16 REPORT—1858. 
further than any terrestrial object whatever ; and the instrument about to be described 
shows how this remarkable power may be utilized for the purposes of telegraphy. 
Heliostats are used in all Government surveys, and their power is well known in 
penetrating haze, and their utility in requiring no ‘sky line.’”” They were also 
habitually employed by the Russians for telegraphy during the Crimean war. But 
all heliostats that have been hitherto used have been fixtures of large dimensions ; 
commonly, a shaded screen, with an aperture in it, was placed at many yards from 
the signaller, who stationed himself in such a way that when he could see the play 
of his flash about the hole in the screen, he might be sure that some of the rays 
which passed through the aperture would be visible at the distant station. At other 
times, a polished ring was used for the same purpose as the screen, but the principle 
was the same. The present instrument dispenses with all fixture ; it is more port- 
able than a ship’s telescope and as manageable as a ship’s quadrant, and may be 
made by a carpenter for 4s., if he possesses a convex spectacle-lens of short focus 
and a piece of good looking- glass. The looking-glass attached to the heliostat is 
about 3 inches by 43 inches, and therefore capable of being seen at distances, which 
may be calculated from the fact, that a mirror 1 inch square is perfectly visible, in 
average sunny weather, at the distance of 8 miles, and that it shows as a brilliant 
and glistening star at 2 miles. Before describing its principle and action, it will be 
necessary to explain clearly the peculiar characteristic of the reflexion of the sun’s 
rays from a mirror. If, for instance, we take a small square looking-glass and throw 
its flash upon a wall 2 or 3 feet off, the shape of the flash will be little different from 
that of the mirror itself, seen in perspective; but if we direct it on an object 3 
or 4 yards off, the angles of the flash will appear decidedly rvunded ;, at 20 or 30 
paces it will appear fairly circular; and if we can manage to see it at 50 or 100 
yards (which can only be effected by selecting some object to throw it on, that is 
naturally of a light colour, but lying under a dark shade), it will appear like a mock 
sun, of identically the same shape and size as the sun itself; and for all greater di- 
stances the appearance remains the same. That is to say, whatever may be the shape 
or size of the mirror, and whatever the irregularity of the distant objects on which 
the flash happens to be thrown, the shape and size of that flash, if it could be seen 
by the signaller, would always appear to him as exactly that of the sun. In fact, 
the flash forms a cone of light, at the blunted apex of which are the mirror and the 
signaller’s eye, and whose vertical angle equals that of the sun’s angular diameter. 
Whoever is covered by the flash sees the mirror, like a small fragment of the sun 
itself, held in the hand of the observer; and the larger the mirror, compared to the 
distance, the larger and the more dazzling does it appear. Now, the hand heliostat 
provides a bright appearance of the sun, which, when the instrument is adjusted and 
looked through, overlays the exact area which is covered by the flash of the mirror, 
which is attached to its side. It is a perfect substitute for that mock sun which we 
can see at 50 or 100 paces distant, but which becomes too faint to be traced much 
further. All we have to do, when we wish to send a flash to a distant object, is to 
make that image of the sun overlay the object, just as may be done in rough sextant 
observations. The principle of the instrument is extremely simple. A convex lens, 
of any focal distance (5 inches is convenient), has a small screen attached to it, 
whose surface is at its focal distance. The mirror is so placed that a small portion 
of its flash impinges upon one end of the lens; say, the right-hand side of it. The 
signaller’s eye looks partly through the other end of the lens, and partly free of it. 
Now the rays from any one point of the sun’s surface are converged by the right 
side of the lens to a bright speck on the screen ; and those rays which radiate from 
that speck and impinge on the left side of the lens are brought back by means of it 
to a state of parallelism with the rays that originally left the mirror. Consequently 
the signaller’s eye sees the bright speck in the precise direction of the vanishing 
point of the mirror’s flash, and he can, by looking partly to the side of the lens, 
refer it to some particular spot in the distant landscape. But what is true for any 
one point on the sun’s disc is true for every point, and accordingly we obtain a 
bright disc upon the screen, which appears of exactly the same shape and size as 
the sun itself, and necessarily overlays the exact area covered by the flash of the 
mirror, It is scarcely possible to describe the instruments that were submitted to 
the Association without drawings. They consisted of a tube of wood 15 inches long, 
