CHAPTER XI.

THE COMPARISON OF SOME TUBE AND OTHER SPECTRA WITH THE SPECTRUM OF THE AURORA.

[In part from an Article in the ‘Philosophical Magazine’ for April 1875.]

[Sidenote: Testing Ångström’s Aurora theory. Battery and spectroscope described. Vogel’s spectrum selected for comparison.]

In order to test Professor Ångström’s theory of the Aurora, referred to in the last Chapter, in an experimental way, I examined, in the winter of 1874, some tube and other spectra, not only for line-positions, but also for general resemblance to an Aurora-spectrum. It did not seem desirable to use powerful currents. A ½-inch-spark coil, worked by a quart bichromate-cell, was found sufficient to illuminate the tubes steadily. The spectroscope used was one made for me by Mr. Browning specifically for Auroral purposes, and of the direct-vision form, being the same instrument as is described _antè_, p. 91, and figured in Plate X. fig. 1. The micrometer was the diaphragm one, also before described and figured on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel’s spectrum for comparison, it being, so far as I am aware, the most accurately mapped, with regard to wave-length, at one observation, of any Auroral spectrum. It seemed an unsafe plan to attempt to obtain an average Aurora by comparison of different observations made at various times by different observers with all sorts of instruments—the difficulty, too, being increased by the suspicion that the spectrum itself at times varies in number and position as well as intensity of its lines.

[Sidenote: Central part only of spectrum mapped.]

In most cases the central part of the spectrum only (corresponding to the central lines of the Aurora) was mapped, the red line in the Aurora not being found to correspond with any prominent line in the gas-spectra examined, and the Auroral line near solar G being so indefinitely fixed as to render comparison almost valueless. (See Plate XIII. fig. 1.)

Dr. Vogel’s spectrum does not comprise the line near G; but I have added this (in an approximate place only) in order to complete the set of lines. For drawing of Dr. Vogel’s spectrum, with its scales attached, see Plate XIII.

_Hydrogen-tube._

[Sidenote: Hydrogen-tube. Colour of glow varied with intensity of current.]

This tube was one of Geissler’s and of rather small calibre. On illumination the wide ends were easily lighted with a silver-grey glow, having a considerable amount of stratification. The capillary part glowed brilliantly with silver-white, bright green, and crimson light, according to the intensity of the current. With the commutator slowly working, white running into green and bright green were the main features of the thread of light; on the current passing more rapidly, the capillary thread became of an intense crimson, at the same time apparently increasing in diameter (an effect probably due to irradiation).

[Sidenote: Spectrum described.]

The spectrum was very brilliant, consisting of the three bright lines usually distinguished as Hα, Hβ, and Hγ, and a number of shaded bands and fainter lines between these, with a bright continuous spectrum as a background to the whole.

[Sidenote: Lines α, β, and γ varied in intensity with colour as seen by eye. Fainter lines or bands described.]

The lines Hα, Hβ, and Hγ were found to vary in intensity with the current, and in accordance with the colour of the light as seen by the eye—a fact, as I think, not without bearing on the question of the Aurora, the varying tints of which are so well known. The fainter lines or bands were mostly stripes of pretty equal intensity throughout, and all about the width of the Hβ line. I did not trace any marked degrading on either side of the lines, though the edges were not uniformly so sharp as Hα and Hβ. Some of the lines were found coincident in position with lines of the air-spectrum.

[Sidenote: Purity of subsidiary lines questioned.]

It is a question whether these subsidiary lines are hydrogen, or are due to some tube impurity. A photograph I have taken of this tube-spectrum shows 17 lines in the part of the spectrum between F and H₂, some of which are repeated in the hydrocarbon-tube spectra.

[Sidenote: Coincidence of lines with Aurora-spectrum.]

No principal line, and one subsidiary line only, actually coincide with the Aurora-spectrum, this last being that to which Dr. Vogel assigns an identical wave-length, viz. 5189. Other of the subsidiary lines, however, fall somewhat near the Aurora-lines 5569, 5390, 5233, and 5004, two faint lines also falling within the band 4694 to 4629.

