286 



NA TURE 



[July 20, 1905 



account, also points out that the second phase is much 

 less well marked in the distant records than in the nearer 

 ones. From the figures given in the paper, it appears 

 that the times taken by the three phases of wave motion 

 to travel from their origin to its antipodes are respectively 

 about 20, 50, and too minutes. 



Physical Society, Jure 30.— Dr. R. T. Glazebrouk, F.R.S., 

 past-president, in the chair. — The comparison of electric 

 fields by means of an oscillating electric needle : D. Owen. 

 This paper describes experiments which show how an 

 " electric needle " may be used to measure electric fields 

 in a manner similar to that in which a magnetic field is 

 measured by an oscillating magnetic needle. The needles 

 used were cylindrical in form, of aluminium or of brass, 

 and were suspended by quartz fibres three or four inches 

 in length. The couple on the needle when disturbed from 

 the direction of the field is proportional to the square of 

 the field strength. For small displacements the needle 

 vibrates isochronously, the frequency being proportional 

 to the electric force. It may be used in alternating as 

 well as in steady fields, and may be applied to illustrate 

 manv of the laws of electrostatics. The disturbing effect 

 of the needle upon the field is considered ; in particular its 

 effect when placed in a uniform field. It is shown by 

 experiments that the disturbing effect falls off rapidly with 

 the distance from the needle, and is inappreciable (in the 

 case of a needle \\ cm. long) at a distance of twice the 

 length of the needle. With regard to the effect of the 

 dimensions of the needle upon the frequency (for given 

 field), while the restoring couple decreases rapidly with 

 decrease of size, yet the moment of inertia decreases more 

 rapidly, so that the smaller the iieedle the greater the 

 frequency, and also the smaller the disturbing effect. The 

 shielding effect of some dielectric materials was examined 

 in the following way : — K needle was suspended centrally 

 in the uniform field between a pair of parallel plates. A 

 thin-walled cylinder of the dielectric was placed around 

 the needle, and the shielding action denoted by a fall in 

 frequency of the needle. Glass and mica w-ere found to 

 effect perfect shielding. Ordinary paper shields ; but when 

 thoroughly dried by heat the electric field is transmitted 

 undiminished only to fall off to zero after a minute or 

 two's exposure to the air. Dry paper soaked in melted 

 paralTin-wax transmits the field perfectly and for an in- 

 definite time. The paper concludes by pointing out that 

 an electric needle suspended between a pair of parallel 

 plates forms a simple means of measuring high voltages, 

 since the frequency of vibration is simply proportional to 

 the voltage between the plates. — The magneto-optics of 

 .sodium vapour and the rotatory dispersion formula : Prof. 

 R. W. Wood. It has been shown in a previous paper 

 (hat the vapour of metallic sodium is an ideal substance 

 for investigating the effect of a strong absorption band 

 on the magnetic rotation of the plane of polarisation. The 

 preliminary work was not very satisfactory, as the method 

 employed did not admit of %'ery accurate determinations 

 of the wave-lengths. Improvements in the methods of 

 observation and design of the apparatus have been accom- 

 panied by an increase in accuracy, and accurate readings 

 have been obtained for as many as nine different values 

 of A between U, and D.. Rotations as great is 1440° 

 (four complete revolutions) have actually been observed, 

 and this with a 10 cm. column of not very dense vapour in 

 a field of 2000 C.G.S. units. In the present paper the 

 magneto-optics of the vapour for light travelling along the 

 lines of force are discussed. The sodium was heated in a 

 tube of thin steel, the ends of which projected from the 

 helices of the magnet. It was found that the field strength 

 within the steel tube did not differ greatly from that 

 obtained when glass tubes were used. A short piece of 

 small brass tubing is brazed into one end of the steel tube, 

 through which the steel tube is exhausted. A good vacuum 

 is essential, all traces of rotation disappearing in hydrogen 

 or nitrogen at atmo.spheric pressure. Light from 'an arc- 

 lamp made parallel by a lens is passed through a Nicol's 

 prism, the steel tube, and a second Nicol, after which it 

 is brought to a focus upon the slit of a spectroscope by 

 means of a second lens. In the present case, a concave 

 grating of 14 feet radius was used instead of a spectro- 

 scope, the observations being made both visually and by 

 NO. 1864, \0L. 72] 



means of photography. The paper then describes the 

 phenomena which are presented when the sodium vapour 

 is formed in the magnetic field. In the case of very dense 

 vapours the rotation has been measured over a consider- 

 able range of wave-lengths, namely, throughout the region 

 comprised between A = 5840 and A = 5g22. The rotation 

 constant of D, was found to be about double that of D,. 

