and F. Towards the red end

the spectrum was terminated by the bright line first mentioned. No

measurements could be taken, as the apparatus had not yet undergone the

above-mentioned alterations, and even the brightest line of the spectrum

did not diffuse sufficient light to be able to perceive the fine cross

wires. The red rays of the Aurora were not examined.

1871, Feb. 11th.—Towards ten o’clock appeared in the north-west a very

bright light-bow of greenish colour as the edge of a dark segment.

Even with a very narrow slit, the line between D and E could be well

recognized and measured. The average of six readings gave 7·11 turns,

equal to 5572 wave-length. In a small spectroscope of low dispersion

which is arranged on Browning’s plan, a few more lines placed further

towards the blue could be recognized (as in October). Towards the red end

of the spectrum no lines were observable. The greatest development of the

Aurora was about midnight. Magnificent rays rose to about 60° elevation;

they had the same greenish colouring as the bow of light, and the

appearance of the spectrum also was exactly the same. I again obtained

two sets of measurements: the average of six readings in the first set

gave 7·10 turns, 5572 wave-length; in another part of the heavens at the

same time 7·10 was the result of four readings.

On Feb. 12, towards eight o’clock, the intensity of the Aurora was

already great enough to allow measurements of the brightest line. The

average of six readings gave 7·09 turns, or 5576 wave-length. Dr. Lohse

took observations later, with the same apparatus, and found from six

readings 7·12 turns, or 5569 wave-length.

Yet the appearance of the spectrum in the spectroscope of low dispersion

was essentially distinct from that of February 11th. The green continuous

spectrum was present; it extended from the bright Aurora-line to the

lines _b_ of the solar spectrum, and was traversed by some bright lines.

Between band _b_ and F, was another line standing alone, out beyond F, in

the blue part of the spectrum, a clear bright stripe; and just before G a

very faint broad band of light was perceived.

Amongst the rays which, later on, shot upwards, and were coloured red

at their ends, another very intense red line appeared in the spectrum

between C and D, yet placed nearer to C[20].

April 9th.—An exceedingly brilliant Aurora, of which the greater

development took place in the early morning hours. Magnificent red

sheaths rose up to the zenith. The spectrum was like that observed on

February 12th, only much more intense, so that the lines could be seen

and measured with the larger spectral apparatus. In the brightest part of

the Aurora was the dark segment; the spectrum consisted of five lines in

the green, and a somewhat indistinct broad line or band in the blue.

The red rays, on the other hand, allowed us to recognize seven lines,

whilst the bright line again appeared in the red part of the spectrum.

I could not again perceive the faint stripe observed on February 12th,

in the vicinity of line G. The mean measurements of four readings on an

average, for each line, gave:—

------+-------+-------+-------+------------------------------------------

Turns | Prob- | | Prob- |

of | able | Wave- | able |

screw.|errors.|length.|errors.| Remarks.

------+-------+-------+-------+------------------------------------------

4·62 | ·0037 | 6297 |·00014 |Very bright stripe. } On

7·12 | 9 | 5569 | 2 |Brightest line of the spectrum; } a

| | | | becomes noticeably fainter at } faintly

| | | | appearance of the red line. } lighted

7·92 | — | 5390 | — |Extremely faint line; unreliable } ground.

| | | | observation. }

8·71 | 21 | 5233 | 4 |Moderately bright.

8·95 | 49 | 5189 | 9 |This line is very bright when the red line

| | | | appears at the same time, otherwise equal

| | | | in brilliancy with the preceding one.

10·06 | 20 | 5004 | 3 |Very bright line.

12·33 | — | 4694 | — |}Broad band of light, somewhat less

12·59 | 22 | 4663 | — |} brilliant in the middle.

12·88 | — | 4629 | 3 |Very faint in those parts of the Aurora

| | | | in which the red line appears.

------+-------+-------+-------+------------------------------------------

April 14th.—Faint Aurora; only the bright line in the green could be

recognized in its spectrum. The mean of two readings gave 7·12 turns, or

5569 wave-length.

I append a table of the wave-lengths of the brightest line, as exactly

measured on four evenings:—

1871, February 11 5573

” 12 5573

April 9 5569

” 14 5569

Therefore the average result (if only half-weight is allowed to the

last observation, because it only depends upon two readings) gives for

the wave-length of the brightest line 5571·3, with a probable error of

·000·92. According to Ångström[21], the wave-length of this line is 5567;

according to Winlock[22], on the other hand, 5570.

