PARIS Academy of Sciences, August 29.—M. Decaisne in the chair.—M. Faye presented the first volume of his “Cours d'Astronomie de l'Ecole Poly technique,” treating of the diurnal motion, the theory of instruments and errors, organisation of great observatories, mathematics and geodesy. The second volume will be devoted to the solar system.—Dioptric studies, by M. Zenger. lie constructs tables which give, in algebraic form, the relation between the radii of curvature and refractive indices of two media forming the objective of a microscope or telescope. Any one may make his own telescope or microscope, without calculation, taking a lens of quartz or crown glass, and a mixture of aromatic substances giving it a dispersion twice as great, or equal for all spectral rays. The lens being corrected, it is combined with one or two other symmetrical lenses, according to the well-known process for getting an aplanetic and achromatic doublet or triplet.—MM. Tresca and Breguet were requested to represent the Academy at the inauguration of the monument to Frederic Sauvage in Boulogne on the I2th inst.—On a very old application of the screw as an organ of propulsion, by M. Govi. This was by Leonardo da Vinci, about the end of the fifteenth century. In one of his works is a sketch of a device for rising in the air, consisting of a helix formed of wire and cloth to be rotated about a vertical axis. He seems to have made small paper models actuated by thin slips of steel, twisted, then left to themselves. Another sketch shows that Leonardo da Vinci conceived the idea of the, parachute.—On some new cases of equipoteutial figures, realised electro-chemically, by M. Guébhard.—On the absorption of ultra-violet rays by some media, by M. de Chardonnet. Two methods are described. The liquids which circulate in plants or impregnate roots and fruits show a great avidity for chemical rays. Fluorescence does not appear to be in direct ratio to the intensity of actinic absorption; thus, e.g. the decoction of radish is a less powerful absorbent than that of potatoes; yet the former is fluorescent, the latter not. White wine is weakly fluorescent, red wine lacks the property. The few animal substances studied gave very varied results. While blood, even very dilute, is a strong absorbent, the (fresh) aqueous humour of a calf's eye and the albumen of hen's eggs have no action on the chemical rays (at least up to 20 mm. thickness). Distilled water, alcohol, sulphuric ether, normal collodion, and solution of cane-sugar are also without action. Gelatine appropriates readily all the actinic rays. An object-glass of Dallmeyer projected an invisible spectrum 25 to 40 per cent longer than one of Darlot, of Paris, of equal focus.—Figures produced by fall of a drop of water holding minium in suspension, by M. Decharme. Minium, in fine powder, is mixed with water and spread uniformly on a horizontal glass plate; then a drop of the mixture is let fall on this layer. Figures resembling those of the three systems Caladni observed on vibrating plates are produced; the three types usually coexist, but one or other may be made to predominate at will.—On the composition of buck-wheat, by M. Lechartier. Marked differences appear between the crops of 1879 and of 1880. Thus the ashes of the straw in 1880 had twice as much potash as in 1879, and phosphoric acid was still more increased; and there was also more chlorine. The composition of the grain is little modified. The straw may contain more of mineral matter than the grain. Buck-wheat removes more of the fertilising principles from the foil than corn.—On hydrosulphurous acid; reply to M. Schutzenberger's note, by M. Bernthsen.—On the dissolution of silver in presence of alkaline iodides, by M. Ditte.—On the constitution of glyceric ether, and on the transformation of epichlorhydrine into normal propylic alcohol, by M. Silva.—On pyruvic alcohol and its derivatives, by M. Henry.—Action of triethylamine on epichlorhydrine; compounds of oxallyltriethylammonium, by M. Reboul.—Biological evolution of the pucerons of the alder tree, by M. Lichtenstein.—Observations on a new enunciation of the second law of Gay-Lussac concerning combinations of gas, by M. Garcia de la Cruz. He indicates some of the numerous exceptions to M. Verschaffel's proposition: “The space occupied by a gaseous compound is always double the space occupied by that one of the components which enters with less volume into the combination.” This law he regards as less general than the laws of contraction long accepted.