[Sidenote: Comparison of the lines.]

The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when compared, as under:—

{4694} Aurora 5569 5390 5233 5189 5004 { to } band. {4629} Hydrogen 5555 5422 ... 5189 5008 4632

I remarked that a line (5596) described by Dr. Vogel as “very bright” in his H spectrum does not appear in my tube, though in most other respects our H spectra agree.

[Sidenote: Effect of distance on the spectrum.]

I thought this tube afforded a good opportunity for testing the effect of distance upon the spectrum. The slit was made rather fine. At 6 inches distance from it the line α in the blue-green (F solar) was very bright. The lines marked β, γ, δ, ε, and ζ also survived, but were faint. At 12 inches from the slit α and γ were alone seen, and at 24 inches α stood by itself upon a dark ground. I also noticed that the red and yellow parts of the spectrum first lost their light on the tube being withdrawn from the slit; and this appeared to account for β disappearing while γ survived. For drawing of the hydrogen-tube spectrum see Plate XIV. spectrum 1.

The question of effect of distance upon the spectroscopic appearance of a glowing light, as tested for this and other tubes, seems an important one. It may possibly account for the generally faint aspect of the lines in the more refrangible part of the Auroral spectrum.

_Carbon- and Oxygen-tubes._

[Sidenote: Carbon- and oxygen-tubes. Tubes described. Carbon-tubes lighted up. Spectra of the carbon-tubes described.]

The following tube-observations were taken together, because my friend Mr. Henry R. Procter (to whom I am indebted for many profitable hints and suggestions in Auroral work) was disposed to regard the spectra found in the carbon-tubes and in those marked “O” as identical, suggesting that pure O, with the ordinary non-intensified discharge, gives only a continuous spectrum; and that the O tubes are in fact generally lighted up by a carbon-spectrum, as the result of impurity from accidental causes. The tubes examined for the purpose of comparison were as follows:—A coal-gas tube, a tube marked “C.A.,” three O tubes (two of, I believe, London make, and the third from Geissler), and an OH₂ tube, also from Geissler. The carbon-tubes were both brilliantly and steadily lighted by the current. The C.A. tube glowed with a peculiar silvery-grey green light in the capillary part, and with a grey glow, considerably stratified, in the bulbs. The coal-gas tube illumination was whiter and still more brilliant than the C.A., and with even finer stratification in the bulbs. The spectra of both tubes were conspicuous for the same three well-known principal bright lines or bands in the yellow, green, and blue (with one fainter in the violet), all shading off towards the violet, and in both cases with fainter intervening bands or lines. These last bands or lines only partially coincided when the two tubes were compared.

The spectra in both cases were rich and glowing, with a certain amount of continuous spectrum between the lines; and the three principal bands or lines showed well and distinctly their respective place-colours.

[Sidenote: Tubes tested for distance.]

_Tubes tested for distance._—In the case of the C.A. tube, at 18 inches from the slit the continuous spectrum and fainter lines disappeared, while the four principal lines still shone out, that in the green being the strongest. At 24 inches the same lines were still visible, though somewhat faintly.

In the case of the coal-gas tube, at 24 inches the whole spectrum was quite brilliant, the four principal lines being very bright and even preserving their distinctive colours. The H line, near the line or band in the blue, was also plainly seen.

[Sidenote: O tubes lighted up.]

The O tubes, when treated by the same current as the carbon-tubes, were found to be all three identical in general features. The discharge lighted up each of the tubes feebly and somewhat intermittently. Grey in the bulbs and a faint but decidedly pinkish white in the capillary part were the distinguishing light colours; while nothing could be more marked than the difference in brilliancy between these and the preceding carbon-tubes.

[Sidenote: OH₂ tubes lighted up. O tubes spectra described.]

