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20   Life and Letters of Francis Galton

as many as it does of his `hyperscope,' a forerunner of the modern periscopes. Galton's hand-heliostat while described in 1858 in a form which he himself carried in a large waistcoat pocket and which he considered efficient up to ten miles and said would on many occasions have been most valuable to him in Damaraland was, in a larger size and with a stand, ultimately manufactured by Messrs Troughton and Simms under the name of Galton's SunSignal. According to the former Admiralty Hydrographer, Sir William Wharton, it was used quite recently in nautical surveys to enable shore parties to make their exact whereabouts visible to those on the ship'. The principle on which Galton based his heliostat is a fairly simple one. He intercepted a small part of the flash and by aid of it created an image of the sun in the field of view of a telescope; this image was then thrown on any required point of the landscape, and when so thrown any one at that point would see the flash. In his paper to the British Association 1858, Galton describes his own rough model' which he says any carpenter could make for four shillings, indeed the tube was of wood, the lens a convex spectacle glass, and there was a piece of good looking-glass 3" x 4L". The mirror turns on an axis perpendicular to that of the tube, and the lens partly in and partly out of the tube brings a portion of the flash to a focal image of the sun on a small screen inside the' tube. When the image of the sun covers the point to which the flash is to be sent, then the flash will be seen at that point;



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Fig. 1 explains the working; M is the mirror with the sun's rays falling on it and reflected in direction D, F-is the screen at the focus of the lens, which is seen by the eye as superposed. on the object at D. Fig. 2 is a simple pocket form; Fig. 3 a more elaborate form, which has a theodolite telescope A, and a plain tube B as a finder.. Fig. 4 shows the section of Fig. 3 at C with a holder which can be screwed on to a camera tripod. A fairly

' Both are arrangements rrangements of parallel mirrors set at an angle of 45° to the axis of a square tube (generally of card!) with a hole in the opposite walls facing each mirror. Galton designed .them in order to see a ceremony over the heads of a crowd or to inspect what lay beyond a high wall. The instrument has also been called the altiscope, and the principle of the modern periscope of the submarine is identically the same. Hyperscopes, probably not under this name or with any knowledge of Galton's early work, were used in the trenches in the course of the recent war.

11 Memories, p. 165.   ' In the Galton Laboratory.