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IN the preface to the first of the above books, the author mentions a question asked him by a young woman friend: “When you began this work you had no idea that anything so clever would come out of it, had you?” This quotation certainly sums up the impression conveyed after reading (1). Schrodinger, however, uses it to bring out the point that in a set of papers in which a single theme is developed, the results of the later papers were more or less unknown when the earlier ones were written. This must be borne in mind when judging these collections. (1) Collected Papers on Wave Mechanics. By Prof. E. Schrodinger. Translated from the second German edition. Pp. xiii + 146. (London and Glasgow: Blackie and Son, Ltd., 1928.) 25s. net. (2) Four Lectures on Wave Mechanics: delivered at the Royal Institution, London, on 5th, 7th, 12th, and 14th March 1928. By Prof. Dr. Erwin Schrodinger. Pp. viii + 53. (London and Glasgow: Blackie and Son, Ltd., 1928.) 5s. net. (3) Selected Papers on Wave Mechanics. By Louis de Broglie Dr. Leon Brillouin. Authorised translation by Winifred M. Deans. Pp. vi + 151. (London and Glasgow: Blackie and Son, Ltd., 1928.) 15s. net.

SOME thirty or forty years ago geometricians were much interested in the endless band of paper to which one half twist had been given before joining the ends. This gave the figure having only one surface and one edge. At that time those who studied this figure were so obsessed with the consequence of cutting down the middle line of such a band or of a band with two or more half twists that I believe no one noticed the result which I wish now to describe. It is the doubling up in a proper manner of an endless band to which four half twists have been given so as to produce the endless band first described but of double thickness. The first band is shown at A, as an endless belt connecting two crossed shafts for which, as is well known, it is exactly fitted. B shows the band with the four half twists all on one side in the form of two complete loops, and to be uniform with the other it is shown as an open band connecting two parallel shafts. The only object of putting in the shafts and pulleys is to assist the perspective. They are not wanted in making the experiment. Now if B, which appears sufficiently uncompromising, is folded up properly it becomes A but of double thickness.

MR. BURKE'S observation regarding the reflexion from paper as viewed by the two eyes separately may have an explanation other than that suggested by Dr. Harrison (NATURE, May 30, p. 613). A simple comparator I have in use has magnesium carbonate block reflectors set substantially vertically and parallel to the plane of a large, tall window, so that these very excellent diffusing reflectors receive skylight at roughly 45° incidence. The light reflected normally is received on two first-surface mirrors arranged in a casing and viewed through a tube at one side, so that a square field divided by a barely perceptible vertical septum is observed. Customarily, this instrument is set up so that the observer has the window at his left-hand side. Thus the left eye is somewhat exposed to direct light from the window, whereas the right eye is appreciably protected by shadow thrown by the nose. On changing from left to right eye a distinct difference is noticed in the blueness or redness of any near-white field examined, and this is greatly reduced by placing a screen so as to prevent much light from the window striking the left eye.

IT may be allowed to me to remark that the error mentioned on pp. 172 and 223 of NATURE has its origin really in Helmholtz's renowned paper (“Ueber die Erhaltungder Kraft,” 1847, p. 67), and that it thence found its way into most of the textbooks on electricity. It has sometimes been detected and hinted at; for the first time, I believe, in C. Neumann's paper published in the Ber. d. k. sachs. Ges. d. Wiss, at Leipzig (1871), “Elektrodynamische Untersuchungen mit besonderer Rucksicht auf das Princip der Energie.” There we find (p. 436) the formula—which is identical with the equation of energy given in NATURE, p. 223, if we put—

OSBORNE REYNOLDS, in a short paper entitled “On the Action of Rain to Calm the Sea"1, discussed the action of rain in calming a rough sea. He expressed the opinion that each drop of rain produces a vortex ring which, on descending into the water, transfers momentum from the surface layers to the underneath layers, thus reducing the relative motion of the layers. It was suggested to us by Prof. A. H. Gibson that further experiments were desirable to confirm the above theory.

I HAD no idea that the short paper you did me the honour to print on this subject would have led any one to suppose that a claim was made for the discovery of a new principle in physics, or that the microscope was to be used for the first time in questions on refraction.

Two years ago W. Kolhorster published a short paper l on the gamma rays of potassium, which he ascertained partly by measurements in the Stassfurt mines and partly by analysing the radiation of a greater quantity of sylvin supplied by Messrs. C. A. F. Kahlbaum.

MANY measurements of the diffuse X-ray and γ-ray background1–4 indicate that it consists of an inverse power law spectrum with index ∼2.0. Spectral features or excesses are reported in the vicinity of ∼40 keV and ∼1 MeV. When plotted on linear–linear paper the 1 MeV excess is seen to consist of a pronounced peak in the 0.5 to 2.0 MeV region with a feeble tail extending on to higher energy (Fig. 1). In this paper a simple explanation is presented for the 1 MeV peak in light of new experimental and theoretical advances made in the past year. No explanation is presented for the higher energy excess and it is presumed to come from other mechanisms. I hypothesise that the peak arises from the astrophysical production of deuterium by the two body recombination process (p+n→D+2.2 MeV γ ray) in an environment transparent to the emitted γ rays. The hypothesis is shown to lead unambiguously to two remarkable numerical results: a calculation of the terrestrial n(D)/n(H) ratio; and a prediction of QSO cutoffs at z∼3.

