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Nature volume 57, pages 2324 | Download Citation

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LONDON. Physical Society, October 29.—Mr. Shelford Bidwell, President, in the chair.—Prof. Stroud exhibited and described the Barr and Stroud “range-finder.” The problem of finding the distance of a given object at sea, or in the field, is complicated by shortness of the trigonometrical “base,” and by restrictions of time. As a rule, the apparatus must be self-contained, and “snap-shot” readings are obligatory, i.e. the range has to be determined from a single instrument and from a single observation. At 3000 yards the errors must not exceed 3 per cent. In foggy weather, or when viewing a nebulous object, this degree of precision is difficult to attain, but under favourable circumstances the authors have determined ranges, at that distance, within 1 per cent, of accuracy. At shorter ranges measurement is more exact; thus an object at about 2000 yards may be estimated to within about 12 yards. Prof. Stroud gave some account of the history and of the general methods employed in these instruments. Two images of the distant object, preferably of a line such as a flag-staff, are received respectively upon two mirrors, two lenses, or two prisms, placed one at each end of a fixed support. From each of these, the light is then directed towards the middle of the instrument, where the two images, after further reflection, are viewed by one eye-piece. The optical system has finally to be adjusted so that the two images, as now seen in the eye-piece, lie in the same straight line. In the instrument designed by the authors this coincidence is attained by translating a small prism parallel to the axis of the supporting rod. The extent of this translation is a measure of the range. Both eyes are used: the right for bringing the two images into alignment; the left for “finding” the object through a small field-glass, and for reading the scale of distances. At nighty sightings have to be taken from “points” of light, and as these are unsuited for measurement, the authors convert them into “lines” by the use of cylindrical lenses, Various devices are introduced to prevent overlapping of the images. The instrument is about five feet long, and tubular in form; it is made of copper, so as to have high thermal conductivity to reduce differential heating. Within the outer tube is the interior supporting rod, designed to equalise so far as possible the effects of interior radiations. Several forms of “separating” prisms were exhibited, the best for the purpose consists of two “reflecting” prisms; these receive the two rays and direct both of them into a third prism, whose angle lies in the space between the angles of the others. Mr. Barr drew attention to the gimbal arrangement and the three struts that keep the supporting rod centred in the tube. To give some idea of the precision and scope of the range-finder, he observed that they were there using the equivalent of a 25-feet “circle,” and their measurements were comparable to the measurement of 20 sees, of angle on such a circle. The instrument is handled by ordinary seamen, and stands rough usage on board ship for years without injury.—Prof. Stroud then exhibited “a folo-meter and spherometer.” He explained that in determining curvatures and focal lengths, some telemetric method was necessary, and that, owing to want of parallelism of the beam, and duplication of images, a short-focus telescope was always an inefficient telemeter. For the measurement of inaccessible lengths it was therefore better to use some simple form of “range-finder.” Such an apparatus could be made with a set of small mirrors arranged in such a manner as to direct two images of the distant object into an eye-piece, with a fixed prism in the path of one of the incident beams. By sliding this instrument along the optical bench one position could always be found at which the two images, as seen through the eye-piece, were in coincidence. He also described a method for determining curvature by interposing a plate of plane glass between the curved o mirror and a source of light.—Mr. Ackermann exhibited two experiments. (1) The blowing-out of a candle-flame by the air from a deflating soap-bubble. The bubble was blown at the mouth of an inverted beaker by breathing into a hole cut out at the top. This hole was then presented to the flame, arid the flame was immediately quenched. But if the bubble was blown from ordinary air, with bellows, the flame was merely deflected without being extinguished. (2) It was shown that a miniature boat, provided with a false stern, consisting of a linen diaphragm, could be propelled by filling the hollow stern-space with ether, or with some liquid similarly miscible with water. The motion is due to the continuous release of surface-tension behind the boat. Prof. Boys said that when he tried, some years ago, to blow out a candle with a soap-bubble filled with common air, he found the operation very difficult—so difficult that, having once succeeded, he never repeated the attempt. It had not occurred to him, as it had to Mr. Ackermann, that the CO2 present in the breath played a part in the quenching. With regard to the second experiment, he had seen a smalljboat propelled by dissolving camphor astern, but he thought the use of a liquid for that purpose was a novelty.—The President proposed votes of thanks, and the meeting was adjourned until November 12.

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