the wires being equal to their diameter; by inclining the grating relatively to the wave-front, the apparent clear spaces available for the passage of radiation could be reduced to 1 mem. The wavelengths investigated ranged from about of the above unit down. to that of red light. The method employed was a somewhat elaborate spectrobolometric one; it was exclusively applied to the non-diffracted part of the incident radiation, corresponding to the so-called "central image." It was found that all wire gratings polarize perpendicularly to the direction of the wire so long as the wave-length is very small; for larger wave-lengths the action is, however, reversed. The intermediate value of the wave-length, at which the grating does not polarize, i. e. the "neutral point," is independent of the geometrical constants of the grating, but characteristic for the metals constituting its wires. Its value increases in the order Pt, Ag, Au, Fe, Cu. The wire gratings behave like polarizers in every respect. Malus' law, properly modified, was found to hold for the polarizing action in different azimuths. The polarizing action increases as the apparent clear space diminishes: the ratio of the intensities of the two perpendicular components of radiation, which measures that action, could be pushed up to over 2. The results were checked by a simpler method of a more qualitative character, which may prove useful for a repetition or demonstration of the experiments. The authors then give a general discussion of the phenomenon, its possible relations to other phenomena, the influence of diffraction, and, lastly, the possible explanation by it of the properties of æolotropic dielectrics. This particular part of the paper presents certain analogies with Lord Rayleigh's theoretical discussion of "the influence of obstacles arranged in rectangular order upon the properties of a medium " (Phil. Mag. Dec. 1892). However, the principal interest attaches to the experiments described, when viewed in the light of the electric theory of radiation and of Hertz's discoveries: the diminutive polarizing gratings in the present case being quite analogous to the large electromagnetic wave-polarizers first introduced by that great investigator. Since there can hardly remain a doubt that in the electromagnetic theory, the electric vector must be considered perpendicular to the plane of polarization, the magnetic vector parallel to it (Trouton, Klemenčič, Righi), the action of polarizing gratings in general may now be described as follows: As long as the wave-length keeps below a certain value characteristic of the metal, the grating allows the greater fraction of incident radiation to pass in case the electric vector be parallel to the wires; whereas for longer waves the transparency is greater if the magnetic vector have that direction. For further particulars the original may be referred to.-Wiedemann's Annalen, xlix. p. 593 (1893). MAGNETIZATION OF HOLLOW AND SOLID CYLINDRICAL CORES. BY H. DU BOIS. The vexed question of hollow v. solid cores is once more discussed in this note. Von Feilitzsch's well-known result that the magnetic moment of short hollow cores is hardly inferior to that of solid ones (other circumstances equal and saturation not being approached) has been elegantly explained by Prof. Silv. Thompson in a general qualitative way by the simple remark that the external air-resistance is approximately the same in both cases. There is no need whatever, therefore, for the assumption that the central fibres of cores are less easily "reached" by temporary steady magnetization than those near the skin: such a state of things would only be proved if the same result were obtained with hollow endless rings, which is improbable. Von Feilitzsch's experiments having been lately repeated and extended by Prof. Grotrian (Wied. Ann. 1. p. 705, 1893), the author seized the occasion for attempting to go into the matter somewhat more fully. The particular case of a cylinder being one of those in which the modern "magnetic circuit" notion does not lead up to quantitative results so well as the more orthodox theory of induction, the latter had to be applied, and immediately yielded the following general rule : If for cylindric or prismatic bodies of given length and any shape of section the "demagnetizing factor" be proportional to the latter and not too small, then the magnetic moment will be practically independent of the section for fields which are not so strong as to produce an approach towards saturation. It is further shown that both premisses of this rule apply to thin hollow cores, for which it is thereby easy to draw the curves of magnetic moment and impressed field up to thorough saturation for any given dimensions. An illustrative diagram of such curves is given, which presents exactly the same aspect as the experimental diagram of Prof. Grotrian's paper. Finally, the question of hollow cores for dynamos is touched upon. Signor Ascoli has since independently criticised Prof. Grotrian's results much on the same lines.-Wiedemann's Annalen, li. p. 529 (1894). MOLECULAR ENERGY OF GASES. A CORRECTION. On page 420, line 9, the word "every" is wrong. It is only because collisions are occurring in every direction that on the average the original difference of energy diminishes with each collision.