QUANTUM theory originated in the work of Prof. Max Planck of Berlin in the year 1900. It is significant that the new theory arose out of a discrepancy between theory and experiment in a somewhat recondite field of investigation, described by Lord Kelvin as one of the clouds over nineteenth century physics. The subject of research was the character of the heat radiated from a hot body—at first glance not a very promising field for anyone looking out for startling discoveries. But when we reflect that this ‘radiation problem’ involves the whole question of radiation and its relations to tangible matter, its importance becomes evident. Planck's paper “On the Distribution of Energy in the Normal Spectrum” was read before the German Physical Society on December 14, 1900. He had already announced his radiation formula, and in this paper he described a method of deducing it from the hypothesis that the rectilinear oscillator (dipole) of Hertz, regarded as a source of radiation, can possess energy only in integral multiples of the quantum hv. The universal constant, h, of the radiation law, being the product of energy and time, was called the elementary quantum of action. (1) Theory of Electricity and Magnetism. Pp. xii + 247. 10s. 6d. net. (2) Theory of Light. Pp. vii + 216. 10s. 6d. net. (3) Theory of Heat. Pp. viii + 301. 12s. net. Being Vols. 3, 4 and 5 of Introduction to Theoretical Physics. By Prof. Max Planck. Translated by Prof. Henry L. Brose. (London: Macmillan and Co., Ltd., 1932.)

OUR knowledge of the nature of muscular motion has hitherto been divided into two main fields of successful analysis. First there was the discovery of the phosphorylation cycles whereby energy is transferred from carbohydrate breakdown to the muscle fibre; here the earliest landmarks were the investigations of Fletcher and Hopkins1 and of Harden and Young2 (see the papers of Parnas3 and D. M. Needham4 for up-to-date reviews). Secondly, there was the discovery of the elongated or anisometric character of the particles of muscle-globulin (myosin), demonstrated in the classical paper of v. Muralt and Edsall5, leading to the application of X-ray techniques to the problem, and the establishment by Astbury6,7, that muscular contractility is essentially a molecular contractility of protein chains. The exact connexion between these two orders of fact, however, still remains obscure.

IN a recent paper L. Tonks1 has criticized the application of the Bethe method to the solution of problems in adsorption. I. Langmuir2 had earlier mentioned the criticism and supported it. I have already given an answer3 to Langmuir's remarks so far as was possible before the appearance of the paper by Tonks.

THE model1,2 presented by R. T. Jones for a swivel wing supersonic transport does not reveal the inherent stability of trim when the wing is rotated. This stability may very simply be demonstrated by a paper model (Fig. 1). A straight strip of paper is taken and trim tabs are cut as shown. For the subsonic flight condition, wing tip fins are bent up at right angles to the span and the nose weight, a wooden match split halfway along its length, is set parallel to the fins. To convert to the supersonic swivel winged condition, new fins are bent at 45° to the span and the match is moved to a new position parallel to the fins. Straight flight is achieved by bending the fin tips and trim for level flight by bending up the trim tabs and adjusting the extension of the match stick.

THE purpose of this letter is to point out an error in sign in Prof. A. Sommerfeld's original paper (1909) on the attenuation of radio waves1. This error in sign has recently been reflected in Bruno Rolf's graphs2 of the Sommerfeld formula, predicting dips to zero in the field intensity at finite distances from a radio transmitter and other anomalous phenomena. This error in sign has been corrected in Prof. Sommerfeld's 1926 papers3 and also does not occur in the derivation by B. van der Pol and K. F. Niessen4. In this latter paper an exact expression is given for the potential of a vertical infinitesimal dipole (equation 21). After expanding this expression, I found that most of the terms are negligibly small at moderately low frequencies for distances from the source greater than a wave-length, giving for the potential function of a vertical dipole over a plane earth: where e is the dielectric constant of the ground referred to air as unity, is the conductivity of the ground in electromagnetic units, c is the velocity of light in cm. per sec., is the wave-length in cm. and r is the distance in cm.

IN a paper on “The Genesis of Pleochroic Haloes” (Phil. Trans. R.S., vol. ccxvii.) by J. Joly, a theory is advanced accounting for certain structural features of the halo on the assumption that, reversal of the. halo-image is possible, and may take place under conditions defined in the paper. In support of this a drawing of a halo is given in which an evident inversion or change from positive to negative has occurred, the inner region being light, the outer dark.