2. _On the Spectra of some Gases in Geissler’s Tubes, as well as on the

Spectrum of the Atmospheric Air._

Numerous experiments have been made in order to find out some admitted

connexion between the spectrum of the Aurora and the spectra of the

principal gases composing the atmosphere. I limit myself to noticing some

of the often-repeated observations in Plücker’s tubes, which contained

oxygen, hydrogen, and nitrogen, as well as the observations of the

spectrum of the air under different conditions. The experiments were

made with a small inductive apparatus, in which the length of the spark

between platinum points in ordinary air was 15 millims. at the most. As

Zöllner (in the pamphlet mentioned) comes to the conclusion, that if

the development of the light in the Aurora, according to the analogy of

gases brought to glow in rarefied spaces, is of an electric nature, it

must belong to very low temperature—in order to bring the gases enclosed

in the tubes to glow at the lowest possible temperature, I have always

employed such weak currents that the gas was only just steadily alight.

The following observations have been repeated often and at various times.

The figures are averages of the indications of the micrometer-screw, so

that the uncertainty of the figures will, in the rarest cases, amount

to no more than 0·015 turn of the screw, and must be reckoned somewhat

more highly only in the case of completely faint misty lines. The

spectrum apparatus was that described above, and the slit was nearly

the same in every experiment, and so narrow that the sodium-lines could

be seen separated. The measurements, for the most part, extend only to

the Fraunhofer line G, as I feared lest, through further turning the

telescope by means of the micrometer-screw, too great a pressure might be

exercised on the worms of the latter.

I. _Oxygen._

_a._ In the narrow part of the Plücker tube.

+------+------------+--------------------------------------------------+

|Screw.|Wave-length.| Remarks. |

+------+------------+--------------------------------------------------+

| 3·97 | 6562 |Moderately bright. |

| 5·04 | 6146 |Very bright. |

| 6·98 | 5603 |Very bright, misty towards the violet. |

| 8·19 | 5332 |Faint. |

| 8·95 | 5189 |Moderately bright. |

|10·97 | 4870 | ” ” |

|11·02 | 4863 |Faint. |

|11·26 | 4829 |Bright; misty towards the red end of the spectrum.|

|13·30 | 4583 |Very faint. |

|14·05 | 4506 |Moderately bright. |

|15·55 | 4372 | ” ” |

+------+------------+--------------------------------------------------+

_b._ In the wide part of the Plücker tube.

+------+------------+------------------+

|Screw.|Wave-length.| Remarks. |

+------+------------+------------------+

| 6·98 | 5603 |Very faint. |

| 8·95 | 5189 |Very bright. |

|11·26 | 4829 |Moderately bright.|

+------+------------+------------------+

The lines near 3·97 and 11·02 belong to hydrogen. Probably traces of

aqueous vapour were present in the tube, which were decomposed by

the galvanic current. These two lines are not to be found in a lower

temperature in the broad part of the tube. It is striking that the red

nitrogen-line near 5·04 is also missing there. In the narrow part of the

tube the lines stand out in the green on a very dimly-lighted ground,

whilst in the wider part they appear on a perfectly dark ground.

II. _Hydrogen._

_a._ In the narrow part of the tube.

+------+------------+--------------------------------------------------+

|Screw.|Wave-length.| Remarks. |

+------+------------+--------------------------------------------------+

| 3·98 | 6558 |Very bright. |

| 6·16 | 5813 |Moderately bright, on both sides very faint lines.|

| 7·01 | 5596 |Moderately bright.} On a dimly lighted ground, |

| 7·18 | 5555 |Moderately bright.} which becomes fainter towards|

| 7·77 | 5422 |Faint. } the violet. |

| | | |

| 8·95 | 5189 |Moderately bright.} |

|10·03 | 5008 |Faint. } On a faint steadily bright |

|10·55 | 4929 |Moderately bright.} ground. |

| | | |

|11·04 | 4861 |Very bright. } From 11·5 to 12·9 a bright |

|12·86 | 4632 |Moderately bright.} ground, which towards the |

| | | } violet becomes very bright. |

| | | |

|13·32 | 4581 |Very faint. } |

|14·05 | 4506 | ” } On a dull ground. |

|15·90 | 4342 |Very bright. } |

+------+------------+--------------------------------------------------+

_b._ In the broad part of the tube.

