Auroræ: Their Characters and Spectra

The Aurora of October 24th, 1870, came at a time when spectroscopes of a direct-vision form were being introduced, and a number of observations were communicated at the time to ‘Nature.’

A correspondent, T. F., writing from Torquay, saw, with a direct-vision spectroscope, one strong red line near C, one strong pale yellow line near D, one paler near F, and a still paler one beyond, with a faint continuous spectrum from about D to beyond F. The C line was very conspicuous and the brightest of the whole. It was intermediate in position and colour to the red lines of the lithium and calcium spectra. Plainly there were two spectra superposed, for while the red portions of the Aurora showed the four lines with a faint continuous spectrum, the greenish portions showed only one line near D on a faint ground. W. B. Gibbs saw, in London, only two bright lines, one a greenish grey, situate about the middle of the spectrum, and the other a red line very much like C (hydrogen). Thomas G. Elger, at Bedford, on the 24th and 25th, saw:—(1) a broad and well-defined red band near C; (2) a bright white band near D (same as Ångström’s W.L. 5567), on 25th visible in every part of the sky; (3) a faint and rather nebulous line, roughly estimated to be near F; (4) a very faint line about halfway between 2 and 3. The red band was absent from the spectrum of the white rays of the Aurora, but the other lines were seen.

With a small Browning direct-vision spectroscope on the 24th, I found no continuous spectrum, but two bright lines, one in the green (like that from the nebulæ, but more intense, and considerably flickering), the other in the red (like the lithium line, but rather duskier: Plate V. fig. 6). The latter was only well seen when the display was at its height; it could, however, be faintly traced wherever the rose tint of the Aurora extended. The line in the green was well seen in all parts of the sky, but was specially bright in the Auroral patches of white light.

Mr. Browning also saw the red line, but found comparison difficult. On the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston, used a chemical spectroscope of the ordinary form, with one prism and a photographed scale illuminated with a lamp. Four Auroral lines were seen at points of his scale numbered 61, 68, 80, and 98. These were reduced to wave-lengths by Professor Pickering, with the following results:—

[61 is evidently wrong, and was probably a mistake for 63.]

George F. Barker, observing at New Haven (U.S.A.), saw, on November 9th, a crimson and white Aurora, which he examined with a single glass-prism spectroscope, by Duboscq, of Paris. The line positions were obtained by an illuminated millimetre scale. In the white Aurora were four lines (the red one being absent); in the red Aurora five. The wave-lengths of the Aurora-lines were run out as follows:—

• (1.) Between C and D, 6230 (Zöllner’s 6270).
• (2.) ” D and E, 5620 (Ångström’s 5570).
• (3.) ” E and b, 5170 (Winlock’s 5200).
• (4.) ” b and F, 5020.
• (5.) ” F and G, 4820 (Alvan Clarke’s, jun., 4850).

Mr. Procter’s Aurora-lines will be found noticed in connexion with the spectrum of oxygen; and Lord Lindsay’s lines, with a comparison scale drawing, are separately described further on in this Chapter. The Aurora of February 4th, 1872, had many observers; some of whom communicated at the time spectroscopic notes. Professor Piazzi Smyth minutely describes the display as seen in Edinburgh, and saw “Ångström’s green Aurora-line perpetually over citron acetylene[13] at W.L. 5579, and the red Aurora-line between lithium a and sodium a, but nearer to the latter, say at W.L. 6370.” Extremely faint greenish and bluish lines also appeared at W.L. 5300, 5100, and 4900 nearly.

The Rev. T. W. Webb, with a very fine slit, saw the green Auroral line even in the light reflected from white paper. With a wider slit he saw a crimson band in the brighter patches of that hue, and beyond an extent of greenish or bluish light, which he suspected to be composed of contiguous bands.

R. J. Friswell, coming up the Channel at 9.40, with a Hoffman’s direct-vision spectroscope (the observing telescope removed), saw the green line, a crimson line near C, and faint traces of structure in the blue and violet.

The Rev. S. J. Perry observed at Stonyhurst four lines, and, on examining one of the curved streamers, found the red line even more strongly marked than the green. A magnetic storm was observed to be at its height from 4 to 9 P.M. of the same day.