IN a paper communicated to the Royal Society in December last (Roy. Soc. Proc., A, vol. lxxx., p. 178), an abstract of which appeared in this journal (vol. lxxvii., p. 314, April 2), I directed attention to a peculiar form of prominence which had been photographed with the spectro-heliograph of the Solar Physics Observatory in the “K” light of calcium. This prominence, situated towards the south pole of the sun in the eastern quadrant, about position-angle 137°, was recorded on two separate negatives taken at the times 3h. 14m. p.m. and 3h. 50m. p.m. G.M.T. on July 17, 1907. Although on each photograph images of other prominences were recorded, no particular attention was directed to them, as they did not present any unusual features. It may, however, be incidentally remarked that the most intense prominence recorded on both the photographs, and situated near the south pole in the western quadrant about position-angle 218°, was reproduced in, the paper in Plate iii., Fig. 5. I did not think it necessary for the purpose of that communication to reproduce the whole limb of the sun, but confined myself to the disturbed area in the south-eastern quadrant. The presence of a large prominence in the south-western quadrant has raised questions of identity, so the complete limb is now here reproduced (Fig. 1) to show the relationship between the two prominences and the sun's south pole.

THE recent paper of Prof. Milne1 on “The Analysis of Stellar Structure” has excited great interest, and several papers by various investigators have appeared on the subject. The knowledge of the opacity coefficient for the different physical states in which matter can exist in the stellar interiors is of importance in these developments. The value of the opacity coefficient for the non-relativistic non-degenerate case was first worked out by Eddington following Kramers' treatment of X-ray absorption. The non-relativistie degenerate case has been recently treated by several authors.

IN a recent paper, Holloway and Moore1 have given a curve showing the average specific ionization, as a function of the residual range, for protons in air. On this curve the ionization maximum occurs at 1.9 mm. residual/range, and in the course of the paper the absolute value of the maximum specific ionization is quoted as 2,320 ion pairs per mm. The shape of the ionization/range curve was deduced from earlier measurements of Parkinson, Herb, Bellamy and Hudson2, and the absolute value of the ionization was obtained on the assumption that, in the region of 3 cm. residual range, the mean specific ionization produced by a proton is a quarter of that produced by an α-particle of the same velocity. On this basis Holloway and Moore inferred that the maximum specific ionization is about one third as great for protons as for α-particles in air (6,760 ion pairs per mm.). Now, while there is overwhelming evidence for the correctness of this last assumption (the 1:4 ratio of ionizations for particles of the same velocity, when the ratio of the effective charges is constant at 1:2), it also appears that the conclusions, regarding the position and height of the ionization maximum for protons, to which the subsequent argument leads, run contrary to accepted evidence from other experiments. If this be admitted, then one must clearly assume that the results of Parkinson, Herb, Bellamy and Hudson will not bear the detailed analysis which Holloway and Moore place upon them, when protons of the smallest energy are concerned. It is the object of the present note to point out the contrary evidence referred to above.

IN the more or less popular accounts which have recently been given of Prof. Arrhenius' theory of cometary tails and the aurorae, it is generally stated that the smaller the diameter of the corpuscle upon which the light is falling the greater the excess of light-pressure over gravitational force. This explanation, however, holds only so long as the diameter is greater than the wave-length of light. If the diameter becomes of the same order as the wave-length, the ratio between light-pressure and gravitation follows an entirely different law. This has recently been demonstrated by Prof. Schwarzschild by an exhaustive mathematical treatment of the question in a paper entitled “Der Druck des Lichtes auf kleine Kugeln und die Arrhenius'sche Theorie der Cometenschweife” (Sitzungsberichte der k. b. Academie der Wissenschaften zu Munchen, 1901, Heft iii.). The conclusions arrived at in this paper are of considerable importance in so far as they show that the effect of gravitation is exceeded by that of the pressure of light only so long as the diameter of the corpuscle is greater than about 0.07.μ For this limiting value the two forces are exactly balanced; but for smaller values of the diameter the light-pressure becomes rapidly less, so that it is then always exceeded by gravitation. It would appear from Prof. Schwarzschild's computations that the globular corpuscles thrown off in the tails of comets should have diameters not smaller than 0.07μ and not exceeding 1.5μ, supposing the specific gravity of the corpuscle to be that of water. Now these values far exceed the limits assigned to the dimensions of the molecules. According to our present knowledge, based on theory as well as on experiment, we are forced to conclude that the diameter of a molecule cannot be larger than 0.003μ. On the other hand, the specific gravity of a molecule of air, for instance, appears to be very great, viz. five times that of water (see Meyer, “Die kinetische Theorie der Gase”). This maximum value of the diameter of a molecule is so enormously smaller than the values demanded by Prof. Arrhenius' theory that the latter appears to be incompatible with any assumption which regards the cometary matter as being of a gaseous constituency. In order to explain the repulsion of matter in the tails of comets by the pressure of the sunlight, this matter must be assumed to consist of small drops, each of a bulk sufficiently large to harbour at least one million molecules within its bounding surface. Whether such an assumption can be justified appears to me very doubtful. At any rate, Prof. Schwarzschild's profound mathematical investigation makes it absolutely clear that the idea of minute electrically-charged corpuscles—of about one-thousandth the size of a hydrogen atom (see Observatory, February 1902, p. 103)—being propelled by the sun's light towards the earth and causing the various phenomena of aurorae, Gegenschein, &c, receives no support from the mathematical point of view. But, even apart from these difficulties, it can hardly be said that the ingenious theory of Arrhenius settles the question as to the nature of the force acting on the cometary matter. So far it offers no explanation of the remarkable phenomenon of the contraction of the coma with the approach towards the sun. Doubtless this contractile force is also of solar origin. But can it be identified with the force which repels the cometary matter in the direction of the radius vector? It appears to me that the theory in its present form only removes one difficulty by introducing several others.

