Aurora Spectrum

NATURE of highly trained and experienced geologists than to that of gentlemen, who, whatever thetr attainments m1.y be, certainly do not in this Report evince much acquaintance with geology. J. G. Reports of the Mim'ng Surveyors and Re-gistrars for Quarter ending March 3 r, r872. Victoria. THERE is nothing in these Reports calling for special notice. The total quantity of gold got respectively from alluvia (or, as the RP.ports have it, alluviums) and quartz reefs during the quarter were as follows :-Alluvial, 171,851 oz. 10 dwt.; quartz, 164,670 oz. 8 dwt.; total, 336,521 oz. r8 dwt. The quantity of gold, the produce of the colony, exported during the same period was 398,131 OZ. 10 dwt. LETTERS TO THE EDITOR [The Editor does not hold himself responsible for opini<Jns expressed by his correspondents. No notice is taken of anonymous communications.] Aurora Spectrum IN connection with my letter in last number of NATURE, I have in a diagram approximately placed the auora lines side by side with the spectrum of hydrogen and of some of the principal air lines (as given in Dr. Watt's index) and with the following results : Line No. 1. Close upon, if not identical with, an air-line marked by Huggins N 0, and Pli.icher 0. No. 2.-Not apparently coincident with any prominent airline. The coincidence with a line of oxygen noted by so careful an observer as Mr. Proctor is puzzling ; and if the instrumental power used was sufficient to ensure absolute identity, seems to indicate a sec.ond or unusual spectrum of that gas. No. 3 is not near any principal air-line. No. 4 is nearly coincident with a faint line of oxygen (confirmatory of Mr. Proctor's observations). No. 5 corresponds to a rather strong N line. No.6 does not coincide with any prmcipal air-line, very faint lines of 0 and N being the nearest. .. . . No.7. Upon clooe examination the posttwns. of thts lme as respectively fixed by Mr. Pre>ctor and Lord l:mdsay are t;J-Ot inconsistent and the line closely corresponds wtth a strong hne of oxygen situate on the less refrangible s!de_of sol:u G.. . :Wtth Nos. 1, 4, 6, and 7 fairly <;Orrespoud .m representative air-lines. None of the ltnes are tdenttfied h H a, H 13, or H -y, and it would appear that the aurora, tf a selects and spectra of atmospheric gases, ignores H a and the stronger N ltnes. The modtficatwn of compound spectra by conditions of and pressure, is however only a partially explored subject, and we have moreover no certain data of conditions in the case of the aurora, will assi:>t us in bringing it to bear. of sa-ne of I accidentally omitted from tl:e the zodiacal light that of Prof. Ptazzt Smyth, whose obs.ervattons in the south may be said to have concl<Isively demoltshed the supposed identity of the light, and. the aurora _(at least so fa_r as bright lines are concerned) made tt extrt;mely tmprubable that anything beyond a continuo lS spectrum wtll ever be seen m the pure zodiacal light, though a further search should be by no J. RAND CAPRON means neglected. Guildford, Jan. 10, 1873 Polarisation of the Zodiacal Light and of the Aurora IN the interesting article by Mr. Rand Capron in the last nnmber of NATURE-after collating the various of _the spectroscopic examinations of the aurora and zodtacal ltght which have appeared at different times in your pages, together with those which have been coilected by Dr. Sche!len-he terminates his analysis of the general results by remarkmg that he is " not aware whether the zodiacal light and the aurora have been examined with the and the "light, though faint, might be tested Wlth a N 1col s priSm and Savart's bands." I would refer him to a paper in the March number of tl,.e "Monthly Notices of the Astronomical Societ:r.".for 1871 1 in I 201 which an observation by Mr. Burton (late Msistant to the Earl of Rosse) on the polarisation of the zod acallight is described. Mr. Burton was one of the eclip>e party stationed at A'l'osta in Sicily. He m1de use of a Savart's polariscope, set so"as give a centre wher": the bands :were parallel to the plane of polansatlon. On lookmg to the bnghtest parts of the zodiacal light Mr. B<1rton believed that he could detect faint traces of polarisation, sufficiently strong to enable him just to recognise that the bands were black centred when their direction coin• cided with the axis of the cone of light, that is, when the direction passed throu!!h the position of the sun. To make sure that he not examining- the remains of air polarisation given by the slight remaining twilight, he examined the light of other parts of the heavens, but was un1ble anywhere else to detect any trace of b:tnds. In contradistinction, however, to this must be set an observation of my own, yieldiu()' a neo-ative result, though made on the same evening and with."a similar instrument, as well as with the same Sa.vart used by Mr. Burton. I was, however, unable to detect any trace of ban is etther upon the cones of zodiacal brightness or upon the adjacent parts of the sky; but it is very possible that Mr. Burton's eye may be more sensitive to faint lights than my own. In February last I also met with. a negative re•mlt in examining a faint trace of the zodiacal light visible in England. I then used a double-image prism as well as a Savart, thinking that its two oppositely polarised fields in juxtaposition might afford a more delicate test for so faint an obj <:ct. Capt. T<1pman while cruising in the Mediterranean has also I believe, repeatedly obtained negative results when makinnof a Savart on the zodiacal light. " And I understand that Mr. Lockyer, together with the other observer> of the Indian Eclipse of December 1871, totally failed to detect any traces of polarisation in the brilliant displays of the zodiacal light which they observed while crossing the Indian Ocean. I am therefore disposed to conclude that any traces of polarisation must be very slight, if indeed any polarisation at all is to be attributed to the zodiacal light itself and not to the veil of atmospheric impurities lying between us and objects near to the horizon. Certainly we may conclude that there is no such polarisation as is fo·und in the light of the solar corona or- -as we might expect-if the zodtacalligltt were caused by a great cloud of cosm\cal dust made up of particles smaller in diameter than the wave-length. Indeed there cannot be as great a percentage of polarisation, or, to spe:1k more exactly, as great a difference between the component radial to the sun's place and the component at right angles, as in the case of a sunbeam dispersed by the dust in our own atmosphere. For if any one will examine the track of a sunbeam passing through a room with a Savart, he will not fail to be struck with the distinctness of the band•. We seem therefore justified iu concluding, that if tl:te zodiacal light is composed of cosmicai dust, such dust particles must be considerably coarser than those which float in our own atmosphere. As to the polarisa (...truncated)