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being equal, depends upon the existence of a temperature in the interior which renders possible the processes of chemical decomposition and combination, from which the luminous body proper results. In ordinary cases the more or less cold gases formed from the illuminating material rushing up from beneath are heated sufficiently by the temperature developed in the exterior zone of combustion to produce the change necessary to luminosity.—35 C, IV., 1875, 220.

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INFLUENCE OF PRESSURE ON COMBUSTION. Some interesting observations have been made by Cailletet in burning different substances under pressure. He finds that pressure slightly augments the temperature at which combustion occurs, and that the luminous and actinic rays emitted by the burning body are considerably intensified. When a candle is made the subject of experiment, the base of the flame, ordinarily bluish and transparent, becomes white and very luminous. Soon, however, clouds of smoke are formed, due to incomplete combustion. Under similar circumstances the flame of phosphorus is not sensibly aug. mented in brilliancy, but sulphur, potassium, alcohol, and carbon disulphide burn much more vividly than in free air. -Annales de Chimie et de Physique, November, 1875.

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THE ABSORPTION OF SOLAR HEAT BY THE ATMOSPHERE.

The discussions of the observations of the black-bulb-invacuum thermometer, made in England during the past five years, have led Mr. Stowe to the following conclusions in reference to the influence on solar radiation of the aqueous vapor in the atmosphere. First, when the tension of vapor is small, the radiation is greater than the average. Second, the north and northwest winds, which contain little vapor, show a greater intensity of solar radiation than the south and southeast winds. Third, the hourly changes, due to the varying altitude of the sun above the horizon, are well marked, and allow the approximate determination of the solar radiation as unaffected by the absorption in the atmosphere; this latter varying from a minimum of ten per cent. to a maximum of twenty per cent. A change in the elevation of the station, from 470 up to 1800 feet, diminishes the absorption by five per cent.—7 C, II., 57.

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THE MATHEMATICAL THEORY OF LIGHT. An exhaustive memoir on the interference of light, by Professor Lommel, is contained in the Proceedings of the Physical Society of Erlangen, in which he brings the phenomena of thick and thin plates of glass and of curved surfaces into one point of view. Some of the diagrams given by him remind one strongly of those multiplex curves treated of by Professor Newton in the Connecticut Academy of Sciences for 1874.-Sitzb. Physikal.-Med. Gesells., Erlangen, 1875, 106.

MATHEMATICAL THEORY OF LIGHT. The connection between the elliptic polarization of light reflected from mirrors, and the refraction and dispersion of light when passing through transparent media, is developed in a very elaborate manner by Ketteler, who deduces a complete theory from the consideration of the expression A+BV

-1, which expression Fresnel met with in his investigation of the subject of total reflection. The occurrence of this expression, to which the term "complexe" is given by the French and German mathematicians, is shown by Ketteler to result from the fact that the wave of light reflected from any surface may be considered as a complex wave, consisting of a superposition of two partial waves differing from each other by one quarter of a wave-length. The interpretation which Fresnel himself seems to have given is submitted to a rigid demonstration. Ketteler states that we are led more and more to the conviction that in all dioptrical phenomena we have to do not so much with a special constitution of the optical ether as with the synchronous vibration of the atoms of ether, and those of material bodies; and that, in fact, the ether itself may have the same inertia and density within as without the transparent body. His memoir is especially devoted to the development of the idea that there is a vibration of the ponderable atoms corresponding to the vibrations of the ether.- Verhandl. Naturhist. Vereins, Bonn, XXXII., 1 to 224.

DEEP-SEA SOUNDING BY PHOTOGRAPHY, Dr. Neumayer has presented to the Geographical Society of Berlin a remarkable photographic apparatus for determin

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ing the temperature and the direction-current at any particular depth in the ocean. It consists of a brass box, hermetically closed, and having attached to it an apparatus resembling a vane or rudder. Within this box a thermometer and a magnetic needle are contained, behind each of which is placed sen. sitive photographic paper, and in front of each of which is a small nitrogen vacuum tube. The box contains also a small induction coil. When the apparatus is lowered to the required depth, the rudder causes it to take a direction parallel to the current there existing, and hence a definite direction with reference to the needle within. The thermometer soon acquires the temperature of the water outside, and becomes stationary. At this instant an electric current is sent to the box, which, by means of the induction coil inside, lights up the little nitrogen tube, the violet light of which, photographically very intense, prints, in about three minutes, the position of the needle and the height of the mercury column upon the prepared paper. The current is then intermitted, the apparatus raised, the photographic tracing fixed, examined, and placed upon record.—3 B, Sept. 3, 1874, 6.