With a Browning 7-prism direct-vision spectroscope I saw the green line in all parts of the Aurora, attended with a peculiar flickering movement. I did not see the other lines.

In a letter to ‘Nature,’ dated November 9th, 1872, I catalogued the lines observed up to that date as follows:—

1. A line in the red between C and D. W.L., Ångström, 6279.

2. A line (the principal one of the Aurora) in the yellow-green, between D and E. W.L., Ångström, 5567.

3. A line in the green, near E (corona line?). W.L., Alvan Clarke, jun., and Backhouse, 5320.

4. A faint line in the green, at or near b. W.L., Barker, 5170.

5. A faint line or band in the green, between b and F. W.L., Barker, 5020 (chromospheric?).

6. A line in the green-blue, at or near F. W.L., Alvan Clarke, jun., 4850.

7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350.

8. The continuous spectrum from about D to beyond F.

Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel, and since of the Astrophysical Observatory, Potsdam, made several observations of the Auroral lines, October 25th, 1870. Besides the bright line between D and E, he found several other fainter lines stretching towards the blue end of the spectrum on a dimly-lighted ground. February 11th, 1871, he observed the same set of lines, and an average of six readings gave 5572 as the W.L. of the Ångström line. February 12th gave 5576 as Dr. Vogel’s reading, and 5569 as Dr. Lohse’s. April 9th gave 5569, and April 14th 5569. The Aurora of April 9th, 1871, was exceedingly brilliant, so that micrometer measurements of the lines were taken. The spectrum consisted of one line in the red, five in the green, and a somewhat indistinct broad line or band in the blue. The lines are thus described:—

Table of Dr. Vogel’s lines. Aurora, April 9th, 1871.

A translation of Dr. Vogel’s interesting paper will be found printed in extenso in Appendix E, and his lithographed drawings of the spectrum in the green and red portions of the Aurora respectively on Plate VI. figs. 2 and 3. The observations of April 9th by Dr. Vogel are probably, up to the present time, the most exact of any one Aurora, and I have therefore in most cases used them for comparison.

Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue of lines, gave the following as the latest determinations from his own observations:—

(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr. Backhouse never saw a line at 5320 again. He found the continuous spectrum to reach from No. 2 to No. 7, being brightest from a little beyond No. 2 to No. 6. This part of the spectrum did not give him so much the idea of a true “continuous spectrum” as of a series of bright bands too close to be distinguished.

I have subsequently, in another section of this Chapter, added a full catalogue of the Auroral lines, prepared by myself from the foregoing and other sources and observations; and I also append to it a Plate [Plate XII.], in which these lines are positioned and the wave-lengths and names of observers are given. The numbers of the lines on the Plate correspond with those in the catalogue. The solar spectrum and the spectrum of the blue base of a candle-flame are added for purposes of comparison. [The telluric bands in the solar spectrum are shown more distinctly than they actually appear, and do not profess to give details.]

Flickering of the Green Line.

A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable circumstance connected with the appearance of the single line observed on this occasion was the flickering and frequent changes with which it rose and fell in brightness; apparently even more rapidly than the swiftly travelling waves, or pulsations of light, that repeatedly passed over the streamers, near the northern horizon, towards which the spectroscope was directed.” In the spectrum of the Aurora of 20th October, 1870, I saw and noted the green line as “considerably flickering;” and in the Aurora of 4th February, 1872, I again saw and noted “the peculiar flickering” I had remarked in 1870. I have not seen the peculiarity noted by other observers.

Mr. Backhouse’s graphical Spectra of four Auroræ.

Mr. Backhouse has been good enough to supply me with some details of four several Auroræ seen by him at Sunderland, accompanied by drawings, showing in a graphical way the spectrum of each display as seen with a spectroscope with rather a wide slit and as drawn by eye. I have reduced the four drawings to the same scale, and in this way they are extremely interesting for comparison (Plate V. fig. 4). The line on the left in each spectrum is Ångström’s bright Auroral line, and is supposed to be considerably prolonged. The height of the lines denotes intensity.

April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which Mr. Backhouse had not perceived before. No. 5 had not been visible lately, and Mr. Backhouse thought it must belong to Auroræ of a different type from those which had appeared latterly.