IN investigating modes of vibration of truncated conical shells of paper, aluminium, and glass, the frequency of a certain resonance group was found to be dependent upon the dimensions only. Table 1 gives some of the data obtained when the cones were driven in a large, highly damped room, by a coil 2.5 cm. in radius wound on a short paper tube.

IN a paper which has recently attracted a considerable amount of attention, Kapitza1 has proposed a standing electromagnetic wave description of ball lightning. His paper presents a good qualitative explanation of this effect in terms of the resonance absorption of a standing wave due to a conducting ionized sphere of plasma, occurring when the wavelength of the radiation is approximately four times the diameter of the fireball.

IN a paper published in 19411 measurements were given of scotopic and photopic sensitivity, for which nine observers were used. These measurements were made in the range between the mercury lines 709 and 302 mμ. In the paper by Goodeve, Lythgoe and Schneider published in 19422 measurements were given of scotopic sensitivity, for which six observers were used, including one with an aphakic eye. Their measurements were made at the mercury lines at 365 and 546 mμ. It is of interest to compare the results obtained after dark adaptation of one hour, with the results of Goodeve et al., obtained after dark adaptation for 10 minutes.

A LETTER in NATURE, November 28, p. 785, under the above title by Dr. G. C. Abbot, discusses a recent paper of mine in the Proceedings of the Royal Society. It refers to two of the subjects investigated in my paper, the 27–day interval, and the relation between corresponding daily values of S (the solar constant), as published by Dr. Abbot, and of C the international character figure, intended to supply a measure of magnetic disturbance. To take the second question first, Dr. Abbot's statement is as follows: “Dr. Chree does find indications of magnetic disturbance associated with low values of the solar constant. Inasmuch as higher solar constant values are generally associated with numerous sunspots and abundant magnetic disturbances, he thinks this paradoxical finding of low solar constants associated with magnetic disturbances is non significant. On the contrary, it is exactly what we should expect…the passage of an individual sunspot group over the central meridian of the solar disc is almost always associated with decreased values of the solar constant, and doubtless frequently with floor terrestrial magnetic disturbances. Hence it is prevailingly with low rather than high solar constant values that individual magnetic disturbances will be found associated.”

THREE recent letters raise the question as to whether there is any experimental basis for applying Kramers' equation for continuous absorption to astrophysical problems1. Ditchburn cites the anomalous phenomena shown by alkalis at the absorption series limits as the only direct experimental evidence. A series of papers2 by me on the emission spectra of caesium vapour gives direct evidence as to the transition probabilities for the continuous spectra and line spectra of the levels 6 P and 5 D. The first paper indicated that the probability of recombination depended on the pressure but subsequent work showed this to be erroneous, and the last paper (published after the appearance of Ditch-burn's letter) shows that the continuous transition probabilities remain nearly constant with electron concentrations ranging from 1012 to 1016 per cm.3 and with pressures from 0.001 mm. to 17 mm. of mercury.

UNDER this heading, in your issue of 5th inst., you report a paper by Prof. Osborne Reynolds, M.A., read at a meeting of the Manchester Literary and Philosophical Society, Nov. 29 last. This paper sets forth that the tails of comets, the solar corona, the aurora, and the Zodiacal lights are due to the ether which “fills” space. Comets' tails, as stated by the Professor, in his paper, are an effect due to the medium through which it passes being heated and illuminated by the comet; and that the other phenomena are also due to the ether.

IN a paper by E. Bruche on “Wirkungsquerschnitt und Molekulbau”1 it is stated on p. 1105 that the curve which shows the variation of the effective cross-section of methane molecules with respect to slow electrons for different velocities of the electrons (we shall call these curves Q-V-curves, since the velocity is given in terms of volts) is of the argon type, namely, that as the velocity of the electrons decreases from 4 volts to 1 volt (that is, from 2V to 1V) the Q-V-curve passes through a maximum and then rapidly sinks to a much lower value. This result was also obtained by R. B. Brode.2 In a later paper by C. Ramsauer and R. Kollath,3 Bruche's Q-V-curve is extended to still lower electronic velocities (to about 0.39V) and is found to pass through a minimum. We have now determined the Q-V-curve for methane by Townsend's diffusion method and have also found that the absolute value of the effective cross-section Q of methane molecules is very low, like that of argon, and that the Q-V-curves of argon and methane closely resemble each other over the range of velocities investigated, which in our case was from 1.74V down, to 0.3V. The minimum Q obtained for methane by Ramsauer and Kollath was 4.7 cm.2/cm.3 at a velocity of 0.62V; our value is 4.3 cm.2/cm.3 at a velocity of 0.52V. The minimum in our curve is a little less sharp than in the curve of Ramsauer and Kollath.

