December 3, 1891] 



NATURE 



19 



have been used as solvents, and many remarkable results 

 obtained. The importance of the paper may be gathered from 

 the fact that in it is described " a method by which vulcanized 

 india-rubber of any quality or character whatever, as well as the 

 undecomposed or reclaimable part of rub'ier-waste, may be dis- 

 solved or liquefied in a reasonably short time, the solutions pos- 

 sessing any desirable degree of viscosity or diluteness, from 

 which india-rubber may be regained on evaporation of the 

 solvents." — Report of the examination by means of the micro- 

 scope of specimens of infusorial earths of the Pacific coast of 

 the United States, by Dr. Arthur M. Edwards. Seven new 

 fluviatile fossiliferous deposits from Oregon, California, and 

 Washington are described. — The Tonganoxie meteorite, by E. 

 H. S. Bailey. An analysis of the meteorite gave the percentage 

 composition : Fe 91 'iS, Ni 793, Co 0-39, Po'io, and a trace of 

 copper. The weight is 23^ lbs., specific gravity 7 '45, shape an 

 irregular triangular pyramid 9^ inches long by 6i inches wide 

 by 4^ inches deep. A fine figure showing numerous pittings on 

 the surface of the meteorite accompanies the paper. — Proposed 

 form of mercurial barometer, by W. J. Waggener. — Colour 

 photography by Lippmann's process, by Charles B. Thwing. 

 The results obtained seem to indicate — (l) that mixed colours 

 may be reproduced with a fair degree of accuracy ; (2) that an 

 exposure sufficiently long to give a clear image of the red is 

 quite certain to obliterate the blue by over-exposure ; and 

 (3) that an over-exposure may completely reverse the colours, 

 causing the original colours to appear on the reverse, and the 

 complementary colours on the film side of the plate. — New 

 analyses of uraninite, by W. F. Hillebrand. From the analyses 

 it appears that the species may be broadly divided into two 

 groups, one characterized by the presence of rare earths and the 

 almost invariable presence of nitrogen, the other containing 

 little or no nitrogen and no rare earths. Varieties of the farmer 

 group occur in more or less well defined crystals, whilst mem- 

 bers of the latter group are usually devoid of crystalline form. — 

 The Tertiary silicified woods of Eastern Arkansas, by R. Ells- 

 worth Call. The investigation has led to the following con- 

 clusions : — (i) The silicified woods of Eastern Arkansas are all 

 of Tertiary age. (2) They are derived from the beds of Eocene 

 clays that underlie the sands and gravels in which they com- 

 monly occur. (3) They are silicified lignite ; the process of 

 silicification has occurred either while they were still in clays, or 

 most often after they were removed and buried in the sands or 

 gravels. (4) They possess as yet no taxonomic value in deter- 

 mining the relative ages of the members of the Tertiary series. — 

 Occurrence of sulphur, orpiment, axd realgar in the Yellow- 

 stone National Park, by Walter H. Weed and Louis V. 

 Pirsson. — Mineralogical notes, by L. V. Pirsson. Some spe- 

 cimens of cerussite, hematite, and c.issiterite, gypsum, and 

 pennine are described. — Peridot dykes in the Portage sand- 

 stones, near Ithaca, N.Y. , by J. F. Kemp. — A new locality 

 of meteoric iron, with a preliminary notice of the discovery 

 of diamonds in the iron, by A. E. Foote. The existence 

 of black and white diamonds in the meteorite appears to be 

 established by indifference to chemical agents and hardness. 

 Carbon in the form of an iron carbide also occurs with the 

 diamonds. The meteorite was found in Canon Diablo, Ari- 

 zona. Three figures accompany the paper, — The South Trap 

 Range of the Keweenawan series, by M. E. Wadsworth. — 

 Geological facts noted on Grand River, Labrador, by Austin 

 Gary. 



SOCIETIES AND ACADEMIES. 

 London. 

 Mathematical Society, November 12. — Prof. Greenhill, 

 F.R.S., President, in the chair. — The President announced the 

 recent decease of Mr. H. M. Jeffery, F. R.S., who was elected 

 January 14, 1875. — The following gentlemen were elected to serve 

 on the Council for the ensuing session : Prof. Greenhill, F. R.S., 

 President ; Dr. J. Larmor, Major P. A. MacMahon, F.R.S., and 

 J. J. Walker, F.R.S., Vice-Presidents ; A. B. Kempe, F.R.S., 

 Treasurer ; M. Jenkins and R. Tucker, Hon. Sees. ; other mem- 

 bers, Messrs. A. B. Basset, F.R.S., E. B. Elliott, F.R.,S. J. 

 Hammond, C. Leudesdorf, A. E. H. Love, S. Roberts, F.R.S., 

 Drs. A. R. Forsyth, F.R.S., J. W. L. Glaisher, F.R.S., and 

 M. J. M. Hill. — The following communications were made : — 

 On selective and metallic reflection, by A. B. Basset, F.R.S. 

