the author's intention to make a thorough investigation of the vast field thus opened. Lundquist has given the results of his calculations to determine the distribution of heat in the normal sun spectrum, founded on certain measurements of Lamansky's. He represents the intensity of this heat graphically, and gives curves in which the ordinates represent intensities, and the abscissas wave lengths. It appears from these curves that in the normal spectrum of the sun the maximum of heat is situated about in the middle of the luminous spectrum, and diminishes on both sides of this point," thus confirming entirely the experimental results obtained by Dr. John W. Draper in 1872. In the electric spectrum, however, assuming Tyndall's results as data, calculation gives a curve in which the maximum of heat is near the line A. In this case the distribution of heat is not equal in both halves of the visible spectrum. Hoorweg has repeated with great care the experiments of Tyndall and Magnus upon the diathermancy of moist air, with a view to reconcile the discrepancies in their results. The general arrangement of the apparatus was similar to that used by Wild, a Leslie's cube being placed on either side of a Melloni's pile furnished with its conical reflectors. But for the introduction of the moist air between the cube and pile on the one side and the dry air on the other, two cylinders were used, the one filled with moistened pumice, the other with calcium chloride. These were placed beneath the line joining the pile with the source of heat, so that, whenever a current of air was driven through them, moist air rose at one end of the pile and dry at the other. With a very delicate galvanometer no deviation could be detected. A pair of tubes, each 25 centimeters long, open at the ends, and bored laterally with fine holes, was then substituted, but with scarcely an appreciable result. Both tubes were now placed on the same side of the pile, and a slight but distinct deviation was observed, amounting to 1.7 per cent. The tube was now increased to a meter in length, a heated copper plate being used as the source of heat. The absorption of moist air was 2 per cent. The experiments were repeated with various sources of heat; the absorption by moist air varied from 3 to 0.4 per cent. Alcohol vapor absorbed, under like conditions, from 6 to 27 per cent. of the heat. Hoorweg con cludes with Tyndall that aqueous vapor has an appreciable absorbing power for heat, though it is much less than Tyndall supposed. The controversy between Tyndall and Magnus was a very natural one, Tyndall, on the one hand, having overestimated this absorption through neglecting condensation, or vapor-hesion, as Magnus called it; and Magnus, on the other, having denied its existence because the tube he used was entirely too short to make it apparent with the galvanometer employed in his experiments. OPTICS. In Optics, Crookes has published some curious and delicate experiments, in which carefully suspended disks of pith were set in motion in vacuo, apparently by the action of light. In a public lecture, however, given in Edinburgh, Professor Dewar, after explaining the method adopted by Professor Tait and himself for obtaining very perfect vacua by taking advantage of the power that charcoal has of condensing gases, stated that these vacua were so perfect that it was impossible to force through them an electric spark between electrodes one quarter of an inch apart, even when a powerful coil is employed; and hence that such vacua were, therefore, eminently proper to repeat the investigation recently made by Mr. Crookes upon the action of a beam of light on a disk at the end of a delicately suspended glass fibre. Such an investigation has been made by Dewar, and he finds that the movements of the disk are due entirely to radiant heat, and not to any mysterious agency, as Mr. Crookes seems to imply. The sensitiveness of the disk increases with the perfection of the vacuum. The sides of the glass receiver must be quite thin. If the disks are covered with lampblack, they are affected much sooner than if left white. The conductivity of the suspended body for heat, and the nature of the residuum gas within the vacuum, determine the density of the gas corresponding to the neutral point observed by Mr. Crookes. The intensity of the movements of the disk increases in proportion to the inverse square of the distance of the source of radiation. If we interpose between the light and the disk a substance opaque to heat rays, although transparent to light, the movements of the disk immediately cease. If we interpose a substance transparent to heat, but opaque to light, the deflection of the disk is large. If two disks are taken, one of rock-salt Wibel has made additional experiments upon the cause of the luminosity of flames. He finds the results of Knapp confirmed, that nitrogen, hydrogen chloride, carbon dioxide, and other indifferent gases, act like air to destroy the luminosity of gas used in a Bunsen burner; but he also finds, curiously enough, that this luminosity may be wholly or partially restored by heating the tube to redness