February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw seven lines, all that he had ever seen. (The red line, not shown in the diagram, makes the seventh.)

The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr. Backhouse had only once before seen No. 4, and it became quite invisible between 7.45 and 7.55, though the other lines were as bright as before and the red line had appeared.

October 3rd, 1874. This spectrum was examined, and diagram made between 10 and 10.25 P.M. Five lines only are indicated.

It is mainly distinguished from the two preceding spectra by the brightness of the continuous spectrum on which the lines 2, 3, and 4 lie, and by the weakness of No. 5.

October 4th, 1874. Taken between 11.10 and 11.20 P.M.; distinguished, like the last, by a considerable amount of continuous spectrum and by a faint line (No. 3), not seen in the last spectrum, while No. 3 in the last is missing in this spectrum.

Mr. Backhouse, as to both these last spectra, remarks that the lines were very variable in intensity, and sometimes some were visible and sometimes others. They varied also in relative brightness in different parts of the sky at the same time. Mr. Backhouse, in a communication to ‘Nature,’ referring to a statement of Mr. Procter’s, that the bands of the Auroral spectrum are seldom visible, except the bright line at 5570, says that he always found two bands, “doubtless Winlock’s 4640 and 4310,” to be invariably visible when the Aurora was bright enough to show them. Of thirty-four Auroræ examined by Mr. Backhouse, fourteen showed the lines 4640 and 4310, and three others at least one of these, while eight showed the red line. (Ångström only once saw this line.) In five Auroræ, all more or less red, he saw a faint band, the wave-length of which he placed at 5000 or 5100. He never saw the line 5320 (also Winlock’s coronal line), unless it were once, probably from want of instrumental power. With regard to these observations, I may say that with a Browning’s miniature spectroscope I saw only two lines (the red and the green) in the grand display of the 24th October, 1870; and with an instrument of larger aperture the green line only on the 4th February, 1872; while I saw the green line and three others towards the violet with the same instrument during the Aurora of 4th February, 1874. (See description of this Aurora, antè p. 21, and drawing of spectrum, Plate VI. fig. 1 a.)

Lord Lindsay’s Aurora-Spectrum, 21st October, 1870.

Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on the night of the 21st October, 1870. It commenced about 9.30, reached its maximum about 11, and faded away suddenly about 11.30 P.M.

A spectrum obtained in the north-west gave five bright lines with a Browning’s direct-vision spectroscope—two strong, one medium, two very faint. A tallow candle was used to obtain a comparison spectrum of sodium and carburetted hydrogen.

A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1 is a sharp well-formed line visible with a narrow slit.

No. 2, a line very slightly more refrangible than F. The side towards D is sharp and well defined, while on the other side it is nebulous.

No. 3, slightly less refrangible than G, is a broad ill-defined band, seen only with a wide slit.

No. 4, a line near E, woolly at the edges, but rather sharp in the centre. This, says Lord Lindsay, should be at or near the position of the line 1474 of the solar corona.

No. 5, a faint band, coincident with b, extending equally on both sides of it.

The lines are numbered in order of intensity. It is questionable, from observations with instruments carrying a scale, whether the line-positions are exact; but the description of their characters is valuable.

As a candle blue-base spectrum is at times a ready and handy mode of reference in Auroral observations (as was found in this instance), I have, on Plate XI. fig. 5, given a representation of it as seen with my Auroral spectroscope. Dr. Watts’s corresponding carbon-spectrum is added on the lower margin. The numbers on the upper margin refer to my scale.

Spectrum of the Aurora Australis.

Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874 (antè, p. 27), with the spectroscope, found three prominent lines in the yellow-green, green, and blue or purple, but not the red line. In the Aurora of March 3rd, 1874 (p. 27), he could trace four lines, three bright and one rather faint. They must have been exceedingly bright to show so plainly in full-moon light.

The spectroscope used was a Grubb single-prism with long collimator. A needle-point in the eyepiece marked the position of the lines; and a corresponding needle-point, carried on a frame by a screw movement in concord with the point of the eyepiece, scratched the lines on a plate of blackened glass. Two plates were taken. On the first were scratched the auroral lines and the solar lines as seen in the moonlight; on the second plate were scratched the auroral lines, the Solar lines from the moon, and the carbon lines in a spirit-lamp.