The OH₂ tube presented very much the same character; but the discharge occasionally varied from a pinkish white to a yellow colour, somewhat like that which artists call brown-pink, and reminding one of the “golden rays” in certain Auroræ. These O spectra presented, in common with the carbon-tubes, three principal bright lines or bands in the yellow, green, and blue, with a fainter one in the violet, all shading off towards the violet. The bands, however, showed but very little trace of local colour, and the whole spectrum had a faint and washed-out look, very different from the carbon-spectra. (I certainly, by a little management, subsequently succeeded in getting the same look to the C.A. spectrum; but it was only by removing the tube to some distance from the slit, and thus depriving the spectrum of very much of its brightness.) The hydrogen line (solar F) was bright, more so than any of the O lines.

The intensity of the three principal lines seemed to me to run in the following order:—

Yellow. Green. Blue.

Coal-gas β α γ Oxygen γ α β

Between the lines γ and α in the Geissler O tube I found a rather bright line, which I shall have occasion to refer to hereafter.

At 12 inches distance from the slit the O spectrum lost nearly all its light; the H line, and the three lines γ, α, and β, alone faintly remaining, α being decidedly the brightest. At 24 inches no spectrum at all was to be seen.

[Sidenote: Comparison of spectra of coal-gas and O tubes.]

I carefully compared together the three principal lines of the two spectra of coal-gas and O by means of:—

1st, the photographed micrometer before described;

2nd, a comparison-prism on the slit plate;

3rd, a piece of very fine brass foil cut as a pointer and fixed in the focus of a positive eyepiece.

The lines or bands in both tubes were found to be slightly nebulous towards the less-refrangible end (where they were measured), and the O tube was not bright under a moderately high power (positive eyepiece). Subject to these remarks, the three principal lines in both tubes were found to correspond in position within the limits of my instrument. The spectra did not, however, I am bound to say, _look_ alike.

[Sidenote: Dr. Vogel’s O spectrum reduced and compared.]

Puzzled by these observations, it then occurred to me to reduce Dr. Vogel’s spectrum of O, given in his memoir, to the same scale with my own. This I did independently, and I then compared the result with my own spectrum as mapped out. From the comparison, I judge that if my O tubes, one and all, showed a carbon-spectrum, the learned Doctor’s tube must have been subject to a similar infirmity, as the tubes all agreed in main features.

There is, however, one point to which I desire to draw attention, which is this, that common to both the Doctor’s and my own Geissler spectrum I found the before-mentioned rather bright line between γ and α. This line I found no equivalent for in either of the carbon-tubes. For spectra of coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4.

[Sidenote: Tube- and flame-spectra of carbon do not correspond.]

In comparing the spectra, it should be remembered that the tube- and flame-spectra of carbon do not correspond. Compare, for instance, the spectrum of coal-gas or CO₂ in tube, and the well-known lines or bands in the blue base of a candle-flame. The sharper edge of the yellow line or band of the carbon-tubes will be found about midway between the two bright yellow candle-lines or bands. The first of the very beautiful group of lines or bands in the green in the candle-flame falls considerably behind the sharper edge of the green line or band in the tube, while the third bright band in the tube, alone of the three, corresponds with a very faint band in the candle-flame. A line or band in the violet in the tube-spectrum finds no equivalent in the candle-spectrum. For comparison of the carbon-tube and flame spectra (the principal lines of the tube being alone shown), see Plate XVI. spectra 6 & 7.

[Sidenote: Prof. Piazzi Smyth’s measurements of the components of the citron-band in a coal-gas flame.]

_Note._—Prof. Piazzi Smyth has been good enough, at my instance, to measure the components of the citron band of the carbo-hydrogen spectrum (near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged with common air.

The spectroscope used had prisms giving 22° of dispersion between A and H, and the observing telescope magnified 10 times. The following is a table of the results communicated to me by the Professor:—

Reading of Intensity. Micrometer.

Reference line, lithium β 4 16·55 ” sodium, α1 10 18·45 ” ” α2 10 18·51

Citron band. Carbo-hydrogen.

Line 1, exquisitely clear 6 21·28 2, ” 5 21·88 3, ” 3 22·44 4, faint but clear 2 22·95 5, faint 1 23·38 6, faint and hazy 1 23·70 7, doubtful ? 23·92 Reference line, thallium α 10 25·08

[Sidenote: Comparison of Dr. Vogel’s O lines and Dr. Watts’s carbon-lines.]

