Must the numerous accidents mentioned be attributed to defects in the metal employed? Were they due to defective hooping? Were they due to some one of the numerous inconveniences inherent to the cylindrico-prismatic system of closing (_Rundkeilverschluss_)? They were doubtless owing to such causes combined.--_La Nature_. * * * * * COLORS OF THIN PLATES. The Right Hon. Lord Rayleigh lately delivered a lecture at the Royal Institution upon "The Colors of Thin Plates," a term which he explained was applied to thin films of substances, such as oily films on the surface of water or the equally familiar soap bubble. Although the reflection of colors from the surface of a soap bubble is probably the most noticeable, yet the "plate" which lends itself most readily for experiment is a film of air confined between two sheets of glass. If a ray of white light be reflected from the surface of the film upon a screen, the so-called Newton's rings, a series of colored concentric rings, are obtained. If, instead of reflected light, the ray of light transmitted through the film of air be allowed to fall upon the screen, the same phenomenon is observable, but the effect is very considerably minimized, owing to the great preponderance of white light, which overlies as it were the colored rings. Even in the first instance, as the lecturer was able to show later on, the colors are not nearly so intense as they may be obtained, owing to some white light being reflected from the surfaces of the two sheets of glass. With regard to the appearance of the phenomenon, it is observed that the part which corresponds to the thinnest part of the film is considerably darker than the rest of the spectrum; around this is a bright ring of white, succeeded by constantly increasing concentric rings of different colors apparently repeating themselves. Lord Rayleigh also obtained the same results with a film of a solution of soap and glycerine, but in this case the dark portion was observed at the top of the spectrum, the other colors arranging themselves in order in the soap film thinned by the force of gravitation, thus showing that the colors vary according to the thickness of the film. Another form of the experiment called forth a considerable amount of applause from the audience. Lord Rayleigh caused a gentle stream of air to play obliquely upon a soap film, so that the part struck was moved forward and the whole film rotated. Then with the alteration of the force of the current of air, which of course regulated the centrifugal force, alternating thicknesses of film were obtained, causing a varying display of beautiful colors and combinations of colors. This last experiment also tended to prove that the bands of color are not arranged in a certain order, but vary according to the thickness of the film, a conclusion arrived at by Brewster, who observed that if a film reflecting certain colors be carefully inverted so as not to disturb the