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DISPERSION OF LIGHT BY PRISMS. Dr. Eugene Block attempts, in his Inaugural Dissertation, to make some additions to the theory of the refraction of light passing through systems of prisms. He says with apparent correctness in his preface that the many occasions on which the spectral analysis is applied in every branch of science has in the last few years brought about important changes in the construction of the apparatus. The greater part of these changes have been prompted, not by theoretical investigations, but by experimental methods, whereby it has happened that many errors have been suffered to remain in the construction of the apparatus which may have a material and deleterious influence upon the sharpness of the spectrum. Dr. Block has therefore made careful investigation and comparison of such instruments as are now in use, and has developed the theory of the refraction of light in systems of prisms. He has extended his investigations to converging and diverging bundles of rays, as also to the influence which imperfect prisms, or the inclinations of the prisms can have upon the purity of the spectrum. Among

the theses defended by him at the close of his inaugural dissertation, we note that the Ketteler formula for dispersion is maintained by him as the most correct that is known; as also that the appearance of the aurora is dependent upon local climatic conditions.— Block's Inaugural Dissertation, 1873.

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SPECTRUM DISPERSION OF GASES. Lorenz communicates to the Royal Academy of Copenhagen an experimental and theoretical investigation into the chromatic dispersion of gases. His observations have been made by means of a small spectroscope, and were in part directed toward the constituents of the atmosphere, for which he gives the coefficients of refraction for each of the eight principal lines of the spectrum, and the necessary changes due to the moisture and temperature of the atmosphere. Pure oxygen, nitrogen, hydrogen, the vapor of water, alcohol, ether, chloroform, iodaethyl, carbon-sulphide, and ammonia are successively investigated by him in different degrees of density and temperature. - Mem. Kongel

. Danske Vidensk. Selsk., Copenhagen, X., 486.

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IMPROVEMENT IN SPECTROSCOPES. Mr. Madan gives a simple method by which we may correct the curvature of the spectrum lines, a defect inherent in all spectroscopes as at present made. This curvature is due to the fact that the rays from different parts of the spectrum fall on the prism under different vertical angles. Mr. Grubb has lately proposed to correct the curvature by a method which had been already employed by Mr. Madan for more than a year past, and which consists in making the slit itself curved instead of straight, the curvature being so arranged as to neutralize the distorting effect of the prism. Mr. Madan's experience shows that the curved slits give perfectly satisfactory results, and that they may be easily adapted to any spectroscope; so that by having several such slits in succession, we may secure straight lines in the field of view for varying powers of dispersion. He finds that slits whose edges are curved to a radius of 21 centimeters, sensibly correct the distortion of a single carbon bisulphide prism, the

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reflecting angle of which is 60 degrees. A slit whose radius is 10 centimeters corrects the distortion of a train of two such prisins.

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SPECTRUM APPARATUS FOR THE NEW OBSERVATORY AT

POTSDAM,

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The maker of the optical apparatus for the Bothkamp Observatory has lately finished the spectrum apparatus for the new observatory at Potsdam, which probably is the most complete instrument of its kind. The maker, H. Schroeder, says of it that the apparatus consists of 21 single prisms combined into a system according to Rutherford's method, they being moved automatically and in such a way that the motion is accomplished with mathematical accuracy and with the greatest ease. This automatic movement allows of exact differential measurements with hitherto unattained accuracy, and is the first apparatus of this kind that has been constructed as an exact instrument for measuring. According to Dr. Vogel, the measurements are perfectly trustworthy to the one-hundredth part of the interval between the double line D of sodium, the optical performance of this spectroscope being such that the sodium line is separated into nine fine lines. Almost all the principal lines of the spectrum are resolved into groups of lines, while new lines are seen among those hitherto known.—7 C, XI., 55.

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THE ATMOSPHERIC LINES OF TUE SOLAR SPECTRUM, J. B. N. Hennessey, of the Trigonometrical Survey of India, communicates to the Royal Society of London a short memoir on the atmospheric lines of the solar spectrum as observed by him on the point known as Vincent's Hill, among the Himalaya Mountains, in the north west province of India, at an altitnde above the sea of 1700 feet. His observations were made not only with an old spectroscope, as used in 1868 to 1871, but with a newer and finer instrument supplied by the Royal Society in 1872; and he publishes a map of the solar spectrum, showing the atmospheric lines as observed with the latter instrument. A comparison of the solar spectrum, as seen when the sun was within two hours of the meridian and as seen at sunset, shows the very

strik ing changes introduced by atmospheric absorption. Thus

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