 Drudo, in his " Lehrbuch der Optik," has given two 

 ■forraul<T? for the magnetic rotatory dispersion, the first of 

 which, developed from the hypothesis of molecular currents, 

 calls for an anomalous effect on crossing the band, and 

 does not apply to sodium vapour. The second, developed 

 from the Hall-effect hypothesis, predicts rotations of 

 similar sign and equal magnitude for wave-lengths 

 symmetrically situated in the spectrum, with respect to 

 the centre of the absorption-band. It seems likely that the 

 molecular currents play some part, and that the formula 

 built up on the hypothesis of the Hall-effect is incomplete. 

 However, the latter formula represents the rotation out- 

 side of the D-lines with great accuracy, while between the 

 lines it gives in some cases a curve which is elevated 

 somewhat above the experimental curve. The paper con- 

 cludes with an account of the bright-line spectrum pro- 

 duced by magnetic rotation which presents itself when the 

 Kicol's prisms of the apparatus are crossed. The spec- 

 trum, which at first could only be seen with difficulty, was 

 finally obtained of such brilliancy that it could be photo- 

 graphed with a 14-feet concave grating. A good vacuum 

 was found to be an essential condition, the presence of 

 inert gases causing a faintness of the lines. — The fluor- 

 escence of sodium vapour : Prof. R. W. Wood. The 

 fluorescence of sodium vapour has been investigated by 

 allowing light of various wave-lengths to illuminate the 

 vapour, and then studying the light emitted with a spectro- 

 scope. .Approximately homogeneous light of any desired 

 wave-length is obtained by means of a monochromatic 

 illuminator. .Some sodium is placed in a horizontal steel 

 tube fitted with steel ends, in one of which is a circular 

 aperture bored just above the centre. The tube is heated 

 and the vapour rises until it reaches the hole. The light 

 from the monochromatic illuminator passes through the 

 hole and falls upon the vapour. The fluorescent light is 

 then observed by means of a spectroscope either visually or 

 by photography. It is essential that the incident light 

 should not traverse an appreciable amount of the vapour, 

 or the fluorescent effects are masked by those of absorp- 

 tion. The bright lines of the fluorescent spectrum are bv 

 no means the exact complement of the absorption spectrum. 

 Very remarkable effects have been observed when the 

 vapour is illuminated with a very narrow band of approxi- 

 mately homogeneous light, the' lines in the fluorescent 

 spectrum changing their position and appearing to dance 

 about with the slightest change in the wave-length of 

 the exciting light. The motion is of course only an 

 illusion, lines disappearing and others re-appearing,' like 

 the sparks of a spinthariscope. Stokes's law is violated 

 in a most flagrant manner, bright lines coming out on 

 both sides of the excited region. The behaviour of the 

 spectrum indicates that we are dealing with a number of 

 groups of electrons, each group containing a large number 

 of vibrators. The excitation of one of these vibrators sets 

 the whole group going, but does not start disturbances in 

 the other groups. 



F.DINISL'RGII. 



Royal Society, June 19.— Dr R. H. Traquair in Ihe chair. 

 —A comparative study of the dominant phanerogamic 

 and higher cryptogamic flora of aquatic habit : George 

 West. Ihe paper referred to three loch areas of Scot- 

 land, namely. Loch Ness, the district between Nairn and 

 Forres, and the Island of Lismore. In the first district 

 the waters were peaty, in the third thev were heavily 

 charged with lime and were free of peat, while in th'e 

 second district^ the waters were neither Hmev nor peatv, 

 but were turbid and unwholesome in appearance, due to 

 the presence of marsh gas. These characteristics influenced 

 in a marked degree the habit of the aquatic flora, the 

 distribution and growth of which were also dependent on 

 the direction of the prevailing winds. Interesting details 

 were given.— Les concri5lions phosphat^es de I'Agulhas 

 Bank (Cape of Good Hope) : Dr. \Aon W. Collet ; aver 



