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to recognize those which appear most frequently. For this purpose they should be made large, and the slit should be a wide one.

For ordinary miniature spectroscopes, two holes on the red and five on the blue side of the slit tor of an inch apart, would suffice for recognitions and even for very useful measurements. The jaws of the slit can be cut with a fine saw across the middle about to or ģ of an inch deep each way; and a piece of copper foil, provided with the row of holes and a sufficiently wide slit across it, can be fastened to one of them inside, opposite to the crosscut and adjoining the edges of the spectroscope jaws.

Some other means may be found of piercing the jaws of a pocket spectroscope at regular intervals; but as a simple plan I have found this very efficient, in finer divisions, for laboratory use. The holes are pierced at tto of an inch apart; and thirty of them include the whole visible spectrum. Sodium-light, which is common in laboratory flames, exhibits the punctures with admirable distinctness; and each fifth hole being punched double, the scale is very easily read off. There are ten holes on the red, and twenty on the blue side of the slit. If the mechanical difficulty of perforated jaws could be overcome, nothing perhaps could be better suited for examining auroras than a pocket spectroscope so prepared with a few close but clear and tolerably open holes on each side of the slit.

The secondary auroral lines can only be seen in small spectroscopes) with a pretty broad slit; and the strength of the yellow line might then prove embarrassing. I would abolish it, if so, by a blue* glass nearly covering one half, and a red glass the other half of the slit-the blue and red parts of the spectrum respectively, not in its immediate neighbourhood, being freely transmitted. The slit might also be made longer than usual for auroral study.

I have been bere supposing that special spectroscopes would be provided for Arctic observers. But it is quite certain that much may be done with common pocket spectroscopes without any such provision. They should have adjustable slits and good dispersion, as the secondary lines are faint; and though abundant enough in the blue to make the spectrum there pretty luminous, they can only be individualized by varying the slit-aperture. On the ouly occasion when I have seen this spectrum (in Pebruary 1872) they seemed to run into each other, and presented a light so nearly continuous in the blue part that, although the slit of the Browning's pocket spectroscope which I was using was extremely fine, and was focused on the

* Some care would be necessary in selecting the blue glass, as these generally transmit a yellow ray closely corresponding with the auroral line.

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yellow line, no interruption or appearance of lines could be made out. It was probably also through not opening the slit that I missed seeing a red line which another observer, using a similar instrument and looking with me at the aurora, saw very plainly. Although its red colour was intensely brilliant, I failed to see the slightest trace of light on the red side of the yellow line. Had I opened the slit, or perhaps opened and closed it alternately (as the yellow line, though tine, was still very bright), the result would probably have been different.

I send with this a copy of the Plate of spectra accompanying Ångströni's original paper (see fig. p. 71); and I can confirm the appearance of the negative or "cathode" spectrum which he gives, from the results of some examinations of it which I have lately made. On projecting the recorded lines in wave-lengths, there is a very exact agreement with the chief lines and shadings as figured in the Plate. Some fainter lines, however, are visible, which Ångström has perhaps omitted purposely, to avoid encumbering the drawing. As regards the aurora spectrum, considering the numbers of lines which have been mapped and measured, I am not so well prepared to say how complete Angström's selection of its principal ones may be. It may, I think, be taken for granted that his map includes every line of whose existence, approximate place, and brightness there is really no doubt, and that it does represent the normal spectrum of the aurora when sufficiently bright. During the years 1871 and 1872 there were several résumés of the subject, accompanied with new measurements, in Poggendorff's Annalen and the American Journal of Science' (by Vogel, Barker, and others), with which, from his

his comments on them, Angström was evidently familiar. Adopting his selection, therefore, and the descriptions which he gives, from his own and other observers' notes, of relative brightness and appearance, I have endeavoured to condense the information in the annexed Table as a guide for further observation.

Professor Piazzi Smyth, who has given much attention to the auroral spectrum, bas published, in the introduction to vol. xiii. of the Edinburgh Astronomical Observations,' a set of simple comparison spectra, with notes of desiderata, which would be of great service to observers well furnished with instrumental means and applying them to measurements of the aurora ; and something similar would very much assist observers using directvision spectroscopes to map their spectra, where auroras frequent and of great brightness, in high latitudes.

Hoping that these notes may be of use for the purpose of comparison with occasional observations of auroras,

I remain, &c., Newcastle-on-Tyne, July 24, 1874.

(Signed) A. S. H.