From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube as under:—

Yellow. Green. Blue. Dr. Vogel’s oxygen-lines 5603 5189 4829 Dr. Watts’s carbon-tube bands or lines 5602 5195 4834

Now these wave-length differences are so small that they raise a presumption of the possibility of the spectra being identical. On the other hand, assuming the spectra are not identical, the comparison tells the other way, viz. that the differences are so minute as to escape detection in instruments of moderate dispersion. With my own instrument I found the O spectrum too faint to increase the dispersive power with advantage. Considering the extremely different character of the two discharges, the identity of all the O tubes, and the presence of the line found between γ and α in the O spectrum, I think the two spectra are independent, though I admit there is room for doubt.

[Sidenote: O and CO₂ spectra photographed.]

_Note._—Since this examination I have photographed both spectra side by side (see ‘Photographed Spectra,’ Plate XXXI., text, pp. 69, 70). The pictures include, of course, only the blue and violet parts of the spectrum; but they are widely different in aspect, and show that, photographically at least, in this part of the spectrum there is a complete want of identity. Subsequent investigations, however, by Schuster and others (detailed later in this Chapter), go to establish that the principal lines shown in mine and Dr. Vogel’s tubes were due to (probably hydrocarbon) impurity. The exception is the single line common to mine and Dr. Vogel’s tubes, but absent from the coal-gas spectrum. This line proves to be oxygen. Compare oxygen-tube spectra (Plate XIV. spectra 3 and 4) with Schuster’s oxygen-tube spectrum (Plate XVIII. fig. 15). The line in question is found identical in the three tubes.

The tube OH₂ was found to give the principal lines of the O and H spectra combined on a faint continuous spectrum.

_Geissler Mercury-tube_ (Plate X. fig. 7) _and Barometer Mercurial vacuum_.

[Sidenote: Mercury- and barometer-tubes examined. Mercury-tube described. Barometer-tube.]

I next examined two vacuum-tubes of an entirely different character. The one was a tube from Geissler of stout glass, some fifteen inches long, without electrodes, and an inch across. Within this tube was a second of uranium glass, with bulbs blown in it. In contact with both tubes a quantity of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this tube with the hand brilliant flashes of blue-white light, like summer lightning, flashed out. These were discernible (though faintly) even in daylight. The fine terminal wires of the coil being wrapped round each end of this tube, when the current passed, a bright and white induced discharge, with a considerable amount of stratification, was seen in the tube. The other tube was that of a mercurial siphon-barometer. This being placed in a stand, one terminal wire was placed in the mercury in the short leg of the siphon, while the other terminal was made into a little coil and placed on the upper closed extremity of the barometer-tube. On passing the current, the entire short space above the mercury was filled with a grey-white light, not stratified, but showing a conspicuous bright ring just above the level of the mercury.

[Sidenote: Spectrum of both these tubes described.]

Both these tubes, when examined with the spectroscope, showed four bright rather uniform bands (the central one being the brightest), which I assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6).

The Geissler tube was probably filled designedly with coal-gas. In the case of the barometer-tube the spectrum must be assumed to be the result of some carbon impurity.

No lines of mercury could be detected in either case.

An effort was made to examine the light of the Geissler mercury-tube as excited by motion only, but the spectrum could not be kept in the field; the four lines were, however, seen to flash out as the light passed before the slit.

_Air-tubes._

[Sidenote: Air-tube illuminated.]

The first tube I examined was an ordinary Geissler tube charged with rarefied air. The bulbs, on passing the discharge, were filled with the well-known rose-tinged light like to the Aurora-streams. This in the capillary part was condensed into a brighter and whiter thread, while the platinum wire of the negative pole was surrounded by its characteristic mauve or violet glow.

[Sidenote: Spectrum described.]

The spectrum, even with a weak current, was quite bright, and consisted mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ, and some of the intermediate lines of the H tube.

The double line α was undoubtedly the brightest in the spectrum when taken in the capillary part of the tube. After this followed β, and then γ(H), δ, and ε. I was, however, uncertain as to the relative brightness of the last three, and marked their intensities with hesitation. I tested them several times independently with differing results, and suspected them of variability with the current.

