Nov. 28, 1859.] GALTON'S SUN SIGNALS FOR TRAVELLERS. 15 



clear day, the Bide of the effective area of the mirror must subtend 



1" 1" / — Jt" 



X . sin ; but, on a hazy one, x . sin • a / 



In most cases, a still further increase of size becomes necessary, 

 because the landscape that forms a background to the station of the 

 signaller, when seen through a mass of luminous haze, ceases to be 

 of its natural dull colour, and may assume an appearance, nearly as 

 bright as that of the sky itself. 



The degree of brilliancy of the sun above head, has not much 

 influence on the visibility of the flash — for the brighter the day, the 

 more luminous the landscape, and the contrast between the flash 

 and the surrounding tints is but little aff'ected. 



It follows from all this, — though we have no space in this abstract 

 to enter into the details, — ^that a mirror of a few square inches in size, 

 even though considerably inclined, is amply suflficient not only to 

 be seen for distances far exceeding those ever used in ordinary 

 telegraphy, but, also, to attract attention through the brilliancy of 

 its flashes, whenever the high land, distant ship, &c., where the 

 signaller may be standing, is itself, even dimly, visible. 



The difficulty is to direct the flash aright, for, as the rays of the 

 sun are reflected from a mirror in a cone precisely similar to that 

 which reaches it, the mirror itself (whose size may be disregarded) 

 being the apex of the cone and the sun's disc its base, it follows 

 that, to the signaller, whose eye is near the mirror, the place 

 where the cone of reflected rays falls on the distant landscape would 

 always appear to him as a disc of simply the same shape and size 

 as the sun itself. In the author's heliostat, an image of the sun is 

 produced, which precisely overlays the area on which the flash of 

 the mirror falls. It is contrived on the following principle. Fig, 1 

 is a tube with a lens across one of its ends to whose true solar focus 

 a screen of white paper, f, is adjusted ; a mirror, M, turns on an 

 axle attached to the tube, which allows it movement in one 

 direction, while the rotation of the entire instrument in the hand 

 gives movement in the other. When the mirror is so adjusted that 

 the reflected (parallel) rays from any one point of the sun's disc 

 impinge on the lens, they are brought by its means to a focus on 

 the screen, and form a minute speck of light. Kays radiate from 

 this in all directions, and those that strike the lower end of the 

 lens are reduced, by its means, back again to parallelism with the 

 rays that originally left the mirror. Consequently an eye, looking 

 down the tube, sees a bright speck of light on the lens, which it 

 refers to the same distant point, d, in the landscape seen to the side 



