HELIOGRAPHY is the name applied to the method of communicating between distant points in which visual signals are obtained by reflecting the rays of the sun from a mirror or combination of mirrors in the required direction. This method can of course be only employed to advantage in places where the sky is free from clouds and the atmosphere clear for considerable periods of time, and the fact that an atmospheric change may indefinitely delay the transmission of a message is an insuperable objection to the establish-ment of permanent heliographic stations in most climates.
In favourable localities, however, heliography possesses important advantages for military signalling over other methods, the principal being the portability of the appara-tus, the great distance to which messages can be sent with-out retransmission, and the fact that the signals are visible to those only who are on the direct line of signalling.
The signals are produced by causing the reflected beam to appear and disappear or be obscured alternately at the distant point, the intervals of appearance and obscuration being usually varied in length, so as to produce the com-binations of long and short signals known as the Morse alphabet. This is done in two distinct ways. In the first of these, known as signalling by "appearance," the reflected beam or "flash" is obscured from the receiving station except when a message is being transmitted, the flashes then giving the signals or " dots " and " dashes " ; while in the second method, or signalling by " obscuration," the flash is kept continually on the receiving station, and only obscured for intervals forming signals. The first method is more easily read by learners; the second is less fatiguing to the eye, and possesses the advantage of enabling the signaller to correct the adjustment of his instrument at any time.
Glass mirrors with a plane surface are employed, hence the angle of divergence of the extreme rays in the reflected beam is the same as that subtended by the sun's diameter at the point, or about 32 minutes of arc; this small diver-gence rendering the flash visible to great distances.
The distance through which signalling by this method can be carried on depends on the size of the mirror em-ployed and the angle of reflexion of the rays, a large mirror giving a more intense " flash" than a small one, since it reflects more rays, and being therefore visible to a greater distance ; also when the angle of reflexion is nearly a right angle the flash will be more intense than when the rays are only diverted through a small angle from the greater extent of surface exposed to the sun's rays. The range, however, depends to so great a degree on the state of the atmosphere that it is impossible to assign any limits to the distance that the flash might be seen through in exceptionally clear weather. From the Himalayas a 5-inch mirror has been found to give very distinct signals over a distance of 60 miles; but when the air was at all misty a very much larger mirror had to be employed.
The more perfect reflexion when the angle between the direct and reflected rays is large is not sufficient to compensate for the loss of intensity caused by the small number of reflected rays. Hence when this angle is greater than a right angle two mirrors are used, the sun's rays being reflected from one back to the other and thence to the distant point. This is called the duplex method of working.
Two forms of instrument are at present used in the British army. In the one known as Begbie's field heliostat the mirrors are square, the sun mirror having a side of 5 inches, and the signalling mirror 4 inches. The object of this difference is to ensure the latter receiving the flash over its whole extent of surface. The mirrors are screwed on to tripod stands, the stand for the signalling mirror having a sighting bar with a frame of cross wires at the end, by means of which the alignment can be accurately taken by looking through a hole in the centre of the mirror. When this has been found a small white disc is put into a hole in the centre of the cross wire frame, and a black disc in the central hole in the signalling mirror, or, if two mirrors are used, the white disc in the signalling and the black disc in the sun mirror. The flash is then brought on the true line by making the shadow corresponding to the black disc fall on the white one; this is accurately obtained by means of slow-motion screws in a vertical and horizontal direction, by which means also the sun's apparent motion is continually corrected. The obscuration is effected by a screen placed on a tripod in the alignment of the flash, pivoted horizontally, and worked by a handle. In the heliograph patented by Mr Mance the mirrors are circular, the obscuration being effected by giving a small angular motion to one of the mirrors, causing the reflected beam to travel through twice the angle and completely disappear from the distant point. This is more easily manipulated than obscuration by a screen. Its working is made very similar to that of an ordinary Morse key. The sun's motion is corrected by slow-motion screws, and the arrange-ments for directing the flash are very similar to those in the heliostat. Two tripod stands are employed: one is for the signalling mirror; the other in single working carries a jointed arm, fitted with a small white metal tablet having a black spot on which is thrown the shadow from an un-silvered hole in the centre of the signalling mirror, while in duplex working a piece of paper in the centre of the second mirror answers the same purpose.
In addition to its usefulness as a signalling instrument, the heliograph has been found of great service in defining exactly distant points for large surveys, such as the triangulation of India. For this purpose it was constantly employed by the late astronomer-royal at the Cape, Sir T. Maclear, in his verification of the arc of the meridian. (P. C.)