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The photographic news
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- 35.1891
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- 1891
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- Bandzählung
- No. 1693, February 13, 1891
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132 THE PHOTOGRAPHIC NEWS. LFEBRUARY 13, 1891. THE ROYAL INSTITUTION. Lord Rayleigh on some Applications of Photography. Last Friday night Lord Rayleigh lectured at the Royal Institution upon “ Some Applications of Photography.” Sir Frederick Bramwell presided, and among those present were Professor Alexander Herschel, Dr. William Huggins, Mr. William Crookes, Mr. J. W. Swan, Capt. W. de W. Abney, Sir William and Lady Thomson, Lord Justice Sir W. Fry, Professor Hughes, Sir Henry Trueman Wood, Professor Oliver Lodge, Mr. Francis Galton, Professor A. Rucker, Mr. Frederick Varley, Mr. Friese Greene, Professor Dewar, Sir Frederick Abel, Mr. John Spiller, Professor Edward Frankland, Mr. Gambier Bolton, Mr. Roberts-Austen, Mr. Bevan, and Mr. J. R. Gotz. The theatre of the Institution was well filled. Lord Rayleigh stated that there is no such thing as instantaneous photography, because the taking of any photograph must occupy time, however short that time may be. Several methods may be employed to take pho tographs quickly. Mr. Muybridge had shown in that theatre how he had taken them by means of rapid shutters ; another plan is to take them by means of a flash of light. He then caused a flash from a magnesium lamp to fall upon a wheel divided into black and white sectors. This wheel was made to revolve rapidly by means of multi plying gear, and when the wheel was illuminated by the flash the sectors did not appear to be motionless, showing that the flash was not of particularly short duration. It occupied, he said, about one-tenth of a second ; other magnesium lamps will give flashes of shorter duration, up to, it is said, a fiftieth of a second. He then exhibited a method of obtaining a much shorter flash by means of three Leyden jars, charged by means of a Wimshurst’s electrical machine. The afore mentioned revolving disc was driven at high velocity, and when illuminated by the spark duo to the discharge of the Leyden jars, the sectors appeared to be absolutely motionless ; they were also sharply defined. The electric spark is, therefore, enormously “more instantaneous” than the magnesium flash. About twenty years ago, a scientific man in New York investigated the duration of the discharge of the Leyden jar, and found that the result depended somewhat upon the circumstances of the case; a small jar, for instance, gives a quicker spark than a large one. The duration of the spark is about 6,001,000 of a second of time. These figures scarcely enable anyone to realise the velocity except by analogy; one million seconds occupy about twelve days and nights, and twenty-five millions occupy about one year; therefore, the duration of the electrical spark is, to the second of time, about what one second is to a year. His method of photographing was to obtain the electric discharge in a box with a lantern condenser in front; outside the box, and in front of the condenser, was the transparent object to be photographed; facing this arrangement was a camera, with its lens focussed upon the object. With this arrangement he had photographed bubbles of gas rising from a nozzle and then ascending through a liquid ; in this case, the bubbles form almost at the first moment of leaving the nozzle. With a water-jet in air, the form is at first cylindrical; then the jet breaks into drops at some distance from the nozzle, in consequence of the water-cylinder being in a state of unstable equili brium, or in what is known in the medical world as a “ varicose ” state; the varicosity goes on increasing until disruption results. Another case of unstable equilibrium is that of sensitive flames. The flame is not an essential part of the arrange ment, for it acts more as an indicator. He caused a solution of permanganate of potash to play into a solution of protosulphate of iron ; the latter was used to quickly decolourise the purple liquid from the jet. This purple jet could be disturbed by shakes and sounds, and the instantaneous photographs taken prove that such jets become sinuous. There is an analogy between these jets and those which produce sensitive flames; there is also a very great difference between them in the forces under which they break up : symmetrical forces will not produce sinuosity. The Photographing of Bursting Soap-films. The lecturer next spoke of a difficult feat, namely, that of photographing bursting soap-bubbles; the chief diffi culty is to obtain the bursting of the bubble and the electric flash at the same instant. He caused two electro magnets to hold up two iron bullets, so that when the current was stopped the two balls should fall at the same time, and one of them fall through the soap-film at the same time that the other caused the electric discharge; this did not answer, for one ball would lag a little behind the other in leaving the electro-magnet. He then tried another device, which we shall describe next week, illus trated by diagrams, and accompanied by diagrams of the results obtained. Another difficulty in this work is, that it is not easy to break a soap-film when you want to do so. Several bullets may sometimes be dropped in succession through a soap-film without breaking it, for they will go through without making a hole. He proved this by experiment. The passing bullet extends the film where it touches it; the film next closes up behind the bullet. Lorn Rayleigh overcame this difficulty by wetting the bullets with alcohol. In these experiments he used an electrometer to indicate when the Leyden jars were sufficiently charged for the purpose. The Connection between Aperture and Optical Definition. The speaker next remarked that we cannot get as high definition as we please by constantly reducing the aperture. With too small apertures the definition gets worse. An optical image of even a mathematical luminous point has considerable extension, and the mathematical point cannot be correctly given at another point by any optical means ; there is sure to be a certain amount of diffusion. He had prepared a series of photographs of gratings showing how, as the aperture of the lens was decreased, the images at first improved; but, as the apertures were still farther diminished, the definition began to get worse. Images of these photographs were then projected by him upon the screen by means of the electric lantern. When the aperture is much restricted, a lens is of no use whatever; as good an image can be obtained without as with it. Lord Rayleigh closed his lecture by exhibiting a photo graph of some cedars, which he had taken without a lens by means of a hole in a shutter in a dark room, with the plate at a distance of seven feet from the hole. These photographs were of a pleasing nature when viewed at the proper distance of seven feet.
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