-O. J. L. ON A FUNDAMENTAL QUESTION IN ELECTRO-OPTICS. To the Editors of the Philosophical Magazine. GENTLEMEN, In the April number of the Philosophical Magazine Dr. Kerr has published a paper, "Experiments on a Fundamental Question in Electro-Optics.' Dr. Kerr finds the velocity of light changed by electric stress in electrostatically strained liquids only for light polarized perpendicular to the lines of force. My paper on the same subject is not mentioned in which, eleven years ago, I treated this question with the identical methods employed by Dr. Kerr. (Cf. G. Quincke, "Electrische Untersuchungen, IX. Ueber die Aenderung der Brechungsexponenten von Flüssigkeiten durch electrische Kräfte," Wiedemann's Annalen, xix. pp. 773-782, 1883.) A proof of this paper I sent immediately after its publication to Dr. Kerr, who first observed the double refraction of light by electric stress. But the results of my experiments were different from those of Dr. Kerr. I observed sometimes an increase, sometimes a diminution, of the velocity of light whose plane of polarization is parallel to the lines of force. I supposed that the reason of this ambiguous change were two forces of opposite character, i. e. the heating of the liquid by vortex motion and the electric pressure, both excited at the same time by the electric forces. Heidelberg, Physical Laboratory of the G. QUINCKE. MOLECULAR ENERGY OF GASES. To the Editors of the Philosophical Magazine. With reference to the remarks made by Professor Lodge in the Philosophical Magazine for April, page 419, I only need to notice the chief point and to say:-It appears to me that the proposition that the amounts of mechanical energy of the molecules of different gases are equal, is of so fundamental a character that it ought, for the advancement of science, not to be limited to algebraic expressions readable only by mathematicians, but it should, in addition, be explicitly expressed in simple words (as Avogadro's law usually is) in every text-book of physics and monograph on gases, for the benefit of all persons. Birmingham, April 6, 1894. I beg to remain, Gentlemen, P.S. Had the "statement" I asked for in my previous letter (not an algebraic expression) existed in any of the text-books of science I should almost certainly have seen it, and that letter would not have been necessary. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [FIFTH SERIES.] 1. GOUY JUNE 1894. LII. On Interference Phenomena. OUY † and RAYLEIGH ‡ have recently discussed the question of interference phenomena observed with a source of white light, and have arrived at important results. When the two interfering beams pass through the slit of a spectroscope, the spectrum is seen to be crossed alternately by bright and dark bands, and if high resolving powers are used, these interference effects may be observed, although the difference of path of the two original beams is considerable. This well-known fact has always been taken to prove that the vibrations of the white light are to a certain extent "regular," and efforts have been made to find by experiment the limits of that regularity. The futility of these efforts has been completely demonstrated by Gouy and Rayleigh, who have proved that the bright and dark bands alluded to would appear with sufficient resolving power, even if light consisted of perfectly irregularly distributed instantaneous impulses. These investigations would seem, therefore, to lead to the conclusion that a source of light is completely defined by the distribution of energy in the * Communicated by the Author. + Gouy, Journal de Physique, (2) v. 1886. Rayleigh, Encyclopædia Britannica, "Wave-theory," $7; Phil. Mag. xxvii. p. 463 (1889). Phil. Mag. S. 5. Vol. 37. No. 229. June 1894. 2 N spectrum, and that there is no room for any such property as regularity" independently from that implied in the connexion between energy and wave-length. A spectrum which possesses the same energy for all wave-lengths would prove complete irregularity of the source, while one in which the energy vanishes except for one definite wave-length would be due to a completely regular source. I confess that on first reading through the investigations which I have quoted, some difficulties presented themselves which it seemed necessary to clear up before these conclusions could be accepted in their full generality. There could be no doubt that Gouy's and Rayleigh's results were correct in the case specially examined by them, in which the analysing spectroscope has a resolving power which is gradually increased; but if we go to the other limit and gradually diminish the resolving power, would it necessarily follow that all sources of white light behave in the same way, and that the bright and dark bands would in every case disappear at the same time, the difference of path being kept constant? A test case (7) occurred to me, which seemed to leave a loophole through which we might still save some of our inherited notions about "regularity" of vibration. Although further investigation proved that the difficulties were only imaginary, and that we must accept the new views without restriction, I think that the importance of the subject deserves a more detailed treatment than has yet been given to it. In order to put the question at issue as clearly as possible, it is necessary shortly to allude to the history of the subject. 2. The fact that rays of light emanating from two different sources do not produce so-called interference phenomena is fundamental, and was always recognized as such. It naturally suggested the idea that the vibrations of luminous particles are frequently disturbed, and this, again, led to the further conclusion that interference between rays emanating from the same source should cease if the difference in path were sufficiently increased. Fresnel looked on each particle of matter as vibrating during a certain time according to the simple pendulum law, and considered the group of waves sent out by the particle. If the group is divided into two, one being retarded as compared with the other, "interference" will take place. If the difference in path is equal or greater than the length of the group, Fresnel argued that interference should cease. "But," he adds, "another cause much sooner prevents our observing the mutual interference of two systems of waves, when the difference of path is somewhat great: it is the impossibility of rendering light sufficiently homo |