The rest of the lines were very much of the same intensity. (For drawing of spectrum of air-tube in capillary part see Plate XV. spectrum 1.)

_Violet [negative] Pole, same tube._

[Sidenote: Violet (negative) pole: spectrum described.]

I next turned my attention to the violet or negative-pole glow; and here a remarkable change took place in the spectrum, not only in the position of the principal bands or lines, but in their relative intensity (see Plate XV. spectrum 2).

The double line α in the capillary part was replaced in the violet glow by a shaded band of second intensity β, the sharp edge of which was extended towards the red, and formed (except for some faint indications) the limit of the spectrum in that direction. The somewhat faint line next α in the capillary tube had its faint representative in the violet pole; but the next two lines (capillary) were represented by the bright band γ in the violet pole lying in a position between them. Next γ in the violet pole came three faint lines, representing β, γ, and δ in the capillary spectrum; and then the bright band α, which was the brightest of the violet-pole group, and represented a medium-intensity band in the capillary spectrum. After this was a faint band near α, representing two rather bright ones in the capillary spectrum, this last being succeeded by other bands in the violet. α, β, and γ in the violet pole were examined carefully for relative brightness, and were, I believe, correctly marked.

_Red [positive] Pole._

[Sidenote: Red (positive) pole: spectrum described.]

The red [positive] pole was next examined, but presented no peculiar features. It appeared as a fainter representation of the capillary air-spectrum, with some few lines or bands absent, and (as will be seen after) was also a fair representation of a diffused air-spectrum (see Plate XV. spectrum 3).

Examined for comparative intensity, at 24 inches from the slit, the whole capillary air-spectrum showed faintly. The marked lines in the centre of the spectrum generally retained their prominence; but after α I judged ε next in brightness. On examining the violet pole at 12 inches from the slit, the whole spectrum was faint and the bands α and β were alone distinctly seen.

_Aurora (air)-tube._ (Plate XV. spectrum 4.)

[Sidenote: Aurora-tube: discharge described. Spectrum described.]

Next to the Geissler air-tube I examined an “aurora”-tube, about 15 inches long and 1¼ inch across, with platinum terminals, and of the same diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red colour, and the long flickering stream from pole to pole certainly much reminded one optically of an auroral streamer. Spectroscopically examined, the discharge presented a faint banded air-spectrum similar to that of the positive pole (see Plate XV. spectrum 4); but the relative intensity of the lines was somewhat altered, while a very bright line in the green (seen also in the tube next described) was characteristic of the spectrum, and in this respect distinguished it from the ordinary air-spectrum.

_Phosphorescent tube._

[Sidenote: Phosphorescent tube described. Discharge described. Spectrum described.]

Following this last tube I examined one purchased as “phosphorescent.” It was rather short (6½ inches), of equal calibre, and about the size of the bulb of a Geissler tube. It was filled with a white powder (probably one of the Becquerel compounds). On passing the current between the electrodes, a bright rose-coloured stream appeared; and wherever this was in contact with the powder, the tube glowed with a brilliant green light. On stopping the current, the tube still continued to shine, but with a fainter green glow, which gave only a continuous spectrum. When examined in full glow, the tube-spectrum was also in the main continuous and of a green tinge; but upon it were bright lines in the blue and violet portions of the spectrum, while in the red, yellow, and green a faint but distinct air-spectrum was seen; and with this was also found the same bright line in the green which distinguished the “aurora”-tube. [Five out of six of the lines in the blue and violet will be also found in Schuster’s oxygen-tube, violet pole (Plate XVIII. fig. 15). The air-spectrum probably arose from impurity.]

_Spark in Air._

[Sidenote: Spark in air: spectrum described.]