through which the mixture passes. Hence he concludes, 1st, that the absence of luminosity in a Bunsen flame is not due to dilution of the gas; 2d, that it is due to the cooling effect of the inert gas, since, if this be heated, the luminosity returns; 3d, that the luminosity of a flame depends upon the temperature existing in its interior; and, 4th, that ordinary illuminating materials are such because the rising gases and vapors are sufficiently heated in the exterior combustion zone to cause their decomposition. Gariel has described some simple apparatus for explaining by construction the elementary laws and formulas of optics. Cornu has communicated to the Academy a valuable paper on the velocity of light, in which he gives the results of the new measurements made between the Paris Observatory and the tower of Montlhéry, twenty-three kilometers distant, under the direction of the council of the observatory. As a mean of 504 experiments, he finds the velocity of light in vacuo to be 300,400 kilometers, or 186,700 English miles, with a probable error below one thousandth in relative value. This gives for the solar parallax, as found by the equation of light, 8.878", and by the phenomena of aberration, 8.881". The same author has described a new measuring instrument for minute quantities, called a reflection lever, which consists of a beam like a balance beam standing on four points, two on the line where the knife edge is usually placed, the other two at the ends of the beam, all four being accurately in one plane. To the centre of the beam is attached transversely a mirror, by means of which any displacement from the horizontal may be detected and measured by the reflected image of a distant scale. The readings are made with a telescope. Pickering and Strange have investigated photometrically the amount of light absorbed by the sun's atmosphere. By means of a porte lumière carrying a black mirror and lens, an image of the sun 40 centimeters in diameter was thrown on a screen 230 centimeters from the aperture. A circular hole was cut in the screen, and behind this the photometer disk was placed. By moving the mirror any portion of the sun's image could be thrown on the photometer, and its light measured. The results were thus given: The probable error does not exceed one per cent., except close to the edge. The light at the edge is about 0.4 that at the centre. The variations in brightness are nearly those which would be produced by a homogeneous atmosphere whose height is equal to the sun's radius, and its opacity such that only twenty-six per cent. of the light is transmitted. There appears to be a slightly different distribution of the light along the polar from that along the equatorial diameter. If the sun's atmosphere were removed, the brightness of the sun's disk would be uniform, and 3.83 times that of the centre of the disk at present. Moreover, the total amount of light would be increased 4.64 times. Curtis has published a method of showing the phenomena of extraordinary reflection. Upon a horizontal circular stage, movable around the axis and adjustable in height, a crystal of Iceland spar well polished is placed. A beam of light falls on the crystal at such an angle that after refraction and reflection within it the beam shall pass from it, making the same angle on the other side of the normal. Five images of the opening through which the beam of light comes will be seen, one formed by reflection at the upper surface, and the other four by double reflection within the crystal. Williams has made a photometric investigation into the intensity of twilight when the sun is at various distances below the horizon. The percentage of error in the instrument employed was about three. The results of the photometer readings were reduced to the light given by a standard candle as unity, when burning at a distance of one meter from the disk. By a graphical construction of the actual results a curve was obtained, and a table deduced which gives the percentage of light, compared with that at sunset as unity, for any number of minutes after sunset up to 34. At 1 minute it is 0.95; at 10 minutes, 0.290; at 20 minutes, 0.064; at 30 minutes, 0.009; and at 34 minutes it is 0.004. Crosby, also, in the Massachusetts Institute of Technology, has made some photometric determinations of the light of the sky at different distances from the sun, adjusting the mirror and lens which were employed so that the sun's image would fall on the disk, and then measuring the intensity of the light at regular intervals thereafter. In some cases this method was reversed. The results represented graphically show a logarithmic curve, when the intensities are taken as ordinates and the natural sines of the sun's angular distance as abscissæ. The author calls attention to the meteorological importance of his results. Giraud-Teulon has discovered a new method of measuring distances optically, and has constructed a telemeter based upon it. A double image of the object is produced by a division of the eye-piece, one half moving by the other by means ject this tance teles in the Jac laris 100 per cent |