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Positions and General Characters of Principal Lines in the Auroral

Spectrum, according to Ångström.
Red 1. Brightness 0 to 4 or 8. Wave-length 6300.

Relative position. Atmospheric absorptiou-line

a near C in the solar spectrum. General description of source and frequency. Seen

chiefly, if not only, in red auroras; a clearly defined line, sometimes intense ; no other red

line visible. Identification with lines of electrical air-spectra.

Coincides with a red band in the negativeYellow glow discharge. or citron- a. 'Brightness 25. Wave-length 5570. green.

Position. Second separable line in the first or

citron band of blue gas-flame spectrum. Description &c. The inost characteristic auroral

line; constant and conspicuous in all auroras ;

not divisible; sharp and bright. Identification 8c. Not identified; possibly a

phosphorescent or fluorescent light emitted when air is subjected to the action of electrical

discharge.

3. Brightness 2 or 0? to 6.... Wave-length5225. Greenish

0-8 (increases
with red line)

5170–5190. blue. 3b. 2 or 0? to 8....

5000. 0 to 4 ?

4820–1870. Position. 3 and 3 a closely adjoin the solar line

b and the second or green band of the blue gasflame spectrum. 36 is at (6,F); and the

line or lines 3 c are near F. Description &c. The first three are distinct lines ;

the first most frequently observed; the second and third less commonly; lines in the fourth place (3 c) noted by Alvan Clark, jun., Barker,

and Angströin. Identification &c. 3, 3 b coincide with lines in

the negative glow, 3 b that of nitrogen in the nebulæ ; 3a with a constant strong line in the spark-discharge. The latter and 3 c, it may be, are only seen in auroral streamers of low

elevation.

4. Brightness 3–6 (fainter Full blue. with red line)..

Wave-lenyth 4663-4740. 4a.

4630-4665.

3a.

blue or

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3 c.

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Position. Middle and latter half of the third or

blue band in the blue gas-flame spectrum. Description &c. A double band, consisting of two

lines; the first rather more frequently noted

than the second in auroral spectra. Identification &c. The principal line and its com

panion agree well in position with the principal

band in the negative-glow spectrum.
Indigo,
blue and

5. Brightness 0-6. Wave-length 4285.
6.

4110.
violet.

Position &c. 5 coincides nearly with G and with

the fourth or indigo band of the gas-flame spectrum; 6 is between G and H, near the

hydrogen line h in the solar spectrum. Description &c. 5 is a frequent but somewhat

difficultly visible line; and, from its position, it is possibly the limit of vision for pocket spectroscopes ; 6 was measured once by Lemströin

at Helsingfors. Identification &c. 5 corresponds exactly with a

strong band in the violet in the negative-glow spectrum.

Remarks and Suggestions. The general character of the subspectrum appears to be a series of bright lines, bands, and shadings, more or less dimly visible on a faint field of light in the blue region of the spectrum, the greatest concentration occurring apparently most frequently at about the positions stated above. They arise, according to Ångström, from discharges of electricity from the denser to the more rarefied strata of the upper air, producing there on a great scale what is seen in artificial discharges of electricity in rarefied air as a blue cap round the negative pole. The appearance in the aurora of only one red line in the place of the many red bands of the negative glow, scarcely less bright than the principal one, is remarkable; and fresh observations are very desirable to confirm it, or to detect other red lines if they exist. Very red auroras should be examined with a wide slit, covered (if of advantage) with light-red glass to shut off all other light as much as possible.

Mixed with the lines of the negative glow, Ångström suppose that lines of the positive disruptive spark or brush discharge may appear in flashing auroras, especially near the base or arch

HH2

45

as distinguished from the tops of the streamers, giving to the subspectrum a different appearance according to the strength and agitation of the streamers: this may perhaps be traceable in the appearance and disappearance

of the lines 3 a, 3 c, and perhaps of other faint lines, whose positions should be noted. Such lines should also be searched for in quiescent parts, such stationary auroral bands and the tops of very bright streamers.

A correct recognition of some one or more of the lines de. scribed above, other than the citron line, is, however, of chief importance in observing with

small spectroscopes, as the leading lines themselves must supply the only standard intervals of comparison for eye-estimations of such faint spectra.

as

Ångström's representation of the Spectrum of the glow discharge round the negative pole of Air-vacuum Tubes,

and its comparison with the Spectrum of the Aurora.

Spectra of (1) negative pole in air, (2) aurora, (3) olefiant gas.
D

51 Wave-lengths, in hundred-thousanaths of a millimetre.

53 55 57

59

05

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