IN a paper under this title which has recently appeared,1 we have described and discussed observations which have led us to the conclusion that the light quantum possesses an intrinsic spin equal to one Bohr unit of angular momentum. In the four weeks which have elapsed since that paper was put into print, the experimental technique has been much improved in the direction of attaining greater precision. It appears desirable forthwith to report our newer results, which confirm the conclusion stated above. Chandrasekhara Venkata Raman is probably most famous for the discovery of the effect that bears his name — the Raman effect describes the change in frequency and phase of light as it is scattered in a medium. In 1930, Raman won the Nobel Prize in Physics for this work, and two years later he was still concerned with the passage of photons through materials. With his colleague S. Bhagavantam, he performed a careful study of the degree to which light becomes depolarized as it Rayleigh-scatters through gaseous oxygen, carbon dioxide and nitrous oxide. Their conclusion was clear and fundamental —"the light quantum possesses an intrinsic spin equal to one Bohr unit of angular momentum".

THE letter (NATURE, May 19) of Messrs. Takamine, Dieke, and Suga, reporting certain results in connexion with the analysis of the band spectrum of helium, was of peculiar interest to me, in view of the fact that I was proposing shortly to incorporate many of them in Part V. of a series of papers dealing with this subject. Mr. A. Harvey, working in collaboration with me, has measured and interpreted a number of new bands, chiefly in the less refrangible region, and, so far as can be gathered from the letter in question, has arrived at substantially similar conclusions. There appears to be at least one important difference of interpretation, but discussion of this had better await detailed publication. Meanwhile it may avoid confusion to remark that in all probability the band 3X → 2P (‘ortho-He’) of Takamine, Dieke, and Suga, is that near λ5885 already described in Part IV. (Proc. Roy. Soc, A, 118, p. 157), whilst one designated 4Z→2P by Dr. Jevons and myself in a paper at present awaiting publication is actually the next series member to their 3Z→2P. It is remarkable, and fortunate, that the same symbols X and Z have been respectively chosen in both cases for the new levels. The latter was employed in our case because of the large and unusual Zeeman effect exhibited by the lines of this band.

IN a letter to NATURE of October 31, 1907 (vol. lxxvi., p. 661), a copy of which arrived here recently, Mr. Barkla criticises a paper of mine which was published in the Philosophical Magazine for October. In that paper I tried to show how closely the properties of γ and X-rays were in agreement with the hypothesis that they consisted mainly at least of neutral pairs, and I pointed out that even the peculiar polarisation effects which Mr. Barkla had shown to exist might be explained, not unreasonably, as a consequence of the rotatory motion which such pairs would probably possess. I suggested that a pair might be more likely to become entangled with and deflected by an atom revolving in the same plane as itself.

MAY I place on record what is, I think, the most direct evidence that the modified scattered radiation is due to the super-position of unmodified scattered radiations? When using a certain primary X-radiation, the scattered radiation from air was found to be totally unmodified radiation, i.e. a radiation with accurately the same absorbability as the primary radiation exciting it. The radiation scattered from paper or paraffin-wax was very definitely a modified scattered radiation, or contained a modified radiation, i.e. differed considerably in absorbability from the primary radiation. Also these two radiations scattered from paper and from paraffin-wax were equally modified—within a small possible error.

GAMOW1 suggested in a classical paper that the present element abundances (in particular the light elements H, D, He and Li) may have been formed during the first few minutes of the birth of the universe. This suggestion received further support after the 3 K background radiation, also implied in that paper and numerically predicted by Alpher and Herman2, was detected by Penzias and Wilson3. There are, however, still serious objections to a primordial origin for helium.

THE point developed in a new direction by Prof. Raman had been noted by Lord Rayleigh, and was mentioned very cursorily in the last sentence of my paper in the Phil. Mag. to which his letter refers. The main purpose of that paper was to express the view that, so far as I understand, independent scattering of light by the molecules of a homogeneous medium, so dense that there are very many molecules per cubic wave-length—for example, in the atmosphere—must arise from the thermal motions of the molecules rather than from irregularity of their spacing. In directions, however, that are nearly coincident with the transmitted ray there can be no sensible dispersal of phase from either cause; disturbances, therefore, completely conspire, and the light scattered by the molecules in such directions is, in Lord Rayleigh's phrase, specially favoured. Prof. Raman points out that if the phases in directions near that of the ray did not in fact thus agree, the molecules of the material medium could take no concordant part in the transmission of the energy of the main beam, and regular propagation would be impossible. It is involved in this remark that each molecule will exert its full effect on the index of refraction, however irregular the distribution may be, provided it is not so dense that the molecules will obstruct each other; and, moreover, the thermal motions will not disturb this effect. The specially favoured directions, for disturbances passed on by the molecules, must be almost coincident with the ray—must, in fact, belong to the ray after the manner of diffraction, when it is regarded as a physical filament of light rather than as a geometrical line.