+-------+------------+------------------+

| Screw.|Wave-length.| Remarks. |

+-------+------------+------------------+

| 5·30 | 6063 |Faint. |

| 7·00 | 5598 |Bright. |

| 8·96 | 5187 |Very bright. |

| 11·28 | 4828 | ” |

| 14·04 | 4507 |Moderately bright.|

+-------+------------+------------------+

The lines appeared on a perfectly dark ground.

The tube shows in the narrow part the hydrogen-spectrum of the first

order; the lines in the green do not coincide with the lines of the

nitrogen, though some lines belonging to nitrogen are found. Here, too,

most probably small particles of aqueous vapour have been enclosed in

the tube and are decomposed. Very striking is the spectrum in the broad

part of the tube; nothing is to be seen of the bright shining lines Hα

3·98, Hβ 11·04, Hγ 15·90; on the other hand, four very bright lines and

one quite faint one are in the red end of the spectrum, which appear,

in opposition to the spectrum of the narrow part, not on a partially

lighted, but on an entirely dark ground. The appearance is very striking

if we bring the tube in front of the slit; and so, by degrees, at first

the light in the narrow part, then the light at the connecting-point of

the narrow and wide parts, and, finally, the light in the latter fall

upon the slit. At the connecting-point of the wide ends of the tube the

three well-known hydrogen-lines decrease in intensity, the continuous

ground of some parts of the spectrum disappears, and a new line appears

near 11·28, which has about the same brilliancy as Hβ.

A comparison with the spectrum of oxygen shows the bright lines which are

in the spectrum in the wide end of the tube as belonging to that element.

The heat evolved by the current appears insufficient to bring the

hydrogen to glow, whilst by it the oxygen, which is of a more rarefied

character, becomes incandescent. An alteration of the direction of the

current has no influence on the appearance.

III. _Nitrogen._

_a._ In the narrow part of the tube.

+------+------------+--------------------------------------------------+

|Screw.|Wave-length.| Remarks. |

+------+------------+--------------------------------------------------+

| 3·84 | 6620 |}Several faint, broad, close lines, increasing in |

| 4·85 | 6213 |} brilliancy as they approach the violet end. |

| | | |

| 5·30 | 6063 |}Broad bright lines, |

| 5·51 | 6000 |} so close together |

| 5·69 | 5948 |} that the intervening spaces appear |

| 5·87 | 5896 |} like fine dark lines. This part of |

| 6·04 | 5846 |} the spectrum is very bright, but |

| 6·20 | 5802 |} not uniform, being brighter towards |

| 6·43 | 5741 |} the violet end. |

| | | |

| 6·96 | 5607 |}Group of faint but at the same time |

| 7·13 | 5567 |} very broad lines. The |

| 7·28 | 5532 |} last is the brightest. |

| | | |

| 7·55 | 5470 |}The dark intervening spaces are somewhat broader,|

| 7·74 | 5428 |} the bright lines somewhat more intense than |

| 7·92 | 5389 |} in the preceding group, and all of almost equal |

| 8·09 | 5353 |} brilliancy. |

| | | |

| 8·32 | 5306 |} |

| 8·50 | 5272 |}Very faint fine lines. |

| 8·69 | 5237 |} |

| | | |

| 9·01 | 5178 |Very bright broad misty line. |

| | | |

| 9·67 | 5066 |Very bright line. } |

|10·25 | 4975 |” ” } |

|10·66 | 4913 |” ” } The bright lines |

|11·03 | 4862 |Very faint line. } are sharply |

|11·41 | 4811 |Bright line. } defined towards |

|12·11 | 4721 |” ” } the red end of |

|12·57 | 4666 |Faint line. } the spectrum, |

|12·57 | 4644 |Bright, broad, misty line. } fading away towards |

|13·42 | 4570 |Very bright line. } the other end of |

|14·24 | 4487 |” ” } the spectrum. |

|15·02 | 4417 |Bright line. } |

| | | |

|15·66 | 4363 |Bright lines. } Bright lines sharply defined |

|15·72 | 4357 | ” } towards red end, indistinct |

|15·87 | 4345 |Bright line. } towards other end of |

|16·72 | 4273 | ” } spectrum. |

+------+------------+--------------------------------------------------+

Here follow several lines.

