20 Life ami L^'tttrn of Francis (ialton 



as many as it does of his 'hyperscope,' a forerunner of the modern periscope'. 

 Galton's hand-heliostat while descriU^d in 1858 in a form which he himself 

 carried in a large waistcoat pot'ket and which he considered efficient up to 

 ten miles and said would on many occasions have been most valuable to 

 him in Damaraland wjls, in a larger si/e and with a stand, ultimately nianu- 

 facture<l by Messrs Troughton and Sinuns under the name of (jullon's Sun- 

 Signal. According to the former Admiralty Hydrographer, Sir William 

 Wharton, it was used (juite 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 flasli and by aid of it created an image of the sun in the 

 field of view ol" a telescope ; this image was then thrown on any required 

 point of the landscixpe, and when so thrown any one at that point would see 

 the fljvsh. In his paper to tlie Briti-sh Association 18.')8, (Ialton describes his 

 own rough model' which he stiys 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 4^". The mirror turns on an axis 

 perpendicular to that of the tube, and the lens pai'tly in and partly out of 

 the tube brings a portion of the fljish 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. 



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 supeiposed on the object at D. Fig. 2 is a 

 simple pocket form; Fig. 3 a more elalxjrate form, which has a theodolite 

 telescope A, and a plain tube B asn. 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 of parallel mirrors set at an angle of 45° to the axis of a sijuare 

 tuJw (generally of card!) with a hole in the opposite walls facing each mirror. Galtxjn designed 

 them in order to see a ceremony over the heads of a crowd or U> inspect what lay beyond a high 

 wall. The instrument has hIso Ijeen called the altiscope, and the principle of the modern peri- 

 Bcope of the submarine is identically the same. Hyperscopes, prol»ably 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. 



* Metnories, p. 165. * In the Galton laboratory. 