In a previous paper 1 it has been shown that if the stress components at a point of a fluid depend only on the state of flow in a small element about that point at the instant considered, then in general, two physical parameters are required to define completely the flow properties of the fluid. One of these is the viscosity and the other will be called the normal-stress coefficient. Each of these may or may not be dependent on the state of flow. In the present paper, a method similar to that adopted by Kramers 2 to calculate the intrinsic viscosity of a high-polymer solution has been employed to calculate its normal stress coefficient. It is shown that appreciable values of the normal-stress coefficient may be expected in concentrated solutions and that its non-vanishing results from the orientation of the molecules in the flowing solution.

IT is interesting to find that an X-ray bulb having a rhodium antikathode gives off a strong, sharply defined (and therefore very homogeneous) beam which is reflected from the (100) face of rock-salt at a glancing angle of 6·2°. Its mass absorption coefficient in aluminium is 3·2. A second weaker beam is reflected at an angle of 5·8°, and this appears to complete the rhodium X-spectrum. Assuming the correctness of my son's determination of the spacing of the atoms of rock-salt (in a paper read before the Royal Society on June 26), the wavelength of the stronger beam is 0·61 × 10–8, and of the weaker 0·57 × 10–8. It can be calculated that radiation of about this wave-length should be emitted by a rhodium antikathode; the argument is given in a paper recently read before the Royal Society (see abstract on p. 496 of this issue).

GAYDON1, in a recent paper on a new Green band system of nitrogen, has made some observations concerning the effect of excitation on the intensity distribution, from which he concludes that the new bands are a consequence of ionic rather than electron-collision excitation. He bases his conclusions on the results of Smyth and Arnott2 on the vibrational intensity distribution in the N2 band systems. It appears that Gaydon had not seen a more recent investigation of Tawde3 on the intensities in the second positive system of nitrogen. The results of Smyth and Arnott are merely the visual and densito-meter estimates of photographic blackenings ; while those of Tawde are the accurate quantitative intensity values of bands derived by rigorous methods of photographic photometry expressed in terms of energy units of a calibrated standard lamp. It has been shown by the present author and his collaborators4 in several papers that mere visual or densitometer estimates can be wrong by several percent. Hence it is doubtful how far the view put forward by Gaydon regarding the mechanism of the excitation of the new bands can be sustained.

CLERK MAXWELL used to say that it was a sign of progress when we began to overhaul the foundations of our science and return to the very beginnings. Prof. Armstrong is calling upon us to do this in electricity. All I want to suggest to him at present is that he is rather over-emphasising the ro1e of electrolysis and conduction generally, and not recognising the full value of electric displacement, so much emphasised by Faraday and Maxwell. Conduction necessarily involves some dissipation of energy, something analogous to friction; it does not store energy, and it gives no recoil. A pure dielectric dissipates no energy, and it recoils perfectly. A dielectric slab brought near a charged body becomes polarised, positive on one face, negative on the other: there has been a displacement of electricity through it, but not by conduction. It may even be worth while to ask Prof. Armstrong to refer back to an ancient paper of mine in the Phil. Mag. for November 1876, in which I design models to illustrate Maxwell's theory. This paper, I was delighted to find, pleased Clerk Maxwell sufficiently to induce him to write me a most interesting letter about it—humorously suggesting, I recollect, lubrication with Canada balsam as suitable for § 10 of that paper,—a letter which, to my long-standing regret, has suffered from “moving accidents” and got itself lost. If the elastically supported buttons or beads on the cord of that model slip on the cord, there is conduction; but if they grip it tight, they represent a dielectric.

OF all the notable contributions to the record of the Cavendish jubilee given in the issue of Dec. 18, perhaps the most interesting to many is the one by Sir Joseph Larmor on Clerk Maxwell; for it suggests the way in which his equations, like those of the writer himself, contained the germs of many future developments, and incidentally directs attention to a Royal Society abstract which was unfortunately omitted from Maxwell's “Collected Papers.” It was customary at that period for the Royal Society to publish not only the complete paper in the Phil. Trans., but a thorough abstract also in the Proceedings—a practice which, perhaps unfortunately, has been departed from. The abstract referred to appeared in Dec. 1864, and gave in concise form the substance of one of the most remarkable memoirs of last century, “A Dynamical Theory of the Electromagnetic Field.”

THE modified form of the ionic deflexion theory of wave propagation, discussed by Messrs. Nichols and Schelling (NATURE, March 7, vol. 115, p. 334), appears to have been suggested in England and the United States almost simultaneously. That the effect of the earth's magnetic field had to be taken into account in calculating the phase-velocity was pointed out in a paper on “Geophysical Influences on the Transmission of Wireless Waves” read at a joint discussion of the Physical Society of London and the Royal Meteorological Society in November 1924 and already published. The formula for the phase-velocity for transmission along the earth's magnetic field and the calculation of the critical frequencies were there given, and some consequences of these effects have been discussed in a paper communicated to the Cambridge Philosophical Society. Since the earth's magnetic field is strong enough to affect the phase velocity to a sufficient extent, all the rotatory and double refraction effects familiar in physical optics are appreciable. But probably the most interesting possibility is that the reciprocity relation between two wireless stations may not hold, for the extra forces on the moving electrons due to the earth's magnetic field are, to a certain extent, independent of the direction of propagation of the waves and thus produce relatively different effects for the two directions of transmission.