_b._ In the wide part of the tube.

+-------+------------+------------------------------+

| Screw.|Wave-length.| Remarks. |

+-------+------------+------------------------------+

| 6·20 | 5802 |Faint, indistinct, broad line.|

| 7·72 | 5433 |Dull stripe. |

| 8·20 | 5330 |Faint line. |

| 8·94 | 5191 |Very faint line. |

| 9·03 | 5175 |Broad band of light. |

| 9·90 | 5029 |Dull band of light. |

| 10·68 | 4911 |Moderately bright line. |

| 11·42 | 4809 |Faint line. |

| 12·59 | 4663 |Bright line. |

| 13·43 | 4569 | ” |

| 14·07 | 4504 |Moderately bright line. |

| 14·25 | 4486 |Very bright line. |

| 15·85 | 4347 | ” ” |

| 16·76 | 4273 |Moderately bright line. |

+-------+------------+------------------------------+

_c._ At the aura of the negative pole.

+-------+------------+------------------------------------------------+

| Screw.|Wave-length.| Remarks. |

+-------+------------+------------------------------------------------+

| 5·18 | 6100 |} Broad, moderately bright stripe, indistinct |

| 5·70 | 5945 |} at the edges. |

| | | |

| 7·60 | 5159 |} Broad, moderately bright stripe. |

| 8·41 | 5289 |} |

| | | |

| 8·76 | 5224 |{ Very bright line, somewhat indistinct towards |

| | |{ the violet. |

| | | |

| 9·19 | 5147 |Faint line. |

| 10·00 | 5004 |Bright line, indistinct towards the red. |

| | | |

| 10·67 | 4912 |{ Somewhat fainter than the last, indistinct |

| | |{ towards the red. |

| | | |

| 11·43 | 4808 |Very faint line. |

| 12·25 | 4704 |Very intense, broad, indistinct towards |

| | | the violet. |

| 12·73 | 4646 |Very faint line. |

| 13·43 | 4569 |Moderately bright, indistinct towards the |

| | | violet. |

| 14·25 | 4486 |Like the last. |

| 15·03 | 4417 |Quite a faint line. |

| 15·86 | 4346 |Moderately bright line. |

| 16·76 | 4275 |Very bright line. |

+-------+------------+------------------------------------------------+

Here follow several other lines.

The observations in the different parts of the tube show plainly the

dependence of the spectrum on the temperature. The aura of the negative

pole gives the line near 10·07 so characteristic of the air-spectrum.

This is the same line which is met with in the spectra of most of the

nebulæ. The very striking groups of lines in the red and yellow in the

spectrum of the narrow part of the tube disappear entirely in the wide

part. If we compare the spectra with those above quoted, of oxygen and

hydrogen, we find line Hβ very faint in the spectrum of the narrow part

of the tube near 11·03; on the other hand, oxygen-lines appear in the

broad part near 8·20, 8·94, and 14·07. Thence I would conjecture that

the tube was not filled with pure nitrogen, the appearance of which is

precise, but with dry rarefied air, since Wüllner’s researches have

proved that dry air yields the same spectrum as nitrogen gas. Perhaps

the air in the tube examined by me had not been thoroughly dried, and

thus the appearance of some lines of the elements before named is to be

explained.

I must further mention that the electrodes of the tubes consisted of

aluminium; yet a comparison of the spectra observed and the aluminium

spectrum has shown no connexion between them.

IV. _Atmospheric Air._

+-----------+------------+------------------------------------------+

| Screw. |Wave-length.| Remarks. |

+-----------+------------+------------------------------------------+

| 5·88 | 5892 |Very bright double line (Na). |

| 6·67 | 5680 |Very bright line. |

| 7·20 | 5550 |Faint line. |

| 9·00 | 5180 |Very bright line. |

| 9·79 | 5047 |Fine faint line. |

| 10·03 | 5008 |} Very bright double line. |

| 10·07 | 5002 |} |

| 11·43 | 4803 |Faint confused line. |

| 12·69 | 4651 |} |

| 12·84 | 4633 |} Faint line not sharply defined. |

| 13·04 | 4612 |} |

| | | |

|From 14·61 | 4453 |} Confused band of light, which ends with |

|to 15·88 | 4444 |} a broad washy line. |

+-----------+------------+------------------------------------------+

Here follow several other lines.

