« PreviousContinue »
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 companion agree well in position with the principal band in the negative-glow spectrum.
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 Lemström 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 red many 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 supposes that lines of the positive disruptive spark or brush discharge may appear in flashing auroras, especially near the base or arch
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.
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 as stationary auroral bands and the tops of very bright
A correct recognition of some one or more of the lines described 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
VIII: Notices respecting New Books.
Nature Series. Polarization of Light. By WILLIAM SPOTISWOODE, M.A., LL.D., F.R.S., &c London: Macmillan and Co. 1874 (crown 8vo, pp. 129).
HIS work contains an admirably clear and exact account of the phenomena of Polarized Light. The author explains, in the first place, the various ways in which light is made to undergo the process of plane polarization (by passage through a doubly-refracting crystal and by reflexion and refraction by glass), and then describes the double-image prism, Nicol's prism, and Norremberg's apparatus. The subjects of the interference of plane-polarized light, the methods of producing circularly polarized light, and the interference of circularly polarized light are next discussed at considerable length with reference to pencils of parallel rays. These subjects fill two thirds of the volume; but, as well as the main subject, several collateral points of great interest come under notice in the course of the discussion: such are the various forms of the saccharometer, Sir C. Wheatstone's apparatus for producing circularly polarized light by reflexion at a metallic surface, and exhibiting the effects of passing light in that state through crystal films, the phenomena resulting from the action on polarized light of glass when unequally cooled, or when in a state of stress, the phenomenon of atmospheric polarization, the polar clock, &c. The student who has made out this part of the volume will be able to understand easily the account which follows of the results arising from the use of a pencil of divergent rays, and the beautiful phenomena of coloured rings produced by the action of various crystals on such a pencil. In several places the author notices the results of an examination of the spectrum of the pencil of interfering light which issues from the analyzer; and a good deal of attention is paid to the production of complementary colours by polarized light. In fact the last chapter of the volume is a reprint of the author's paper "On Combinations of Colour by means of Polarized Light," originally published in the Proceedings of the Royal Society,' vol. xxii. p. 354.
It will be seen from the above brief account of its contents that the work before us passes in review all the main points of interest in the subject. The author speaks of his book as a talk rather than a treatise on polarized light," and says that it contains "the substance of lectures delivered at various times to my work-people." It is therefore so far a popular book, that the subject is discussed without the aid of mathematical symbols; and doubtless, when the experiments described were actually exhibited, the lectures would be very interesting to any intelligent audience. This, however, does not prevent the book from being decidedly hard reading; and it would be easy to point out passages which could not be fully understood by a reader wholly without the mathematical learning which the author has at command. On the other hand, a reader
o already has some acquaintance with the mathematical treatt of the subject will find the time usefully spent which he de
votes to the study of a work like the present, which gives with great clearness, accuracy, and brevity the facts of the subject clothed, as far as may be, in the language of daily life.
The illustrations are sufficiently numerous: some of them are little more than diagrams; others are elaborate drawings of instruments, and are very good, e. g. those on p. 60 and p. 102. There are also two whole pages of coloured figures (which are very beautifully done), showing the rings and brushes produced by polarized light in the case of six crystals, two complementary figures for each crystal. In conclusion we can cordially recommend the book to the somewhat select class of readers who take an intelligent interest in this beautiful branch of science.
IX. Proceedings of Learned Societies.
[Continued from vol. xlviii. p. 546.]
February 4, 1874.-His Grace the Duke of Argyll, K.T., F.R.S.,
The author first described the general physical characters of the valley of the Rhine, and discussed some of the hypotheses which have been put forward to explain them. His own opinion was that during portions of the Miocene epoch the drainage through the great valley between the Schwarzwald and the Vosges ran from the Devonian hills north of Mainz into the area now occupied by the Miocene rocks of Switzerland. Then after the physical disturbances which closed the Miocene epoch in these regions, the direction of the drainage was reversed, so that, after passing through the hill-country between the lake of Constance and Basel, the river flowed along an elevated plain formed of Miocene deposits, the remains of which still exist at the sides of the valley between Basel and Mainz. At the same time the Rhine flowed in a minor valley through the upland country formed of Devonian rocks which now constitute the Taunus, the Hundsruck, and the highland lying towards Bonn; and by the ordinary erosive action of the great river the gorge was gradually formed and deepened to its present level. In proportion as the gorge deepened, the marly flat Miocene strata of the area between Mainz and Basel were also in great part worn away, leaving the existing plain, which presents a deceptive appearance of having once been occupied by a great lake.
2. On the Correspondence between some Areas of Apparent Upheaval and the Thickening of Subjacent Beds." By W. Topley, Esq., F.G.S., Geological Survey of England.
The author referred to many instances in which beds have unequal development, being much thicker in some places than in others; and the main object of his paper was to show that such thickening and thinning of beds has an important effect in producing the apparent dip of overlying beds. The thinning of any one bed may
have an appreciable effect in producing or increasing its own apparent dip; but where a whole series of beds thin constantly in one direction, the amount of the dip of one of the higher beds, due to the sum of the thinnings of the underlying beds, is often very considerable.
In illustration of this the author indicated the Lower Jurassic rocks between Leckhampton Hill and Burford. The Inferior Oolite, and the Upper, Middle, and Lower Lias thin out rapidly to the east along this line; the base of the Middle, Lias is nearly a horizontal line, the easterly "dip" of the Great Oolite being due to the easterly thinning of the Middle and Upper Lias and the Inferior Oolite. The base of the Lower Lias has a westerly dip.
It is generally supposed that the dip of any bed is due to great movements of the earth's crust; from the facts mentioned the author argued that our inferences as to such movements will vary according to the beds which happen to be exposed at the surface. In the example given we assume a westerly upheaval because we see the Great Oolite dipping to the east. If over this area the whole of the higher strata had been removed down to the Middle Lias, we should perceive the beds to be flat; if the denudation had exposed the base of the Lias, we should probably suppose that along this line there had been an easterly upheaval.
Under London the entire series of strata between the Paleozoic rocks and the base of the Upper Cretaceous is absent; as we recede from the London Basin the intermediate beds necessarily come in and gradually thicken, producing the dip of the Cretaceous beds towards the London Basin.
The Paleozoic rocks under London are about 800 feet below sea-level. We now know, by the sub-Wealden boring, that under the centre of the Weald the Palæozoics are also below sea-level. Supposing that they should be found at about the same depth as at Kentish Town, then the Paleozoic floor will be approximately a straight line, whilst we know that the whole of the Wealden and other cretaceous beds dip to the north, their dip being thus wholly accounted for by the gradual thinning out of all the beds as they approach London. In the case of the Weald, some of the strata are of freshwater origin; we must suppose that the area of water within which they were deposited had some limit to the north, and that the Paleozoic floor, which is now approximately flat, rose up to the north as a bounding ridge. But even if this be so, it is evident that the present dips are no exact measure of the amount of upheaval which the beds have undergone.
Other examples were quoted in which the observed general dip corresponds in direction with the known or inferred general thinning; and it was shown that in all such cases we are liable to serious error in inferring the amount of upheaval from observed dips.
It is, however, evident, from the faults intersecting strata, that upheavals and disturbances have taken place; but unless we assume every bed to have been deposited on a perfectly horizontal plane, we cannot infer the amount of such upheaval from the present position of the bed. In all cases we must take into account the actual or possible thinning of underlying beds.