I AM sorry that I overlooked the papers cited by Prof. Stetter. In excuse I may state that the first paper is published under the title ” Eine neue Methode zur Messung dcr Iononbewcglichkcit”, the second is unobtainable here, the third is published in a journal not very much read, and the fourth was published after my letter was sent to NATURE.

MAY we be permitted to direct attention to a certain aspect of Einstein's three recent papers (Berliner Berichte, pp. 217, 224, 1928; Feb. 1929) on distant parallelism which came to light in a discussion with Prof. D. J. Struik? The avowed aim of these papers is to develop an improved unified field theory of electricity and gravitation. A much more pressing need of general relativity theory is a harmonisation with quantum theory, particularly with Dirac's theory of the spinning electron. On the basis of Levi-Civita's parallelism the task seems hopeless, inasmuch as we have no adequate means of comparing spins at different points. On the other hand, the notion of a parallelism valid for the whole of space and of Einstein's n-uples enables us to carry over the Dirac theory into general relativity almost without alteration. All that we need do is to interpret Dirac's P0,P1,P2,P3 not as differentiation with respect to four variables x,y,z,t defined throughout space-time, but as differentiation along the lines of the quadruple (Einstein's ‘4-Bein’). That is, the quadruples need not be integrable so as to furnish us with a co-ordinate system throughout space, for such a co-ordinate system is absolutely inessential in the proof of the invariance of Dirac's equations under a Lorentz transformation.

WITH reference to the interesting letter on this subject by Prof. Rutherford in last week's NATURE, I should like to point out that in my paper “On the positive electrification of α rays and the emission of slowly moving kathode rays by radio-active substances” (Proc. Camb. Phil. Soc., xiii., p. 49) I have described experiments which demonstrate the communication of a positive charge of electricity to bodies struck by α rays from polonium or radium. I had considerable difficulty in disentangling this positive charge from the copious streams of slowly moving negatively electrified corpuscles which I found were given out by these substances, and the experiments in which I finally succeeded in doing this were not completed until a few days after the reading of the paper on November 14, and are not referred to in the abstract quoted by Prof. Rutherford. A description of them will be found in the paper which has lately been published. I may take this opportunity of saying that I have recently found that uranium also gives out slowly moving corpuscles, so that this effect seems a general property of radio-active substances. The velocity of these corpuscles is very small compared with that of the β rays, and is more nearly of the order of the velocity of the corpuscles emitted by metals when exposed to light.

IN a letter in NATURE (August 2, 1906) I gave an account of some experiments which I considered proved that the particle as initially expelled is not charged, and I also gave an account of the same work in a paper read before the British Association at York last August. Although I have no reason to doubt the accuracy of the experiments published in my letter, I do not now consider them sufficiently conclusive, as some recently published researches on the particle have to be taken into account in their interpretation. I refer chiefly to a paper published by Rutherford shortly after my letter (Phil. Mag., October, 1906, p. 348), in which the view is put forward that the particle carries two atomic charges.

IN a paper to appear shortly in the Proceedings of the Royal Society, I have reported observations on the absorption of the radiations produced in beryllium and other light elements by bombardment with polonium α-particles. The experiments indicated that the radiations emitted from beryllium, boron, and fluorine in the ‘forward’ direction, that is, in roughly the same directions as the α-particles producing them, were less absorbable than those emitted in the backward direction. In my paper I show that this observation is very difficult to reconcile with conservation of momentum and energy if the radiations are assumed to be γ-radiations corresponding to the energy available.

IN NATURE of March 29 (p. 515), Dr. Stoney, in referring to a paper by the writer in the January number of the Astrophysical Journal, raises the question as to the correctness of the use of Maxwell's distribution of velocities in computing the escape of gases from the earth's atmosphere. He maintains that this distribution does not hold at its attenuated limits. In my paper I have not taken conditions which may exist there, but boundary conditions, which are much more favourable to the escape of the molecules of a gas, and certainly compatible with the kinetic theory, if we are to accept such a theory at all.

TAWDE has criticized a part of the discussion in my paper describing a new Green band system in N2. He appears to think that I had not seen his paper on the intensity distribution in the second positive system of N2. I had seen this paper but did not see any reason for quoting it ; there was no discussion of whether the excitation in the various sources was due to ions or electrons, and absence of data about the gas pressures employed prevented full use being made of his results.

IN a previous paper, Bhabha and Chakrabarty1 discussed the general theory of the production of cascade showers. The solution obtained there was finally evaluated by the saddle-point method, and it was shown that the same expression cannot be used for small values of t and y0, where t is the thickness of the material in radiation units and y0 is log (E0/β), where E0 is the energy of the primary particle and β is the critical energy for the material. Consequently in that paper numerical values were obtained only for values of y0 ≥3 and t ≥ 2.