Rarefied air saturated with aqueous vapour.

+----------+------------+----------------------------------------------+

| Screw. |Wave-length.| Remarks. |

+----------+------------+----------------------------------------------+

| 3·97 | 6562 |Moderately bright line. |

| (5·88)| 5892 |Bright double line (Na). |

| (6·25)| 5789 |Bright line (H). |

|From 7·03 | 5591 |}Broad dull band of light; near 7·03 a |

|to 7·55 | 5470 |} somewhat brighter line. |

| 7·59 | 5461 |Bright line (H). |

| 8·72 | 5231 |Dull stripe. |

| 8·96 | 5187 |Broad misty stripe.} On a dull steady ground. |

| 10·07 | 5002 |Faint line. } |

| 11·05 | 4859 |Very bright line. |

| 12·21 | 4709 |Moderately bright line.} On dimly lighted |

| 12·75 | 4644 |{Line fainter than the } ground, becoming |

| | |{ preceding. } fainter towards |

| 13·28 | 5585 |Very faint line (H). } the violet. |

| (15·71)| 4358 |Very bright line (H). |

| 15·90 | 4341 | ” ” |

+----------+------------+----------------------------------------------+

Here follow several more lines.

In the first observations, the electric spark, about 1 centim. in length,

was allowed to pass between platinum points in ordinary air.

The sodium-line near 5·88 appeared continually. The bright double line at

10·03 and 10·07, with a weaker current or longer spark, was no longer to

be recognized as a double line, but appeared as a broad somewhat confused

line, of which the brightest part was near 10·05. No lines belonging to

the platinum spectrum appeared. Ordinary rarefied air, under a pressure

of 25 to 30 millims., and which was enclosed by mercury in a tube 8

millims. wide, showed exactly the same lines as Plücker’s nitrogen-tube

(_b_), except that some lines belonging to the spectrum of mercury also

appeared.

This observation may be regarded as a confirmation of the conjecture

above expressed as to the condition of Plücker’s tube III. (nitrogen).

In the observations described under _b_, the air saturated with aqueous

vapour was under a pressure of 22 millims. Besides the sodium-lines,

lines of the mercury-spectrum appeared at 6·25, 7·59, and 15·71. The

spectrum of rarefied air under similar pressure was found to accord

completely with the spectrum of the light in the broad part of Plücker’s

tube.

III. (Nitrogen _b._)—A comparison of the spectrum of rarefied air

saturated with aqueous vapour with the former shows the striking

alterations in the spectrum which are brought about by the presence of

the aqueous vapour.

3. _Comparison of the Aurora-Spectrum with the Spectra of Atmospheric

Gases and of Inorganic Substances._

In the next place, I turn to the comparison of the observed spectra

of different gases and of the air with the spectrum of the Aurora.

The first band of light in the red part of the Aurora-spectrum most

probably coincides with the first system of lines in the spectrum of

nitrogen (_a_). Probably only the bright part of this group of lines

is perceptible, on account of the extreme faintness of the Aurora; and

as in nitrogen the increase of the brilliancy of the spectrum takes

place towards the violet end, the absence of the intermediate spectrum

towards this direction would find its explanation. The most intense line

of the Aurora-spectrum at 7·12 is to be also found in the spectrum of

nitrogen (_a_)—as a very faint line, however. That this line appears

in the Aurora by itself, and with intensity relatively great, need not

appear strange, considering the great alteration of the gas-spectra under

different conditions of pressure and temperature. The third line of the

Aurora-spectrum, very vaguely defined on account of its great faintness,

coincides in the same way with a nitrogen-line.

The line at 8·71 is met with in the nitrogen-spectrum (_c_), as well

as in the air-spectrum (_b_). The third line of the oxygen-spectrum at

8·95, which _seems to appear under very different conditions_, is found

again, as the fifth line in the spectrum of the Aurora. _Moreover, the

sixth line in the Aurora at 10·06 coincides very exactly_ with the known

nitrogen-line appearing in the spectra of some of the nebulæ. Lastly, as

to the broad band of light in the Aurora-spectrum from 12·33 to 12·88,

several lines are found in this place in the spectrum of nitrogen as well

as the air-spectrum (_a, b_); so that here, too, a coincidence between

the spectra may be regarded as probable.