I next took a ½-inch spark in air between platinum terminals (see Plate XV. spectrum 6). The principal lines in this spectrum were the line α (by far the brightest), corresponding to γ in the violet pole; next was β, a line in the yellow, not appearing in the tube-spectrum, and then other lines of less intensity. In the “aurora” and “phosphorescent” tubes was found, as before mentioned, a line in the green prominent for its brightness, and, indeed, in the “aurora”-tube the only one which survived when it was moved away from the slit. This line also appeared in the spark-spectrum, but there only of an average brightness. I examined it carefully for position in the respective tubes; and on comparing them by means of a pointer in the eyepiece, found it coincident with the ridge or centre of the wedge-like bright-green broad band which is so conspicuous in the air-tube spectrum.

I think this edge-like centre has actually a line coincident with the line I refer to; but if so, its intensity little exceeds that of the band itself.

_Spark over Water._

[Sidenote: Spark over water: spectrum described.]

To complete the set of air-experiments, I examined the same spark taken from the surface of a small meniscus of water, placed in a glass cup upon the lower platinum wire. In this case the air-spectrum was plainly, but not brightly, seen at the violet end of the spectrum—the red, yellow, green, and blue being filled with a continuous spectrum, through which some of the air-lines faintly showed (see Plate XV. spectrum 7).

_Phosphoretted-Hydrogen Flame._

[Sidenote: Phosphoretted-hydrogen flame.]

This was obtained from a hydrogen-bottle fitted with glass tubing, two or three minute pieces of phosphorus being placed with the zinc. The flame was of a bright yellow colour, with a cone of vivid green light in its centre.

[Sidenote: Spectrum described.]

The spectrum was found to consist mainly of three bright bands in the yellow, green, and green-blue respectively (see Plate XVI. spectrum 3).

[Sidenote: Mons. Lecoq de Boisbaudran’s remarks on the spectrum increasing in brilliancy when the flame is cooled.]

The central band was very striking in its emerald-green colour, while all the bands were remarkable as being very broad in proportion to the slit (which, however, was not fine). The yellow band had a rich glow of colour. My spectrum was mapped out at ordinary temperature, and I found the bands sufficiently bright; but Mons. Lecoq de Boisbaudran, in his ‘Spectres Lumineux’ (texte, p. 188), has described how the brilliancy of these bands is increased when the flame is artificially cooled (_refroidie_).

The idea of cooling the flame was due to M. Salet, who effected it either by a jet of water or by an air-blast.

The less refrangible bands seem the most susceptible to increase of brilliancy.

Mons. Boisbaudran also makes the important remark that the relative intensities of the bands are in such case altered, adding:—“La plus importante de ces modifications consiste en un renforcement très-considérable de la bande rouge δ 97·03 (W.L. 5994) qui devient vive de presque invisible qu’elle était en l’absence du refroidissement artificiel de la flamme.”

Full details of the changes are given by M. de Boisbaudran.

The bearing of these observations as connected with the variable character of the red line in the Aurora-spectrum seems to me in the highest degree noteworthy.

_Iron-Spectrum._

[Sidenote: Iron-spectrum.]

A comparison of this spectrum suggested itself, partly from the suspected relations between the Aurora and solar corona, and partly from a consideration of the views expressed by M. Gronemann and others in favour of the Aurora having its origin in the fall of an incandescent meteoric powder.

[Sidenote: How obtained. Spectrum described. Mons. Lecoq de Boisbaudran’s spectra also given.]

The spectrum was obtained from a spark taken over a solution of perchloride of iron in a small glass cup, and was remarkable for its brightness in and about the green region. The lines varied considerably in intensity, and with a fine slit the principal ones were sharp, distinct, and clear. A group of three lines (α) stood out boldly in the green as the most marked, and next to these a group of three others more towards the violet end of the spectrum (see Plate XVI. spectrum 4). By the side of my phosphoretted-hydrogen and iron spectra I have placed the principal lines of Mons. Lecoq de Boisbaudran’s same spectra (reduced to my scale), and with figures of wave-lengths for comparison with the Aurora-spectrum (see Plate XVI. spectra 1 and 2).

[Sidenote: Comparison of iron- and Aurora-spectrum.]

A difficulty in comparing the iron-spectrum with that of the Aurora arises from the large number of fine lines found in the former spectrum. In a photograph (taken with the same prism as before described) of a small piece of meteoric iron fused in an electric arc by the aid of 40 Grove cells, about 154 lines are easily counted in the blue and violet parts of the spectrum. Double this number at least would be seen with a spectroscope of moderate dispersion in the region comprising the entire set of auroral lines.

