Swiss rose gold chronograph with telemeter scale Circa 1905

Rose gold, diamond-set keyless lever by Vacheron & Constantin
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A very fine and very rare Swiss 18 carat rose gold and diamond-set keyless lever pocket watch with chronograph and telemeter scale, circa 1905. Frosted gilt movement, 23 jewels, straight-line lever escapement, cut bimetallic compensation balance, blued steel Breguet balance spring, index regulator, visible chronograph work. Hinged gold cuvette. White enamel dial with Breguet numerals, outer minute track with Arabic five-minute numerals, concentric fifths of a second divisions, outermost red telemeter scale, subsidiary seconds and original gold poire hands. Polished case, the back with an applied foliate gold monogram DG set with rose-cut diamonds. Dial, case and cuvette signed, case and movement numbered. Accompanied by the original V&C fitted box.
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Diameter: 51 mm
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Signed: Vacheron & Constantin Genève
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Using a telemeter scale it is possible to measure the distance of a phenomenon that is both visible and audible. The chronograph is started as soon as the phenomenon is seen and is stopped when the sound is heard. The position of the hand on the scale then shows the distance in kilometres or miles separating the phenomenon from the observer. The calibration is based on the speed with which sound travels through air – approximately 340 metres or 1,115 feet per second.