The observations show with some certainty that at least one line at 10·06

of the Aurora-spectrum coincides with the maximum brilliancy of the

air-spectrum, and that the other lines appear with great probability in

the spectra of atmospheric gases.

In the very great difference of the gas-spectra under varying conditions

of pressure and temperature, it would indeed be difficult to succeed

in producing artificially a spectrum which should resemble that of the

Aurora in all parts. Moreover, it must be admitted, under the hypothesis

that the Auroræ are electric discharges in rarefied air-strata, that

these strata, qualified for the transmitting of electricity, will have a

very considerable thickness.

In this case the conditions of pressure on these air-strata are

themselves so different that, within certain limits, each will yield its

own peculiar spectrum; but we shall see the sum of collective spectra, so

to speak, spread out behind each other; and therefore the impossibility

of attaining a perfect agreement between the Aurora-spectrum and the

artificially exhibited spectra of mixed gases is evident.

A comparison of the Aurora-spectrum with the spectra of inorganic

substances may be easily worked out by the help of the above-quoted

wave-lengths of the single lines of the former, with due regard to

probable errors, and with the aid of Ångström’s Atlas of the Solar

Spectrum. Here the perfect harmony of the brightest Aurora-line (which

was fixed with an exactitude of about one seventh of the separation of

the sodium-lines) with the lines of the iron-spectrum is especially

striking. The wave-lengths in the above-cited observations of the bright

Aurora-line vary between 556·9 and 557·3, whilst, according to Ångström,

two lines of the iron-spectrum are situated at 556·85 and 557·17.

Iron-lines corresponding to the other Aurora-lines, within certain limits

of accuracy, are also to be found, as will be seen from the following

comparison:—

+--------------+--------------------+----------------------+

|Aurora-lines. | Lines of the | Remarks. |

| | iron-spectrum. | |

+--------------+--------------------+----------------------+

| | { 630·08 } | Moderately bright. |

| 629·7 | { 629·85 } | |

| | | |

| | { 539·60 } | |

| | { 539·92 } | Mostly very faint. |

| 539·0 | { 539·05 } | |

| | { 538·85 } | |

| | | |

| | { 523·43 | Very faint. |

| 523·3 | { 523·21 | Moderately bright. |

| | { 522·90 | Very faint. |

| | | |

| | { 519·79 | ” |

| | { 519·40 | ” |

| 518·9 | { 519·16 | Moderately bright. |

| | { 519·06 | ” ” |

| | { 518·51 | Very faint. |

| | | |

| | { 500·65 } | |

| | { 500·52 } | |

| 500·4 | { 500·49 } | Very faint. |

| | { 500·30 } | |

| | { 500·20 } | |

| | | |

| From 469·4 } |

| to 462·9 } 3 stronger and 4 very faint iron-lines. |

+----------------------------------------------------------+

Yet this agreement, though remarkable, can only be considered as

complete proof of the presence of iron-vapour in the atmosphere when we

shall have succeeded in showing by observation analogous modifications of

the relative conditions of brilliancy in the iron-spectrum by alterations

of temperature and density; and in this way explain the appearance of

relatively very faint iron-lines in the Aurora-spectrum, or, on the other

hand, the absence of the most intense lines.

It will meanwhile remain far more in accordance with probability to

regard the _Aurora-spectrum as a modification of the air-spectrum_;

since we are already aware, in the case of gases, of the alteration of

the spectra by conditions of temperature and pressure; and an agreement,

at any rate, quite as certain between the spectrum in question and the

spectra of atmospheric gases has been proved above.

[I am indebted to Miss Annie Ludlam for a translation from the

German of the above Memoir.—J. R. C.]

FOOTNOTES

[1] βόθυνος, a hollow.

[2] πίθος, a cask.

[3] χάσμς, a chasm.

[4] M. Lemström (Swedish Expedition, 1868) concludes that the corona of

the Aurora Borealis is not entirely a phenomenon of perspective, but that

the rays have a true curvature, that they are currents flowing in the

same direction and attract each other. There is also an account [_antè_,

p. 16] of an Aurora at Melville Island (Parry’s first voyage), in which

two arches were seen curving towards each other.