_Spectrum of Mercury._

[Sidenote: Mercury-spectrum. How obtained.]

This spectrum is given as useful for comparison with the bright and principal Aurora-line. It is easy to obtain with a small coil, the metal being used as one electrode. The yellow lines are distinct and steady; but the green, which is very bright, is apt to flicker as the spark moves on the surface of the metal (see Plate XVI. spectrum 5).

_The following Table was compiled for the purpose of comparing the foregoing results with the Aurora-spectrum._

[Sidenote: Table of coincidences.]

TABLE showing comparative position of Aurora-lines with the principal lines in the examined spectra. C. means coincident within the limits of my instrument and scale, N. near, and VN. very near.

-------------+--------+------+------+------+------+------+--------+------ {| | | | | | | 4694 | Aurora-lines{| 6297 | 5569 | 5390 | 5233 | 5180 | 5004 |to 4629 | 4350? {| β. | α. | ζ. | δ. | δ. | γ. | ε. | ε. -------------+--------+------+------+------+------+------+--------+------ | | | | | C., | | Band | Hydrogen-tube| | N. | | N. | same | |includes| | | | | | W.L. | |2 lines.| -------------+ +------+------+------+------+------+--------+ | | | | | | | Band | Coal-gas | | | | N. | VN. | |includes| tube | | | | | | |1 line. | -------------+ +------+------+------+------+------+--------+ Oxygen-tube | | | | | VN. | | | -------------+ +------+------+------+------+------+--------+ | | Band | | | | Band | Air, | | includes | | N. | |includes| capillary | | | | | |2 lines.| -------------+ +------+------+------+------+------+--------+ | | | | |Band | | Band | Air, violet- | * | | | | in- | C. |includes| ** pole | | | | |cludes| |1 line. | -------------+ +------+------+------+------+------+--------+ Air, red-pole| | See Air, capillary. | -------------+ +-------------------------------------------+ Aurora-tube} | | See Air, capillary; and note bright line. | and phos- } | | | phorescent} | | | tube } | | | -------------+ +------+------+------+------+------+--------+ | | | | | | | Band | Air, spark | | N. | | | N. | C. |includes| | | | | | | |2 lines.| -------------+ +------+------+------+------+------+--------+ Air, spark | | Continuous spectrum and faint | Band | over water | | air-lines. |includes| | | |2 lines.| -------------+ +------+------+------+------+------+--------+ Phosphoretted| | N. | Faint| Band | | | | hydrogen | | | band.| in- | | | | | | | |cludes| | | | -------------+ +------+------+------+------+------+--------+ Iron | | VN. | N. | VN. | VN. | | | -------------+--------+------+------+------+------+------+--------+------

* No results in the examined spectra; but see Plate XIII. fig. 2.

** Too uncertain in position for comparison (see Plate XIII. fig. 1).

Tested by coincidence, or close proximity of lines to those of the Aurora, we arrange the spectra in the following order:—(1) iron, (2) air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) carbon and oxygen.

The air-tube spectrum might perhaps stand higher in the scale but for its broad bands, which make comparison doubtful. Lines of oxygen possibly escape detection in the Aurora from the faint character of its spectrum.

The phosphorus and iron spectra are especially interesting in connexion with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546).

[Sidenote: Additional Table of compared spectra.]

As an addendum to the foregoing, on Plate IX. fig. 1 will be found a Table I have prepared, in which a type Aurora and also Vogel’s and Barker’s Auroræ are compared with eight other spectra, viz.:—

S. Solar spectrum. N. Nitrogen (air): Watts. O. Oxygen (air): Watts. C.H. Carburetted-hydrogen vacuum-tube: Watts. C.I. Carburetted-hydrogen flame: Watts. C.C. Blue base of candle-flame: Capron. O.P. Oxygen vacuum-tube: Procter. I. Iron: Watts.

The divisions and vertical lines will guide the eye in making comparison of the spectra.