16 



GALTON'S SUN SIGNALS FOR TRAVELLERS. [Nov. 28, 1859. 



of it, as that to which the unobstructed rays from the mirror are 

 being flashed. If a telescope be used, the white spot actually 

 appears to overlay the distant point. Now what is true for any 

 one point in the sun's disc is true for every point : therefore the 

 signaller sees a luminous disc, and not a mere point, in the field of 

 view of the instrument, and this exactly overlays the locm of the 

 flash. By gently rotating the hand, the image can be made to cover 

 or to forsake any given object that may be desired, and, when that 

 is done, the rays of the mirror will produce an appearance of flashes, 

 as seen from that object. 



Very small instruments, of great efficiency, can be made as in 

 Fig. 2. Their tubes should pull out to not less than 4 inches, or it 

 will be difficult to make signals when the sun is low and behind 

 the back, on account of the shadow of the head. 



Inches 



A much more perfect instrument is shown in Fig. 3. (Fig. 4 is a 

 section of Fig. 3 through cc.) The lower tube b is a plain tube, 

 and simply used as a "finder;" the upper tube, a, is a theodolite 

 telescope, and affords means of signalling with the utmost precision. 

 When the sun's image covers the distant station at the time that 

 the latter is seen at x in the plain tube, then, on looking through 

 the telescope above it, the sun's image will be found to overlay the 

 object, whenever the latter is brought into the field of view. The 

 adjustment, by which this result is ensured, is by drilling the eye 

 hole of the plain tube so that a line passing through it and through 

 X shall be parallel to the axis of the telescope.. By pushing the 

 slide, s, backwards or forwards, the quantity of light that can reach 

 the lens is regulated at will, and the image of the sun can be 

 toned down to any required shade. A little practice with the 

 instrument makes it exceedingly easy to bring the image of the sun 

 on to the field of view in the first instance. It is done by grasping 