REFERRING to the abstract of Mr. F. W. Aston's very interesting paper read before the Royal Society on December 13, 1906, “On Experiments on the Length of the Kathode Dark Space with Varying Current Densities and pressure in different Gases,” published in your issue for April 11 (p. 574), may I point out in reference to the therein contained statement that “the stream of positive ions may be strikingly shown by a rotatory mica mill mounted inside the dark space which rotates violently in the opposite direction to the familiar ones designed to show the motion of kathode rays away from the electrode,” that in my two papers “On the Circulation of the Residual Gaseous Matter in a Crookes' Tube,” read before the Physical Society, and published in the Philosophical Magazine for October, 1898, I showed similar results, i.e. that mica mill wheels which turned in one direction under kathode-ray bombardment, turned in the opposite direction when so placed as to be just outside of the stream of kathode rays, thus indicating a current of particles proceeding towards the kathode, which particles I found to be charged positively?

IN discussing the negative bands of carbon, Deslandres mentions (Comptes rendus, 137, p. 460, 1903) the two strong bands λ2883.86 and 2897.11 which “are very intense, are diffuse on both sides, are of a different structure and otherwise very curious”, and suggests that they may probably be due to oxygen. These bands have also been noticed by Prof. Fowler while studying the comet-tail bands, but he makes no mention of them in his papers. He calls them the “H and K” bands of carbon because of their striking resemblance to the two solar lines H and K. In a recent paper Johnson (Proc. Roy. Soc., A, 108, p. 343, 1925) observes that each of these bands is resolved into two components, and he has allocated two of them to the regular negative band system. This allocation seems to be of a rather doubtful nature, for the structure of these bands is peculiar and they can be obtained alone, unaccompanied by the other members belonging to the same system.

IN your issue of July 29 your reviewer devotes some space to my paper on the origin of the planetary system (Astronomische Nachrichten, No. 4308), and closes by asking, “Why, for instance, on the hypothesis of capture, are the vast majority of the orbits near the plane of the ecliptic and their motion direct?” This is because our system was formed by the unsymmetrical meeting of two streams of nebulosity or by the mere gravitational settling of a single nebula of curved and unsymmetrical figure, giving a rotating cosmical vortex, or spiral nebula, but without hydrostatic pressure as imagined by Laplace. In Lick Observatory Publications, vol. viii., Plate 38, you will find an illustration of H.V. 2 Virginis, a spiral nebula of unsymmetrical figure just beginning to coil up and form a system. What will happen in the later stages of this nebula is sufficiently shown in the Lick photographs of other nebulae given in this volume. As the mass whirls and condenses under resistance, it will necessarily retain and draw down most of the nebulosity into the plane of motion. This is exactly what has given the planar arrangement of the bodies in the solar system. In Astronomische Nachrichten, Nos. 4341–2, your reviewer will find a fuller explanation of the method of capture, and other papers yet to come will make the theory so clear that it need not take up more of your valuable space at present.

FROM time to time the point is brought home that factors should be discarded only when exact calculation proves them to be negligible. A case in point is the recent paper by Dr. K. Burker (Pfluger's Archiv fur die gesamte Physiologie, vol. 195, p. 516). In this interesting paper it is shown that in mammals the weight of haemoglobin per square micron of surface of the erythrocytes is apparently a constant equal to 31.7 × 1014 gm. Dr. Burker has, however, assumed that the surface area of the cells is equivalent to twice the area of a circle having for its diameter the large diameter of the cell. The general opinion is that in mammals (the camel excepted) the shape of the cells is a bi-concave disc, having a circle for its horizontal projection, and a flat bi-concave ellipse for the vertical projection of which the minor axis is about one-third of the major axis (E. Ponder, Proc. Roy. Soc. 94B, p. 102). The surface area of such an erythrocyte would then be equal to that of an ellipsoid of revolution around the minor axis. It can be shown by the integral calculus that the area of such an ellipsoid (if the minor axis is equal to one-third the major) is 1.09 times as great as that of two circles with the major axis for diameter. Dr. Burker's constant is therefore equal to 31.7/1.09 or 29 × 1014 gm. haemoglobin per square micron of surface of erythrocyte instead of 31.7 × 1014 gm.

To the reader of the numerous papers that have recently been communicated from various sources on the subject of Dr. Rontgen's great discovery, considerable obscurity is caused by the confusion of the above terms. Until quite recently what has been meant by 'kathode rays” or “the kathodic discharge” has been that discharge of matter from the negative electrode in a highly-exhausted vacuum tube, which can be deflected by a magnet, produce heat, mechanical energy, and phosphorescence, can be brought to a focus by using a curved kathode, and in this case will project an inverted image of the kathode upon either the inside walls of the tube, or upon a phosphorescent screen placed inside the tube to receive it. As is well known, this discharge has been very thoroughly investigated abroad by Hittorf, Puluj, and others, and in England by Crookes, and has been called by him the discharge of “radiant matter.” The X-rays of Dr, Rontgen are said to be generated at the spot where the kathodic discharge of radiant matter impinges upon an obstacle, be it the phosphorescent walls of the vacuum tube, or a plate of metal similarly placed to receive it. The distinction is perfectly clear in Dr. Rontgen's paper, as the following extracts from the translation, published in this journal on January 23, show.