 It is well known that most transparent substances, which pro- 

 duce anomalous dispersion, exercise a strong selective absorption, 

 NO. I 153. VOL. 45] 



and at the same time strongly reflect rays of the same periods as 

 those which they absorb. Thus in fuchsine the order of the 

 colours going up the spectrum is blue, indigo, violet ; then there 

 is an absorption band, followed by red, orange, yellow. 

 The experimental laws relating to substances of this class may 

 be summarized as follows : (i) the rays which are most strongly 

 absorbed, when light is transmitted through the substance, are 

 most strongly reflected ; (2) when the incident light i? plane 

 polarized in any azimuth, the reflected light is elliptically 

 polarized ; (3) when sunlight is reflected, the colour of the re- 

 flected light, when viewed through a Nicol's prism whose principal 

 section is parallel to the plane of incidence, is different from 

 what it is when viewed by the naked eye. The phenomena of 

 absorption, anomalous dispersion, and the like, have formed the 

 subject of numerous theoretical investigations by German mathe- 

 maticians. It is not the object of the present paper to propose 

 any new theory upon the subject, but to discuss and extend the 

 theory of von Helmholtz. The theory of von Helmholtz is an 

 elastic-solid theory, which is based upon certain assumptions re- 

 specting the mutual reaction of ether and matter. The potential 

 energy of the system may be conceived to consist of three distinct 

 portions, viz. Wj, Wg, Wg, of which Wj is the ordinary expres- 

 sion for the potential energy of an isotropic elastic solid ; Wj 

 is a homogeneous quadratic function of the displacements of the 

 matter ; and W3 is a similar function of the relative displace- 

 ments of ether and matter, and is supposed to arise from the 

 mutual reaction of ether and matter. Having obtained the ex- 

 pression for the energy of the system, the equations of motion 

 can he at once written down ; and it will be found, on in- 

 tegrating them, that the index of refraction, /*, of light of period 

 T, is given by the equation — 



fi-= - - — ^ ; I + o iia w. iJTS-o C • • \^i 



Po 4^^ po ( A-x'Pi^k'^ - T-) - a'k-T^ 1 



In this equation p is the density of the ether when loaded with 

 matter, p^ is the density of the ether in vacuo, and pj is the 

 density of the matter ; k is the free period of the matter vibra- 

 tions, and o is a constant depending on the mutual reaction of 

 ether and matter. If we suppose that the value Tj of t, which 

 makes the denominator vanish, corresponds to the double sodium 

 line D of the spectrum, whilst a value r^, which makes n — o, 

 corresponds to the hydrogen line F, /a- will be negative when t 

 lies between D and F, and (i) accordingly represents a trans- 

 parent medium (such as fuchsine) which has a single absorption 

 band in that portion of the spectrum. Moreover, the dispersion 

 is anomalous, since the value of fi when t is a little greater than 

 T,, is much greater than its value when t is a little less than r„. 

 To explain selective reflection, I have provisionally adopted Sir 

 W. Thomson's hypothesis, that the ether is to be treated as an 

 elastic medium, whose resistance to compression is a negative 

 quantity, whose numerical value is slightly less than |rds of its 

 rigidity. Under these circumstances, the amplitudes of the re- 

 flected light will be given by Fresnel's sine and tangent formulae, 

 according as the incident light is polarized in or perpendicularly 

 to the plane of incidence. When fj? is a negative quantity, 

 these formulae become complex quantities of the form e'^'-^I'^ 

 and e^'^/i/\ and this indicates that reflection is total, and is 

 accompanied by a change of phase ; moreover, since the changes 

 of phase, /, /i, are different, according as the incident light is 

 polarized in or perpendicularly to the plane of incidence, it 

 follows that if the former is polarized in any azimuth the re- 

 flected light will be elliptically polarized. From these results it 

 appears that the colour of the reflected light is of a greenish 

 yellow when viewed by the naked eye ; but when it is viewed 

 through a Nicol, whose principal section lies in the plane of 

 incidence, a considerable portion of the yellow rays are refused 

 transmission by the Nicol, and the light under these circum- 

 stances is of a much richer green. Cauchy's formula; for metallic 

 reflection may be obtained from Fresnel's sine and tangent 

 formulas, by assuming that fi (= sin z'/sin r) is a complex quantity 

 of the form Re'* ; but the experiments of Jamin, and the calcu- 

 lations of Eisenlohr, show that the real jiart of fi- must be 

 negative, which requires that o should lie between 45° and 

 90. In fact, for silver, Eisenlohr finds that = 83^ Lord 

 Kayleigh, on the other hand, has shown that, if we attempt to 

 explain metallic reflection by introducing a viscous term into the 

 ordinary equations of motion of an elastic solid, physical con- 

 siderations require that the real part of m" should be positive ; 

 he has also shown that a similar objection lies against attempt- 

 ing to explain metallic reflection on the electro-magnetic theory. 