The next morning the solar lines were verified in sunlight. I subjoin (Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the auroral lines being marked A, the solar lines by the usual designating letters, and the carbon by Car. To these spectra I have added for comparison Dr. Vogel’s spectrum of the Aurora Borealis. Captain Maclear could not account for the different positions of the auroral lines in the two plates; for the prism, as far as he was aware, was not moved during the observations. As the solar lines are indicated in the same place in both spectra, the case would seem one of actual change of position of the auroral lines during observation. A comparison of the two spectra gives the impression that the lower one is the same as the upper, except that the dispersion is greater, the lines remaining relatively in position. One does not, however, see how the dispersion could have so varied in a single-prism instrument, and the position of the solar lines is adverse to such an explanation.

There is a suspicion that Auroræ are not always identical in position of some of the lines; but the line in the green (considerably out of place in the lower Australis spectrum) has always, within small limits, the same position. It will be noticed how much further the Australis spectrum runs into the violet than Vogel’s Borealis, the latter having no lines much beyond F.

The faint line (No. 2) mentioned by Captain Maclear possibly corresponds with Dr. Vogel’s band. The absence of the four lines of the Aurora Borealis in the green part of the spectrum of the Australis is peculiar; and in this respect, too, the two Australis spectra agree.

The nearest approaches to Captain Maclear’s lines (of the upper spectrum) which I can find are:—

But the comparisons are not by any means close. Further observations of the Australis spectrum are very desirable.

Prof. Piazzi Smyth’s Aurora-Spectra.

Prof. Piazzi Smyth, in volume xiv. of the ‘Edinburgh Astronomical Observations,’ 1870-77, has compared simultaneously the Aurora-spectrum with the sets of bright lines seen in the blue base of flame—the lines of potassium, lithium, sodium, thallium, and indium being also introduced for comparison. The spectra are drawn as seen under small dispersion, and will prove most useful in cases where an Aurora is not bright enough to admit of the lines being measured by micrometer, and the eye and comparison spectrum are obliged to be resorted to.

Author’s Catalogue of the Auroral Lines.

(See Plate XII.)

1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zöllner. Seen only in red Auroræ; stands out on a dark ground, without other lines near it. Character of line sharp and well defined; varies in colour from dusky red to bright crimson. Intensity, Herschel, 0 to 4 or 8. According to same, position coincident with atmospheric absorption-group “a” in solar spectrum (between C and D). I confirm this position according to my scale of solar lines, and a drawing of the coincidence (in which, and in Plate XII., the absorption-lines are drawn too dark) is given on Plate XIII. fig. 2. Herschel says this line coincides with a red band in the negative glow-discharge, but its identity is doubtful. Its isolation and want of adjacent lines seem to separate it from the air-spectrum and gas-spectra in general. At the appearance of this line, 5569 (No. 2) becomes noticeably fainter. When this line is bright, 5189 (No. 5) is bright also (Vogel).

If we propose to assign to this line, as well as to 5569, a phosphorescent origin, it would be strongly confirmatory of such a theory (in connexion with the phosphoretted-hydrogen spectrum) to find it brighten at low temperatures.

Note.—Sir John Franklin says, in his ‘Polar Expeditions,’ that a low state of temperature is favourable for the production of brilliant coruscations. It was seldom witnessed that the Auroræ were much agitated, or that the prismatic tints were very apparent, when the temperature was above zero.

2. Line in the yellow-green. Brightest of all lines in the Aurora-spectrum. W.L. 5567, Ångström; 5569, Vogel. Intensity 25, Herschel. To me more pale green than yellow, sometimes flickering and changing in brightness (Herschel and Capron). Seen in all Auroræ usually sharp and bright, but Procter has once recorded it nebulous. Its character as to width, sharpness, and intensity, if carefully observed, might indicate height and structure of Aurora. Becomes noticeably fainter at appearance of red line (Vogel). Found by me to correspond in position with a faint atmospheric absorption-band (see Plate XIII. fig. 2). According to Ångström and Herschel, arising from a phosphorescent and fluorescent light, emitted when air is subjected to the action of electrical discharge.