[5] A brilliant display in December 1870, on the east coast of Sicily,

was followed by very violent storms, with the overflow of the Tiber and

the flooding of Rome.

[6] Some curious instances have been recently (January 1879) given in

the ‘Times’ of such electric phenomena, comprising, amongst others, gas

lighted by the finger in Canada, points of flame seen on the ironwork of

Teignmouth Bridge, and similar points seen on the alpenstocks and axes of

a party making a mountain ascent in Switzerland.

[7] On the occasion of the Aurora of September 24, 1870, Dr. Allnatt

says, “the air seemed literally alive with the unwonted phosphorescence.”

[8] See, however, Dr. Schuster’s article “On the Spectra of Lightning,”

Phil. Mag. May 1879, p. 316.

[9] The proof from occulted stars merely goes to the fact that the moon

possesses no atmosphere _appreciable in that way_. It may still be a

question whether there does not exist something of the kind, lying low

and close to the surface, and possibly of a rarefied character, which

would scarcely make itself visible by its effects in occultations.

Cloud-vapour might form in an atmosphere of inconsiderable density.

[10] This observation is not without a certain amount of confirmation by

more recent ones, in which certain lunar objects and regions have been

suspected of mist or vapour. Mr. Birt (‘English Mechanic,’ vol. xxviii.

no. 725) mentions two—the cloud-like appearance of the white patch west

of Picard, and the interior of Tycho, which at one time always misty and

ill-defined, is now become perfectly distinct and sharply defined.

December 4, 1878, 4h 45m. I observed Klein’s crater as a dull dark spot,

larger than the true object; and while definition was good and other

objects were well defined, “the floor of Klein’s object, the oval spot

near, and also Agrippa (especially), all had _an odd misty look as if

vapour were in or about them_” (‘English Mechanic,’ vol. xxviii. no.

727). The mystery of different observers seeing and not seeing Klein’s

object on the same night is hardly to be accounted for by the angle of

illumination.

[11] Some doubt has been cast on this observation, on the ground that

nothing unusual was seen, and that the appearances were only those

ordinarily presented by the moon at its then phase. I simply give the

account as it appears in the scientific journal in which it was published.

[12] The question of a connexion between the waxing and waning of the

solar corona and the prevalence of sun-spots is now being mooted, and

may have an important bearing on the subject of the constitution of the

corona. It would seem that when the corona has been examined about the

time of minimum of sun-spots, it has proved fainter though more extended,

while the bright lines of the spectrum have been absent, indicating a

change or variance in the gaseous part of it at those periods.

[13] There seems to be some confusion as to the W.L. here given; 5567 is

usually accepted as Ångström’s line, while Prof. Smyth refers to it as

5579. The position, too, when examined with a spectroscope of greater

dispersion, is not exactly over the citron-line of acetylene, both the

above referred to lines lying somewhat more towards the violet end of the

spectrum (see Plate V. fig. 7).

[14] Ångström’s drawing, in giving this character to the two Aurora-bands

which are said to correspond with violet-pole bands about 47 and 43, is

incorrect, and calculated to mislead by giving the Aurora-bands a feature

corresponding to the violet-pole bands which they do not possess. I am

not aware of any Aurora-line or band which is described as distinguished

by degrading towards the violet.

[15] The tubes generally seem marked Si Fl instead of the ordinary

notation Si F. Si Fl₆ is probably, in fact, Si F₄.

[16] Dr. Schuster has found that while the line-spectrum of lightning is

attributable to N, it has also a band-spectrum, which he considers due to

O and a slight trace of CO₂ (Phil. Mag. 5th ser. vol. vii. p. 321).

[17] _In these observations some suggestions made by Mr. Capron have been

incorporated._

[This was Mr. Lockyer’s note. In point of fact, the Author was

responsible for the verbatim paragraphs comprised between the letters _A_

and _B_, and _C_ and _D_, in the instructions as now reprinted.]

[18] Communicated by the author to the Royal Saxon Academy of Science,

1871.

[19] Reports of the Royal Saxon Academy of Science, Oct. 31, 1871.

[20] This red line was first noticed by Zöllner.

[21] Recherches sur le Spectre Solaire, p. 42.

[22] American Journal of Science, lxviii. 123.