3. Line in green near last. W.L. 5390. An extremely faint and unreliable observation (Vogel). Seen only by him, unless Alvan Clarke’s 5320 (coronal?) be the same.

4. Line in green-blue. W.L. 5233, moderately bright (Vogel); 5200, Winlock. Intensity, 2 or 0? to 6, Herschel. Coincides with line in the negative glow according to same. Frequently observed.

5. Line in green-blue. W.L. 5189. This line is very bright when the red line appears at the same time; otherwise equal in brilliancy with No. 3 (Vogel); Winlock, 5200. Not so frequently observed as No. 3. Barker gives a band extending from 5330 to 5200. Intensity of 5189, 0 to 8, Herschel, who considers it coincident with a constant strong line in the spark-discharge.

6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker (coronal?). Intensity 2 or 0? to 8, Herschel. Coincides with line of nitrogen in the nebulæ according to same. Barker gives a band extending from 5050 to 4990.

7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. W.L. 4850, Alvan Clarke; 4820, Backhouse and Barker. Intensity, Herschel, of 4820-4870, 0 to 4? Herschel suspects this and No. 4 to be seen only in Auroral streamers of low elevation. Barker gives a band extending from 4930 to 4850.

8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the middle; very faint in those parts of the Aurora in which the red line appears (Vogel). Intensity 3-6 (Herschel). A double band, consisting of two lines, the first rather more frequently noted than the second in Auroral spectra, agrees well in position with the principal band in the negative glow-spectrum (same). Barker gives a band extending from 4740 to 4670; Backhouse and Winlock give a line at 4640, situate within the same.

9. There seems a good deal of confusion about a fairly bright line (intensity 0-6, Herschel) seen in most Auroræ (not, however, by Vogel, April 9th, 1871), and situate somewhere near G in the solar spectrum. Alvan Clarke places it at 4350, on the less refrangible side of G; Backhouse and Barker at or very near to G; while Lemström and others position it on the more refrangible side of G. Accurate observations, for which a quartz spectroscope might be useful, are much wanted. Herschel makes this line, at 4285, correspond with a strong band in the violet in the negative glow-spectrum.

Herschel also refers to an apparently additional line near the hydrogen-line, or between G and H₁, in the solar spectrum, as mentioned once by Lemström at Helsingfors. I am not aware of any other observation of this line, which must be considerably beyond that at or near G, and would probably be difficult to detect, except in instruments specially adapted for examination of the violet end of the spectrum.

Theories in relation to the Aurora and its Spectrum.

Lemström (1):—That the Polar light is caused by an electric current passing from the upper rarefied layers of the air to the earth, producing light-phenomena that do not arise in the denser layers of the air. (2) That there are nine rays (lines or bands) in the Aurora-spectrum, which in all probability agree with lines which belong to the gases of the air. (3) That the Aurora-spectrum can be referred to three distinct types, which depend on the character of the discharge.

Dr. Vogel:—(1) That the Auroræ are electric discharges in rarefied-air strata of very considerable thickness. (2) That the Aurora-spectrum is a modification of the air-spectrum, involving the question of alteration of the spectrum by conditions of temperature and pressure.

Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final cause in electrical discharges in the upper strata of the atmosphere, and that these, whether disruptional or continuous, take place sometimes on the outer boundary of the atmosphere, and sometimes near the surface of the earth.

(2) That the Aurora has two different spectra.

(3) That the green line is due to fluorescence or phosphorescence, and that there is no need to resort to Dr. Vogel’s variability of gas-spectra according to circumstances of pressure and temperature.

(4) That an agreement exists between the lines of the Aurora (except the red and green before mentioned) and the lines or bands of the violet light which proceed from the negative pole in dry air.

Zöllner has pointed out that the temperature of the incandescent gas of the Aurora must be exceedingly low, comparatively, and concludes that the spectrum does not correspond with any known spectrum of the atmospheric gases—only because, though a spectrum of our atmosphere, it is one of another order, and one which we cannot produce artificially.

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

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

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.

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.

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).

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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:—

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.